Improving the Performance of Diesel Engines by Bore Profile Control under Operating Conditions

The cylinder bore in an engine block is deformed under the assembling stress of the cylinder head and thermal stress. This distortion exacerbates the piston skirt friction and piston slap. Through a numerical and experimental study, this article analyzes the effect of an optimized bore profile on the engine performance. The piston skirt friction was estimated in a three-dimensional elastohydrodynamic (EHD) friction analysis. An ideal cylindrical bore under the rated load condition was assumed as the optimal bore profile that minimized the piston skirt friction without compromising the piston slap. The simulation study revealed that secondary motion of the piston immediately after firing the top dead center can be mitigated by narrowing the piston–bore clearance at the upper position of the cylinder. After optimizing the bore profile, enlarging the clearance from the middle to the lower part of the cylinder reduced the friction in the piston skirt to cylinder interface by an estimated 30% from that of the baseline. The CAE prediction was validated by measuring the bore deformation and piston secondary motion under the firing condition. Further improvement was expected by changing the design parameters of the piston and piston rings according to the bore-profile change. Experiments on a four-cylinder diesel engine verified the engine performance for various cylinder blocks with different bore shapes. The friction force, noise level, blow-by gas flow, and lubricating oil consumption were reduced after controlling the bore profile. This study proved that controlling the bore deformation under operating conditions largely improves the engine performance.