Scavenging by Large Valve-Overlap Increases Power and Economy

SINCE the power output of an engine is practically proportional to the weight of the charge, the object has been to increase the weight of the charge burned. The weight of charge inducted by an aircraft engine and the supercharger power required to supply this charge depend among other factors upon how completely the engine is scavenged.

In the conventional four-stroke-cycle engine only the exhaust gases in the displacement volume are forced out of the cylinder by the piston on the exhaust stroke; consequently, the engine cannot induct a charge of greater volume than that of the displacement volume, whereas if the clearance volume could be scavenged also, the engine could induct a charge equal to the displacement plus the clearance volume.

A description of the equipment used for conducting a series of tests to determine the performance of an engine that was completely scavenged as compared with one having only the displacement volume scavenged is given by the author, who explains also the method and scope of the investigation and discusses the test results. Curves are presented which resulted from the test data obtained. The mechanical considerations which pertained during the tests are presented and commented upon, and three conclusions are stated.