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Optimisation of piston assembly tribology for automotive applications
Tribological Research and Design for Engineering Systems D. Dowson et al. (Editors) 9 2003 Elsevier B.V. All rights reserved.
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Optimisation of Piston Assembly Tribology for Automotive Applications M. Priest Institute of Tribology, School of Mechanical Engineering, The University of Leeds, UK
Current trends in engine design, such as higher operating temperatures and the enhanced use of exhaust gas recirculation (EGR) in diesel engines, and legislative and consumer pressures on lubricants, such as the requirement for reduced levels of phosphorous and extended oil drain intervals, are detrimental to the piston assembly. Future engines will require more wear resistant surfaces in the piston assembly with minimal scope for rapid running-in of the components to establish optimisation on behalf of the engine designer and compatibility issues for the lubricant formulator as iron based materials become less common. In this paper a review is presented of the current understanding of piston assembly tribology in order to contribute to methods of future design optimisation.
1. INTRODUCTION The piston assembly is a key engineering system in the reciprocating internal combustion engine and central to its efficient and reliable operation. It is at the heart of the engine, forming a vital link in transforming the energy generated by combustion of the fuel and air mixture into useful kinetic energy. A typical piston and piston ring pack from a modern automotive engine is shown in Figure 1. The top two rings are referred to as the compression rings and utilise gas pressure to supplement the inherent elasticity of the ring to maintain an effective combustion chamber seal. The top compression ring is the primary gas seal and, as the ring nearest the combustion chamber, encounters the highest loads and temperatures. It usually has a barrel-faced profile with a wear resistant coating such as chromium, flame-sprayed molybdenum, metal or cermet mixtures 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. As such it has a taper-faced, downward scraping profile which is not normally coated. The bottom ring in the pack is the oil-control ring which has two running faces, or lands, and a spring element to enhance radial load. As its name suggests, the role of this ring is to limit the amount of oil transported from the crankcase to the combustion chamber and by design it has no gas sealing ability.
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The periphery of the oil-control ring lands and, occasionally, the flanks are often chromium plated. Neale [1] gives an excellent summary of the piston ring types in common use. The primary role of the piston ring pack is to maintain an effective gas seal between the combustion chamber and the crankcase. The rings of the piston ring pack, which together effectively form a labyrinth seal, achieve this by closely conforming to their grooves in the piston and to the cylinder wall. The small quantity of gas that does find its way into the crankcase, blow-by, is normally piped back to the inlet valve and fed back into the cylinder. In addition to causing a dramatic increase in pressure, the combustion event generates a large amount of heat. Much of this thermal energy is convected into the piston causing a marked increase in the temperature of the piston, which is dissipated by heat transfer to adjacent components and the engine coolant. The secondary role of the piston ring pack is to transfer this heat from the piston into the cylinder wall and thence into the coolant. The final function of the piston ring pack is 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 the oil consumption of an engine and leads to an increase in harmful exhaust emissions as the oil mixes and reacts with the other contents of the combustion chamber [2]. The desire to extend service intervals of engines and minimise harmful exhaust emissions to meet ever more stringent legislative requirements, means that the permissible oil consumption levels of modern engines are very low compared to their predecessors of twenty years ago [3]. The piston ring pack must fulfil these three roles with a minimum of frictional power loss, most notably at the sliding interface with the cylinder wall, and a minimum of wear in order to maximise component life. Unfortunately, the piston assembly is one of the largest sources of friction in the internal combustion engine over the normal range of engine speeds and loads encountered in service [4-7]. Exact figures vary from engine to engine, but typically the piston assembly, comprising both the piston rings and the piston skirt, accounts for 40-50% of total engine friction. Piston ring pack friction losses are greater than those of the piston skirt at low to moderate engine speeds but the situation may be reversed at high engine speeds due to the large
wetted area of the piston skirt contributing to viscous friction [8]. In terms of wear, there is insufficient understanding of the interaction with the lubrication process. So, even though manufacturers can produce rings that have an excellent life expectancy, these components may be far from optimum from a lubrication and friction standpoint [9]. Interactions between the piston, piston rings, cylinder and the lubricant are complex and sensitive to a whole range of engine configuration and operating conditions. The need to maximise gas sealing in the combustion chamber and minimise oil consumption with low friction and wear presents the engine developer with a number of directly conflicting design parameters. Those technology drivers for the automotive industry relevant to engine tribology were recently addressed by Korcek et al [10] and are summarised in Figure 2.
