Four Stroke Cycle Spark Ignition (petrol) Engine

Gasoline or petrol engines are also known as spark-ignition (S.I.) engines. Petrol engines take in a flammable mixture of air and petrol which is ignited by a timed spark when the charge is compressed. The first four stroke spark-ignition (S.I.) engine was built in 1876 by Nicolaus August Otto, a self-taught German engineer at the Gas-motoreufabrik Deutz factory near Cologne, for many years the largest manufacturer of internal-combustion engines in the world. 

As the name suggest the Four Stroke Petrol Engine uses a cycle of four strokes and petrol as the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely:

1.An Intake Stroke

2.A Compression Stroke

3.A Combustion Stroke also called Power Stroke

4.An Exhaust Stroke 

1.Intake Stroke

As the name suggests in this stroke the intake of fuel takes place. When the engine starts, the piston descends to the cylinder's bottom from the top. Thus the pressure inside the cylinder reduces. Now the intake valve opens and the fuel and air mixture enters the cylinder. The valve then closes.

2.Compression Stroke

This stroke is known as compression stroke because the compression of the fuel mixture takes place at this stage. When the intake valve closes (exhaust valve is already closed), the piston forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is around 1/8th of the original volume. An engine is considered more efficient if its compression ratio is higher.

3.Combustion/Power Stroke

Now in case of petrol engine when the fuel mixture compresses to the maximum value the spark plug produces spark which ignites the fuel mixture. The combustion leads to the production of high pressure gases. Due to this tremendous force the piston is driven back to the bottom of the cylinder. As the piston moves downwards, the crankshaft rotates which rotates the wheels of the vehicle.

4.Exhaust Stroke

As the wheel moves to the bottom the exhaust valve opens up and due to the momentum gained by the wheel the piston is pushed back to the top of the cylinder. The gases due to combustion are hence expelled out of the cylinder into the atmosphere through the exhaust valve. 

The exhaust valve closes after the exhaust stroke and again the intake valve opens and the four strokes are repeated.