Automotive tribology overview of current advances and challenges for the future

Tribology International 37 (2004) 517–536 www.elsevier.com/locate/triboint

Automotive tribology overview of current advances and challenges for the future Simon C. Tung
, Michael L. McMillan General Motors Research and Development Center, USA Received 3 April 2003; received in revised form 26 December 2003; accepted 7 January 2004

Abstract This keynote address will provide a comprehensive overview of various lubrication aspects of a typical powertrain system including the engine, transmission, driveline, and other components, as well as the integration of these lubrication and surface engineering concepts into a unified automotive powertrain system. In addition, this presentation will focus on the current status and future trends in automotive lubricants including discussion of current and anticipated future requirements of automotive engine oils. This presentation will also review the current standard ASTM (American Society for Testing and Materials) test methods for engine lubricants and other compilations of automotive standards. In addition to engine oil test development, industrial researchers are developing light-weight materials such as non-ferrous materials (Al, Mg) for engine and drivetrain materials to replace the current heavy-weight cast iron blocks. Recent industrial developments include high strength and high density of composite materials, high volume liquid molding and hydroforming technology, structural adhesive boding, and the ability to mold large structural components. Industrial researchers have also developed processing improvements for forming more complex stamped aluminum parts or panels, more robust stamping, and improved casting techniques. In this paper, our insights and perspectives on future trends in light-weight tribological material and nonotribology will also be reviewed. # 2004 Elsevier Ltd. All rights reserved.

1. Introduction The petroleum and automotive industries are facing tough international competition, government regulations, and rapid technological changes [1–3]. Ever increasing government regulations require improved fuel economy and lower emissions from the automotive fuel and lubricant systems. Higher energy-conserving engine oils and better fuel-efficient vehicles will become increasingly important in the face of both the saving of natural resources and the lowering of engine friction [4–7]. The United States Department of Energy has conducted a workshop [8] in which the focus was on industry research needs for reducing friction and wear in transportation. Reducing friction and wear in engine and drive train components could save the US economy as much as US$ 120 billion per year. There are many hundreds of tribological components, from bear


Corresponding author. E-mail address: [email protected] (S.C. Tung).

0301-679X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2004.01.013

ings, pistons, transmissions, clutches, to gears and drivetrain components. The application of tribological principles is essential for the reliability of the motor vehicle, and in the area of powertrain technology has led to enormous advances in the field of tribology. For the purpose of classifying the tribological components, we can divide the motor vehicle into engine, transmission, traction drive, continuously variable transmission (CVT), drive train, joints, and ancillaries. In the following sections, each automotive component is discussed in detail. Required lubricants used with these automotive tribological components are described in the last section of this review paper.

2. Automotive tribology 2.1. Engine The reciprocating internal combustion engine as shown in Fig. 1 is the most important component in

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Fig. 1.

Main engine components in an internal combustion engine.

the motor vehicle, as well as in many ground and sea transportation devices including motorcycles, scooters, mopeds, vans, trucks, buses, agricultural vehicles, construction vehicles, trains, boats and ships. The popularity of the reciprocating internal combustion engine is due to its performance, reliability and versatility. However, there are also some major drawbacks. Thermal and mechanical efficiencies are relatively low, with much of the energy of the fuel dissipated as heat loss and friction. The internal combustion engine is also a contributor to atmospheric pollution through hydrocarbon, particulate and NOx (nitrogen oxides) emissions and to the greenhouse effect via carbon dioxide emissions. A viable alternative with the required portfolio of attributes including versatility and cost, however, has yet to be found, and hence the reciprocating internal combustion engine will still dominate the road vehicle market for the foreseeable future. 2.1.1. Importance of engine tribology The engine tribologist is required to achieve effective lubrication of all moving engine components. In order to reduce friction and wear, with a minimum adverse impact on the environment. This task is particularly tough given the wide range of operating conditions of speed, load, and temperature in an engine. Improvements in the tribological performance of engines can generate the following benefits: . . . . . .

Reduced fuel consumption, Increased engine power output, Reduced oil consumption, A reduction in harmful exhaust emissions, Improved durability, reliability and engine life, Reduced maintenance requirements and longer service intervals.

With such large numbers of reciprocating internal combustion engines in service, even the smallest improvements in engine efficiency, emission levels and durability can have a major effect on the world fuel economy and the environment in a long-term. The energy derived from combustion of the fuel is distributed in an engine and a powertrain system. In a recent published paper by Andersson [1] showed the distribution of fuel energy for a medium size passenger car during an urban cycle. Only 12% of the available energy in the fuel is available to drive the wheels, with some 15% being dissipated as mechanical, mainly frictional, losses. Based on the fuel consumption data in his publication, a 10% reduction in mechanical losses would lead to a 1.5% reduction in fuel consumption. Concerning energy consumption within the engine as shown in Fig. 2, friction loss is the major portion (48%) of the energy consumption developed in an engine [24]. The other portions are the acceleration resistance (35%) and the cruising resistance (17%). If we look into the entire friction loss portion as shown in the right pie chart of Fig. 3 [34], engine friction loss including piston skirt friction, piston rings, and bearings is 66% of the total friction loss, and the valve train, crankshaft, transmission, and gears are approximately 34%. Concerning powertrain friction loss only, sliding of the piston rings and piston skirt against the cylinder wall is undoubtedly the largest contribution to

Fig. 2.

Energy consumption developed in an engine.

Fig. 3. Distribution of energy consumption in a light-duty vehicle [34].

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friction in a powertrain system. Frictional losses arising from the rotating engine bearings (notably the crankshaft and camshaft journal bearings) are the next most significant, followed by the valve train (principally at the cam and follower interface), and the auxiliaries such as the oil pump, water pump and alternator. The relative proportions of these losses, and their total, vary with engine type, component design, operating conditions, choice of engine lubricant and the service history of the vehicle, i.e. worn condition of the components. Auxiliaries should not be overlooked, as they can account for 20% or more of the mechanical friction losses. 2.1.2. Lubrication regimes in the engine Any powertrain system designed to operate with a liquid lubricant; the key operating tribological parameter in a powertrain system is the lubricant film thickness separating the interacting component surfaces. More precisely, it is the relative magnitude of the lubricant film compared to the combined surface roughness of the two surfaces, the so-called film thickness ratio, or parameter, k, k¼

h ðr2surface 1 þ r2surface 2 Þ1=2

ð1Þ

Where h is the film thickness calculated through the application of classical thin film analysis taking the surfaces to be smooth, and r is the root mean square (rms) surface roughness. A related version of this equation uses the center line average surface roughness values, ðRasurface 1 þ Rasurface 2 Þ, in the denominator in place of the root mean square term. Fig. 4 shows a plot of the relationship between the coefficient of friction and the oil film thickness ratio or Summerfield number (¼ viscosity  speed=load). The diagrams at the top of the figure provide a visual example of the lubrication of two surfaces that are in relative motion to each other and that are separated by an oil film. At the box on the top left side there is surface contact; at the box on the bottom left a fluid film

Fig. 4. Lubrication regimes for engine components and their relationships to friction.

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separates the surfaces, and between these two extremes partial or intermittent contact occurs. The curved line below the lubrication regimes indicates the relationship between the friction coefficient and the Summerfield number. Different automotive components rely upon different modes of lubrication to achieve acceptable performance, and each may experience more than one regime of lubrication during a single cycle. Generally, journal and thrust bearings are designed to operate in the hydrodynamic lubrication regime in which bearing surfaces are separated by a lubricant film. Actual metal-to-metal contact is expected to take place only at low speeds and high loads and with low-viscosity lubricants. In contrast, valve train, piston ring assembly and transmission clutch sliding generally take place under mixed or boundary lubrication conditions; surface contact occurs, and chemical films or reaction products may be an important means of surface protection. In addition, the importance of different lubrication regimes for each engine component may change with the surface roughness at the interface, wear of critical interacting surfaces, and lubricant degradation. 2.1.3. Engine bearings In an operating engine, rotating journal bearings are used to support the camshaft, the crankshaft and the connecting rod. The basic configuration is a split halfshell bearing fitted into a bore and incorporating some form of locating notch. The shells are formed from a steel backing strip to give strength and tight dimensional tolerance and overlaid with a relatively soft bearing material such as tin–aluminum or lead–bronze. A further thin soft overlay is then coated onto the bearing material to ensure a degree of initial conformability [22]. Shafts designed for running against engine bearings are generally made of heat-treated steels or spheroidal graphite irons, with hardness approximately three times that of the principal bearing material. Lubricant is directed to the bearings either by jet impingement or by passing through the bore of a hollow shaft. If the bearings are adequately lubricated, bearing wear is low after an initial stage of running in period. However, shaft misalignment or particulate contamination of the lubricant supply can lead to excessive wear. An additional failure mechanism is bearing corrosion. The tribology of journal bearings is complicated by issues such as lubricant supply, thermal effects, dynamic loading and elasticity of the bounding solids. The mobility technique for the analysis of dynamically loaded engine bearings was established some 30 years ago and remains the most common approach [3,4]. The technique is robust and has proved a simple computer analysis. It can calculate the cyclic minimum film thickness between the journal and the bearing, which is an