Figure 2: Technology drivers for a major automotive vehicle manufacturer, after Korcek et al [10] The need to produce vehicles which use resources wisely, have minimal impact on the environment and that customers will want to own and drive presents major challenges to engine designers and lubricant formulators. For example, the automotive industry is currently developing the next generation of engine lubricants, with important and potentially conflicting implications for piston assembly tribology. The new lubricant specification, known as ILSAC GF-4, places strict demands on fuel economy and its retention as the oil ages in service and the level of phosphorus permitted in the formulation. Phosphorus is known to contaminate catalytic after-treatment devices in the exhaust,
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reducing their efficiency, but in the form of zinc dialkyldithiophosphate (ZDDP) is the most costeffective anti-oxidant and anti-wear agent currently available. The conflicting requirements for enhanced fuel economy, emissions system compatibility and extended useful lubricant life demands improvements in the
between the asperities on the two opposing surfaces, which is set equal to four times the composite root mean square surface roughness (a) in accordance with the stochastic mixed lubrication and empirical boundary lubrication models incorporated in the software. This limit is the transition between the mixed and full fluid film lubrication regimes. 10
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Novel materials and coatings to control wear with less opportunity for running-in. A shift away from iron based materials is likely.
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A major personal reason for writing this paper was to contribute to methods of future design optimisation for piston assemblies.
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2. SOFTWARE TOOLS The tribological design of modern piston assemblies for automotive applications is greatly assisted by the use of sophisticated predictive mathematical models. Numerical analyses for the piston and the ring pack are generally discrete models. Predictive analyses for the dynamics and lubrication of sliding interface between the piston skirt and the cylinder wall have been proposed by a number of research groups, for example [8, 11-13]. The primary outputs are piston secondary motion, tilting and lateral translation within the bore, and friction. Attempts have also been made to predict piston slap, as a precursor of piston derived noise [14]. Piston ring lubrication models are numerous, for example [15-19], reflecting the ongoing problems encountered by industry with piston ring lubrication and associated phenomena. They provide vital information on lubricant film thicknesses, component dynamics, friction losses and lubricant transport paths. Figure 3 presents some exemplar film thickness results from such an analysis for the top compression ring and oil-control ring of a diesel engine [9]. The data is presented as the minimum lubricant film thickness between the piston ring and cylinder at any instant (hmin) plotted against crank angle where 0 degrees is top dead centre firing. Also shown is the upper limit of surface contact
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Figures 3 (a) and (b) show the film thickness behaviour of the top compression ring and oilcontrol ring in isolation, assuming a plentiful supply of lubricant at the inlet, so called fully-flooded lubrication. In reality, however, the performances of adjacent rings are interdependent as the lubricant available at the inlet is to a great extent that left behind on the cylinder by the preceding ring. Figure 3(c) shows the effect this interaction has on the predicted film thickness for the top compression ring, drastically reducing the overall film thickness magnitudes throughout the engine cycle. As a consequence, this consideration of oil flow continuity within the ring pack is known as starved lubrication. Such software tools for pistons and piston rings are in widespread use in the automotive industry and commercial software is available from a number of vendors. The analyses continue to become more sophisticated, building incrementally upon previous versions. For example, piston ring models have been enhanced to consider detailed lubricant rheological characteristics such as shear thinning [20]. Figure 4 highlights the significance of such behaviour.
experimental data also shown in Figure 5 is encouraging.
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The complex dynamics of piston rings within their grooves in the piston have also been considered [21]. Figure 5 illustrates the predicted influence on inter-ring gas pressures developed between adjacent piston rings, which strongly affects radial load, and the axial motion between the top ("up") and bottom ("down") of the groove. The correlation with
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The validated predictions of fluttering in Figure 5(b), an unstable axial reciprocation of the ring within the groove, are invaluable to engine designers as this phenomenon is associated with a loss of gas and oil flow control in the piston assembly. As a final example of recent advances in piston ring modelling, the detailed of modelling of Tian [22] is exemplified by the predictions of variation in piston ring film thickness around the circumference of the piston of Figure 6.
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However sophisticated such models become, their origins remain in mechanical dynamics and fluid film lubrication analyses. As such they can only perform realistic predictions when provided with component geometry and surface texture after running-in and the local lubricant performance characteristics at the engine running time of interest. Unfortunately such data is not readily available to the engine designer. The main reason for this shortcoming is the complexity of the operating environment of the piston assembly as illustrated in Figure 7.
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....R I N G F A c E L U B R I C A T I O N 9 friction 9 lubricant transport 9 cavitation 9 links to wear
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There are clearly a large number of inter-related mechanical, metallurgical, rheological, chemical and environmental parameters influencing the performance of piston rings in an internal combustion engine.
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Figure 7" Complex operating environment of the piston ring At the ring face in Figure 7 there are uncertainties surrounding the nature of the lubricant present in the interface and how this changes with
running time. Similarly the complex interactions between lubrication and wear at this juncture are challenging to predict with any confidence.
