Fuel Consumption and Consumption Optimization [2 ed.] 3662664488, 9783662664483

Table of contents :
1
Preface
Contents
978-3-662-66449-0_1
1 Fuel Consumption and Consumption Optimization
1.1 Fuel Consumption Optimization as an Economic Necessity
1.2 Fuel Consumption Optimization for the Environment
1.3 Energy Consumption Optimization for ALL Drive Systems
978-3-662-66449-0_2
2 Vehicle and Energy Loss
2.1 Motion Resistance
2.2 Energy Consumption in a Vehicle with Combustion Engine
2.3 Energy Consumption of a Vehicle with an Electric Drivetrain
2.4 Factors that Influence Consumption
978-3-662-66449-0_3
3 Vehicle Technology
3.1 Engine
3.1.1 E-machine
3.2 Drivetrain
3.2.1 Transmission
3.2.2 Axles
3.3 Auxiliary Consumers
3.4 Tires and Rolling resistance
3.5 Aerodynamics
3.5.1 Aerodynamics of the Driver’s Cab
3.5.2 Optimization of Aerodynamics
3.6 Technical Aids for the Driver
3.6.1 Predictive Systems
3.7 Curb Weight
3.8 Full Trailers, Semitrailers and the Load
978-3-662-66449-0_4
4 Operating Conditions of the Vehicle
4.1 Topography of the Route
4.2 Weather and Temperature
4.2.1 Vehicle with a Warmed-Up Engine
4.3 Traffic
4.3.1 Convoy Driving—Platooning
4.4 Speed
4.5 Loading
4.6 Consumption Reductions Through Optimized Operating Conditions
4.7 Reduced Consumption Through Optimized Logistics Concepts
978-3-662-66449-0_5
5 The Influence of the Driver on Energy Consumption
5.1 Consumption Values of New Vehicles
978-3-662-66449-0_6
6 Maintenance of the Vehicle and Service Fluids
6.1 Tire Pressure
6.2 Energy Content of the Fuel
6.3 Lubricating Oils
978-3-662-66449-0_7
7 Concluding Remarks on the Topic of Energy Consumption
7.1 Measurement of the Fuel Consumption
7.2 Effect of External Factors on Fuel Efficiency
7.3 Scope of the Energy Efficiency Considerations
1 (1)
Comprehension Questions
Abbreviations and Symbols
References
Index

Citation preview

Commercial Vehicle Technology Series Editor Michael Hilgers, Weinstadt, Baden-Württemberg, Germany

Michael Hilgers

Fuel Consumption and Consumption Optimization Second Edition

Michael Hilgers Daimler Truck Weinstadt, Baden-Württemberg, Germany

ISSN 2747-4046 ISSN 2747-4054  (electronic) Commercial Vehicle Technology ISBN 978-3-662-66448-3 ISBN 978-3-662-66449-0  (eBook) https://doi.org/10.1007/978-3-662-66449-0 © Springer-Verlag GmbH Germany, part of Springer Nature 2021, 2023 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer Vieweg imprint is published by the registered company Springer-Verlag GmbH, DE, part of Springer Nature. The registered company address is: Heidelberger Platz 3, 14197 Berlin, Germany

Preface

For my children Paul, David and Julia, who derive just as much pleasure from trucks as I do, and for my wife, Simone Hilgers-Bach, who indulges us.

I have been working in the commercial vehicle industry for many years. Time and again I am asked, “So you work on the development of trucks?” Or words to that effect. “That’s every boy’s dream!” Yes, it really is! Armed with this enthusiasm, I have attempted to create as complete a picture of truck engineering as possible. In the course of this, I realized that one has not really fathomed out the facts of any subject until one is able to logically explain those facts. Or to put it in a nutshell, “To be able to learn something properly, you have to teach it.” So, in the course of time I started to write down in my own words as many of the technical aspects of commercial vehicle technology as possible. Many of the articles on the subject of commercial vehicle technology deal with individual technical systems and assemblies. However, this booklet deals with just one characteristic of the vehicle: energy consumption. As will be shown, this characteristic is of paramount importance for the truck as a capital good. In addition, for the conventional drivetrain with combustion engine the subject of fuel consumption and CO2 emissions is of upmost relevance in today’s society. Finally, fuel consumption and the numerous technical solutions that optimize fuel consumption are an interesting and inspirational topic from the standpoint of the engineer. This booklet provides a compact and easily comprehensible survey of fuel and energy consumption and the numerous aspects that affect it. At this point I would like to thank the publisher, Springer Verlag, for their friendly cooperation, which has made the end result possible. Last but not least, I have a small request on my own behalf. It is my intention to maintain continual further development of this text. Dear readers, I would greatly welcome your help in this regard. Please send any technical comments and suggestions for improvements to the following email address: [email protected]. The more v

vi

Preface

tangible your comments, the easier it will be for me to comprehend them and, where appropriate, integrate them into future editions. And now, I hope you have lots of fun saving energy. Weinstadt-Beutelsbach Beijing Aachen August 2022

Michael Hilgers

Contents

1 Fuel Consumption and Consumption Optimization. . . . . . . . . . . . . . . . . . . . 1.1 Fuel Consumption Optimization as an Economic Necessity. . . . . . . . . . . . 1.2 Fuel Consumption Optimization for the Environment . . . . . . . . . . . . . . . . 1.3 Energy Consumption Optimization for ALL Drive Systems. . . . . . . . . . . .

1 1 4 4

2 Vehicle and Energy Loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Motion Resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Energy Consumption in a Vehicle with Combustion Engine. . . . . . . . . . . . 2.3 Energy Consumption of a Vehicle with an Electric Drivetrain . . . . . . . . . . 2.4 Factors that Influence Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 7 8 10 10

3 Vehicle Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 E-machine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Drivetrain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Axles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Auxiliary Consumers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Tires and Rolling resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Aerodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Aerodynamics of the Driver’s Cab. . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Optimization of Aerodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Technical Aids for the Driver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Predictive Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Curb Weight. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Full Trailers, Semitrailers and the Load. . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 14 16 17 17 19 21 24 25 30 33 39 40 42 42

4 Operating Conditions of the Vehicle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Topography of the Route. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Weather and Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Vehicle with a Warmed-Up Engine . . . . . . . . . . . . . . . . . . . . . . . . .

45 45 46 46 vii

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Contents

4.3 Traffic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Convoy Driving—Platooning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Consumption Reductions Through Optimized Operating Conditions. . . . . 4.7 Reduced Consumption Through Optimized Logistics Concepts. . . . . . . . .

46 48 49 49 49 50

5 The Influence of the Driver on Energy Consumption. . . . . . . . . . . . . . . . . . . 5.1 Consumption Values of New Vehicles. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53 54

6 Maintenance of the Vehicle and Service Fluids . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Tire Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Energy Content of the Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Lubricating Oils. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55 55 56 57

7 Concluding Remarks on the Topic of Energy Consumption. . . . . . . . . . . . . . 7.1 Measurement of the Fuel Consumption. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Effect of External Factors on Fuel Efficiency. . . . . . . . . . . . . . . . . . . . . . . 7.3 Scope of the Energy Efficiency Considerations. . . . . . . . . . . . . . . . . . . . . .

59 59 59 60

Comprehension Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

63

Abbreviations and Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

67

Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1

Fuel Consumption and Consumption Optimization

1.1 Fuel Consumption Optimization as an Economic Necessity Fuel consumption—or in our days we better talk about energy consumption—has been a major driver of technical progress in the trucking industry for decades. As the commercial vehicle is an investment good, purchased and operated to earn money, total cost and the share of the different cost factors have always played a major role. For the last decades in most markets fuel consumption was and still is the second most important cost factor in trucking business, second only to the cost of the driver. In long-distance haulage, diesel consumption accounts for 30% of the costs calculated over the entire service life of the vehicle. Figure 1.1 shows the cost breakdown of a European long-distance trucking company. The exact proportions of the individual costs does of course differ from trucking company to trucking company, because the exact cost breakdown depends on the vehicle and vehicle body, and in particular on the specific operational use of the vehicle. Road tolls, personnel costs and taxes vary from country to country. The cost breakdown shown below corresponds with an evaluation by the Bundesverband Güterkraftverkehr, Logistik und Entsorgung (BGL) e. V. (Federal Association of Road Haulage, Logistics and Waste Management) of German trucking companies in 2007 [42]. The same evaluation in 2015 produced very much the same result. Similar illustrations can also be found elsewhere [8, 35]. Figure 1.2 shows that the price of diesel in Germany increased over decades. Hence fuel-efficient trucks are important for the profitability of trucking companies for decades. The price of diesel is determined by the global crude oil prices and taxation. The decade of 2010 to 2020 saw a phase of a sideways movement in the diesel price. The average diesel price was more or less constant but showed a significant volatility. See Fig. 1.3. showing the US as an example. Long term further rising diesel prices can be assumed and might even be desired from a climate protection point of view. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_1

1

2

1  Fuel Consumption and Consumption Optimization

Fuel and oil: 27.2%

Driver costs: 28.6%

Tolls, taxes and insurance: 11.5%

Other costs and interest: 4.2% Administrative costs: 10.6%

Depreciation: 8.5% Maintenance, repairs: 7.2%

Tires: 2.3%

Cost of diesel [Cent/l]

Fig. 1.1   Cost breakdown of a European long-distance trucking company in 2007. From [42]

May 2008

150

100

50

0 1950

1960

1970

1980

1990

2000

Year Fig. 1.2   Diesel price trend in Germany (including taxes) from 1950 to 2008. Source Verkehrsrundschau 25/2008 (transport review)

Since a few years, growing concerns about the environmental impact of burning (fossil) fuels and the worldwide problem of climate change added an additional motivation to reduce energy consumption—see below. The less fuel is burnt the better for the environment. Long-term the conventional drivetrain using fossil fuel will be replaced by alternative drive concepts—see [4]. For the alternative drivetrains, optimizing energy consumption will be extremely important as well:

1.1  Fuel Consumption Optimization as an Economic Necessity

3

Fig. 1.3   Diesel price trend in the U.S. (including taxes) in the last 10 years. Source U.S. Energy Information Administration

Hydrogen is one of the potential fuels of the future. Today hydrogen is expensive (relative to fossil fuel with the same energy content) and currently there is little reason to assume that Hydrogen will become a cheap energy source. So reducing energy consumption is a good idea for hydrogen driven machines. The energy storage (battery) in battery electric vehicles is costly and heavy. Battery size directly translates into driving range before recharging is required. Therefore, there is always the trade-off to be made between battery cost and weight versus driving range. Optimized energy consumption is a must for battery electric trucks to offer a certain driving range in a cost-efficient way. From today’s knowledge it must be assumed that alternative liquid fuels (synthetic fuels), that might replace diesel, will not be cheaper than diesel. If they were, they would already have replaced the fossil diesel. So efficient usage of those fuels is required. Whatever the fuel is, whatever the energy is we are using to propel our trucks it will always be a high priority quest to reduce energy consumption in a truck.

4

1  Fuel Consumption and Consumption Optimization

1.2 Fuel Consumption Optimization for the Environment In addition to the commercial importance for the trucking company, reduced energy consumption also means active environmental protection. This is true for all drive systems: smaller batteries have a smaller environmental footprint, less hydrogen production needed or less synthetic fuel consumption will be beneficial for the environment. Diesel from fossil deposits contribute to climate change. As already mentioned, long term the diesel engine running on (fossil) diesel will be replaced. But obviously diesel trucks will be around for quite a while. In this while it is our task to reduce diesel consumption as much as possible. The less diesel the engine consumes, the less carbon dioxide (CO2) it generates. Every liter of diesel fuel saved represents a reduction of about 2.6 kg in the greenhouse gas CO2 for our environment. If we assume a truck traveling 150,000 km a year with an average fuel consumption of 34 l/100 km (realistic for a 40 ton truck) a 3% fuel reduction results in fuel savings of 1500 L and it saves approx. 4000 kg CO2 per year to our environment.1 As environmental concerns have been growing over the last decades not only truck manucfacturers but also public programmes are researching for better more efficient vehicle technologies—see [55] as an example.

1.3 Energy Consumption Optimization for ALL Drive Systems Many measures to reduce energy consumption are independent of the particular drive system or the fuel: Better energy efficiency through optimized aerodynamics, the correct tires, anticipatory driving, optimized logistical concepts and an optimized infrastructure are not only expedient for present-day conventional trucks with diesel engines but are transposable to all future technologies that are currently under discussion. The consumption-lowering possibilities available for design and operation of the conventional vehicles with which we are familiar today can be used just as well for vehicles with alternative fuels, hydrogen drives or battery electric vehicles. Many of the possibilities to optimize energy efficiency are combinable: An aerodynamically optimized vehicle can be running on low friction tires propelled by a e-motor steered by a foresighted driver on a logistically optimize route at times of the day when traffic density is low. It is worthwhile to work on all aspects of energy consumption.

1 Example

from the US for a similar application: One gallon of fuel (3.785 l) generates 9.842 kg, or approximately 22 lb of CO2. If fuel savings of 3% are possible on a fully laden truck with a gross weight of 80,000 lbs traveling 100,000 miles per year, the environment will be relieved of over 9000 lbs of CO2 each year, based on an average consumption of about 7 miles per gallon.

1.3  Energy Consumption Optimization for ALL Drive Systems

5

However some of the solutions discussed in this booklet focus on aspects that are specific to the fuel consumption optimization on conventional vehicles. It can be safely assumed that diesel powered vehicles will be around and need to be optimized for at least the next decade.

