33

Paper Il(iii)

Tribological design -The automotive industry

P. A. Willermet

Rapid and continuing change within the automotive industry demands continual improvement in the quality, performance and reliability of vehicles. At the same time, competitive forces demand shorter product cycle times and new organizational approaches to the design process. The introduction of new technology and increased reliance on suppliers demand better methods of evaluating designs and materials. All of these factors lead to increased opportunities for the introduction of improved tribological design methods as well as the introduction of improved designs and materials.

1 INTRODUCTION

The automotive industry is in a period of rapid change. This change is driven by pressures from several sources: increased international competition, higher customer expectations, and social pressures in the form of regulations relating to emissions, fuel economy and safety.

Automotive technology has advanced significantly in the last few years in response to these needs. These advances include items such as electronic engine controls and catalytic converters which are not tribological in nature, but which may impact tribological components. Other advances include the implementation of design changes which directly involve tribology (see appendix).

The challenge faced by automotive manufacturers, however, is not simply one of achieving a certain level of technical sophistication. The challenge is also to develop better ways to implement continual product improvement within the constraints unique to the industry.

This paper will focus on current applications of tribological principles to automotive design, and on ways technology transfer might be improved. Because manufacturers have different approaches and organizational structures, and because much of the technology is proprietary in nature, the discussion will necessarily be somewhat general. However, it is to be hoped that the presentation will be of use to those attempting to facilitate the translation of research results to a practical end use.

2 APPLICATION OF TRIBOLOGY TO DESIGN

2.1 m e desien - D rocesg

The general characteristics of a new model and its placement in the product cycle are almost always defined by a management consensus process. This process must take into account many non technical factors, including marketing, financial considerations and the technological vision the corporation aims to imprint on the future product.

Advanced engineering teams propose and evaluate design alternatives. Many factors will enter into the choices made, many of them not immediately obvious. For example, space limitations may exclude certain powertrain options, especially for low drag coefficient front wheel drive vehicles.

In the past, such forward looking teams have tended to carry out the task with only minimum input from outside sources. This approach has been found to lead to inefficiencies in implementing and producing the final design as well as to design shortcomings which are difficult to correct after the fact. The current trend is to bring other organizations into the design process early on. These organizations may include manufacturing, component suppliers, tooling suppliers for manufacturing, and design support groups. In the case of joint ventures, which are becoming increasingly common, even other automotive manufacturers may be included. Ultimately, needs for quality, manufacturing efficiency, and shorter product cycles lead to the adoption of simultaneous engineering, in which product and manufacturing engineering decisions proceed in parallel.

In the final design stage, the selected design is finalized, refined and released for manufacturing. This may be the responsibility of forward model teams, which retain responsibility for monitoring the vehicle in production. These teams do detailed development, calibration for performance and emission objectives and durability testing.

2.2 The role of triboloizy

At this point one may ask: “Who does the tribology?” . Tribology was not explicitly mentioned at any point. Tribology is an organic part of the process, and tribology specialists are brought into the design process as described below.

CornDonen t SUDDl iers; Many parts which are subject to friction and wear may be designed and furnished by component suppliers (Table 1). Accordingly, design responsibility may be rather diffuse. When a supplier assumes responsibility

34

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 1. Partial list of powertrain parts having tribological significance which are often obtained from suppliers

Engines

Camshaft Tappets Valves Crankshaft Rods Pis tons Rings Seals Pumps Bearings Engine Oil

- - - - - - - - - - - - Transmissions Axles

Clutch Plates CV Joints Transmission Bearings f h i d Seals Bearings Gear Oil

. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ - - - - - - - - - - - - - - - - -

Seals

Brakes Tires

Friction Tires materials Seals

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . for the design of a part, the automotive manufacturer relies on his knowledge. It is not always easy to know how sophisticated various suppliers are because of a wish to protect proprietary technology on the part of the supplier and lack of a perceived need to know on the part of the automotive manufacturer. Obviously, there will be wide variations. For example, some suppliers have established reputations for depth of understanding in tribology which rests on a substantial body of publications in the open literature. Clearly, visible competence in this area makes a supplier more attractive to the vehicle producer. Other suppliers may not have established reputations in this manner, but the fact that the components supplied function with low failure rates suggests at least some empirical knowledge. Unless the automobile manufacturer has retained expertise in design of the particular component and applies it to evaluate designs and supplier capabilities, it will be difficult to know whether the part functions as well as it could. This will continue to be an issue, as competitive pressures lead to greater reliance on supplier research and expertise.

