Enhanced mixture preparation by liquid phase injection on to the port could promote the application of liquefied petroleum gas (LPG) in spark ignition (SI) engines. Mixture distribution and flame propagation of the liquid phase LPG injection (LPLI) engine with a large bore size were investigated in a single-cylinder optical engine which had optical accesses through both sides of the cylinder liner and bottom of the piston. LPG fuel distribution was visualized quantitively by the acetone planar laser-induced fluorescence (PLIF) method. In addition, a series of bottom-view images were taken by direct visualization using a high-speed camera. Flow conditions were varied with the shapes of intake port and piston geometries characterized by different swirl and squish flows. The effects of fuel injection timings were also studied to characterize the mixture preparation for open-valve injection and closed-valve injection.
Stronger swirl with a swirl ratio of 3.4 and squish flow with a cylindrical piston bowl shape showed faster flame propagations under the open-valve injection condition with preferable mixture formation. On the contrary, closed-valve injection caused the undesirable mixture distribution of the mixture at the ignition, which led to a leaner mixture distribution near the spark plug.