A Five-Speed Starting Clutch Automatic Transmission Vehicle

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SAE TECHNICAL

PAPER SERIES 2003-01-0248

A Five-Speed Starting Clutch Automatic

Transmission Vehicle

Chi-Kuan Kao, Anthony L. Smith and Patrick B. UsoroGeneral Motors Corp

Reprinted From: Transmission & Driveline Systems Symposium 2003(SP-1760)

2003 SAE World CongressDetroit, Michigan

March 3-6, 2003


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ABSTRACT

A wet multi-plate clutch, designated as the starting

clutch, is used to replace the torque converter in the

automatic transmission in order to improve vehicle fuel

economy. The transmission ratio spread must be

increased to compensate for the torque multiplication of

the torque converter and avoid penalizing the 0-60 mph

acceleration performance. The main challenge of thisconcept is the control of the starting clutch to ensure

acceptable vehicle drivability. This paper describes the

system of a five-speed starting clutch automatic

transmission vehicle and shows vehicle test results.

Vehicle test data show that (i) the fuel economy benefit of

the starting clutch is significant, and (ii) a starting clutch

transmission can be designed to equal or better the 0-60

mph acceleration performance of a torque converter

transmission by proper selection of the gear ratios.

INTRODUCTION

A torque converter is a hydro-mechanical device thatconnects the engine with the transmission in an

automatic transmission. The function of the torque

converter is to provide fast and smooth vehicle launch

through its torque multiplication and driveline torsional

damping capabilities. One major drawback of the torque

converter is its relatively lower efficiency over the operating

cycle. The torque converter efficiency can be improved by

controlling the torque converter slip with a torque converter

clutch (TCC) [1]. Further efficiency improvement can be

achieved by replacing the torque converter with a wet

clutch [2], designated as the starting clutch. The

torsional damper spring is retained for its primary function

of filtering engine-firing pulses when the clutch is locked.

A viscous converter clutch (VCC) [3] is also used in some

passenger cars to lockup torque converter clutch early

with viscous damping to improve fuel economy without

sacrificing drivability. A viscous converter clutch is a

torque converter clutch with a viscous damper located

between the torque converter clutch and turbine to improve

drivability by damping engine torque fluctuation when early

lockup the torque converter clutch [3]. Its drawback is that

it adds additional cost and it maintains a small slip even

when the torque converter clutch is locked.

The concept of using a starting clutch in automatic

transmissions is not new [2]. Borg-Warner used a starting

clutch for a 3-speed automatic transmission in 1949 [4]. In

the 1980s they put the starting clutch at the output of a

continuously variable transmission (CVT) as a start-up

and protective device [5]. The Borg-Warner starting clutch

with CVT design was successfully used in Suzukis

production vehicles with 1000cc and 1300cc engines [6]

Honda introduced a mass production starting clutch CVT

called Multi-Matic transmission for 1500cc engines [7].

Although there is currently no production discrete-step

transmission with a starting clutch, prospects for such a

transmission are not far fetched if wide-ratio-spread five

(or more-) speed automatic transmission can be designed

and torque-converter type of vibration-free drivability can be

achieved with intelligent starting clutch slip control as

explained below.

To improve vehicle fuel economy without sacrificing

vehicle acceleration performance and drivability when

replacing the torque converter with a starting clutch, the

following two issues must be addressed. First, the overalgear ratio spread must be increased to provide a deep

underdrive to emulate the torque multiplication of a torque

converter during launch and an overdrive to maintain fue

economy in highway driving. Since the overall gear ratio

has to be increased, to maintain small ratio steps for shif

quality, it is necessary to add an extra speed ratio to the

transmission; consequently, a 5-speed automatic

transmission is typically required. Second, the starting

clutch slip needs to be effectively controlled to achieve

smooth vehicle launch, good engine torsional isolation,

and fuel economy improvement. Thus, a well-synthesized

starting clutch control system is essential to the success

of the system.

In this paper, a GM passenger car equipped with a V6

3300 engine and a wide-ratio spread (6:1) experimental 5-

speed automatic transmission was used to evaluate

vehicle drivability and fuel economy improvement. Genera

issues such as cost, integration challenges, and

customer acceptance were outside the scope of this

work.

2003-01-0248

A Five-Speed Starting Clutch AutomaticTransmission Vehicle

Chi-Kuan Kao, Anthony L. Smith and Patrick B. UsoroGeneral Motors Corp

Copyright 2003 SAE International


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STARTING CLUTCH CONTROL ALGORITHM

The schematic of a starting clutch vehicle is shown in Fig.

1. The torque generated by the engine is transmitted

through the wet multi-plate starting clutch, the damper

spring, the transmission gearset, and the axle to the

vehicle. A damper spring is placed after the starting clutch

to provide filtering of engine torque disturbances such as

engine firing pulses when the starting clutch is locked.

This damper spring is similar to that of a production torqueconverter clutch.

