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Detailed analysis of oil transport in the piston assembly of a gasoline engineR.J Gamble, M. Priest* and C.M. TayloraaInstitute of Tribology, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UKReceived 3 March 2002; accepted 22 August 2002More realistic and useful models of piston ring lubrication can only be achieved if there is a better understanding of the complexmechanisms by which oil flows in this region of the engine. The volume of oil in the piston assembly andits residence time in this high-temperature environment are crucial in determining the quantity and quality of oil available to lubricate the piston rings. Typicallymodels of piston ring pack lubrication focus upon the oil flowing through the piston ring/cylinder interface. However, a number ofadditional oil flow paths and interactions with gas blow-by have been observed in the piston assembly. This paper presents a modelthat includes a number of such mechanisms and evaluates their influence on the lubrication of a piston ring pack from a typicalautomotive gasoline engine. The results indicate that such additional mechanisms are needed to give improved predictions of oiltransport they highlight the relative importance of several of these mechanisms and help guide future research.KEY WORDS: blow-by, gasoline engine, lubricant transport, piston, piston rings1. IntroductionExtraction and analysis of oil from piston assembliesof internal combustion engines 14 has shown thatsignificant degradation of the lubricant occurs in thiszone. It is here that the lubricant reaches its highestoperating temperatures and comes into contact with thecombustion gases flowing through the ring pack. Thesegases contain a number of products, such as oxides ofnitrogen, which are known to be highly influential in thedegradation of oil 5.Key factors controlling the degradation of the lubri-cant in the engine are, therefore, the time that the oilremains in the harsh environment of the piston assemblyand the volume of oil resident at any given time.To study the chemistry of lubricant degradation inthis region, it is therefore necessary to determine howmuch oil there is, how long it remains there, known asthe residence time, and what gases it will come intocontact with. Hence, detailed knowledge of oil and gasflow mechanisms in the piston assembly is required.Gas flow in the piston ring pack has been studiedusing orifice and volume models such as Ting and Meyer6,7 and Ruddy et al. 8, where fixed volumes areformed between adjacent rings and gas passes betweenthem via the ring gaps. Such research has highlighted thehighly dynamic nature of the flow both down and upthrough the piston ring pack, at velocities that may besupersonic at the ring gaps, driven by pressure changesin the combustion chamber. The additional complica-tion of piston secondary motion across the cylinder,which varies the ring gap throughout the engine cycle,has recently been investigated 9. This effect has beenshown to be influential on gas flow, the relative impor-tance being dependent upon the operating radial clear-ance between the piston and the cylinder and thecircumferential position of the ring gaps.Most models of piston ring pack lubrication onlyanalyse particular aspects of oil transport in the pistonassembly, primarily oil flow on the cylinder wall betweenthe piston ring outer diameter, the ring face, and thecylinder wall, which is easily obtained from a hydro-dynamic analysis. Some models consider further trans-port mechanisms, such as that presented by Edwards10, which includes analysis of the flow of oil on thecylinder wall through the ring gaps. In a different vein,Gulwadi 11,12 and Ma et al. 13 incorporated inter-ring oil accumulation in their studies of ring packlubrication. However, none of these models are trulycomprehensive in their treatment of oil flow in the pistonassembly.A schematic representation of an internal combustionengine piston assembly is given in figure 1. It consists ofthe piston and the ring pack, consisting typically of twoupper single-piece compression rings and a lower multi-piece oil-control ring. The top compression ring is theprimary gas seal and the oil-control ring limits theupward movement of lubricant, with the second com-pression ring assisting in both these roles.Oil flowing on the cylinder wall is not the only meansby which oil can be transported in this region of anengine. Oil flow has been observed experimentally on