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At the ring flank there is even less capability to predict long term performance and occasional reported instances of piston material transfer to the ring flank in service, so-called "micro-welding", is indicative of the severity of the tribology at this interface [23, 24]. The highly dynamic operating conditions in the piston assembly and the large range of materials and surface treatments used for pistons, piston rings and cylinder bores, simply adds to the difficulty of optimising design using predictive software tools. As examples of ways to improve this situation, two complexities at the ring face are considered further. 3.1. Wear and Lubrication at the Piston Ring Face and Cylinder Bore Interface
Optimisation of piston assembly tribology is currently achieved through early-life wear of the tribological surfaces, the process of running-in. Most notably at the cylinder wall and the piston ring outer surface that slides against it, the piston ring face. This reveals not only the desire to minimise manufacturing costs but also the shortfall in our understanding of the tribology of this system. The interactions between lubrication and wear during the running-in process are complex with significant changes in the 9
Overall shape of the piston ring face
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Surface texture of the piston ring and the cylinder wall
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Surface texture of the piston skirt, to a lesser extent.
The modelling of wear at the piston ring / cylinder wall interface receives relatively sparse attention in the literature with just a handful of notable analyses [9, 25-29]. An example from the work of the current author [9] is given in Figure 8. The measured and predicted wear of the top compression ring from a diesel engine after 120 hours running at constant load and speed are compared. This is a barrel-faced, chromium-plated, cast iron ring. The measured new and worn ring profiles are shown overlaid using two different methods. Firstly by geometry, visually matching the unworn outlying regions of the profile, and secondly by mass, converting the weight loss of the ring to an evenly distributed volume loss around the ring circumference. The predicted ring profile, after 120 hours simulated running with multiple interactions
between wear and lubrication models is also shown and this correlates well with the measured data. In terms of assessing the significance of this wear on performance, the new ring was predicted to operate for 41% of the engine cycle in the mixed or boundary lubrication regime whereas for the worn profile this fell to just 7% [9]. The friction power loss was also predicted to decrease, by some 22%. 20
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Figure 8: Predicted and measured top compression ring profiles, after Priest et al [9] Such models offer the possibility of predicting piston assembly performance variation with running time and their development should continue. They are currently limited by over-reliance on empirical wear data from laboratory tribometers, interpretation of such data in the mixed lubrication regime and the inability to predict at the microscopic scale, for roughness as well as profile change. To add further difficulties, many of the coatings and surface finishing processes used in engines have surface topographies, even after running-in, which are not modelled satisfactorily using standard fluid film and mixed lubrication models [30]. New strategies are therefore required to analyse the response of such surfaces. To illustrate this point, consider the rapid, early-life wear of the second compression ring measured in a modern gasoline engine, Figure 9(a). This is a plain cast iron, Napier scraper ring with compression and oil-control functions. The complex surface topography of this piston ring as new, with the fine turning marks deliberately left in place, and after 2 hours rapid wear is apparent. Figure 9(b) gives the measured brake specific fuel consumption (bsfc) of the engine for the early stages of running and shows a dramatic fall during the first hour.
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Although, there are other mechanisms involved, such as changes in the lubricant, it is argued that this dramatic reduction in fuel consumption, and hence friction, is mainly attributable to the rapid wear of the second compression ring [30]. The distinctive topography of the surface of this ring is not readily analysed used typical mixed and boundary lubrication models, which are stochastic. Similar problems are faced when considering the top compression ring of the same engine; a barrelfaced, spheroidal graphite cast iron ring with a flame-sprayed molybdenum coating, Figure 10. The open porous structure of the coating produces discrete features which persist even after 120 hours running [30].
Much effort has been devoted to sensitising models of piston assembly tribology to bulk lubricant properties such as shear thinning and piezoviscosity. However, precise characterisation of the lubricant present in the piston assembly presents difficulties with respect to 9
Knowledge of local temperatures
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Lubricant composition with the likelihood of fuel dilution, soot contamination and local thermal degradation
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Interaction of the lubricant with the surfaces, most notably anti-wear behaviour and boundary friction modification
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Background deterioration in the sump bulk lubricant properties with time.
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Degradation of the lubricant, especially around the top compression ring, has been a major cause for concern in recent years. Saville et al [31], Picken et al [32] and Taylor and Bell [33] all examined the chemical and physical changes in samples of lubricant drawn from the top ring zone of firing diesel engines. Clear differences were found in the degraded samples when compared to fresh lubricant, most notably a significant increase in viscosity. The degradation process is related to the residence time of the oil in the top ring groove. This in turn is dependent on the oil circulation mechanism in the piston assembly and hence the oil consumption and blow-by patterns [34]. Recent research associated with the current author has improved the prediction of lubricant transport within the piston assembly as a means to
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predict lubricant residence times [35]. The quantification and interaction of all conceivable oil transport mechanisms in the piston assembly was considered, Figure 11(a). As shown in Figure 11(b), the model is capable of predicting differences in the volume of oil present in the piston assembly of a gasoline engine with new and worn piston rings. Comparison of the flow predictions with residence time experiments using a hydrocarbon marker and sampling of oil extracted from the top ring zone of the same gasoline engine has been undertaken [36]. The results overall are encouraging although a need for more research, especially into oil mist as a transport mechanism has been identified.