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important design parameter. However, it is important to note that many simplifying assumptions are implicit in the use of the mobility method, which means that the predictions can only be used as a benchmark. The mobility method and many other alternative methods of solution assume that engine bearings operate entirely in the hydrodynamic lubrication regime. For a satisfactory minimum lubricant film thickness in an engine, the benchmark prediction was used to be approximately 2.5 lm. Today, minimum film thickness predictions in the range 0.5–1.0 lm have made for engine bearings in passenger cars, suggesting that asperity interaction may occur between the journal and bearing for part of the engine cycle. To design engine bearings operated in the mixed lubrication regime will be a critical and significant approach to improve engine performance. ([29, 43]). 2.1.4. Piston assembly The heart of the reciprocating internal combustion engine is the piston assembly, forming a critical linkage in transforming the energy generated by combustion of the fuel and air mixture into useful kinetic energy. The piston assembly includes the ring pack, which is essentially a series of metallic rings, the primary role of which is to maintain an effective gas seal between the combustion chamber and the crankcase. The rings of the piston assembly, which form a labyrinth seal, achieve this function by closely conforming to their grooves in the piston and to the cylinder wall. A secondary role of the piston ring is to transfer heat from the piston into the cylinder wall, and thence into the coolant, and to limit the amount of oil that is transported from the crankcase to the combustion chamber. This flow path is probably the largest contributor to engine oil consumption and leads to an increase in harmful exhaust emissions as the oil mixes and reacts with the other contents of the combustion chamber. The objectives to extend engine service intervals and minimize harmful exhaust emissions for meeting more stringent legislative requirements are to reduce the permissible oil consumption to a low level compared to their predecessors of 20 years ago. The left side of Fig. 5 is a schematic representation of a piston assembly from a modern automotive engine. From a tribological perspective, the main piston features are the grooves, which hold the piston rings, and the region of the piston below the ring pack (the piston skirt), which transmits the transverse loads on the piston to the cylinder wall. The top two piston rings are referred to as the compression rings. As shown in the right side of Fig. 5, firing pressure pushes these rings out until the entire ring face engages the cylinder wall. Gas pressure is provided to supplement the inherent elasticity of the ring and maintain an effective combustion chamber seal. The top compression

Fig. 5. Piston assembly and piston ring function from an internal combustion engine.

ring is the major gas seal and encounters the highest loads and temperatures as the ring nearest the combustion chamber. The top compression ring usually has a barrel-faced profile with a wear resistant coating such as flame-sprayed molybdenum on the periphery and occasionally on the flanks. The second compression ring, which is sometimes referred to as the scraper ring, is designed to assist in limiting upward oil flow in addition to providing a secondary gas seal. The right side of Fig. 5 illustrates how the second compression ring scrapes surplus oil from cylinder walls and how the ring rides on a film of oil and presents a lower edge to cylinder wall. Because of these functions, the second compression ring has a taper-faced, downward scraping profile that is not normally coated. The bottom ring in the ring assembly is the oilcontrol ring, which has two running faces, or lands, and a spring element to enhance radial load. As its name defines, the role of this ring is to limit the amount of oil transported from the crankcase to the combustion chamber, and it has no gas sealing ability. The periphery of the lands and, occasionally, the flanks are often chromium plated. There are a large number of configurations in the basic ring design, a good summary of which may be referenced from the paper published by Neale [25]. Some practical examples are the compression ring with tapered sides designed to prevent ring sticking due to deposit formation in the piston ring grooves of diesel engines, the internally stepped compression ring, which imposes a dishing on the ring when fitted to improve oil control, and the multi-piece, steel rail, oil-control ring. In contrast to the one-piece oil-control ring, the multi-piece oilcontrol ring has smaller land heights, which increases conformability to the cylinder wall, and hence oil control, with reduced spring force. The piston ring is the most complicated tribological component in the internal combustion engine to analyze because of large variations of load, speed, temperature and lubricant availability. In one single stroke of the piston, the piston ring interface with the cylinder wall may experience boundary, mixed and full fluid film lubrication [30]. Elastohydrodynamic lubrication of piston rings is also feasible in both gasoline and

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diesel engines on the highly loaded expansion stroke after firing. A typical prediction from a mathematical model for the cyclic variation of minimum film thickness between a piston ring and cylinder wall, where zero degrees of crank angle is top dead center firing has been made by Ruddy et al. [30]. The prediction is assuming a plentiful supply of lubricant for the top compression ring of a four-stroke gasoline engine. In this case, the combined surface roughness of the piston ring and cylinder wall was approximately 0.1 lm. The solution exhibits a characteristic curve with small film thickness around the dead center positions where the sliding velocity and lubricant entrainment velocity are low. In his analysis, a large film thickness was observed at the mid-stroke positions where the sliding and entrainment velocities are large. During the engine cycle the piston itself exhibits a complex secondary motion, transverse movement toward the cylinder wall and tilting about the main piston pin axis [20]. This movement results in fluid film or mixed lubrication between the piston skirt and the cylinder wall. Modern, light-weight pistons have skirts that may deflect elastically during interaction with the cylinder wall, leading to the application of elastohydrodynamic lubrication theory to these components [27,28]. Frictional losses between the skirt and the cylinder wall are significant, estimating for about 30% of total piston assembly friction. The piston skirt interaction with the cylinder wall can lead to noise generation, so-called piston slap. Gray cast iron, malleable/nodular iron and carbides/malleable iron are the most common base materials for all types of compression piston rings and single piece oil-control rings. However, steel is getting popular as a piston ring material due to its high strength and fatigue properties. Steel is used for top compression rings and the rails of multi-piece oil control rings. There are a variety of surface treatments and coatings for piston ring running-in processes. Chromium plating and flame sprayed molybdenum are the most common wear resistant coatings, although plasma sprayed molybdenum, metal matrix composites, cermets and ceramics are getting their popularity as their technology progresses. Cylinder bores are generally manufactured from gray cast iron, either plain or with the addition of alloying elements or an aluminum alloy. The surface treatment and coating of cylinder walls is less common than with piston rings in industry but a lot of wear resistant coatings such as Nikasil plating or thermal sprayed coatings [38] are applied to aluminum bores. Aluminum–silicon alloy is the major material used for automotive pistons, with additions of other elements to enhance particular properties, e.g. copper to increase fatigue strength. Piston skirt coatings, based on materials such as polytetrafluroethylene (PTFE), or graphite, are applied. In contrast to piston ring and

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cylinder liner coatings, piston skirt coatings are generally intended to reduce friction rather than wear. Controlling friction and wear of the piston assembly is crucial to successful engine performance as shown in Fig. 6. Several research opportunities in piston ring/ cylinder bore contact can be explored by investigating low friction coatings, wear resistant coatings for aluminum bores, surface texture control, and better surface finish or treatments. Manufacturers have come to rely upon early life wear of the piston rings and cylinder wall to modify the profile and roughness of the interacting surfaces to achieve acceptable performance as part of the running-in process. However, a clear understanding of the complex interactions between lubrication and wear of these components is important for lubrication engineers. In addition to wear as a consequence of mechanical interactions, corrosive wear may occur in the upper cylinder region during short-trip service in a winter climate. Manufactured surface finish of piston rings and cylinders can have a major influence on wear behavior and thereby the success or failure of an engine. Despite of a wide range of surface finishing techniques has evolved for both piston rings and cylinder liners, the objectives of these processes appear to be the same: to improve oil retention at or within the surface, to minimize scuffing and to promote ring profile formation during the running-in period. Plain cast iron piston rings are often used in a fine turned condition and electroplated; flame sprayed and plasma sprayed rings are generally ground to the desired finish. The poor wetability by oil of chromium plated piston rings can be overcome by etching a network of cracks and pores into the surface by reversing the plating current, chemical etching, grit blasting, lapping or depositing the plate over a surface with fine turned circumferential grooves. Cylinder bores can be honed with ceramic stones, diamond hones, rubber tools, cork tools or composite stones of cork or diamond. Piston skirt textures using a turned finish have been proved to give outstanding fuel economy and reduced risk of scuffing.

Fig. 6. Research opportunities for improving friction and wear reduction in an engine design.

