world of hybrids - schaeffler group · existed are equivalent to the volume of mount fuji ... nal...

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World of hybrids –A difficult choice

Wolfgang ReikDierk ReitzMartin Vornehm

As a result of the threats from greenhouse gasesand urban air pollution, that have in some casesreached almost intolerable levels, efforts arebeing made throughout the world to stem vehi-cle emissions. Conferences on climate protec-tion are trying to combat impending globalwarming by setting limits on harmful substancesand CO2. The self-imposed obligation on theautomotive industry to reduce CO2 output pres-ents new challenges to powertrain engineering,which will require and drive forward new con-cepts.

The CO2 output of a vehicle in particular isdirectly related to its fuel consumption. Successin the battle against this greenhouse gas canonly be achieved, therefore, by reductions in fuelconsumption.

However, careful use of oil reserves is importantfor another reason. Oil reserves are not infinite.Estimates vary as to how long they will last,depending on which difficult to extract reservesare included in the calculations. There is littledoubt, however, that reserves are slowly com-

ing to an end. A striking comparison was pre-sented in [1]. All the oil reserves that have everexisted are equivalent to the volume of MountFuji (Figure 1).

Approximately half this volume has alreadybeen used up. The remainder will have to bedivided up between an ever-increasing numberof people. Even if countries such as China orIndia achieve only a fraction of our standard of living, oil reserves will dwindle rapidly.Working of deposits such as oil sand and oilshale is more difficult, forcing up the costs ofproduction.

Estimates of the remaining life of oil reservesvary widely but are probably within the rangepresented in Figure 2 [2]. Taking account ofincreasing population and especially theincreases in the numbers of oil consumers, the situation will become critical between 2020and 2050. Alternative fuels such as biodieseland liquid fuels derived from natural gas willpresumably not be able to fill the gap betweensupply and demand.

In addition to fuel consumption, emissionsare playing an increasingly important role.Unlike other harmful exhaust gases, CO2 can-

not be removed bysubsequent treat-ment using a cat-alytic converter, and this thereforerestricts the contin-ued unlimited use offossil fuels.

Ozone concentrationhas increased sub-stantially in recentyears (Figure 3) [3]. If this should actu-ally be attributed toCO2 and not to natural fluctuationsin climate, it willnot be possible toavoid strong pres-sure for reductionsin CO2 emissionsand therefore sav-ings in fuel con-sumption.

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Introduction

Figure 1 All available crude oil reserves correspond to the volume of Mount Fuji [1]

Savings in fuel con-sumption thus have atriple effect:

• protection of theenvironment

• extended avail-ability of oil

• cost savings

Modern vehiclesmust therefore bedesigned for very lowfuel consumption inorder to eke out thevaluable resource ofoil for as long as pos-sible. In this way it isalso possible to reduce emissions, in particularof CO2.

On this point, achieving these necessary objec-tives will not be aided by simply playing off dif-ferent consumption-reducing measures againsteach other, e.g. hybrids, diesel versus petrol,engine modifications such as new combustionmethods with variable valve control etc. Thedecisive question in the future will not be“either or” but “not only but also”. Modernvehicles will therefore incorporate a skilfulcombination and integration of very differentmeasures for reducing consumption. Hybridtechnology is ideal for combination with numer-ous other measures.

Optimum consump-tion can be achievedunder the followingpreconditions:

• the vehicle musthave low aerody-namic drag androlling resistance

• low vehicle mass

• internal combus-tion engine withfavourable fuelconsumption

• basic transmis-sion with veryhigh efficiency

• automatic start/stop system for switchingoff the internal combustion engine when-ever it is not required to generate tractionpower

• braking energy regeneration.

The hybrid vehicles on the market already exhib-it this holistic approach, in an attempt to createan overall concept with favourable consumptionthrough a skilful combination of different meas-ures.

This paper deals with the typical hybrid-relatedissues of braking energy regeneration andstop/start in combination with an optimised effi-


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Figure 3 Trend of ozone concentration in Europe

Figure 2 Rapid growth in global population and collapse in oil production between theyears 2020 (pessimistic forecast) and 2040 (optimistic forecast)

ciency basic transmission. A wide range of differ-ent concepts will be described.

