air compressed into the air chamber 10 below piston 8 can be bypassed as shown into the volume of the combustion chamber above the piston as the piston reaches the bottom of its stroke allowing this amount of otherwise unused air to assist in the cooling and scavenging process this bypass vent 31 can be in the form of an external line as shown or simply be a channel cut into the cylinder wall at this location fig4 shows the combustion chamber check valve 24 open during the return of the power piston 8 due to the accompanying pressure drop in the combustion chamber 2 air from the scavenging plenum chamber 4 passes through the open combustion chamber check valve 24 and flows through the combustion chamber 2 and out the exhaust valve 16 scavenging exhaust gases with it simultaneously the power piston 8 starts to return by spring 30 or other means to its top or starting position drawing in air into the air chamber 10 below it through the air inlet valve 32 while forcing exhaust out of the combustion chamber 2 through the exhaust valve 16 above it pressure in the scavenging plenum chamber 4 is dropping at this time and air is beginning to flow.