The Development of an Engine with a Higher Compression Ratio

Abstract

In all internal combustion engines working on a cycle in which a combustible mixture is compressed prior to the ignition and expansion strokes, the efficiency is limited to the efficiency at which the maximum compression temperature is equal to the ignition temperature of the fuel used. For the internal combustion engines, such as are used in the automotive and airplane field, the fuel is gasoline. This has a flash point at about 600 degrees Fahrenheit, depending on the gravity of the fuel. Compression temperatures higher than this, cause preignition. In fact compression temperatures approaching this limit are difficult of realization because of detonation or knocking. It is evident on analysis that the higher is the expansion ratio the higher will be the efficiency of any engine; for the larger is this ratio, the more will the heat of combustion be utilized in doing useful work. Due to mechanical reasons it is customary in engine work to keep the expansion ratio practically equal to the compression ratio, so that the efficiency is limited to that determined by the compression ratio, which is in turn limited by the final temperature of compression. It has been the purpose of the investigation to develop a method which has been proposed for lowering the maximum compression temperature so that an increase in the expansion ratio could be realized. In this method cooling of the charge during the compression stroke is obtained by connecting the cylinder with an auxiliary chamber through a valve during a portion of the compression stroke. The pressure in the auxiliary chamber builds up until the flow from the chamber when the connecting valve is first opened is equal to that forced back into the chamber during the time the valve is open. The auxiliary chamber is fitted with cooling tubes so that cooling in the cylinder is affected by means of these gases and the lower compression ratio during that portion of the stroke when the auxiliary valve is open.