Abstract: To give a view of the impact of automatic engine air supply, the geometric model with sealed chamber and without sealed chamber was established,and the working process of them were simulated using renormalization group(RNG) k-ε turbulence model. Simulation represented the change of in-cylinder flow field with the changes in the structure,e. g. cylinder pressure,exhaust flow. Calculation results show that the structure with sealed chamber improved the pressure difference of the exhaust valve' s upper and lower surfaces,which came from the high pressure gas in the process of inlet,improving the feature of the use of high-pressure gas pressure to automatically close the exhaust valve. Meanwhile,it made the gas flow more smoothly through the inlet valve port, and reduced the loss of inlet generated by the gas reflux and the loss of leak generated by the gas exhaust out of cylinder in the inlet process.