CTI SYMPOSIUM 2018: 17th International Congress and Expo 3 - 6 December 2018, Berlin, Germany [1st ed. 2020] 978-3-662-58865-9, 978-3-662-58866-6

Table of contents :
Front Matter ....Pages i-ix
A 25 kW 48 V Mild Hybrid Motor and Inverter (Lawrence Alger, James Haybittle, Anthony D. Wearing, Cedric Rouaud, William D. Drury)....Pages 1-19
Compact and Efficient Electric Propulsion Systems Enabled by Integrated Electric Controllable Clutches (Carl Beiser)....Pages 20-42
Transmission Durability Objective Assessment: A Method to Estimate Durability Requirement (G. Camboni, M. Ferraccioli, P. Gili)....Pages 43-72
HRAM: Hybrid Rear Axle Module, an Innovative Hybrid Differential for P3 and P4 Applications (Carlo Cavallino, Sergio De Santis, Peter Riemer)....Pages 73-79
P2 Hybridization - Tailored Solutions (Martin Dilzer)....Pages 80-86
The Drivetrain for the New Mercedes-Benz Compact Cars (Christoph Dörr, Servane Lessi, Marcus Adolph, Volker Marx, Florian Veik)....Pages 87-96
Anthropometric Analysis in Automotive Manual Transmission Gearshift Quality Perception (Edson Luciano Duque, Plinio Thomaz Aquino Jr.)....Pages 97-109
48 V Hybridization (Thomas Eckenfels, Andreas Kaksa, Christian Marek)....Pages 110-119
Development of a Technical Solution Aimed to Improve Customer Acceptance of Reverse Gear Engagement Quality (Pietro Esposito, Gianluca Camboni, Salvatore Manolio)....Pages 120-134
Evaluation of Wear in an Automotive Transmission Using Powder Metal (PM) Gears (Anders Flodin)....Pages 135-142
From Vehicle Manufacturer to Mobility Service Provider – Business Challenges of Electromobility for OEM and Supplier (Wilfried Funk, Benedikt Strigel)....Pages 143-152
Lubricant Concepts for Hybrid Electric Vehicle (HEV) Transmission (Michael Gahagan)....Pages 153-162
The Next Generation DCT-Transmission for Mercedes-Benz Compact Vehicle Family (Alexander Harsch, Benjamin Kemmner, Andreas Ertel, Anton Rink)....Pages 163-175
Hybridization Requires New Clutch Systems (Joachim Hoffmann, Karl-Ludwig Kimmig)....Pages 176-186
Modular E-Drive Concepts for Light to Heavy Electric Trucks and Buses (Martin Huber, Heimo Schreier, Jürgen Tochtermann, Martin Ackerl)....Pages 187-195
100 Experts, 1 Opinion: Predicting Future Electric Vehicle and Powertrain Component Sales (Malte Jaensch, Hannes Bantle)....Pages 196-209
Game-Changing Lightweight E-Motor Design Enables Unrivalled In-Wheel Drives and Other Applications (Roland Kasper)....Pages 210-225
New All-Wheel Drive Systems in the Conventional and Electrified SUV Drivetrain of Mercedes-Benz (Ralf Koesling, Sven Stöhr, Volker Marx)....Pages 226-242
Development of a Virtual Reality Simulator Test Bench Capable of Validating Transmission Performance of Drivability using a Virtual Engine (Hiroki Kumashiro)....Pages 243-252
The Pedelec as a Plug-In Hybrid - Innovation Through an Automotive Technology Transfer (Bernhardt Lüddecke, Joerg Brandscheid, Stephan Rebhan, Hermann Meyer)....Pages 253-264
Design of a Hyper-High-Speed Powertrain for EV to Achieve Maximum Ranges (Marco Mileti, Patrick Strobl, Hermann Pflaum, Karsten Stahl)....Pages 265-273
Development of a New Hybrid Transaxle for 2.0L Class Vehicles (Nobuhito Mori, Yuki Hiura, Mitsutaka Matsumura, Kazuya Shiozaki)....Pages 274-281
Ultra-High Accuracy Technology for Measuring Transient Transmission Efficiency (Shintaro Ohshio)....Pages 282-291
Lightweight Forging: Potentials in the (Hybrid) Powertrain with Forged Components (Hans-Willi Raedt, Thomas Wurm, Alexander Busse)....Pages 292-299
Leveraging Connectivity and Automation to Improve Propulsion System Energy Sufficiency (Darrell Robinette, Bo Chen, Pradeep Bhat Joe Oncken, Josh Orlando, Neeraj Rama)....Pages 300-309
GKN’s ActiveConnect AWD System as an Energy-Efficient Solution for Electrified Vehicles (Christoph Schmahl, Michael Höck, Dirk Ressin, Mathias Kesseler)....Pages 310-318
Critical Safety Issues for EV, FCV and Useful Testing Machines to Solve Them (Takashi Shibayama)....Pages 319-333
Benefits of Hybrid P2 Off-Line Module Compared to Other Architectures (Norberto Termenon, Gilles Lebas, Thibault Meert)....Pages 334-346
Topology Comparison of 48 V Hybrid Drivetrains with Manual Transmission (Matthias Werra, A. Ringleb, J. Müller, F. Küçükay)....Pages 347-355
Back Matter ....Pages 357-358

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Proceedings

CTI SYMPOSIUM 2018 17th International Congress and Expo 3–6 December 2018, Berlin, Germany

