Abstract
Advanced piston technology for motorsport applications is driven through development of lightweight pistons with preferentially compliant short partial skirts. The preferential compliance is achieved through structural stiffening, such that a greater entrainment wedge is achieved at the skirt’s bottom edge through thermo-elastic deformation, whilst better conforming contact geometry at the top of the skirt. In practice, the combination of some of these conditions is intended to improve the load-carrying capacity and reduce friction. The approach is fundamental to the underlying ethos of race and high-performance engine technology. Contact loads of the order of 5 kN and contact kinematics in the range 0–35 m/s result in harsh transient tribological conditions. Therefore, piston design requires detailed transient analysis, which integrates piston dynamics, thermo-elastic distortion and transient elastohydrodynamics. The paper provides such a detailed analysis as well as verification of the same using non-invasive ultrasonic-assisted lubricant film thickness measurement from a fired engine under normal operating conditions, an approach not hitherto reported in literature. Good agreement is noted between measured film thickness and predictions.
Highlights
Frictional losses in the piston skirt-cylinder liner conjunction account for approximately 3 % of the input fuel energy, whereas piston ring pack losses account for a further 4 % [11]
A detailed methodology to predict piston skirt-liner conjunctional performance is presented, which includes the effect of all major causes contributing to the thermo-elastic deformation of the contiguous contacting solids under transient dynamic conditions
The elastohydrodynamics of the contact is embedded within the transient dynamic analysis
Summary
Frictional losses in the piston skirt-cylinder liner conjunction account for approximately 3 % of the input fuel energy, whereas piston ring pack losses account for a further 4 % [11]. These losses are primarily due to viscous shear of the lubricant film and asperity interactions of contiguous surfaces. For most of the piston cycle, the regime of lubrication in the skirt-cylinder liner conjunction is dominated by hydrodynamic or soft elastohydrodynamic (iso-viscous elastic) regimes of lubrication [2, 16]. Aside from piston reversals at top and bottom dead centres, where mixed regime of lubrication can ensue, friction is usually generated through viscous shear of a lubricant film. One may encourage piezo-viscous action of the lubricant, leading to elastohydrodynamic conditions, which would yield the lowest friction [12, 35]
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