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Integrating of the Auto Shift Gearbox April 9, 1999

with the electrical machine

Dr. Robert FischerGunter HirtLuK Getriebe-Systeme, Bühl

IntroductionAgainst all predictions, manual transmissions still have in Europe a market shareof more than 80%. This can be attributed to the high efficiency and the smallspace required for installation together with low weight and low cost.

In the meantime, there are indications that the manual shift transmission can besuccessfully automated and thus additional advantages can be gained at very littleexpense.

The basis was the automatic clutch, as provided by LuK and Bosch for theMercedes-Benz A-class. In the meantime, we have the first Auto Shift Gearbox(ASG), which is based on a manual shift transmission [1], [2]. Eliminating theinterruption of the tractive force during shifts promises major potential forimprovement.

As shown in other lectures, not only can the starter generator fulfill the demandsof future on-board electrical systems, but major energy-related advantages canalso be achieved. According to calculations by LuK, energy savings as high asapprox. 20% can be achieved in comparison to the currently available manualshift transmission, thanks to the start-stop automatic system, and the recovery ofbraking and coast energy [3] to [7].

By combining the ASG and the starter generator in an intelligent manner, LuK isaiming to achieve more than just the sum of the advantages of these systems –the goal is also to eliminate the interruption in the tractive force. LuK calls such atransmission Electric Shift Gearbox (ESG).

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Arrangements of the starter generator in the ASGFigure 1 depicts an overview of the different possible arrangements for theelectrical machine in the drive train with a manual transmission.

YES

YES

YES

a)i

b)i

c)i

d)i

starting and

generating

recovery powershifting

automationnecessary

Figure 1: Arrangements of the starter generator in the drive train

In the first possibility, the starter generator can be attached to the crankshaft(Figure 1a). With this arrangement, starting and generating are possible. Inprinciple, recovery is also possible, however, it does not utilize the full potential ofthe starter generator, since the internal combustion engine must always berotated.

The electrical machine can be placed onto the transmission-input shaft (Figure1b). To do so provides the advantage that the electrical machine can bedisengaged from the engine, therefore allowing the full potential for recovery. Incomparison to the previously mentioned solutions that were attached to theengine side, the potential saving is approx. 10% higher.

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The internal combustion engine can be started either with an engaged clutchdirectly or with an impulse start, where the starter generator is accelerated firstand the clutch is subsequently engaged. Thus, the electrical machine does nothave to deliver the maximum drag torque at low temperatures, what allows theuse of a smaller electrical machine.

For this solution it is necessary to automate the manual transmission, since tostart the vehicle, it must be taken out of gear and the clutch has to be engaged;and to recover, the vehicle has to be put in gear and the clutch has to bedisengaged.

By keeping the electrical machine on the input shaft, the weak point of the ASG –an interruption of the tractive force during the shifting process – is retained.

To change this, the electrical machine must impinge on the power take-offtrain, (Figure 1c). In this case, recovery is also possible. Here, too, a connectionwith an automated clutch or an Auto Shift Gearbox is required. With thisarrangement, however, the internal combustion engine can no longer be startedwith the electrical machine, and a generating process is possible only duringdriving.

In order to fulfill all requirements, i.e., starting, generating, recovering and powershifting, the electrical machine must be able to either affect the input shaftor the power take-off (Figure 1d).

Therefore, the desired consumption advantages and starter generator functioncan only be achieved if the electrical machine can affect the transmission input.Therefore, we will concentrate only on variations 1b and 1d.

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Shifting gears using individual synchronizations

Having the electrical machine on the transmission input shaft, the challenge lies inthe shifting process. To illustrate this, we will first describe the shifting process ofan Auto Shift Gearbox (ASG).

On the right side, Figure 2 depicts (using symbols) the drive train with the manualshifting transmission, i.e., the ASG. The clutch connects the internal combustionengine and the input shaft. Between the input shaft and the output shaft twotoothed gear pairs act with different ratios and a shiftable dog clutch. A singlesynchronization affects each dog clutch. A tire at the output shaft symbolizes thevehicle mass.

A B C D

Time

Time

Ou

tput

torq

ueS

pee

ds

Input shaft

Engine

Figure 2: Up-shifting the ASG using individual synchronization

In the top diagram in figure 2, the speed characteristics of engine and the inputshaft are depicted over time; and in the bottom diagram, the corresponding torqueat the drive shaft.

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What happens during the shifting of the gears? First, a look at the process withoutthe starter generator.