thesurface of the piston between adjacent piston rings, thepiston lands, by a number of researchers. For example,*To whom correspondence should be addressed. e-mail: M.Priestleeds.ac.ukTribology Letters, Vol. 14, No. 2, February 2003 (# 2003)1471023-8883/03/0200-0147/0 # 2003 Plenum Publishing CorporationThirouard et al. 14 and Nakshima et al. 15 detectedcircumferential flow of oil around the piston lands.Inagaki et al. 16, using a fluorescence technique, foundoil on the piston being blown axially upwards throughthe top ring gap into the combustion chamber. Thisoccurs when the gas pressure below the top piston ringexceeds that above it. At this point gas will flow from thepiston assembly to the combustion chamber, a processknown as reverse blow-by.The way in which lubricant is conveyed is, therefore,a potentially complex interaction between a number ofmechanisms on the cylinder wall and on the piston aspostulated in figure 2.This paper presents and evaluates models that includea number of mechanisms by which oil transport mayoccur around the piston assembly and at the ring/cylinder interface. Using as an example operating con-ditions for a gasoline engine as typical input data, anumber of cases are analysed with the piston ring packconditions varied in each case. The effect of thesetransport mechanisms on the operation of the pistonassembly is considered.The basis of this work was an existing piston ringlubrication model 17, which was developed to include amore detailed analysis of oil flow at the ring/cylinderinterface and a model for lubricant flow in the pistonassembly.2. Models for lubricant flow on the pistonThe piston and piston ring pack form a labyrinth sealwith a series of ducts along which oil and gas may flow,each connecting adjacent ring gaps as shown in figure 3.Two piston rings and the clearance between the inter-mediate land and the cylinder wall define each duct.Each inter-ring volume is then formed by two of theseducts, representing the two possible flow paths aroundthe piston.Lubricant and gas may then flow up or down thepiston assembly via the ring gaps. The lowest pistonring, the oil-control ring, is assumed to be flooded withoil on the downstroke, and the crown land, the pistonland directly above the top piston ring, is considered tohave no retained lubricant. All lubricant passing the topring on the piston is thus considered lost to the com-bustion chamber and exhaust.2.1. Circumferential flow of oil on piston landsGas flowing over the lubricant in the ducts exerts ashear stress at the interface between gas and lubricant,driving oil film flow circumferentially around the pistonlands. Given the highly dynamic nature of the gas flow,a full analysis of the complex interactions between thegas and the oil throughout the engine cycle is a majorchallenge. In the context of the current study, it wasPiston Land Piston Piston Ring Cylinder Wall Figure 1. Schematic of a piston assembly.Sump Piston Assembly Cylinder Oil mist Oil mist ? & Scraping Oil mist, Gas & InertiaDriven Flow Spray / Splashing Gas Driven FlowFigure 2. Interaction of transport mechanisms.Inlet Outlet Piston Cylinder wall= Gas/Oil FlowDuct 1Duct 2yFigure 3. Connected ducts.148R.J. Gamble et al./Oil transport in the piston assembly of a gasoline enginedeemed appropriate to make the simplifying assump-tions that the oil film is concentrated on the pistonrather than the piston ring sides, the flanks, and that theoil surface remains smooth at all times. The system canthen be represented as a two-phase flow in a rectangularduct, formed by the piston land, the cylinder and theadjacent piston rings, as illustrated in figure 4.It has been shown by Akagawa et al. 18 that anasymmetric gas-velocity distribution is found in two-phase flows, with the gas close to the oil interfaceflowing more slowly than that near the top of the duct.However, in this model the oil film was not consideredto have an influence on the gas flow and a uniformvelocity was assumed.Solutionoftheone-dimensionalNavierStokesequation, with boundary conditions of the oil velocitybeing zero at the piston surface and equal to the gasvelocity at the oil/gas interface, yieldsQD bh22?i?dpdyh6?;1where QDis volume flow rate of lubricant on the pistonland, b is the duct width perpendicular to the flow and ?iis the shear stress at the interface between the oil filmand the gas.A value for the interfacial shear stress is required tosolve for the flow rate of the oil in equation (1). If thevelocity of the