Proceedings

Ein stetig steigender Fundus an Informationen ist heute notwendig, um die immer komplexer werdende Technik heutiger Kraftfahrzeuge zu verstehen. Funktionen, Arbeitsweise, Komponenten und Systeme entwickeln sich rasant. In immer schnelleren Zyklen verbreitet sich aktuelles Wissen gerade aus Konferenzen, Tagungen und Symposien in die Fachwelt. Den raschen Zugriff auf diese Informationen bietet diese Reihe Proceedings, die sich zur Aufgabe gestellt hat, das zum Verständnis topaktueller Technik rund um das Automobil erforderliche spezielle Wissen in der Systematik aus Konferenzen und Tagungen zusammen zu stellen und als Buch in Springer.com wie auch elektronisch in Springer Link und Springer Professional bereit zu stellen. Die Reihe wendet sich an Fahrzeug- und Motoreningenieure sowie Studierende, die aktuelles Fachwissen im Zusammenhang mit Fragestellungen ihres Arbeitsfeldes suchen. Professoren und Dozenten an Universitäten und Hochschulen mit Schwerpunkt Kraftfahrzeug- und Motorentechnik finden hier die Zusammenstellung von Veranstaltungen, die sie selber nicht besuchen konnten. Gutachtern, Forschern und Entwicklungsingenieuren in der Automobil- und Zulieferindustrie sowie Dienstleistern können die Proceedings wertvolle Antworten auf topaktuelle Fragen geben. Today, a steadily growing store of information is called for in order to understand the increasingly complex technologies used in modern automobiles. Functions, modes of operation, components and systems are rapidly evolving, while at the same time the latest expertise is disseminated directly from conferences, congresses and symposia to the professional world in ever-faster cycles. This series of proceedings offers rapid access to this information, gathering the specific knowledge needed to keep up with cutting-edge advances in automotive technologies, employing the same systematic approach used at conferences and congresses and presenting it in print (available at Springer.com) and electronic (at Springer Link and Springer Professional) formats. The series addresses the needs of automotive engineers, motor design engineers and students looking for the latest expertise in connection with key questions in their field, while professors and instructors working in the areas of automotive and motor design engineering will also find summaries of industry events they weren’t able to attend. The proceedings also offer valuable answers to the topical questions that concern assessors, researchers and developmental engineers in the automotive and supplier industry, as well as service providers. More information about this series at http://www.springer.com/series/13360

Euroforum Deutschland GmbH Editor

CTI SYMPOSIUM 2018 17th International Congress and Expo 3–6 December 2018, Berlin, Germany

Editor Euroforum Deutschland GmbH Düsseldorf, Germany

ISSN 2198-7432 ISSN 2198-7440  (electronic) Proceedings ISBN 978-3-662-58865-9 ISBN 978-3-662-58866-6  (eBook) https://doi.org/10.1007/978-3-662-58866-6 Springer Vieweg © Springer-Verlag GmbH Germany, part of Springer Nature 2020 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, express 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. Verantwortlich im Verlag: Markus Braun 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

Diversity in Intelligent, Electrified Automotive Drives

The rapid progress of digital transformation and faster evolution in automotive development will have a long-term effect on automotive drives. In a few years’ time, machines will hold meaningful conversations with humans and will ‘know’ their habits and moods. This will enable user-oriented driving functionality, and hence optimized vehicle operating strategies. Already, large companies are moving fast to extend their digital competence by buying up smaller digital specialists. The diversity of concepts, energy sources and charging modi in automotive drives is stronger than ever, which in turn increases their complexity. The next five years will see an ‘electromobility rollout’ with a broad portfolio of electric vehicles that enable ranges of 500 km or higher, plus significant extensions to infrastructure. By 2025 at the latest, more and more BEVs will meet customer expectations – and not just because of the medium-range price parity with conventional automobiles that experts predict in specific segments. Nevertheless, combustion engines – whether diesel or petrol – will still be the central pillar of automotive drives for a long while. Together with legal framework parameters, ‘reality checks’ from the energy and automotive industries continue to pour cold water on the euphoria generated by some electromobility players. However, significantly more electrified drive concepts – from 48 V mild hybrid through to plug-in hybrid drives – will enter mass production. Long term, some large carmakers now predict over 70% market share for hybrid drives by 2050. In addition to numerous add-on hybrid concepts based on familiar MT, AMT, DCT and CVT transmission concepts, the future will also see more DHT concepts such as multi-mode, powersplit or serial hybrid. Once again, this CTI SYMPOSIUM in 2019 addressed issues relating to all aspects of the automotive, transmission and drive industry. In addition to new concepts for transmissions, hybrid and electric drives, this includes production technology and component-related topics for passenger automobiles and commercial vehicles, and the influence of connectivity and automated driving on tomorrow’s drives: • Market and regulatory developments and their impact on drives • New hybrid, electric and fuel cell drives for passenger cars, trucks and buses, in part connected and automated v