Mild hybrid or full hybridDepending on their electric power rating, hybriddrives are broadly classified as micro hybrids,mild hybrids or full hybrids (Figure 4).

In micro hybrids, the electric power rating is suf-ficient to restart the engine automatically after ithas stopped. Braking energy regeneration isonly possible to a very small extent. Due to therelatively low additional costs of a micro hybrid,such systems will nevertheless continue toappear in vehicles in the future.

If braking energy regeneration is also to beused, electric power ratings must be in therange 10 ... 15 kW. This allows normal deceler-ation to be covered and a large proportion ofthe total deceleration energy to be regenerat-ed. A further increase in the electric power rat-ing would allow regeneration of more severedeceleration, but since this occurs relativelyrarely, an increase in the available electricpower does not seem economically advisablepurely in terms of braking energy regenera-tion.

A mild hybrid does not allow electric driving butonly support (boost) to the internal combustionengine.

In terms of fuel consumption savings, this sys-tem can achieve practically everything: approxi-

mately 5 % through start/stop and approxi-mately 10 % with regeneration. A mild hybridwould therefore be sufficient if there is norequirement for electric driving or for significantpower support to the internal combustionengine.

A further reduction of 4 % in fuel consumption isobtained if the internal combustion engine isswitched off under operating conditions with lowefficiency and the vehicle is propelled by theelectric motor alone.

However, this additional consumption benefitcomes at a price. The electric power rating is, at 30 ... 50 kW, considerably higher thanthat of a mild hybrid; costs will be correspond-ingly higher. In order to achieve increasedpower, therefore, full hybrids are often devel-oped.

A full hybrid can also be used, however, toachieve downsizing of the internal combustionengine. The resulting drive comprises an inter-nal combustion engine that relies permanentlyon the active assistance of the high-power elec-tric motor in order to deliver its full perform-ance.

Overall, there are numerous variants and, inaddition, there is a transitional area betweenmild and full hybrid that for example allows elec-tric manoeuvring but not electric driving at high-er speeds.

Hybrid conceptsClassification in terms of micro, mild or fullhybrid may give an indication of functionality,but says little about the actual configuration andarrangement of the electric motor in the power-train. In some hybrid concepts, it is simply thedimensioning of the electric motor that deter-



Figure 4 Classification of hybrid systems and theirfunctionality

Figure 5 Simple schematic of powertrain to illustratetorque flows during drive and coast operation

mines whether the concept is classified as a mildor full hybrid. An attempt will therefore be madeto develop the arrangement of the electric motorin the powertrain and the torque flows as differ-entiation factors.

This will be presented using simplified schemat-ics of the powertrain (Figure 5). The transmissionindicated can be taken to represent any one ofthe established transmission types.

The generator asa starter generator Probably the simplest solution is a microhybrid for stop/start, which can be combinedwith all existing powertrains, with the conven-tional generator designed also for operation asa motor (Figure 6). It can then also function asa starter and restart the internal combustionengine with little noise, which is not possiblewith a conventional starter. This allowsstop/start, but not electric driving or regenera-tion. Typical power ratings for the electricmotor are in the range 4 ... 6 kW. This arrange-ment is only suitable as a mild hybrid.

Special attention must be paid to the belt drive.The torque flow in the belt during start-up is thereverse of the torque flow during generator oper-ation. The belt guidance and tensioning systemsmust therefore be designed to allow for switch-ing back and forth between taut strand and slackstrand.

In diesel engines particularly, the cold starttorques required may be well over 100 Nm,beyond the capacity of a normal belt drive. In such cases, the conventional starter canprovide cold starting of the engine. Stop/start will be used from an exhaust gas perspec-

tive and only on a warm engine. The torquerequired is well below 100 Nm and can beapplied by the starter generator via the beltdrive with little noise. This is one of the mainrequirements for acceptance of stop/startsystems.

Further it is possible to integrate a planetarygear set into the belt drive for cold start [5].

Crankshaft starter generatorIn the crankshaft starter generator, the electricmotor sits directly on the crankshaft where theflywheel is normally mounted. The rest of thepowertrain is downstream (Figure 7).