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4. OPTIMISATION Given the complexity of the piston assembly system and particularly the important role of lubricant chemistry in controlling wear and lubricant rheology, empirical data from laboratory and engine tests will remain a key input to the design optimisation process in the medium term. As has been demonstrated in this paper, sophisticated software tools to predict tribological performance of the piston assembly have been developed and continue to be enhanced. However, the demands from industry for new solutions to the problems posed by this complex system are growing more rapidly. There is a danger that asthe models become ever more complicated and detailed, the developers and users lose perspective in terms of the overall aim of designing successful piston assemblies. Therefore, in parallel, more systematic engineering design tools must be developed to capture the capability of and utilise the results from the new generations of detailed mathematical models. This will ensure rapid technology transfer from research to design.
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5. CONCLUSIONS Sump
9 Validated mathematical models of piston assembly tribology have proved excellent aids to the designer for a tribological system that presents many design conflicts.
(a) Interaction of transport mechanisms
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9 The extremely complicated operating environment of the piston assembly presents major challenges when developing such predictive models. 1.0
9 Input data and modelling problems exist with respect to the evolution with time of the component surfaces, form and roughness, and the lubricant, both physical and chemical behaviour.
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9 Research is progressing rapidly but industry demands are changing even more quickly. Future generations of engines will demand novel tribological solutions. 9 To this end, alongside research, engineering tools to achieve design optimisation must be developed.
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REFERENCES
1. Neale, M.J., "Drives and Seals, A Tribology Handbook", Butterworth-Heinemann, Oxford, 1994 2. Gazzard, S.T., Eastham, D.R., Jakobs, R.J. and Lunsford, R.L., "Piston System Design for Low Emissions", Leading Through Innovation, T&N Symposium 1995, Paper 20, 1995 3. Munro, R., "Emissions Impossible- The Piston & Ring Support System", SAE Paper 900590, 1990 4. Monaghan, M.L., "Engine Friction - A Change in Emphasis", Instn. Mech. Engrs., 2nd BP Tribology Lecture, 1987 5. Monaghan, M.L., "Putting Friction in its Place", 2nd Int. Conf.: Combustion Engines - Reduction of Friction and Wear, IMechE conf. pub. 1989-9, Paper C375/KN1, 1989, pp. 1-5 6. Andersson, B.S., "Company Perspectives in Vehicle Tribology - Volvo", 17th Leeds-Lyon Symposium on Tribology- Vehicle Tribology, Elsevier (Tribology Series 18), 1991, pp.503-506 7. Taylor, R.I. and Coy, R.C., "Improved Fuel Efficiency by Lubricant Design: A Review", Jour. Engineering Tribology, Proc. Instn. Mech. Engrs., Part J, 214, J 1, 2000, pp.1-15 8. Chittenden, R.J. and Priest, M., "Analysis of the Piston Assembly, Bore Distortion and Future Developments", Chapter 10, Engine Tribology, ed. C.M. Taylor, Elsevier, Amsterdam, 1993, pp.241-270 9. Priest, M., Dowson, D. and Taylor, C.M., "Predictive Wear Modelling of Lubricated Piston Rings in a Diesel Engine", Wear, 231, 1, 1999, pp.89-101 10. Korcek, S., Sorab, J., Johnson, M.D. and Jensen, R.K., "Automotive Lubricants for the Next Millennium", Industrial Lubrication and Tribology, 52, 5, 2000, pp.209-220 l l.Li, D.F., Rohde, S.M. and Ezzat, H.A., "An Automotive Piston Lubrication Model", ASLE Trans., 26, 2, 1982, pp. 151-160 12.Knoll, G.D. and Peeken, H.J., "Hydrodynamic Lubrication of Piston Skirts", J. Lubrication Tech., Trans. ASME, 104, 1982, pp. 504-509 13. Oh, K.P., Li, C.H. and Goenka, P.K., "Elastohydrodynamic Lubrication of Piston Skirts", J. Tribology, Trans. ASME, 109, 1987, pp. 356-362 14.Wong, V.W., et al., "A Numerical Model of Piston Secondary Motion and Piston Slap in
