This series shows a cut-away diagrams of a Four-Stroke Petrol 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). A mixture of Air and Petrol 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 and petrol is compressed to about 5:1 (20%) of its original volume. The act of compressing the air heats it tremendously.

This happens on every second upward stroke of the piston.



The spark plug fires and the combination of the spark and the high temperature of the mixture 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.

Thanks to Dr Gary Zimmer who pointed out the compression ratio was way out for the engines described here. I appreciate it when people who discover errors on the pages point them out so they can be corrected!