COMPRESSION-RATIO AND THERMAL EFFICIENCY OF AIRPLANE ENGINES

Appreciating the fundamental relation of the compression-ratio to the thermal efficiency, the National Advisory Committee for Aeronautics sponsored a comprehensive investigation of this subject at the Bureau of Standards. Every effort was made to measure the engine performance so completely as to make possible an analysis that would not only explain the results of this particular series of tests but form a sound basis for predicting the effect of changes in the compression-ratio on the thermal efficiency of any engine. The experimental work is as yet incomplete and only some of the more salient results are presented in this paper.

An eight-cylinder airplane engine was used, having pistons allowing compression-ratios of 5.3, 6.3, 7.3 and 8.3. The differences in compression-ratio were effected by crowning the piston-heads by different amounts. The tests were made in the altitude-chamber to secure typical altitude conditions of air temperature and pressure, and those presented in this paper were made under full load at an engine speed of 1600 r.p.m. The resultant data are exhibited in charts and these are analyzed.