All About Engines

228 All About Engines through the valve in the piston with the air and petrol. Some of it was flung out through the exhaust valve while that which remained was liable to be charred by the temperature of explosion. This caused premature ignition, interfered with regular spark- ing by depositing soot on the sparking plug, and choked up the exhaust valve. In spite of these defects it was by far the most popular aeroplane engine in 1912, when the makers introduced several important improvements. The valve in the piston was abolished, ports were provided in the cylinder at a point where they were covered and uncovered by the piston. The exhaust valve remains open for a short time after the waste gases have been expelled, and a certain amount of air enters the cylinder. The valve then closes, the ports in the cylinder are un- covered, and a rich mixture of petrol and air is forced in by a pump. With these alterations the speed can be varied by means of a throttle valve from 1,000 to 200 revolutions a minute, and though powerful rivals have arisen the engine is still used on aero- planes to-day. At the military trials in England in 1912 the prize for speed was won by S. F. Cody on a biplane of his own construction, driven by a 120 horse-power Austro-Daimler motor. The following year, in the War Office reliability trials of aeroplane engines, only one succeeded in passing the test—a continuous run of twenty-four hours—and that was the Green engine, All the wonderful progress made during the war was under the stress of military necessity, and the details