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.
The exhaust stroke is shown at D and is one of the up strokes of the piston. The cam opens the exhaust valve and the piston forces the burnt gases of the fuel mixture out of the exhaust port, and that clears the cylinder for the next stroke, which will be the A stroke over again, and so on through the same four cycles just described.
The Carburetor and What It Does.—This apparatus is connected to the fuel tanks, which usually contains gasoline, by a supply pipe. The gasoline is forced out of the tank by compressed air being pumped into the latter and thence it passes into the carburetor.
The carburetor changes the liquid fuel into a fine spray, or vapor, and mixes it with air, and this is drawn into the cylinder when the inlet valve opens and the piston is making its suction stroke.
The Magneto Electric Machine.—This is an electrical device which is simply a little dynamo. When it is driven by the engine it generates a high tension current of electricity which will jump between the ends of two wires ⅛ inch apart, and this makes a spark.
The magneto is connected to a spark-plug which is screwed into the head of the cylinder. A timer connected to the magneto and the spark-plug closes the circuit each time the compression stroke is completed; the instant the circuit is closed the current generated by the magneto makes a spark at the business end of the spark-plug and this fires the fuel mixture.
But as good as the gasoline engine is for motor cars, power boats, and airplanes, it has been found sadly wanting as a power plant for submarines; this is due not to any fault of the engine but to the explosive nature of the gasoline which is used.
Gasoline is a very volatile liquid, that is, at ordinary temperatures and pressures it tends to change from its liquid state to a vapor which is really a gaseous state. You may have noticed this if you have been near a place where gasoline is stored, for the whole air is saturated with the vapor given off by it and this is what you smell.
Further, it is quite impossible to store a fuel like gasoline, and to use it for firing an engine in such a confined space as there is on a submarine, without the air becoming charged with the vapor, which is injurious to those who breathe it, and which, should it be accidentally ignited, would explode with such force that it would wreck and sink the submarine.