General remarks Although internal combustion engines began to be used in industry long ago - explosive engines about 70 years ago, slow combustion engines about 50 years ago - but their theoretical study lagged behind, as it often happens when introducing new ways of technical progress.The main problem of theoretical research in the field of heat engines in general is to determine the maximum values of thermal efficiency of engines at work without heat losses caused by side causes, not by the principle of conversion itself.