modular hybrid drive systems

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15 ATZ 09/2005 Volume 107 Authors: Michael Bielefeld and Norbert Bieler Modulare Hybrid-Antriebssysteme You will find the figures mentioned in this article in the German issue of ATZ 09/2005 beginning on page 738. Modular Hybrid Drive Systems Hybrid vehicle designs can vary greatly. In order to cover the complete variance of individual ap- plications, Siemens VDO Automotive has opted for a system with a toolbox of modules for hy- brid designs. The first two stages of this hybrid toolbox have been completed on the back- ground of practical experience stemming from joint projects with vehicle manufacturers and are being used for demonstration cars. This pa- per reports on the current stage of development and gives an outlook on the next development steps to come. 1 Introduction For years hybrid vehicles, combining a combustion engine and an electric ma- chine or two electric machines within one drive system, have been an exotic solution or a concept study more than anything else. Right now, however, this drive technol- ogy is in its breakthrough phase. There are a number of drivers, which make it very likely that hybrid vehicles will become widely used. Ever more demanding emis- sion and fuel economy targets (e.g. the ACEA CO 2 targets for 2008, 2012 and be- yond), climbing fuel prices necessitate the search for even more efficient drive tech- nologies, which can be combined with the combustion engine that will continue to be the dominant drive technology. Tax bene- fits for particularly environmentally friendly vehicles are to be expected (and are already in place in the U.S.). The well mar- ketable “green” image of hybrid electric ve- hicles and the growing demand for envi- ronment-friendly cars will do one more thing for hybrid solutions. Add to that the new functions which hy- brid vehicles can offer and the driving fun owed to the dynamic torque performance. In this situation several renowned market analysts have predicted a progressive growth for hybrid vehicles. Even a moder- ate prognosis suggests that the annual worldwide market volume for hybrid vehi- cles could grow to 700,000 units by 2012 [1]. The more aggressive scenario of the same study suggests 1.1 million hybrid vehicles. At the same time hybrid vehicles pose a consid- erable technological and economic chal- lenge as they deeply affect the complete dri- vetrain once the hybrid system is designed to have a certain performance and functional level. To be able to design such an intrusive system requires broad technical expertise on the developer’s side and close cooperation

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Page 1: Modular hybrid drive systems

15ATZ 09/2005 Volume 107

Authors:Michael Bielefeld and Norbert Bieler

Modulare Hybrid-Antriebssysteme

You will find the figures mentioned in this article in the German issue of ATZ 09/2005 beginning on page 738.

Modular Hybrid Drive Systems

Hybrid vehicle designs can vary greatly. In orderto cover the complete variance of individual ap-plications, Siemens VDO Automotive has optedfor a system with a toolbox of modules for hy-brid designs. The first two stages of this hybridtoolbox have been completed on the back-ground of practical experience stemming fromjoint projects with vehicle manufacturers andare being used for demonstration cars. This pa-per reports on the current stage of developmentand gives an outlook on the next developmentsteps to come.

1 Introduction

For years hybrid vehicles, combining acombustion engine and an electric ma-chine or two electric machines within onedrive system, have been an exotic solutionor a concept study more than anythingelse. Right now, however, this drive technol-ogy is in its breakthrough phase. There area number of drivers, which make it verylikely that hybrid vehicles will becomewidely used. Ever more demanding emis-sion and fuel economy targets (e.g. theACEA CO2 targets for 2008, 2012 and be-yond), climbing fuel prices necessitate thesearch for even more efficient drive tech-nologies, which can be combined with thecombustion engine that will continue to bethe dominant drive technology. Tax bene-fits for particularly environmentallyfriendly vehicles are to be expected (and arealready in place in the U.S.). The well mar-ketable “green” image of hybrid electric ve-

hicles and the growing demand for envi-ronment-friendly cars will do one morething for hybrid solutions.

Add to that the new functions which hy-brid vehicles can offer and the driving funowed to the dynamic torque performance.In this situation several renowned marketanalysts have predicted a progressivegrowth for hybrid vehicles. Even a moder-ate prognosis suggests that the annualworldwide market volume for hybrid vehi-cles could grow to 700,000 units by 2012[1].