2

Vehicle and Energy Loss

2.1 Motion Resistance Various external forces act on the vehicle: the rolling resistance, the aerodynamic drag and the gradient resistance when driving uphill. These forces decelerate the vehicle. To maintain speed, the drivetrain of the vehicle must deliver an equal driving force. If the vehicle is to not just maintain a constant speed, but even accelerate, an accelerating force must be added to this. This results in a total force (Eq. 2.1):

FTotal = FAero + FRoll + FGrade + FAcceleration

(2.1)

The total driving power required for a vehicle moving through windless air is (Eq. 2.2):

PDrive = FTotal · v = FAero · v + FRoll · v + FRoll · v + FGrade · v + FAccelaration · v 1 = · ρ · v 3 · A · cd 2 + mTotal · g · cRoll · cos(α) · v

(2.2)

+ mTotal · g · sin(α) · v + mTotal · a · fRot · v where mTotal is the mass of the vehicle, v is the speed and g is the gravitational acceleration. The angle α denotes the inclination of the road surface from the horizontal. fRot is the so-called rotational inertia coefficient, which takes into consideration that for acceleration an additional force is required to overcome the moment of inertia of the rotating masses (wheels, propeller shaft, etc.). The terms for the aerodynamic drag FAero and rolling resistance FRoll are defined in detail in Eq. 2.2. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_2

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2  Vehicle and Energy Loss

The energy that is expended to overcome the aerodynamic drag and the rolling resistance is frictional energy that must be considered directly as lost energy. In contrast, the energy that is invested to overcome the gradient resistance (uphill driving) and to accelerate the vehicle is subsequently present in the vehicle in the form of potential or kinetic energy. This energy is consumed (later) during a rolling phase or consumed by braking.

2.2 Energy Consumption in a Vehicle with Combustion Engine The energy source in a conventional vehicle is the diesel. The diesel engine converts the chemical energy of diesel fuel into mechanical energy. Diesel consists of liquid hydrocarbons. The exact composition of diesel is inhomogeneous. It depends on the original material fed into the diesel production process. The minimum requirements of diesel fuel are defined in the German standard DIN EN 590. Tab. 2.1 shows the density and the calorific value of diesel fuel. On the basis of the values in Tab. 2.1, a liter of diesel contains approximately 10 kWh of energy,1 Eq. 2.3: (2.3)

0.838 kg/l · 42.6 MJ/kg = 35.7 MJ/l = 9.9 kWh/l

The process that converts the chemical energy of the diesel into kinetic energy of the vehicle is subject to loss. In Fig. 2.1 it is analyzed to where the energy of diesel fuel goes. Shown here by way of the example of a modern long-distance tractor semitrailer combination operated on the Stuttgart–Hamburg–Stuttgart (Germany) route. The table of losses shown is a little different for each route. There are routes on which braking is more frequent and those on which the average speed is lower, and therefore the energy loss due to aerodynamic drag has a somewhat smaller proportion. The breakdown shown is based on an experienced driver who practices anticipatory driving and controls the vehicle optimally in terms of fuel consumption.

Tab. 2.1  Properties of diesel fuel

1 A

Diesel

Value

Range

Density

0.838 kg/l

0.82–0.845 kg/l

Calorific value

42.6 MJ/kg

39–43.2 MJ/kg

joule is equivalent to a watt-second. A kilowatt hour is equal to 3.6 · 106 W-seconds, which is equal to 3.6 · 106 MJ.

2.2  Energy Consumption in a Vehicle with Combustion Engine

9

Total energy of the diesel fuel consumed 100% Mechanical energy at the crankshaft 44.2%

Engine brake: 3.4%

Engine drag resistance 1.8%

Braking energy 10.9%

Service brake 5.7%

Motion resistance 29.2%

Rolling resistance 15.6%

2.7%

Aerodynamic drag 13.6%

Alternator, power steering pump 0.3% Fan 0.3% Air compressor 0.8%

21.3%

Heat dissipation 2.5%

Energy in the exhaust gas 31.3%

Energy entrainment into the cooling system (coolant and charge air cooling)

Water pump, oil pump and injection pump 0.7%

1.4%

Drivetra in

Axle losses 1.5%

Auxiliary consumers

Thermodynamic losses and mechanical losses

Transmission losses 1.2%

Losses in the engine 55.8%

Fig. 2.1   Analysis of the energy losses in accordance with [8]. A fully laden Mercedes-Benz Actros-3 tractor semitrailer combination (40 t) on the Stuttgart–Hamburg–Stuttgart route (1517.2 km) was analyzed. A Mercedes-Benz OM501LA engine with 320 kW, 2100 Nm (435 hp) and the emissions standard EURO 5 was considered. The average speed was 83.2 km/h and 532.7 l of diesel were consumed on the entire route, resulting in an average consumption of 35.1 l per 100 km. The representation of the different losses is not shown to scale

In the breakdown shown, the engine achieves a mean mechanical efficiency of approximately 44%. Fifty-six percent of the combustion energy of diesel fuel is lost in the form of heat. The engine needs about 1% of the diesel fuel energy to sustain its own function. Losses due to auxiliary consumers and due to friction in the drivetrain sap the mechanical energy provided by the engine at the crankshaft; so ultimately, roughly 40% of the chemical energy of the diesel is available at the wheels. This energy serves to overcome the motion resistance and accelerate the vehicle. For other combustion engine technologies that will be used in the future (for example hydrogen combustion engines) a picture similar to that of Fig. 2.1 will emerge. The mean mechanical efficiency might be somewhat different but the overall breakdown of the energy consumption is similar. In electric drivetrains however there is an additional very important mechanism to be considered: recuperation. Part of the braking energy will be used to recharge the battery. This effect must be taken into account in analyzing energy usage of an electric truck.

10

2  Vehicle and Energy Loss

Fig. 2.2   Schematic analysis of the energy losses in an electric truck. Here a distribution truck with high amount of recuperation is shown. Auxiliaries summarizes energy used for heating and cooling for cab and aggregates, steering pump, air pressure unit, 24 V net and the like

2.3 Energy Consumption of a Vehicle with an Electric Drivetrain Figure 2.2 shows a schematic analysis of the energy losses in an electric truck. A distribution truck with a high amount of recuperation is shown here to illustrate the difference to a conventional diesel driven truck. If the vehicle is braking part of the energy is recovered and fed back to the battery. Heating and cooling the cab and the battery must be driven by electricity from the battery. This energy is included in “auxiliaries” in Fig. 2.2 whereas in the conventional drivetrain the energy for heating is taken from the energy disposed to the cooling system. So the “auxiliaries” part in Fig. 2.2 represents a bigger portion of the overall energy consumption than in the conventional ICE-propelled drivetrain.

2.4 Factors that Influence Consumption The diesel consumption of conventional trucks as well as the energy consumption of vehicles with alternative propulsion systems is determined by four different factors [6]. These are dealt with in separate sections in this booklet:

2.4  Factors that Influence Consumption

11

1. Vehicle technology: Modern vehicle technology makes the most important contribution to consumption-optimized transport. Low rolling resistance, optimized aerodynamics, auxiliary consumers with reduced power loss and an efficient drivetrain are the technical ingredients for an energy-efficient vehicle. Besides the tractor, the semitrailer or full trailer also contributes decisively to the total consumption. The degree of efficiency of the diesel engine of 44% in Fig. 2.1 dates back to around 2008 [8]. Since then it has been improved and engineers still work on improving conventional engines. Components of the electrical drivetrain will be constantly improved as well. 2. The conditions under which the vehicle is operated: The energy consumption of the vehicle depends on the conditions under which the vehicle is operated. Hilly terrain will cause an increase in consumption. The weather affects vehicle losses. The load influences consumption because the gross weight of the vehicle has a linear effect on the rolling resistance. Traffic events force the driver to perform braking procedures and the subsequent acceleration phase results in additional fuel consumption. Hilly terrain and braking has a less pronounced influence in energy consumption for electric vehicles compared with conventional combustion vehicles as energy recuperation recovers some of the dissipated energy. Frequently, the operating conditions can only be altered within narrow limits, although they are actually responsible for a large proportion of the fuel consumption. 3. The driver: The third important influence is the driver. Even a technically-optimized vehicle will achieve good consumption values only if the driver operates the vehicle in a fuel-efficient manner. A good driver drives with anticipation, making braking procedures and subsequent acceleration as minimally as necessary. 4. The condition of the vehicle: The fourth and last group of consumption-determining influencing factors is the condition of the vehicle. Maintenance and care of the vehicle have a practical effect on all types of losses shown in e.g. Fig. 2.1 and Fig. 2.2.

3

Vehicle Technology

EUROI

Consumption of naturally aspirated engines Consumption of supercharged engines Consumption of intercoolers

EUROII

EUROIII

EUROIV

EUROV

CO2 emission in g/ton-kilometer

Diesel consumption in the test [l/100 km]

As fuel consumption is an important topic for the truck customer, competition between the various vehicle manufacturers ensures that new, more efficient vehicles are constantly being offered. So far, competition to offer the most efficient truck to the customer was solely on conventional trucks with combustion engines. Other trucks with alternative drivetrains were only used in very small niches. This is about to change. Both the drivetrain and the vehicle have been and are being continually optimized to reduce fuel consumption. Consequently, it has been possible to reduce the consumption of fully laden 40-t trucks by about a third between 1965 and 1995—see Fig. 3.1.

Year Source: Lastauto Omnibus (trade magazine)

Fig. 3.1   The fuel consumption of heavy trucks (40-t load) has been significantly reduced from 1965 to 1995. See [34]

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_3

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3  Vehicle Technology

The diagram also shows that less progress has been made in improving the fuel consumption since about 1990. This is attributable to the requirements of the emissions legislation, some of which requires technical solutions that result in increased fuel consumption. Furthermore, the better and more advanced the technical solution is, the more difficult it will be to find and economically implement further improvements. Sections 3.1 to 3.3 focus on optimizing the engine, its drivetrain and its auxiliaries to design the most fuel-efficient truck. The technical solutions depend on the drive type. Obviously, a lot of attention has been paid to the drivetrain with conventional combustion engine in the past. Starting with Sect. 3.4 technical solutions are discussed that will influence the energy consumption of every truck, be it propelled by fossil fuel or by electricity that might be supplied by the grid (via the battery in the vehicle) or by a fuel cell on board of the vehicle.

3.1 Engine The engine accounts for the most obvious proportion of the fuel consumption—the diesel being combusted in the engine. Various leaps in technology have reduced the specific fuel consumption of engines. The specific fuel consumption defines what mass of diesel has to be expended to perform a defined amount of mechanical work. The specific fuel consumption is usually specified in grams per kilowatt-hour (g/kWh). In addition to the specific consumption at the optimal point, an even more important factor for real use is how wide the range (in rpm and torque) is within which the engine operates efficiently. Figure 3.2 shows the so-called engine map of a modern truck engine. The engine map shows the specific fuel consumption (usually in g/kWh) in the rpm-torque range of the engine. A fuel-efficient engine must possess an engine map with a wide range on the torque-rpm-diagram where it operates with high efficiency. It is here in particular that significant progress has been achieved over the past 20 years. Figure 3.3 provides an overview of the specific diesel consumption trend. In this case, the specific fuel consumption of truck engines is illustrated at the most efficient operating point. The engine improvements that have led to fuel savings are the changeover from the chamber combustion process to direct injection, the exhaust gas turbocharger (renders usable some of the energy in the exhaust gas), intercooling (which increases the effectiveness of turbocharging), the introduction of four-valve technology, significant increase of injection pressures and SCR technology. SCR technology allows stringent exhaust gas limits to be met and at the same time, allows the engine to operate within the consumption-optimized range. In Figs. 3.1 and 3.3 it is also shown that the engine trend—parallel to the endeavor to increase efficiency—must also take into consideration the ever stricter and intensifying emissions legislation that is being passed in quick succession.

3.1 Engine

15

Fig. 3.2   Sketch of a fuel map of an engine with a speed range of up to 2200 rpm and a maximum engine torque of close to 2600 Nm. The lines consist of points with the same fuel consumption. Inside those fuel consumption lines the fuel consumption is below the value of the line. Not only the best point of the engine is decisive for a good vehicle fuel economy but also the size of the area with low fuel consumption. Note that the lines shown on a fuel map are usually progressive: the fuel consumption difference of adjacent lines in the middle of the map are much smaller than the difference between two lines in the lower part of the map. Shown is not a real engine map; however real engine maps of today’s heavy duty truck engines look very similar. Today’s best engines (2022) have a best point with a specific fuel consumption below 180 g/kWh. Note: The numerical value given to each of the lines with constant fuel consumption depends on the fuel used to measure the engine map. The value of chemical energy contained in one gram of diesel fuel is not a fixed value but can be in a certain range (see Table 2.1 in Chap. 2). Hence the mass of diesel required to produce one kWh of mechanical output varies depending on the diesel quality

Start-stop function The start-stop function that is standard in passenger cars since quite a while can also be realized for truck engines. For longhaul trucks that do not stop frequently the benefit is small so that start-stop functions are usually not used. In distribution trucks, driving through congested urban areas the truck engine might have a substantial idling time at red traffic lights. In this case, a start-stop function might make sense and can result in a fuel consumption reduction of up to 1% depending on the driving profile.

16

3  Vehicle Technology

Fig. 3.3   The specific consumption of truck engines has been significantly reduced over the past few decades. Technological leaps and continual improvements have succeeded in reducing consumption. Source Daimler, until 2008 see [8]

The prospect of further engine optimizations Possible further improvements for even more efficient engines are envisaged in a partial homogenization of the diesel-air mixture in the partial-load range and in a reduction of auxiliary losses. Figure 3.3 also shows that the internal engine measures to improve the specific fuel consumption have led to increasingly smaller savings. On the combustion side, it is likely that only minor improvement steps will be possible in the future. The diesel engine optimizations of the future might therefore deal more intensely with utilization of the residual energy in the exhaust gases. One wants to utilize a higher proportion of the unused energy that is lost with the heated exhaust gases. Around 30% of the chemical energy of the diesel fuel is lost via exhaust heat, see Fig. 2.1. Waste heat recovery systems try to make use out of the heat in the exhaust stystem.

