Small off-road gasoline engines have advantages of light weight, low cost and simple operation, which have prove successful when extensively used in generator garden machinery outboard machinery, etc.A turbulent flame combustion model was used to present the effect of turbulent on combustion. The influences of geometry of the cylinder, ignition timing, and gas flow and tube composition upon heat release were taken into account in this model. The basic idea of the model is that the combustion process is regarded as a flame front (usually assumed to be spherical) spreading through the unbaked mixture at certain turbulent burning rate