This work investigates the flame propagation and combustion characteristics of LPG (Liquefied Petroleum Gas) fuel. To clarify the combustion process of the heavy duty LPG engine, the flame propagation and combustion characteristics were investigated using a CVCC (constant volume combustion chamber) and a port injection type heavy duty LPLi (Liquefied Petroleum Liquid injection) engine system. Both the laser deflection method and the high-speed Schlieren photography method were employed to measure the flame propagation speed of LPG fuel. In addition, the single cylinder heavy duty LPLi engine was manufactured to analyze the combustion characteristics of the LPG. The experimental results indicated that the laser deflection method showed the measuring accuracy of this method to be less than 5% when compared with the result of the high-speed camera. According to the CVCC and heavy duty LPLi engine experimental results, the flame propagation reached a maximum speed at the stoichiometric equivalence ratio, regardless of operating conditions, and the effect of the equivalence ratio on both flame propagation and combustion characteristics was greater than that of ambient conditions. In addition, we found that the coefficient of variation of combustion duration increased when the equivalence ratio decreased. Furthermore, the combustion stability worsened as the equivalence ratio moved into the lean region. (c) 2005 Elsevier Ltd. All rights reserved.