3.1.1 E-machine In case of an electric truck (be it battery-electric or a truck with a fuel cell), the efficiency of the e-motor is influencing the total energy consumption. Luckily, efficiencies of e-motors are high. Figure 3.4 shows a schematic efficiency of an e-motor. Sometimes the efficiency map of the e-machine might be shown including additional components like the inverter or a gear set to convert torque and rotational speed. It can be seen from Fig. 3.4 that the efficiency of the e-motor depends on rotational speed and output torque. Therefor electric drive trucks might be equipped with a transmission with different gear steps to keep the e-motor in an ideal range of the speedtorque map. However, generally spoken one can say that the required transmission of an e-drive is simpler than the typical transmission for a heavy truck with combustion engine.

3.2 Drivetrain

17

Fig. 3.4   Illustrative example of an efficiency map for an e-motor. In principle, there are several efficiency maps: one for drive mode and one for regenerative mode and as an e-motor (different from combustion engines) can turn clockwise and anti-clockwise there might be different efficiency maps for the two turning directions

3.2 Drivetrain To move the vehicle, the mechanical energy that the engine provides must reach the wheels. Unfortunately, this is not free of losses. This section discusses how transmission and axles contribute to the losses in the vehicle.

3.2.1 Transmission The transmission contributes to the fuel-optimized vehicle in two ways. First, the transmission and the transmission control unit chooses the appropriate gear ratio so that the engine operates within the optimal range. Only with the correct gear ratio and selection of the correct gear can the combustion engine achieve the mean efficiency of higher than the 44% shown in Fig. 2.1. Transmission automation helps with selection of the correct gear. The automated transmission electronically selects the correct gear and executes an optimal gear change. Although a very good driver in a test will succeed in driving in the correct gear as well as perform perfectly timed gear changes, even the best driver will tire and not be able to maintain his or her performance with the constancy of a technical solution. Transmission automation therefore contributes to lower fuel consumption in everyday use.

18

3  Vehicle Technology

Secondly, mechanical power is lost in the transmission. The objective, as always, is to minimize these losses. Various transmission designs have different efficiencies. The transmission solution that offers the highest efficiency and, at the same time, meets long service life requirements is the spur gear transmission. Transmissions with 12 or 16 gears are installed on heavy trucks to achieve the high gear ratio spread that is necessary due to the high gross weights of commercial vehicles [4]. The spur gear transmission suffers from two loss types: 1. Drag losses, which are losses that occur, whenever the transmission rotates independently of the transmitted power. These include frictional losses in the bearings and synchronizations, splashing losses, frictional losses in the seals etc. 2. Frictional losses of the gear wheels under load. So-called direct-drive transmissions are preferred in the commercial vehicle sector to keep the frictional losses low under load. These are transmissions in which the highest gear is a direct drive through the transmission. Figure 3.5 shows the average transmission efficiency for each gear of an Europeanstyle 12 gear transmission with a planetary set and a direct gear in the main transmission. Transmission efficiency depends on the load. As some of the losses inside the transmission are independent of the transmitted power the efficiency of a transmission is generally somewhat higher at higher loads. The exact efficiency depends on the rotational speed and on the torque transmitted. Therefore, for each gear one can measure a rpm/ torque efficiency map (see the example for axles in the axle Sect. 3.2.2). Moreover, the transmission efficiency depends on the oil used in the transmission and on the transmission oil temperature. EcoRoll function Depending on the terrain, a proportion of the route can consist of sections on which the vehicle requires no tractive force from the drivetrain. The vehicle can simply roll by disengaging the combustion engine from the drivetrain during these rolling phases. As, for safety reasons, one would not like to drive with the clutch disengaged, the gearshift is moved to neutral position. The disengaged combustion engine continues to idle. If the speed drops below the preset cruise control speed or the driver presses the accelerator pedal, a gear is engaged again and the combustion engine drives the vehicle again. Alternatively, the fuel supply to the engine can be stopped and the vehicle is coasting downhill with the wheels turning propshaft, transmission and engine. In this scenario, no fuel is consumed at all, but the internal friction is much higher (the engine must be turned). Whether it is more beneficial to disengage the engine and let the engine run in idle or to turn the engine in coast mode from the wheels depends on the topography. In both cases the engine is still turning as this is required for safety reasons. The turning engine is still driving auxiliaries and can be started quickly if required. Putting the drivetrain in neutral AND cutting fuel supply to the engine (engine stalls) is not recommended.

3.2 Drivetrain

19

Retarder Retarders are used as wear free braking element in many trucks. The hydraulic retarder is basically a turbine wheel that is connected with the drivetrain and that is spinning through the retarder housing—see [4]. If braking is required, the retarder housing is filled with liquid (usually retarder oil). In non-braking operation, the liquid is expelled from the retarder and the turbine wheel is spinning freely through the air. To avoid the remaining energy loss from the turbine wheel spinning through air, retarders that are completely disconnectable with a clutch are used. A reduced fuel consumption of 0.2 to 0.5% compared to a conventional retarder without clutch seems realistic.

3.2.2 Axles The axle gear also has an efficiency of less than 100%. Planetary hub reduction axles that comprise two gear reductions and thereby have to twice overcome friction in the gear teeth, by their very nature have a worse degree of efficiency than single reduction axles. Planetary hub reduction axles were therefore replaced in the 1990s—at least in long-distance vehicles—by single reduction hypoid axles. The losses in the axle consist of: • • • •

gear teeth losses, losses in the various bearings (hub bearings, pinion bearings, …), frictional losses at the seals (pinion seal, wheel hub seal), and oil splashing losses.

The efficiency of an axle is not constant, it depends on the rotational speed and the torque transmitted. As the efficiency of an axle is different for different torques and speeds the overall efficiency depends on the route, the load and the driving behavior. The same by the way is true for transmissions. Figure 3.6 shows the efficiency map of a rear axle for a heavy duty truck. The transfer efficiency of the axle depends on the quality of the gears itself, the internal geometry of the moving parts and the housing and on the oil used. Synthetic oil is usually better than mineral oil. Multi stage axles or drive through axles have a lower efficiency as simple hypoid axles as they have more moving parts and gears. In the case of the single reduction hypoid axle, the hypoid gear influences the gear teeth losses. A low hypoid offset reduces the losses. The efficiency of an axle might undergo some changes during the useful life. Wear and tear might improve the efficiency. Towards the end of the technical lifetime the efficiency might fall again. Two “identical” axles might have different internal friction and different efficiency due to different bearing clearance, different bearing preload and slight differences in surface finish. In addition to the internal losses in the axle and transmission, the correctly selected transmission-axle combination also plays an important role in the design of the

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3  Vehicle Technology

Fig. 3.5   Average efficiency of a heavy duty long haul truck transmissions for the 12 different gears. The efficiency depends on the load. At higher power throughput, the efficiency is usually somewhat higher. Depending on the design of the transmission, you find somewhat different efficiencies for the different gears. The transmission shown hear has a two gear splitter, a three gear main transmission and a two gear range group (planetary gear set). In gears 7 to 12 the planetary gear set is not contributing and the efficiency is higher. The direct gear in the main transmission (6th and 12th gear) also allows for higher efficiency. See [4] for details

Fig. 3.6   Example of an efficiency map for a single reduction heavy duty rear axle. The efficiency map shows the ratio of mechanical power output to mechanical power input to the axle. The lost power is heat that is generated by ubiquitous friction phenomena. At high rotational speed and at high torque the efficiency is usually best

3.3  Auxiliary Consumers

21

consumption-optimized vehicle. Through the selection of the transmission–axle combination, it is defined at which point on the engine fuel map the engine operates in specific driving situations. An excessively short reduction drivetrain in an easy topography will lead to a high engine speed level and to greater fuel consumption: in the case of modern engines, an engine speed level that is too high by approximately 100 rpms will cause an additional consumption of 1%. A drivetrain with a ratio that is too long in difficult topography will increase the frequency of downshifts: the vehicle will seldom be traveling in direct gear. The consequence is additional consumption due to the drivetrain having a too long drivetrain ratio for the application in question.

3.3 Auxiliary Consumers The diesel engine not only drives the wheels of the vehicle but also numerous auxiliary components (see Fig. 2.1). The power consumption of these consumers is optimized to reduce the fuel consumption. Some examples include: Engagement of the fan The power consumption of the fan at full air delivery is over 30 kW for a heavy long-distance truck! This power must be provided by the diesel engine. A fan that is activated only when required has therefore been standard for a long time. A fan with continuously variable control is even more consumption optimized. The fan with variable rotational speed is engaged by means of a variable friction coupling and therefore consumes only the drive energy required to cool the engine (Fig. 3.7). Optimization of the compressed air system Intelligent control of the charging of the air reservoir will reduce energy consumption:

Fig. 3.7   The fraction of fuel consumption that is needed to drive the fan to cool the engine depends on the ambient temperature, the load of the vehicle and the route. Other factors that influence the fan activation time is the vehicle configuration, average speed and even the wind conditions … In rare cases (high load, high ambient temperature, difficult route) the engine fan can cause up to 2% of the total fuel consumption. In light condition, only 0.1 to 0.2% of fuel is used to propel the fan

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3  Vehicle Technology

A simple control of the air reservoir charging process involves the air compressor pressing air into the reservoir up to a defined cut-off pressure. If the reservoir pressure drops below the cut-in pressure through air consumption (braking, etc.), it is replenished again by the air compressor. The point in time of charging the reservoir is determined only by the reservoir pressure. A fuel consumption-optimized controller takes into consideration the current drive status of the vehicle. If the vehicle is moving in overrun mode, the air reservoir is replenished even if the cut-in pressure has not yet been attained. In addition, in overrun mode the air pressure in the reservoir is charged well above the cut-off pressure. As a result, the compressed air supply of the vehicle is much less of a burden to the diesel consumption. A fuel consumption reduction of about 0.3% in comparison to a simple control can thereby be achieved in long-distance haulage. The connectable/disconnectable air compressor provides a hardware-based improvement in energy consumption for the compressed air system. The conventional air compressor is permanently connected mechanically to the diesel engine. The crankshaft of the air compressor always rotates and air delivery continues even when the supply pressure required in the air reservoirs has been attained. To avoid the power loss associated with this, there are air compressors that have a pneumatically actuated multi-disc clutch, which is seated between the air compressor’s crankshaft and the drive gear from the diesel engine [24]. Upon reaching cut-off pressure, the air compressor is disconnected from the diesel engine. If the air pressure in the reservoir drops, the clutch is reengaged and the air compressor again delivers air into the compressed air system. The savings actually achieved do of course depend on the quality of the air compressor used. In the case of an optimized air compressor with only a low power loss on no-load operation, a disconnectable air compressor will enable only minor energy savings. In the case of air compressors with a high frictional resistance, the positive effect of the clutch is much more pronounced. The positive accompanying features of a disconnectable air compressor should be a longer service life of the air compressor, reduced oil entrainment into the braking system and reduced noise emission. The disadvantage of the disconnectable air compressor is that there is an additional cost-intensive component, namely the clutch, that has to meet high reliability requirements. Two-speed or variable coolant pump The coolant or water pump is driven mechanically by the engine and is a considerable energy consumer. In the simplest case, the water pump revolves at a fixed rate relative to the engine speed. The maximum capacity of the water pump is designed to meet the maximum cooling requirement. However, the full pumping capacity is often not required. The power consumption of the water pump is reduced by designing the water pump with several engagement stages or so that it is continuously variable. The water pump operates at full or reduced capacity as required. Power savings of up to 1 kW are achievable.

3.3  Auxiliary Consumers

23

Power steering pump Conventional power steering pumps constantly generate a flow rate that is depending on the engine speed, and that does not take into consideration whether there is any actual steering activity. In the case of a heavy truck, an uncontrolled power steering pump causes a power consumption of between several hundred and 1000 W. A mechanical power steering pump with a variable flow rate can reduce this power consumption. The electrically-driven power steering pump, which is independent of the engine speed and delivers power steering fluid only when the steering is actually being used, goes a step further. Oil pump with variable displacement Analogous to the optimization of the power consumption of the power steering pump and the water pump it is also possible to improve the oil pump in the engine. It is possible to vary the volume flow of the oil in the engine by means of an adjustable oil pump. [49] indicates a fuel saving potential of up to two percent for a regulated oil pump compared to a conventional oil pump. Generator The generator (alternator) is a mechanical load on the combustion engine. With intelligent alternator control functions (sometimes also called LIN generators, because the alternator is controlled via a LIN connection), the fuel consumption caused by the alternator can be reduced. In overrun mode the alternator charge voltage is increased so that the alternator delivers a significant voltage surplus and the batteries are charged. The mechanical load produced by the alternator is then high. In other driving mode phases (rolling phases, traction phases, etc.) the alternator voltage is reduced so that the mechanical power consumption of the alternator is low and the combustion engine is subjected to only a slight load by the alternator. In this phase the battery is of course not charged. Electric power consumption The more electrical power the alternator has to deliver, the greater the load on the combustion engine. Each electrical consumer on and in the vehicle puts an additional burden to the battery and the generator and hence also contributes to the fuel consumption. LED lights that are very energy efficient in principle reduce the energy consumption of the vehicle although the effect is small. Electric appliances running in the vehicle (coffee machine) have some influence on the power consumption as well. A well isolated cab reduces the energy needed to cool the cab in summertime.