gas is much greater than that of the oil,the system may be approximated by a single-phase gasflow over a stationary oil film and the interfacial shearstress 19 is?if?gU2m2;2where f is the friction factor at the interface between thegas and the oil film 19, ?gis the gas density and Umthemean gas velocity.The method of Ruddy et al. 20 was adopted topredict the circumferential gas pressure in equation (1)through the application of compressible flow theory.An initial estimate of the volume of oil present in aduct, as represented by the lubricant film thickness hpresent on the piston, figure 4, is also necessary for thedetermination of the oil rate from equation (1). Basedon the experimental investigations of Thirouard et al.14 and the predictions of Burnett et al. 21, values inthe range 5 to 10 ?m were considered appropriate.These two extremes were chosen for the computationspresented in this paper.2.2. Axial oil flow on the piston through the piston ringgapsA similar two-phase flow is found at the ring gaps onthe piston. Here the gas flowing through the ring gapswill transport oil between the lands on either side of thepiston ring as illustrated in figure 5. This mechanism canmove oil up or down the piston assembly depending onthe direction of the gas flow.This oil flow can be modelled using similar assump-tions to those used for circumferential flow on the pistonlands. The lubricant volume flow rate is then given byQGP ch22?i?h6dpdx?:3An additional assumption is that all of the oil flow isassumed to reach the next land, such that none of theflow through the gap is accumulated in the piston ringgroove, figure 5.As with the model for film flow in a duct, the filmthicknesshandthepressuregradient(dp/dx)arerequiredto determine the flow rate. Oil film thickness in the pistonring gap is taken to be that on the piston land from whichthe gas is flowing. The pressure gradient is assumed to belinear across the ring between the known pressures in theinter-ring volumes above and below the ring. PistonCylinder PistonRing Gas FlowOil FlowPistonCylinder Circumferential zyh b Figure 4. Oil flow in a duct.Ring Groove Land 1Land 2 Ring Oil Flow No Oil Gap z x c Figure 5. Oil flow through the ring gap on the piston.R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine1493. Models for lubricant flow on the cylinder wallAs noted previously, virtually all piston ring analysespredict the oil transported between the piston ring faceand the cylinder wall. However there exists the possi-bility of oil accumulating in the inter-ring volumes dueto flow imbalances past adjacent rings and for gas-driven flow on the cylinder wall through the ring gaps ina similar manner to that observed on the piston. Theseaddition mechanisms are considered in detail below.3.1. Oil accumulationIn general, piston ring lubrication models take noaccount of any build-up of oil that may occur ahead of apiston ring on the cylinder wall, which has beenobserved in experiments 14,22. This phenomenon wasincluded in the models of Ma et al. 13 and Gulwadi11,12, who considered such accumulation as an addi-tional constant-thickness thin film of lubricant on thecylinder wall in the inter-ring region. The approachtaken here is somewhat different and reflects moreclosely what was observed in experiments.If all of the oil film ahead of the ring does not flowbeneath the ring face as the piston moves along thecylinder, oil will begin to accumulate in front of theleading edge of the ring, figure 6. The fact that oilaccumulates directly in front of the ring is mostimportant. When analysis of piston ring lubrication ismade it is necessary to determine at which point the oilfilm contacts the ring face, point A in figure 6. If oil startsto build up in front of the ring, contact occurs furtherforward and higher on the ring, point B. This accumu-lation of oil therefore impacts on the solution of thehydrodynamic equations for the film thickness and oilflow between the pistonringface andthe cylinder wall. Inaddition it supplies extra lubricant to other transportmechanisms, such as oil flow through the ring gaps.During time t a piston ring with velocity ? will travela distance x. The section of the cylinder traversed in thistime will have a film thickness h1on its surface, figure 6,while the ring will leave a film of thickness h2behind iton the liner. The total accumulated volume of oil aheadof the ring is therefore simplyVacl h1? h2x?d:4The accumulated oil volume can then be added to the oilavailable to