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Detailed analysis of oil transport in the piston assembly of a gasoline engine R.J Gamble, M. Priest* and C.M. Taylora aInstitute of Tribology, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK Received 3 March 2002; accepted 22 August 2002 More realistic and useful models of piston ring lubrication can only be achieved if there is a better understanding of the complex mechanisms by which oil fl ows 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. Typically models of piston ring pack lubrication focus upon the oil fl owing through the piston ring/cylinder interface. However, a number of additional oil fl ow paths and interactions with gas blow-by have been observed in the piston assembly. This paper presents a model that includes a number of such mechanisms and evaluates their infl uence on the lubrication of a piston ring pack from a typical automotive gasoline engine. The results indicate that such additional mechanisms are needed to give improved predictions of oil transport 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 rings 1. Introduction Extraction and analysis of oil from piston assemblies of internal combustion engines 14 has shown that signifi cant degradation of the lubricant occurs in this zone. It is here that the lubricant reaches its highest operating temperatures and comes into contact with the combustion gases fl owing through the ring pack. These gases contain a number of products, such as oxides of nitrogen, which are known to be highly infl uential in the degradation of oil 5. Key factors controlling the degradation of the lubri- cant in the engine are, therefore, the time that the oil remains in the harsh environment of the piston assembly and the volume of oil resident at any given time. To study the chemistry of lubricant degradation in this region, it is therefore necessary to determine how much oil there is, how long it remains there, known as the residence time, and what gases it will come into contact with. Hence, detailed knowledge of oil and gas fl ow mechanisms in the piston assembly is required. Gas fl ow in the piston ring pack has been studied using orifi ce and volume models such as Ting and Meyer 6,7 and Ruddy et al. 8, where fi xed volumes are formed between adjacent rings and gas passes between them via the ring gaps. Such research has highlighted the highly dynamic nature of the fl ow both down and up through the piston ring pack, at velocities that may be supersonic at the ring gaps, driven by pressure changes in 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 eff ect has been shown to be infl uential on gas fl ow, the relative impor- tance being dependent upon the operating radial clear- ance between the piston and the cylinder and the circumferential position of the ring gaps. Most models of piston ring pack lubrication only analyse particular aspects of oil transport in the piston assembly, primarily oil fl ow on the cylinder wall between the piston ring outer diameter, the ring face, and the cylinder wall, which is easily obtained from a hydro- dynamic analysis. Some models consider further trans- port mechanisms, such as that presented by Edwards 10, which includes analysis of the fl ow of oil on the cylinder wall through the ring gaps. In a diff erent vein, Gulwadi 11,12 and Ma et al. 13 incorporated inter- ring oil accumulation in their studies of ring pack lubrication. However, none of these models are truly comprehensive in their treatment of oil fl ow in the piston assembly. A schematic representation of an internal combustion engine piston assembly is given in fi gure 1. It consists of the piston and the ring pack, consisting typically of two upper single-piece compression rings and a lower multi- piece oil-control ring. The top compression ring is the primary gas seal and the oil-control ring limits the upward movement of lubricant, with the second com- pression ring assisting in both these roles. Oil fl owing on the cylinder wall is not the only means by which oil can be transported in this region of an engine. Oil fl ow has been observed experimentally on the surface of the piston between adjacent piston rings, the piston lands, by a number of researchers. For example, *To whom correspondence should be addressed. e-mail: M.Priest leeds.ac.uk Tribology Letters, Vol. 14, No. 2, February 2003 (# 2003)147 1023-8883/03/0200-0147/0 # 2003 Plenum Publishing Corporation Thirouard et al. 14 and Nakshima et al. 15 detected circumferential fl ow of oil around the piston lands. Inagaki et al. 16, using a fl uorescence technique, found oil on the piston being blown axially upwards through the top ring gap into the combustion chamber. This occurs when the gas pressure below the top piston ring exceeds that above it. At this point gas will fl ow from the piston assembly to the combustion chamber, a process known as reverse blow-by. The way in which lubricant is conveyed is, therefore, a potentially complex interaction between a number of mechanisms on the cylinder wall and on the piston as postulated in fi gure 2. This paper presents and evaluates models that include a number of mechanisms by which oil transport may occur 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, a number of cases are analysed with the piston ring pack conditions varied in each case. The eff ect of these transport mechanisms on the operation of the piston assembly is considered. The basis of this work was an existing piston ring lubrication model 17, which was developed to include a more detailed analysis of oil fl ow at the ring/cylinder interface and a model for lubricant fl ow in the piston assembly. 