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2.1.5. Valve train A modern valve train system should include valves, valve springs, valve spring retainers, valve keys, rocker arms, piston rods, lifter/tappets, and a camshaft, as shown in Figs. 7 & 8. The major function of the valve train system is to transform rotary camshaft motion into linear valve motion in order to control fluid flow into and out of the combustion chamber. The secondary function is to drive ancillary devices such as distributors, fuel pumps, water pumps and power steering pumps. Many different styles of valve train mechanisms have been used on engines. Three basic valve train systems are (1) poppet valve; (2) sleeve valve; and (3)

Fig. 7.

Valvetrain system.

Fig. 8. Cross-section through a V6 engine with a valvetrain mechanism of center pivot rocker arm or center pivot rocker with a lifter.

rotary valve. In a valve train system, the poppet valve configuration is the most popular and is used by virtually all the major automobile manufacturers for the inlet and exhaust valves of reciprocating internal combustion engines. A cam driven from the crankshaft to ensure synchronization of the valve motion with the combustion cycle and piston movement invariably controls the opening and closing of the valves. Common mechanisms are generally classified and referred to throughout the automotive industry as Type 1 through Type 5 [26]. The basic mechanisms for Type 1 through Type 5 include the following: (1) direct acting bucket valve train; (2) end pivot valve train; (3) center pivot; (4) center pivot with cam follower; (5) overhead (OHV) pushrod. These types may be designed using either roller or sliding friction at each contact interface. However, there is a large range of mechanisms in use to transmit the motion of the cam to the valve. These include push-rod operated, center-pivoted rocker, finger follower, direct acting bucket follower, and roller follower. Other designs such as rotary or sleeve valves have proved difficult to lubricate, cause excessive oil consumption, have poor sealing properties and generate excessive friction. A comparison of the friction, effective mass or valve weight, maximum engine speed, and overall engine package among these different types of valve train systems is shown in Table 1. Friction also plays an important role in valve train type selection. The best configuration is roller follower, which can significantly reduce valve train friction. This is because the coefficient of friction at the camshaft/follower interface is less by an order of magnitude for rolling compared to sliding friction. The direct acting system has poor frictional characteristics because it is not equipped with roller followers. The most important characteristics for friction control of rollers are a function of the lowest mass, least number of components, and the lowest number of friction interfaces. The friction associated with the valve train is generally considered to be a small component of the mechanical losses, typically 10% compared to the 50% linked to the piston assemblies. However, valve train friction is seen to rise significantly to 20%–25% at low engine speeds and for larger, slower running engines associated with the prestige car market. To introduce friction modifier additives such as molybdenum dithiocarbamate (MoDTC) into the engine oil has been one of the main drivers behind which strive to reduce boundary friction [19]. The valve train presents a wide spectrum of potential problems to the tribologist in relation to the cam and follower, valve guides, valve stem seals, valve seats, hydraulic lash adjusters, lifter guides, pivots, camshaft bearings, belt drives and chain drives. The most critical interface in the valve train is that between the cam and follower, a contact that has proved difficult to lubricate

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Table 1 Comparison of effective mass, engine speed. Friction, and overall engine package among these five types of valvetrain systems

Effective mass valve weight (gr.) Maximum RPM Friction (A–E) Overall engine package (A–E)

Direct acting

End pivot rocker arm

Center pivot rocker arm

Center pivot rocker arm with lifter

Push rod

140–160

80–120

120–160

130–170

240–290

6500++ E D–E

6500++ A D–E

6000+ B B

6000+ C–D C

4000–6000 C–D A

A, best; E, worst.

effectively with all designs of valve train. Traditional design philosophy assumed that the cam and follower operated entirely in the boundary lubrication regime. Therefore a lot of attentions have been limited to the materials and surface treatments of the components and the lubricant additives intended to produce wear resistant surface films by chemical reaction, e.g. zinc dialkyl dithiophosphates (ZDDP or ZDTP). Tribological investigations over the last 20 years or so, however, have shown that mixed and elastohydrodynamic lubrication have a significant role to play in the tribological performance of cams and followers [31,35]. The most popular materials for cams and followers are irons and steels with a variety of metallurgies and surface treatments to assist running-in and prevent early life failure. Ceramic followers are, however, becoming more common, especially in direct acting valve trains, in an attempt to reduce frictional losses [9]. Wear has been a problem with cams and followers for many years, especially with finger follower configurations. Recent technology advances in our understanding of the link between kinematics, lubrication and wear of valve trains are now helping to overcome these difficulties [2], as is the trend toward direct acting and roller follower systems. The failure modes of cams and followers are pitting, polishing and scuffing, all of which are influenced by materials, lubrication, design and operating conditions. The durability and type of failure can vary considerably depending on the combination of materials chosen, their surface treatment and the lubricant and its additive package. Wear is also a persistent problem at the valve seat interface; the valve ‘recesses’ into the seat and results in loss of engine timing. Loading is from a combination of the dynamic closing of the valve under cam action and the application of the firing pressure on the closed valve face, and therefore the seat is subjected to both high static and dynamic stresses. Temperatures in the region of v the valve are high (typically 300–500 C), and lubrication tends to be from small quantities of oil that flow past the valve guide. The valve may be allowed to rotate (usually slowly at about one to two revolutions

per minute) under the action of the cam on the bucket. To reduce wear an insert of hardened steel is shrunk fit into the head. Valve and seat materials should have high strength, wear resistance, high temperature stability, and corrosion resistance. In general, inlet valves are made from hardened low alloy martensitic steel for good wear resistance and strength. The exhaust valves are subjected to higher temperatures and are often made from precipitation-hardened, austenitic stainless steel for corrosion resistance and hot hardness. The valve seats are made from cast or sintered high carbon steel. In some high performance engines, the seat is formed by the induction hardening of the cylinder head material. 2.2. Transmission and drive line 2.2.1. Transmission The tribological characteristics of the transmission clutch and band are crucial because they control transmission shift performance and clutch and band durability. From a tribological standpoint, an automatic transmission clutch consists of two basic elements: the friction lined clutch plates and the steel reaction plates. The clutch plate assembly consists of three major components: a steel core, an adhesive coating, and the friction facings. In most US automotive clutches, the steel core is a part onto which the friction facings are bonded. The core is blanked from medium carbon steel with a Rockwell C hardness of 24 minimum. The core hardness ensures maximum tooth contact area. The minimum compressive load requirement can reduce the potential for wear or fretting on both spline surfaces. The friction facing is bonded to one or both sides of the steel core with an adhesive. The adhesive, a thermosetting organic resin, is selected to withstand high shear forces and a wide range of operating temperatures. The friction material must have the required friction characteristics for effective engagement, a pleasing (to the customers) shifting of gears and durability. It also must withstand a broad range of operating temperatures as well as high shear forces and compressive loads. Friction material is produced from a variety of

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Fig. 9.

Transmission torque converter clutch assembly.

fibers, particle fillers, and friction modifiers. These materials, with properly blended composition, can provide the necessary material strength, bulk uniformity for successful manufacturing, as well as provide a heat resistant material with the required friction properties. The porosity of the friction material allows the transmission fluid to be stored near the friction interface as a lubricant reservoir, and the resilience of the friction material permits it to conform to transmission mating surfaces. In automatic transmissions such as shown in Fig. 9, multiple plate clutches, band clutches, and the torque converter clutches are used either to transmit torque or to restrain a reaction member from rotating. The function of an automatic transmission clutch and band is to serve as a lubricated brake, e.g., for a planetary gear element, to transmit torque between transmission elements and the torque converter pump (input) and turbine (output). Dynamic and static band and clutch friction torque levels, and the difference between static and dynamic friction torque are important clutch performance criteria. They are closely dependent upon transmission fluid characteristics. The transmission fluid can have a deep impact on shift-feel, chatter, static holding capacity, and the friction interaction between the transmission fluid and clutch material. All automatic transmission fluids recommended by vehicle manufacturers are formulated using a base stock plus a variety of additives, including frictionmodifying additives to produce the desired transmission operation. Friction characteristics of the friction modifiers are described. Clutch friction characteristics for a base stock containing no friction-modifying additives are undesirable. Although a high static coefficient of friction is desirable for good clutch holding capacity, smooth engagement is difficult to achieve when static friction is greater than dynamic friction. As the clutch engagement approaches lock up (decreasing sliding speed) with such friction characteristics, the increasing coefficient of friction tends to produce unstable stickslip action between the rubbing surfaces and create abrupt and harsh clutch lock-ups. If these undesirable friction characteristics are too severe, shift-feel can be