Figure 1. Schematic of a Starting Clutch Vehicle

The starting clutch controller regulates the hydraulic

pressure on the starting clutch to allow smooth torque

transmission from the engine to the vehicle, and control

the slip between the engine and transmission for torsional

vibration isolation and improved fuel economy. The overall

transmission control block diagram is shown in Fig. 2.

Figure 2. Transmission Control Block Diagram for the

Starting Clutch Vehicle

The gearshift control algorithm can be found in [8, 9]. As

shown in Fig. 2, the starting clutch control system usesthe engine torque (Te), engine speed (

e ), starting clutch

speed (transmission input speed (t

)), throttle position

( ), and brake information to control the starting clutch

pressure (Psc). This information can also be used to

control engine spark timing and throttle. In this paper,

spark control and electronic throttle control were not used

to control the starting clutch.

VEHICLE IMPLEMENTATION AND RESULTS

The starting clutch control algorithm was implemented in

a GM passenger car with a V6 3.3L PFI engine and a five

speed wide-ratio spread (6:1) experimental transmission.

A starting clutch with a damper spring was used to

replace the torque converter. The stick diagram of the

automatic transmission is shown in Fig.3.

The starting clutch and the damper spring fit in the torqueconverter compartment. A 32-bit microprocessor is used

to control the transmission with a 5 msec loop time. The

control software was written in MODULA-GM and

extensive instrumentation was employed in the vehicle to

measure engine, starting clutch, and transmission

speeds, axle torque, pressures internal to the

transmission, brake signal, gas pedal and throttle

positions so that the transmission control system could

be debugged, tested and verified. A linear solenoid valve

was used to control the starting clutch pressure.

Figure 3. Stick Diagram of the Experimental Five-Speed

Starting Clutch Automatic Transmission

Two different baseline transmissions were available fo

installation in the same vehicle for comparison. One was

the 3T40 three-speed production transmission with torque

converter clutch (TCC), and the other was a five-speed

5.5:1 ratio spread experimental transmission equipped

with a viscous converter clutch (VCC) [3] to enable early

clutch apply. Ratio comparisons for the vehicles equipped

with the three transmissions are listed in Table 1.

VEHICLE TEST RESULTS

The fuel economy, wide-open-throttle (WOT) acceleration

performance, and part-throttle launch drivability of the

starting clutch vehicle in comparison with torque converter

vehicles are discussed in this section.

INPUT

OUTPUT

F1

CB5

CBRC345

C123

CREV

CB24

ST. CL.


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Table 1. Ratio Comparisons for the Starting Clutch and

Torque Converter Vehicles

3-Speed

with TCC

5-Speed with

VCC

5-Speed with

Starting Clutch

Gear

Ratio

N/V Gear

Ratio

N/V Gear

Ratio

N/V

1 2.84 95 2.667 162 2.909 177

2 1.6 53 1.556 95 1.636 100

3 1 33 1 61 1 61

4 - - 0.667 41 0.667 41

5 - - 0.485 30 0.485 30

Chain

Ratio

0.84 1.53 1.53

Fuel economy

The fuel economy tests were performed at the GM

Powertrain Warren West Emissions Laboratory using thesame vehicle, engine, driver, dynamometer, and

preparation procedures during a one-week test period to

ensure consistent and reliable results. Three

transmissions were installed in the same vehicle, one at a

time, for the tests. These include (i) a three-speed 3T40

(THM 125C) production transmission, (ii) a five-speed

experimental transmission with viscous converter clutch

using very aggressive fuel economy shift schedule, with

very early viscous converter clutch engagement starting in

second gear at 9 mph for zero throttle, and (iii) the starting

clutch automatic transmission. The fuel economy

improvements of the starting clutch equipped vehicle over

the other two baseline vehicles are listed in Table 2.

The table lists fuel consumption during the EPA II hot city

and highway cycles, and the 55/45 combined cycles. This

data shows that the starting clutch transmission provides

fuel economy improvements of 11.4% over the three-

speed 3T40 production transmission and 7.2% over the 5-

speed experimental transmission with viscous converter

clutch for the combined 55/45 EPA cycles. These results

are consistent with simulation predictions and

expectations.

Launch Performance

Figure 4 shows a comparison of the vehicle acceleration

versus time traces during a wide-open-throttle (WOT)

launch for the starting clutch vehicle and a production

3T40 torque converter vehicle. This data shows that the

peak vehicle acceleration for the torque converter vehicle

reaches 0.56 g because of the torque multiplication from

the torque converter and starts to decrease when the

torque converter approaches the coupling mode where

there is no torque multiplication. The peak vehicle

acceleration for the starting clutch vehicle reaches 0.54 g;

this high acceleration is maintained because of the high

N/V (engine speed in rpm over vehicle speed in mph) ratio

in first gear (Table 1).

The 0-60 mph time comparison of these vehicles and the

5-speed VCC experimental vehicle is presented in Table

3. This data shows that the starting clutch 0-60 mph

performance is better than the production 3T40

transmission vehicle because of the increased ratio

spread (Table 1) and slightly worse than the 5-speed VCC

experimental transmission vehicle because of the highfirst gear N/V along with the torque converter. Clearly, a

starting clutch transmission can be designed to equal or

exceed the 0-60 mph performance of a torque converter

transmission by proper selection of gear ratios.

