39 s inlet pressure corresponding to the piston back pressure is equal to the optimum p back defined by the optimum s this optimum represents the most advantageous distribution of charge expansion enthalpy between the piston component of the gas generator unit and the expander unit based on the design optimization consideration p back and hence the charge pressure during the exhaust stroke is to be maintained at a value higher than p super since the intake process to the cylinder cannot take place until the charge pressure within the cylinder drops to the input pressure there can be no overlap of operation of the exhaust and input valves or exhaust through the input valve would occur instead the exhaust valve must close almost at top dead center to lower piston pressure without adversely affecting the p back for the turbine however an additional scavenging exhaust valve is provided as illustrated in fig2 this valve opens simultaneously with the closing of the exhaust valve 40 and opens to a manifold 44 maintained at a pressure slightly lower than p super and remains open until adequate scavenging is effected thus the exhaust process includes the following steps 1 the exhaust valve opens a few crankcase degrees before bottom dead center 2 the exhaust valve closes immediately after the piston reaches top dead center 4 charge scavenging continues as the intake valve opens at a crank angle after top dead center as the scavenging manifold is at a pressure below p super exhaust continues to be removed from the cylinder 5 the scavenging exhaust.