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The geometry and area of the notch in the swirl control valve installed in the intake port were varied to analyze the effects on HC emissions. A swirl control valve functions to promote the formation of a homogeneous mixture, enabling the amount of liquid fuel supplied to the cylinder to be reduced. For this reason, it is difficult to obtain an added effect through the combined use of a swirl control valve and an auxiliary-air type of injector for assisting fuel atomization. Tumble (vertical swirl) flow fields are effective in shortening the combustion period. This results in a higher exhaust gas temperature at an equivalent level of combustion stability. It was thought that swirl flow fields produce residual gas flow in the cylinder after the completion of the main combustion period. It is surmised that the residual gas flow functions to diffuse and promote after-burning of the unburned HC layer.