The piston having thus been driven down to the limit of its stroke, it commences another upward movement, at which moment another valve opens and lets out the hot waste gases which have resulted from the burning of the oil. Thus the cylinder is cleaned out ready for a fresh supply of pure air to be drawn in on the next ensuing downstroke. The engine thus works upon a series or cycle of operations which are repeated automatically over and over again. First comes a downstroke, drawing in air: then an upstroke, compressing it: then a second downstroke, during which the fuel burns and the power is generated: and, finally, a second upstroke during which the waste products of the burning are ejected. Power, it will be noticed, is only developed in one out of the four strokes: the other movements having, in single cylinder engines, to be performed by the momentum of the flywheel. In most cases, however, the engine has several cylinders in which the cycles are arranged to follow in succession. Thus, if there are four cylinders, there is always power being developed by one of them. The valves are operated automatically by the engine itself just as is the case with steam-engines. The engine also works a small pump which provides the very highly compressed air necessary to blow the oil jet into the cylinder. Arrangements are often provided whereby the engine when working stores up a reserve of compressed air which can be used to start it. From the very nature of its working such an engine cannot develop power until it has accomplished at least four strokes or two revolutions, so that it cannot possibly start itself. If, however, compressed air be admitted to the cylinders to give it a vigorous push or two and so get it going, it can then take up its own work and go on indefinitely. In some cases this is not necessary and that of an engine in a submarine is one of them. In that instance, the electric motor, which drives the boat when submerged, can be made to give the engine a start. By altering the rotation in which the valves act the direction can be reversed. A very simple mechanism can be made to effect this change, so that reversing is quite easy. Aircraft are mostly, if not entirely, driven by petrol engines, some of which are very little different from those of a motor-car or motor-cycle. These motor-car engines are so well known that little need be said about them. It may be well to explain, however, that they, like the Diesel engines, work on a cycle of four strokes, as follows:-- First stroke (down) draws in a mixture of air and gas. Second stroke (up) compresses the mixture. Just at the top of this stroke an electric spark fires the mixture, causing an explosion which drives the piston downwards, thus making the Third stroke (down), during which the power is developed. Fourth stroke (up) expels the waste products of the explosion.