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3  Vehicle Technology

3.4 Tires and Rolling resistance As already addressed above (Eq. 2.2), the vehicle has to overcome the rolling resistance. Where the angle of ascent is α, the following equation results:

FRoll = cRoll · FN = cRoll · mTotal · g · cos(α)

(3.1)

The rolling resistance accounts for about 16% of the total energy consumption in long-distance haulage and consumes 35% of the energy available at the crankshaft—see Fig. 2.1. The magnitude of the rolling resistance depends on the tires and on the road surface condition. The rolling resistances of different tires vary considerably, and thus can affect consumption—see Table 3.1. The energy lost in this process is needed in the deformation of the tires while rolling. In the process, the tires heat up quite measurably, as can be seen in Fig. 3.8.1 As of 2012, an EU regulation requires that the rolling resistance for newly approved truck tires must be below a specific limit value. From 2016 on, the limit value is applied to all tires that go on sale as new tires. In 2016 the limit values were tightened up for newly approved tires and in 2020 were tightened up for all tires. Traction tires with rough treads for off-road use are exempted from the limit values. Measuring tire rolling resistance is quite complex. Usually rolling resistance is assumed to be independent of vehicle speed. This helps simplifying measuring but is (in most cases) not true. See for example [52]: Usually the rolling resistance shows a clear dependence on vehicle speed. Regional tires, long-distance transport tires and wide base single tires are used on the drive axle of long-distance tractor units. Regional tires are often used because they achieve very high mileages in long-distance haulage, however, at the cost of a significantly higher fuel consumption. All axles are important for the optimization of rolling resistance. For the purpose of orientation: in the typical US application, about 42.5% of the rolling resistance of a laden tractor-semitrailer occurs on the trailer axles, 42.5% on the drive axles and 15% on the front axle. In an European 4 × 2 tractor trailer combination, 60% of the rolling resistance occurs in the trailer, 30% on the (single) drive axle and 10% on the front axle. Tread depth The deformation and flexing work decreases with a diminishing tread depth. This reduces the consumption. The effect of the tread depth on the consumption is quantified in [19]. In the case of regrooved tires, the material flexed during travel is less thick; a slightly reduced fuel consumption is associated with this.

1 In

addition to the flexing work in the tire, the waste heat of the vehicle (wheel brake, engine, exhaust system) also contributes to the heating up of the tires.

3.5 Aerodynamics

25

Table 3.1  Examples of the coefficient of rolling resistance cRoll in % of long-distance haulage tires in accordance with [21]. Among other things, long-distance haulage tires were investigated. Listed here are long-distance haulage tires for drive axle tires and steering axle tires with the dimensions 315/80 R22.5. The differences between the brands amount to as much as 20% Brand

G

C

D

M

T

Coefficient of rolling resistance of steering axle tires in %

0.44

0.46

0.48

0.49

0.54

Coefficient of rolling resistance of drive tires in %

0.66

0.56

0.57

0.58

0.70

30

Temperature [ºC] left-hand

Speed [km/h] right-hand 90

20 60

10 0

30

-10 Outside air temperature

-20

0 0

1

2

3

4

5

Travel time [h]

Fig. 3.8   Heating of a tire during travel: the tire temperature increases by about 45 °C above the ambient temperature. The tire cools down again when it is stationary. Measurement takes place in winter testing at an outside temperature of about −20 °C. Date source Daimler

3.5 Aerodynamics Aerodynamics has a major effect on fuel consumption. In addition, aerodynamic optimizations also often have reduced wind noise as a positive secondary effect. This pleases the driver, local residents and is good for the environment. Figure 3.9 shows the power that is required to overcome the motion resistance of a heavy truck on a flat road. The aerodynamic drag is part of the motion resistance and contributes to vehicle losses. Aerodynamic drag is composed of the compressive force and the skin friction (from the air sweeping along the surface). In vehicle development the compressive forces are dominant. The compressive forces originate from pressure differences when the vehicle is pushing the air away and from the low pressure region behind the vehicle pulling the vehicle back. Part of the airdrag is not because of the air flowing around the vehicle but is due to the air flowing through the grille, the radiators and the engine compartment for cooling

26

3  Vehicle Technology 120

Power loss [kW]

100 Increase P ~ v3 in aerodynamics

80 60 40 20

Linear increase P ~ v in rolling resistance, inner friction

0 0

10

20

30

40

50

60

70

80

90

Speed [km/h]

Fig. 3.9   The power that is required to overcome the motion resistance of a heavy truck on a flat road. The motion resistance force F is composed of the (here assumed to be approximately) constant rolling resistance and the quadratically increasing aerodynamic drag. The necessary power P equals P = F · v. A linear term and a term proportional to v3 are thus obtained for the power consideration

purposes. The contribution of air drag due to cooling is around 5 to 8% of the total air drag [54]. How large the proportion of the aerodynamic drag is of the total motion resistance depends on the road condition and the speed. In European long-distance haulage, there is a rule of thumb that a 4% lower aerodynamic drag results in about a 1% reduction in fuel consumption ([20] suggests a factor of 3.5). The aerodynamic drag FAero is a force that is given by the Eq. 3.2:

FAero = 1/2 · ρ · v2 · A · cd

(3.2)

• Where FAero is the force of the aerodynamic drag. • cd denotes the coefficient of drag. The coefficient of drag cd is a dimensionless number that denotes the aerodynamic quality of a body. • A is the frontal area of the vehicle. In the case of a modern long-distance truck, this is about 10 m2, as the vehicle has a width of approx. 2.5 m and a height of 4 m. Figure 3.10 provides typical values for the frontal area and the cd value of present-day vehicles. • The variable ρ from Eq. 3.2 denotes the density of the air. At standard temperature and pressure of 0 °C and 1013 hPa the density of the air is 1.293 kg/m3. The density of the air is subject to the strong weather fluctuations, which is why the aerodynamic drag of a vehicle also varies. At 30 °C, 992 mbar and high air humidity, the density of the air is 8% lower than at 10 °C, 1016 mbar and 20% air humidity! In [11] the significance of the air density ρ for the measurement of aerodynamic drag is acknowledged

3.5 Aerodynamics

27

Coefficient of drag cd cd =1.1 plate (round or square)

1 0.9 0.8 Long-distance drawbar combination Long-distance tractor semitrailer combination

0.7 0.6 0.5

Van

Passenger car

0.4

Compact car

0.3

Distribution truck

SUV

Long-distance bus

Sports car Limousine/coupes

0.2

0,2 – Ungefähre limit Grenze realistische ccdw==0.2—approximate forfür realistic seriesSerien-PKW production

0.1 cd = 0.05—tear plate = theoretical optimum

0.0 0

2

4

6

8

10

Area A [m2]

Fig. 3.10   Typical values for the cd value and the frontal area of modern vehicles. Technical progress is constantly pushing the lower edge of the cd values further down. On a bus, cd values of 0.33 are attained [29]. Regardless of this, vehicles that have significantly worse values are still encountered on the roads

in detail. The influence of the weather on fuel consumption is mentioned again at another point in the text (Subsect. 4.2). Moreover the density of the air depends on the geodetic altitude. • v denotes the speed of the vehicle. Aerodynamic drag increases quadratically with increasing speed. That is why aerodynamics are more important for commercial vehicles with a high average speed (above all in long-distance haulage). Equation 3.2 describes the ideal case of a vehicle moving through still air at a speed v. In realistic conditions, the influence of the wind has to be taken into consideration. If the incident flow on a body is at a yaw angle, the aerodynamic drag of the body alters. The total wind force acting on the vehicle results from the movement of the vehicle vDrive and the movement of the air vWind (wind). Coefficient of drag cd If the vehicle is moving at a constant speed in direction x (along the vehicle longitudinal axis) the forces on the vehicle are in equilibrium. The vehicle applies a force that works against the drag in the vehicle’s longitudinal direction and it must also apply a counterforce to compensate for the lateral force arising from the airflow, see Fig. 3.11.

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3  Vehicle Technology

Fig. 3.11   Equilibrium of forces on vehicle with sidewind

x The force FAir along the vehicle’s longitudinal axis is considered as the aerodynamic resistance. This is the force that inhibits the motion of the vehicle and is taken into account in the vehicle performance consideration and in the fuel consumption calculation (as a reminder, energy is the scalar product between force and distance traveled, hence force perpendicular to a movement does not contribute to physical work). The total force acting on the vehicle is greater. The force component transverse to the direction of travel is balanced mainly by the lateral stability of the tires. For crosswind flow the coefficient of drag cT is defined as follows:

cT (β) =

1 2

x FAir (β) · ρ · (vAir )2 · Ax

(3.3)

Direction x and index x denote the longitudinal axis of the vehicle, hence the direction of travel. vWind is the wind speed relative to the ground. vDrive is the vehicle speed relative to the ground. Then vAir is the wind speed of the vehicle relative to the air and can be derived from vWind and vDrive see Eq. 3.4: 2 2 2 = vDrive + 2vDrive vWind cosα + vWind vAir

(3.4)

In the definition of cT the frontal area of the vehicle in travel direction Ax is used, even if the wind strikes a different (usually larger) surface. For clarification purposes we have denoted the area in Eq. 3.3 as Ax. The air flow around the vehicle changes its behavior at different wind angles. x Therefore FAir (β) and cT (β) depend on the angle β. With larger β usually FxAir (β) and cT (β) gets bigger. cT (β) has been defined so that it is easily measurable in a wind tunnel using a rotatable balance—see Fig. 3.12 on the left-hand side. In wind tunnel

3.5 Aerodynamics

29

a

b vWind,x = |vWind| • cos

Wind, vA

Vehicle is rotated by the weighbridge

vWind,y = |vWind| • sin

α

⃗ Wind ⃗ Drive β

⃗ Air Total speed vehicle versus air: 2= vAir vDrive2 + 2vDrive vWindcos

vWind2

β

Wind

Weighbridge Measures in the longitudinal direction of the vehicle

Driving direction Fx

Fig. 3.12   a Measurement of cT (β) in a wind tunnel: the vehicle is rotated with the force balance. b The → → v Drive and the wind speed − v Wind (wind speed relative to the speed of the vehicle relative to the ground − → v Air to which the vehicle is being subjected. The vecground) provide the resulting incident flow speed − tors drawn here illustrate the case when the wind is blowing from the front at an angle of 45° relative to the direction of travel and the wind speed is 83 of the vehicle speed (e.g. vehicle speed of 80 km/h, wind speed 30 km/h). The incident flow angle β of the air is then about 12°

measurements and in CFD-simulations cT (β) depending on β is determined. The resulting diagram is called polar diagram. Figure 3.13 shows polar diagrams and illustrates how intensely cT, and therefore the aerodynamic drag, increases under various yaw conditions. If α is the angle between the direction of travel and the wind direction (see Fig. 3.12, right-hand side), and with the definition of cT, Eq. 3.5 describes the definition of the aerodynamic drag in wind: x FAero = FAir

= 1/2 · ρ · (vAir )2 · Ax · cT 2 2 = 1/2 · ρ · (vDrive + 2 · vDrive · vWind · cos(α) + vWind ) · Ax · cT

(3.5)

30

Coefficient of drag cd and cT

a Tractor semitrailer combination 150

100

50 -17°

-15°

-10°

0°

5°

10°

15°

17°

10°

15°

17°

10°

15°

17°

Flat bed truck (solo)

150

100

50 -17°

-15°

-10°

-5°

0°

5°

Incident flow angle

c Coefficient of drag cd and cT

-5°

Incident flow angle

b Coefficient of drag cd and cT

Fig. 3.13   Curves of the cd value and cT value of vehicles with an oblique incident flow. a Shows the relative change on a tractor-semitrailer combination without roof wind deflectors or side deflectors. b Relative change of the cd/cT value on a solo vehicle with a box body without roof wind deflectors. c Relative change on a drawbar combination. The data complies with [11, 12]

3  Vehicle Technology

Drawbar combination 150

100

50 -17°

-15°

-10°

-5°

0°

5°

Incident flow angle

The incident flow angle β (see Fig. 3.12) results as follows

tan (β) =

vWind · sin (α) vDrive + vWind · cos (α)

(3.6)

3.5.1 Aerodynamics of the Driver’s Cab The basic concept of a European long-distance truck has remained unchanged since the 1960s. The length restriction in Europe has ensured that what is in principle a cubic driver’s cab has been implemented by all vehicle manufacturers. In doing so, the European long-distance truck exposes a large flat area of approximately 2.5 m · 4  m = 10  m2 to the wind. Modern trucks in a tractor semitrailer configuration thereby achieve an A · cd value of approximately 4.7 to 5.7 m2. The most important aerodynamic parameters that are applied to the basic shape of the driver’s cab are:

3.5 Aerodynamics

• • • •

31

the radius of the driver’s cab at the A-pillar, the inclination of the windscreen, the shape of the roof—a fleeing forehead versus a steep front face of the roof, and the taper.

Figure 3.14 illustrates this. The design objective for aerodynamic trucks competes with other requirements, such as the loading volume and a sufficiently large driver’s cab. Since a large interior compartment often has high priority in the design of the driver’s cab, the sweep and taper are only slightly formed. The styling and functional considerations, for example, the field of vision, the heating-up of the cab in sunshine and convenient handling, also play a role in assessing ideas for aerodynamic improvements. Figure 3.15 shows a heavy duty longhaul tractor offered in the Chinese market that strives to improve aerodynamics with a strong inclination of the windscreen and a fleeing forehead. a

b Contour of semitrailer

Shape of roof

Curvature

Inclination of the windscreen

c Sweep

Inclination of the windscreen

Corner radius

Taper

Fig. 3.14   a and c show the important geometric parameters that affect the basic shape of the driver’s cab. It must be considered that the aerodynamic performance of the whole combination is of interest and that this performance is co-determined by the body/semitrailer. The latter usually utilizes the width of 2.55 m, making the curvature less suitable for aerodynamic optimization, as seen in b

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3  Vehicle Technology

Fig. 3.15   HD tractor presented to the Chinese market in 2021 with a strong inclination of the windscreen and a fleeing forehead as the shape of the roof in order to achieve good aerodynamics. Other product features like e.g. interior space have to compromise. Photo CNHTC

In the USA where vehicles are not restricted in length,2 the conventional vehicle (cab-behind-engine), which was also usual in Europe until well into the 1960s, has asserted itself. Conventional cab vehicles offer the possibility of configuring aerodynamically-favorable vehicle fronts. [27] compares European vehicle shapes with US shapes and specifies an aerodynamic advantage of the cd value of approximately 0.05, which represents a percentage improvement of about 8 to 10%. The average driving speeds in US long-distance haulage are higher than in Europe, which means that the aerodynamic drag plays an even more important role in the US. Figure 3.16 shows on the right-hand side a modern US long-distance truck that uses the conventional cab shape, designed in a wind tunnel, to achieve aerodynamically-optimized favorable consumption values. Shown on the left as a comparison is a traditional American conventional vehicle, which in terms of the design concept originates from a time in which diesel was low in cost and fuel consumption savings were less of a concern. Sharp edged corners, steeply arranged radiator grilles and steep windscreens, protruding exhaust pipes, protruding air filters, rugged vehicle sides and numerous attachments on the roof indicate that aerodynamics have not been prioritized here. In [28] numerous potential aerodynamic optimizations for typical North American long-distance vehicles have been assessed. Many ideas for developing trucks in accordance with aerodynamic aspects have been developed in the past 20 years.

