To correspond to the issues of climate change and energy security, the engine thermal efficiency has been enhanced by expanding combustion limit. The current main technology to enhance combustion is high tumble which strengthens turbulence intensity in the combustion chamber. However, the existence of combustion limit still remains even though the combustion limit is expanded. Therefore, the chemical reaction of fuel molecules is focused on in this research and it is clarified that some fuel molecules contribute to enhance combustion. This paper describes the effect of the difference of fuel molecules on combustion characteristic.