The starter generator must be designed suchthat cold starting of the engine is possible sinceit is not really feasible to accommodate a con-ventional starter. Typical power ratings are in therange 10 ... 15 kW. This requirement marked thefirst step into hybrid technology for many auto-motive manufacturers. In general, however,some system-related weaknesses prevented itsadoption for volume use. Stop/start can beachieved neatly, with little noise and quickly.However, regeneration and electric driving areonly possible to a limited extent since the inter-nal combustion engine cannot be decoupledfrom the starter generator.

In both regeneration and electric driving, theinternal combustion engine must be draggedalong against its internal friction. As a result,there is almost nothing left to be regenerated.

In principle, the crankshaft starter generator iscompatible with all transmission types. It mustbe noted, however, that for the vehicle to startquickly and move off immediately, all the nec-



Figure 6 Belt starter generator Figure 7 Crankshaft starter generator with electric motoron the crankshaft

essary torque-transmitting elements of thetransmission must already be in a positionready for operation. This is not the case in con-ventional automatic transmissions, in whichthe pressurised oil required for the actuation issupplied by an oil pump driven by the internalcombustion engine. For restart, the pump mustfirst move the oil, the pipes must be filled andelasticities preloaded before pressure can bebuilt.

Significant fractions of a second are lost in thisprocess. When restarting, the driver experi-ences an unacceptable delay. An (additional)electrically-driven oil pump must be fitted insuch automatic transmissions. This impairs theefficiency of the powertrain and increasescosts.

The crankshaft starter generator category alsoincludes the Honda Insight CVT. As shown in Fig-ures 8 and 9, the crankshaft starter generator iscombined with a CVT. This combination is per-haps not very imaginative, but is notable for itssimplicity and low consumption of approximate-ly 4 l/100 km.

An advantage of the CVT compared to a steppedtransmission is that driving is frequently possi-ble in overdrive ratios that can be set withoutcomfort-impairing gearshifts. Due to the lowengine speed, drag losses are reduced andregeneration is possible, with restrictions, evenwithout a separating clutch between engine andelectric motor.

All the Honda system lacks is pure electric driv-ing. In addition to a further clutch, this wouldrequire even more additional outlay for the CVTclamping system. The hydraulic pump is normal-ly driven by the internal combustion engine, so itwould need an alternative power source in elec-tric driving.

Starter generator betweentwo clutchesIf regeneration and electric driving is required, itmust be possible to decouple the internal com-bustion engine from the electric motor and therest of the powertrain. This gives rise to therequirements in Figure 10. The internal combus-tion engine can now be shut down and decou-pled from the rest of the powertrain. The decel-eration energy to be regenerated can be used infull to drive the generator. In electric driving, thefriction losses of the internal combustionengine no longer have an unfavourable influ-ence.

Depending on the drive rating, this concept canbe realised as a mild or full hybrid.

With electric motor ratings up to approx. 12 kW,only stop/start and regeneration are possible. Ifthe rating is 20 kW or higher, electric driving canalso be achieved.



Figure 9 Cutaway of CVT with starter generator in theHonda Insight

Figure 8 Combination of crankshaft starter generator andCVT in the Honda Insight

Figure 10 A powertrain with a starter generator betweentwo clutches

In principle, all transmission types can be oper-ated with this system.

The restrictions mentioned in the preceding sec-tion are valid here too. Restart can only beachieved with acceptable promptness if eitheran electrically-driven pressurised oil pump isprovided or, even better, the actuator systemsrequired for the gearshift and clutches are drivendirectly by electric motors.

In practically all projects, the clutch separatingthe internal combustion engine and transmis-sion is a dry clutch. This is possible becausethis clutch is not used as a starting clutch, sovery little frictional energy must be trans-ferred. Furthermore, it must have a low dragtorque. This can be best achieved with a dryclutch.

A dry clutch always has one side with a highmass moment of inertia, namely the side withthe cast friction elements, and one side with theclutch plate, which has a low mass moment ofinertia.

It is now possible in design terms to allocateeither the high or low mass moment of inertia tothe crankshaft. If the high mass moment of iner-tia is on the crankshaft, this operates like a nor-mal flywheel (Figure 10). The engine can there-fore run even when the separating clutch isopen.

During restart, however, the relatively largemass moment of inertia of the clutch mustaccelerate as well. This means a higher work-ing load on the clutch, a longer start time and a sensible deceleration of the vehicle, if the motor is started using vehicle momen-tum.

The reversed arrangement was therefore alsoconsidered (Figure 11).

