Wave Action following the Sudden Release of Air through an Engine Port System

Abstract

As an intermediate step in the analysis of the scavenging process in two-stroke engines, a ported opposed-piston engine liner, discharging cold air through the exhaust ports into a conventional exhaust system, is considered. The ports are sealed by cellophane membranes which, when pierced by a solenoid-operated needle, give a sudden discharge. The process of discharge, wave propagation in the exhaust pipe, and partial evacuation of the cylinder by the returning rarefaction wave, is analysed by methods developed from the work of Bannister and Mucklow (1948)^‡ on waves of finite amplitude, and good agreement between the experimental and theoretical investigation is obtained, particularly with regard to the duration and magnitude of the depression in the cylinder. In addition, the coefficient of discharge of the exhaust ports is examined under different release conditions and a relation, based on dimensional analysis, is established from which a coefficient of discharge under any given set of operating conditions may be deduced. The results of the present investigation may be summarized under three headings:—( 1) Wave Action. The various phenomena associated with the discharge of compressed gas from the cylinder into the pipe through the exhaust ports may be accounted for satisfactorily by an extension of the theory of waves of finite amplitude. ( 2) Coefficient of Discharge. The coefficient of discharge of the exhaust ports varies with cylinder pressure, port opening, and temperature, and a general relation, based on dimensional analysis, is established between the coefficient of discharge and these variables. ( 3) Depression in the Cylinder. The duration and magnitude of the depression in the cylinder following wave action in the exhaust pipe may be evaluated satisfactorily with the aid of a step-by-step analysis based on the finite wave theory. While there is no scavenging action, in the accepted sense, in the experimental apparatus, owing to the permanently closed inlet ports, the present analysis may readily be extended to simultaneous flow through the inlet and exhaust ports, as it occurs in the running engine.