1999-01-1259_Kluger_SWRI

400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

SAE TECHNICALPAPER SERIES 1999-01-1259

An Overview of Current Automatic, Manual andContinuously Variable Transmission Efficiencies

Michael A. Kluger and Denis M. LongSouthwest Research Institute

International Congress and ExpositionDetroit, Michigan

March 1-4, 1999

and Their Projected Future Improvements

Licensed to Eaton CorporationLicensed from the SAE Digital Library Copyright 2008 SAE International

E-mailing, copying and internet posting are prohibitedDownloaded Tuesday, November 18, 2008 4:42:13 AM

Author:Gilligan-SID:12627-GUID:14163039-192.104.67.122

The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.

SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.

Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.

To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.

ISSN 0148-7191Copyright 1999 Society of Automotive Engineers, Inc.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.

Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.

Printed in USA

All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database




1

1999-01-1259

An Overview of Current Automatic, Manual andContinuously Variable Transmission Efficiencies

and Their Projected Future Improvements

Michael A. Kluger and Denis M. LongSouthwest Research Institute

Copyright © 1999 Society of Automotive Engineers, Inc.

ABSTRACT

This paper will overview current production manual, auto-matic, and continuously variable transmission (CVT) effi-ciencies and efficiency variations across the industry. Forautomatic transmissions, efficiencies associated with thepump and the gearbox components will be highlightedalong with areas for improvements. Efficiencies associ-ated with various types of pumps such as internal-exter-nal, gerotor, hypocycloidal, and variable displacement willbe compared. For CVT’s a comparison of efficiencies forbelt type and toroidal types will be provided, along withan examination of external-external and variable dis-placement type ball pumps.

1.0 MANUAL TRANSMISSION

Manual transmissions consist of two parallel gear shaftseach typically containing thirteen gears and four synchro-nizers. Most gear shafts are supported across theirlength by three bearings. Though the type of bearingsused differ dramatically among the various manufactur-ers. The gears are helical cut and are typically manufac-tured from through hardening alloy or carburizing gradesteel and heat treated to an Rc 58 value.

In non-direct gears the powerpath of the transmissiontakes engine input speed through a gear ratio to the“lower” lay shaft which is operated at a speed 35 per centlower than engine speed. The ratio used dramaticallyeffects the torque loading in the downstream gearing andthe gear speed. This particular gear ratio is one of thefactors differentiating manual transmissions produced bydifferent manufacturers. It in turn represents a significantvariable differentiating the efficiency of a particular trans-mission.

1.1 MANUAL TRANSMISSION EFFICIENCY – Whenoperated through a representative combined city/highwaycycle, the time spent in each gear for a typical five-speedmanual transmission and its corresponding efficiency isas follows:

An average efficiency for this transmission would be96.2 per cent. Note that in a direct-drive gear (Gear 4)the efficiency is 3-5 per cent higher than for nondirect-drive gears.

1.2 MANUAL TRANSMISSION EFFICIENCYIMPROVEMENTS – Since torque-dependent losses arethree times speed losses, it is better to transmit powerthrough the layshaft at high speeds. Increasing the lay-shaft speed helps increase manual transmission effi-ciency. Methods for reducing wind-age and churning willbe discussed in the proceeding para-graph. Similarly, useof a 35-mm, cast iron center section for improved bearingsupport will reduce shaft distortion and improve geartooth meshing and reduce bearing losses.

In addition, manual transmission efficiency values can beimproved by reducing windage losses which are gener-ated by rotating gears through air saturated with fluid. Fora typical 2.3L transmission application operating at 3,000rpm, a repre-sentative drag torque is 1.4 N•m. This valuecan be improved by assuring that the inner surface of thetransmission case is smooth and continuous and freefrom casting imperfections. These imperfections providenucleation sites where fluid can cling to and subse-quently “rain” back into the air rotating about and with thegears further saturating the rotating air.This improvement can be further enhanced with theapplication of an oil-shedding, dry film lubricant on theinner case surface that again prevents fluid from attach-

Table 1. Five-Speed Manual TransmissionPercent Time in Gear and Efficiency in EachGear

Timein Gear

RepresentativeEfficiency

Current ProductionEfficiency Variation

Gear 1 8% 93.5% 92-96%Gear 2 10% 92.0% 92-97%Gear 3 21% 94.0% 93-97%Gear 4 20% 97.4% 93-99%Gear 5 41% 93.8% 92-97%




2

ing itself to the inner surface. Frictional losses betweenthe gears can be reduced by incorporating an oil-retaining dry film lubricant on the gear teeth to reduce thesliding friction occurring during gear meshing.