vi

Diversity in Intelligent, Electrified Automotive Drives

• 48 V Mild Hybrid for various drives and markets • New AT, DCT, CVT and MT concepts with innovations to boost comfort and efficiency, and their modular hybridization • Various DHTs (Dedicated Hybrid Transmissions) for new hybrid drives • Drives with up to 3 ratios for EV drives, e-axles, high-rpm concepts • Conventional and electrified all-wheel drives, torque vectoring • Semiconductors, electric motors and batteries as key technologies in electrified drivetrains • Next-generation oils and lubricants for electrified drivetrains • Virtual, customer-oriented development using AI to determine efficiency, driveability and longevity • Test beds for EV and FCEV drives to examine and safeguard safety-relevant states • Compact, efficient driveaway and shift elements, eClutch, claw clutch, absorber solutions, torque and pressure sensors • Production: lightweight battery construction, electric motors, transmissions, new solutions with additive production, powder metallurgy The wide range of topics was addressed in a comprehensive programme that included 10 expert plenary talks and 16 parallel lecture series. In the panel discussion, vehicle, mobility and drive experts were addressed a topic cluster titled ‘Which energy source will power tomorrow’s mobility? What are its implications for the drivetrain?’. All this was flanked by CTI SYMPOSIUM EXPO – the ‘Hands-On Technology Market for Innovative Products’ with 125 exhibitors. Together with the introductory day for industry newcomers and the annual CTI TEST DRIVE with various drive and transmission concepts, the 17th CTI SYMPOSIUM in Berlin offered outstanding opportunities for international transmission and drive specialists to exchange ideas and opinions. It also provided invaluable updates on where we stand – and fascinating insights into what lies ahead. More: www.drivetrain-symposium.world Save the date:

Contents

A 25 kW 48 V Mild Hybrid Motor and Inverter. . . . . . . . . . . . . . . . . . . . . 1 Lawrence Alger, James Haybittle, Anthony D. Wearing, Cedric Rouaud, and William D. Drury Compact and Efficient Electric Propulsion Systems Enabled by Integrated Electric Controllable Clutches. . . . . . . . . . . . . . . . . . . . . . . . 20 Carl Beiser Transmission Durability Objective Assessment: A Method to Estimate Durability Requirement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 G. Camboni, M. Ferraccioli, and P. Gili HRAM: Hybrid Rear Axle Module, an Innovative Hybrid Differential for P3 and P4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Carlo Cavallino, Sergio De Santis, and Peter Riemer P2 Hybridization - Tailored Solutions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Martin Dilzer The Drivetrain for the New Mercedes-Benz Compact Cars. . . . . . . . . . . . 87 Christoph Dörr, Servane Lessi, Marcus Adolph, Volker Marx, and Florian Veik Anthropometric Analysis in Automotive Manual Transmission Gearshift Quality Perception. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Edson Luciano Duque and Plinio Thomaz Aquino Jr. 48 V Hybridization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Thomas Eckenfels, Andreas Kaksa, and Christian Marek Development of a Technical Solution Aimed to Improve Customer Acceptance of Reverse Gear Engagement Quality . . . . . . . . . . . . . . . . . . . 120 Pietro Esposito, Gianluca Camboni, and Salvatore Manolio

vii

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Contents

Evaluation of Wear in an Automotive Transmission Using Powder Metal (PM) Gears. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Anders Flodin From Vehicle Manufacturer to Mobility Service Provider – Business Challenges of Electromobility for OEM and Supplier . . . . . . . . . . . . . . . . 143 Wilfried Funk and Benedikt Strigel Lubricant Concepts for Hybrid Electric Vehicle (HEV) Transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Michael Gahagan The Next Generation DCT-Transmission for Mercedes-Benz Compact Vehicle Family. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Alexander Harsch, Benjamin Kemmner, Andreas Ertel, and Anton Rink Hybridization Requires New Clutch Systems . . . . . . . . . . . . . . . . . . . . . . . 176 Joachim Hoffmann and Karl-Ludwig Kimmig Modular E-Drive Concepts for Light to Heavy Electric Trucks and Buses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Martin Huber, Heimo Schreier, Jürgen Tochtermann, and Martin Ackerl 100 Experts, 1 Opinion: Predicting Future Electric Vehicle and Powertrain Component Sales. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 Malte Jaensch and Hannes Bantle Game-Changing Lightweight E-Motor Design Enables Unrivalled In-Wheel Drives and Other Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Roland Kasper New All-Wheel Drive Systems in the Conventional and Electrified SUV Drivetrain of Mercedes-Benz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Ralf Koesling, Sven Stöhr, and Volker Marx Development of a Virtual Reality Simulator Test Bench Capable of Validating Transmission Performance of Drivability using a Virtual Engine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Hiroki Kumashiro The Pedelec as a Plug-In Hybrid - Innovation Through an Automotive Technology Transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Bernhardt Lüddecke, Joerg Brandscheid, Stephan Rebhan, and Hermann Meyer Design of a Hyper-High-Speed Powertrain for EV to Achieve Maximum Ranges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 Marco Mileti, Patrick Strobl, Hermann Pflaum, and Karsten Stahl

