the two cycle engine described is a preferred form of the engine it is understood that most of the features illustrated and described may also be applied to four cycle engines this is desirable for high speed engines as it provides more time for the introduction of the new working fluid the principle modification of normal four cycle practice is a change in the exhaust valve timing instead of closing the valve at the end of the exhaust cycle the valve is held open during the entire exhaust and intake cycle so that if nothing is added to the cylinder the exhaust gas simply moves out into the exhaust manifold and is then drawn back into the cylinder again on the other hand if a full charge of working fluid is pumped into the cylinder during the intake cycle the exhaust does not re enter at part load operation it is desirable to introduce the working fluid in a pulsed flow which terminates at the end of the intake cycle but has a variable beginning the time depending on the quantity of working fluid introduced the rate of flow should match the displacement of the piston in this manner the exhaust gas and new working fluid do not enter the cylinder at the same time to become mixed together there is no vacuum cycle and the control and operation of the.