In this case, only the extremely light clutchplate sits on the crankshaft. Together with thecrankshaft, connecting rod and piston, itsmass moment of inertia is not sufficient tokeep the engine turning when the separatingclutch is open. However, this is not required inany case.

The advantage of this arrangement takes effectin the restart. With a small power level split offfrom the powertrain, the internal combustionengine can be abruptly brought up to speed.Extremely rapid starts are thus possible.



Figure 11 Arrangement as in Figure 10, but the light side ofthe clutch is linked to the crankshaft

Figure 12 Dry clutch between crankshaft and electric motorin automatic transmission with converter. Heavyside of clutch on crankshaft

Figure 13 Separating clutch between crankshaft and electric motor with light side of clutch on crankshaft

However, the separating clutch must be capableof transmitting without slippage not only thestatic engine torque but also the torque fluctua-tions caused by the non-uniformity which, espe-cially in diesel engines, can be several times theengine torque. This is not an easy requirementfor the clutch but is one that can only be fulfilledusing a dry clutch.

In the arrangement “electric motor between two2 clutches”, there are therefore projects withthe heavy side and with the light side on thecrankshaft. Figures 12 und 13 show examples ofsuch clutches with the associated electricmotors.

Serial hybrid with clutch between two electric motorsIn the serial hybrid (Figure 14), the powertraincan be completely separated in mechanicalterms. Power can be transmitted by purely elec-tric means.

The two electric motors arranged in series, thefirst acting as a generator to give electricalpower that is converted in the second back intomechanical energy, can also be regarded,when the clutch is open, as an electrical trans-former or an electric, continuously variabletransmission. Such arrangements have longbeen used in diesel-electric units in locomo-tives.

This is of particular interest for the CVT, whichrequires considerable outlay for a reverse gearwith a planetary indexing set and a sepa-rate clutch. In the serial hybrid, appropriatedimensioning of the two electric motorsallows a reverse gear to be realised by regulat-ing the output to give reverse motion (Figure15).

In a further variant of the serial hybrid, the firstelectric motor is driven by the accessory driveand thus replaces the normal generator (Figure16).

The power level is limited, however, because thepower required for a full hybrid certainly cannotbe transmitted by means of a belt drive.

Hybrids with power splittingA quite different path was pursued by Toyotasome years ago. The power flow of the internalcombustion engine is split along two paths by aplanetary gearbox (Figure 17).



Figure 15 Reverse travel in serial hybrid with CVT

Figure 17 Power splitting in the Toyota Prius

Figure 16 Serial hybrid with first electric motor arrangedinstead of normal generator in accessory drive

Figure 14 Serial hybrid

The first path leads directly to the output, theother to a generator. Depending on how muchthe generator curbsthis path, a corre-sponding speedoccurs at the output.The output speed caneven be continuouslyregulated by meansof the braking torqueof the generator.

During braking, thegenerator of courseproduces a largequantity of electricalpower that cannot bestored (completely)in the battery. Anelectric motor istherefore mountedon the input thatfeeds this electricalpower back into thepowertrain.

Power is thus trans-mitted via a mechani-cal path direct to theoutput as well as anelectrical path.

This concept isimpressive for itssimple mechanicaldesign but requirestwo high-power elec-tric motors with atotal power rating ofapproximately 50 kW(Figures 18 and 19). A

further disadvantage lies in the large quantity ofpower flowing via the electrical path that, due tothe numerous conversion steps (mechanical toelectrical energy, frequency converter, back tomechanical power) does not have optimum effi-ciency and thus, at least to a certain extent, hasa sapping effect on the fuel consumption sav-ings.

As a result, there has been no lack of effort tocompensate at least partially for this disadvan-tage.

It was found that progress toward the objectivecould be achieved by splitting the power in mul-



Figure 18 Schematic design of Toyota Prius hybrid transmission

Figure 19 Toyota Prius transmission, cutaway

Figure 20 Power splitting with multi-range transmission [5]

tiple ranges. In simple terms, this is bestdescribed as follows: instead of a planetary setwith a fixed ratio, various planetary sets withdifferent ratios are used. Depending on the run-ning range (engine speed, torque, vehiclespeed), the planetary set is used that gives thesmallest possible flow of electrical power (Fig-ure 20).

