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Saturday, November 14, 2009

4 stroke cycle


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.



• Used for heavy vehicles because of high torque output
• Improved fuel economy because diesel fuel much cheaper than petrol fuel (gasoline)


• Air pollution
• Heavy weight engine
• Slower acceleration
• Very high cost
• Diesel engines are noisy and unrefined

There are two classes of diesel (and gasoline) engines: two-stroke and four-stroke. Most diesels generally use the four-stroke cycle, with some larger diesels operating on the two-stroke cycle, mainly the huge engines in ships. Most modern locomotives use a two-stroke diesel mated to a generator, which produces current to drive electric motors, eliminating the need for a transmission. To achieve operational pressure in the cylinders, two-stroke diesels must utilize forced aspiration from either a turbocharger or supercharger. Diesel two-strokes are ideal for such applications because of their high power density--with twice as many power strokes per crankshaft revolution compared to a four-stroke, they are capable of producing much more power per displacement.

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