In order to release the dog clutch of the old gear, the torque at the transmissioninput is reduced to zero (point-in-time A). To achieve this, the engine torque isdecreased and the start-up clutch is disengaged, which has a direct effect on theoutput. The transmission input shaft and the internal combustion engine aredecelerated very slowly. By synchronizing the process at the dog clutch of thenew gear, the gear input shaft is very quickly brought to the speed level of thesubsequent gear via the synchronization on the dog clutch of the new gear (rangefrom B to C). Then the new dog clutch can be engaged (Point D). Finally, with thehelp of the start-up clutch, the speed is synchronized between the engine and thetransmission input shaft. Afterwards the torque of the new gear acts on thetransmission. The thin lines depict the speed curves that would result withoutadjusting by synchronizing and re-engaging the clutch.

If the starter generator is connected with the input shaft, then not only the rotatingmass of the input shaft, but also that of the electrical machine must beaccelerated during the synchronization phase (range from Point B to C). Thiswould lengthen the phase and would stress the synchromesh more. To givesupport, the electrical machine could receive power. The power requirements arevery high however, if the vehicle is to be shifted within the same time-frame aswhen no additional rotating mass is present. A few attempts to integrate thestarter generator into the ASG were not successful due to this problem.

Therefore, we are looking for a synchronization that takes into account the massof the electrical machine. The following two sections show an approach and asolution.

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Synchronization using the engine brake against the housing.

The principle introduced here is already being currently used in trucks (Figure 3).

Time

A B

Out

pu

t to

rqu

eS

pee

ds

Time

Figure 3: Synchronization using the engine brake against the housing

Synchronization of the speeds of the transmission input shaft, the internalcombustion engine and the electrical machine is achieved by a powerful brakeacting against the housing.

Therefore, it is not necessary to synchronize the individual dog clutches. Aninterruption in the tractive force, however, cannot be avoided even during this kindof synchronization.

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Synchronization using the engine brake against the power take-off

In the above principle of the engine brake, the torque is supported at the housing.During the braking process, the kinetic energy of the internal combustion engineand the electrical machine is transformed into heat and is lost to the drive.

A B

Time

Ou

tpu

t to

rqu

eS

pee

ds

Time

Figure 4: Synchronization using the engine brake against the power take-off

It is preferable to deliver this torque to the drive train, using a power shifting clutchand thus to transfer portions of the kinetic energy of the internal combustionengine and the electrical machine during the shifting process to the vehicle(Figure 4). This is the basic principle of the Electric Shift Gearbox (ESG).

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Here, the power shift clutch fulfills two functions:

• Synchronizing the transmission input shaft, internal combustion engine andelectrical machine

• Avoiding the interruption in the tractive force by delivering the torque to thedrive train

In comparison to the ASG, using this principle to adjust the speed eliminates thedelay times that occur between disengaging the dog clutch and beginning thesynchronization process, as well as between the completion of thesynchronization process and the re-engagement. The gear shifting elementsmove simultaneously in the interlocking phase and independently of thesynchronization process by the power shift clutch. The gradient of thetransmission input shaft speed during the synchronization process may now beless than that of the ASG (Figure 2), without it lengthening the time it takes to shiftgears. This simplifies the demands on the electrical machine.

The thin line in Figure 4 again compares the speed curve, as it would be withoutsynchronization by the power shift clutch.

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The Electric Shift Gearbox – ESG

Design of the ESG

Figure 5 depicts how, in principle, an ESG could be designed.

5 4 3 2 1 R 6

Selecting

Actuation

i

Actuation

Shifting

Figure 5: ESG design

Synchronizers are eliminated, and now an additional power shift clutch is addedto the start-up clutch. This power shift clutch connects the input shaft with theoutput shaft via an auxiliary gear pair – here depicted for the sixth gear. Thedemands made on the power shift clutch regarding torque and performance aresimilar to those made on a start-up clutch. In addition to being used for powershifting, this clutch can also be used as a locking mechanism for parking, withoutany additional expense! To do this, the power shift clutch is engaged after a gearhas been selected – this puts the drive train into a torque-lock condition.

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As mentioned above, in order to transmit the inertia effect and the torque of theinternal combustion engine to the transmission and thus via the power shift clutchto the vehicle, the start-up clutch remains engaged during the shifting processes.

This means that during the start-up only the start-up clutch is used and when thegears are shifted, only the power shift clutch is used. This is the basis of the ideato operate these two clutches with a common actuator and to design them as acombi clutch (Figure 6).

6 5 4 3 2 1 R

i

i

Selecting

Actuation

Shifting

Figure 6: ESG with one actuator for the start-up and power shift clutch

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The corresponding characteristic curves for torque are depicted in Figure 7.

Start-up clutch Power shift clutch

Actuator travel

CBA

Torq

ue

Figure 7:The torques of the start-up clutch and power shift clutch versus actuatortravel

If the actuator is located at Point A, then both clutches are disengaged. On theway from Point A to Point B the start-up clutch will be engaged and the vehiclestarts to move. If the actor is at Point B, it can either move left and disengage thestart-up clutch and it can realize the torque follow up strategy or it can move to theright (in the direction of Point C) and then initiate the shifting process by activatingthe power shift clutch, whereby the start-up clutch remains completely engaged.