Contents

ix

Development of a New Hybrid Transaxle for 2.0L Class Vehicles. . . . . . . 274 Nobuhito Mori, Yuki Hiura, Mitsutaka Matsumura, and Kazuya Shiozaki Ultra-High Accuracy Technology for Measuring Transient Transmission Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 Shintaro Ohshio Lightweight Forging: Potentials in the (Hybrid) Powertrain with Forged Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Hans-Willi Raedt, Thomas Wurm, and Alexander Busse Leveraging Connectivity and Automation to Improve Propulsion System Energy Sufficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 Darrell Robinette, Bo Chen, Pradeep Bhat Joe Oncken, Josh Orlando, and Neeraj Rama GKN’s ActiveConnect AWD System as an Energy-Efficient Solution for Electrified Vehicles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 Christoph Schmahl, Michael Höck, Dirk Ressin, and Mathias Kesseler Critical Safety Issues for EV, FCV and Useful Testing Machines to Solve Them . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Takashi Shibayama Benefits of Hybrid P2 Off-Line Module Compared to Other Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 Norberto Termenon, Gilles Lebas, and Thibault Meert Topology Comparison of 48 V Hybrid Drivetrains with Manual Transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347 Matthias Werra, A. Ringleb, J. Müller, and F. Küçükay Author Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357

A 25 kW 48 V Mild Hybrid Motor and Inverter Development of High-Power, 6-Phase, Frameless 48 V IPM Motor and an Oil-Cooled Inverter with Low Thermal Resistance Lawrence Alger, James Haybittle(*), Anthony D. Wearing, Cedric Rouaud, and William D. Drury Ricardo UK Ltd., Shoreham Technical Centre, Shoreham-by-Sea BN43 5FG, UK [email protected]

Abstract.   A novel inverter and a corresponding high-powered electric machine (“motor”) were developed for a hybrid vehicle. The inverter is a compact oil cooled silicon MOSFET design rated at 25 kW that drives a frameless 6-phase 8 pole, interior permanent magnet (IPM) motor with oil (Automatic Transmission Fluid, ATF) cooling and novel windings. Both Oil- and watercooled inverter variants were explored. Both the inverter and the motor were designed, built and tested to A-sample level. A major aim of the design was to make it possible to cool the motor, inverter and gearbox from the same oil supply. The focus of the work presented in this paper is reducing the thermal resistance of the inverter through the use of impingement jet cooling. Cooling of the electrically live tab of MOSFETs is well known to reduce thermal impedance, hence improving performance; this was achieved in the oil-cooled design by exploiting the oil’s electrical insulating property. Keywords:  Mild hybrid · Motor · Inverter · High power · Hybrid vehicle

1 Introduction An inverter and motor were developed for a 48 V [1] automotive hybrid transmission application. The inverter, motor (designed to fit into the same space as an existing 15 kW induction machine to enable testing in the existing gearbox) and the ancillary systems supporting it need to be small as well as cost-effective. Using oil as the cooling medium for the semiconductor devices and the rotor/ stator end windings, using the transmission’s existing cooling system eliminates the need for a separate coolant loop of 50:50 mix water-glycol, hereafter referred to solely as “water”, and the additional space and cost this incurs. For the oil-cooled version of the inverter to be compact, it requires the total thermal resistance between the © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Euroforum Deutschland GmbH (Ed.): CTI SYMPOSIUM 2018, Proceedings, pp. 1–19, 2020. https://doi.org/10.1007/978-3-662-58866-6_1

2     L. Alger et al.

semiconductor junction and the oil to be very low, due to its lower thermal conductivity compared to water. Using electrically non-conductive oils, the isolator in the device construction can be eliminated. This results in a live semiconductor packaging tab but reduces the thermal resistance of the semiconductor junction to the case (θJC) and enables more effective heat extraction from the electrically live surface. The design of the printed circuit board (PCB) needs to be considered and has a significant impact on the overall assembly costs of the system. One side of the PCB will be in an oil flow created by a submerged jet pointed at the base of the metal oxide semiconductor field effect transistor’s (MOSFET’s) heatsink. Various topologies, flow rates and temperatures are considered and compared. For the motor, thermal management of the magnets and end windings were key considerations.

2 Inverter Cooling Concept Development 2.1 Overview of the Cooling Technologies Evaluated There are many techniques and topologies that have been developed for the cooling of power electronic systems, both for automotive [2, 3] and non-automotive applications [4, 5]. This paper considered single-sided cooling of the devices from the tab-side only. Flip-chip [6] or dual-sided [7] cooled devices were not considered. The low thermal inertia and, consequently, very low thermal time-constant of the devices, gave rise to the need for good thermal diffusivity during high power demand operation. This is most commonly delivered using a high coolant flow rate and a highly optimized low thermal resistance across the layers of the semiconductor package and PCB. All these requirements provide a juxtaposition of excellent and novel technologies and techniques with a cost-effective solution. In the concept stage of this work, multiple approaches were considered, both as potential technologies and for comparison purposes, as shown in Fig. 1. Of the four arrangements shown, which are not to scale, the first 1) represents the typical layout used for a water-cooled inverter with this type of construction [8], including a thin ‘prepreg’ layer and some thermal adhesive. The second 2) shows the topology of the electrically live design that was selected, whilst the third 3) shows another possible layout for a water-cooled design, with FR4 fibreboard acting as a water-tight insulating layer. The fourth option 4) is the one that was arrived at as a fair comparison with option 2), as it has a similar structure including an insulating layer, to allow water cooling. 2.2 Option 1: Conventional Water-Cooled Design In this ‘industry baseline’ arrangement the MOSFETs are soldered to a copper track mounted on a sheet of FR4 fibreboard, through which numerous small holes (vias) are be drilled. The track will be both thick and wide, as it passes current from one side of the MOSFET to the phase connector. The vias’ function is to transfer heat through the PCB to a second layer of copper below, known as a ‘flood’, from which the heat is removed [8]. In a conventional water-cooled arrangement, there will be an insulating

