What is Compression Ratio – Petrol and Diesel Engine?

 A compression ratio (CR) of an I C engine is the ratio of the total volume of the combustion chamber To the volume left after complete compression i.e. clearance volume. In simple words, it is the ratio between the total volume of the combustion chamber which is left when the piston is at its bottom dead center and the volume left inside the combustion chamber when the piston moves to its top dead center.

For example- Let’s consider an engine having 1000cc total volume out of which 900cc is the swept volume i.e. the volume covered by the piston when it moves from BDC to TDC, and having 100cc clearance volume i.e. the volume left inside the cylinder when the piston reached to its TDC. So the compression ratio of this engine will be 1000:100 or 10:1.

It is found that greater the compression ratio more will be the power output of the engine.

The compression ratio of diesel engine is much higher than that of petrol engine. i.e. for petrol engine the CR varies from 10:1 to 14:1 and for diesel engines, the CR varies from 18:1 to 23:1.

1. Petrol Engine-

If we talk about 4-stroke petrol engine the compression ratio has its own significance that are-

  • As we all know the in petrol engine air-fuel mixture enters the combustion chamber during the suction stroke and for proper mixing and for proper combustion of this air-fuel mixture compression of this mixture is required which is done by the engine in its compression stroke, so a good compression ratio of the petrol engine is required for proper combustion of the air-fuel mixture which in turn provides better thermal efficiency.
  • The pressure inside the cylinder increases during compression stroke which in turn raises the temperature of the air-fuel mixture that leads to complete or proper combustion of the fuel when the spark plug produces spark which in turn provides better fuel economy and also prevents the engine from various defects like knocking.
  • The petrol engine with proper CR provides a balanced amount of power and speed.
  • The petrol engine usually comes with 10:1 to 14:1 compression ratio depending upon the application and design requirement.

2. Diesel Engine-

When it comes to diesel engines the compression ratio has its greater significance as-

  • In diesel engine a high CR is required  as the diesel engine doesn’t  have any spark plug so the combustion of the fuel completely depends upon the compression of the air provided by the compression stroke of the diesel cycle because of which diesel engine is also known as compression ignition engine.
  • The diesel engine with high compression ratio provides the engine with high compression i.e. provides high pressure rise, which is required to increase the temperature of the compressing air to the extent of the auto ignition temperature of the fuel that is to be sprayed by the fuel injectors which in turn provide complete or proper combustion of fuel.
  • The diesel engines are known for providing high power output which is due to the high compression ratio of the diesel engine, as we know higher the CR higher will be the thermal efficiency or work output.
  • A diesel engine with high CR provides high fuel economy due to the higher thermal efficiency provided by the high compression combustion.
  • Diesel engines usually come with higher compression ratios varies from 18:1 to 23:1 depending upon the application and the design requirement.

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