Abstract: A reciprocating piston machine comprising a main piston secured to a piston rod, a cylinder in which the main piston can slide, the main piston and cylinder defining a combustion chamber, a scavenging plate disposed in the combustion chamber and axially movable therein to expel combustion gases from the chamber, at least one subsidiary piston fixed on the piston rod below the main piston and slidable in a cylinder defining pumping chambers by means of which gas can be forced into the combustion chamber above the main piston at superatmospheric pressure, the arrangement being such that movement of the scavenging plate away from the cylinder head is by means of the compressed gas which then forms the fresh charge for introduction into the combustion chamber, the scavenging plate being provided with a one-way valve through which the fresh charge passes to enter the combustion chamber proper.