Effects of applying a Miller cycle with split injection on engine performance and knock resistance in a downsized gasoline engine

Downsizing a spark ignition (SI) engine with boost pressure is a proven way of increasing the thermal efficiency and reducing CO2 emissions. However, the occurrence of abnormal combustion problems such as knock and super knock could seriously damage the downsized SI engine and limit its efficiency improvement. In this study, the effects of the Miller cycle on anti-knock and engine power are experimentally investigated using a single-cylinder gasoline engine, the speed corresponded to 1600 rpm and air-fuel ratio corresponded to 1. The Miller cycle provides a good anti-knock performance because of its low in-cylinder temperature induced by the lower effective compression ratio, however, it has a poor dynamic output. To increase the intake charge of the Miller cycle, boost pressure was applied. The results showed a clear improvement in both the thermal efficiency and torque of the engine. Furthermore, the knock tendency of the engine was reduced. However, compared with the Miller cycle without boost pressure, the knock tendency with boost pressure was significantly higher. Therefore, to improve both engine power and knock resistance, this study combined split injection to the Miller cycle with boost pressure. The result indicates that the combined method can effectively decrease the knock tendency and increase the engine torque. The best secondary start of injection timing for the Miller cycle with boost pressure and split injection is 100 CAD BTDC. In conclusion, the Miller cycle with intake boost pressure and split injection has a considerable potential for achieving both a better knock resistance and higher engine power.