Switching between planetary sets is similar tothat in a stepped automatic transmission. Thegreat advantage, apart from the smaller loss-es, is the considerably reduced power ratingsof the electric motors, in the range of approx.20 ... 30 kW. This significantly reduces the sizeof the power electronics, reducing the costsinvolved.

The change between the different ranges may bea disadvantage if they are not designed to besufficiently comfortable.

A design for 400 Nm called SEL (Stufenlos Elek-trisch Leistungsverzweigt = Continuous ElectricPower Splitting), is shown in Figure 21 [4]. How-ever, the cost and design envelope content ofthe two electric motors is still considerable and italso includes many components of an automatictransmission (pump, hydraulics, planetary sets,clutches, brakes).

Through the roadA further interesting variant is shown in Figure22. The front wheel drive, transverse powertrainarrangement remains unchanged. In addition,the rear axle is driven by an electric motor. Both

electric driving and regeneration are carried outvia the rear axle as a function of the batterycharge condition.

In order to achieve highly dynamic stop/start,the vehicle is first started up under high load inelectric-only mode via the rear axle while theinternal combustion engine is started in paral-lel.

In addition to the hybrid functions, it is possibleto achieve all-wheel drive in which the torquescan be split between the front and rear axle inany proportion required, within the power limitsof the two drives.

The power flow is via the road in different drivingsituations, for example in generator mode withdriving under engine power. Hence the descrip-tion “through the road”.

Hybrid with double clutch transmission The double clutch transmission (Figure 23) canbe expanded to a hybrid drive by inserting anelectric motor and a separating clutch for the



Figure 21 Hybrid with power splitting with multi-rangetransmission according to Tenberge [5]

Figure 23 Schematic of double clutch transmission

Figure 22 Hybrid drive “through the road”

internal combustion engine (Figure 24). In accor-dance with the subdivision, this arrangementwould fall into the category “between two clutch-es”.

This can be simplified considerably if the electricmotor is mounted on the sub-transmission forthe even ratio gears. No additional clutch is thenrequired since the existing double clutch canachieve separation from the internal combustionengine (Figure 25).

This gives a hybrid variant named ESG that ful-fils all hybrid functions with particularly lowoutlay, but is only appropriate in conjunctionwith a double clutch transmission [5-7]. With this arrangement of the electric motor,the axial length of the complete drive isunchanged from the basic drive which, consid-ering the restricted space conditions in thefront wheel drive, transverse powertraindesign, is a major advantage in comparisonwith coaxial mounting between the engine andtransmission.

Figure 25 describes the restart process, in whichthe electric motor starts the internal combus-tion engine via the clutch for the even ratio

gears while the first gear clutch is simultaneous-ly activated in order to split off part of the torquewhich is fed to the wheels. The driver thereforesenses an immediate response from the vehicleeven before the internal combustion engine isfully started. The good response behaviour ismade possible by an electric actuator systemthat allows complete closing of the clutcheswhile stationary irrespective of the powertrainspeed.

Figure 26 shows the torque flow during regener-ation in schematic form. Since the climate con-trol compressor is directly connected to the elec-tric motor, regeneration can be carried out aselectrical energy or into the climate control com-pressor. This arrangement also allows stationaryclimate control since the electric motor can drivethe climate control compressor with an openclutch and synchronisation devices in the trans-mission.

In terms of powertrain dynamics, the most criti-cal situation is the switching on and switchingoff of the internal combustion engine duringdriving. In particular, restart as a reaction tooperation of the gas pedal requires very rapidresponse behaviour.

The internal combustion engine is switched offafter a specified time by transferring the torquefrom the active clutch K2 onto the electricmotor. Once the clutch is opened, the internalcombustion engine decelerates to zero and theelectric motor picks up the coast torque as agenerator.

Figure 27 shows the precise structure of the pow-ertrain in which the two subtransmissions nor-mally used in double clutch transmissions are



Figure 24 Expansion of double clutch transmission into hybrid

Figure 25 Simplified arrangement of electric motordouble clutch transmission

Figure 26 Regeneration, either in electric motor or climatecontrol compressor

nested in a singletransmission. Theelectric motor islocated with a parallelaxis to the transmis-sion and is driven viatwo cylindrical gearsby the 4th gear fixedwheel and a ratio ofapprox. 1,2.

