A study of fluid mechanical effects on unburned hydrocarbon generation has been made in a single compression-expansion model automobile engine. Full optical access has allowed color schlieren observations of gas motion inside the engine cylinder. Motion pictures of the gas motion and flame propagation have been taken at a rate of seven thousand frames per second for the following cases: 1) intake stroke 2) exhaust stroke 3) compression power stroke with combustion and blow-down with appropriate exhaust valve opening. Unburned fuel concentrations were measured by means of a gas chromatograph. The results show that turbulent motion of the mixture increases the amount of unburned fuel. It is implied that the rolled-up vortices play an important role on wall flame quenching processes in an engine.