Effect of initial start conditions on combustion and rotation characteristics of first firing cycle during direct-start process for a GDI engine

The first cycle combustion is important for the start process of spark ignition engines and plays an important potential role in the direct-start process. In the experimental work, the effects of initial start conditions, such as initial piston position (IPP), coolant temperature (CT) and fuel injection pressure (FIP), on the combustion and rotation characteristics of the first firing cycle during direct-start process had been investigated. For different IPPs and CTs, the combustion efficiency (CE) increased obviously with the improved excess air coefficient (λ). The most advanced combustion process and fastest heat release velocity appeared for the IPP of 90 and 100°CA. With the λ of 0.7-1.0, the CE was gradually improved for the IPP changed from 40 to 90°CA and the difference was not obvious for the IPPs of 90-120°CA. During the IPP range of 60-110°CA, the engine could get higher peak rotation speed in a relatively wide λ range of 0.6-0.9 and only when the λ was too large or too small, the peak rotation speed decreased obviously. The peak in-cylinder pressure and heat release rate improved, the combustion duration shortened, the heat release velocity increased and the CE enhanced with the CT and FIP improvement. However, the engine still could get bigger peak rotation speed using a relatively smaller λ when the CT and FIP were low. Compared with the two direct-start modes, the IPP range for successful direct-start had a certain overlap and also some differences. The range expanded to 85-130°CA with combined using two direct-start modes and increased and then narrowed with the enhanced CT, but the effect of FIP on it was weak. Controlling the temperature near 70 °C made the successful realization range biggest.