Partially Flooded Elastohydrodynamic Skirt Lubrication", SAE Paper 940696, 1994 15.Furuhama, S., "A Dynamic Theory of Piston Ring Lubrication (lst report, calculation)", Bull. JSME, 2, 7, 1959, pp. 423-428 16.Rohde, S.M., "A Mixed Friction Model for Dynamically Loaded Contacts with Application to Piston Ring Lubrication", Surface Roughness Effects in Hydrodynamic and Mixed Lubrication, ed. S.M. Rohde and H.S. Cheng, ASME pub., 1980, pp. 19-50 17. Ruddy, B.L., Dowson, D. and Economou, P.N., "A Review of Studies of Piston Ring Lubrication", Proc. 9th Leeds-Lyon Symp. on Tribology: Tribology of Reciprocating Engines, Paper V(i), 1982, pp. 109-121 18. Jeng, Y.R., "Theoretical Analysis of Piston-Ring Lubrication Part I - Fully Flooded Lubrication", Tribology Transactions, STLE, 35, 4, 1992, pp.696-706 19. Jeng, Y.R., "Theoretical Analysis of Piston-Ring Lubrication Part II - Starved Lubrication and its Application to a Complete Ring Pack", Tribology Transactions, STLE, 35, 4, 1992, pp.707-714 20. Taylor, R.I., Brown, M.A., Thompson, D.M. and Bell, J.C., "The Influence of Lubricant Rheology on Friction in the Piston Ring-Pack", SAE Paper 941981, 1994 21. Mierbach, A., Hildyard, M.L., Parker, D.A. and Xu, H., "Piston Ring Performance Modelling", Leading Through Innovation, T&N Symposium 1995, Paper 15, 1995, pp.15.1-15.13 22. Tian, T., "Dynamic Behaviours of Piston Rings and their Practical Impact. Part 2: Oil Transport, Friction and Wear of Ring/Liner Interface and the Effects of Piston Ring Dynamics", Jour. Engineering Tribology, Proc. Instn. Mech. Engrs., Part J, 216, J4, 2002, pp.229-247 23.Barrell, D.J.W., Priest, M. and Taylor, C.M. "The Axial Motion of the Piston Ring in the Top Ring Groove of a Gasoline Engine", Tribology Research: From Model Experiment to Industrial Problem, Proc. 27th Leeds-Lyon Symposium on Tribology, Elsevier, 2001, pp.641-649 24.Barrell, D.J.W., Priest, M. and Taylor, C.M. "Impact and Sliding Wear in the Top Piston Ring Groove of a Gasoline Engine", Proc. ITC Nagasaki 2000, International Tribology Conference, Japanese Society of Tribologists, Nagasaki, 2001, pp. 2029-2034
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25.McCallion, H., Lloyd, T. and Large, J., "The Formation of Piston Ring Profiles", Tribology Convention 1972, Instn. Mech. Engrs. conf. pub., Paper No.C93/72, 1972, pp. 55-61 26.Ting, L.L. and Mayer Jr, J.E., "Piston Ring Lubrication and Cylinder Bore Wear Analysis, Part I - Theory", J. Lubrication Tech., Trans. ASME, Paper 73-LUB-27, 1974, pp. 305-313 27.Ting, L.L. and Mayer Jr, J.E., "Piston Ring Lubrication and Cylinder Bore Wear Analysis, Part II - Theory Verification", J. Lubrication Tech., Trans. ASME, Paper 73-LUB-25, 1974, pp. 258-266 28.Nautiyal, P.C., Singhal, S. and Sharma, J.P., "Analysis of Wear Mechanism of Piston Ring", International Conf. on Wear of Materials: Wear of Materials 1979, ASME, April 16-18, 1979, pp. 124-131 29. Hannoschoeck, N., "Zur Tribologie des Kolbenring", MTZ Motortechnische Zeitschrift, December, 46, 12, 1985, pp. 503-506, 509-511 30.Priest, M. and Taylor, C.M., "Automobile Engine Tribology - Approaching the Surface", Wear, 241, No.2, 2000, pp. 193-203
31.Saville, S.B., et al., "A Study of Lubricant Condition in the Piston Ring Zone of SingleCylinder Diesel Engines under Typical Operating Conditions", SAE Paper 881586, 1988 32. Picken, D.J., et al., "Lubricating Oil - Extraction and Chemical Analysis", Experimental Methods in Engine Research and Development 91, I.Mech.E Seminar, HQ, December, 1991, pp.115-120 33.Taylor, R.I. and Bell, J.C., "The Influence of Lubricant Degradation on Friction in the Piston Ring Pack", Dissipative Processes in Tribology, Proc. 20 th Leeds-Lyon Symposium on Tribology, 1993, pp.531-536 34. Burnett, P.J., "Relationship between Oil Consumption, Deposit Formation and Piston Ring Motion for Single Cylinder Diesel Engines", SAE Paper 920089, 1992 35. Gamble, R.J., Priest, M. and Taylor, C.M., "Detailed Analysis of Oil Transport in the Piston Assembly of a Gasoline Engine", Tribology Letters, In Press 36. Gamble, R.J., "The Influence of Lubricant Degradation on Piston Assembly Tribology", PhD thesis, The University of Leeds, UK, 2002
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Optimisation of Piston Assembly Tribology for Automotive Applications M. Priest Institute of Tribology, School of Mechanical Engineering, The University of Leeds, UK
Current trends in engine design, such as higher operating temperatures and the enhanced use of exhaust gas recirculation (EGR) in diesel engines, and legislative and consumer pressures on lubricants, such as the requirement for reduced levels of phosphorous and extended oil drain intervals, are detrimental to the piston assembly. Future engines will require more wear resistant surfaces in the piston assembly with minimal scope for rapid running-in of the components to establish optimisation on behalf of the engine designer and compatibility issues for the lubricant formulator as iron based materials become less common. In this paper a review is presented of the current understanding of piston assembly tribology in order to contribute to methods of future design optimisation.