LuK has built a proto-type with such atransmission in con-junction with a 1,3litre diesel engine.The electric motor inthe prototype (Figure28) has a power rat-ing of 10 kW, is entire-ly adequate for thisperformance classand this vehicle forthe purposes ofregeneration andthus represents amild hybrid. If the same construction isretained but the electric motor is scaled up, afull hybrid can be achieved. The climate controlcompressor is positioned directly under theelectric motor and is driven by means of a poly-vee belt. With this arrangement, interior cli-mate control can be maintained during regener-ation and also while stationary.

The controllers for the internal combustionengine, transmission, starter generator andenergy storage facility communicate via theexisting CAN. Co-ordination of the hybrid func-tions and energy management was integrated inthe transmission management system.

As mentioned, the dynamics involved in restart-ing the internal combustion engine and buildingup wheel torque are decisive in determiningacceptance of the stop/start function in conjunc-tion with an automated transmission. Figure 29shows measurement of creep behaviour afterrelease of the brake. The electric actuator systemprepares for the next start during the stationaryphase by completely closing clutch K2 and set-ting start-up clutch K1 to creep torque (cf. Figure26). After magnetisation of the electric motor (TT˜ 70 ms), the starting torque rises to the value ofapprox. 140 Nm necessary for acceleration of theinternal combustion engine. In parallel, clutch K1directs 10 Nm to the output via the engaged 1stgear. 140 ms after release of the brake pedal, theclearances in the drive are overcome and thevehicle starts to accelerate. This leads to signifi-cantly better response in comparison with drivesdesigned on the basis of ASG. Approximately



Figure 27 Structure of double clutch transmission with electric motor and climate controlcompressor

Figure 28 Prototype transmission with electric motorflange mounted on side

290 ms after release of the brake pedal, theinternal combustion engine achieves idlingspeed and ignition occurs.

Time is just as critical in restart of the internalcombustion engine from a coast phase. Figure30 shows the activation of the internal combus-



Figure 29 Start-up from stop/start (measurement)

Figure 30 Start-up of internal combustion engine after regeneration phase (measurement)

tion engine at a travel speed of 55 km/h. This isinitialised by activating the gas pedal. First, theelectric motor is decoupled from the output bydeselecting the active gear and switched fromgenerator to motor torque. Clutch K2 is closedrapidly, accelerating the crankshaft. In additionto the power supplied by the electric motor, acti-vation of clutch K1 feeds additional power fromthe output to crankshaft in order to furtherreduce the time until ignition of the internal com-bustion engine. However, this torque can only bemaintained for a short period, otherwise thedriver will experience this as a delayedresponse. The initial reaction to activation of thegas pedal felt by the driver is a reduction in thecoast torque to zero. Approximately 340 ms afteractivation of the gas pedal, the crankshaft reach-es the speed of transmission input shaft 1. Fromthis point, torque is built up at clutch K1 and,after 380 ms, the vehicle begins to accelerate.

The behaviour of the vehicle under accelerationfrom standstill can also be significantly improved.The torque available from the electric motor is firstfed to the internal combustion engine in order toreach start-up speed quickly. In accordance withthe increasing torque of the internal combustionengine, the torque of the electric motor is thenreduced. The boost function is only active duringperiods of low internal combustion enginespeeds. The maximum torque of the internal com-bustion engine is not exceeded, in order to givethe driver reproducible acceleration behavioureven over several cycles. The vehicle accelerates

from stop/start by means of the boost function,also with highly dynamic characteristics. Figure 31shows the acceleration advantage to 70 km/h ofapprox. 1,1 s compared to the base vehicle.

Following function testing, consumption savingswith the prototype vehicle were measured asshown in Figure 32. This is based on the doubleclutch transmission, already with significantlyimproved consumption behaviour, and thehybrid functions of stop/start and braking ener-gy regeneration gave a further advantage ofapprox. 12 % fuel saving over the mixed cycle.

Comparison of conceptsFigure 33 shows an attempt to compare the vari-ous concepts. It should be pointed out first of allthat this was only possible to a limited extentsince, for each concept, there is a large numberof possible designs that cannot be included insuch an assessment. Nevertheless, some signif-icant statements can be made that may assist inreaching a preliminary decision on selecting theright hybrid system.