The more aggressive scenario of the samestudy suggests 1.1 million hybrid vehicles. Atthe same time hybrid vehicles pose a consid-erable technological and economic chal-lenge as they deeply affect the complete dri-vetrain once the hybrid system is designed tohave a certain performance and functionallevel. To be able to design such an intrusivesystem requires broad technical expertise onthe developer’s side and close cooperation

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DEVELOPMENT Alternative Drives

16 ATZ 09/2005 Volume 107

with the vehicle manufacturer as well as sev-eral drivetrain suppliers. It does not help ei-ther that there are currently no standard-ized definitions of different types of hybriddesign. Considering the numerous require-ments which can be met by hybrid drives ofthe most diverse designs, it is perfectly possi-ble that there will be no such standard defi-nitions in the mid-term. That is why SiemensVDO Automotive is developing a toolbox forhybrid drive solutions. By using the compo-nents from this toolbox additional electricperformance of up to 15 kW have alreadybeen put on the road. In total the system willprovide solutions for up to 75 kW electricoutput and will thus also cover so called fullhybrid designs.

2 Rationale of the Modular Approach

The decision for a modular solution is a con-sequence of practical experience with defin-ing hybrid drives. As hybrid solutions can beused to achieve very different targets (fueleconomy, down-sizing of the combustion en-gine, driving dynamics, additional electricenergy to electrify function and pure electri-cal zero emission driving in cities etc.) thereis hardly a match between any two require-ment profiles. A purpose of this paper andthe solution presented in it is to begin to de-fine a hybrid drive system with the vehiclemanufacturer’s individual targets in mindand by considering the corresponding mar-ket needs. By precisely defining the drive sys-tem targets the hybrid component require-ments are also clearly specified.

Many design parameters, such as the elec-tric drive voltage and the issue of potentialinfluences on the installation space, directlydepend on the requirements that the electricmachine has to meet. As a consequencethose parameters are a result rather than apoint to start from. By defining the targetswhich the hybrid drive needs to fulfil, the so-lution can be achieved at a much morefavourable relation of effort and result be-cause over-designing or under-designing isavoided at an early stage. A toolbox of systemcomponents and simulation tools is requiredif one wants to put such a target oriented hy-brid design together in a flexible way and topredict which effects individual decisionscan have on system behaviour at differentlevels of abstraction (cf. section 5.5).

3 Definition of Key Hybrid Designs

As there are so many varying targets for hy-brid solutions this paper suggests differenti-ating very clearly between hybrid designs ofdiffering scope. Three typical hybrid solu-

and may have to be provided by two electricmachines in the case of four-wheel drive ap-plications. Also a traction energy storage de-vice of corresponding energy content isneeded. The considerably higher output lev-els require a much higher voltage level. As aconsequence to the more extensive andmore complex electric drive, the tractionbattery has to be sized appropriately as therequested electric power increases. Today’s14 V vehicle electrical systems can be main-tained and will be complemented by a trac-tion circuit (also referred to as intermediatecircuit). An integrated DC/DC converterlinks the two electrical systems. The re-quired number of functions will define thevoltage level that is needed in the electriccircuit of the hybrid drive. Voltage levels ofover 60 V require special measures of protec-tion such as fuses, isolation and monitoringto ensure safety.

4 Present Stage of Practical Solutions

Siemens VDO already has several years of ex-perience with developing and realizing hy-brid drive solutions. As an example a mildhybrid was developed and qualified to serieslevel (C sample) together with a European ve-hicle manufacturer as early as 2002. Some ofthe numerous other projects carried out incooperation with several vehicle manufac-turers (to A, B and C sample status) date backas far as 1998 and span electric drive solu-tions for both hybrid passenger cars andcommercial vehicles. The electric power lev-els range from a 105 Nm engine (at 10,000rpm) up to twice 200 Nm electric torque andrange from 114 V to 450 V operating voltage.Among those projects are parallel hybrid ve-hicles as well as fuel cell electric vehicles. Inmost cases the development scope also in-cluded the vehicle and drive control soft-ware.

