Effects of Pulsating Flow on Exhaust Port Flow Coefficients

Five very different exhaust ports of diesel and gasoline engines are investigated under steady and unsteady flow to determine whether their flow coefficients are sensitive to unsteady flow. Valve lift is fixed for a specific test but varied from test to test to determine whether the relationship between steady and unsteady flow is lift dependent. The pulse frequency is chosen to correspond to the blow-down phase of an engine running at approximately 6000 rpm, but the pressure drop across the port is much smaller than that present in a running engine. Air at room temperature is used as the working fluid.

It is shown that unsteady flow through the five exhaust ports causes, at most, a 6% increase or a 7% decrease in flow coefficient. When the change in flow coefficient for a given exhaust port is averaged over the range of valve lifts most important in determining engine performance, the average increase in port flow coefficient ranges from 0.5-2.5% which only minimally affects engine performance. This finding affirms that it is reasonable to measure exhaust port flow coefficients under steady flow conditions and then use these flow coefficients in gas exchange simulations to optimize valve timing and the exhaust manifold for state of the art flow efficiency under running engine conditions.