unsatisfactory to the vehicle’s passengers. Objectionable shift-feel is a major cause of customer complaints about vehicle/transmission performance. The most desirable friction characteristics [41] for smooth clutch engagement are those described by a low static friction coefficient and followed by a high and stable dynamic friction coefficient [33]. High dynamic friction promotes a rapid clutch engagement, and static friction that is lower than dynamic friction provides a smooth transition to clutch lock-up. However, the static friction coefficient must be high enough to provide good static clutch torque holding capacity. Friction modifier additives can produce low static and high stable friction characteristics. Clutch torque capacity is reduced under both dynamic and static conditions [33]. During clutch engagement, the amount of clutch energy dissipated can affect both transmission fluid and clutch friction material life. In a transmission power shifting clutch or band, torque capacity must be great enough to produce a rapid clutch or band engagement. Otherwise, the energy dissipated as heat can produce clutch surface temperatures that contribute to the deterioration of both clutch materials and transmission fluids. Typical clutch surface temperatures have been v measured and found to be on the order of 95 C above sump fluid temperatures for wide-open-throttle conditions [33]. A good transmission fluid can help the transmission clutch to shift satisfactorily and provide a long clutch life. If an excessive clutch slippage results from an inability of the clutch to transmit sufficient torque to lock up the clutch in a reasonable time, the transmission will lose its function. If the energy can produce dynamic torques higher than the clutch can transmit, the clutch will not lock up, and the resulting energy will be so high that cellulose clutch materials are likely to burn or char. Such deterioration can occur in only a few severe clutch engagements. A surface v temperature as high as 230 C above sump temperature has been reported for automatic transmission power shifts [33]. High clutch surface temperatures will deteriorate clutch durability due to severe energy dissipation. In summary, clutch and band friction characteristics, transmission fluid viscosity, fluid oxidation, fluid shear stability, and wear properties of friction materials all play a crucial role in the overall performance and durability of the transmission. Because of the important characteristics of automatic transmission fluids in controlling transmission shift performance and long-term durability, the requirements for transmission fluid are described in greater detail in the section on automotive lubricants. 2.2.2. Traction drive The development of the CVT by vehicle manufacturers has increased dramatically to take advantage

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Fig. 10. Diagrams for the typical 4 speed automatic transmission and the generic CVT design.

of the fuel economy and better drivability benefits that can be achieved with traction drive components. An advantage of the traction CVT [14] compared with the current automatic transmission is that it can replace gears, clutches, bands, etc., with ‘‘a variator’’, permitting a smooth and continuous ratio change over the entire range of operation as shown in Fig. 10. A unique advantage of the CVT is that it allows an engine to operate over a range of speeds and loads independent of the torque requirements that are placed on the wheels by the vehicle and the driver [11]. Traction drive CVT was applied to vehicles in the 1930s and was started by the Toric transmission [15]. Three basic types of commercial CVTs [18] have been developed as shown in Fig. 11, as follows: (1) belt (steel, fabric, push, and pulley type) [42]; (2) chain type, (3) half or full toroidal traction. The two commercial CVTs (belt-pulley and chain) are the most common CVT drive units. Production vehicles equipped with steel belt-pulley CVTs have been in the automotive market for more than 10 years. However, the full potential for reduction of fuel consumption could not be realized until the fully integrated electronic control system became available. The belt-pulley CVT relies on a metal link belt developed by Van Doorne’s transmission and is typically limited to use with engines of 2.0 liter displacement or smaller [13]. Around 1990, a full toroidal traction drive device as shown in Fig. 12 was developed to supplement the belt-pulley type because of the traction drive’s ability to handle larger engines. The toroidal traction drive provides better mechanical efficiency because of a very

Fig. 12. Traction CVT design concepts.

efficient rolling motion that operates in the elastohydrodynamic regime [7]. The current traction drive transmission transmits power through a rolling contact via forces that are varied with the radius at which the traction force is applied. Thus, power is transmitted in a continuous manner. This concept relies on two rolling elements that place a thin film of lubricant into an extreme shearing condition. The thin film between the two rolling bodies experiences high load and extreme shearing force, which produces complex elastohydrodynamic phenomena. The viscosity of the traction fluid increases significantly as a consequence of the high pressure, and the fluid becomes almost glass-like. The tangential force that is transmitted between the driving and driven elements is greatly enhanced by the traction fluid. The surface contact area between the two rolling elements is a finite area whose orthographic projection is an ellipse [18]. Within the ellipse, fluid is trapped between the rollers and is subjected to high compressive stresses of 0.7–3.5 GPa (100,000–500,000 psi). These stresses increase the instantaneous viscosity of the fluid by several orders of magnitude. This semi-solid lubricant can then transmit torque through the drive. Basic research was conducted with application to different traction drive units [11]. However, a traction drive CVT was not applied in practical use until 1990 due to three major barriers [21]: . The traction drive rolling element materials were not sufficiently reliable due to high temperature and high pressure at the traction contact point. . There were no bearings that could support high speed and large axial load. . There were no traction fluids that could meet all traction component requirements.

Fig. 11. Belt and chain continuously variable transmission (CVT) systems.

A traction drive requires not only higher traction coefficient to meet power transmission requirements but also better lubrication and hydraulic lubricity requirements. Traction elements are generally made of

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materials similar to those used for ball or roller bearings for which the surface must withstand high loading and long duty cycles. This requirement also demands hardened steel or hardened materials for the contact surfaces. Since the traction drive operates at high speeds, it therefore provides high input and output ratios. Advanced tribological materials in the rolling element have made the life of the traction contact point long enough for use in high power automotive transmissions. The efficiency of the traction drive depends greatly on the lubricant that is used. A high traction coefficient is required even at high contact temperatures. The temperature of a typical traction drive can rise to over v 140 C. Besides the traction requirements at high temperatures, traction fluids must not deteriorate when subjected to repeated shear or oxidation conditions. It has been indicated by traction fluid researchers ([10,36]) that naphthenic base compounds and several types of synthetic fluids have better traction characteristics than paraffinic or aromatic compounds. Research on traction fluids has also indicated that naphthenic compounds provide a higher rotational barrier and stronger molecular stiffness compared to paraffinic or aromatic compounds [21]. Other variables affecting traction properties include rolling speeds, lubricant temperature range, quality of the roller surface finish, Hertzian contact pressure, degree of spinning, and the geometry of the rolling bodies. A substantial amount of CVT research is focused on traction fluid formulation and evaluation to determine whether the fluid can meet traction drive system performance requirements. Future research will be focused on traction fluid formulation to optimize overall system performance and to apply the traction fluids to more automotive applications. 2.2.3. Universal and constant velocity joints Universal joints and constant velocity joints [26] are used to transmit power from the main driveshaft to the vehicle’s wheels). There are several different designs for joints. All have the ability to transmit power between two shafts with some degree of axial misalignment. The joints may be of the fixed or plunging type. The plunging type incorporates a spline coupling to allow axial approach of the shafts. Typically the joints consist of an array of rolling elements held by a retaining cage between two raceways. The whole assembly is packed with grease and sealed inside a rubber boot. The contacts within the joint are between the rolling elements (balls, needles, or rollers), the raceway grooves and the cage. The rolling elements are usually made from high carbon steel which is through hardened. The raceways are made from medium carbon steel, and the load bearing areas are induction hardened or carburized. When the shafts are aligned, the

rolling elements are stationary. When the shafts are misaligned, the rolling elements undergo a low oscillation reciprocating motion. The load on the ball race contact is high, and the entrainment velocity is low. Since the surfaces tend to be relatively rough, boundary lubrication is the dominant lubrication regime. In general, the most common failure mode in the field is by damage to the boot. This results in loss of lubricating grease and failure by gross damage to the joint components. However, contact fatigue is also observed. Cracks can initiate at the surface and propagate to form spalls in the near surface region. Extended use of the joint will result in wear of the balls and raceways leading to loss of dimensional accuracy. 2.2.4. Wheel bearings Traditional rolling element bearings are commonly used in the wheel hubs. These standard manufactured components are hardened high carbon steel, machined and ground to a high tolerance and surface finish. In general deep groove bearings or a back-to-back angular contact bearing assembly are used. The bearings are packed with grease on assembly and sealed with elastomeric lip seals. The most common wheel bearing failure mode is by damage to the surface caused during incorrect fitting or following impact during operation (the wheel striking a curb). The surface damage can generate an initiation site for rolling contact fatigue failure to propagate. 2.2.5. Drive chains Some automotive engine designs use chain drives for timing and driving ancillary components. Chains have the advantage of a high capacity, increased durability, and are relatively cheap and simple. The basic components of a chain drive are the chain, sprockets, chain guides, and tensioners. There are two main types of drive chain: the roller chains consist of link plates with a pin, bush, and roller assembly; silent chains consist of an array of shaped link plates and pivot pins. The chain is lubricated by two means, firstly by splash as it dips into the sump during its travel and secondly through jets which impinge onto the sprocket/chain entry. The rollers in the case of a roller chain, and the link plates in the case of a silent chain, locate in the teeth of the sprockets. In the roller chain the pin slides inside the bush during articulation, while the roller rolls across the sprocket tooth. The pin bush contact can therefore undergo excessive wear causing chain elongation. For this reason the pins and bushes are usually case hardened and ground smooth, while the link plates are usually unhardened steel. With silent chain the link plate slides across the sprocket tooth; this can result in wear of the link plate surface.