2 In

the USA the overall length of the vehicle is not limited as it is in Europe. U.S. vehicle configurations are intensely characterized by the “bridge formula”, which envisages minimum spacings between axles to conserve bridges [46].

3.5 Aerodynamics

33

Fig. 3.16   On the left: a traditional U.S. cab-behind-engine vehicle (in the illustration on the left a Western Star) with sharp-edged styling that attaches little importance to aerodynamic optimization. On the right a Freightliner Cascadia (first presented in 2007), which is aerodynamically optimized and uses the advantages of the cab-behind-engine shape for the aerodynamic drag. Photos Daimler 2008

3.5.2 Optimization of Aerodynamics The optimization of aerodynamic details in the course of the model changeover of the heavy MAN tractor unit in 2007 have—according to the manufacturer—led to a reduction in the cd value of about 4% [33]. This is equivalent to a reduction in fuel consumption of about 1%. Aerodynamic optimization is usually in conflict with other design objectives. The conflict between aerodynamically-optimized shapes and the maximum interior space in the driver’s cab has already been addressed. Another detailed example of such a conflict is the A-pillar: on the one hand, the vehicle manufacturer would like to design the A-pillar and side mirror so that they have as little aerodynamic drag as possible; on the other hand the mirror and side windows being kept free of dirt. These two requirements cannot be optimally met at the same time. Figure 3.17 shows attachments and optimizations that come into consideration for the aerodynamic improvement of the vehicle both the tractor and the semitrailer. In aerodynamics trailer and tractor must be optimized jointly. So the section on aerodynamics cover both measures on the tractor and on the trailer. A wind deflector on the roof of the driver’s cab (spoiler) A roof spoiler (fairing) is very important for optimal aerodynamics and therefore for the lowest possible fuel consumption. Figure 3.18 shows the great influence of a correctly set roof spoiler on the fuel consumption in long-distance haulage. In the illustration a tractor semitrailer combination is considered that is pulling a semitrailer with a height of 4 m. Incorrectly set roof spoilers cause unnecessarily high diesel consumption. Despite this, you can still frequently see tractor-semitrailer combinations on which the setting of the spoiler is not matched to the height of the semitrailer or tractors that do not have a roof spoiler at all.

34

3  Vehicle Technology Side deflectors

Roof fairing

Rear end taper

Rounded edges of the body

Aero bumper

Tractor to trailer gap

Trailer skirt

Aerodynamic side paneling

Additional fuel consumption [%]

Fig. 3.17   Attachments and vehicle contours on which the optimization of aerodynamic details is usually focused

4 3

Optimal setting of the spoiler

2 1 0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

Catch setting

Fig. 3.18   The influence of a correctly set spoiler on the fuel consumption in long-distance haulage. The additional fuel consumption at various settings of a roof spoiler on an LH driver’s cab of a MercedesBenz Actros from 2008. The spoiler can be fixed in various catch positions. Catch 11 is the correct one for a semitrailer with a height of 4 m

If a flat semitrailer is hitched lower than the driver’s cab, it is obvious that the roof spoiler makes no sense. It only adds to the front face area that the vehicle presents to the wind. Side extenders The side extenders ensure that the air flow passes from the side surface of the driver’s cab onto the side surface of the semitrailer without any harmful disturbances. The side deflectors show a positive effect on the aerodynamic drag, with the result that present-day long-distance semitrailer trucks virtually always have side deflectors. Publication [11] shows the influence of side deflectors on vehicles made in the 1980s at all yaw angles. The magnitude of the effect does of course depend on the aerodynamic drag resulting from the width and shape of the driver’s cab and semitrailer.

3.5 Aerodynamics

35

Tractor-trailer gap A gap, causing air turbulence and aerodynamic drag, results between the tractor and trailer. In [11] the effect of this gap on the aerodynamic drag is dealt with in detail. [25] suggests that this should be filled out with inflatable rubber spoilers. These rubber spoilers are inflated by the truck’s compressed air system. At low speeds and during turning maneuvers, the air is released from the rubber spoilers to re-create free movement between the tractor and the semitrailer. The closing of the gap between the tractor and semitrailer should enable an improvement in the cd value of 1.5%. Rounding of body corners The cd value of a cuboid improves if the corners of the cuboid are rounded. Therefore, the edges of the body are often rounded. There are studies on the aerodynamically positive effect on rounded edges, for example, in [12, 17]. Air dam The air dam (or front spoiler/front bumper) optimizes the airflow in the lower front area of the vehicle. The air should flow under the vehicle and laterally along the vehicle with as little drag as possible. It is shown in [11, 12] that the front skirt on the specific vehicle considered there enables a clear 3 to 4% improvement in the A · cd value. Grille shutter The proportion of the air that is conducted under the driver’s cab and through the engine compartment to cool the radiator and circulate air around the engine also contributes to the aerodynamic drag of the vehicle. The flow through the radiator and engine compartment increases the aerodynamic drag. Grille shutters consist of adjustable vanes in front of the radiator that allow greater or less intensive flow through the engine compartment, depending on the cooling requirement. In the aerodynamically, most favorable case, the radiator shutters are closed and the air flows around the vehicle. If a high cooling performance is required, the shutters are opened. The aerodynamic quality of the vehicle (the cd-value) changes according to the specific driving situation. Side skirts Sometimes side paneling of the vehicle frame lowers the aerodynamic drag and therefore the fuel consumption. There are various types of side skirts. The side paneling types that are commonly available nowadays usually leave the wheel cut-outs uncladded. Partly paneled or even fully cladded wheel cutouts are more aerodynamic, though. Figures 3.19 and 7.1 show fully cladded wheel cut-outs on a semitrailer. In [25] a tractor semitrailer combination is presented on which the drive axle of the tractor also has fully cladded wheel cut-outs. In addition to reducing the aerodynamic drag, the side skirts also provides safety advantages. These are shown in [14]. Flat chassis paneling provides additional safety for pedestrians and two-wheeled vehicle drivers, because in the event of an accident they are turned away better by closed surfaces than by underrun guards, which do not protect the whole of the area in question. In addition, the side paneling reduces spray water and fine spray streaks. This achieves not only additional safety for the truck driver but also a lot more safety for other road users. Side skirts also reduce external noise [14]. The disadvantages of side skirts are increased costs and increased weight. Wheel covers (hub caps) are also mentioned time and again for reducing the aerodynamic drag of the vehicle. However, the achievable reduction in aerodynamic drag is small [28]. On the other hand, wheel covers add additional costs and additional weight to the vehicle. As smaller and smaller aerodynamic improvements become attractive in the

36

3  Vehicle Technology

Fig. 3.19   An aerodynamically optimized tractor-semitrailer combination: a series-production tractor with a trailer perfectly matched to this vehicle [9]. Photo Daimler

future the usage of wheel covers might increase. There are primarily optical reasons for the frequent use of wheel covers on coaches. Underbody paneling (skirt) An aerodynamically-optimized underfloor makes it possible to reduce the cd\cT value. The concept vehicle in [25] has a holistically-optimized underfloor. There are reports of a reduction in the cd value of 2.5% by optimizing the underfloor of the tractor and, in addition, a reduction in the cd value of 3% is indicated by optimizing the underbody of the semitrailer with a diffuser at the rear. Trailer tail A trailer tail or vehicle tail (sometimes called boat tail or rear end taper) also improves the cd value of the vehicle. [15, 25, 31] use the rear end taper to reduce the aerodynamic drag. If the tail is implemented within the loading volume, space is lost due to the tapering rear end. This affects the transport capacity and therefore the operating efficiency for the trucking company. In addition, the cubic geometry that is usual nowadays on bodies and semitrailers is easily realizable in production. A box body or a flatbed (platform semitrailer) with tarp can easily be made up from flat and rectangular standardized surface elements. A tapering rear end causes additional costs in production. The rear end taper can be much more easily configured and with a neutral effect on the volume, if additional air deflectors are added at the rear of a conventional trailer. This opening up of the statutory length specification is already nationally possible today (2015) in some European countries. Approval for cross-border traffic in Europe is expected. In [25] the trailer tail is represented by an additionally inflatable rear spoiler made of rubber.

3.5 Aerodynamics

37

A foldable rear extension is shown in [45, 51] that is attached to the rear doors and improves the airflow at the rear of the semitrailer. In folded condition the rear doors can be opened without difficulty. The overall length of the vehicle is also not increased much by the rear extension in its folded condition. When folded out, the rear extension should enable a clear reduction in fuel consumption on the freeway. Curved roof A significant improvement in the cd value can be achieved by an overall drop shaped tractor trailer combination (tear drop trailer). This path has been taken by various concept vehicles for the purpose of studying consumption optimization [31]. Figure 3.20 illustrates the curved roof concept. To achieve a comparable loading volume on a semitrailer with a curved roof with the same length, the semitrailer with the hunchroof must be higher than the present-day usual cuboid semitrailers. This increases the frontal area A of the overall tractor trailer combination; and must then be offset against the cd value improvement. In the overall effect the changes in cd and A must be such that the aerodynamic drag FAero is reduced to make a drop shaped form worthwhile. Presentday semitrailers usually fully utilize the height limitation of 4 m. This height limitation of 4 m has manifested itself in the infrastructure in many countries. Many bridges and structures have a headway of 4 m. The curved roof therefore has poor prospects in countries with a 4 m height limit. In some countries (for example, the UK) semitrailers are allowed to be significantly higher. This makes the curved roof trailer marketable. Trailer manufacturers there offer this shape for semitrailers and full trailers [47]. Front hood: In addition to the rear tail and the curved roof, the front hood is another approach for the aerodynamic optimization of commercial vehicles. The vehicle front of cab-over-engine vehicles, which is nowadays designed to be relatively flat, can be improved in terms of the aerodynamic drag by adding a front hood. This is sketched in Fig. 3.20. In the case of short hoods, which are possible in Europe since 2020, the aerodynamic effect tends to be only slight. Nevertheless, as this new design freedom will result in new cabs, that will be developed with a higher focus on energy efficiency, better cd values can be expected in the future. Real engine hoods, as are usual on classic American long-distance vehicles, have more distinct effects—compare Fig. 3.16. The decisive factor for the success of hood shapes is that the transition surface between the hood and the windscreen is aerodynamically shaped. A stepped transition can be aerodynamically unfavorable.

Front hood

Hunched back

Trailer tail aper

Fig. 3.20   Vehicle contours that can contribute to aerodynamic optimization. The changes shown here interfere with the dimensional concept of present-day trucks and in some cases would require an amendment of the statutory regulations (length, height) for their implementation

38

3  Vehicle Technology

Step covers reduces air turbulence around the steps. The cover extends the door downwards (therefore sometimes called door extension). In markets with a high level of criminality, covering of the steps is not used for aerodynamic reasons, but it prevents unwanted guests from jumping onto the steps at traffic lights. In the area of the doors, flush-mounted doorhandles are also shown as aerodynamic optimizations. In this case, however, the cost–benefit ratio is more than questionable. Flush-mounted doorhandles can be considered more of a design element. Attachments Wing mirrors are aerodynamically very prominent. They place a surface area in the wind that is not negligible and are attached to the vehicle at an aerodynamically important postion. Instead of wing mirrors, cameras looking rearwards are provided on consumption-sensitive long-distance vehicles [48]. This so called mirror cam system (MCS) is a recurring idea on concept trucks (see, for example, [32, 50, 53]) and is now finally available in series production trucks—see Fig 3.21. The camera images are shown on display screens in the driver’s cab. Wing mirrors are no longer needed. If (small) cameras replace large wing mirrors, better side visibility will be achieved as a positive side effect. The visibility blind spot caused by large mirrors disappears. In addition, the camera display screen system can be enriched with other functions: orientation lines can be superimposed on the display screen and areas of the image that are dependent on the driving situation can be displayed. Attachments to the vehicle—such as air horns, separate headlights on the roof and sun visors above the windscreen—are all consumption increasing factors. Optimization of semitrailer details The semitrailer can also be aerodynamically improved in detail. For example, a covering with clean design lines for the pallet box is likely to improve the flow around the vehicle. In the case of curtainsiders, the strap buckles of the side tarpaulins and the loosely hanging ends of the ratchet straps of the side

Fig. 3.21   First mirror cam system in series production: a truck without main mirrors b smoke probe in the wind tunnel shows how the main mirror disturbs the air flow c displays in the cab interieur show the camera’s images. Photo Daimler

3.6  Technical Aids for the Driver

39

tarpaulins, which often flap about in the wind, provide for unfavorable air turbulence. Various semitrailer manufacturers have developed solutions in which the traditional strap buckles have been dispensed with. High potential for aerodynamic improvement—see Subsect. 3.8—is provided by resolutely optimizing the semitrailer in combination with the tractor. Dynamic changes to the vehicle Further ideas can change the truck or tractor trailer combination, depending on the driving condition, thereby improving the aerodynamics. For example, a tractor trailer combination can be lowered as of a certain speed (i.e. the ground clearance is reduced) and in this way the aerodynamic drag is reduced. This method is used on long-distance buses to reduce the aerodynamic drag. This aerodynamic trick is based on the assumption that buses and trucks drive faster only on good, flat roads. To lower the vehicle at a certain speed, trucks and full trailers or semitrailers must have air suspension on all axles. The tractor semitrailer gap spacings could also be adapted as a function of the speed. The gap spacings on tractor-semitrailer combinations are dimensioned so that the bodies do not strike one another in tight bends or when cornering. When traveling fast, though, the gap spacings are not required; they only increase the aerodynamic drag. A technical solution that reduces the gap spacings when traveling fast would reduce consumption. However, the technical realization of this seems to be very complex.

