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10PFL-0711

CVT-Based Full Hybrid Powertrain Offering High Efficiency at

Lower Cost

Patrick Debal, Saphir Faid, Laurent Tricoche and Steven BervoetsPunch Powertrain

Brecht Pauwels and Kevin VerhaeghePsiControl Mechatronics

Copyright 2010 SAE International

ABSTRACT

In 2007 Punch Powertrain started the development of a full hybrid powertrain concept based on its CVT. A

performance and efficiency analysis proved that a post-configuration offered the best solution. In parallel to themechanical and electrical development an advanced, Matlab/Simulink simulation system was established. A

robust powertrain strategy was developed and implemented into the simulation system. Results show a potential

of 30% to 70% fuel efficiency improvement depending on the cycle (type approval and real world cycles). A

higher saving potential is possible as a plug-in.

The fuel efficiency improvement is reached while meeting other important targets. First of all, the powertraincost premium needs to match the saving. Next to keeping the transmission cost under control the electric drive

technology and the batteries are cornerstones of the powertrain development. A dedicated switched reluctance

electric motor/generator is developed at a partner. Switched reluctance combines high efficiency and dynamicbehaviour with a low cost potential. Special care has been taken to iron out some drawbacks like torque ripple

and noise. LiFePO4 is the preferred battery chemistry. It offers the best combination of performance and costwithout the safety risk of classic lithium ion or polymer cells.

Additionally, the powertrain size is very restricted. The development team at Punch Powertrain managed to

keep the powertrain length, width and height within the size of the conventional counterpart. This enables astraightforward integration into most engine bays. As such, Punch Powertrain offers a fairly easy hybridization

path for conventional cars.

During the development project the search for auxiliary components was a continuous effort to find affordable

components that do not jeopardize the development targets for mass, space and efficiency. For most

components a hybrid compatible solution was found. The hydraulic pump for the transmission was oneexception. A parallel development was initiated during the project.

A demonstrator vehicle was built to drive as EV mid 2009 and as full hybrid by the end of 2009. In parallel a

second powertrain is built to undergo a series of tests on bench to validate and optimize the powertrain strategy

By exchanging powertrain control modifications between the demonstrator and the test bench driveability willbe guaranteed while further optimizing the fuel economy. Experience from both test platforms will be used in

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the industrialization project. This project will result in a production ready powertrain design as well as a flexibleproduction system for small to medium production series.

INTRODUCTION

In 2006 Punch International took over the Belgian CVT transmission plant of ZF with a clear target: the

development of hybrid powertrains for small and medium passenger cars. The company was renamed Punch

Powertrain. As Tier-1 supplier Punch Powertrain set ambitious targets to assure the hybrid powertrain is

competitive and appealing to OEMs at the time of introduction and during the following years. Next to fuelefficiency improvements the development project also targets low cost and easy vehicle integration.

This paper focuses on the chosen parallel topology, the general optimization strategy, the technology and

components selection and the control system development. Simulations for different target vehicles are

performed with detailed component maps. The fuel consumption target is well within reach. Functionalhardware tests have started in 2009. Performance tests will be performed in 2010.

DEVELOPMENT TARGETS

The strategic view of Punch Powertrain is to develop a next generation hybrid powertrain. This powertrain

needs to lower the current barriers for OEMs to offer hybrid versions of their vehicles. To reach this goalambitious targets were set at the beginning of the project.

FUEL SAVING

The main target is a fuel saving of minimum 25%/15% on the NEDC-cycle with gasoline/diesel cars whilerealizing similar savings in real traffic. These savings need to be realized by the powertrain only without

recharging the battery. This implies no other changes are applied to the vehicle than those required for

accommodating the hybrid powertrain. Additional measures taken by the OEM like engine efficiency

improvement, mass reduction and streamlining allow realizing further fuel consumption reduction. The similarsavings of greenhouse gases can help to meet the CO2emission target of 130 g/km set forward by the European

Commission or equivalent standards in other regions.