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The chain drive is subjected to dynamic loading. Chain guides and tensioners are incorporated to reduce the associated vibration. The guides are generally made of a polymer (typically Nylon 66) and either bolted in place or loaded against the chain by a spring or hydraulic tensioner. The chain tension and impact forces between the chain and guides can result in wear of the guide faces. Wear debris or dust in engine oil can become embedded in a polymeric tensioner and can act as an abrasive agent on the timing chain. Fuel components or fuel reaction products may enter engine oil and may soften a polymeric tensioner. Wear occurs on the guide face where it contacts the link plate edge, forming ‘tramlines’. In some cases, this continues until the grooves formed are deep enough that the rollers make contact with the guide; the load is then distributed to a greater extent and wear is reduced.

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. Develop a quantitative understanding of failure mechanisms such as wear, scuffing, and fatigue. This is important both for developing improved computational design codes and for developing bench tests to predict accurately and reliably the tribological behavior of full-scale automotive components. . Develop a variety of affordable surface modification technologies [39] that are suitable for various vehicle components that are used with different fuels or lubricants under a variety of operating conditions. . Develop a better understanding of the chemistry of lubricants and how additives affect the interactions between lubricants and rubbing surfaces. This will provide a foundation for developing new lubricants that will be longer-lasting, environmentally friendly, capable of handling increased soot and acid loading from EGR (exhaust gas recirculation), compatible with catalysts, and compatible with new, lightweight non-ferrous materials.

2.3. Future development trends 2.3.1. Automotive tribology Future recommendations for automotive tribology development and desirable areas for future development trends including advanced technical development, and research goals related to fuel efficiency, emissions, durability, and profitability of powertrain systems are as follows:

Stringent federal legislation calling for better fuel economy and reduced emissions is the driving force for improved fuel efficiency and development of new engine technology. Among the various approaches for improving fuel efficiency as shown in Fig. 13, the use of energy conserving engine oil is the most economic way in the automotive industry to acquire the necessary gains, compared to complicated hardware changes [38].

Fig. 13. Powertrain components and applications for minimizing total energy power loss, achieving fuel efficiency, and engine reliability are closely related to friction and wear issues.

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In addition, design and legislative pressures for cleaner, more efficient engines with higher specific power outputs is forcing tribological engine components to be operated with generally thinner oil films. One notable trend is the move toward lower viscosity engine oils, e.g. SAE (Society of Automotive Engineers) 5W-20 and 0W-20, in an effort to improve fuel economy. While this helps to reduce friction losses, it also leads to potential durability problems and a more critical role for the surface topography of engine components [29]. In addition, there is a drive to extend engine service intervals. The engine has therefore to withstand an increasingly contaminated and degraded lubricant. The ability to incorporate more and more aspects of the physical behavior of lubricants into analytical modeling is an important and a fast developing field [5]. Uppermost in this regard are the reduction in viscosity at high shear rate, particularly with polymer-containing multi-grade lubricants, the rise in viscosity at elevated pressure and the boundary friction and wear behavior in the mixed and boundary lubrication regimes. Models of the rate of degradation of oil additives are also becoming available. One of the biggest challenges facing the engine tribology community is to make an effective link between the physical tribology of the components [40] and the complex chemical behavior and degradation of engine lubricants, both in the bulk fluid [17,23] and at the surface [19]. The engine oils are expected to last longer and simultaneously reduce engine losses. 2.3.2. Light-weight materials development Industrial researchers are developing light-weight materials such as non-ferrous materials (Al, Mg) for engine and drivetrain materials to replace the current heavy-weight cast iron blocks. Over the last few years many automotive applications were focused on thermal sprayed aluminum liners, or metal-matrix composites to replace cast iron blocks [38–40]. However, the current automotive engine lubricant formulations are designed for cast iron blocks, not for aluminum blocks. Based on several bench and engine test results [38,39], aluminum blocks have poor wear resistance. The current aluminum surface or coatings are not compatible with engine oil formulation. The aluminum cylinder bores require cast iron sleeves inside the cylinder bores. These are expensive to manufacture, however. At

Fig. 14. Comparison of density of materials.

present, researchers are developing wear and scuffing resistant coatings on piston skirts or aluminum cylinder bores for alternatives. In addition, a new class of amorphous carbon coatings with exceptional low friction and wear properties was developed for reducing wear and improving durability of engine components (Erdemir et al. 2000, 2001). Light-weight materials have been widely used in vehicles. Vehicle weight has been dramatically reduced in recent years as shown in Figs. 14 and 15. In North American vehicles including those produced by the big three the aluminum and magnesium content has increased in the last few years. General Motors has the highest Al and Mg content in vehicles. In addition, the density of light-weight materials is always a big concern for material strength. The general trend is as shown in Fig. 15, which illustrates the lower density associated with the light-weight materials. Recent industrial developments include high strength and high density of composite materials, high volume liquid molding and hydroforming technology, structural adhesive boding, and the ability to mold large structural components. Researchers have also developed processing improvements for forming more complex stamped aluminum parts or panels, more robust stamping, and improved casting techniques. Recently, USCAR participants are developing light-weight Al and Mg materials for auto parts and car bodies. Over the years, several industrial-academic consortia have been formed to develop new generation vehicles utilizing advanced powertrains, lighter, stronger materials, and environmentally friendly lubricants.

Fig. 15. Trends of average vehicle weight in pounds.

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2.3.3. Nanotribology development and industrial needs The automotive components in lubricated contacts, as the component scale moves from macro to micro to nano, are dominated by surface adhesion forces that normally are not well-understood by mechanical means. Therefore, nanolubrication needs to be taken into account to determine unique mechanisms which are different from those in conventional lubrication. Recently, Stephen Hsu of NIST organized a nanotribology workshop to discuss this nanotribology approach and compare this new concept with the traditional lubrication methods [16]. In addition, several researchers have proposed various nanometer scale lubricating film designs as a means to control the properties of surfaces at nano/micro scales. As the scale of the device becomes smaller and smaller, the need to lubricate and protect the surfaces of such components increases. The authors also conducted a literature search in an effort to identify the industrial needs for nanotribology or nanolubrication technology development. The following needs were identified: 1 Nanostructured materials ‘‘by design’’ with stronger, harder, self-repairing, and safer characteristics: . Structural carbon and ceramic materials 10 times stronger than conventional steel. . Polymer materials three times stronger than conventional materials and which will not melt above v 100 C. . Multi-functional materials to reduce friction, wear, and corrosion. . Addition of nanoparticles to convert aluminum alloys to wear and scuffing resistant materials. . Nanocoatings on metallic surfaces to achieve super-hardening, and low friction. . Nanoparticle-reinforced materials that can replace metallic components in cars. . Models/simulations incorporating multi-scale computation and leading to materials by design. 2 For efficient energy conversion and storage . Catalysts for fuel cells, such as zeolites with pore sizes in the nanometer range, which can serve to more efficiently break down or crack large hydrocarbon molecules to form gasoline or synthetic lubricants. . Thermoelectric energy-conversion devices and hydrogen storage devices for future fuel cells.