3.6 Technical Aids for the Driver Vehicle manufacturers are developing technical aids that are aimed at supporting the driver in achieving a consumption-optimized driving style. The automation of the transmission addressed above is an important part of these technical aids. The cruise control system provides the driver with a distinct increase in convenience; in fact it was developed as a convenience feature. The exact configuration of the cruise control system also affects fuel consumption, though. A so-called hysteresis, which allows the cruise control system to deviate from the speed set by the driver, reduces fuel consumption. When driving downhill, the so-called overrun hysteresis allows the vehicle to roll faster than specified by the driver. Only when the vehicle has reached the cruise control setting speed plus overrun hysteresis does the cruise control function intervene and brake the vehicle. As a result, the free kinetic energy of the hill descent is used to better effect, the vehicle rolling a bit further after leaving the downhill stretch and thus reducing the vehicle’s consumption. The traction hysteresis allows the cruise control system to subceed the set speed if, for example, when traveling uphill, an acceleration of the vehicle up to the set speed would require greatly increased fuel consumption. The engine torque at which the vehicle is accelerated to the set cruise control speed is another parameter that affects the consumption influence of the cruise control system. A simple controller applies the maximum available engine torque (or let’s say 80% of the available torque) at all times. Acceleration phases with a reduced engine torque are suitable

40

3  Vehicle Technology

for lowering the fuel consumption. A reduced engine torque when accelerating or undershooting the set speed (traction hysteresis) will of course cause a slightly reduced average speed. The driving trainer, which gives the driver tips on how driving style can be optimized in order to drive as fuel-efficient as possible, is also helpful for the driver. Besides providing support for the pure driving function, the driver can also be given other helpful information, for example, a prompt to switch off the (energy consuming) air conditioning system, if the driver is driving with an open window. Another attempt would be to just inform the driver better about the energy consumption he can directly influence: The vehicle could just inform the driver that the current usage of the AC consumes x Watts resulting in y % of the total energy consumption of the vehicle. For battery electric vehicles displaying the contribution of heating to the momentary energy consumption might be educational for the driver.

3.6.1 Predictive Systems Predictive systems can contribute substantially to the reduction of energy consumption. The basic principle is to charge the vehicle’s energy storage if the topography ahead indicates to do so. The largest and most important energy storage unit for a conventional vehicle is the kinetic energy of the vehicle, or put simply, the momentum of the vehicle. For electric vehicles with battery energy can be stored back to the battery (see Chap. 2, Fig. 2.1). The so-called electronic horizon (eHorizon) makes an effective predictive driver assistant possible: Three-dimensional information about the course of the road is used. The topography ahead is determined by means of the GPS data of the navigation system and digital maps that include road grade data. Digital maps containing the requisite information are available for Europe, North America and China for example. On the basis of the topographical data, the speed set in the cruise control system varies within a prescribed speed band so that driving is performed fuel-efficiently. For example, at the end of a downhill stretch the vehicle (within the framework of the set hysteresis) can be allowed to roll faster than the cruise control speed requires, so that a longer rolling phase can be achieved on the subsequent flat stretch or ascent. Before reaching the crest of a hill the predictive driver assistant reduces the engine torque and, to save fuel, drives over the crest at a slightly lower speed. Not only is the cruise control hysteresis fully utilized by this system, but there is also the possibility of predictively determining the gear selection (see, for example [22]). If a digital preview of the route profile is available, additional energy storage can be integrated together with their functions. When traveling downhill, the compressed air supply can be specifically increased, the starter battery charged (Subsect. 3.3) and the charge status of the high-voltage battery in an electric or a hybrid-electric vehicle can be improved.

3.6  Technical Aids for the Driver

41

The regeneration cycles of the exhaust gas aftertreatment system can be controlled by way of a predictive system so that they have only a minimal effect on the consumption and performance of the vehicle. Control of the cooling system can be integrated into a predictive energy management system. Extensions to predictive systems In the case of predictive thermal management, an attempt is made to optimize activation of the actuators for the engine cooling (radiator shutters, fan and water pump) on the basis of the predicted route topography. For example, the engine coolant temperature can be predictively lowered to avoid fan activation when approaching a grade. The objective of this control feature is to minimize the overall mechanical power consumption of the cooling over a specific segment of a route. Additional map and traffic information known from the navigation are integrated into predicitive systems. If the system knows that a traffic situation will follow that requires a speed reduction, for example, a roundabout (traffic circle), a T–intersection or a crossroads at which priority has to be given, provision can be made at an early stage for a rolling phase. Considerations relating to communication between the vehicle and the infrastructure extend into the future: if the vehicle knows within what time frame a traffic light will indicate green or if a railway barrier is opened, the speed profile of the vehicle can be adjusted over a period of several minutes so that the traffic light or railway crossing barrier is reached at the ideal point in time.3 It is also conceivable for the air-conditioning system—in the event of an energy access, i.e. a steep downhill stretch of road—to charge an air-conditioning storage unit (a thermal storage unit), later support the air-conditioning from this storage unit and thereby have a lower power consumption. Predictive systems require exact route data. This route data can be improved in terms of precision, if a learning system is set up: the actual route (gradient, etc.) and the route stored in the map are compared. Any deviations are corrected in the map data. The map continually becomes more exact. Such a system is ideally designed as an off-board system that continually creates better data from the data of many vehicles. The improved updated data is fed back to the vehicle fleet. Even temporary obstacles and changes to the normal route like construction sites can be considered in such a learning off-board system. Besides the pure route data, predictive systems must make a prediction about the route that the vehicle is going to take. This prediction could be improved by estimating probabilities from past routes as to which turnoffs will probably be selected.

3 There

have been inverted systems for several decades in the public transport system: there, an approaching regular bus signals that it requires a green traffic light.

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3  Vehicle Technology

3.7 Curb Weight The weight of the truck affects its fuel consumption. How additional weight converts into additional fuel consumption depends on the route profile. An estimate of the fuel consumption effect of the weight can be calculated with the aid of Fig. 2.1 for the specific route studied in the illustration. The weight has a linear effect on the rolling resistance. In addition, the energy that is consumed in braking procedures (and has previously been put into the vehicle) is linearly dependent on the mass of the vehicle. The loss types—rolling resistance, service brake and engine brake—are therefore dependent on the weight. According to Fig. 2.1 these losses consume 56% of the mechanical energy that is available at the crankshaft (15.6% + 5.7% + 3.4% = 24.7% of 44.2%, resulting in 56%). If the weight is reduced by 1%, the consumption on the route under consideration in this case is decreased by 0.56%. In the case of a 40-t vehicle, a weight reduction of 400 kg, or 1%, amounts to a fuel saving of 0.56% or about 0.2 l. Different values result on other routes with a different load, etc.

3.8 Full Trailers, Semitrailers and the Load As mentioned before a large proportion of the motion resistance is caused by the full trailer or semitrailer. Accordingly, for optimization of the tractor trailer combination, full trailers and semitrailers also have to be considered with regard to fuel consumption. The parameters described in Subsect. 3.5.2 for optimizing the aerodynamic drag also relate to semitrailers and full trailers. For aerodynamically-optimized results it is necessary to optimize the tractor and semitrailer as a unit. Figure 3.19 shows an aerodynamic tractor-semitrailer combination on which the semitrailer and tractor vehicle have been jointly optimized. The combination of an aerodynamically good, series-production tractor and a trailer perfectly matched to this vehicle brings about an 18% better aerodynamic drag (A · cd) [9]. In the case under consideration, a reduced fuel consumption of 4.5% is reported. From the fuel consumption aspect, different driver cab heights are recommended, depending on the type of semitrailer [33]. The design of the trailer affects the consumption. A box body has a different aerodynamic drag to that of a flatbedbody with tarp. With a tarp body it is important that the semitrailer tarp is properly lashed. Flapping tarpaulins increase the aerodynamic drag (and they wear out more quickly). A corrugated side surface of a shipping container has a higher aerodynamic drag than the smooth, uniform surface of a refrigerated van body. The aerodynamic behavior of open trailers is different again. A drawbar combination has a significantly worse cd value (0.55 to 0.85) than a tractor semitrailer combination (0.45 to 0.75). The reason for this is that the tractor semitrailer combination has one space between the two units that creates turbulence in the air flowing past—namely, the transition between the driver’s cab and the semitrailer—while the

3.8  Full Trailers, Semitrailers and the Load

43

drawbar combination has two such turbulence zones: the transition between the driver’s cab and the platform and the space between the motorized vehicle and the full trailer. The number of axles of the semitrailer affects the rolling resistance and therefore the consumption. Selection of the trailer and vehicle configuration obviously is not made under consumption aspects only. Initially the transport task and the logistics concept (see Subsect. 4.7) determine the properties of the tractor trailer combination and body. It is not until the second stage that the consumption aspects are taken into consideration by the trucking company. Full trailers and semitrailers with a refrigeration unit require a considerable amount of energy consumption to refrigerate the freight space. Optimized insulation ensures that the energy consumption is reduced.

4

Operating Conditions of the Vehicle

4.1 Topography of the Route

Consumption [l/ 100 km]

The topography is a very obvious energy consumption factor. Operation in hilly terrain and on long uphill stretches requires a lot more energy than operation in flat terrain. The difference between a demanding topography and flat stretches is shown in Fig. 4.1 for a diesel truck. On freeway routes in particular, the difference in consumption between flat stages and hilly terrain is very big. The topographical effect is less on national highways because other interference factors come into play, for example, traffic light stops and urban throughways, that require a reduction in speed and in the process consume valuable energy (momentum).

50 40 30 20

42.5

39.5

37.5

28.0

10 0

S Severe national highway (the Black Forest)

2. Qrtl. National highway

Severe freeway operation

Light freeway operation

Fig. 4.1   Consumption values of a European standard tractor semitrailer combination with a gross weight of 40 t on various operating routes (status 2008)

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_4

45

46

4  Operating Conditions of the Vehicle

Generally it can be said that the influence of the topography on energy consumption is bigger for diesel powered trucks than for electrically propelled trucks. The reason is that the electric truck will recover some of the energy used to go uphill on a subsequent downhill stretch.

4.2 Weather and Temperature The natural weather conditions are another decisive contributing factor to consumption. As already mentioned above, the density of the air (the air pressure) has a direct effect on the aerodynamic drag (Eq. 3.1). However, the air density also affects the combustion in the engine. Air density is affected by the weather and by the altitude above sea level of your current location. A roadway wet with rain causes additional fuel consumption. Ultimately, the energy required to produce the spray clouds from the vehicle and its tires comes out of the diesel fuel tank. The increased rolling resistance, that the water causes, results in an additional consumption of up to 5%. Puddles and water-filled pot holes increase the rolling resistance even further. Very hot ambient temperatures might require higher fan activation to cool the engine and hence increase fuel consumption see Fig. 3.7. For battery electric trucks high temperatures might require additional battery cooling and very low temperatures might require battery heating. In both cases energy consumption is affected unfavorably.

4.2.1 Vehicle with a Warmed-Up Engine In addition to the outside temperature, which has a fuel consumption-altering effect, the inner temperature of the vehicle components is also of great significance. A vehicle with a warmed-up engine consumes significantly less fuel than a vehicle with a cold engine. The viscosity of the engine oil, transmission oil and rear axle oil increases as the oil sump temperature increases in the course of the journey. A clearly measurable reduced fuel consumption is associated with this [16]. The rolling resistance of the tires is also temperature dependent.

4.3 Traffic The traffic that the vehicle is subjected to has a great influence on consumption. Braking procedures and subsequent acceleration cause additional consumption. Figure 4.2 illustrates how much diesel fuel a truck with a 40-t load consumes in the acceleration phase. In the acceleration stretch of 1 km (0.6 miles), 1.1 l (approx.

4.3 Traffic

a

47

Acceleration travel 100 Speed [km/h]

80

Consumption [l]

1.2 1.0 0.8 0.6 0.4 0.2

Speed

60 40 20

Consumption

200

b

400

600

800

100 Speed [km/h]

Consumption [l]

1.2 1.0 0.8 0.6 0.4 0.2

Speed

80 60 40

67 s

Distance traveled [m]

1000

Constant travel at 85 km/h

Time required for 1 km:

Consumption

20 200

400

600

800

1000

Time required for 1 km:

42 s

Distance traveled [m]

Fig. 4.2   Consumption of a fully laden tractor semitrailer combination (40 t) with a standard configuration (430 bhp) for long-distance haulage. The vehicle needs a distance of approximately 1 km to accelerate from 0 to 85 km/h. This first kilometer is covered in 67 s and consumes roughly 1.1 l of diesel. At a constant speed of 85 km/h, one km is covered in 42 s and 0.25 l of diesel are consumed

0.30 gallons) of diesel is consumed during which the vehicle accelerates from 0 km/h to 85 km/h (53 mph). If driven at a constant speed, this stretch just about consumes 0.25 l (0.07 gallons). The consumption difference of 0.85 l (0.23 gallons) of diesel is used to build up the kinetic energy of the vehicle. This represents a primary energy value of about 30 MJ (see Table 2.1). The kinetic energy of a tractor semitrailer combination fully laden with 40 t (more than 80,000 lbs) at 85 km/h (53 mph) is 11.14 MJ (Eq. 4.1):

Wkin = =

1 · m · v2 2

1 · 40, 000 kg · (23.6 m/s)2 2 = 11.14 MJ

(4.1)

This results in an average efficiency of the diesel engine during the acceleration phase of about 37%: a plausible value. It is noteworthy that not only the traffic that forces braking procedures, such as traffic in the vehicle’s own lane and intersecting traffic, causes additional consumption, but also oncoming traffic causes an increase in the aerodynamic drag. Opposing (truck) traffic pushes a wall of air onto the vehicle (“our vehicle”) throwing the vehicle back. This is an effect that one can sometimes feel while driving.

