All About Engines

The Oil Engine 243 any form of solid fuel, there are obviously advan- tages apart from the economy of the engine. The amount of attention is, indeed, reduced to a mini- mum. But it must not be assumed, as we have already had occasion to point out, that any less intelligence is desirable in its management. The attendant is relieved of a good deal of manual labour, and the strain of continuous watching oil, air, and water supply is transferred to automatic devices. It is still necessary, however, for him to see that these devices are acting properly, and to know how to make adjustments when necessary. The man must, in fact, not only understand his engine, but he must also understand the little accessories which control its motion. He does not have to act so fre- quently, but when he does act it must be with wider knowledge and deeper understanding than before. In addition to the single-acting, four-stroke engine that has been described, two-stroke and double-acting engines have been made. The two-stroke engine acts on the same principle as the two-stroke gas and petrol engines explained in previous chapters. Near the end of the explosion stroke the piston uncovers a number of openings in the walls of the cylinder through which issues a stream of compressed air. This air acts as a scavenging charge, sweeping out the waste gases, and leaving in sufficient air for the compression stroke. In this way the piston receives an impulse every revolution. The cam shaft, which operates the valves, must now rotate at the same