practice of the invention it is understood that the advancement of the exhaust valve opening shown as the 60 degrees presented in the last graph of fig4 is a fixed amount provided in one embodiment of the invention wherein the engine has a relatively simple design such that the advancement is fixed for all values of rotational speed of the crankshaft however in accordance with a second embodiment of the invention wherein the invention is implemented in an engine having a variable timing capability the advancement in the time of the opening of the exhaust valve could be varied in accordance with the rotational speed of the crankshaft and the load on the engine for example at an engine speed of 1500 rpm the advancement in the exhaust valve opening could be less than 60 degrees prior to bottom dead center and at an engine speed of 4500 rpm the advancement in the exhaust valve opening could be greater than 60 degrees prior to bottom dead center it is a function of how many degrees of crankshaft rotation it takes to release the gas pressure on the piston it is understood that the foregoing values of advance as a function of engine speed are given by way of example and that more accurate values are obtained by experimentation with a specific form of engine to be evaluated thus in accordance with the second embodiment of the invention a range of values of advancement of the exhaust valve opening namely a range of 40 to 80 degrees of crankshaft angle indicated at b in fig3 provides for more efficient operation of the engine over a range of engine speeds.