High performance naturally-aspirated internal combustion engines require effective use of exhaust pressure waves during the gas exchange process to maximize volumetric efficiency and torque. Under certain conditions sudden increases, or steps, in exhaust runner diameter are used to control pressure wave reflections to provide appropriately timed low pressure waves to the cylinder that reduce pumping work and improve air scavenging. This research evaluates gas exchange performance for an exhaust port and an attached stepped-tube primary using unsteady conditions with 1-D and 3-D CFD. The objectives of this research are to (1) discuss the importance of using unsteady flow simulations in the design of high performance exhaust systems, (2) describe the use of stepped-runners to provide performance gains, and (3) discuss the influence of runner step geometry and the number of steps on gas exchange. Simulations are correlated with experimental data to ensure accuracy of the results. A correlation is found between the step size and the magnitude as well as phase of tuning effects. The number of steps is also found to have a direct impact on tuning. The pumping work of the cycle was significantly affected by the stepped primary design, while the scavenging efficiency was not.

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