A pump piston in a two-stroke engine is formed integral with or attached to one working piston, supplies scavenging air to a reservoir surrounding the cylinders and thence to the working cylinder. The inventor describes an engine in which two pistons reciprocate in opposite directions in the same cylinder. A piston carries the pump piston and scavenging air is compressed in the crank-chamber and transferred through valves and ports to reservoir spaces.
Admission and exhaust ports are formed in the cylinder. In one arrangement shown the pistons are connected to the same crankshaft. The pump piston is attached to the crosshead which may work in slots in the cylinder wall. The air is conveyed to the reservoir through a passage or through valves in the piston.
In a further modification, the piston rods work in glands in the partition, the space alone being used as the reservoir. The pistons are prevented from rotating by a recess in the scavenging piston engaging a projection on the casing. Baffles tend to prevent loss of lubricating oil splashed by the cranks. In multi-cylinder engines the reservoirs of the several engines may be interconnected to reduce the variations in pressure.