Effects of injection timing on mixture distribution were investigated in a liquid phase LPG injection engine for a heavy-duty vehicle. An optically accessible engine was made for planar laser induced fluorescence and flame imaging. Mixture distributions around the spark plug were quantified and compared for open and closed valve injection. Two-dimensional flame images were taken to investigate the effect of mixture stratification on flame propagation. In addition, the overall engine performance and emissions were compared for the injection timings at a selected condition. Unlike the case of closed valve injection, where a nearly homogeneous mixture is formed near the spark plug and the flame moves with swirl, in the case of open valve injection, a cloud of richer mixture than overall excess air ratio is formed near the spark plug and the flame moves in the direction of the rich mixture, which extends the limit of lean burn operation.