Tuesday, May 14, 2013

Four Stroke Petrol Engine - Working and Principle with P-V Diagram

Physics | Reference Article
Working of a Four Stroke Petrol Engine and its Principle
For : Class 11

Principle of a Four Stroke Petrol Engine
The principle used in a four stroke petrol engine is commonly known as Otto Cycle. It states that there would be one power stroke for every four strokes. Such engines use a spark plug which is used for the ignition of the combustible fuel used in the engine. Most of the cars, bikes and trucks use a 4 stroke engines.
In every Otto cycle there is an adiabatic compression, addition of heat at constant volume, an adiabatic expansion and the release of heat at constant volume. The P-V diagram for a 4 stroke engine is as follows:

Working of a Four Stroke Petrol Engine
A stroke is the movement of the piston from the top, to the bottom of the cylinder. 

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 

The steps involved are as follows:

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.

Note : This article might not be for good answer to score good marks in the exam. we have posted it only for the reference. This was described in easy language with picture diagram so we hope it will be helpful for your study.

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  1. Hello , you really did very well in your post.The post is full of information and pictures also shared in the post.
    Thank you so much...
    2 Post Car Hoists


  2. Hello this is really nice post.
    Thanks for sharing this.
    Workshop Equipment

  3. It's nice for study to undarstand the working of otto engine ....k.k (m.e)

  4. It's nice for study to undarstand the working of otto engine ....k.k (m.e)

  5. Really good and informative article ..hope you write about Carnot engine too


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