Desien ennineers; Few engineers have extensive prior training in tribology. Generally speaking, knowledge is obtained from design practice documents, from colleagues, from vendors and from experience. Forward thinking organizations actively encourage continued education through participation in seminars and courses. In such organizations, design engineers are usually competent practical tribologists in their areas of responsibility.

However, the move to ever higher standards of reliability and durability will demand more sophisticated approaches than are commonplace today. In addition, design engineers will need to take the lead in evaluating suppliers for competence in design of tribological components.

Desien analvsts: Mathematical analysis of tribological interactions is critical for highly stressed contacts such as cam/tappet contacts, journal bearings and gearing. Design analysis computer programs fill these needs and are progressively becoming more powerful and flexible. These will be discussed more fully below.

Triboloev mecialists; These are understood to include metallurgists, lubrication specials and the like as well as tribologists per se. These resources are typically employed during a specific design exercise if and when a need is perceived by design engineers. If no problems are encountered, these resources often are not employed. If the link between design engineer and specialist is too weak, design issues which would benefit from deeper understanding may be dealt with by an extrapolation of existing methods. The tribology specialist should accordingly make sure that lines of communication with his design colleagues remain open.

Lubricant sumliers; The role of the lubricants supplier is somewhat special, particularly in the case of engine oils. Gear oils, which are rarely replaced, are specified by manufacturers. Automatic transmission performance is critically dependent on the frictional behavior of the fluid/friction material couple. 'Accordingly, the properties of these fluids are also specified and controlled by manufacturers. Engine oils are periodically replaced by the vehicle owner. They usually are described generically for better customer identification, especially in North America. Suppliers and manufacturerd work together to define test methods and sometimes also specifications covering viscosity and performance properties such as antiwear protection, oxidation resistance, bearing corrosion, etc. Because of the quasi-generic nature of automotive lubricants, properties are usually taken as typical for the current generation of fluids during the design process. When possible, unique fluids are avoided because of service availability concerns.

2.3 The design process - summary

Competitive pressures are leading to shorter product cycles, simultaneous engineering practices, and to greater demands for product quality, performance and economy. At the same time, improved productivity on the part of design groups will be required.

The automotive industry relies heavily on the design capabilities of suppliers, and this reliance is growing. Accordingly, ensuring that the supplier community is using the latest design methods and technology should have a high priority .

Knowledge of applied tribology is widespread among design engineers, although few have much formal academic training in the subject. Even so, the drive for continual improvement in the product as well as the importance of critically assessing supplier designs show the need for increased sophistication.

Shortening of the product cycle and implementation of simultaneous engineering make it essential that tribological concepts be applied to the design process at the very beginning.

. Improving the sophistication and capabilities of design tools

. Introducing new tools and methods

. Helping design engineers to upgrade skills

. Evaluating and improving supplier

This can be done by:

35

capab i 1 it ie s Improving links between specialists and design engineers

.

The tribology community can participate by:

Working with automotive manufacturers and suppliers to develop new and upgraded design tools. Organizing courses and seminars aimed at design engineers. consultants, suppliers and in-house experts as well as by universities and professional societies. Communicating research results and tribology concepts in forms accessible to the general engineering community'.

2.4 Triboloeical design methods and design

This can be done by

issues

Design tools, generally in the form of computer programs or reference handbooks, are the best means of formalizing the incorporation of tribology concepts and methods into the design process. Considerable advances can be made by application of information already available to the tribology community. Some open issues, however, will require new information for satigfactory resolution.

