This series shows a cut-away diagrams of a Four-Stroke Diesel engine. This engine design is more complex mechanically of two and four stroke as it requires synchronisation of moving parts. The description "four stroke" comes from the fact that the engine fires (burns fuel) on every second upward stroke (travel of the piston from bottom of the cylinder to the top), thus there are four strokes for every ignition of fuel, two upward and two downward. The first stroke moves from top to bottom, where air is drawn in, the first upward stroke compresses the air and fuel is sprayed in, the air and fuel ignite and begin the third stroke where the piston is forced back downwards by the explosive force of the fuel igniting. On the fourth stroke the piston moves upwards again forcing the spent exhaust gasses out of the cylinder.



In the diagram the piston moving towards BDC (Bottom Dead Centre - meaning it is at the lowest point of travel within the cylinder). Air is being drawn through the inlet valve in the top of the cylinder.



The piston starts its upwards movement and the air intake 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.

Just before reaching Top Dead Centre (TDC) fuel is sprayed into the cylinder.

This happens on every second upward stroke of the piston.



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.



At the end of the downward stroke when the piston reaches Bottom Dead Centre (BDC), the exhaust port opens, and the cylinder is swept clean of burnt fuel by the force of the piston rising in the cylinder.

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