In this study, a novel variable-length intake manifold is designed to increase the volumetric efficiency of a four-cylinder gasoline engine. The proposed intake manifold is designed with four independent throttle bodies on its inlet duct, which are separately connected to the atmosphere. By opening and closing each one of these throttle bodies, a different length of the intake manifold is obtained. The novelty of this study lies in the fact that in addition to the variable-length system at each engine speed, it uses a different combination of opened throttle bodies to further increase volumetric efficiency. To demonstrate the efficiency of the novel design, a one-dimensional model of the Stock engine is developed using the GT-Power software in wide-open throttle steady-state condition and then validated with experimental results. Five vital criteria are defined for evaluating all of the thermo-fluid, heat, and mass transfer characteristics of the engine. The result of using the novel proposed design indicates that the volumetric efficiency, brake torque, brake specific fuel consumption, brake thermal efficiency, and heat release rate of the engine compared to the stock intake manifold were improved by 6.33%, 7.23%, 0.83%, 1.77%, and 11.79% respectively. Based on the obtained results, the best performance is achieved at 3500 rpm almost in all conditions. At this condition, the air and fuel mass flow rates and brake power are 0.0574 kg/s, 0.0041 kg/s, and 56.42 kW, respectively.

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