At present three vehicles serve to demon-strate the solutions micro hybrid, Figure 4,and mild hybrid, Figure 5. A full hybriddemonstration car with an electric outputof 75 kW will probably be available from2006. This practical background providesthe technical foundation of the modulartoolbox system. Its development so far hasheavily benefited from the long-term expert-ise with electric machines, being partly contributed by other business fields andthe central research department withinSiemens AG.

5 Toolbox Scope

The toolbox that has been developed to dateand will be further developed over the next

tion requirement levels are classified as mi-cro hybrid, mild hybrid and full hybrid asfollows.

3.1 Micro HybridMicro hybrid denotes a solution with anelectric machine that is integrated into thebelt drive instead of a conventional genera-tor and typically delivers around 4 kW ofelectric output at 14 V voltage, Figure 1. Incomparison to claw-pole machines induc-tion machines are particularly well suited asthey facilitate additional functions. As far asthe electric machine goes, it is the micro hy-brid, which can be most likely integratedwithout a need for additional installationspace. On the function side it offers a stop-start mode (rapid combustion engine re-start within <300 ms and up to 600 rpm dur-ing cranking) plus an additional energy sup-ply (generator mode) with a higher systemefficiency of up to η ∼ 85 %. Cold start, faststart and battery management are addition-ally implemented in the Siemens VDOdemonstration car. Experience on the roadso far has shown that the stop-start mode of-fers a 5 % better fuel economy in the drivingcycle (NEFZ). As there is more electric energyavailable further benefits can be exploited,e.g. by electrifying functions that are hy-draulically actuated.

3.2 Mild HybridMild hybrid denotes a solution which in-cludes electric boosting of the combustionengine’s torque in specific driving situationsplus energy re-gaining during overrun (coast-ing) and braking (regenerative braking).These solutions were developed over the lastfive years. At a system efficiency of up toη ∼ 88 % they top the micro hybrid levelslightly. The combustion engine can becranked up to idling speed during start. Mildhybrids offer up to 15 kW electric output andcan be realized at a low-level voltage of 60 Vthat falls within a minimum protectionclass. Such a mild hybrid requires a compactelectric machine that is typically integratedinto the drivetrain, Figure 2. The SiemensVDO demonstration cars are also equippedwith cold start <800 ms, a fast start function<300 ms and traction energy storage man-agement. The fuel efficiency can be in-creased by 15 to 18 % in this case.

3.3 Full HybridFull hybrid denotes designs that offer addi-tional pure electric driving. Therefore thedevelopment extends far into the completevehicle, as the full hybrid requires a secondclutch on the combustion engine side, Fig-ure 3. The electric power demand is higher

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17ATZ 09/2005 Volume 107

years comprises hardware and softwarecomponents that facilitate solutions for themany requirement profiles, voltage and out-put levels mentioned above. In more detailthe toolbox scope includes:– a solution for the functional integration

in the drivetrain (Integrated PowertrainManagement, IPM)

– the development of electric drive func-tions (algorithms)

– electric machines as the physical ele-ment to generate torque and electric en-ergy

– the electronics for various control unittypes

– energy storage solutions.

5.1 Top Control LevelThe complex interaction of combustion en-gine control, transmission control if in-stalled and electric drive in a hybrid electricvehicle requires coordination at a highercontrol level. For that purpose Siemens VDOhas further developed the modular, inte-grated drivetrain management (IPM), Figure6, which also provided the basis for the con-trol solutions in the current hybrid demon-stration cars. The modular software solutionis based on an open system architecture andcan be integrated in the transmission con-trol unit, for example.

Serving as a coordinator and operationalmode manager IPM integrates the individualcontrol units in the drivetrain and providesthe algorithms for specific hybrid functions.These include stop-start, electric traction en-ergy management (including an interface tovehicle energy management), the controlover converting kinetic energy into electricenergy during regenerative braking (inclu-sive of all controls needed to adjust the brak-ing torque requested by the driver) and con-trol of electric torque boosting.