Changing from steel ball bearings to ceramic ball bear-ings will reduce parasitic losses by approximately0.07 N•m for a 40 mm I.D., 70 mm O.D., and 20 mm widebearing operated at 2,000 rpm. Typically, ceramic bear-ings have about one-third less resisting torque than an allsteel bearing. This is due to a smaller contact pressureellipse and lower skid within the contacts.

1.3 IMPROVED MANUAL TRANSMISSIONEFFICIENCY VALUES – By incorporating the aboveimprovements the efficiency in each gear would be as fol-lows:

This would provide an overall transmission efficiency rat-ing of 96.7 per cent or a 0.5 per cent improvement.

2.0 AUTOMATIC TRANSMISSIONS

Automatic transmissions consist of three major compo-nents: the pump, torque converter, and gearbox. Foreach of these components there are major differences intheir configurations as produced by different manufactur-ers. For example, the variation in the components used inthe gearbox portion only, are shown in the figure below.

The differences as they affect efficiency values will bediscussed in subsequent sections.

2.1 AUTOMATIC TRANSMISSION EFFICIENCY –When operated through a representative combined city/highway cycle the time spent in each gear and its corre-sponding efficiency are as follows:

An average efficiency for this transmission would be 85.3per cent.

2.2 PUMPS – The pumping system used in a step ratioautomatic transmission can consume up to 20 per cent ofthe total power required to operate a typical automotivetransmission through a representative city cycle. Acrossthe operating range, these pumps require pumpingtorques of 4.0 - 9.0 N•m.

2.3 FIXED DISPLACEMENT PUMPS – There are threeconventional fixed displacement pump types that are typ-ically used in automatic transmissions. They are as fol-lows:

• Internal External Crescent Type

• Gerotor

• Hypocycloidal

The pump with the best overall and volumetric efficiencyis the hypocycloidal type as it is approximately 5-10 percent more efficient at any operating conditioning than agerotor or internal external crescent type. A comparisonof overall efficiency for these three pumps is shown in thefigure below.

Figure 1. A Comparison of Overall Efficiency for ThreeFixed Displacement Pump Types at 1.0 MPa

Outside of conventional improvements to reduce axialand radial leakage in the pump with tighter tolerances,areas for improvement in the overall pumping systeminclude improved porting arrangements which improveefficiency most predominately at high speeds.

Table 2. Future Manual Transmission Efficiency Values

Efficiency Efficiency ImprovementGear 1 94.7 % 1.2%Gear 2 93.1 % 1.1%Gear 3 94.4% 0.4%Gear 4 97.8% 0.4%Gear 5 94.4% 0.6%

Table 3. Automatic Transmission Component Variations

Planetary GearsetTypes

2 and 3 Sets Simpson, Ravignaux

Clutch Packs 3, 4, 5, and 6Bands 0, 1, 2, and 3One-Way Clutches 0, 1, and 2Bearings Ball, Roller, and Bushings

Table 4. Five-Speed Automatic Transmission Per CentTime in Gear and Efficiency in Each Gear

Timein Gear *

Efficiency Current ProductionEfficiency Variation

Gear 1 9% 70.3% 60-85%Gear 2 4% 78.1% 60-90%Gear 3 5% 86.0% 85-95%Gear 4 32% 86.2% 85-95%Gear 5 40% 88.7% 83-94%

* Excludes 10% for idle




3

2.4 VARIABLE DISPLACEMENT PUMPS – Theoreti-cally, the variable displacement pump is the best pumpconfiguration based upon energy consumption as itmatches the delivery of the pump to the transmission’sflow requirements. Though not uniformly used throughoutthe industry, resistance to its use has been associatedwith control pressure instability. Additionally, current vari-able displacement applications have built in extra leakageto obtain improved stability which tends to reduce theadvantages of this pump type in practice.

In comparison to fixed displacement pumps, variable dis-placement pumps require 1 N•m less pumping torque atlow speeds and 5 N•m less at high speeds as shown inthe figure below.

Figure 2. Average Pumping Torque Differences for Fixedand Variable Displacement Pumps

2.5 CLUTCH PACK DRAG – Of considerable interest tothe industry are spin losses because of their large influ-ence on coastdown performance and the fact that theyrepresent a constant parasitic loss during vehicle opera-tion. For a given friction material groove pattern, clutchpack drag is a function of many variables. These vari-ables and improvements that can be realized for each areshown below.

2.6 WINDAGE AND CHURNING LOSSES – Internalfluid movement inside a transmission greatly affects thewindage and churning losses. Routing of fluid directly offthe rotating components as it directly exits the rotatingdrums into a collector that returns the fluid immediatelyand directly to the sump has the potential to reduce wind-age and churning losses by 10 per cent as shown in thefigure below.

Figure 3. Windage and Churning Drag LossImprovement with a Fluid Collector System

2.7 GEARBOX PARASITIC LOSS DISTRIBUTION – Abreakdown of the losses within a 3.0L automatic trans-mission gearbox is shown in the figure below.

