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To design an optimum engine intake system, a flow model for the intake manifold was developed by the method of characteristics. The flow in the intake manifold was one-dimensional, and finite difference equations were derived from the governing equations of flow. The thermodynamic properties inside a cylinder were found by the first law of thermodynamics, and the boundary conditions were formulated using a steady flow model. By comparing the calculated results with experimental data, the appropriate boundary conditions and convergence limits for a flow model were established. From this model, design variables for the intake system were investigated. The optimum manifold length became shorter when the engine speed were increased. The effect of intake valve timings on inlet air mass was also studied by this model. Advancing intake valve opening decreased inlet air mass slightly, and the optimum intake valve closing was found.

A flow model is a convenient tool for developing the engine intake system. Two flow models for the branched engine intake were developed by the finite difference method and the method of characteristics. The results from the models were compared with the experimental data and the appropriate boundary conditions were established for each model. Modeling the flow at the intake and exhaust valves with a cylinder and at the pipe branches were the most critical part of the flow models affecting the accuracy of the solutions. From two models, it was found that the finite difference model was simpler than the characteristic model in formulation with the better accuracy. The effects of valve timings and intake geometry were studied by the flow models to design the optimum intake system.