The IPM software is a generic platformfor different voltage and output classes. Itsfunctional architecture is a hierarchy of thethree levels of fundamental conditions(such as pedal position), operating state con-trol and torque management.

5.2 Electric MachinesIn most cases the physical backbone of theelectric drive is a three-phase machine,which is either integrated in the belt drive(offering typically between 2 and 4 kW ofoutput) as shown in Figure 1, or is directlymounted on the crankshaft (up to 75 kWoutput), cf. Figure 2 and 3. It should be keptin mind in this context that even 15 kW elec-tric output results in a considerable torqueincrease. So far mostly induction machines(asynchronous machines, ASM) of a cage de-

sign type are being used [2, 3]. In this casethe rotor follows the stator’s rotational mag-netic field in an asynchronous mode. In de-signs with just one clutch the engine speedsensor will suffice to provide the position in-formation, a separate sensor is not needed.

Induction machines have clear advan-tages, as they are robust, electrically verysafe, they offer low noise levels and are eco-nomic. At the same time Siemens VDO is de-veloping new permanent magnet synchro-nous machines (PSM), which offer the addi-tional benefit of greater low-rpm efficiency(lower battery output at cranking). PSM de-signs are also advantageous in their dimen-sions, weight and allow larger air gaps whencompared to ASM designs.

However, such PSM machines requiremagnetic materials which will work at anoperating temperature of up to 200 °C with-out showing an unwanted demagnetizingeffect. The development outlook in section 6offers more details on this issue and alsocovers the question of person safety in PSMdesigns. It also contains a brief outlook on anew type of synchronous machine with vari-able permanent-magnetic field.

5.3 ElectronicsHybrid solutions require new, scalable elec-tronic control units (ECUs), which can dissi-pate much more heat, facilitate greater pack-aging density and smaller designs despitehigher electric output. During driving thecurrent will have to be kept at a continuous200 A which can require higher voltage lev-els between 12 V and 400 V depending on thepower demand level in the span of 4 to 75kW. The toolbox comprises new 32 bit ECU06control boards with synergies from engineand transmission management and newgeneric lead frame power boards which meetthe above requirements.

Both board types are in the transitionfrom qualification to sampling with assem-bled boards and they are installed in a singlecommon housing. The powerboard technol-ogy in particular will make it necessary forthe housing design to either have a connec-tion to the engine coolant circuit or a sepa-rate cooling cycle or a fan-cooling. To facili-tate the use of the engine cooling cycle de-spite its high coolant temperature a hightemperature solution is under development.

5.4 Energy StorageIn today’s hybrid electric vehicles NiMHaccumulator batteries are the most widelyused technology as they reach high cyclestability and offer good energy and powerdensity. Double layer capacitors (DLC) aresuitable complementary traction energy

storage devices showing a high energydensity and a very high number of cycles.They are particularly well suited to absorbthe brief high currents during the energyregaining process of re-generative brakingor overrun.

5.5 Tools for the System Design PhaseSiemens VDO uses a three-step process to de-fine the requirements of a hybrid solutionin detail. In this process the vehicle targets(vehicle in total) establish the top layer. Thecorresponding simulation tools help to eval-uate the effects of different hybrid solutionson various driving cycles at an early stage.

On the intermediate layer (drivetrain) theeffects of different drivetrain strategies canbe simulated. On the lower tool layer (hybridsystem) with the lowest level of abstractiondifferent electric parameters can be checkedas to whether they are suitable for a certainsolution. These tools help to find scenariosduring the decision making phase. The con-sistent top-down approach that is used is agood way of revealing in growing detail theinterdependencies between top strategic tar-gets and specific design parameters. Thetools provide a survey of causal connectionsin the correct cascade structure.