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4  Operating Conditions of the Vehicle

4.3.1 Convoy Driving—Platooning Convoys of trucks are often formed (of necessity), especially on freeways. When driving in convoys, the air flow conditions of the vehicles mutually affect each other. Following vehicles drive in the slipstream of the trucks ahead. Provided that the distance is small enough, this leads to a reduction in consumption; and provided that the distance is sufficiently small, the lead vehicle also experiences a positive effect. The dynamic pressure, which builds up in front of a following vehicle, pushes the leading vehicle ahead. In [16], in the case of vehicles from the 1980s, a clear reduction in consumption is indicated with a distance of 70 m from the truck ahead. However, the measurements were performed with vehicles that were aerodynamically less favorable than nowadays. The positive aerodynamic effects of driving in convoys have also been proven on vehicles from the 1990s [1]. In the case of aerodynamically-optimized vehicles, a clearly less pronounced slipstream and a less pronounced dynamic pressure results, which means that nowadays the consumption reducing effects of driving in a convoy with aerodynamically favorable vehicles, does not become effective until the distances are short. Figure 4.3 shows current values for the reduction of consumption from driving in a convoy derived from a study of American long-distance vehicles at a distance of approximately 10 m [40]. Such short distances are not possible in normal road traffic. Regulations for a minimum distance vary state by state, however in many cases exceeds the following distance needed to save fuel. In addition, a greater distance from the truck ahead helps the driver to drive with anticipation. At a sufficient distance, the driver is able to drive relaxed and economically, and does not have to react frantically to the speed changes of the truck ahead. However, if automated systems allow several vehicles to drive in a convoy with a short distance between them in the future, driving in a convoy will provide aerodynamic advantages. The formation of a platoon with a small distance of course requires certain conditions: The vehicles must be connected to each other by a reliable quick wireless information exchange in order to exchange signals extremely quickly: If the vehicle in

3%

7%

6%

Fig. 4.3   The reduction in consumption through convoy driving—determined from simulations—that would result, if American long-distance vehicles could follow one another at a distance of 10 m and at a speed of 60 mph (96 km/h). The fuel consumption of the vehicle ahead is also improved. The illustration follows data from [40]

4.6  Consumption Reductions Through Optimized Operating Conditions

49

front brakes, the following vehicle must react and brake within a few hundredths of a second. It might be necessary, to take the different braking behavior of the different vehicles into account. The effects of automated vehicles driving in a platoon on other road users must also be taken into account. Traffic participants in adjacent lanes must be able to pass through the platoon in order to use on- and off-ramps or junctions, for example. Opening the convoy at each exit and each junction to give space for other road users and closing the gap again afterwards might jeopardize fuel efficiency gains. Although discussed from time to time currently the idea of truck platoons is not pursued anymore.

4.4 Speed The speed with which a truck is moved obviously also have an effect on the consumption. Moving a truck with 55 mph (which corresponds to 88 km/h, the usual cruising speed in European long distance haulage) instead of 50 mph (corresponding to 80 km/h speed, permitted in most European countries) increases consumption by about 10%. For this reason there are increasingly more trucking companies that are reducing the maximum speed of their trucks. This is a procedure that has been common practice in the USA for a long time. In many U.S. states, the allowed maximum speed of 65 mph (roughly 105 km/h) is distinctly above the European speed limit. As consumption increases overproportionately at high speeds, trucking companies in the U.S. limit the maximum speed of their vehicles. In China trucks with certain technical attributes are allowed to travel considerably faster than the standard 80 km/h. However, for economic reasons few truck drivers use this option.

4.5 Loading The loading status of the vehicle has a marked effect on the fuel consumption. Most of the values mentioned in this section apply to a tractor semitrailer combination that is laden to a gross weight of 40 t, the maximum permissible gross weight in Germany and a lot of European countries. Unladen vehicles consume less.

4.6 Consumption Reductions Through Optimized Operating Conditions The operating conditions are commonly accepted as being unchangeable. However, on closer consideration this is not the case. Using optimized operating times and smart route selection, the trucking company can attempt to optimize the traffic density that a vehicle will encounter.

50

4  Operating Conditions of the Vehicle

Fig. 4.4   Road construction affects energy consumption and smooth traffic flow

Based on the assumption that fuel efficiency is a topic of general interest, there are more options to help commercial vehicles to achieve optimized energy consumption, such as: • good roads that enable traffic flow without frequent braking, • optimized road surfaces, and • intelligent traffic light switching phases and well thought-out traffic routing. These are just a few things that help to reduce the energy consumption of highway transportation. The (energy) efficiency of traffic can also be improved when new freeways are being constructed. Instead of following the contour of the hills in hilly landscapes, the road can be smoothed by means of tunnels and bridges, thereby making fluid and low consumption traffic possible—see Fig. 4.4. The big advantage of such kind of improvements is, that they will improve the energy efficiency of all vehicles. New ones and old ones. It is undisputed that the road construction work will initially be more expensive as a result of this. Railroad freight carriers enjoy very smooth track routing (also for technical reasons).

4.7 Reduced Consumption Through Optimized Logistics Concepts For most considerations, it is not the consumption per vehicle that is decisive but the consumption per transported ton or consumption per transported cubic meter of capacity. Firstly, it is important to utilize as much of the transport capacity of a vehicle as possible. It is obvious that a fully laden truck consumes less diesel per ton/kilometer than a vehicle that is only semi-laden. Considered overall, a reduction in empty runs also brings

4.7  Reduced Consumption Through Optimized Logistics Concepts

51

about a reduction in fuel consumption. Secondly, increasing the payload or transport volume also reduces the consumption. In the case of transport goods with a critical volume, the consumption per transported load unit can be reduced by selecting optimized semitrailers. For example, there are low-loading double-deck trailers that enable a significant additional volume compared to a conventional box semitrailer. However, these low-loading semitrailers are considerably more expensive to purchase. Figure 4.5 shows examples how to increase the volume on a standard European Trailer. Concepts that increase the utilizable length of the semitrailer from the present-day length of 53 ft (US) or the de facto 13.6 m (EU) also increase the loading volume. Such semitrailers are operated with a special permit. In Europe, trucks with overall lengths of the truck trailer combination of 25.25 m are currently the subject of public discussion. In the case of bulky freight, the larger loading length alone enables a reduction in consumption (per transported volume unit). A field test involving long trucks has been performed in Germany in 2012, which envisages trucks with a length of 25.25 m with an unchanged gross weight of the truck trailer combination of 40 t [41]. Long truck-trailer combinations have been permitted in Sweden and Finland for quite some time and are allowed a gross weight of 60 t for the combination. A 60 t vehicle obviously consumes more diesel than a 40-t vehicle. However, the consumption per ton of transported goods is clearly reduced. [34] indicates a fuel saving per ton/kilometer for the EuroCombi of about 15–20%, [39] indicates a fuel saving per ton/kilometer of about 16%. In its interim report, the German field test arrives at a reduced fuel consumption of 15 to 25%. [25] suggests that the payload should be increased by allowing an increased gross weight of 44 t for all tractor semitrailer combinations, not only for intermodal transport journeys. This results in 15% more payload when fully laden.

Fig. 4.5   Examples to increase the transport volume of a European standard trailer keeping outer dimensions unchanged. For bulky cargo 10% more trailer volume basically translated to close to 10% less fuel consumption per unit transported

52

4  Operating Conditions of the Vehicle

However, the greatest consumption savings through optimized logistics concepts result, if driving distances can be avoided. Optimal routing at different loading and unloading points reduce vehicle mileage and thus consumption. Empty runs are particularly unproductive.

5

The Influence of the Driver on Energy Consumption

Even a technically-optimized vehicle will achieve good consumption values only if the driver exhibits fuel-efficent driving behavior. All vehicle manufacturers confirm that trained drivers can achieve considerable fuel consumption savings with present-day trucks in long-distance haulage. In [8] savings through an improved driving style of up to 16% are indicated in individual cases. On average, an effect of the magnitude of 6 to 8% can be expected. If one realizes what enormous technical efforts are necessary to achieve a 6% fuel saving through vehicle technology, it becomes obvious that driver behavior is an important factor for the reduction of energy consumption both for diesel powered vehicles and for electric vehicles. The driver can be encouraged by driver training courses to learn an energy-efficient driving strategy. It is most important to learn anticipatory driving: unnecessary braking, stopping and subsequent acceleration must be minimized. An anticipatory driver can also make use of the topography. Maintaining a sufficient distance from the vehicle ahead helps to reduce the number and severity of braking procedures. In addition to the training, the right thing to do is encourage the driver in his daily work to drive conscious of energy consumption. On diesel fueled vehicles with manual transmissions, simple but very effective encouragement can consist of indicating to the driver when it would be favorable to change gear from the fuel consumption point of view. In vehicles with AMT or electric vehicles the driver is not involved in gear changing anymore. Suitable driver assistance or telematic systems (see [2]) allow the driving style of the driver to be analyzed and, where applicable, to detect potential improvements. Telematic systems allow the operating severity of the route to be assessed and to thereby make a justified assessment of the driving style, even if the many drivers of a trucking company are operating on very different routes. This opens the possibility for the trucking company to give incentives to the drivers that adopt a energy saving driving style.

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_5

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5  The Influence of the Driver on Energy Consumption

5.1 Consumption Values of New Vehicles When a new vehicle is handed over to a driver, the effect of initially wanting to experience the performance potential of the vehicle, and therefore driving less economically at the beginning, is frequently perceivable. A new vehicle will exhibit an increased driving resistance anyway because the inner friction of the drivetrain, which has not yet been run-in, is higher. Brand-new tires also have an increased rolling resistance. In addition, brand-new tires also suggest an apparent increased consumption. As the tire circumference is greater, a shorter driven distance is indicated than with a worn tire. This results in a calculated (not real!) additional consumption. This calculation error can be avoided by not determining the distance driven on the basis of the vehicle’s speedometer, but doing so on the basis of the GPS data of a navigation system. Autonomous driving vehicles will eventually replace the driver and thus the driver’s contribution to fuel consumption.

6

Maintenance of the Vehicle and Service Fluids

Perfect condition and professional maintenance of the vehicle are required for it to be driven with optimized energy efficiency. There are a variety of maintenance errors and neglected maintenance tasks that can cause additional unnecessary energy dissipation i.e. unnecessary fuel consumption: 1. Well lubricated and intact bearings should be obvious. 2. [36] indicates that an axle misalignment of 1° can cost an increase of up to 3% in fuel. 3. Grinding brakes increase the driving resistance (and wear) unnecessarily. 4. A leaking compressed air system requires longer operating times of the air compressor. The power consumption of the air compressor is then far above the 0.8% in Fig. 2.1 and causes additional consumption. 5. Blocked or defective oil, air or fuel filters prevent the vehicle from being operated at its best. In particular, a defective air intake can have considerable effects on the performance characteristic and fuel consumption of the engine.

6.1 Tire Pressure Excessively low tire pressure will lead to a reduced service life of a tire and increased fuel consumption. Insufficiently inflated tires flex more intensely (and heat up more intensely), which means greater rolling resistance and increased consumption. If a tractor trailer combination has a tire pressure that is 20% too low, the result is an increased consumption of more than 2%. The relationship between tire pressure and consumption is illustrated in Fig. 6.1. With this in mind, tire monitoring systems not only contribute to road safety but also contribute to being able to operate the vehicle with optimized fuel consumption.

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_6

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6  Maintenance of the Vehicle and Service Fluids Additional fuel consumption [%]

56

8% 6% 4% 2% 0% -2% -60%

-50%

-40%

-30%

-20%

-10%

0%

10%

20%

30%

40%

Tire air pressure compared to target value

Fig. 6.1   The influence of the air pressure of the tires on fuel consumption: if all tires of a long-distance tractor semitrailer combination have a pressure that is 20% too low, the fuel consumption increases by 2%. The illustration follows [8]

6.2 Energy Content of the Fuel All fuels that do not only contain one molecule only (like pure Hydrogen) but consist of a mixture might have some variations in their energy density. Natural Gas For natural gas low caloric and high caloric natural gas are distinguished. The high caloric gas has a higher energy content per volume and per weight. Measured gas consumption in e.g. kg per 100 km are of course lower for the high caloric gas even if the energy efficiency of the truck might be the same. Diesel Fuel The quality of the diesel fuel obviously also contributes to the measured fuel consumption. Table 2.1 shows that diesel from different sources still in conformity with the respective standards can have an energy content that might differ by 10%. Different diesel fuels that meet the same standard can cause differences in consumption. Biodiesel causes (in 2010) an additional consumption of about 5%. A globally active mineral oil corporation claims that it can reduce consumption by 3% through the addition of a suitable diesel fuel additive [37]. Diesel fuels that do not meet the established DIN/ISO standard can cause considerably higher diesel fuel consumption.

6.3  Lubricating Oils

57

6.3 Lubricating Oils The most important task of lubricants in the engine, transmission and axle gear is to build up a lubricating film.1 A good lubricating film between two surfaces enables relative movement that is as friction-free as possible. This prevents wear (or at least reduces it) and ensures low power loss. This low power loss is the contribution of the lubricant to the fuel efficiency of the vehicle. Manufacturers of premium lubricants claim that optimized lubricants, so-called low-friction oils, enable a reduction in fuel consumption of up to 4%. Even if the real effect is probably less, an influence of the lubrication on the fuel consumption is plausible (see also [38]).

