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Aiming at the working process research of independently designed 2.4L gasoline engine, RNG k-ε model, Weller combustion model, PISO arithmetic and moving meshes techniques were employed to carry out three-dimensional dynamic simulations of the flow-field and combustion process in the cylinder. The results of three-dimensional dynamic flow-field simulations were applied into three-dimensional dynamic combustion simulations as the initial conditions. Thus, original field error supposed by experience can be avoided, and combustion simulations can approximate to the actual working conditions more. It was discovered that tumbles formed in large-angle pent-roof combustion chambers do not break into turbulence during compression. By optimization design of ports and combustion chamber structures, high turbulence intensity can still be obtained, and the combustion process can be perfected even when tumbles are maintained. Compared with experimental result of layoutⅡ, cylinder pressure of simulated result is almost identical with the experimental data.