2.4. Automotive lubricants 2.4.1. Introduction of automotive lubricants The lubricant must protect the automotive component that it lubricates. In some cases this protection is in the form of a fluid film that keeps opposing surfaces

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separated. In other cases, the lubricant provides wear protection by forming a chemical film on a surface, to generate boundary lubrication protection. Automotive lubricants protect against corrosion by virtue of alkaline agents to neutralize acids that form in hot spots. The lubricant transports protective chemicals to the sites where they are needed and transports waste products away from the sites where they are generated. This diversity of function strongly influences the choice of chemical composition and physical properties for a given type of lubricant and dictates that a variety of lubricants are required to perform the various lubrication functions in a vehicle. That is, different engine components require substantially different kinds of protection. Transmission fluid must withstand high temperatures and loads. Engine oil must remain effective despite the fact that fuel and combustion products can enter the oil under all driving conditions. In particular, during short-trip cold-start conditions, fuel and water may be present in oil at concentrations greater than 5%. During high temperature operation engine oil must not evaporate or degrade excessively. Various types of bearings require the presence of a fluid film that separates a rotating shaft from its opposing bearing surface. Brake fluid must remain in place and continue to function, even if the vehicle drives through pouring rain or encounters puddles of slush on the road. Under highly loaded, high temperature conditions, fluid film lubrication may not be sufficient to provide complete wear protection such as those experienced by piston rings at the top of the cylinder under the influence of the burning fuel. In such cases, the lubricant must contain additives that interact with rubbing surfaces to form antiwear films. These chemical films form on hot iron surfaces during vehicle operation as long as the lubricant has not degraded excessively. However, these chemical films may be partially scraped away during severe engine service. Once the additives that provide this chemical film (typically, zinc dialkyl dithiophosphate, ZDP, and other antioxidant/antiwear agents) are sufficiently degraded, the engine oil needs to be changed, otherwise engine wear will accelerate during use. Automotive lubricants must also inhibit corrosion in addition to providing a fluid film and a chemical surface film, since partially burned fuel can be acidic. Engine oil itself, when exposed to hot spots in the presence of oxygen, can form acids that promote engine corrosion, especially if the oil’s alkaline anticorrosion additives have been degraded as a consequence of extended use without an oil change. A lubricant typically reduces friction, but in some cases an automotive lubricant is designed to provide a desired amount of traction, as is the case with traction fluids in some CVTs as described earlier. Traction

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fluids have a chemical structure that, under stress, forms a three-dimensional network that resists flow. A mixture of lubricant and air-conditioning fluid must have sufficient lubricating properties to protect the air-conditioning pump, but the fluid/lubricant combination must not become aggressive to hoses or to other materials in the air-conditioning system. These examples illustrate the complexity of the issues that must be addressed when attempting to provide adequate automotive lubrication. In some cases, standard test methods document whether a lubricant provides appropriate protection to the hardware. In other cases, vehicle, component, or lubricant manufacturers designate the lubricant that, in their estimation, best fits a given application. 2.4.2. Standard test methods for automotive lubricants A variety of standard tests are needed to guarantee that a given automotive lubricant performs as it should. Those tests include: engine tests that mimic various driving styles under a variety of test conditions, physical and chemical characterization of the lubricant, corrosivity of a used lubricant, wear protection provided by the lubricant, and remaining effectiveness of the lubricant’s additive package. Additional desired information with regard to lubricant characterization includes: tendency to form deposits, extent of oil oxidation, fuel efficiency, and emission system protection (including limits on phosphorus in engine oil, since phosphorus can adversely affect catalyst performance). For some lubrication functions, standard test methods are available to ensure that the various products in the marketplace provide the desired protection. For other lubrication functions, there are no standard tests, but a vehicle manufacturer may require that the lubricant of choice must pass the manufacturer’s (or some other) designated tests [32]. Each different type of automotive lubricant has its own unique test methods that address the issues relevant to the service needs of that particular lubricant. For example, engine tests are used to determine whether a new candidate engine oil formulation meets current certification requirements. Bench tests can often be used to indicate whether certain fundamental lubricant properties are within a desired range (for example, tests for viscosity, pour point, volatility, shear stability, wear rate, wear depth, wear mechanism, etc.). Chemical and physical tests measure the composition and properties of a lubricant, the changes that occur as the lubricant ages, and the characteristics of any contaminants or wear debris that may have entered the lubricant. The following sections describe major issues relative to various types of automotive lubricants and the test methods that have been developed to address those issues [32,37].

2.4.3. Automotive engine oils 2.4.3.1. Introduction. One of the most difficult challenges facing automobile and lubricant manufacturers is to identifying appropriate test methods for representing evaluation of engine oils. The reason for the difficulty is not because of any inherent characteristics of engine oil. The challenges in the development of engine oil arise because the environment in which engine oils must operate is particularly harsh. Combustion gases, fuel and water in the oil, and outside contaminants (dirt, sand, other airborne materials) accelerate oil degradation, cause unique filtration problems, and result in much shorter change intervals compared to other typical automotive lubricants. Engine oil properties and composition also influence vehicle fuel economy (as is the case for transmission fluids and rear axle lubricants), and engine oil even influences vehicle emissions, as a consequence of emission system contamination and catalyst poisoning effects. A lot of attention has been focused on lubricant properties and performance tests in the technical literature as well as in the standard test-development arena. The following discussions describe engine oil performance designations (from both a historical and a current perspective) and standard test methods for evaluating engine oil performance, including both engine and bench tests. The discussions are limited primarily to performance designations and associated test methods commonly used in North America. 2.4.3.2. Engine oil performance designations—past and present. Many organizations have contributed to the development of engine oil performance standards over the years. Among these organizations is the American Society for Testing and Materials (ASTM), the American Petroleum Institute (API), the Engine Manufacturers Association (EMA), the Coordinating European Council (CEC), the Japan Automobile Manufacturers Association (JAMA), and the International Lubricant Standardization and Approval Committee (ILSAC). ILSAC includes automotive and engine builders JAMA, EMA, and the former American Automobile Manufacturers Association (AAMA, whose members were General Motors, Ford, and Chrysler). When AAMA was dissolved in 1999, it was replaced by a new organization—the Alliance of Automobile Manufacturers (the Alliance), which includes the former members of AAMA plus six other manufacturers. The Alliance, however, is not actively involved in development of automotive lubricant standards. The designation of engine oil on the basis of performance began in 1947. Prior to that time, oils were differentiated only on the basis of viscosity. In 1947, API (American Petroleum Institute) adopted a classification system based on the intended use of the oil.

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Three types of oil were defined: regular (termed ML), Premium (MM), and Heavy Duty (MS). Regular oils generally were straight mineral oils, Premium oils contained some oxidation inhibitor (but no other performance additives), and Heavy Duty oils were blended with oxidation inhibitors and detergents/dispersants. This system was modified in 1952 to create different performance requirements for diesel oils than those for gasoline engine oils and to rank the severity of the service. However, many vehicle and engine manufacturers found it necessary to add additional performance requirements to ensure adequate protection for their engines even with the ML, MM, and MS designations indicating the quality of the engine oil. This led to the development of lubricants tailored to a given manufacturer, and complicated the manner in which engine manufacturers informed their customers which oils were currently recommended. Finally in 1969 three organizations—API (American Petroleum Institute), ASTM (American Society for Testing and Materials) and SAE (Society of Automotive Engineers, now called SAE International)—cooperated to establish a performance designation system that is still being used. In this system, two series of engine oil performance and service categories were established—the ‘‘S’’ series (for use with gasoline-fueled, 4-stroke-cycle, spark ignition engines) and the ‘‘C’’ series (intended for use in compression–ignition diesel engines, both 2-stroke-cycle and 4-stroke-cycle). Table 2 shows the evolution of light-duty gasoline engine oil performance designations. A similar evolution occurred for heavy-duty diesel engine oil performance. A detailed description of the history of both gasoline and diesel engine oil performance categories can be found in SAE Standard J183, ‘‘Engine oil performance and engine service classification’’. The SA through SD designations listed in Table 2 were adopted in 1971 to describe oils which had been available previously and which were available in 1971. API category SA was adopted to describe oils generally Table 2 A brief history of engine oil performance designations Performance designation

Period of use

ML, MM, MS SA SB SC SD SE SF SG SH/ILSAC GF-1 SJ/ILSAC GF-2 SL/ILSAC GF-3

1950s and later 1900–1930 1931–1964 1964–1967 1968–1971 1972–1979 1980–1988 1988–1992 1993–1996 1996–2001 2001 and beyond

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available in North America between 1900 and 1930. These oils contained no performance additives. Such oils had previously been designated as ‘‘Regular’’ or ‘‘ML.’’ SA oils can still be found in some retail establishments that sell quarts of oil for make-up purposes. API category SB was adopted to describe oils generally available in North America between 1931 and 1963. These oils were comparable in performance to oils known as ‘‘Premium’’ or ‘‘MM’’ prior to 1971. They contained a minimal level of performance additives. API category SC described oils typically available between 1964 and 1967 in North America. These oils were probably the first oils to contain a full range of performance additives, and were designated ‘‘MS’’ prior to 1971. API category SD described oils of the performance level generally available in 1971. Although these oils were also described as ‘‘MS’’ prior to 1971, they generally contained a higher additive treatment level than API SC oils. Table 1 shows the historical development of these engine oil categories. API ‘‘S’’ categories (SE through SL) were developed as needs arose for greater engine protection. API SE oils addressed high temperature engine oil thickening problems in the field. API SF oils addressed customer concerns related to engine wear. API SG oils addressed oxidation and sludge formation concerns which had arisen in the field. Throughout this period (1972–1992), automotive engines had undergone tremendous changes to accommodate increasingly more stringent emission control requirements, both nationwide and in California beginning in 1972, and US Corporate Average Fuel Economy (CAFE) requirements originating in 1978. The desire of vehicle manufacturers to specify oils which might help them address these requirements, as well as their desire to have a greater voice in establishing engine oil performance specifications, led to the formation of ILSAC (International Lubricant Standardization and Approval Committee) in 1987. Originally a union between the Motor Vehicle Manufacturers Association (MVMA) and the Japan Automobile Manufacturers Association (JAMA) for the purposes of establishing lubricant standards, ILSAC has expanded to include the EMA. MVMA later became the AAMA, and subsequently became the Alliance of Automobile Manufacturers (the Alliance) in 1999. However, only the original three members of AAMA (General Motors, Ford, and DaimlerChrysler) retained membership in ILSAC when AAMA was dissolved. The first ILSAC engine oil standard (ILSAC GF-1) was published in 1993, as indicated in Table 2. ILSAC standards apply only to certain viscosity grades, namely SAE 0W-XX, 5W-XX, and 10W-XX viscosity grades, in which ‘‘XX’’ represents a viscosity designation such as 20, 30, 40, etc. ILSAC standards also include minimum levels of fuel economy performance