1. INTRODUCTION The piston assembly is a key engineering system in the reciprocating internal combustion engine and central to its efficient and reliable operation. It is at the heart of the engine, forming a vital link in transforming the energy generated by combustion of the fuel and air mixture into useful kinetic energy. A typical piston and piston ring pack from a modern automotive engine is shown in Figure 1. The top two rings are referred to as the compression rings and utilise gas pressure to supplement the inherent elasticity of the ring to maintain an effective combustion chamber seal. The top compression ring is the primary gas seal and, as the ring nearest the combustion chamber, encounters the highest loads and temperatures. It usually has a barrel-faced profile with a wear resistant coating such as chromium, flame-sprayed molybdenum, metal or cermet mixtures 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. As such it has a taper-faced, downward scraping profile which is not normally coated. The bottom ring in the pack is the oil-control ring which has two running faces, or lands, and a spring element to enhance radial load. As its name suggests, the role of this ring is to limit the amount of oil transported from the crankcase to the combustion chamber and by design it has no gas sealing ability.
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The periphery of the oil-control ring lands and, occasionally, the flanks are often chromium plated. Neale [1] gives an excellent summary of the piston ring types in common use. The primary role of the piston ring pack is to maintain an effective gas seal between the combustion chamber and the crankcase. The rings of the piston ring pack, which together effectively form a labyrinth seal, achieve this by closely conforming to their grooves in the piston and to the cylinder wall. The small quantity of gas that does find its way into the crankcase, blow-by, is normally piped back to the inlet valve and fed back into the cylinder. In addition to causing a dramatic increase in pressure, the combustion event generates a large amount of heat. Much of this thermal energy is convected into the piston causing a marked increase in the temperature of the piston, which is dissipated by heat transfer to adjacent components and the engine coolant. The secondary role of the piston ring pack is to transfer this heat from the piston into the cylinder wall and thence into the coolant. The final function of the piston ring pack is 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 the oil consumption of an engine and leads to an increase in harmful exhaust emissions as the oil mixes and reacts with the other contents of the combustion chamber [2]. The desire to extend service intervals of engines and minimise harmful exhaust emissions to meet ever more stringent legislative requirements, means that the permissible oil consumption levels of modern engines are very low compared to their predecessors of twenty years ago [3]. The piston ring pack must fulfil these three roles with a minimum of frictional power loss, most notably at the sliding interface with the cylinder wall, and a minimum of wear in order to maximise component life. Unfortunately, the piston assembly is one of the largest sources of friction in the internal combustion engine over the normal range of engine speeds and loads encountered in service [4-7]. Exact figures vary from engine to engine, but typically the piston assembly, comprising both the piston rings and the piston skirt, accounts for 40-50% of total engine friction. Piston ring pack friction losses are greater than those of the piston skirt at low to moderate engine speeds but the situation may be reversed at high engine speeds due to the large
wetted area of the piston skirt contributing to viscous friction [8]. In terms of wear, there is insufficient understanding of the interaction with the lubrication process. So, even though manufacturers can produce rings that have an excellent life expectancy, these components may be far from optimum from a lubrication and friction standpoint [9]. Interactions between the piston, piston rings, cylinder and the lubricant are complex and sensitive to a whole range of engine configuration and operating conditions. The need to maximise gas sealing in the combustion chamber and minimise oil consumption with low friction and wear presents the engine developer with a number of directly conflicting design parameters. Those technology drivers for the automotive industry relevant to engine tribology were recently addressed by Korcek et al [10] and are summarised in Figure 2.
Figure 2: Technology drivers for a major automotive vehicle manufacturer, after Korcek et al [10] The need to produce vehicles which use resources wisely, have minimal impact on the environment and that customers will want to own and drive presents major challenges to engine designers and lubricant formulators. For example, the automotive industry is currently developing the next generation of engine lubricants, with important and potentially conflicting implications for piston assembly tribology. The new lubricant specification, known as ILSAC GF-4, places strict demands on fuel economy and its retention as the oil ages in service and the level of phosphorus permitted in the formulation. Phosphorus is known to contaminate catalytic after-treatment devices in the exhaust,
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reducing their efficiency, but in the form of zinc dialkyldithiophosphate (ZDDP) is the most costeffective anti-oxidant and anti-wear agent currently available. The conflicting requirements for enhanced fuel economy, emissions system compatibility and extended useful lubricant life demands improvements in the
between the asperities on the two opposing surfaces, which is set equal to four times the composite root mean square surface roughness (a) in accordance with the stochastic mixed lubrication and empirical boundary lubrication models incorporated in the software. This limit is the transition between the mixed and full fluid film lubrication regimes. 10
9
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Mechanical design of the piston assembly
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Novel materials and coatings to control wear with less opportunity for running-in. A shift away from iron based materials is likely.
4
A major personal reason for writing this paper was to contribute to methods of future design optimisation for piston assemblies.