lubricate the ring during the next time step.In the model presented here, an assumption is maderegarding the shape of this accumulated oil ahead of thering, whereas Ma et al. 13 and Gulwadi 11,12 addedthe extra volume to the film h1throughout the inter-ringregion.Basedonthelaser-inducedfilm-thicknessexperiments of Seki et al. 22 and Thirouard et al. 14, itwas assumed that the wave in front of the ring takes theform of a parabola. Given the uncertainties involved,three levels of parabolic curvature a and hence waveshape were studied in the current computations to eval-uate the influence on the lubrication of the ring pack.3.2. Oil flow on the cylinder through the piston ring gapsIn addition to driving oil flow on the piston assembly,blow-by gas can also contribute to oil flow on thecylinder wall. As the gas flows through the piston ringgaps it will exert a shear stress on the oil film surface inthe same way as the gas-driven flows on the piston.The oil flow that takes place due to this mechanismcan therefore be modelled using equation (5), in amanner similar to that of Edwards 10, the oil filmthickness in the piston ring gap being that preceding thepiston ring on the cylinder. Once again the pressuregradient is assumed to be linear across the ring betweenthe known pressures in the inter-ring volumes above andbelow the ring.QGC ch22?i?dpdxh6?:54. Model for oil mist generationIn addition to driving flows of oil on the surfaces ofthe piston and the cylinder wall, blow-by gases may alsotransport oil more directly. It is possible that when thegas is travelling with high velocity that droplets of oilmay be torn from the surface of the oil films as it passes.Thoiroud et al. 14 observed this experimentally, par-ticularly through piston ring gaps.To determine oil entrainment fully in this system withhighly dynamic gas flows which regularly change direc-tion and can accelerate to supersonic velocities in thering gaps is a major analytical challenge and inap-propriate in the broader context of the current research.An empirical approach was therefore sought based onv Oil Accumulation A B h1 h2 x y Figure 6. Oil accumulation ahead of a piston ring.150R.J. Gamble et al./Oil transport in the piston assembly of a gasoline enginepublished experimental studies. Though a number ofthese exist, none are based on the flow of gas over a thinoil film, in a rectangular duct, the situation found in thepiston assembly. However, two correlations fit thissituation at least partially.Ishii and Mishima 23 presented an empirical corre-lation for low-viscosity fluids, such as water or oil athigher temperatures, in a circular pipe, while Akagawaet al. 24 derived a correlation for the fraction of thetotal fluid flowing in the system as a mist for waterflowing in a rectangular duct system.5. ComputationsThe new models were evaluated using geometric andbasic operating data from a Ricardo Hydra engine asinputs. This is a single-cylinder gasoline engine based ona General Motors 2.0 litre 4-cylinder engine, which usesa standard production piston and piston rings. Thepiston ring pack comprises three rings, made up of twocompression rings and an oil-control ring. As such thepiston assembly is very typical of modern automotivegasoline engines. The engine data and the operatingconditions considered are summarised in table 1.To study the sensitivity to top piston ring gap sizeand the initial assumed lubricant film thickness on thepiston lands, several variations of the input data set wereused as summarised in table 2. Piston ring gap size iswell recognised as a key parameter in controlling gasand oil flows in piston assemblies. For the cylinder wallflow only the results for case 1 of table 2 are presented asthe ring pack configuration had very little influence onthe results. The effect assumed shape of the accumulatedoil on the cylinder wall at the leading edge of the ringwas investigated for both new and worn ring face pro-files to determine the influence of changes in ring shapepresented to the cylinder wall on lubricant accumulationand hence lubrication. The cases considered are definedin table 3.Figure 7 shows the oil flow rate between the pistonring face and the cylinder wall for the top two pistonrings for the operating conditions shown in table 1 usingthe standard ring gaps, ring 1 being the top compressionring. This provides a baseline against which to comparethe additional flow mechanisms. Positive oil flow istowards the combustion chamber and zero degrees ofcrank