Figure 4. Automatic Transmission Gearbox ParasiticDrag Torque Distribution

2.8 AUTOMATIC TRANSMISSION IMPROVEMENT –By reducing the viscosity of the fluid, parasitic losses canbe reduced. At normal transmission operating tempera-tures a reduction in fluid viscosity of 1 cSt will reducedrag torque by approximately 0.2 N•m.

Changing seal material from cast iron to Teflon andPEEK will reduce drag torque by approximately 0.07 N•mper seal. Considering that a typical transmission utilizes 6seals on average, a total reduction in drag torque of 0.4N•m can be realized.

Table 5. Clutch Pack Variables and Improvements

Clutch Pack Variable ImprovementPack Clearance By Increasing Pack Clearance from

0.75 to 1.5 mm, Drag Torque WillDecrease 75%

Flow Through the Pack At High Speeds a 40% Decrease inFlow Through the Pack Will ReduceClutch Pack Drag Torque by 40%

Speed Reduce Relative Speed Across TheFriction Material and Separator Plate

Stationary Friction Mate-rial or Separator Plate

Will Increase Drag Torque by 35%

Direction of Rotation ofthe Friction MaterialPlate

Positive Rotation

3

3.5

4

4.5

5

5.5

6

0 1000 2000 3000 4000

Input Speed (RPM)

Dra

gT

orqu

e(N

-m)

ConventionalTransmission DragTorqueImproved CollectorsystemDrag Torque

Bushing/Bearings

Seals

Gear Geometry

Windage

Clutch Pack Drag




4

2.9 IMPROVED AUTOMATIC TRANSMISSIONEFFICIENCY VALUES – By incorporating the aboveimprovements the efficiency in each gear would be as fol-lows:

This would provide an overall efficiency value of 86.3 percent or a 1.0 per cent improvement.

3.0 BELT TYPE CVT’S

In general, CVTs offer the promise of better efficiencythan conventional transmissions, particularly at part load.A CVT’s advantage is that it allows an engine to operateover a wide range of speeds and loads while in manycases being independent of the speed and torquerequirements placed on the wheels by the vehicle andthe driver

For belt type CVT’s, the amount of power which can betransmitted is determined by either the tensile strength inthe bands as the belt is squeezed, or by the transversebuckling strength of the belt as it is squeezed betweenthe two halves of the sheave. Tension in the steel bandsneeds to vary enough to prevent buckling of the stack ofmetal blocks and to hold them in contact with the sheavefaces with sufficient normal force to generate adequatetangential strength.

Typical efficiency for belt-type CVT’s is shown in the fig-ure below.

Typical belt type CVT losses are 5 to 30 N•m.

3.1 CVT PUMP – One type of pump used in current pro-duction belt type CVT’s is the external-external gear type.Typically these pumps produce flows of 10 to 40L/minthrough pressure ranges of 3.0 to 5.0 MPa. Typical pump-ing torques range from 2 to 5 N•m. Typical overall effi-ciency for an external pump is shown in the figure below.

Figure 5. External-External Pump Overall Efficiency

3.2 BELT TYPE CVT IMPROVEMENT – Because belttype CVT’s require pressures of up to 3 MPA, the pump-ing power requirements represent a very large portion ofthe overall CVT power losses. This is particularly a prob-lem at low-power conditions typical of the major operatingperiods during city and highway driving, where CVT effi-ciency values are approximately 85 per cent. Variabledisplacement pumps provide a mechanism to improvelow end power consumed by matching the power require-ment of the CVT and the power required by the pump.

The pumping requirement for a belt type CVT is for lowflow and high pressures. Conventional variable displace-ment, vane type pumps are not well suited for theserequirements. A better variable displacement pump typeis a radial ball pump or a radial piston pump. Suchdevices have overall efficiencies as shown below.

Figure 6. Variable Displacement Ball Pump Efficiencyfor Different Displacement Positions

Table 6. Future Automatic Transmission EfficiencyValues

Efficiency Efficiency ImprovementGear 1 72.1% 1.8%Gear 2 80.9% 1.8%Gear 3 87.1% 1.1%Gear 4 86.7% 0.5%Gear 5 89.5% 0.8%

Table 7. Belt Type CVT Efficiency Values at VariousOperating Conditions

Low-SpeedRatio

High-InputTorque

Mid-SpeedRatio

Mid-InputTorque

High-SpeedRatio

Low-InputTorque

Low-Speed 84% 86% 77%Mid-Speed 86% 89% 80%High-Speed 83% 85% 76%

Overall CVT Pump Efficiency

0

10

20

30

40

50

60

0 1000 2000 3000 4000 5000 6000Input Speed (rpm)

%O

vera

llE

ffici

ency

Outlet pressure = 3.0 Mpa

Outlet pressure = 5.0 Mpa




5

4.0 TOROIDAL TRANSMISSION

The potential for high mechanical efficiency is character-istic of traction drives because rolling motion is intrinsi-cally very efficient. The overall efficiency curve of atraction drive is quite flat over most of the torque loadingregime.