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2. SOFTWARE TOOLS The tribological design of modern piston assemblies for automotive applications is greatly assisted by the use of sophisticated predictive mathematical models. Numerical analyses for the piston and the ring pack are generally discrete models. Predictive analyses for the dynamics and lubrication of sliding interface between the piston skirt and the cylinder wall have been proposed by a number of research groups, for example [8, 11-13]. The primary outputs are piston secondary motion, tilting and lateral translation within the bore, and friction. Attempts have also been made to predict piston slap, as a precursor of piston derived noise [14]. Piston ring lubrication models are numerous, for example [15-19], reflecting the ongoing problems encountered by industry with piston ring lubrication and associated phenomena. They provide vital information on lubricant film thicknesses, component dynamics, friction losses and lubricant transport paths. Figure 3 presents some exemplar film thickness results from such an analysis for the top compression ring and oil-control ring of a diesel engine [9]. The data is presented as the minimum lubricant film thickness between the piston ring and cylinder at any instant (hmin) plotted against crank angle where 0 degrees is top dead centre firing. Also shown is the upper limit of surface contact
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Figures 3 (a) and (b) show the film thickness behaviour of the top compression ring and oilcontrol ring in isolation, assuming a plentiful supply of lubricant at the inlet, so called fully-flooded lubrication. In reality, however, the performances of adjacent rings are interdependent as the lubricant available at the inlet is to a great extent that left behind on the cylinder by the preceding ring. Figure 3(c) shows the effect this interaction has on the predicted film thickness for the top compression ring, drastically reducing the overall film thickness magnitudes throughout the engine cycle. As a consequence, this consideration of oil flow continuity within the ring pack is known as starved lubrication. Such software tools for pistons and piston rings are in widespread use in the automotive industry and commercial software is available from a number of vendors. The analyses continue to become more sophisticated, building incrementally upon previous versions. For example, piston ring models have been enhanced to consider detailed lubricant rheological characteristics such as shear thinning [20]. Figure 4 highlights the significance of such behaviour.
experimental data also shown in Figure 5 is encouraging.
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The complex dynamics of piston rings within their grooves in the piston have also been considered [21]. Figure 5 illustrates the predicted influence on inter-ring gas pressures developed between adjacent piston rings, which strongly affects radial load, and the axial motion between the top ("up") and bottom ("down") of the groove. The correlation with
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The validated predictions of fluttering in Figure 5(b), an unstable axial reciprocation of the ring within the groove, are invaluable to engine designers as this phenomenon is associated with a loss of gas and oil flow control in the piston assembly. As a final example of recent advances in piston ring modelling, the detailed of modelling of Tian [22] is exemplified by the predictions of variation in piston ring film thickness around the circumference of the piston of Figure 6.
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However sophisticated such models become, their origins remain in mechanical dynamics and fluid film lubrication analyses. As such they can only perform realistic predictions when provided with component geometry and surface texture after running-in and the local lubricant performance characteristics at the engine running time of interest. Unfortunately such data is not readily available to the engine designer. The main reason for this shortcoming is the complexity of the operating environment of the piston assembly as illustrated in Figure 7.
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....R I N G F A c E L U B R I C A T I O N 9 friction 9 lubricant transport 9 cavitation 9 links to wear
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There are clearly a large number of inter-related mechanical, metallurgical, rheological, chemical and environmental parameters influencing the performance of piston rings in an internal combustion engine.
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Figure 7" Complex operating environment of the piston ring At the ring face in Figure 7 there are uncertainties surrounding the nature of the lubricant present in the interface and how this changes with
running time. Similarly the complex interactions between lubrication and wear at this juncture are challenging to predict with any confidence.
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At the ring flank there is even less capability to predict long term performance and occasional reported instances of piston material transfer to the ring flank in service, so-called "micro-welding", is indicative of the severity of the tribology at this interface [23, 24]. The highly dynamic operating conditions in the piston assembly and the large range of materials and surface treatments used for pistons, piston rings and cylinder bores, simply adds to the difficulty of optimising design using predictive software tools. As examples of ways to improve this situation, two complexities at the ring face are considered further. 3.1. Wear and Lubrication at the Piston Ring Face and Cylinder Bore Interface
Optimisation of piston assembly tribology is currently achieved through early-life wear of the tribological surfaces, the process of running-in. Most notably at the cylinder wall and the piston ring outer surface that slides against it, the piston ring face. This reveals not only the desire to minimise manufacturing costs but also the shortfall in our understanding of the tribology of this system. The interactions between lubrication and wear during the running-in process are complex with significant changes in the 9
Overall shape of the piston ring face
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Surface texture of the piston skirt, to a lesser extent.