angle is top dead centre firing.5.1. Results for the piston modelFigure 8 shows the oil flow rates predicted for cir-cumferential flow on the piston second land, betweenrings 1 and 2. This mechanism is prevalent when com-bustion gases are flowing quickly through the ring packafter the combustion event. Clearly the flow rates pre-dicted are large when compared to those of flow beneaththe rings of figure 7. The results also highlight theimportance of top ring gap size and the initial assump-tion for oil film thickness on the piston.The results for the flow around the piston lands plusthe gas-driven flow through the ring gaps are presentedin figure 9. This shows the change in the volume of oil onTable 1Ricardo hydra single-cylinder gasolineengine data.ParameterValueNumber of Rings3Cylinder Bore86mmPiston Stroke86mmFitted Gap (Top Ring)0.60mmFitted Gap (Second Ring)0.58mmEngine Speed2500rpmBMEP5barLubricantSAE 30Table 2Ring pack configurations (piston model).CaseRing gapsInitial film thicknesson piston lands (?m)1Standard5250% increase for top ring53Standard10450% increase for top ring10Table 3Assumed profiles for the accumulated oil ahead of each ring.CaseRing profilesAssumed parabola profile foraccumulated oil (m?1) z ax25Newa 16Newa 107Newa 208Worna 19Worna 1010Worna 20Figure 7. Axial oil flow beneath the piston rings.R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine151the second land of the piston for cases 1 to 4 from theinitial assumed value as a percentage. These combinedmechanisms only predict transport of oil out of theupper piston region, either through the top ring gap andinto the combustion chamber or down past the secondring towards the bottom of the piston assembly. Clearlythis cannot be the case in reality, with no additional oilflow reaching the piston, as the upper part of the pistonassembly would quickly be totally starved of oil.The amount of oil predicted to flow through the topring gap into the combustion chamber is of interest asthis is a major contributor to oil consumption. Table 4shows the oil volume and the percentage of the totaltransport that takes place from the piston to the com-bustion chamber during one engine cycle for cases 1 to4. As the net gas flow in an engine cycle is down thepiston assembly, the majority of oil is also moved downthe piston. However, with an increased top ring gap, notonly is more oil transported out of the second land, alarger proportion of this is carried into the combustionchamber, contributing to oil consumption. As would beexpected, with a larger initial film on the piston lands,cases 3 and 4, a larger volume of oil is transported to thecombustion chamber. However, the oil transport to thecombustion chamber is a smaller proportion of the total.This is due to the larger oil flow along the piston lands incases 3 and 4. Gas-driven, circumferential oil flow on thelands is increased with the thicker oil film, as equation(1), moving oil towards the lower ring gap. Hence, moreoil becomes available to be transported down the pistonthrough the second ring gap.5.2. Results for the cylinder wall modelFigure 10 shows the gas-driven oil flow on thecylinder liner, through the piston ring gaps for case 1.The high pressures in the combustion chamber after topdead centre firing, zero degrees of crank angle, lead tohigh gas flow rates down through the top ring gap and aconsequently large oil flow down through the ring gap,negative values in figure 10. The gas flow changesdirection, reverse blow-by, when the pressure below thering exceeds that above it at around 60 degrees of crankangle. At this point the oil flow changes direction, and asmaller amount of oil is transported towards the com-bustion chamber. As the gas flow rate through the sec-ond ring is less than that for the top ring, the oil flowrate through this gap is smaller. In addition, reverseblow-by does not occur through this ring and so the oilflow is always down the cylinder, towards the crankcase.Again this mechanism is clearly significant compared tothe baseline flow between the piston ring face andcylinder wall.Figure 8. Circumferential flow of oil on piston second land for cases 1to 4.Figure 9. Change in oil volume on piston second land for cases 1 to 4.Table 4Oil transport from the piston to the combustionchamber during one engine cycleOil transport to combustion chamberCaseVolume (m3)Proportion of Total (%)11:04 ? 10?1126.321:74 ? 10?1134.732:31 ? 10?1115.144:11 ? 