The Theory of Sudden Enlargements Applied to the Poppet Exhaust-Valve, with Special Reference to Exhaust-Pulse Scavenging

Abstract

It is shown that the simple theory of incompressible flow through a sudden enlargement may be extended to describe fully the compressible flow of a perfect gas. The generalized theory has useful bearing on the performance of air ejectors, although its greatest value is in furnishing a standard framework of description for any flow system in which the presence of a sudden enlargement is evident. The flow properties of the poppet type of exhaust valve are examined on this basis, and it is verified that an optimum design will differ little from common practice: a chart is given which shows the overall compressible-flow characteristics of such an optimum form. Further charts are developed which give a graphical representation of the basic theory, and assist in the solution of problems of steady and transient flow through systems containing a sudden enlargement. Special attention is directed to the solution of problems arising from the exhaust and scavenging processes of a two-stroke internal-combustion engine. Examples given in appendix form show how the “blow-down” and scavenging periods may be separately analysed, and it is shown how the ability of such an engine to remain self-sustaining on pulse scavenging alone may be examined at the design stage.