2. Models for lubricant fl ow on the piston The piston and piston ring pack form a labyrinth seal with a series of ducts along which oil and gas may fl ow, each connecting adjacent ring gaps as shown in fi gure 3. Two piston rings and the clearance between the inter- mediate land and the cylinder wall defi ne each duct. Each inter-ring volume is then formed by two of these ducts, representing the two possible fl ow paths around the piston. Lubricant and gas may then fl ow up or down the piston assembly via the ring gaps. The lowest piston ring, the oil-control ring, is assumed to be fl ooded with oil on the downstroke, and the crown land, the piston land directly above the top piston ring, is considered to have no retained lubricant. All lubricant passing the top ring on the piston is thus considered lost to the com- bustion chamber and exhaust. 2.1. Circumferential fl ow of oil on piston lands Gas fl owing over the lubricant in the ducts exerts a shear stress at the interface between gas and lubricant, driving oil fi lm fl ow circumferentially around the piston lands. Given the highly dynamic nature of the gas fl ow, a full analysis of the complex interactions between the gas and the oil throughout the engine cycle is a major challenge. In the context of the current study, it was Piston Land Piston Piston Ring Cylinder Wall Figure 1. Schematic of a piston assembly. Sump Piston Assembly Cylinder Oil mist Oil mist ? 1 where QD is volume fl ow rate of lubricant on the piston land, b is the duct width perpendicular to the fl ow and ?i is the shear stress at the interface between the oil fi lm and the gas. A value for the interfacial shear stress is required to solve for the fl ow rate of the oil in equation (1). If the velocity of the gas is much greater than that of the oil, the system may be approximated by a single-phase gas fl ow over a stationary oil fi lm and the interfacial shear stress 19 is ?i f?gU2 m 2 ;2 where f is the friction factor at the interface between the gas and the oil fi lm 19, ?gis the gas density and Umthe mean gas velocity. The method of Ruddy et al. 20 was adopted to predict the circumferential gas pressure in equation (1) through the application of compressible fl ow theory. An initial estimate of the volume of oil present in a duct, as represented by the lubricant fi lm thickness h present on the piston, fi gure 4, is also necessary for the determination of the oil rate from equation (1). Based on the experimental investigations of Thirouard et al. 14 and the predictions of Burnett et al. 21, values in the range 5 to 10 ?m were considered appropriate. These two extremes were chosen for the computations presented in this paper. 2.2. Axial oil fl ow on the piston through the piston ring gaps A similar two-phase fl ow is found at the ring gaps on the piston. Here the gas fl owing through the ring gaps will transport oil between the lands on either side of the piston ring as illustrated in fi gure 5. This mechanism can move oil up or down the piston assembly depending on the direction of the gas fl ow. This oil fl ow can be modelled using similar assump- tions to those used for circumferential fl ow on the piston lands. The lubricant volume fl ow rate is then given by QGP c h2 2? ?i? h 6 dp dx ? :3 An additional assumption is that all of the oil fl ow is assumed to reach the next land, such that none of the fl ow through the gap is accumulated in the piston ring groove, fi gure 5. As with the model for fi lm fl ow in a duct, the fi lm thicknesshandthepressuregradient(dp/dx)arerequired to determine the fl ow rate. Oil fi lm thickness in the piston ring gap is taken to be that on the piston land from which the gas is fl owing. The pressure gradient is assumed to be linear across the ring between the known pressures in the inter-ring volumes above and below the ring. Piston Cylinder Piston Ring Gas Flow Oil Flow Piston Cylinder Circumferential z y h b Figure 4. Oil fl ow in a duct. Ring Groove Land 1Land 2 Ring Oil Flow No Oil Gap z x c Figure 5. Oil fl ow through the ring gap on the piston. R.J. Gamble et al./Oil transport in the piston assembly of a gasoline engine149 3. Models for lubricant fl ow on the cylinder wall As noted previously, virtually all piston ring analyses predict the oil transported between the piston ring face and the cylinder wall. However there exists the possi- bility of oil accumulating in the inter-ring volumes due to fl ow imbalances past adjacent rings and for gas- driven fl ow on the cylinder wall through the ring gaps in a similar manner to that observed on the piston. These addition mechanisms are considered in detail below. 3.1. Oil accumulation In general, piston ring lubrication models take no account of any build-up of oil that may occur ahead of a piston ring on the cylinder wall, which has been observed in experiments 14,22. This phenomenon was included in the models of Ma et al. 13 and Gulwadi 11,12, who considered such accumulation as an addi- tional constant-thickness thin fi lm of lubricant on the cylinder wall in the inter-ring region. The approach taken here is somewhat diff erent and refl ects more closely what was observed in experiments. If all of the oil fi lm ahead of the ring does not fl ow beneath the ring face as the piston moves along the cylinder, oil will begin to accumulate in front of the leading edge of the ring, fi gure 6. The fact that oil accumulates directly in front of the ring is most important. When analysis of piston ring lubrication is made it is necessary to determine at which point the oil fi lm contacts the ring face, point A in fi gure 6. If oil starts to build up in front of the ring, contact occurs further forward and higher on the ring, point B. This accumu- lation of oil therefore impacts on the solution of the