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Cost comparison of ESG to ASG

As many components as possible were used from the ASG during the design ofthe ESG. They have the same number of the following components:

• Start-up clutch • Clutch actuator

• Shifting actuators • Interior shiftingmechanism

• Input shaft • Output shaft

• Gear sets • Dog clutches

• Housing • Release bearing

No additional actuator is required, despite power shifting and the electricalmachine. In addition, the following components are required:

• Connection to thetransmission

• Electrical machine /Electronics

• Larger battery • Wiring

• Power shift clutch

To compensate, the following components are eliminated:

• Synchronizers • Generator

• Starter • Belt drive

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ESG system requirements

Controls

With ESG, triggering the system, in particular the clutches, is a considerablechallenge.

In order to fulfill the synchronization function, the power shift clutch, the internalcombustion engine and the electrical machine must be controlled in a coordinatedmanner. In addition to the shifting strategies the additional functions of theelectrical equipment (starting, supplying power to the on-board net) must beguaranteed.

The power shift clutch slips during the shifting phase and thus dominates thepower torque. This requires very sensitive control.

Another challenge lies in disengaging and engaging the dog clutches. During thedisengagement process the precise point in time must be found where the dogclutch is torque-free. During the engagement process the speed must besynchronized, however, at the same time the speed gradients should be asequally high as possible.

Comprehensive total system knowledge is necessary to coordinate engine,clutches and transmission. LuK already has gained expertise in this area fromdeveloping the ASG.

Power shift clutch design

High friction performance and good modulation capability is demanded from thestart-up clutch as well as from the power shift clutch. The energy savings couldbe even higher than that of the start-up clutch, which would make wear-adjustment a sensible idea. If the clutches are designed as a combi clutch – start-up clutch and power shift clutch in one module – then this poses a particularly bigchallenge. In addition, both clutches must be reduced in force in order to use asmall clutch actuator. Certainly that would be the right task for the clutchdevelopment departments at LuK.

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On the road to ESG:The Uninterrupted Shifted Gearbox – USGIn the revolution of the vehicle on-board system, with the EMST introduced here,we can offer a totally optimized transmission system.

In preparation, partial aspects of this technology can already be introduced withoutmuch additional expense. The introduced concept of the power shift clutch canalready be used as an independent alternative to the manual transmission, i.e.,the ASG, without integrating the electrical machine. LuK calls this system theUninterrupted Shifted Gearbox (USG).

Design of an USG

Figure 8 depicts the design of an USG with a combination clutch that is activatedwith only one actuator.

6 5 4 3 2 1 R

Actuation

i

i

Selecting

Shifting

Figure 8: Design of the USG

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LuK took the first step with the prototype illustrated in Figure 9. Here, the powershift clutch is attached to the fifth gear in an existing ASG. Only in the nextdevelopment step are the two clutches to be united into a combination clutch.

Figure 9: LuK prototype for the USG

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Cost comparison of the USG to the ASG

ASG and USG have the same number of the following components:

• Start-up clutch • Clutch actuator

• Shifting actuators • Internal shiftingmechanism

• Input shaft • Output shaft

• Gear sets • Dog clutches

• Housing • Release bearing

Despite increasing the ASG function to that of the USG, no additional actuator isrequired. Only one additional component is required, whereby it can be madecost-efficiently by integrating it into the existing clutch:

• Power shift clutch

The following components are eliminated:

• Synchronizers

This comparison shows that despite the advantages, the USG does not have tobe more costly than the ASG. Upgrading the USG to an ESG is possible withoutmajor effort.

We see an additional advantage by having the transmission manufacturersretaining the existing investments, which is made possible by the majority of theparts being the same.

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Summary of the characteristicsIn many ways, the ASG is the best in its class. Highest efficiency and lowestmanufacturing costs, combined with lowest weight in the smallest space, whichlead to lowest consumption and promise a long life for the ASG and thus theinvestments made in the manual transmissions (Figure 10).

ESG

ASG

USG

AuStSc

Installation space

EfficiencyWeightConsumption

ComfortCostsInvestment

Power shiftingParking lock-up

Multi-use of componentsStart & stopGenerationRecoveryHigher onboard network powerBoosting

Figure 10: Characteristics of ASG, USG and ESG

For the USG, all these advantages are preserved. It offers additional functions,however, such as power shift and parking lock-up at no additional cost. Therefore,the USG is a stand alone system.

