the flame front penetrates and is quenched by the surrounding layer of air the temperature within the cylinder is high enough to start oxidation of the co and hc into carbon dioxide and water this oxidation continues into the following exhaust stroke no particles of unburned fuel air mixture emerge from alongside the piston and from in back of the top compression piston ring to adhere to the cylinder wall because of this layer of air when the injection of air into the cylinder occurs near the close of the expansion stroke the residual hydrocarbons are oxidized and a layer of air remains at the bottom of the combustion chamber part of this air layer fills the chamber at the close of the following exhaust stroke during the exhaust stroke depicted in fig1 exhaust valve 48 is opened there is sufficient residual pressure within the combustion chamber to keep piston valve 18 closed and the suction of the piston head acting within pumping chamber 19 opens cylinder valve 27 to fill this chamber with air for the following intake stroke the exhaust gases 58 are discharged past the exhaust valve the temperature of these gases while still within the cylinder is high enough to continue oxidation of any remaining co and hc and when exhaust scavenging is complete with the piston at the top dead center position of fig1.