A 25 kW 48 V Mild Hybrid Motor and Inverter    3

‘pre-preg’ layer over the flood that seals the back of the PCB and prevents water getting in [9]. A heatsink is glued to this pre-preg layer with the result that the heat would be drawn through the two insulating layers into the heatsink for removal. The thermal resistance of this arrangement would be quite high due to the high thermal resistance of the two insulating layers, even though they would be as thin as possible. This is option 1) in Fig. 1. 2.3 Option 2: New Oil-Cooled Arrangement A new arrangement is used with the MOSFETs soldered to a copper track etched from a layer mounted on a sheet of FR4, through which numerous vias are drilled. The vias are filled, as much as possible, with copper by plating to connect the flood layer both thermally and electrically. A pin-fin heatsink is soldered to the copper flood, maintaining the thermal and electrical contact with the MOSFET and ensuring a low thermal resistance. The cooling plate is live and, as such, the coolant medium must be electrically inert. This is shown as option 2) in Fig. 1. Similar ideas have previously been presented [10, 11] but the authors are unaware of this being applied within automotive applications.

Fig. 1.  Four possible power board topologies; 1) Conventional water-cooled approach, 2) Live cooling of the flood with oil, 3) Alternative flood isolation with water cooling, 4) Water cooled heatsinks bonded and isolated with adhesive

2.4 Option 3: First Alternative Water-Cooled Design Another arrangement was considered, whereby a continuous unbroken but thin layer of FR4 would be used to ensure that there could be no leakage through the board. This would be as thin as possible, with the pin-fin heatsink glued on the wet side and

4     L. Alger et al.

is option 3) in Fig. 1. The layer of FR4 provides electrical isolation but will increase θJC. 2.5 Option 4: Second Alternative Water-Cooled Design The thermal resistance of a water-cooled design would be lowest if a thin layer of high thermally conducting adhesive was used to seal the structure, unlike preceding water-cooled topologies. However, the manufacturability and reliability of such an approach is costly due to the requirement of sealing in addition to having to isolate the copper flood from the coolant chamber. This is option 4) in Fig. 1.

3 Thermal Analysis of the Inverter 3.1 Starting Assumptions The design of the inverter considered losses, calculated using estimates of temperatures, resistances and heat flux. The thermal design of any power electronic system is paramount to the overall performance, gravimetric and volumetric power densities. The preliminary thermal analysis uses the losses calculated for the selected MOSFET during conduction and switching events. As with all modelled approaches, manufacturing variances in addition to non-parameterised effects result in the need to correlate simulation with real-world practical results. Heat flux generated by each device is assumed to be uniform across its tab surface area, since the MOSFET has a significantly lower thermal mass and lower junction to case thermal impedance than the rest of the PCB construct. All the devices in the inverter are cooled the same way and their interactions with each other do not cause significant differences. Driven by the need to maximise the volumetric power density, the available cooling area is limited. Forced convection by impingement jet cooling would be required to achieve target steady state dT, given the calculated heat flux of the devices. This gives the potential to minimise the size of the inverter power stage. A standard transmission oil of either Shell D97 or 134FE Automatic Transmission Fluid (ATF) is used to cool the devices. Such an oil can be controlled and supplied to the pin-fin cooling plates at a required flowrate and pressure drop. Notably, the oil-cooling system is shared with the gearbox and the motor meaning the temperature at the inverter’s inlet could be higher than usual at 80 °C. Using the cooling concepts described in Fig. 1, the overall flow regime necessary is calculated (the size of the jets and the flow-rate). The minimum jet size is selected to prevent the jet from being blocked by debris, based upon Ricardo experience in the industry. All calculations assumed that the coolant was being sprayed directly onto the heatsink at a predefined position. The jet flow onto the heatsink is in a submerged environment with no air present. The flow rate is low enough to prevent erosion of the aluminium heatsink under jet impact, again based upon Ricardo knowledge.

A 25 kW 48 V Mild Hybrid Motor and Inverter    5

3.2 Modelling Heat Transfer 3D conjugate heat transfer (CHT) analysis is used to estimate the thermal resistance between the MOSFET junction and the lower side of the PCB flood, due to the relatively complex 3D effects of thermal diffusivity as the heat spreads out from the application area, Fig. 2. Thermal resistance of other thin layers such as adhesives are established by 1D solid conduction calculations. The relevant copper elements including thermal vias, basic pin fin heatsink geometry, and a fluid domain are analysed in the models. Four models are developed using the topologies described in Fig. 1. All dimensions and materials are optimised for mechanical integrity to ensure their appropriateness and manufacturability.

Fig. 2.  CHT Simulation to establish Rth improvement due to 3D effects

Oil cooling shows an improved value of thermal resistance despite the predicted poor heat transfer between heatsink and oil. Most of the oil cooling benefit is achieved by using an electrically live heatsink and thereby reducing the number of thermal layers and interfaces contributing to a higher thermal resistance. In comparison, the conventional arrangement showed that a pre-preg layer imposed a much higher total thermal resistance. Despite the incoming oil being hotter, due to the nature of the installation on the vehicle, the steady state junction temperature targets are still met. 3.3 Selecting a Thermally Efficient PCB Structure The water- and oil-cooled solutions show very similar total thermal efficiency. The dielectric properties of ATF allowed electrically insulating layers within the board construction to be removed, reducing the total thermal resistance. The water-cooled approach is competitive with this design because of its superior heat transfer properties over the ATF. For testing purposes, a structure is used that would allow two nearly identical boards to be made. This allows assessment of an ATF version using the existing gearbox cooling on the vehicle. Option 2) was selected as the preferred arrangement, and option 4) as an alternative. The modelling shows the thermal resistance of the water-cooled variant was highly sensitive to the interface material between the heatsink and the copper flood. A high thermally conductive, electrically insulative, and mechanically stable compound between the flood and the heatsink in an immersed fluid environment is required. To control interface gap, an adhesive with spacer beads is used, ensuring the thermal efficiency without compromising electrical insulation. Such an approach is also appropriate for high volume manufacture using existing techniques [12].