Desien euide; Design guides describe and set limits on parameters such as physical dimen- sions, materials, stress levels and the like, which are to be used in designing a component. These are intended to be a compilation of the state of the design art as practiced by the manufacturer. Although such documents are not primarily,-tribological design tools, practices specified therein impact the tribological behavior of the final product. A design guide is, of course, updated periodically and will grow in sophistication as better information becomes available to design engineers. Efforts to encourage the development of expertise through education in its many forms should assist the transfer of new technology into design guides.

Enginehowertrain simulation Drovrams; These allow evaluation of design alternatives early in the design process with a minimum of experimen- tal input. Friction and durability are less important in these global models than combustion efficiency, emissions and power output, but they do play a role. For programs of this sort, reasonable approximations of friction losses or stress levels are more appropriate than precise calculations requiring detailed input. Accordingly, well documented scaling algorithms could be of practical value in this application.

Bearine desien D rovrams; programs calculate the film thickness and temperature rise in dynamically loaded crankshaft journal bearings. This is no small task, since loads and film thicknesses change continually in complex ways (1, 2 ) . These programs are used to optimize a number of design variables, including bearing dimensions. This is particularly important for transversely

Bearing analysis

mounted front wheel drive configurations, which can place severe restrictions on the overall crankshaft length. important in assessing the impact of reduced engine oil viscosity as part of the North American move toward 5W30 oils. While these programs are very useful, additional work needs to be done to establish quantitative correlation with operating engines. This will be increasingly important as higher loads drive bearing operating conditions further into the mixed lubrication regime, and as higher maximum engine speeds put greater thermal stress on lubricants. Other challenges are to incorporate and verify methods of dealing with non-ideal conditions, such as misalignment ( 3 ) .

They have also been

Viscometrics: Improved bearing design programs, lubricant viscometrics and bearing properties are linked issues which are of importance to both automotive manufacturers and suppliers. Lubricant suppliers are particularly interested because engine oil specifications will ultimately reflect the critical parameters determining bearing performance. Improved understanding of the relationships between the performance of multigrade engine oils and viscosity as measured at high temperature and high shear rate (HTHS viscosity) ( 4 ) has led to HTHS limits for factory fill oils, and may lead to a redefinition of API viscosity. However, open issues still remain. Is HTHS viscosity an adequate lubricant parameter for defining film thickness, or will further development of viscometric methods be needed ( 5 ) ? Since EHD film thickness and traction calculations require parame ter s such as pressure-viscosity coefficient and glass transition pressure/temperature, should oil specifications ultimately include similar parameters? The answers are not obvious to this researcher.

Bearing performance is ultimately a function of oil additive composition, bearing materials and surface finish as well as design and viscometrics. Accordingly, it is necessary to not only determine relationships between film thickness and oil viscosity, but also between film thickness, oil and bearing composition, and bearing performance. This is a difficult experimental task being pursued by manufacturers and lubricant suppliers (1, 2, 6, 7).

Low friction oils; Oils formulated to give lower friction losses have resulted in some improvement in fuel economy (see appendix).

There is reason to believe, however, that further improvements in fuel economy may be achievable through improvements in friction reducing additive technology (8). Further improvements in friction reducing oil technology would not only result in improved fuel economy, but would also affect the economics controlling the selection of design options. For example, if the friction losses in sliding cam/tappet contacts were reduced by 50% by low friction oils, then the costbenefit ratio for roller cam followers would increase by a factor of 2. Accordingly, it is important to know what is the ultimate level of friction reduction attainable with practical oil formulations so that the industry can work toward implementation of improved technology.

I. Examples would include Society of Automotive Engineers technical papers and "Lubrication Engineering", published by STLE.

36

Surface uroperties: Although considerable progress has been made in surface measurement techniques, experience has shown that the description of engineering surfaces embodied in specifications is often inadequate.

For example, a finishing method may be given, together with a surface roughness value. If the surface is inadequately specified, the finishing method might go out of control without the knowledge of the operator, resulting in inexplicable field problems. Difficulties have been encountered in the industry when implementing new finishing methods because surfaces which appear the same have performed very differently. Potential problems include noisy cams and failed bearings. Because surfaces need to be statistically controlled in production, an ideal result would entail characterization methods sufficiently straightforward to allow use in factories on a routine basis. Failing that, more complex methods could be useful in problem solving or quality control.

