Transient tribo-dynamics analysis and friction loss evaluation of piston during cold- and warm-start of a SI engine

Abstract

Fuel economy improvement of the Internal Combustion (IC) engine is prompted by the broad promulgation of more stringent vehicle emission regulations. Friction loss during the engine start-up is a longstanding issue because of the high frictional dissipation under cold-start and the heavy wear within warm-start. This study presents a numerical cycle-by-cycle investigation into the piston tribo-dynamics characteristics and friction loss during the Spark Ignition (SI) engine cold- and warm-start based on a coupled piston tribo-dynamics model system. The model system consists of the piston skirt-liner mixed lubrication model and the piston-rod-crank mechanism multibody dynamics model. Thermal distortion of the piston and the liner, and the lubricant viscosity-temperature property are accurately incorporated into the corresponding engine start-up conditions. Analyses and comparisons between the cold- and warm-start are carried out in terms of tribology and dynamics. Results show that frictional energy dissipated by the cold-start is 72% higher than the warm-start, which primarily due to viscous shearing of lubricant oil. At low engine speed, the tight clearance under warm-start results in a lower level of slap noise excitation than under cold-start. It is also found that the low oil viscosity of warm-start leads to a high risk of catastrophic scuffing and seizure failure near the FTDC of the first ignition cycle. In the light of the above analysis, a piston profile optimization scheme is proposed and its influence on piston scuffing resistance and energy efficiency enhancement is assessed. The evaluations indicate that almost half of friction consumption can be reduced for the engine warm-start if the thermal intricacies are properly factored in the piston design.