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.
Although all of them work on this same cycle, in which they resemble the engines of the motor-car, there are several much-used types of aero-engine in which the mechanical arrangement of the parts is quite different. Of these the best known is the famous Gnome engine which has a considerable number of cylinders arranged around a centre like the spokes of a wheel. The centre is in fact a case which covers the crank, and the cylinders are placed in relation to it just as the spokes are placed around the hub of a wheel.
There is only one crank and all the connecting-rods drive on to it. Owing to their position around it they thus act in succession, giving a nice regular turning effort.
Further, these engines differ from all others in that the crank is a fixture while the rest of the engine goes round, exactly the opposite of what we are accustomed to. The engine, in fact, constitutes its own flywheel. Rushing thus through the air, the cylinders tend to keep themselves cool, doing away with the need for cooling water and radiators. Consequently engines of this type are the very lightest known in proportion to their horse-power. A fifty horse-power engine can be easily carried by one man.
It would be possible to go on much longer with this most interesting subject of engines, but having treated the three types which are most used in warfare, it is now time to pass on to something else.