10?1122.0Figure 10. Axial oil flow on the cylinder wall at the ring gaps for case 1.152R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engineIt has been shown by Priest et al. 25 that the ringface profiles wear rapidly in service as running-in pro-gresses and that the resulting changes in profile androughness have a significant effect on the predictedlubrication response at the piston ring/cylinder inter-face. The analyses for oil transport on the cylinder weretherefore undertaken using both new ring face profiles,and for profiles measured after 40hours running at1500rpm and 7.75bar bmep. The new and worn ringprofiles are shown in figure 11.The predicted variation of accumulated oil filmheight ahead of each ring with crank angle is shown fornew and worn ring face profiles in figure 12 and figure 13respectively. For both sets of ring face profiles, it is clearthat accumulation occurs for only part of the enginecycle and the values are generally smaller than antici-pated. For the top ring especially, very little oil builds upwith accumulation only taking place around the rever-sals of direction, a similar result to that predicted byGulwadi 12. This is most probably due to the reductionin the speed of the piston as it nears the end of onestroke or begins another. At these points, the load onthe ring is increasingly supported by the squeeze-filmaction of the ring approaching the cylinder wall. As thering approaches the cylinder wall, less oil will flowthrough the reducing clearance. This leads to theincreaseintheaccumulatedvolumeoflubricantobserved at these points.The second ring is a scraper ring designed to helpcontrol the amount of oil being transported up thecylinder towards the combustion chamber. This role isclearly demonstrated in the accumulation results. Thering allows all of the oil to flow beneath it on the upwardstrokes; however as it travels down the cylinder a largeamount of oil accumulates ahead of the ring and is thustransported away from the combustion chamber.The minimum film thickness between the piston ringfaces and the cylinder wall for each of cases 5 to 10 canbe seen for the top ring and the second ring in figure 14and figure 15 respectively. These also show the fullyflooded case where the rings are lubricated with anexcess of oil for reference.A number of points are immediately evident fromthese results. Firstly, the accumulation of oil has littleinfluence on the lubrication of the piston rings whenthey have unworn profiles. Figure 15(b) also shows thatFigure 11. Measured piston ring profiles, (a) top ring, new (b) second ring, new (c) top ring, 40hours and (d) second ring, 40 hours.Figure 12. Height of accumulated oil ahead of each ring for case 5, newring face profiles and a 1m?1.R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine153this is the case for the second ring with the worn profiledue to the worn shape of the ring. Accumulation onlyoccurs when the second ring is moving down the cylin-der. When the ring is travelling in this direction there isonly a small clearance between the ring inlet and thecylinder wall. Hence the inlet region is easily flooded andthe accumulated oil has no effect on the lubrication ofthe ring.Finally, the main influence of the accumulation onthe lubrication of the ring pack can be seen in figure14(b). There is a large increase in the minimum filmthickness under the top ring, during the first part of thepower stroke. This is to be expected, as it is the point ofgreatest accumulation ahead of the top ring with anassumed parabolic profile for the accumulation, as canbe seen in figure 13. The results in this part of the cycle,both the accumulation of figure 13 and the film thick-ness of figure 14(b), display some minor residualnumerical instability. Efforts will be devoted to elim-inating this phenomenon in the full version of the pistonring flow model with complete links between the variousflow mechanisms.5.3. Oil mist generationFigure 16 shows the maximum volume of oil mistpredicted with the correlation of Ishii and Mishima 23.This occurred at the ring gaps and it can be seen thatmist is only generated during a very small part of theengine cycle, while the volume of mist produced isextremely small. On the basis of this analysis, oil mistwas therefore deemed negligible when compared to theother transport mechanisms.Figure 13. Height of accumulated oil ahead of each ring for case 8,worn ring face profiles and a 1m?1.Figure 14. Influence of assumed shape of accumulation on minimumfilm thickness for the top ring