Valve gear desien uroerams; A considerable effort has been devoted to developing computational methods for the design and analysis of valve trains (9). These have resulted in proprietary computer programs which are used on a routine basis for analyzing the kinematics, stresses and thermal behavior of cam/tappet contacts. Programs are used on a more limited basis for analyzing more complicated issues related to valve train dynamics. Stress levels and flash temperatures are used as criteria to define design limits, for example, to exclude excessively aggressive cam profiles. These criteria can also be used to select materials appropriate for the demands placed on them by a particular design approach. The utility of such programs could be further enhanced by the application of EHD and mixed lubrication theory to allow estimations of oil film thickness and friction losses to be made as well (10, 11).

Improved computer methods are being developed, spurred by developments in computer technology and by increasingly user friendly software. Since this work does not appear to directly impact material specifications, it is less visible than the efforts referred to above, and is generally proprietary. Finite element analysis and CAD/CAM methods are being applied, as are approaches obtained from studies of elastohydrodynamic lubrication.

Valve eear desipn oDtions; Valve train friction is a significant fraction of total engine friction. In the United States, government mandated Corporate Average Fuel Economy (CAFE) objectives make low friction design options like roller cam followers attractive in many designs despite their higher cost. The move to 4-valve engines, however, doubles the cost of this particular approach. Accordingly, minimizing sliding cam/tappet friction by attention to design details deserves careful attention.

Piston rinP/cvlinder bore models: Some manufacturers have in-house models aimed at calculating ringbore friction and wear (12, 13). These do not appear to be widely used as design tools by the automotive industry, because pistons and rings are generally designed by

outside suppliers, who provide needed engineering data to engine designers. Models focussed on evaluating design variables such as side loading, bore distortion and scaling effects, rather than the details of the ringbore interface, could see wider use.

Other desien models; Computer methods are widely used to assist in many design problems. Examples include finite element analysis methods applied to cylinder block distortion and connecting rod stresses. Although these and other issues are not tribological in nature, they may influence tribological behavior. For example, if cylinder bores distort on engine assembly or during operation at temperature, then ring tension must be high to achieve effective sealing. Maintaining round cylinder bores allows the designer to minimize ring tension and thus reduce piston/cylinder bore friction. Similarly, reduced reciprocating mass leads to reduced loads, greater oil film thickness and lower friction in rings and bearings.

Material/surface treatment selection Droerams; Data bases, especially in combination with improved data retrieval methods, may greatly enhance the ability of design engineers and tribologists to choose better materials and surface treatments. Such programs are not in widespread use at the moment, but current developments suggest that they may be developing rapidly into valuable tools. Some programs are being developed on a proprietary basis, but others, in various stages of development, have been presented in the open literature (14, 15). It seems probable that specialized data bases will see widespread use well before a general tribology data base is far enough advanced to be applied to automotive issues.

For the longer term, the idea of a general tribology data base has considerable appeal, even though the task of constructing such a system appears formidable. An effort sponsored by agencies of the United States government is underway, with input from technical and industrial committees (16). At this point, only preliminary work to define program structure has been completed (17), but even this much progress holds out the hope that the vast body of tribological data may eventually be made generally accessible.

2 . 5 Desien methods and desien issues - summarv; A substantial 'body of tribological information, much of it formalized by inclusion in computer programs, is now being applied in the design process. More information must be made available to design engineers to improve design quality, This information must be in readily accessible and easily usable form to ensure its use and to shorten design time. Improved methods for characterizing materials are also needed. These must be sufficiently straightforward to be carried out at a well equipped industrial laboratory and should not require expert interpretation. Additional information in certain areas would also be clearly useful. Some suggestions are as follows :

Well documented engine friction scaling algorithms could be incorporated in engine simulation programs.

37

. Bearing design programs needs validation for high transient load conditions which may push bearings into the mixed lubrication regime.

