A Diesel engine

A Diesel engine is a machine which produces power by burning oil in air which has been quickly compressed to a high pressure by a piston moving in a closed cylinder. In Fig. I4a the piston is shown at its lowest position, about to start upwards on its compression stroke. As it moves upwards it compresses the air above it into a smaller and smaller volume. If the final volume of the air is one twenty-fifth of its original volume, the engine is said to have a compression ratio of 25.

Fig. 14

In Fig. 14a the piston is shown at its highest position. Here the air in the cylinder is highly compressed and is also very hot, (Have you not noticed how уоur bicycle-pump gets hot when you pump up your tyres?) At this stage of the cycle of operation oil is injected into the hot air. The oil ignites instantaneously. The heat of its combustion raises the temperature of the air still higher and so the pressure of the enclosed air is very much increased also.

The piston is therefore forced down again violently. This second stroke of the piston is called the working-stroke, as shown in Fig. 146.

After reaching the bottom of its working-stroke the piston begins to rise again. Its purpose now is to sweep the products of combustion from the cylinder. So a valve is opened (Fig. 14c) to let the spent gases escape to the exhaust pipe as the piston rises to the top of its third stroke.

The fourth stroke of the piston is downwards again. The exhaust valve is closed and an inlet valve is opened to allow fresh air to refill the cylinder (Fig. I4d). After four strokes, two up and two down, the cylinder is again in the condition we began with. We say that it has completed its four-stroke cycle.

A Diesel engine usually has four cylinders which fire in turn. The four pistons are connected to a rotating crankshaft so that the working-stroke of one piston is always available to drive the three simultaneously non-working-strokes of the other pistons. The remaining energy of the working-stroke is available for the external work the Diesel engine has to do.