The efficiency of a toroid transmission is approximately91 per cent over the majority of the operating torque andratio ranges. However, at low torques and low reductionratios the efficiency drops dramatically to 78 per cent asshown in the figure below.

Figure 7. Traction Drive Efficiency as a Function of InputTorque and Reduction Ratio

There are three major loss categories for the toroidaltransmission. The largest category is the normal forcegeneration and reaction, the second category is the con-tact patch whose losses are related to geometry, whilethe third category is associated with the output gear andis essentially fixed.

Breakdowns in the total system losses of a typical toroi-dal transmission are shown below.

Figure 8. Breakdown of Power Losses Within theToroidal Transmission

To improve the efficiency at low input torque conditionsand reduction ratios some sort of mechanical shunt isrequired as shown in the figure below.

Figure 9. Toroid with Mechanical Shunt

This improves the low end efficiency to approximately 98per cent as shown in the figure below.

Figure 10. Toroidal Transmission Efficiency with aMechanical Shunt

The greatest potential for efficiency improvements can berealized with higher Ct fluid. Higher Ct fluids lower thenormal force required, reduce the overall componentsize, bearing losses and pump losses. For example a 50per cent improvement in Ct reduces the normal forceapproximately 15,000N, while reducing the overall toroidsize by 16 per cent

By incorporating a traction fluid with an improved coeffi-cient of traction of 0.12, the overall toroid efficiency wouldbe improved 1.8 per cent. A breakdown of losses andtheir improvements are shown below.

5.0 CONCLUSION

Manual transmissions have overall efficiency values of96.2 per cent which are the highest efficiency values ofany type transmission. Manual transmissions represent avery simple and mature technology with very little roomfor improvement. At best, manual transmissions can beimproved to provide an efficiency of 96.7 per cent.

0.4

0.9

1.4

1.9

Reduction Ratio

0

50

100

150200

250300

Input Torque (N-m)

78808284868890

92

94

Efficiency, %

78808284

86

88

90

92

94

Efficiency, %

Pumping Creep Spin Bearings Seals

0

10

20

30

40

50

60

70

Total Loss

Table 8. Loss Distribution and Changes with TractionFluids Having Ct Values of 0.08 and 0.12

Losses C t = 0.08 Ct = 0.12 ImprovementPumping 67% 59% 8%

Creep/Spin 16% 20% (4)%Bearings 10% 9% 1%

Seals 8% 9% (1)%

* Toroidal Data Based on Machida & Abe, 1996

75

80

85

90

95

100

0 50 100 150 200 250 300 350 400Input Torque (N-m)

i = 0.70

i = 1.00

i = 1.45

i = 2.25




6

Automatic transmissions have overall efficiencies of 85.3per cent which is on average 10 per cent less efficientthan manual transmissions. Automatic transmissions arecomplex mechanisms containing multiple interdepen-dent systems typically consisting of between 500-700parts. Interestingly enough, no transmission manufac-turer in the world today has been able to optimize thesecomponents to produce a transmission with the highestefficiency in each gear, in torque dependent and speeddependent operating scenarios, and in both engine-driv-ing and rear-wheel or coast mode operation. Even so, theideal best current production type transmission could beimproved to provide an efficiency of 86.3 per cent.

Belt type CVT’s have overall efficiencies of 84.6 per centwhich are less than manual transmission efficiency val-ues. However, the major advantage of a CVT is that itallows the engine (a device that has a maximum 35 percent

adiabatic efficiency) to operate in a more fuel-efficientmanner. A CVT-engine combination will improve a typicalpowertrain’s fuel economy by 5-10 per cent. Additionalfuel economy benefits for belt type CVT’s can be realizedby reducing the fluid pumping power requirements, mostnotably at low-power conditions, by replacing currentfixed displacement pumps with a variable displacementtype ball pump to achieve an overall efficiency of 88.4 percent.

Toroidal type CVT’s have overall efficiencies of approxi-mately 91 per cent. This type of mechanism offers greatpromise to manufacturers of high horsepower vehiclesconfigured with engine sizes greater than 3.0L, typicallyused in minivans, sport utility vehicles, and light dutypickup trucks. Efficiency improvements of 1.8 per centcould be realized in toroidal transmissions with the imple-mentation of traction fluids having a coefficient of traction(Ct) of 0.12.




1999-01-1259_Kluger_SWRI

Documents

Transcript of 1999-01-1259_Kluger_SWRI