Programs need verified means of dealing with non-ideal conditions such as misalignment. Friction calculation algorithms for mixed lubrication would also be useful.

. What information is required to adequately characterize a lubricant in terms of its tribological behavior? What are the necessary parameters and how should they be measured? not only viscometrics (perhaps including viscoelastic effects), but also a general method for characterizing friction modifiers.

A complete answer would include

What is the lowest "boundary" friction coefficient attainable with the ultimate friction modified oil? that goal be approached with a practical engine oil?

What set of measurements will adequately characterize a tribological surface? This issue impacts how materials, coatings and surface treatments are selected, specified and controlled in production.

How closely can

. Valve train analysis programs could benefit from the incorporation of friction and perhaps durability calculations based on EHD theory and lubricant chemical effects.

Alternate valve train design approaches need to be explored to reduce friction losses in a cost effective manner.

.

. Piston ring/cylinder bore models focussed on assessing the effects of design variables and dimensional effects could be of value.

. How can we best optimize material combinations, select materials or coatings for an application and screen new materials? Efforts to introduce new materials or to use old materials in new applications are often wasted because information available to experts is not readily available to designers.

2.6 Technoloev transfer

New desims and materials: In order to be easily accepted into a vehicle program, a new technology must be low risk and cost effective. If the material or device is to be supplied by a vendor, production capacity must be available, Little time can be devoted to development once a vehicle program begins, and competitive pressures are reducing the available time even more.

A high degree of confidence in the new technology is required because of economic, marketing and safety considerations. This is more true of the automotive industry than it is of many others, and can induce a certain degree of conservatism.

Accordingly, development and introduction of new technology can be a lengthy process. This can be done by working directly with a

vehicle manufacturer. However, suppliers are often in a better position to perform development work in their area of specialization. As supplier companies come to work more and more closely with their automotive customer, they become more and more able to initiate development work earlier in the product cycle to anticipate needs.

Automotive companies can promote the introduction of new technology by improving technology management practices. That is, identifying areas of new technology which are promising and adopting management structures and methods which facilitate technology development. Originators of new technology should recognize that multiple input points exist, and that these may be either at the automotive manufacturer or at a supplier. A long term commitment and perhaps a substantial investment in testing and development may be required.

Design methods: New design methods can potentially be adopted as they become available, regardless of the timing of the product cycle. The initial input point may be to any of several areas in the organization: design analysis groups, advanced or forward engineering groups, or research and development groups. The new method does not necessarily need to be a completely finished product to be useful, as long as it expands the ability of the customer to design better products or to design more efficiently. Possibilities exist for cooperative development over extended and imperfectly defined time periods.

Effective internal development and transfer of technology requires good communication channels and a cooperative attitude on the working level as well as good technology management practices on the management level. These are sometimes problem areas in large organizations. Much has been written on the subject, and each organization must find its own ways to improve communication and cooperation. These factors are important not only for the transfer of technology developed in-house, but also for the transfer of technology developed outside from the point of input to the point of application.

Design Engineers need to remain current through participation in professional societies and through the stimulation afforded by cooperative research.

industry must communicate with the automotive customer at some input point. While publication in scientific journals is an excellent way to reach researchers, design engineers are often reached more directly by industry publications (eg., the Society of Automotive Engineers).

Those outside the automotive

3 CONCLUSIONS

Significant changes are taking place in the automotive industry and in automotive technology. These changes are driven by intense competition and the resultant need to provide a high quality, economical product in a timely manner. Both the design of tribological components and the design process itself have been affected.

The automotive industry relies heavily on the design capabilities of suppliers and this reliance is growing. Accordingly, ensuring that

the supplier community is using the latest design methods and technology should have a high priority.

Increasing competition, the need for continual improvement in product quality and performance as well as the need to critically assess supplier designs show the need for increased knowledge of tribology. Knowledge can be increased by participation in courses and seminars obtainable at universities, through consultants, by use of in-house expertise and through supplier presentations.

