Combustion and flame propagation characteristics of the LPG liquid phase injection (LPLI) engine were investigated in a single cylinder optical engine. A liquid phase LPG injection is helpful to increase volumetric efficiency and to decrease mixture temperature. Engine operation temperature can be lowered with the aid of low mixture temperature. As a consequence, the exhaust gas temperature and the amount of NOx emissions are decreased. The engine knock tendency is also reduced and it does affect favorably to extend a lean misfire limit. A lean burn operation is important not only to enhance fuel economy but also to reduce the thermal stress of a heavy duty LPLI engine. It has been found that the in-cylinder bulk flow such as swirl and tumble and the injection timing are important parameters in enhancing lean operation. In this study, the effects of piston geometry along with injection timing and swirl on flame propagation characteristics were investigated. A series of bottom-view flame images were taken from direct visualization using a UV intensified high-speed CCD camera. Concepts of flame area velocity, flame propagation patterns were introduced to analyze the flame propagation characteristics.
The results show the correlation between the flame propagation characteristics, which is related to engine performance of lean region, and engine design parameters such as swirl ratio, piston geometry and injection timing. A strong swirl was helpful to promote faster flame propagation under open valve injection. And the flame speed was significantly affected by injection timing even among open valve injection timings. The piston geometry effect was seemed to be primary based on the squish effects.
