The in-cylinder flow of an internal combustion engine has great effect on the major engine performance characteristics. To understand the complex intake phenomena in a small high-speed two-valve-per-cylinder motorcycle engine, a numerical analytic model based on the KIVA-3 code is developed for the three-dimensional transient intake flow, including a moving piston and a moving inclined intake valve. The valve model adopts the body-fitted technique and the dynamic grids induced by the moving valve are automatically generated by the grid remeshing method. Turbulence is represented by k-ε model. Comparison with the measured engine cylinder pressure shows that the simulation result is generally in good agreement with the experiment. The calculated results reveal the formation of the in-cylinder tumble motion, the variation of tumble ratios, turbulence kinetic energy and the cylinder pressure. The effects of engine speeds on the intake process are also investigated. The simulation results provide important information for the design of engine intake system.

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