Tribological design tools include design guides and computer programs aimed principally at calculating parameters affecting bearing and valve train performance. upgraded and validated on a continuing basis. Data bases for the selection of tribological materials are being developed. Opportunities exist for the tribology community to participate in these developments by introducing established methods and information into design practice and by generating new information in critical areas.

New technology can often be best implemented during the design stage. In order to be easily accepted, the new technology should be low risk, cost effective and available. Accordingly, development work must be close to completion before implementation. Originators of new technology should recognize the existence of multiple input points and should be prepared to make a long term commitment to development of the technology, in cooperation with an automotive company or supplier.

design methods may be less constrained in terms of timing and completeness of product development. Long term development of new or upgraded methods in cooperation with industry may be easier to implement than introduction of new technology. Cooperation in research work is one way for universities to develop contacts in the industry.

These are being

The introduction of new

REFERENCES

SPIKES, R. H., and ROBINSON, S. M. 'Engine Bearing Design up-to-Date', paper C1/82, from 'Tribology, Key to the Efficient Engine', I. Mech. E. Conference Publications 1982-1, Mechanical Engineering Publications Ltd., London (1982). GOODWIN, G. and HOLMES, R. 'On Bearing Deformation and Temperature Distribution in Dynamically Loaded Journal Bearings', paper C2/82, ibid. REASON, B. R., and SIEW, A. H., 'A Numerical Solution for the Design and Performance Evaluation of Journal Bearings with Misalignment', paper C9/82 from 'Tribology, Key to the Efficient Engine', I. Mech. E. Conference Publications 1982-1, Mechanical Engineering Publications Ltd., London (1982). American Society for Testing Materials Test Methods D4683 and D4741, Coordinating European Council (CEC) Test Method L-36, Institute of Petroleum (IP) Standard IP- 370. HUTTON, J. F., JACKSON, K. P. and WILLIAMSON, B. P. 'Effects of Lubricant Rheology of the Performance of Journal Bearings', ASLE Trans., 2, 1 (1986) 52- 6 0 .

BATES, T. W., WILLIAMSON, B. SPEAROT, J. A . and MURPHY, C. K. 'A Correlation Between Oil Rheology and Oil Film Thickness in Engine Journal Bearings', SAE Int. Cong. and Exposition, Detroit, MI, USA, February 24-28 (1986), paper 860376.

SCHILOWITZ, A. M. and WATERS, J. L. 'Oil Film Thickness in a Bearing of a Fired Engine - Part IV: Measurements in a Vehicle on the Road', SAE Int. Fuels and Lubricants Meeting, Philadelphia, PA, USA, October 6-9 (1986), paper 861561. WILLERMET, P. A., PIEPRZAK, J. M. and DAILEY, D. P., 'Friction Reduction in Valve Trains: the Influence of Friction Reducing Oil Additives', to be submitted to SAE. FAN Y. CHEN, 'Mechanisms and Design of Cam Mechanisms', 1982 (Pergamon Press, Inc., New York).

(10) STARON, J. T. and WILLERMET, P. A. 'An Analysis of Valve Train Friction in Terms of Lubrication Principles', SAE International Congress and Exposition, Detroit, MI, USA, February 28-March 4 (1983), paper 830165.

(11) DOWSON, D., TAYLOR, C. M., and ZHU, G. 'Mixed Lubrication of a Cam and Flat Faced Follower', paper XX(i), from 'Fluid Film Lubrication - Osborne Reynolds Centenary', D. Dowson, C. M. Taylor, M. Godet and D. Berthe, eds., Elsevier Science Publishers BV., Amsterdam (1987).

(12) PARKER, D. A. and ADAMS, D. R., 'Friction Losses in the Reciprocating Internal Combustion Engine', paper C5/82, from 'Tribology, Key to the Efficient Engine', I. Mech. E. Conference Publications 1982- 1, Mechanical Engineering Publications Ltd., London (1982).

(13) TING, L. L., 'Lubricated Piston Rings and Cylinder Bore Wear', from 'Wear Control Handbook', M. B. Peterson and W. 0. Winer, eds., The American Society of Mechanical Engineers, New York, pg 609-666, (1980).

