Theoretical evaluation of the effects of crank offset on the reduction of engine friction

This study discusses the effects of crankshaft offsets to the piston thrust side on engine friction. An analytical model to interpret some key friction parts of an engine, such as crankshaft bearings, pistons and piston rings, is considered, and the effects of a crankshaft offset on the moving parts is calculated using numerical analysis. Analytical results show that the crankshaft offset has some influence mainly on the side force upon the piston and effects variation in the piston sliding speed. The crankshaft offset can reduce significantly friction loss of the piston skirt, whereas friction loss in other parts is negligible. The optimum offset to minimize skirt friction loss depends on the operating conditions. Upon calculation and measurement it is determined that reduction in friction loss occurs mainly at low engine speed and low engine load. When the speed and load increase, benefit is confined to the lowest offsets, and at higher offsets the friction increases. Analytical and experimental results indicate that crank offset is effective in reducing engine friction and improving fuel economy in the low and medium engine speed region.