Wave Action in Diffusers for Exhaust-Pipe Systems, with Special Reference to the Scavenging of Two-Stroke Engines

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Abstract
The work described was carried out in the Mechanical Engineering Department of Birmingham University and is an extension of the investigation into air flow in a naturally aspirated two-stroke engine described in an earlier paper (Wallace and Nassif 1954)‡. In that paper the scavenging process of an unblown, opposed-piston engine with simple parallel-pipe exhaust and inlet systems was analysed, and the results were compared with experimental observations. Only a brief reference was made to the very marked increase in air flow resulting from incorporation of a diffuser in the exhaust system. The present paper describes first a theoretical investigation of wave effects in the exhaust system using: (1) The small-wave theory; (2) The method of characteristics; (3) An approximate analytical treatment designed to give better accuracy than (1), but to avoid the extremely laborious computations associated with (2); and, secondly, an experimental programme comprising: (1) Air-flow tests with various diffuser arrangements and over a wide range of speeds, and (2) Tests in which indicator diagrams were taken at salient points of the exhaust system. The general conclusions are: (1) The incorporation of a diffuser in the exhaust system leads to an increase in air flow of up to 85 per cent under favourable conditions. (2) The very complex phenomena occurring in the exhaust system can be dealt with satisfactorily by the approximate analytical treatment with great saving in time over the method of characteristics and very much better accuracy than can be achieved with the small-wave theory.

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