FIG. 31. HOW A FOUR-CYCLE GASOLINE ENGINE WORKS.
It consists of (1) a cylinder with valves in it and in which a piston moves to and fro; this is connected to (2) a crankshaft by means of (3) a piston rod as shown in [Fig. 31]. To the cylinder is fixed (4) a carburetor which mixes the gasoline with the air and forms the explosive gas, or fuel mixture as it is called, and (5) a high tension magneto which generates an electric current to make the spark that fires the fuel mixture, etc.
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.