EV-RANGE

An EV-range of at least 13 km (8 miles) at city traffic speeds fits into policies of some major European cities to

reduce harmful emissions in city centers. When benefits from incentives can be gained or taxes (e.g. thecongestion tax in London) can be avoided the cost premium for the hybrid powertrain can be partially or totally

compensated in a short period.

INTEGRATION WITHOUT COMPROMISES

As Punch Powertrain intends to supply the hybrid powertrains to OEMs the impact of the powertrain on the

vehicle should be minimal. Next to the adaptations to make the vehicle hybrid ready other required changesneed to have the smallest possible impact. Consequently, the hybrid version of a vehicle must have the same

functionalities as the conventional counterpart, with respect to luggage space, the full use of folding seats andmodel choice.

Also the powertrain excluding the battery system needs fit in the existing engine compartment. Meeting this

target allows a reduced application development cost and time at the OEM side. Also market adoption will not

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be hindered by hybrid versions of passenger cars with reduced handiness or restricted availability over themodel range.

TARGET SEGMENTS

Punch Powertrain targets the most popular vehicle segments of small and medium passenger cars and small

vans in Europe. The hybrid powertrain can replace conventional powertrains with naturally aspirated gasoline

engines up to 2.8l. Vehicle segments using such conventional powertrains can be provided with the hybrid

powertrain under development.

Both Toyota and Honda have proven that in the compact vehicle segments a considerable number of hybridvehicles can be sold.

COST TARGET

Punch Powertrain has set strict cost targets to lower the barrier for OEMs and their customers to buy vehicles

with this powertrain. Buyers in the target segments are very cost sensitive. Therefore the cost premium for the

hybrid must clearly allow a return on investment.

Figure 1: Hybrid Powertrain by Punch

HYBRID STRATEGY DEVELOPMENT

The fuel saving target immediately ruled out a mild hybrid powertrain. Another choice was less obvious. As

developer of transmissions Punch Powertrain had the freedom to choose between two configurations.

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Figure 3: Combined Efficiency of Engine with CVT

Gradually, the calculation scheme was extended with more complex component maps. This required amigration from a spreadsheet based tool to a Matlab based application. Eventually, the calculation scheme

contained very detailed component maps. Although it was unable to simulate transient events like clutch closing

the calculation scheme yielded realistic fuel consumption results for non-hybrid vehicles. Therefore it wasdecided it could be used for comparative calculations.

Efficiency Engine + CVT - 1000 cc 3 cyl - 3300 rpm

0%

5%

10%

15%

20%

25%

30%

35%

0 25 50 75 100 125 150 175 200 225

Torque [Nm]

Efficiency[%]

EVmode

Generatemode

Conventionalmode

Assistmode

Figure 4: Use of the Hybrid Modes depending on Torque Levels at a Given Speed

The calculation scheme was used for comparing both the PRE and POST configuration. Overall the POST

configuration resulted in higher fuel savings on both type approval cycles and real world cycles. The main

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advantage of the PRE configuration is higher launch acceleration from stand still due the torque multiplicationby the variator. Due the higher fuel saving potential Punch Powertrain opted for the POST configuration.

HIGH-END, DYNAMIC SIMULATIONS

In parallel with the development and use of the calculation scheme, a highly detailed and dynamic hybrid

powertrain simulation was developed in Matlab/Simulink by using the SimDriveLine toolbox. The POST

configuration strategy was carried over from the calculation scheme and further refined. The powertrain

simulation tool uses a forward approach, i.e. a driver action causes the powertrain to change its operation like inreal vehicles. Furthermore, the inertia of all powertrain components as well as highly detailed component

models are included. This allows fully simulating the transient behavior inside the powertrain and adopting thestrategy to obtain a good driveability (low jerk level).