1 Another

task of lubricants is the important contribution of engine oil and transmission oil to the cooling of the units in question.

7

Concluding Remarks on the Topic of Energy Consumption

7.1 Measurement of the Fuel Consumption As mentioned at the beginning of this booklet, even small reductions in fuel consumption can bring about great savings. Exact fuel consumption measurements with great precision are therefore required to compare different technical solutions. Air pressure, humidity, wind speed, wind direction, the road condition, the road surface, traffic incidents, the load, the tire type, the tire air pressure, etc. will affect the fuel consumption of a heavy truck with a magnitude in the percentage range. The bases as to how measurement of the fuel consumption of a truck or bus should take place are defined in the SAE standards J1321 and J1526 for the U.S. and in DIN 70 030-2 [5] for other countries (Europe). [30] gives an idea of the large amount of work that must be done to achieve reliable and comparable consumption measurements.

7.2 Effect of External Factors on Fuel Efficiency An impressive indication of how efficient fuel-optimized commercial vehicles are and how big the influence of external factors, such as the topography, weather and traffic events is, was provided in May 2008 on the test track in Nardo, southern Italy. On this test track, a standard tractor semitrailer combination with a permissible laden gross weight of 40 t achieved an average consumption of 19.44 l per 100 km/h under ideal conditions (Fig. 7.1) [7]. The average consumption achieved in European long-distance haulage at this time was between 30 to 35 l per 100 km for a 40 t truck. So in the year 2008 one could say roughly spoken about 60% of the fuel are needed to overcome driving resistance in perfect conditions. The remaining 40% are due to imperfect conditions like weather and traffic. © Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0_7

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7  Concluding Remarks on the Topic of Energy Consumption

Fig. 7.1   In May 2008, this tractor semitrailer combination with a gross weight of 40 t achieved an average consumption of 19.44 l per 100 km on the test track in Nardo, southern Italy. The average speed was 80 km/h, which is the legally allowed maximum speed in many European countries. Photo Daimler

A standard-production tractor semitrailer combination that was a copy of the Nardo vehicle in terms of equipment and configuration subsequently achieved in operation at a trucking company a fuel consumption that was 10% less than the standard vehicles that were operated with the same transportation task for the purpose of comparison [44].

7.3 Scope of the Energy Efficiency Considerations If vehicle engineers try to improve energy efficiency of a vehicle or a technology, they usually focus (like this book does) on the so called tank-to-wheel chain. That is to say they compare the energy stored on-board the vehicle (in a tank or a battery) with the mechanical energy that is ultimately available for propulsion. This scope of discussion makes sense as it comprises all the levers and options the vehicle manufacturer has, to improve the vehicle. The tank-to-wheel efficiency contains all the design-features and configuration decisions that are fed into a vehicle like aerodynamics, engine efficiency, internal friction, tires and the like: basically all the technology considerations presented in this book. The scope of consideration is called tank to wheel or short TTW. The TTW consideration has the additional advantage of being comparatively easy to measure. At the filling point of the energy carrier into the vehicle (charging station or gas station) there is usually a calibrated measuring device—because the energy has to be paid. This means that the energy taken on board is known exactly.

7.3  Scope of the Energy Efficiency Considerations

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The well-to-tank efficiency determines the ratio of the total energy used to produce and supply the energy used in the vehicle to the energy that ends up in the tank (energy storage) of the vehicle. The well-to-tank view includes for example energy necessary to process the fuel, energy for transportation of the fuel, energy that is used to cool down, pressurize or liquefy the fuel and so on. From an environmental point of view the overall efficiency of a technology is of interest as well. This is described by the so called well-to-wheel consideration or WTW. The well-to-wheel view considers all energy used for producing the fuel—be it diesel, hydrogen, electricity or some other alternative fuel—and all the losses in the entire chain and compares it with the mechanical energy finally usable at the wheels. The total energy necessary and the total efficiency of a technology can be assessed in a WTW view. The energy consumed in the well-to-tank processes and the energy used in the tank-to-wheel process sum up to the well-to-wheel energy. From an environmental perspective even the well-to-wheel view does not show the complete picture. It covers the direct usage of the vehicle but does not give a total life footprint for a vehicle. For this total life-cycle perspective, production, maintenance, repair and disposal, including the positive effects of recycling, must be considered. Some technologies need quite some energy to be produced at first. Some products last for very long and are therefore somewhat more resource efficient.

Comprehension Questions

The comprehension questions serve to test how much the reader has learned. The answers to these questions can be found in the sections to which the respective question refers. If it is difficult to answer the questions, it is recommended that you read the relevant sections again. A.1 Calorific Value A single-family house has an electricity consumption of 6000 kWh per year. a) How much diesel fuel would be required to generate this amount of electrical energy with a diesel engine and a generator? Assume realistic degrees of efficiency. b) How much waste heat is obtained in addition? What problem results, if, for example, the intention is to use it for heating? A.2 Energy Losses Within a Truck a) Where does the diesel energy go to that is not converted into mechanical energy at the crankshaft? b) Where is the utilizable mechanical energy “consumed” that the engine provides. What are the most important motion resistances? A.3 Electric Drivetrain What are the major differences if you analyse the total energy consumed for a electric drivetrain and a conventional drivetrain with a combustion engine? A.4 Transmission and Fuel Consumption a) How does the transmission contribute to the fuel consumption optimized truck? b) Why do different gears in a transmissions have different average efficiencies? c) Why would one require a direct-drive transmission?

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Comprehension Questions

A.5 Axle Why does the average axle efficiency depend on the route the vehicle is driving? A.6 Aerodynamics a) On which vehicles/in which cases are the aerodynamics of the vehicle especially important? b) What are typical values for the frontal area A and the coefficient of drag cd? c) What do sweep and taper describe in the case of the driver’s cab? A.7 Predictive Systems a) Explain how predictive systems can reduce the fuel consumption. b) What predictive systems exist? A.8 Weather How does the weather affect consumption? A.9 Tank to wheel a) Explain the terms tank-to-wheel (TTW), well-to-wheel (WTW) and well-to-tank. b) When is it appropriate to use a TTW approach and when do you propose a WTW view?

Abbreviations and Symbols

The following is a list of the abbreviations used in this booklet. The letters assigned to the physical variables are in conformity with normal usage in engineering and the natural sciences. The same letter can have different meanings depending on the context. For example, a small c is a busy letter. Some abbreviations and symbols have been subscripted to avoid confusion and improve the readability of formulas, etc.

Lowercase Latin letters a acceleration c coefficient, proportionality constant cd coefficient of drag cT coefficient of drag with oblique incident flow f coefficient or correction factor fRot additional load factor in rotary motion g gravitational acceleration (g = 9.81  m/s2) g gram—unit of mass h height (linear dimension) k kilo  = 103 = a thousand times kg kilogram—unit of mass kW kilowatt—unit of active power; 1000 watts kWh kilowatt-hour—unit of energy l length l liter, a volumetric dimension; 1 L = 10−3 m3 m mass m meter m milli  = 10−3 = one thousandth p pressure r radius (linear dimension)

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0

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Abbreviations and Symbols

s distance (linear dimension) t ton—unit of mass—1 t = 1000  kg v velocity

Uppercase Latin letters A area, especially the frontal area BGL Bundesverband Güterkraftverkehr, Logistik und Entsorgung e. V. (German federation for road haulage, logistics and disposal) C Celsius—unit of temperature C coulomb—unit of electric charge CO2 carbon dioxide DIN Deutsches Institut für Normung (German institute for standardization) E energy F force FG weight force J joule, unit of energy LIN local interconnect network M mega  = 106 = million MCS mirror cam system MJ megajoule, unit of energy—one million joules P power T temperature (in kelvin or °C) TTW tank-to-wheel, TTW, means that all the processes are considered from having the fuel in the tank until the mechanical energy to propel the vehicle is available—for e.g. energy efficiency or for emission measurements W watt, unit of active power W mechanical work or mechanical energy Wkin kinetic energy (energy of motion) WTT well-to-tank. WTT takes into account all processes that are necessary during the production and logistic processes until the fuel finally is in the vehicle tank WTW well-to-wheel. WTW takes into account all the process steps from the production of the fuel until it is used to propel the vehicle. WTW is the sum of WTT and TTW.

Lowercase Greek letters α (alpha) angle β (beta) angle µ (mu) stands for micro = 10−6 = a millionth ρ (rho) density

References

General Textbooks 1. Schütz, T. (publ.): Hucho – Aerodynamik des Automobils: Strömungsmechanik, Wärmetechnik, Fahrdynamik, Komfort. Springer Vieweg, Wiesbaden (2013) 2. Hilgers, M.: Electrical systems and mechatronics. 2nd Edition. Commercial vehicle techno­ logy. Springer, Berlin (2023) 3. Hilgers, M.: Transmission and drivetrain design. 2nd Edition. Commercial vehicle technology. Springer, Berlin (2023) 4. Hilgers, M.: Alternative powertrains and extensions to the conventional powertrain. 2nd Edition. Commercial vehicle technology. Springer, Berlin (2023)

Technical Articles 5. DIN 70030-2 November 1986, Kraftfahrzeuge; Ermittlung des Kraftstoffverbrauchs; Lastkraftwagen und Kraftomnibus 6. Hilgers, M.: Wo geht die Energie des Diesels hin? Oder: Wie gestaltet man den verbrauchs­ optimalen Lastkraftwagen? 10th International CV Congress. VDI Reports 2068 (2009) 7. Mercedes-Benz: New Mercedes-Benz actros in the guinness book of records: The world’s most economical series-production truck. Press release Mercedes-Benz Truck, May 2008 8. Zürn, J.: Head of Mercedes-Benz truck development. Talk at record test track drive of the Mercedes-Benz truck in Nardo: Shaping future transportation. Fuel Efficiencies, May 2008 9. Mercedes-Benz: Mercedes-Benz aerodynamics truck & trailer: Saving fuel, cutting emissions. Press release IAA for Commercial Vehicles in Hanover, 21 September 2012 10. Göhring, E., Krämer, W.: Auswirkung aerodynamischer Maßnahmen auf Kraftstoffverbrauch und Fahrleistung moderner Nutzfahrzeuge – Part 1. ATZ 1985, 7/8 (1985) 11. Göhring, E., Krämer, W.: Auswirkung aerodynamischer Maßnahmen auf Kraftstoffverbrauch und Fahrleistung moderner Nutzfahrzeuge – Part 2. ATZ 1985, 12 (1985) 12. Göhring, E., Krämer, W.: Auswirkung aerodynamischer Maßnahmen auf Kraftstoffverbrauch und Fahrleistung moderner Nutzfahrzeuge – Part 3. ATZ 1986, 4/5 (1986) 13. Göhring, E., Krämer, W.: Seitliche Fahrgestellverkleidungen für Nutzfahrzeuge. ATZ 1987, 9 (1987)

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References

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Index

A Additional load factor, 7 Aerodynamic drag, 7, 25 Aerodynamics, 25 Air compressor, 22 Air density, 26 Autonomous vehicle, 54 Auxiliary consumers, 21

C Cab-behind-engine vehicle, 32 Characteristic map, 21 CO2, 4 Compressed air system, 21 Convoy, 48 Convoy driving, 48 Cost breakdown, 1 Costs, 1 Cruise control system, 39 Curb weight, 42 Curved roof, 37

D Diesel, 8 Diesel price, 2, 3 Direct-drive transmission, 18 Drawbar combination, 42 Driver training, 53 Driver’s cab, 30 Driving, anticipatory, 53 Driving style trainer, 40

E EcoRoll function, 18 Efficiency, 9 Emissions legislation, 14 Engine, 14 Engine speed level, 21 EuroCombis, 51

F Fan, 21 Flexing work, 24 Frictional energy, 8 Front (aero) bumper, 35 Fuel consumption specific, 14 Fuel consumption measurement, 59

G Gear ratio spread, 18 Generator, 23 Gradient resistance, 7

H Heat, 9 Hub caps, 35 Hunched back, 37 Hypoid axles, 19

I Incident flow, oblique, 27

© Springer-Verlag GmbH Germany, part of Springer Nature 2023 M. Hilgers, Fuel Consumption and Consumption Optimization, Commercial Vehicle Technology, https://doi.org/10.1007/978-3-662-66449-0

71

72 Increased consumption, apparent, 54

L Loading, 49 Logistics concept, 50 Lost energy, 8 Lubricating oil, 57

M Maintenance, 55 Measurement, 59 Motion resistance, 9

N Nardo, 59 New vehicle, 54

P Planetary hub reduction axles, 19 Platooning, 48 Power steering pump, 23

R Radiator shutter, 35 Rear end taper, 36 Refrigeration unit, 43 Road surface, 50 Rolling resistance, 7, 24 Roof spoiler, 33 Roof wind deflector, 33 Route selection, 49

S SCR technology, 14 Side deflector, 30

Index Side paneling, 35 Side skirts, 35 Slipstream, 48 Speed, 49 Spur gear transmission, 18 Steps, 38 System, predictive, 40

T Tank-to-wheel (TTW), 60 Temperature, 46 Tires, 24 Topography, 45 Tractor semitrailer combination, 42 Traffic, 46 Traffic light stops, 45 Traffic light switching phases, 50 Traffic routing, 50 Trailer tail, 36 Transmission, 17 Transmission automation, 17 Transmission control, 17 TTW. See Tank-to-wheel

U Underbody paneling, 36 Underfloor paneling, 34 Uphill driving, 7 Urban throughways, 45

W Water pump, 22 Weather, 46 Weight, 42 Well-to-wheel (WTW), 61 Wind tunnel, 28 Wing mirrors, 38 WTW. See Well -to-wheel