The first question is which concepts are suitablefor mild or full hybrids. Figure 33 shows thatmost are in principle suitable for both, depend-ing on the dimensioning of the electric motor(s).There may even be a fluid transition between



Figure 31 Vehicle acceleration: PSG versus ESG, 0 – 70 km/h(measurement)

Figure 32 Reductions in consumption measured over cycle

mild and full hybrids in the future, for examplewhere electric manoeuvring but not full electricdriving is required.

The two columns “Mild hybrid” and “Full hybrid”show which of the functions start/stop, brakingenergy regeneration and electric driving are pos-sible.

For example, the variant “between two clutches”can be expanded to mild hybrid or full hybriddepending on the size of the electric motor. Typ-ical electric power ratings for regeneration are 10 ... 15 kW. This is somewhat over-dimensionedfor start/stop. Electric driving would requireapproximately 30 kW or higher.

Some hybrid concepts can only be used in con-junction with particular types of transmission orrequire a transmission type that would not func-tion at all without a hybrid. This applies mainly tothe variants with power splitting.

Costs will play an important role. In addition tothe electric motor, critical issues will be the

power management electronics and power stor-age in the battery and super-capacitors. It is diffi-cult to make reliable assessments here since thecomponents are not yet available on the worldmarket in a form suitable for volume usage.

Nevertheless, some statements can be made. Inapproximate terms, the costs of an electric motorwill be underpropotional to its power. The sameeffect exists for the storage media of batteriesand super-capacitors.

Hybrid concepts using two electric motors will, forthe same total electric power rating, be moreexpensive than if only one unit is used. In this casetoo, there will be additional outlay in the transmis-sion, for example an additional clutch and theassociated actuator system. Alternatively, an elec-trically driven oil pump will be required in automat-ic transmissions to allow a start/stop function.

A whole range of these concepts are alreadyused in small production quantities or are indevelopment, as shown by the reference in thefinal column of Figure 33.



Type Function

mild/micro

strong start/stop

recupe-ration

electricdriving

combi-nable withtransmission

additionalcost

in productionor published

belt drivenstarteralternator

all PSA

crankshaftstarter alternator

all GM

Honda

betweentwo clutches all 2nd clutch AUDI

VW

power split

specialtransmissionrequired

2nd E-Motor

ToyotaGM / DC /BMW

serial all 2nd E-Motor

DC

through theroad all DC

Toyota

ESG

doubleclutchtransmission

LuK

Figure 33 Comparison of hybrid concepts

It should be assumed that, in the next few years,there will be a phase comprising consolidationand concentration on the concepts that promisethe most success. There will also be a quite deci-sive question to be addressed: whether a hybridmust also allow electric driving or will only beused to achieve reductions in fuel consumption.This will decide whether full or mild hybrids willultimately make the breakthrough.

Literature[1] Kawata, K.: Future Trends for Automotive

Transmission Technology, CTI-SymposiumInnovative Fahrzeug-Getriebe Würzburg30.11./01.12.2004

[2] Goll, S.; Domian, H.-J.: Vom konventionellenAntriebsstrang zum Hybrid. 26. Inter-nationales Wiener Motorensymposium28./29.04.2005

[3] Takimoto, M.: Toyota´s Ansatz für nach-haltige Mobilität. 26. Internationales WienerMotorensymposium 28./29.04.2005

[4] Tenberge, P.: Stufenloses Fahrzeuggetriebemit elektrischer Leistungsverzweigung, VDI-Berichte 1346 (1997), 83-114

[5] Pels, Th.; Reitz, D.; Man, L.; Vestgard, B.:Small Starter Generator – Big Impact, 7th LuKSymposium Baden-Baden 11./12.04.2002

[6] Reitz, D.; Dilzer, M.: ESG – vom Paral-lelschaltgetriebe zum Minimalhybrid, VDITagung innovative Fahrzeugantriebe Dres-den 11./12.11.2004, VDI-Bericht 1852

[7] Wagner, U.; Pollak, B.: Elektrisches Schalt-getriebe (ESG). Die konsequente Weiteren-twicklung des Doppelkupplungsgetriebeszum milden Hybridsystem, CTI-SymposiumInnovative Fahrzeug-Getriebe Würzburg30.11./01.12.2004



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