Abstract: The three-dimensional steady flow numerical simulation is studied in the carburetor and the cylinder system of the Model CUB100 four-stroke gasoline engine under maximum of valve lift. The characteristic of the carburetor and the cylinder system compares the carburetor plunger full-opened with semi-opened. The research results reveal that a Lower pressure zone is formed round the carburetor nozzle, the lower pressure in the carburetor plunger full-opened is lowest than semi-opened, but the airflow velocity in the carburetor plunger full-opened is bigger than semi-opened. The throat of a nozzle has the maxi- mum velocity. The twin vortices is obvious in cylinder and inlet pipe entrance. The asymmetry vortices is also obvious in the carburetor plunger semi-opened. The asymmetric velocity still exists on the cross section located the 2 diameter distance form cylinder head bottom. The characteristic of pressure field is sameness with velocity field.

Key words: Engine, Flow field dynamics, Numerical simulation, Three-dimensional steady flow