Petrol 4 Stroke Cycle

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The four stroke petrol engines cycles are very similar to that of the four stroke diesel engine, except from a few differences. Which I will explain on the 4-Stroke Diesel page.

The first stroke of a petrol engine is called the ‘Induction’ stroke, the piston moves down the cylinder, the inlet port opens, and the exhaust port closes up. Fresh air (oxygen) and fuel is drawn into the cylinder. The majority of petrol engines draw in fuel and air. After the cylinder has drawn the air and fuel into the chamber, the inlet port closes, and it goes onto the next stroke, the ‘Compression’ stroke.

The second stroke of a petrol engine is called the ‘Compression’ stroke; on this stroke the piston heads up the cylinder, whilst both inlet and exhaust ports are closed, the mixture of air and fuel is then pushed into the combustion chamber, which is the top end of the cylinder, and because it has nowhere else to go, is compressed.

The third stroke of a petrol engine is called the ‘Ignition’ stroke; this is the stroke that gives all of the power to keep the engine ticking over. Nearing the end of the ‘Compression’ stroke, above, the mixture of air and fuel is very hot due to the massive compression it has just undertaken, a spark plug; sitting at the top of the cylinder will spark and ignite the highly compressed gases inside of the combustion chamber. This ignition causes a powerful explosion, which in turn pushes the piston down the cylinder, moving on to the exhaust stroke.

The fourth stroke of a petrol engine is called the ‘Exhaust’ stroke; on this stroke the piston is heading up the cylinder, this being cause by the crankshaft’s rotation. The inlet port is closed, and the exhaust port is open. The force of the piston coming up the cylinder therefore forces the burnt gases out of the cylinder via the exhaust port. Then the inlet port opens, and the exhaust port closes ready for the next ‘Induction’ stroke.

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