Working Principle of Four Stroke (Diesel/Petrol) Engine

Working Principle of Four Stroke Engine

The various strokes in four stroke engine are 

1.   Suction stroke 
2.  Compression stroke 
3.  Power stroke 
4.  Exhaust stroke 

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 01. Suction Stroke: 

• Piston moves from TDC to BDC 
• Opening of intake valve  
• Suction of the air or air fuel-mixture ( air in diesel engine and air-fuel  mixture in petrol engine) 

In suction stroke what happens, first the piston moves from TDC to BDC. As the piston moves the inlet valve opens and the air fuel mixture in case of petrol engine and only air in diesel engine enters into the cylinder. The exhaust valve remains closed during this stroke.

  

 02. Compression Stroke: 

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• Piston moves from BDC to TDC
• Compression of air or air-fuel mixture (air in diesel engine and air-fuel mixture in petrol engine)
• Intake and Exhaust valve remains closed

In compression stroke, the piston moves from BDC to TDC. The inlet and exhaust valve remains closed during this stroke. As the piston moves upward (from BDC to TDC) the compression of air- fuel mixture in case of petrol engine and only air in case of diesel engine takes place. The compression processes completes when piston reaches to the TDC. The compression is done to increase the temperature of the air or air-fuel mixture. The temperature is increased so that it can easily catch fire during sparking in case of petrol engine and spraying of diesel in case of diesel engine.

03.  Expansion/Power Stroke:

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• Sparking and burning of the air-fuel mixture in Petrol engine.
• Burning of diesel by the hot gases.
• Piston moves from TDC to BDC.
• Intake and Exhaust valve remains closed.

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Petrol engine: The air-fuel mixture is ignited by the spark plug. Due to the ignition the burning process starts. The burning of the air-fuel mixture creates a very high pressure burnt gases. This high pressure burnt gases exerts a thrust on the top face of the piston and it starts to move downward from TDC to BDC. This is the power stroke of the engine. In this stroke we get power which is utilized to run the vehicle. The intake and exhaust valve remains closed during this stroke.

Diesel engine: As the Piston approaches TDC the injection of the diesel in the form of spray by fuel injector takes place. As the diesel sprayed by the fuel injector come in contact with the hot compressed gases it catches fire and burning processes starts. Due to burning high pressure hot burnt gases originates and it puts a very high thrust on the top face of the piston. Due the thrust impact on the piston it starts to move in downward direction i.e. form TDC to BDC. 

 04. Exhaust Stroke: 

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• Piston moves from DBC to TDC.
• Opening of the Exhaust Valve.
• Escaping of the hot burnt gases through exhaust valve.

In this stroke the piston moves upward i.e. from TDC to BDC. As the piston moves upward the exhaust valve opens and all the burnt gases left after power stroke starts escaping out of the cylinder. The burnt gases escape out in the environment through exhaust Valve. When the piston reaches at TDC the exhaust process completes. And after this again all the four stroke repeat themselves. © www.mechanicalbooster.com