First we will learn about the basic parts of IC [Internal combustion] engine,they are

1. cylinder– It is the part where the piston reciprocates
2. piston– It is the main constituents which makes crankshaft move,which in turn moves the vehicle
3. Intake valve– Intakes the fuel mixture (fuel+air)
4. exhaust valve– Releases unburnt gases out
5. spark plug– which ignites the fuel mixture
6. connecting rod– which connects the piston to crankshaft.

You can clearly understand by seeing the above picture.

petrol engine is also called SI(spark ignition) because here fuel mixture is ignited by spark plug. where as disel engine is called CI(compression ignition) engine because the fuel mixture is ignited by compression.

stroke is the movement of the piston from the top, to the bottom of the cylinder.

As the name says 4 stroke petrol engine, it has 4 strokes namely

1. suction stroke or intake stroke
2. compression stroke
3. power stroke or combustion stroke or expansion stroke
4. exhaust stroke

Now we will study each stroke:-

1. Intake stroke or suction 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 creates suction inside the cylinder  and  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. 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.

Click here to see a animation video on 4 stroke petrol engine

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