Abstract: The influences of initial piston position on the engine rotation characteristic and realization of direct-start mode without starter are investigated on a 4G15 GDI engine. The initial piston position range which can ensure the success of direct-start mode under different coolant temperature and spray pressure is explored. Results show that both forward and reverse direct-start modes can succeed only if the initial piston position is in a suitable range. However, the best ranges for the two kinds of direct-start mode have both superposition and difference. The combine using of the two modes can significantly extend the suitable range of the initial piston position of a successful direct-start. Either too low or too high coolant temperature can decrease the range of the initial piston position. Specifically, the suitable range has a maximum value when the coolant temperature is 70 ℃. Besides, improving the spray pressure also helps extend the initial piston position range of a successful direct-start. Generally, it is a wise choice to locate the piston in expansion cylinder in 90 ~ 125 °CA, ATDC, that can achieve successful direct-start in wide ranges of both coolant temperature and spray pressure.

Key words: power machinery and engineering, direct injection gasoline engine, direct-start mode without starter, initial position of piston, engine rotation characteristic