Table 2. Vehicle Fuel Economy Comparison

Vehicle (Avg.)

MPG

(Avg.)

% Impr.

(Avg.)

% Impr.

3-Speed

with TCC

Trans.

Hot City MPG

Hwy MPG

Comb. 55/45

20.24

33.18

24.55

Base

Line

5-Speed

with VCC

Trans.

HotCity MPG

Hwy MPG

Comb. 55/45

21.12

34.21

25.51

4.3%

3.1%

3.9%

Base

Line

5-Speed

Starting

Clutch

Trans.

HotCity MPG

Hwy MPG

Comb. 55/45

23.00

35.58

27.35

13.6%

7.2%

11.4%

8.9%

4.0%

7.2%

.

Figure 4. Comparison of Wide Open Throttle Vehicle

Launches


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Table 3. Vehicle 0 - 60 mph Performance Comparison

Transmission (Avg.)

Seconds

3-Speed (3T40) Production Trans. 10.54

5-Speed with VCC 8.88

5-Speed Starting Clutch Trans. 9.27

Figure 5 compares the 30% throttle vehicle launch for the

starting clutch and the production 3T40 vehicles. The

starting clutch vehicle has a very smooth launch, but with

a lower peak acceleration than the production vehicle.

The higher peak acceleration for the production vehicle is

due to the effects of the torque converter. Notice that while

the acceleration of the torque converter equipped vehicle

decays from the peak value, that of the starting clutch

equipped vehicle sustains a steady value.

This set of tests shows very good starting clutch vehicle

launch drivability achieved with the control strategy

developed in this project.

Figure 5. Comparison of 30% Throttle Vehicle Launches

SUMMARY

A starting clutch control algorithm was developed toachieve vehicle drivability that is comparable with that of a

torque converter equipped vehicle. The starting clutch

control system uses engine speed, starting clutch speed

(transmission input speed), throttle position, and brake

information to control the starting clutch pressure. The

control system was developed and implemented in a GM

passenger car. The vehicle had a 3300 V6 PFI engine and

a five-speed electronically controlled wide-ratio spread

(6:1) experimental transmission. Fuel economy tests were

performed at the GM Powertrain Warren West Emissions

Laboratory using the same vehicle with three different

transmissions: (1) 3T40 production 3-speed automatic

transmission, (2) experimental 5-speed automatic

transmission with a viscous converter clutch, and (3

experimental 5-speed automatic transmission with a

starting clutch. The fuel economy, acceleration

performance, and drivability of the starting clutch vehicle

were compared with that of the two torque converte

vehicles.

Vehicle tests showed that for the combined FTP cyclesthe starting clutch transmission provided a fuel economy

improvement of 11.4% over the production 3T40 3-speed

transmission and 7.2% over the experimental 5-speed

transmission with viscous converter clutch. The viscous

converter clutch apply schedule was very aggressive; GM

proprietary best fuel economy shift schedule was adopted

and the converter clutch apply started in second gear at 9

mph vehicle speed for zero throttle. Vehicle test data

showed that the 0-60 mph time for the starting clutch

vehicle is shorter than the production 3T40 3-speed

transmission vehicle because of the wide ratio spread (6:1

compared to 2.84:1) and slightly longer than the

experimental 5-speed automatic with a torque converterbecause the torque multiplication of the torque converte

was not completely compensated for.

CONCLUSION

Starting clutch control algorithms were developed and

successfully implemented in an experimental five-speed

automatic transmission vehicle equipped with a starting

clutch to replace the torque converter. Vehicle tests

showed that a starting clutch transmission could provide

significant vehicle fuel economy improvement over a

torque converter transmission without penalizing

acceleration performance when a wide-ratio-spread five

speed automatic transmission is used. General issues

such as cost, integration challenges, and custome

acceptance were not examined.

REFERENCES

1. Hiramatsu, T., Akagi, T., and Yoneda, H.,Contro

Technology of Minimal Slip-Type Torque Converte

Clutch, SAE-850460, 1985.

2. Gott, P.G., Changing Gears: The Development of the

Automotive Transmission, Society of Automotive

Engineers, Inc., 1991.3. Tung, S.C. and Linden, J.L., Modeling Torque

Converter Clutch Viscous Damper Performance,

SAE-850459, 1985.

4. Smirl, R., Transmission, U.S. Patent 2,700,312

January 25, 1949.

5. Schneider, K.F., Hydro-electronic Tension Bel

Continuously Variable Transmission for Passenge

Cars, SAE-894079.

6. Hirano, S., Miller, A.L., and Schneider, K.F., SCVT

A State of the Art Electronically Controlled

Continuously Variable Transmission, SAE-910410.


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A Five-Speed Starting Clutch Automatic Transmission Vehicle

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