(14) SYAN, C. S . , MATTHEWS, A. and SWIFT, K. G. 'Knowledge Based Expert Systems in Surface Coating and Treatment Selection for Wear Reduction', Surface and Coatings Tech., 33

(15) SWINDELLS, N. and SWINDELLS, R. J. 'System for Engineering Materials Selection', Met. Mater. (Inst. Met.) 1, 5, (1985) 301-304.

(16) ANON 'A Computerized Tribology Information System', Lubrication Eng., 1987, 42, 4 , 228,

(17) TALLIAN, T. E. 'Tribological Design Decisions Using Computerized Databases', ASME Trans., Journal of Tribology, 1987

(1987) 105-115.

- 109, 381-387.

APPENDIX

A1 RECENT/CURRENT TECHNOLOGY IMPLEMENTATION

Some of the more significant recent applications of tribological principles to automotive design are given below. These show the results of some of the driving forces for change in the industry - the customer driven needs for performance, economy and durability. Also, they illustrate the extent to which the implementation of new technology can depend on cooperation among automotive manufacturers and suppliers.

39

Roller followers. Roller followers have been widely implemented in North America as a means of reducing valve train friction. of rollers has required changes in camshaft metallurgy because of the need for improved fatigue resistance. Where fatigue failure has been eliminated by the proper choice of materials, long term durability has been enhanced. Since both roller followers and fatigue resistant camshafts are more expensive, their use entails significant cost penalties; these are considered worthwhile in view of the improved fuel economy obtainable through reduced friction. If use of roller followers became general, this might eventually lead to reduced antiwear protection requirements for engine oils, resulting in less use of antiwear additives such as zinc dithiophosphates.

Low friction rinP Packs. Low friction rings have been introduced in a number of vehicles. This has been accomplished largely by attention to maintaining bore roundness, thus allowing use of lower tension rings.

5W30 eneine oils. SAE 5W30 engine oils have become an ever larger factor world wide. The reduction in viscosity grade not only improves cold start, but results in lower viscous friction losses for most vehicles.

Introduction

ImDroved friction reducine oils. Since engine oils are replaced periodically by the customer, who may obtain them from any of a large number of vendors, it was necessary to define categories of energy conserving oils in order to assure availability of these oils to the general public. This has been accomplished through the cooperative work of the automotive, oil and additive industries in SAE and ASTM. The American Petroleum Institute (API) defines an energy conserving oil as one providing a fuel economy improvement of 1.6% over a reference oil and energy conserving I1 oil as one providing a 2.7% improvement. Both viscometric properties and additive chemistry are important in formulating energy conserving oils.

Concinuouslv variable transmissions. Contin- uously variable transmissions, long proposed as a means of improving efficiency and performance, are now beginning to enter production in Europe. Current models are based on the Van Doorne metal belt drive, although other types are still under consideration. Friction and wear as well as manufacturing issues have been obstacles to overcome in introducing this technology. In addition to metallurgical, design and manufacturing parameters, transmission fluid composition can also affect the performance of the belt/cone contact by influencing friction and durability.

Lock-up toraue converters. Lock-up torque converters have been introduced in a number of automatic transmissions to minimize viscous losses. These designs, together with the desire to maximize fluid life and to achieve the smoothest possible shifts, have led to changes in the composition and performance of automatic transmission fluids. A smooth engagement together with a firm lock-up requires control of the friction versus sliding speed curve. To avoid shudder, the static to dynamic friction

ratio should be less than 1. If the ratio is too low, however, slipping can occur. Actual fluid requirements depend on design philosophy. If the weight and size penalties are considered to be acceptable, a lower friction coefficient can be accommodated by increasing the friction material area.

ImDroved transmission fluids. Ford has recently introduced a new class of automatic transmission fluids. General Motors is expected to in the near future. The Ford fluid is characterized by closely controlled frictional behavior during clutch engagements and by improved stability over time. The General Motors fluid is expected to emphasize smooth shifts by producing an appropriate friction versus sliding speed curve. These demands have required significant changes in transmission fluid additive packages.