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and contain other requirements (most notably a maximum phosphorus level) to address emission system compatibility and other issues. Even though API categories apply to all viscosity grades and do not include fuel economy requirements, an ILSAC GF-1 oil is roughly equivalent to an API SH quality oil, but in addition the GF-1 engine oil also meets requirements allowing the GF-1 oil to be classified as energy conserving. This means that a GF-1 engine oil probably provides slightly better fuel economy than an SH-quality engine oil. The ILSAC GF-2 standard is similar to API SJ, and has somewhat stricter fuel economy requirements than GF-1 oils. The ILSAC GF-3 standard is similar to API SL, but again, GF-3 oils must provide better fuel economy than GF-2 engine oils. The ML, MM and MS Categories, as well as API SA through API SG, have been declared technically obsolete by SAE. This was done because many of the engine tests necessary to demonstrate that oils meet the performance requirements of these categories cannot be conducted, since reference fuels, reference oils, and engine parts for these tests are no longer available. Because API SH, SJ, and SL oils can be used in applications in which oils having the earlier designations were required, SAE made the decision to declare these earlier categories technically obsolete. API SH, while not technically obsolete, is generally not used except in combination with a ‘‘C’’ category, to describe oils intended for mixed fleet (i.e., diesel and gasoline) service. In such cases, the ‘‘C’’ category designation usually precedes the SH designation. 2.4.3.3. Engine oil performance tests. Beginning with oils designated as API SB, all of the oil performance categories described in Table 2 were or are defined by a series of engine dynamometer performance tests (usually termed engine sequence tests) and bench tests. The complete description of each of these performance categories (both the current categories and those declared technically obsolete) is provided in SAE Standard J183, ‘‘engine oil performance and engine service classification’’. Only the tests used to define the performance categories which became effective in 1996 (API SJ and ILSAC GF-2) and the tests used in the categories effective in 2001 (API SL and ILSAC GF-3) are discussed in this section. Some of these tests have been developed into ASTM Standard test methods; others are in the process of being developed into ASTM Standards. It is not a requirement that all tests used to define an engine performance category be ASTM Standards. However, those tests which are not ASTM Standards are usually advanced to ASTM Standards during the lifetime of a given category. ASTM Subcommittee D02.B0 on automotive lubricants maintains a section devoted exclusively to converting test

methods developed for new lubricant performance categories into ASTM Standards. 2.4.3.4. Issues addressed in test method development. Examples of the types of questions that need to be addressed to ensure that a given engine oil will protect an engine in various types of service are as follows: . If the engine is run at high engine speed and high oil temperature, – Will the engine oil thicken excessively? – Will there be sufficient wear protection? – Will harmful deposits form? . If the engine is driven in city service, will the oil provide sufficient sludge protection? . If the engine is driven in extreme short trips in a winter climate, will the oil provide adequate corrosion protection when fuel, water, and fuel reaction products condense in the engine oil? Each bulleted item in the previous paragraph represents a different driving style: high temperature highload, city service with oil fully warm, and extreme short trip service in which the engine oil never warms completely. Each of these driving styles is associated with a specific ASTM standard test method to ensure that an engine oil provides sufficient protection for that particular type of service. An engine oil must pass all of these tests before its manufacturers are allowed to display a label indicating that the oil meets current specifications. Easy freeway driving (an additional driving style) is not represented in a standard test, since it is assumed that the tests representing the other driving styles are sufficient to characterize lubricant properties. In addition to measuring the effects of different driving styles, an engine-oil test method must create a test environment such that changes in the test fluids and test hardware are generated via the same fundamental mechanisms as occur under the real-world operating conditions that the test is designed to emulate. A given engine test environment should be related to a realworld driving condition in at least three ways: . The chemical changes that take place in the test should be comparable to those that occur in the real world. . The materials used in the test should be equivalent or identical to those in an engine. . The temperatures and loads of the test should be appropriate for the type of service being emulated. For a given test method, a desired chemical environment is created by utilizing an appropriate fuel (including its degradation products) and generating oil degradation products that are of the same type as those generated in the driving style of interest. A desired

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material environment is created by using engine and hardware materials that are representative of the engines that the test method is designed to protect. In addition, hardware should be chosen for the test method such that surface reactions (catalytic reactions and varnish or deposit formation) are chemically similar to those that are generated in an engine environment. A desired thermal environment means that test temperatures, even if they are severe, are not so severe that fundamental mechanisms of damage have changed. Creating an appropriate test environment, but also finding test conditions such that the test does not require an inordinate amount of test time, is often difficult to achieve. An example of this difficulty is as follows. If a lubricant test method is designed to determine the corrosive effects of fuel degradation products on engine materials, the test must be conducted at a temperature such that those corrosive fuel reaction products will not boil off, but will remain in the engine oil to create corrosive conditions. If a test temperature differs significantly from operating temperatures, an investigator must determine whether or not any apparent correlation is accidental. In addition, leaded fuel typically contains chlorine and bromine compounds that can contribute to engine corrosion. If leaded fuel is used in an engine test that simulates a given type of vehicle operation, but current vehicle operation uses unleaded fuel, one must determine whether any correlations observed between the leaded-fuel test and realworld operation are real or accidental and whether the chemical environment in the engine test simulates that in the real world. Such correlation is often difficult to prove. In other words, chemical, thermal, and material effects influence the outcome of a test. Therefore, for a given test method, it is essential to ensure that the observed chemical, thermal, and material effects are appropriate and that the test method correlates to current real-world conditions. When bench tests are used in place of engine tests to measure a given performance parameter, great care must be taken to ensure correlation. 2.4.3.5. Current gasoline engine oil performance categories and associated ASTM test methods. As of the year 2002, only four designations are widely used to describe light-duty, gasoline engine oil performance: API SJ and SL, and ILSAC GF-2 and GF-3. As stated previously, the engine test and bench test performance requirements for API SJ and ILSAC GF-2 are similar with regard to engine test methods, but in addition ILSAC GF-2 oils must also meet stricter energy conserving II requirements. Similarly, API SL requirements as well as energy conserving requirements must be passed before an engine oil can be designated as ILSAC GF-3.

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The appropriate test methods for engine oils must be conducted in accordance with the requirements outlined in the American Chemistry Council (ACC) Product Approval Code of Practice. These requirements include test registration of all tests, use of only calibrated equipment and facilities, and guidelines for acceptable modifications during program development. These requirements were implemented when API SH and ILSAC GF-1 designations for engine oil were adopted in 1993. 2.4.3.6. Current diesel engine oil performance categories and associated ASTM Standard test methods. When diesel engines are operated at high temperature, the viscosity of the engine oil can increase due to oil oxidation and nitration, evaporation of the lighter ends of the oil, and accumulation of soot in the oil. As was the case for gasoline-fueled vehicles, engine oil additives can degrade during long-term use. In addition, soot accumulation can adversely affect engine wear by two fundamentally different mechanisms: by interacting with and sequestering an engine oil’s antioxidant/ antiwear agent so that the antiwear capability of the engine oil becomes diminished or by abrading a protective antiwear film that has formed on heavily loaded regions of an iron surface. These oil-related concerns are addressed using various standard tests. At the time of this writing, there were five active API diesel engine oil performance categories, designated as API CF, CF-2, CF-4, CG-4 and CH-4. (Note: a new category, API CI-4, is in the final stage of approval.) Each of these categories has at least two engine performance tests which must be conducted to demonstrate compliance with category requirements. Various bench-tests are also included. A complete description of each of these categories, including engine and bench tests, as well as associated test limits for each requirement, is documented in ASTM D 4485, standard specification for performance of engine oils and in SAE J183, engine oil performance and service classification. It is anticipated that these five categories will be reduced to three: CF-2, CG-4, and CH-4. Thus, CF and CG-4 will no longer be used. Engine sludge and oil filter plugging tendency are measured. The engine oil aeration test (EOAT), mentioned in the CG-4 specification, is also conducted. 2.4.4. Transmission fluids 2.4.4.1. Introduction. Composition and performance of transmission fluids. Transmission fluids lubricate a vehicle’s transmission. They may be composed of synthetic or mineral oil. Synthetic oil typically shows enhanced resistance to deterioration from exposure to heat. However, synthetic and mineral oil both contain additives that help maintain the desired friction properties, minimize wear, and provide corrosion inhibition to the

