(b) The power that this stroke gives to the flywheel forces the piston up again, and this compresses the air as shown in A, [Fig. 32]. Now when you compress air, it heats it, and the amount of heat developed depends on how much the air is compressed; you can even feel the heat that is set up by compressing the air in a toy pop-gun; or you may have noticed that when an automobile tire is pumped up fast it gets hot. Of course, while the air is being compressed in the cylinder of the engine the compressed air valve stays closed.
Courtesy of Scientific American
THE ENGINE ROOM OF A MODERN SUBMARINE SHOWING THE DIESEL ENGINES
(c) When the fuel inlet valve opens, the heavy fuel is forced into the cylinder by means of compressed air which presses on the fuel in the supply tank. The instant the fuel strikes the hot air in the cylinder it ignites and burns, and as it burns it expands just as the gases of burning powder in a cartridge expand. This forces the piston down and makes the power stroke as shown in B, [Fig. 32].
Here, then, is another great advantage of the Diesel engine over the ordinary gasoline engine: it does not need an electric spark or any other kind of flame to fire it.
(d) As soon as the piston has reached the down end of its power stroke, the exhaust ports are opened and more compressed air flows through the compressed air valve; this blows what is left of the burnt gases out of the cylinder through the ports, and as soon as this is done the piston starts to go up, when it compresses whatever air there is in the cylinder again.
While this stroke is being made the air compressor piston in the bottom of the cylinder, which is a part of the regular piston, has been compressing the air needed to perform the above operations. Since there is only one waste stroke to every power stroke, the engine is a two-cycle one, and herein lies its third big advantage over the gasoline engine.
The reason that the Diesel engine is better adapted to burn heavier oils than the gasoline engine is because the latter uses an electric spark to ignite the fuel mixture and this must be very light and volatile, since the spark is not hot enough to fire the heavier oils.
Again, in a gasoline engine the fuel mixture explodes—that is, it burns very rapidly, like the powder charge in a cartridge—whereas in a Diesel engine the fuel mixture expands when it is ignited, very much as steam expands in the cylinder of a steam engine.