The Diesel Engine - Simply Explained

STROKE 1A: PISTON AT BDC

In the diagram the piston is at BDC (Bottom Dead Centre - meaning it is at the lowest point of travel within the cylinder). A blower is forcing compressed air into the cylinder, the air flow forces spent gases out through the open exhaust valves in the top of the cylinder.

Not all two-stroke diesels have a blower, some rely on the suction of the piston alone to draw in the necessary air. An air compressor can replace the blower.

The blower could be considered a very early style of super charger (a device which forces air into the cylinder).

STROKE 1B: THE INSTROKE

The piston moves upwards and covers the air intake port.

The exhaust valve closes.

The charge of fresh air is compressed to about 5% of its original volume. The act of compressing the air heats it tremendously.

This happens on every upward stroke of the piston.

STROKE 2A: PISTON AT TDC

Just prior to the piston reaching the uppermost portion of it's travel at Top Dead Centre (TDC) atomised fuel is sprayed into the cylinder by the fuel injector.

The high temperature of the compressed air in the cylinder ignites the fuel vapour, the resulting explosion forces the piston back downwards.

STROKE 2B: PISTON NEARING BDC

During the downward stroke the exhaust port opens, and the cylinder is swept clean of burnt fuel by fresh air from the inlet port.

This entire cycle is repeated for every revolution of the crank shaft.