Tritec 1000 3 cylinder + CVT - MOL cycle

0

20

40

60

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120

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Secondary Speed [rpm]

SecondaryTorque[Nm]

Max 31% 30% 29% 28% 27% 25% 23% 20% 15% 10% 5% BYD HEV

Tritec 1000 3 cylinder + CVT - MOL cycle

0

20

40

60

80

100

120

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000

Secondary Speed [rpm]

SecondaryTorque[Nm]

Max 31% 30% 29% 28% 27% 25% 23% 20% 15% 10% 5% BYD HEV

EV

Assist

Conv

Generate

Figure 5: Operating Points and Actual Modes during a Real World Cycle

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The powertrain control logic for the high-end simulation was developed in Stateflow. This tool allows an easymigration of the control logic into embedded software by autocoding.

COMPONENT AND SUBSYSTEM SELECTION

The selection of the batteries and the electric motor/generator are two important cornerstones in how the hybrid

strategy can realize the fuel saving target. It is mandatory that the benefits realized at the conventional side(engine plus variator) largely outweigh the losses at the electrical side. Consequently, efficiency was the major

criterion in the selection process while cost, mass and size were also important.

ELECTRIC MOTOR/GENERATOR DRIVE

The combination of the electric motor/generator and its power electronics need to realize a high efficiency overa wide speed and torque range. At the same time the drive needs to have a high power density and a low cost.

An investigation of available technologies, products and suppliers resulted in the choice for switched reluctance

(SR) motor/generator. This type of electric motor/generator best fits the above requirements.

Punch Powertrain decided to partner with PsiControl mechatronics for the development and production the SRmotor/generator. PsiControl mechatronics has more than 10 years experience in industrial drivelines with SR,

mainly for weaving machines. PsiControl mechatronics has over 300.000 axes installed worldwide.Thisd makes

them an authority in SR

Figure 6: SR Motor - Sample Stator, Rotor and Actual Prototype

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SR motors have the inherent advantage of their robust design, both mechanically and electrically. Theirstraightforward mechanical structure is also advantageous for cost compared to other motor topologies when

produced in similar quantities. An SR motor has no natural boundaries, neither in torque nor speed compared to

conventional motors

In contrast with most industrial applications, a motor/generator in an automotive hybrid powertrain does notstay in a nominal point of operation for most of the time, but varies continuously over the full speed and torque

range. Therefore PsiControl mechatronics needed to implement a novel development approach, especially

concerning efficiency evaluation and comparison, and the thermal modeling. For both, a reference cycle wasused.

PsiControl mechatronics developed a dedicated SR motor/generator capable of 200 Nm and 30 kW (both peakratings, available for 30 s). This torque and power is provided in a package with diameter 225 mm and length

251 mm (external dimensions). The motor has a continuous power rating of 15kW and can deliver 90Nm

continuously at standstill.

From other hybrid projects Punch Powertrain learned that SR motor/generators are generally known for high

torque ripple at very low speeds and for noise. Both these issues were identified as serious barriers for marketacceptance due to reduced driveability at vehicle launch and driving comfort.

Torque ripple is mainly caused by the salient poles and are therefore inherent to the design of the motor, but can

be greatly reduced through the application of advanced control techniques as shown in Figure 6 for low speed

operation. The resulting torque ripple is low enough such that is does not induce vibrations and noise in thesystem, or is felt by the passengers.

Figure 7: Torque Ripple Reduction in SR Motor/Generator

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Although no specification on maximum dB rating was set in advance, because meeting such a limit can be donewhile still producing a noise quality that will not be accepted by OEMs and the public, the motor/generator is

designed with low noise generation in mind. From previous studies was known that the noise is highly

influenced by the mechanical structure and by the control method. Signifying improvements on both aspectswith respect to previous designs were incorporated in the design of the SR motor/generator.

BATTERY SYSTEM

The efficiency and the EV-range target rule out the use of NiMH batteries. A NiMH battery system would betoo large, too heavy and too expensive for the application. The common Lithium chemistries as currently used

for laptops and cell phones pose a potential but serious safety risk especially when these cells are scaled tocapacities as required for hybrid vehicles. Therefore Punch Powertrain opts for the LiFePO4 chemistry. This

emerging chemistry combines sufficient efficiency, usable SoC range, power and energy density, cycle life and

safety.(3)

Figure 8: Prototype Battery System

An evaluation of different suppliers of a combination of LiFePO4cells and battery management systems yielded

a preliminary short list of possible battery system suppliers. One battery system was ordered early in the project.

