Coefficients of Discharge in High Performance Two-Stroke Engines 962534

Coefficient of discharge for a particular flow discontinuity is defined as the ratio of actual discharge to ideal discharge. In an engine environment, ideal discharge considers an ideal gas and the process to be free from friction, surface tension, etc. Discharge coefficients are widely used to monitor the flow efficiency through various engine components and are quite useful in improving the performance of these components.

In modelling the flow through internal combustion engines it is equally important to have accurate values for coefficients of discharge through the combinations of valves, ports and ducts. It is especially important when modelling high performance two-stroke engines, due to the relatively high flow rates and the rapidly changing flow directions. Such an engine relies on the plugging pulse from the tuned exhaust system to ram escaped fresh charge back into the cylinder, prior to exhaust port closure. In this case a high coefficient of discharge is desirable for inflow (reverse flow) through the exhaust port. In a low performance engine which does not use a tuned exhaust system, it is probably desirable to have a low coefficient of discharge for inflow through the exhaust port.

Discharge coefficients can be quite easily measured under steady flow conditions for a range of pressures and flows. In the past they have been successfully applied to the simulation of unsteady flow in a quasi-steady fashion. This paper presents experimental results for coefficient of discharge relating to a high performance two stroke engine. Measurements are shown for various flow conditions at the following geometrical boundaries:

cylinder - exhaust port - pipe,

transfer duct - transfer port - cylinder

inlet - reed valve - crankcase.