The Heat-Transfer Processes in Air-Cooled Engines

Abstract

E temperatures are important factors in limiting the power of air-cooled engines. An insight into the manner in which cylinder temperatures vary is obtained from a study of the heating of the cylinder walls by the engine gases and the cooling of the cylinder walls by the passage of heat through the fins to the fin surfaces where the heat is carried off by the cooling air stream. For any set of engine and cooling conditions the cylinder temperatures attain an equilibrium at which the rate of heating of the cylinder is equal to the rate of cooling. In the subject paper, from a consideration of heattransfer theory, expressions have been obtained for the rate of heat transfer from the engine gases to the cylinder walls and from the cylinder walls to the cooling air. These expressions contain several empirical constants, values for which were obtained from test data for a Pratt & Whitney 1340-H cylinder. The test data were obtained by the N.A.C.A. at its engine laboratories at Langley Field, Va. From these expressions, equations are obtained for the average cylinder head and barrel temperatures as functions of the fundamental engine and cooling variables. Although the maximum head and barrel temperatures are the limiting factors, a good approximation to the variation in the maximum temperatures may be obtained from the variation in the respective average temperatures. The average head and barrel temperatures calculated from the equations were found to check experimental values with a good degree of accuracy. Although the empirical constants apply to a particular engine and cooling arrangement, the equations provide a better understanding of the cooling of air-cooled engines in general. The empirical constants associated with the heat transfer from the engine gases to the cylinder appear at present to apply to other engines of a similar type.