hydrodynamic equations for the fi lm thickness and oil fl ow between the pistonringface andthe cylinder wall. In addition it supplies extra lubricant to other transport mechanisms, such as oil fl ow through the ring gaps. During time t a piston ring with velocity ? will travel a distance x. The section of the cylinder traversed in this time will have a fi lm thickness h1 on its surface, fi gure 6, while the ring will leave a fi lm of thickness h2behind it on the liner. The total accumulated volume of oil ahead of the ring is therefore simply Vacl h1? h2x?d:4 The accumulated oil volume can then be added to the oil available to lubricate the ring during the next time step. In the model presented here, an assumption is made regarding the shape of this accumulated oil ahead of the ring, whereas Ma et al. 13 and Gulwadi 11,12 added the extra volume to the fi lm h1throughout the inter-ring region.Basedonthelaser-induced fi lm-thickness experiments of Seki et al. 22 and Thirouard et al. 14, it was assumed that the wave in front of the ring takes the form of a parabola. Given the uncertainties involved, three levels of parabolic curvature a and hence wave shape were studied in the current computations to eval- uate the infl uence on the lubrication of the ring pack. 3.2. Oil fl ow on the cylinder through the piston ring gaps In addition to driving oil fl ow on the piston assembly, blow-by gas can also contribute to oil fl ow on the cylinder wall. As the gas fl ows through the piston ring gaps it will exert a shear stress on the oil fi lm surface in the same way as the gas-driven fl ows on the piston. The oil fl ow that takes place due to this mechanism can therefore be modelled using equation (5), in a manner similar to that of Edwards 10, the oil fi lm thickness in the piston ring gap being that preceding the piston ring on the cylinder. Once again the pressure gradient is assumed to be linear across the ring between the known pressures in the inter-ring volumes above and below the ring. QGC c h2 2? ?i? dp dx h 6 ? :5 4. Model for oil mist generation In addition to driving fl ows of oil on the surfaces of the piston and the cylinder wall, blow-by gases may also transport oil more directly. It is possible that when the gas is travelling with high velocity that droplets of oil may be torn from the surface of the oil fi lms as it passes. Thoiroud et al. 14 observed this experimentally, par- ticularly through piston ring gaps. To determine oil entrainment fully in this system with highly dynamic gas fl ows which regularly change direc- tion and can accelerate to supersonic velocities in the ring gaps is a major analytical challenge and inap- propriate in the broader context of the current research. An empirical approach was therefore sought based on v 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 engine published experimental studies. Though a number of these exist, none are based on the fl ow of gas over a thin oil fi lm, in a rectangular duct, the situation found in the piston assembly. However, two correlations fi t this situation at least partially. Ishii and Mishima 23 presented an empirical corre- lation for low-viscosity fl uids, such as water or oil at higher temperatures, in a circular pipe, while Akagawa et al. 24 derived a correlation for the fraction of the total fl uid fl owing in the system as a mist for water fl owing in a rectangular duct system. 5. Computations The new models were evaluated using geometric and basic operating data from a Ricardo Hydra engine as inputs. This is a single-cylinder gasoline engine based on a General Motors 2.0 litre 4-cylinder engine, which uses a standard production piston and piston rings. The piston ring pack comprises three rings, made up of two compression rings and an oil-control ring. As such the piston assembly is very typical of modern automotive gasoline engines. The engine data and the operating conditions considered are summarised in table 1. To study the sensitivity to top piston ring gap size and the initial assumed lubricant fi lm thickness on the piston lands, several variations of the input data set were used as summarised in table 2. Piston ring gap size is well recognised as a key parameter in controlling gas and oil fl ows in piston assemblies. For the cylinder wall fl ow only the results for case 1 of table 2 are presented as the ring pack confi guration had very little infl uence on the results. The eff ect assumed shape of the accumulated oil on the cylinder wall at the leading edge of the ring was investigated for both new and worn ring face pro- fi les to determine the infl uence of changes in ring shape presented to the cylinder wall on lubricant accumulation and hence lubrication. The cases considered are defi ned in table 3. Figure 7 shows the oil fl ow rate between the piston ring face and the cylinder wall for the top two piston rings for the operating conditions shown in table 1 using the standard ring gaps, ring 1 being the top compression ring. This provides a baseline against which to compare the additional fl ow mechanisms. Positive oil fl ow is towards the combustion chamber and zero degrees of crank angle is top dead centre fi ring. 5.1. Results for the piston model Figure 8 shows the oil fl ow rates predicted for cir- cumferential fl ow on the piston second land, between rings 1 and 2. This mechanism is prevalent when com- bustion gases are fl owing quickly through the ring pack after the combustion event. Clearly the fl ow rates pre- dicted are large when compared to those of fl ow beneath the rings of fi gure 7. The results also highlight the importance of top ring gap size and the initial assump- tion for oil fi lm thickness on the piston. The results for the fl o