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The working process of a 1.2L low-pressure air-assisted direct injection gasoline engine was studied based on simulation. The flow of working fluid in intake port and cylinder were analyzed. The cylinder mixture concentration distribution at 20°CA between the before and after improvement of combustion chamber under different injection timings were compared. The effect of ignition timing on engine combustion process was discussed. Results show that the overall uniformity of cylinder mixture concentration distribution at 20°CA is better than the original engine, and the mixture concentration for stable ignition could be formed in the two spark plugs gap with the injection timings of 420°CA and 440°CA. With the advance of ignition timing, combustion advances gradually, the maximum cylinder pressure and the maximum pressure rise rate increase. In addition, with the delay of ignition timing, flame development duration decreases gradually, while rapid burn duration decreases at the beginning and then increases.