It is easy to understand how a gas-engine works if you will just remember that for every _power stroke_ there are three other strokes, making four strokes altogether, or _four cycles_ as it is called.
The power stroke of the piston is the stroke made by the explosion of the fuel mixture and this forces the piston down. This is the stroke that turns the crankshaft _one-half_ of a revolution and gives it force enough to carry it around until the next power stroke takes place. Thus the flywheel of a single cylinder four-cycle engine makes two complete turns or revolutions to each power stroke of the piston.
Fig. 31 shows how the inlet and exhaust valves are worked, each one by a little wheel with a lump on one side, or _cam_, as it is called, which is fixed on a _cam-shaft_ and is turned by the crankshaft. The cams, of which there are two for each cylinder, are set directly under the ends of the _valve rods_, and as the cam-shaft revolves, the little lumps on the cam strike the valve rods at the right moments and this lets the fuel mixture into the cylinder and lets the used and burnt-up gases out of it.
Each valve is provided with a stiff spring, which, as soon as the lump on the cam has turned past the valve rod, lets the latter drop again and so closes the valve.
Fig. 31 also shows the complete action of a four-cycle engine. The _suction stroke_ is shown at _A_; as the piston moves down, the cam forces the inlet valve up and the piston sucks the fuel mixture into the cylinder. The exhaust valve is closed while this stroke is taking place.
The _compression stroke_ is shown at _B_; the _momentum_, that is the stored up energy of the flywheel, carries the piston up and forces the fuel mixture into a very small space, that is, it _compresses_ it. By this time the cams have moved past the valve rods and both the inlet and exhaust valves are closed.
The _power stroke_ is shown at _C_, and this is the only one of the four strokes that actually counts. When the fuel mixture is compressed it is exploded by an electric spark and the force of the explosion drives the piston down and gives the flywheel great momentum.
It is the momentum produced by this stroke that not only furnishes enough power to carry the flywheel round until another power stroke takes place, but also furnishes the excess power to do useful work, such as to drive a dynamo or a propeller. Of course both valves are closed when this stroke is being made.