Later on two more arrived for different vehicle and test bench tests as well as for assessments of battery cellsand battery management systems.

Meanwhile different interested companies in Flanders have joined Punch Powertrain in a project for testing and

assessing different cells from a selected group of cell manufacturers. In this additional project also information

and knowledge about batteries and battery management is gathered by and shared between the differentpartners. As such it provides an undeniable support for the hybrid powertrain development.

During the project the short list of battery system suppliers evolves based on new insights and information. It isconsidered as preliminary also because LiFePO4 technology is still immature and important players are still

expected to emerge.

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TRANSMISSION DESIGN

The transmission design is based on the current VT2 CVT. Because this transmission is a thoroughly optimized

for conventional powertrains its adaptation for the POST configuration was not straightforward. The prototype

hybrid transmission currently used on test benches and in a demonstrator vehicle is therefore considered as an

interim solution, mainly for functional testing and demonstration purposes.

A high volute chain is used to connect the electric motor/generator to the secondary shaft of the CVT

transmission. This connection, at the engine side of the transmission, causes minimal changes to the rest of thetransmission and allows a fairly long electric motor/generator. Consequently a large number of parts is carried

over from the conventional CVT.

Figure 9: Hybrid Transmission with Electric Motor/Generator and Chain Drive. New parts in exploded view.

Further changes are related to removing the conventional reverse drive. All reverse driving will be done in EV-

mode. To provide oil in EV-mode to the transmission, the internal, engine driven oil pump is replaced by anexternal, electrically driven oil pump. During the whole development process priority was given to getting a

functional hybrid transmission running as soon as possible. Design changes to apply later versions for

performance improvements and cost reductions were recorded. They will be applied together with the lessonslearned from the functional prototype.

By meeting the tight sizing constraints the hybrid powertrain has the same powertrain length as the conventionalversion. Therefore it will fit in nearly any engine bay that can accommodate a conventional powertrain with

Punch Powertrains CVT. This is illustrated Figure 10.

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Figure 10: Comparison between Conventional and Hybrid Transmission

Once the hybrid powertrain has proven its potential a production intent design will be made. This newtransmission will feature several efficiency improving components and designs. Today most building blocks of

this next design are known.

CONTROL SYSTEM

The simulation tool mentioned earlier does not stand by itself. It is the first stepstone in the development of an

embedded hybrid powertrain control system. The logic developed in the Matlab/Simulink + Stateflowenvironment can be converted into embedded control software with a minimum of hand coding. Punch

Powertrain has acquired different dSpace rapid control prototype hardware and software tools to implement

the hybrid control logic from the simulation in prototype controllers. The hybrid development project is the firstproject at Punch Powertrain where the complete software of an embedded controller will be generated by the

autocoding process.

The hybrid control unit (HCU) is the master controller in the powertrain. The throttle pedal actuation by the

driver is converted by the HCU into a torque demand for both the engine and the motor/generator and a ratio

setting for the transmission. The hybrid powertrain control continuously sets the most efficient operationparameters.

The strategy is to a large extent robust. This implies that in a variety of operating conditions the powertrain canoperate at an efficiency that is substantially higher than the conventional powertrain. Consequently a substantia

fuel consumption reduction is achieved for different cycles in charge sustaining mode.

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CVT

ECU

ICE

EMG

Bat

+

PowEl

TCU MCU BMS

HCU BCU

CAN

POWERTRAIN MANAGEMENT

Figure 11: Powertrain Control Architecture

RESULTS AND SHORT TERM PLANS

The calculations as well as the simulations have shown that the target fuel consumption saving is within reach

Currently results better than the 25% saving on the NEDC-cycle are obtained while other cycles also yield high

savings. The first tests of the SR motor are also promising with respect to torque and efficiency.

