Authors: G. P. Blair, H. B. Lau, A. Cartwright, B. D. Raghunathan, D. O. Mackey
Title: Coefficients of Discharge at the Apertures of Engines
Date: SAE International Off-Highway Meeting - Milwaukee, Wisconsin - SAE 952138
Published: September 1995
Abstract: This paper reports on the experimental evaluation of certain aspects concerning the mathematical modeling of pressure wave propagation in engine ducting. A particular aspect is the coefficient of discharge of the various ports, valves or apertures of the ducting connected to the cylinder of an engine or to the atmosphere. The traditional method for the deduction of the coefficients of discharge employs steady flow experimentation. While the traditional experimental method may well be totally adequate, it is postulated in this paper that the traditional theoretical approach to the deduction of the discharge coefficient from the measured data leads to serious inaccuracies if incorporated within an engine simulation by computer. An accurate theoretical method for the calculation of the discharge coefficient from measured data is proposed.

The paper presents experimental results for the coefficients of discharge for several duct end geometries such as orifices, plain ends, bellmouths and the exhaust port of a two-stroke engine cylinder, and demonstrates that the conventional method for the deduction of the coefficient of discharge may be used only as a comparator for these geometries but leads to serious computational errors if used within a computer simulation. The paper contrasts and compares the values of coefficients of discharge determined by the postulated and the traditional methods, based on the same experimental measurements for the several duct end geometries.

The paper presents further theoretical confirmation that the postulated theoretical approach is justified and accurate, by comparison of the predictions of measured flow rates by the postulated and the traditional theories with the theoretical results from a CFD analysis of an identical duct end geometry. The CFD analysis and the postulated theoretical approach match almost perfectly whereas the traditional method does not.

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