air shroud 17 placed about the cylinder 1 forming a purged air conduit 17 a for routing air from the purging chamber to the combustion chamber the purged air inlet ports 58 allow the air purged from the purging chamber into the combustion chamber when a conduit valve 30 opens in response to pressure from the purged air in this exemplary embodiment and as shown in fig2 the conduit valve is a ring having a width sufficient to block the conduit completely when biased against the cylinder and purging air shroud by a spring 15 placed between the ring and a cylinder head 19 the conduit valve spring is sized to correspond to the conduit valve ring thus providing an equal distribution of force about the ring 39 s circumference the cylinder 1 and purging air shroud 17 are secured between the cylinder head 19 and a cylinder base plate 29 using cylinder head bolts 18 the cylinder head receives and positions an igniter 20 which in this exemplary embodiment is a spark plug a cap screw 22 is positioned on the cylinder head to provide access to the combustion chamber the cylinder base plate has a hole 60 that closely receives and positions a packing gland 11 through which a connecting rod 12 slidably extends the rod being connected to the piston 2 a resilient bias member 4 cooperates with the connecting rod to bias the piston to a first position in the exemplary embodiment of fig1 the connecting rod bias member is a spring 4 positioned about a connecting rod shaft 10 and against structure 4 a such that the spring is stretched when the piston moves from the first position as shown in fig1 to a second position.