Residual low-grade fuels are often used for marine diesel engines for reasons of economy. Some marine engines have been damaged due to the difficulty of combustion control of such fuels. Many factors affect scuffing of the piston ring and cylinder liner. Because of the complexity and difficulties, however, no one can predict scuffing. Problems have to be solved one by one. This study focuses on scuffing that is caused by the fuel itself. A spray flame was observed with a high-speed video camera in a constant volume vessel, in which ambient pressure and temperature were controlled. A previous study found that after-burning and long ignition delay were problems, so this analysis focused on combustion phenomena after the end of injection. A function of the period from the end of injec-tion till quenching of the flame and of the flame length at quench showed good correlation with the degree of scuffing damage. These two parameters are more valuable for predicting scuffing than are other methods before bunkering fuel in the ship.