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Table 1 shows the results from the thermal analyses, and Fig. 3 the breakdown of thermal resistance by layer. It is shown that the effect of the thin FR4 layer in option 3 is too large to ignore, despite the manufacturing advantages it possesses: this option was discounted. 3.4 Analysis of the Cooling Plate and Heatsinks Isothermal analysis of the cooling plate is conducted to understand the fluid movement through the proposed cooling plate that feeds the impingement jets. Further, this enables geometry changes to be made that help to balance the system and eliminate stagnant flow regions. The geometry of a single cooling plate accommodates 24 MOSFET devices and the corresponding impingement jets were simulated providing appropriate resolution of both jet and feed-return chambers. These chambers were large, giving a low system pressure drop and achieve a theoretical jet flow variance of around ±2%. Analysis revealed redundant pins in the design towards the outer edges of the heatsink, where the wall jet had already been diverted. Geometry changes were made to improve pin fin efficiency and to prevent flow sharing between adjacent heatsinks.

Table 1.  Predicted Rth for four topologies Topology option Rth jct to MOSFET [°C/W] Rth MOSFET to Cu + Rth across Cu + Rth of thermal vias + Rth across flood [°C/W] Rth across insulating layer [°C/W] Rth across joint [°C/W] Heatsink base to fluid [°C/W] Rth total [°C/W] Steady state rise [°C] Inlet coolant temperature [°C] Steady state junct. temperature [°C]

1 0.400 0.320

2 0.400 0.320

Pre-preg 0.396 n/a

3

4

0.400 0.320

0.400 0.360

FR4 2.858

n/a

Thermal Soldered 0.067 Thermal Thermal adhesive 0.763 adhesive 0.763 adhesive 0.763 0.276 0.614 0.276 0.355 2.155 53.86 60

1.401 35.02 80 (oil)

4.617 115.40 60

1.878 46.94 60

113.86

115.02

175.40

106.94

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A second isothermal model, including three cooling plates in parallel and the secondary cooled capacitor bank, is required for a full system understanding. This allows the flow balance between the boards and the total system back pressure to be reviewed, Fig. 4. Geometry changes implemented prevented an area of returning turbulent flow.

Fig. 3.  Thermal resistance components shown as a bar graph

Fig. 4.  Inverter system pressure drop established by analysis of the fluid domain

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3.5 Modelling the Worst Case Operating Point A 3D CHT analysis of the PCB assembly is conducted, modelling solids only, to understand the transient temperature response when the device is at full load with no cooling. This identifies the maximum time it would take for the MOSFET to overheat and fail under worst case conditions (such as in a vehicle installation where the coolant has leaked or is not being circulated). In addition, it establishes the MOSFETs’ natural temperature rise over time at maximum load. Initially the thermal response of a single device is modelled with a 1D calculation and a basic 3D model. This provides an understanding of the model convergence criteria, built model confidence, and creates a baseline thermal response time. The 3D CHT model comprises of 12 MOSFETs mounted on a concept PCB with a basic heatsink design. The MOSFETs operating at constant 25 W loss with no active heat removal. This evaluates all solid domains and accounts for heat spreading through the structure, particularly the copper layers, and establishes an acceptable operating time until maximum junction temperature was met. A clear thermal path from the device to the heatsink is observed; importantly the model shows the thermal vias act as a bottleneck in this arrangement and careful design in this area is required. 3.6 Combined Analysis The same 3D CHT model is used with a fluid domain to model steady state thermal and fluid parameters to determine temperature balance across the board and eliminate hotspots. The model indicates acceptable temperature variations between the devices and identified a thermal limitation caused by the track length of the PCB where some MOSFTEs have restricted heat spreading effect, Fig. 5.

Fig. 5.  3D CHT model at steady state.

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3.7 Final Power Board Designs To allow a thin PCB construction and, in effect, shorten the thermal vias whilst maintaining the mechanical stiffness of the power boards, an adhered stiffener board is used. Using conventional double-sided PCB construction and a second process of adhering a stiffener board to it, common manufacturing methods can be adopted to reduce cost, and commonality maintained between both oil- and water-cooled variants. The notable difference is the oil-cooled solution used a soldered copper heatsinks whereas the water-cooled solution uses bonded aluminium ones. The grade of copper for the heatsinks on the oil cooled board was selected based upon good thermal conductivity, acceptable hardness, good machinability for forming pins and a coefficient of thermal expansion very close to pure copper to improve durability of the solder joint due to thermal cycling. The water coolant stipulated an aluminium heatsink, with inferior thermal conductivity, to avoid galvanic corrosion.

