Some Fundamental Aspects of the Discharge Coefficients of Cylinder Porting and Ducting Restrictions

This paper reports on the experimental evaluation of the coefficient of discharge of ports in the cylinder wall of a two-stroke engine and of restrictions within engine ducting. It is required to know such discharge coefficients, as a function of the area of the restriction and the pressure ratio across it, so that in the mathematical modelling process at each juncture during the open cycle, accuracy can be obtained for the mass flow rates of gases, the magnitude of pressure waves created, and the effect on the internal engine thermodynamics. The discharge coefficients, C_d, are required for both inflow and outflow, i.e in both directions, at all such cylinder apertures or ducting restrictions. A fundamental experimental study is conducted of the effect of size of the engine ducting on the discharge coefficient of the cylinder porting aperture and it is found to have no significant influence. The general applicability of this conclusion in design and simulation practice is debated. A study of the discharge coefficients of restrictions or throttles within engine ducting is carried out and the ensuing map is determined to be significantly dissimilar in profile to that of all published data on engine porting or valving.