Tribology of piston skirt conjunction

Frictional losses in the piston skirt to cylinder liner conjunction account for approximately 2.5% of the energy supplied to the modern car [1]. These losses are contributed by viscous shear of the lubricant and asperity interactions on the contiguous surfaces. However, for most of the piston cycle the regime of lubrication is dominated by elastohydrodynamics or hydrodynamics. Hence, friction due to viscous shear is dominant. Most idealistic analyses employ a “cold” piston skirt shape and use either a measured profile or by approximated polynomials as the input shape [2-4]. In reality, however, pistons are subject, not only to contact forces, but also thermo-mechanical distortion. These are as the result of thermal expansion of the piston as well as its global mechanical deformation in situ. They alter the pistonliner conjunctional gap. The piston structure is designed in such a way as to prevent gross localised wear in service by means of skirt profile and structural stiffness modification [5]. Considering the combined effect of global as well as local deformation of the skirt under the influence of contact force, it is vital to take into account the effect of shape and rigidity of both the piston and liner structures in an integrated thermoelastic and elastohydrodynamic analysis. This approach is more representative of the in situ “hot” skirt condition as noted by McClure [6]. This paper shows the significant differences observed in the generated pressures, film thickness and friction by comparing “cold” piston profiles; disregarding large scale global deformation and “hot” thermo-elastically deformed skirt conjunctions with representative skirt stiffness.