4 Prototyping the Inverter The inverter has 6 phases and is built with one phase on each power board. The peak operating current rating is 735 A/phase. With the need for a safety margin of circa. 30%, a design value of 1000 A/phase is developed. The CAD assembly is shown in Fig. 6.

Fig. 6.  CAD assembly of the inverter

4.1 Power MOSFETs The selected MOSFET devices are OptiMOS IPT015N10N5 units from Infineon [13]. The selected units have a VDS of 100 V, and a current rating, ID, of 300 A. There were 12 MOSFETs in total per phase (2× 6-parallel), 72 in the complete inverter.

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The supporting gate drive circuitry was not accommodated on the power boards, primarily to improve manufacturability and increase reliability due to the use of heavy gauge copper tracks on the power board. 4.2 Power Boards The MOSFETs are soldered to the PCBs manufactured specifically for the project. A 0.4 mm thick FR4 board with 2 oz copper foil layers top and bottom are used, with drilled vias. The foil and vias on the board are plated up to 6 oz, and the second FR4 stiffener layer bonded onto the back of the board. The plating is thick to reduce thermal resistance between the top track and the lower flood. The adhered stiffener layer has machined port holes to allow for the heatsink mounting to the floods of the power board. This layer also supports sealing between the top of the power board and the fluid volume and prevented excessive strain on the boards under fluid pressure. Heatsinks and heavy copper busbars are soldered directly to both sides of the PCB using vapour phase reflow. Vapour phase reflow provides a strong joint between the dissimilar copper thickness of the track and busbar which are difficult to join via conventional methods. The rear face of a power board is seen in Fig. 7. The heatsink pin-fin geometry is revised, to allow the pins to be saw cut and eliminate the need for special tooling. The cutting action allows deep grooves to form diamond shaped pins. The maximum pin surface area is maintained to limit any performance compromise. All pins are kept at constant height, to remove additional manufacturing effort but with a slight sacrifice in pin efficiency and mass, both of which are assumed to have relatively little impact on thermal performance.

Fig. 7.  Electrically live heatsinks soldered on floods of the power board with vapour phase reflow technology

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4.3 Power Packs Each power pack assemblies consists of two power boards and corresponding driver boards, which are sandwiched via bolted joint onto a machined cooling plate. The interface between the power board and the cooling plate is sealed with an o-ring located into a groove on the cooling plate and compressed onto the face of the power board. With DC connectors from the power boards orientated downwards from the assembly, and AC busbar orientated upwards, a highly volumetric efficient arrangement is created. 4.4 Inverters The inverter assembly consists of three power packs, a DC link assembly, mezzanine board and an rCube2 unit. The mezzanine board acts as an interface to the rCube2 unit, providing signal isolation to/from the 48 V circuits, pre-processing and interlock functionality amongst others. The rCube2 unit is a universal controller developed by Ricardo for rapid prototyping and was selected for this project to avoid developing additional control boards and to reduce the duration of software development [14]. The inverter is completed with aluminium housings to route the cooling fluid to the power board assemblies, to provide cooling locally to the DC link capacitors, and to seal the system for vehicle application, Fig. 8.

Fig. 8.  Prototype oil cooled inverter

5 Inverter Bench Testing 5.1 Correlation Analysis Testing of the oil cooled variant allows for correlation analysis to be performed. The inverter is run at a two steady-state operating points, set at different part loads. Data logged at approximately 167 Hz was averaged over 10 s after 40 min running. Surface temperature measurements are taken from the top of each of the 12 MOSFETs mounted on two of the PCBs cooled by ATF 134FE.

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Inverter thermal losses are quantifiable from the heat transfer to the oil, using the specific heat capacity of the ATF fluid at the measured temperature and the measured flowrate of the oil. The thermal load of one operating point was applied to the conjugate heat transfer model described in Sect. 3.2, to corelate thermal resistance calculations of the board over a single device. 5.2 Results The peak junction temperature of the device under the considered operating mode was determined by simulation to be 58.8 °C. This was marginally higher than the steady state surface temperatures measured during test, ranging from 53 to 57 °C, Fig. 9. The junction temperature was expected to be higher than the surface temperature measurement; by inferring the junction temperature, a good correlation of the 3D CHT model with test data is established, suggesting that the inverter does indeed have the low thermal resistance intended.

Fig. 9.  Surface temperature measurement results of devices on one PCB used to corelate the conjugate heat transfer analysis of a single device

6 Motor Concept Development 6.1 Motor Design Considerations There were 3 key considerations behind the design: 1. The wide speed range of the motor, particularly given the constraints of VDA320. 2. The desire for the e-machine to be cheap in production without compromising performance excessively.