The USG, in turn, is the optimum basis for expansion with the starter generator tothe ESG. The ESG is a turnkey concept that retains all the advantages of the ASGand USG and that fully utilizes the potential offered by the starter generator atstart-stop and with recovery. All the additional functions of the starter generator,such as energy conversion, higher on-board network, performance and boostingare possible.

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In the ESG the electrical machine supports the power shift clutch of the USG,making the shifting process even more comfortable and quicker (see Figure 11).

ASG

USG

ESG

Time

Veh

icle

acc

ele

ratio

n

Figure 11: Vehicle acceleration during shifting with ASG, USG and ESG

System classificationIn this lecture we dealt with the Uninterrupted Shifted Gearbox (USG) and theElectric Shift Gearbox (ESG) as a further development of the Auto Shift Gearbox(ASG). How do these systems relate to each other and how do they relate incomparison to other transmission systems?

In Figure 12 we attempted to depict a cost-benefit evaluation, whereby the goodcomfort and good performance level represent the benefit for the client.

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HSG

ECM

ASG USG

ESG

CVTAT

EVTCo

sts

Use (comfort, efficiency)

ESG = Electric Shift Gearbox EVT = Electrically variable transmission

USG = Uninterrupted Shifted Gearbox CVT = Continuously variable transmission

ASG = Auto Shift Gearbox AT = Automatic transmission

ECM = Electronic clutch management

MT = Manual transmission

Figure 12: Cost-benefit analysis of various drive train automatic systems

Currently, the manual transmission, the most cost-efficient transmission availabletoday, is taken as the basis. The other known reliable cornerstone is representedby the automatic transmission (AT), with greater benefits, but also at higher costs.According to our estimates, the electronic clutch management (ECM) is at aboutone-third between the two systems mentioned above with regard to the benefitsand costs.

With the ASG the benefit increases markedly in relation to the ECM, withoutmarkedly higher costs.

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The USG is most likely as reasonable in cost as the ASG. However, the comfortlevel continues to increase, since the interruption in the tractive force iseliminated. LuK estimates the benefit of the USG to be equal to that of theautomatic transmission, the focus being, however, on other classes of vehicles.With the USG not quite approaching the shifting comfort of the automatictransmission, the consumption nevertheless is much lower.

The ESG is the further development of the USG. The additional costs for theelectrical machine and the performance electronics may not be assignedexclusively to the transmission, since the many advantages of a starter generatormust be evaluated, in part, independently of the drive train. We see the majoradvantage in the potential for savings in fuel consumption.

The CVT costs approximately as much as an automatic transmission, however, itoffers decidedly more comfort.

If comfort is the focal point during the development, then successors have beenconsidered for the automatic transmission and the CVT. Systems will be used ashave been introduced by, amongst others, Prof. Tenberge in his lecture [8]. Ascan be seen in Figure 12, however, this addresses a completely different marketsegment than ESG does.

ASG, USG and ESG are further developments of the manual transmission. LuK isworking on it!

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Literature

[1] Fischer, R.; Berger, R.: Automatisierung von Schaltgetrieben, [Auto Shift Gearboxes] 6. LuKColloquium 1998

[2] Berger, R.; Fischer, R.; Salecker, M.: Von der Automatisierten Kupplung zum AutomatisiertenSchaltgetriebe [From the automated clutch to the Auto Shift Gearbox]; VDI-Bericht [VDIReport] 1393

[3] Reik, W.: Startergenerator im Antriebstrang [Starter generator in the drive train], LuK-Fachtagung E-Maschine im Antriebsstrang [LuK conference of experts on the electricalmachine in the drive train] 1999

[4] Boll, W.; Antony P.: Der Parallel-Hybridantrieb von Mercedes-Benz [The parallel hybrid driveby Mercedes-Benz]; VDI-Bericht [VDI Report] 1225

[5] Kerschl, S.; Höhn, B.; Pflaum, H.: Einsparpotentiale des Autarken Hybrid-Fahrzeugs [Thesavings potential in autonomous hybrid vehicles], VDI-Bericht [VDI Report] 1459

[6] Buschhaus, W.; Jaura, A.; Tamor, M: P2000 LSR – Fords Systematic and Integrated HEVDevelopment Program, VDI-Bericht [VDI Report] 1459

[7] Dietrich P., Eberle M.: Betriebsverhalten des ETH-Hybrid III Antriebes auf dem dynamischenPrüfstand und im Fahrzeug, [Driving behavior of the ETH-Hybrid III drive on the dynamictesting range and in the vehicle], VDI-Bericht [VDI Report] 1459

[8] Tenberge, P.: Automatisiertes Fahrzeuggetriebe mit elektrischener Regelungà Hybridgetriebe, [Automated vehicle manual shift transmissions with electronic controls àhybrid shift transmissions] LuK-Fachtagung E-Maschine im Antriebsstrang [LuKconference of experts on the electrical machine in the drive train] 1999