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transmission components. In addition, antioxidant additives are required to prolong the life of the transmission fluid. Maintaining stable frictional properties is highly important, since any change in shift characteristics, or generation of shudder after prolonged or severe use, is sure to be noticed by a vehicle owner. To provide the desired shift properties, one of the additives present in the oil is typically a long-chain hydrocarbon that has a polar group on one end. The long chain hydrocarbon portion permits the additive to dissolve in oil, and the polar group on the end is attracted to a metal surface and provides the desired amount of friction. If too much friction is generated, the transmission may not shift smoothly. If too little friction occurs, the transmission may slip. Thus, great care must be taken to ensure that an appropriate additive is chosen for this important function. In addition to appropriate friction characteristics, it is also essential to reduce wear and promote long transmission life. If suitable antiwear additives are not present, there will likely be excessive wear of the transmission under harsh operating conditions. In summary, fluid degradation during prolonged service should not be extensive enough to compromise transmission performance, and wear rates must remain low so that excessive loss of material does not cause shift characteristics to deteriorate. Various standard test methods are available to ensure sustained transmission performance. Tests for automatic transmission fluids include measurement of physical and chemical properties: viscosity, flash point, pour point, corrosion resistance, wear resistance, friction characteristics, and resistance to foaming. Greases provide a great number of highly specialized functions in an automobile, such as lubrication of door locks, electrical connections, various types of bearings, gears, and levers. Greases typically are composed of oil (synthetic or mineral, representing 70% or more of the total composition of the grease), a thickening agent, and various additives to provide wear protection, corrosion protection, and stability. Thus, test methods for automotive greases focus on such parameters as separation of the thickener from the oil, oxidation resistance, wear protection, and ability to withstand high temperature. . Standard test methods. . Physical and chemical properties. . Chemical properties, stability, and contamination. A number of tests are available to ensure that a grease has the appropriate composition and that its properties have not deteriorated excessively during service.

2.4.5. Gear lubricants Gears must be designed to withstand high loads that are concentrated on gear surfaces. As a consequence, high temperatures may be generated at the point of contact. Despite the harsh environment, the gear lubricant must provide sufficient wear and extreme pressure protection to prevent material failure of the gears, and the gear lubricant must remain functional for extended periods of time. If the gear materials are not sufficiently robust and the lubricant is not sufficiently protective, irreversible damage may occur to the gear, which typically means that the equipment in which the gear is located will not perform as designed. Performance criteria for gear lubricants include such factors as ability to withstand the required loads and continuing ability to transmit power. Gear surface finish and gear contour must not deteriorate. In addition, the properties of the gear lubricant must remain stable, including oxidative stability, antiwear protection, maintenance of appropriate friction characteristics, low-temperature viscosity, corrosion protection, and resistance to foam formation. Various standard tests address these concerns. Other test methods for gear oils are also available from CRC (coordinating research council). The CRC L-37 method is a 24-h dynamometer test that is conducted at high torque and low speed and that assesses gear distress. CRC L 42 is a 24-h dynamometer test incorporating high speed shock loading, designed to measure resistance to gear scoring. CRC L-33 is a 7-day motored rust test in which gears are exposed to humidity in an oven. CRC L-60-1 is a test for oxidation and deposits, using motored gears over a period of 50 h. 2.4.6. Axle lubricants Requirements for axle lubricants are described in the American Petroleum Institute API-GL-5 requirements for gear lubricants. However, two of the gear tests (CRC L-37 and CRC L-32) are not required. Desirable properties for axle lubricants include viscosity in an appropriate range (for example, near 14 or 15 cenv tistokes at 100 C). Viscosity stability is measured according to ASTM D 445, standard test method for kinematic viscosity of transparent and opaque liquids (the calculation of dynamic viscosity), as described under the section on engine oils. Low temperature viscosity is measured using ASTM D 2983, standard test method for low-temperature viscosity of automotive fluid lubricants measured by Brookfield Viscometer. In this test, viscosity is measured in the temperature range v from 5 to 40 C, using a rotating spindle. The referenced document in ASTM D 2983 is ASTM D 341, viscosity–temperature charts for liquid petroleum products.

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Performance of the axle lubricant should not degrade excessively during service. The axle lubricant should not damage metal components, nor should the metal components accelerate axle lubricant degradation. In addition, axle lubricants must not damage seals. Seal performance can be measured using ASTM D 5662, standard test method for determining automotive gear oil compatibility with typical oil seal elastomers, as described previously under gear lubricants. Desirable axle-lubricant properties include fuel efficiency, thermal stability, long life, and appropriate limited slip differential performance.

2.4.7. Solid lubricants Solid lubricants are used where it is important for the lubricant to stay in place. Solid lubricants are typically composed of a solid, a binder, and additives such as corrosion inhibitors or solvents. Examples of common solid lubricants include molybdenum disulfide, graphite, and polytetrafluoroethylene (PTFE) or other fluorine-containing polymers. Molybdenum disulfide and graphite have a chemical structure that is like flat plates such that an upper plate can easily slip along the surface of the next plate beneath it. PTFE has a chemical structure that does not attract or hold a variety of common fluids such as water or oil. Solid lubricants typically have a temperature range over which they are effective. Above the optimum range, they may degrade chemically or physically. For example, according to the Tribology Data Handbook (R. M. Gresham, pp. 600–607, CRC Press, 1997), molybdenum disulfide has an upper temperature limit in v the range of 400 C, but it can withstand high loads. v Graphite can withstand 650 C and moderate loads. PTFE cannot withstand high temperatures or high loads, but it provides useful friction characteristics for such automotive components as fasteners. ASTM D 2714, standard test method for calibration and operation of the Falex block-on-ring friction and wear testing machine, provides a test method for determination of friction coefficient of a solid lubricant. Friction coefficient is defined as the ratio of the friction force, F, that resists movement to the normal force, N, that presses the two bodies together. A steel test ring rotates against a steel test block. The specimen assembly is immersed in the lubricant. A normal load of 65 kg is applied. The velocity of the test ring is 7.9 m/min. The test method contains the warning that the user must determine whether the test results mean anything for the particular application of interest. Since any given application may have different materials, different speeds, different loads, and different temperatures, an investigator would be wise not to extend any conclusions beyond those directly obtained for the test

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conditions (materials, lubricants) that were used in the test of interest. 3. Summary and conclusion (1) This paper has provided a comprehensive overview of various lubrication aspects of a typical powertrain system including the engine, transmission, driveline, and other components. We can realize that a lot of outstanding contributions in automotive tribology and lubricants areas have made to improve the efficiency and productivity of powertrain and manufacturing systems. As we face this new century, the tribological issues will be more complex than ever. Stringent federal legislation is calling for better fuel economy and reduced emissions. In addition, design and legislative pressures for cleaner, more efficient engines with higher specific power outputs is forcing tribological engine components to be operated with generally thinner oil films. Then, we need to address the issues of wear resistance and durability of our tribological system. (2) Industrial researchers are developing light-weight materials such as non-ferrous materials (Al, Mg) for engine and drivetrain materials to replace the current heavy-weight cast iron blocks. This keynote paper has described the current status and future trends in automotive light-weight materials to meet their tribological requirements. (3) This paper has discussed the current automotive lubricant development including discussion of current and anticipated future requirements of automotive engine oils. This paper has also reviewed the current standard ASTM test methods for all automotive lubricants and other compilations of automotive standards. (4) The automotive components in lubricated contacts, as the component scale moves from macro to micro to nano, are dominated by surface adhesion forces that normally are not well-understood by mechanical means. Therefore, nanolubrication needs to be taken into account to determine unique mechanisms which are different from those in conventional lubrication. In this paper, our insights and perspectives on the current development and future trends in nonotribology have been shared.

Acknowledgements The authors thank James Spearot, Shirley Schwartz, Robert Olree, Coleman Jones, and Dennis Meyers of General Motors for their suggestions and recommendations for the key development areas. The authors also acknowledge their reviews of this paper.

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