6 Next Steps of Development

The complete toolbox inclusive of the full hy-brid components will be available by 2008. Inaddition long-term research and develop-ment projects focus on further optimizationand technical variations of the hybrid drive:Under the name of Memory Motor SiemensVDO is developing an electric machinewhich combines the induction machine’sadvantages with the benefits of a synchro-nous machine. Compared to PSM machinesthe Memory Motor has the added benefitthat the rotor magnets can be magnetizedand demagnetized via current impulseswithin a few microseconds. The magneticflux can thus be adapted to the current oper-ational requirements [4, 5].

At low rpm the rotor magnets are typical-ly fully magnetized and have a high level ofmagnetic excitation. At higher rotationalspeeds the magnets are partly demagnet-ized which reduces the field intensity ac-cordingly. An AlNiCo alloy is used as mag-netic material in the rotor. It shows neitherirreversible demagnetizing, nor tempera-ture-related demagnetizing, nor corrosionof the magnetic material. The materialchoice increases the level of safety: Synchro-nous machines with NdFeB permanent mag-nets as they are being used today can causehigh over-voltages at medium and high rpmduring fault situations (e.g. failure of the

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DEVELOPMENT Alternative Drives

18 ATZ 09/2005 Volume 107

motor connection), an issue that has to be eliminated withcontrol algorithms otherwise. With the Memory Motor theAlNiCo magnets can be demagnetized during fault via short-circuit currents without affecting the consecutive machineoperation. Installation and service are also made easier by theoption of demagnetizing. A first prototype delivered high ef-ficiencies between 80 and 90 % during partial load.

Another landmark on the roadmap is the development ofa four-wheel drive solution with two electric machines. In thefield of control electronics high clock rates will be part of fu-ture modules and new components as well as new mechani-cal designs will be utilized.

7 Conclusion

The success of hybrid vehicles will depend on two things:On the one hand a healthy relation between effort and re-sult is needed. On the other hand it is a matter of the prior-ity that highly efficient drive solutions will have. The car-bon dioxide emission targets in particular suggest a rapidlygrowing demand for hybrid solutions [6]. As fuel prices arelikely to continue increasing, this will also rapidly changethe relation between the technological efforts a hybriddrive requires in relation to its benefits. As the pure electricdriving option of a full hybrid marks the upper end of theadditional cost level it appears feasible to decide individual-ly whether pure electric driving makes sense. Micro hybridsand mild hybrids on the other hand are solutions with agood balance of effort and result. As an economic solutionthey also have good market potential.

Overall it is clear that hybrid solutions meet three centraldrivers of automobile technology: lower emissions, better fueleconomy and improved driving dynamics at the same time.The toolbox discussed in this paper will be a technically andeconomically optimized, scalable system that facilitates thestep to flexible series development of hybrid vehicles. The tool-box’s high degree of variability allows to put together practi-cally all imaginable configurations from a modular compo-nent range that is highly standardized.

References[1] ABI Research: Automotive System Reports. In: Hybrid Electric Vehicles,

Nr. 11 (2003), Nr. 12 (2003) und Nr. 3 (2004)[2] Schäfer, H.: Starter-Generator for Cars, Based on an Induction Machine

with Field-Oriented Control. SAE Toptec, European Switched ReluctanceMotors and Brushless Technology. Munich, Germany, September 1998

[3] Schäfer, H.; Wächter L.: Moderne Starter-Generatoren für Kraftfahrzeugemit Asynchronmaschinen und Feldorientierter Regelung / Kurbelwellen-startergenerator – Basis für zukünftige Fahrzeugkonzepte. Renningen: Expert-Verlag, 1999

[4] Ostovic, V.: Memory-Motors – a New Class of Controllable Flux PMMachines for a True Wide Speed Operation. In: Proceedings of the 36th IEEE IAS Annual Meeting, 2001

[5] Kruse, R.: Neuartiger Synchronmotor mit variabler Permanentmag-netisierung. In: Schäfer, H.: Innovative Konzepte für Starter-Generatoren.Renningen: Expert-Verlag, 2004

[6] Dudenhöffer, F.: Die Marktentwicklung von Hybrid-Fahrzeugkonzepten. In: ATZ 104 (2005), Nr. 4

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