What is Compression Ratio – Petrol and Diesel Engine?

Here is the complete guide about Compression Ratio. Here we provides Compression Ratio working principle and Advantages of Higher Compression Ratio ect.

Compression Ratio is one of the essential specifications of an Internal Combustion engine. It is the ratio of the volume above the piston when it is at the bottom-most position (BDC) to the extent above the piston when it is at the top-maximum position (TDC). It indicates the quantity to which the air-gasoline mixture is compressed in the engine.

Compression Ratio

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 easy words, it is the ratio among the total volume of the combustion chamber that is left when the piston is at its bottom dead middle and the volume left inside the combustion chamber whilst the piston actions to its top dead center.

Working of Compression Ratio

Since Petrol may be very volatile, the ‘Compression Ratio’ for Petrol engines is generally lower. Thus, it varies from 10:1 to 14:1. The petrol engine compresses the air & gas with a ratio among 10:1 to 14:1. The petrol engine mixes petrol with air & compresses this mixture in the combustion chamber. The higher blending of air and gas with each other makes it homogeneous. An electric powered spark-plug then ignites the compressed air-gas mixture with a spark. Thus, it reasons the gas to burn absolutely and instantly.

In diesel engines, the ‘Compression-Ratio’ varies from 18:1 to 23:1, which depends on engine design & construction. The petrol engines use the ‘Spark Ignition’ method. However, the diesel engine technologies such as ‘Direct Injection’ & ‘InDirect Injection’ & ‘Common-rail Direct Injection’ use the Compression Ignition method. However, the compression ratio remains almost the same for both the petrol engine and diesel engine, respectively, regardless of the engine capacity/displacement.

What is the Need for Compression Ratio?

The compression ratio (CR)  of an I C engine is a design criterion that must be decided through the layout group at the same time as designing the engine; the CR is chosen in line with the output want of the engine as it directly affects the engine output and also the overall size of the engine.

The want of the CR is exclusive for diesel and petrol engines which can be as follows-

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-gasoline combination enters the combustion chamber for the duration of the suction stroke and for correct blending and
    for proper combustion of this air-gasoline combination compression of this combination is needed that’s executed with the aid of using the engine in its compression stroke, so a good compression ratio of the petrol engine is required for proper combustion of the air-gasoline combination which in turn provides better thermal efficiency.
  • The pressure in the cylinder will increase for the duration of compression stroke which in turn raises the temperature of the air-gasoline combination that leads
    to finish or right combustion of the gasoline while the spark plug produces spark which in turn provides better gasoline economic system and also prevents the engine from various defects like knocking.
  • The petrol engine with proper CR provides a balanced amount of energy 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 gasoline absolutely depends upon the compression of the air provided with the aid of using the compression stroke of the diesel cycle due to 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 excessive pressure rise, which is required to increase the temperature of the compressing air to the extent of the automobile ignition temperature of the gasoline that is to be sprayed by the gasoline injectors which in turn provide complete or proper combustion of gasoline.
  • The diesel engines are known for providing high power output which is because of the excessive compression ratio of the diesel engine, as we know better the CR better might be the thermal performance or work output.
  • A diesel engine with excessive CR affords excessive gasoline economic system because of the better 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.

Advantages of Higher Compression Ratio:

The better the compression-ratio, the better is the thermal efficiency of the engine. Thus, the engine can extract more mechanical strength from the given mass of the air-gas mixture. In that context, diesel engines have enhanced gas efficiency for the given amount of gas than petrol engines of the same size.


It means that let’s say; you compare the conventional petrol and diesel engines with the same engine capacity of 1.0L. Then, in the real world, the 1.0L diesel engine will consume much less gas than the 1.0L petrol engine. In other words, a automobile with a 1.0L diesel engine would give higher mileage in comparison to the only with a 1.0L petrol engine under similar driving conditions.

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