A 25 kW 48 V Mild Hybrid Motor and Inverter    13

3. The requirement to fit the 25 kW machine into the same space as a 15 kW machine (Ø166 × 160 mm o/e), with the same cooling (2 l/min ATF at 100 °C max). 6.2 Motor Concept The existing gearbox cooling system sprayed ATF onto the end windings from the rotor, removing 2–3 kW of losses. For a 25 kW machine this implied a need for 90% or better efficiency, which could best be achieved with a PM machine. A high pole count and fractional slot windings were chosen as they permit thin back iron and short end windings, helping to maximise the active volume. This led to the selection of 8 poles and 18 slots. The high frequency resulting from the pole count led to 0.35 mm laminations in 35H250-C5, a reasonable grade of steel, being selected. This steel was relatively expensive, but the required performance could not be achieved without it. By making the stator frameless the parts count and projected manufacturing cost were reduced, and the stator bore could be made larger, increasing the power output. The most cost-effective rotor design with acceptable torque density is an IPM layout as this permits the use of flat magnets in enclosed slots, improving the motor’s 6-sigma score and eliminating both the need for a retaining can and for grinding arced magnet segments. The penalties for this were increased leakage and hence more magnet material. Finite element analysis was conducted, Fig. 9, that suggested that 25 kW could be achieved with this outline design despite the limited space. The idealised model predicted torque up to 60 Nm with the corner point at 4400 rev/min and a maximum speed of 18000 rev/min. 6.3 Designing the Stator Winding Determining the number of phases was an important decision made in conjunction with the inverter design. A true 6 phase design would be very complex to control but would have low dc link ripple, whereas a conventional 3 phase design would draw huge current, requiring too many MOSFET devices in parallel. The solution was to connect the coils as two separate sets of star (wye) connected 3 phase windings. The winding would inevitably have a low turn count (only 6 turns per phase) due to the low voltage, making a hairpin winding attractive. This would make it possible to have very short end windings and would also confer a high slot fill and potentially low manufacturing cost, provided that the number of connections was minimised and rectangular wire used. To minimise the number of connections it would be necessary to avoid parallel (multi-stranded, or bifilar) conductors. Consequently, they needed to be particularly large in section. However, this risked significant ac losses in the windings from the chopping frequency of the inverter. Calculation showed that there would be a noticeable increase in losses but that this could be partly traded-off against the consequences of reducing the 73% slot fill when using parallel conductors. The amount of loss would depend on the chopping frequency and current, the

14     L. Alger et al.

smoothing effect of the inductance, the operating temperature and other variables that were uncertain during the design phase. The chosen design used hairpins made from nested pairs of 8 × 4 mm copper conductor to maximise slot fill and to have the minimum number of connections, which was deemed essential for a production design. 6.4 Designing the Rotor Being an IPM design, the rotor’s lamination stack included pockets for the magnets with cavities either side to reduce magnetic leakage, and their tolerancing was designed so that the magnet’s glue would form a good thermal and mechanical connection to the laminations. The stack was clamped by end caps and fitted to the shaft by interference fit to maximise heat transfer to the ATF-cooled shaft. There was also a resolver fitted at the free end. The rotor design was optimised to minimise eddy current heating caused by current harmonics, Fig. 10, which led to the magnet dimensions and positions being modified to increase torque as the stack was shortened to accommodate the bearings, terminals and end windings. Magnet cooling was also introduced, as outlined in the next section.

Fig. 10.  Early 2D FEA model of the motor

6.5 Motor Cooling Cooling was only applied to the stator at the ends, where ATF was sprayed onto the end windings by the rotor, Fig. 11. This avoided large flows of liquid in the airgap and the churning loss that this would have created, whilst the short stack ensured that heat would reach the ends quickly. The end caps had large ports to allow the ATF to flow out quickly so that coolant would not remain close to the hot areas for long. There was no external cooling to the outside of the stator.

A 25 kW 48 V Mild Hybrid Motor and Inverter    15

Fig. 11.  Rotor PM loss (78 W). 17000 rpm, 78 °ɣ, PM 120 °C, Wdg 110 °C

2 l/min of ATF were fed down the shaft and sprayed onto both end windings, with another path through the rotor body cooling the magnets. These pathways had four nozzles fitted to thoroughly cool each quadrant of the end windings. The cooling was modelled using Motor-CAD [15]. An initial model was developed to support the design process, but many aspects required estimates that would have to be validated on test. Examples of this include: • • • • •

Realistic values for all losses. Estimating thermal contact resistance between the shaft and the rotor laminations. Similarly, between the magnets and the rotor body. Again, between the conductors and the stator stack. Allowing for the possibility that oil may pass over the hot surfaces more than once due to churning. • Allowing for reduced heat transfer in the slots due to incomplete coverage of surfaces by varnish. • Changing coolant flow with temperature and speed. The model was rebuilt after testing, using real values wherever possible. The thermal analysis predicted temperatures throughout the machine to within 5 °C for two measured steady state load points.

7 Motor Bench Testing A back-to-back test rig was built, and two motors and inverters were tested. It was not possible to commission the field weakening software in the time available, limiting the speed and torque ranges that could be tested, so further simulation was carried out using the test results to improve their validity. It was found that the machine is very susceptible to inductance and that this was slightly higher than expected, limiting the power output of the machine tested to a maximum of 22 kW.

16     L. Alger et al. 4 jets on the rotor

4 jets on the drive

4 jets on the free end of the shaft

Fig. 12.  CAD model of the rotor construction showing the rotor cooling jets

Figure 12 shows some test results. The graph matches test data against several scenarios modelled using Siemens’ SPEED software [16] with real dimensions, properties and test data. Lines ‘1’ and ‘2’ are the predicted and specified peak load lines respectively; and the measured load points are shown as red dots ±0.2 Nm, ±1 rev/ min. The highest torques recorded were 34.74 and 33.18 Nm. At 0.081 Nm/Arms this means that the machine needed 431 Arms and 411.6 Arms respectively to produce that torque. The top speed recorded was 14000 rev/min. Load line ‘3’ simulates the test rig arrangement, with ~17.5 kW peak and torque held to