The scavenging pump is mounted on one end of the engine and is driven from the crank-shaft, the cross-head of the pump forming one piece with the piston of the low pressure cylinder of the injection air cylinder. All of the compressor stages are water cooled and fitted with automatic valves. The double acting scavenging pump has a piston valve driven by a link motion for reversing it when the engine is reversed. The air enters the pump through the top valve chamber from a pipe leading into the engine room. The air discharges a pressure of about 3 pounds per square inch in a header that passes in front of all four working cylinders. By means of a valve the air entering the low pressure stage of the compressor can be taken either from the atmosphere or from the discharge of the scavenging pump; taking the air from the latter allows of a greater weight of air taken by the compressor and consequently a higher compression for use in emergencies. As in the ordinary type of two stroke cycle engine, two independent sets of exhaust ports are used, one set being for the scavenging air and the other for the exhaust gases, both sets being at the end of the stroke as usual. The air inlet ports are divided into two groups, however, one group being controlled by the piston of the working cylinder, and the other group by an independent piston valve driven from the cam-shaft. Both sets of ports connect with the main scavenging air header. By means of the valve controlled ports it is possible to admit scavenging air even after the other ports are closed by the piston, which greatly increases the scavenging effect. With the air at 3 pounds pressure the air from the valve controlled ports throw the scavenging air to the top of the cylinder even after the exhaust ports are closed. This valve is provided with a reverse mechanism. A single cam is used for operating the fuel inlet valve and the air starting valve, and the reversal of the engine is obtained by turning the cam shaft through a small angle relative to the crank-shaft, which of course also reverses the lead of the fuel valve. Starting is accomplished by compressed air, with the air valve lever on the cam, and the fuel valve lever off. After turning through a few revolutions, the air valve levers are raised, and the fuel levers dropped back on the cams which results in the engine taking up its regular cycle. By moving the tappet rod of the fuel valve out of or into a vertical position, the time of the fuel valve opening is regulated and the amount of air is controlled. This movement is normally performed by a compressed air motor, but in an emergency hand wheels may be used.