Figure 12: Hybrid Powertrain in Smart ForFour on Chassis Dynamometer

Two prototype powertrains are built. One powertrain will undergo functional testing as well as strategy

validation tests on a dedicated powertrain test bench. Due to the economic climate the investment in this test

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bench has been delayed. In parallel the second powertrain is built into a demonstrator. To show the potential ofthe hybrid powertrain with the focus on its compact size a Smart ForFour was selected. This vehicle was built

on the Mitsubishi Colt platform. Its size is similar to the Ford Fiesta or Toyota Yaris. Currently the powertrain

in the Smart is being calibrated for driveability. When the driveability has been achieved the calibrated controlsystem will be implemented on the powertrain on the test bench. Then the powertrain strategy validation and

efficiency testing will start.

Tests so far with the Smart Forfour as well as a transmission check have revealed no major issues with the

transmission. The SR motor/generator low speed torque ripple improvement as described in the section"Electric Motor/Generator Drive" was applied on the vehicle after the initial assessments revealed animprovement was required. Similarly, the vehicle has been driven in pure electric mode to evaluate the SR

motor/generator noise. Even though the vehicle had no doors and a completely open engine compartment as

shown in Figure 12 the SR motor/generator noise was exceeded by other noises in the vehicle during tests onnormal road surface.

PRELIMINARY CONCLUSION

The hybrid powertrain under development at Punch Powertrain is made to be built into standard vehicles

without too much modifications and compromises at the OEM side. It will provide a substantial fuel saving

compared to similar conventional powertrains under most to all circumstances.

The functional transmission design is a very good basis for the later final design. Only minor modifications are

required. The SR electric motor/generator performs as expected or better with respect to vehicle acceleration,torque ripple and noise. Both the transmission and the electric motor/generator design are in line with the cost

target. The hybrid powertrain is also compact

When the strategy validation proves that the fuel saving can be realized Punch Powertrain and PsiControl

mechatronics have developed an efficient hybrid powertrain at a lower cost than systems available today.

REFERENCES

1. Pasquier, M., Continuously Variable Transmission Modifications and Control for a Diesel Hybrid ElectricPowertrain, SAE-Paper, CVT 2004-34-2896, 2004

2. Van Mierlo, J. and G. Magetto, Innovative Iteration Algorithm for a Vehicle Simulation Program, IEEETransactions on Vehicular Technology, Vol. 53, No. 2, March 2004.

3. Bauer, S., "New Lithium Ion Technologies, Performance Characteristics for Different Fields ofApplications", Advanced Battery Technologies, July 2008

CONTACT INFORMATION

ir. Patrick Debal, Project Manager R&D - Development Hybrid Powertrain

PUNCH Powertrain

Industriezone Schurhovenveld 4 125, BE-3800 Sint-Truiden, BelgiumTel. +32 11 679 266 - Fax +32 11 679 230

e-mail patrick.debal@punchpowertrain.com

ir. Brecht Pauwels, R&D Engineer

PsiControl mechatronics

Karel Steverlyncklaan 13, B-8900 Ieper, Belgium

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Tel: +32 57 409 635 - Fax: +32 57 409 697e-mail: brecht.pauwels@psicontrol.com

ACKNOWLEDGMENTS

The development of the hybrid powertrain at Punch Powertrain is supported by the Flemish Government as an

IWT industrial research and development projects. The IWT is the Institute for the promotion of Innovation by

Science and Technology in Flanders.

DEFINITIONS/ABBREVIATIONS

Bat Battery systemBCU Brake control unit

BMS Battery management system

CO2 Carbon dioxide

CVT Continuously variable transmissionECU Engine control unit

EMG Electric motor/generator

EV Electric vehicle

HCU Hybrid control unitICE Internal combustion engine

LiFePO4 Lithium iron phosphateMCU Motor control unit

NEDC New European drive cycle

NiMH Nickel metal hydrideOEM Original equipment manufacturer

PowEl Power electronics

SoC State of charge

SR Switched reluctanceTCU Transmission control unit