for cases 5 to 10, (a) new and (b) wornprofiles (units of a are m?1).Figure 15. Influence of assumed shape of accumulation on minimumfilm thickness for the second ring for cases 5 to 10, (a) new and (b)worn profiles (units of a are m?1.154R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine5.4. Total oil volume in the piston assemblyFinally, the variation of oil volume in the pistonassembly with crank angle is presented in figure 17 fornew and worn profiles assuming a 10 ?m film on thepiston and a top ring gap increased by 50%. In this casethe volume of lubricant is taken as all oil between thetop of the oil-control ring and the bottom of the topcompression ring. The volume of oil on the cylinder walldominates the results, and the effect of oil being trans-ported from the piston to the combustion chamber isnot observed against this background. Running-in ofthe piston assembly, including the cylinder wall, clearlyis predicted to reduce the volume of oil in the pistonassembly on the basis of the model presented.6. DiscussionIt can be seen from the results presented that thegeometry of the piston ring pack is influential forlubricant transport in the piston assembly. For oiltransport on the cylinder wall the shape of the pistonring profile has an impact on the level of oil accumula-tion seen ahead of the piston rings. This is especially thecase for the second, or scraper ring where the ringscrapes significantly less oil down the cylinder wall witha well run-in profile.Representing the accumulation as a parabolic filmavailable to lubricate the ring during the next crankangle impacts upon lubrication of the top piston ring. Alarge increase in the minimum oil film thickness beneaththe top ring is observed after top dead centre firing forthe worn profile. No change in the minimum oil filmthickness was predicted for the second ring, due toaccumulation occurring at the points in the cycle whenthis ring is already operating in the flooded lubricationcondition.The top ring gap size and the starting lubricant filmthickness on the piston are both significant factors forlubricant flow on the piston. Increasing the top ring gapsize and hence the gas flow through the piston assemblycauses lubricant to flow from the piston assembly morerapidly. A greater amount of reverse blow-by is alsoseen with the larger top ring gap. This leads to anincrease in oil flow reaching the combustion chamber,where it contributes to oil consumption.The assumed starting lubricant film thickness on thepiston is clearly highly significant in determining theamount of lubricant transport in the piston assembly.Doubling the film thickness from 5 to 10 ?m increasesthe amount of oil reaching the combustion chamber inone cycle by around 130%.It is clear then that the detailed design of the pistonassembly is important in determining the length of timethat oil will remain resident in the piston assembly. Assuch, this in turn will be a central factor in the degra-dation of lubricant in this region.Further, it is evident that the transport mechanismsproposed in this study cannot be solely responsible forthe way in which lubricant is carried around the pistonassembly. If this were the case the upper regions of thepiston ring pack would eventually become totallystarved of oil. Other mechanisms clearly have a role toeither transport more oil from the crankcase to thepiston or from the cylinder wall to the piston. Theinfluence of the piston axial acceleration on oil dis-tribution on the piston surfaces is a one factor thatneeds to be investigated. Also, the simple way in whichoil mist generation has been predicted also needs to bereconsidered given the accepted wisdom in the auto-motive industry that oil mistingis an importantoperational parameter. The ultimate objective is tointeractively couple all the significant mechanisms toanalyse in detail the flow into, out of and within thepiston assembly.Figure 16. Predicted oil mist generation at the ring gaps on the cylinderwall using the Ishiii and Mishima 23 correlation with standard ringgaps.Figure 17. Variation of oil volume in the piston assembly with crankangle, 10?m initial film on the piston lands and a top ring gapincreased by 50%.R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine1557. Conclusions(i) An existing piston ring pack lubrication model hasbeen extended to include additional oil-transportmechanisms at the ring/cylinder interface and in thepiston assembl
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