The influence of the crankshaft offset on the piston position, the indicated specific fuel economy and the emissions of a direct-injection diesel engine

Abstract

The effect of offsetting the crankshaft from the cylinder centre-line by 20 mm towards the thrust side was investigated. The effect on the piston top-dead-centre and bottom-dead-centre positions, the valve timing requirements, the indicated performance and the emissions was examined. The investigation was carried out on a common-rail direct-injection diesel engine. The compression ratio was maintained constant, but the piston’s top-dead-centre position and the piston’s bottom-dead-centre position occur 5.5° crank angle later and 10.5° crank angle later respectively than for the zero-offset case. Camshaft position changes were used to retain either the same valve overlap or the same inlet and exhaust valve opening times as for the zero-offset case. The results show that the indicated performance changes are small and that the direction of change can vary with the operating condition. The largest changes in the nitrogen oxide emissions were consistently increases; the largest changes in the carbon monoxide emissions and the filter smoke number were consistently reductions. Small increases in the indicated specific fuel consumption occurred in the low-load cases and a larger reduction occurred in one of the high-load cases. The indicated mean effective pressure fell in the low-load condition and increased or was unchanged in the high-load condition. Overall, the effect of the offset on the indicated performance is minor and secondary to the effect on the rubbing friction; the results are similar when either the valve overlap or the valve opening time was maintained when the crankshaft was offset.