The modelling of wear at the piston ring / cylinder wall interface receives relatively sparse attention in the literature with just a handful of notable analyses [9, 25-29]. An example from the work of the current author [9] is given in Figure 8. The measured and predicted wear of the top compression ring from a diesel engine after 120 hours running at constant load and speed are compared. This is a barrel-faced, chromium-plated, cast iron ring. The measured new and worn ring profiles are shown overlaid using two different methods. Firstly by geometry, visually matching the unworn outlying regions of the profile, and secondly by mass, converting the weight loss of the ring to an evenly distributed volume loss around the ring circumference. The predicted ring profile, after 120 hours simulated running with multiple interactions
between wear and lubrication models is also shown and this correlates well with the measured data. In terms of assessing the significance of this wear on performance, the new ring was predicted to operate for 41% of the engine cycle in the mixed or boundary lubrication regime whereas for the worn profile this fell to just 7% [9]. The friction power loss was also predicted to decrease, by some 22%. 20
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Figure 8: Predicted and measured top compression ring profiles, after Priest et al [9] Such models offer the possibility of predicting piston assembly performance variation with running time and their development should continue. They are currently limited by over-reliance on empirical wear data from laboratory tribometers, interpretation of such data in the mixed lubrication regime and the inability to predict at the microscopic scale, for roughness as well as profile change. To add further difficulties, many of the coatings and surface finishing processes used in engines have surface topographies, even after running-in, which are not modelled satisfactorily using standard fluid film and mixed lubrication models [30]. New strategies are therefore required to analyse the response of such surfaces. To illustrate this point, consider the rapid, early-life wear of the second compression ring measured in a modern gasoline engine, Figure 9(a). This is a plain cast iron, Napier scraper ring with compression and oil-control functions. The complex surface topography of this piston ring as new, with the fine turning marks deliberately left in place, and after 2 hours rapid wear is apparent. Figure 9(b) gives the measured brake specific fuel consumption (bsfc) of the engine for the early stages of running and shows a dramatic fall during the first hour.
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Although, there are other mechanisms involved, such as changes in the lubricant, it is argued that this dramatic reduction in fuel consumption, and hence friction, is mainly attributable to the rapid wear of the second compression ring [30]. The distinctive topography of the surface of this ring is not readily analysed used typical mixed and boundary lubrication models, which are stochastic. Similar problems are faced when considering the top compression ring of the same engine; a barrelfaced, spheroidal graphite cast iron ring with a flame-sprayed molybdenum coating, Figure 10. The open porous structure of the coating produces discrete features which persist even after 120 hours running [30].
Much effort has been devoted to sensitising models of piston assembly tribology to bulk lubricant properties such as shear thinning and piezoviscosity. However, precise characterisation of the lubricant present in the piston assembly presents difficulties with respect to 9
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Degradation of the lubricant, especially around the top compression ring, has been a major cause for concern in recent years. Saville et al [31], Picken et al [32] and Taylor and Bell [33] all examined the chemical and physical changes in samples of lubricant drawn from the top ring zone of firing diesel engines. Clear differences were found in the degraded samples when compared to fresh lubricant, most notably a significant increase in viscosity. The degradation process is related to the residence time of the oil in the top ring groove. This in turn is dependent on the oil circulation mechanism in the piston assembly and hence the oil consumption and blow-by patterns [34]. Recent research associated with the current author has improved the prediction of lubricant transport within the piston assembly as a means to
746
predict lubricant residence times [35]. The quantification and interaction of all conceivable oil transport mechanisms in the piston assembly was considered, Figure 11(a). As shown in Figure 11(b), the model is capable of predicting differences in the volume of oil present in the piston assembly of a gasoline engine with new and worn piston rings. Comparison of the flow predictions with residence time experiments using a hydrocarbon marker and sampling of oil extracted from the top ring zone of the same gasoline engine has been undertaken [36]. The results overall are encouraging although a need for more research, especially into oil mist as a transport mechanism has been identified.
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4. OPTIMISATION Given the complexity of the piston assembly system and particularly the important role of lubricant chemistry in controlling wear and lubricant rheology, empirical data from laboratory and engine tests will remain a key input to the design optimisation process in the medium term. As has been demonstrated in this paper, sophisticated software tools to predict tribological performance of the piston assembly have been developed and continue to be enhanced. However, the demands from industry for new solutions to the problems posed by this complex system are growing more rapidly. There is a danger that asthe models become ever more complicated and detailed, the developers and users lose perspective in terms of the overall aim of designing successful piston assemblies. Therefore, in parallel, more systematic engineering design tools must be developed to capture the capability of and utilise the results from the new generations of detailed mathematical models. This will ensure rapid technology transfer from research to design.
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5. CONCLUSIONS Sump
9 Validated mathematical models of piston assembly tribology have proved excellent aids to the designer for a tribological system that presents many design conflicts.
(a) Interaction of transport mechanisms
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9 The extremely complicated operating environment of the piston assembly presents major challenges when developing such predictive models. 1.0
9 Input data and modelling problems exist with respect to the evolution with time of the component surfaces, form and roughness, and the lubricant, both physical and chemical behaviour.
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9 Research is progressing rapidly but industry demands are changing even more quickly. Future generations of engines will demand novel tribological solutions. 9 To this end, alongside research, engineering tools to achieve design optimisation must be developed.
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REFERENCES
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