What is Compression Ratio? | Compression ratio of Petrol Engine and Diesel Engine

What is the Compression Ratio?

The compression ratio (CR) of an IC engine is a ratio between the maximum and minimum values ​​of the engine cylinder and the combustion chamber. In simple words, the ratio between the total volume of the combustion chamber that remains when the piston is at BDC to the volume that remains in the combustion chamber when the piston moves to TDC is known as the compression ratio.

compression ratio

Types of Compression ratio

The compression ratio is calculated in the following two different methods:

  1. Static compression ratio
  2. Dynamic compression ratio

1) Static Compression Ratio

The static compression ratio is measured according to the volume of the combustor when the piston is at its top stroke and according to the relative volume of the combustor and cylinder when the piston is at the bottom of its stroke.

2) Dynamic Compression Ratio

The dynamic compression ratio is very hard to calculate because it also includes the gas entering and leaving the cylinder during the compression process.

Let me explain with an example; imagine an engine with a total volume of 2000cc. In this 2000cc, the 1900cc is the swept volume (the distance covered by the piston when it travels from BDC to TDC), and the clearance volume is 100cc (the remaining volume in the cylinder when the piston reaches TDC). Therefore, the CR of this engine is 2000:100 or 20:1.

The engine performance increases by increasing the compression ratio. As you know, the diesel engine doesn’t contain a spark plug, and the ignition process occurs due to the high compression of the air-fuel mixture. Therefore, the diesel engine compression ratio (18:1 to 23:1) is higher than the petrol engine compression ratio (10:1 to 14:1).

Design Criteria on which the compression ratios depend

The compression ratio depends on the following parameters:

1) Stroke length

The engine stroke length is the length of the combustor or the distance between the bottom dead center and the top dead center of the engine cylinder.

The CR varies according to the length of the stroke. It increases by increasing the stroke length. The longer the engine’s cylinder stroke length, the higher the CR.

2) Bore Diameter

The engine cylinder has a cylindrical shape. The bore diameter is the diameter or engine cylinder’s internal diameter in which the piston reciprocates.

The CR is also dependent on the bore diameter (i.e., the larger the engine bore diameter, the higher the compression ratio).

3) Square Engine

The square engine has an engine’s cylinder bore diameter equal to the cylinder stroke length, providing the right balance of power and speed performance.

4) Number of Cylinders

The compression ratio highly depends on the number of cylinders of the engine. This is because the number of pistons increases by increasing the cylinders. Therefore, the compression ratio is higher in an engine with a larger number of pistons; the number of cylinders also affects the CR of the engine.

Therefore, from the above discussion, we can easily conclude that a large engine will have a higher CR than a small engine.

Due to the large size requirements of the I-engine with a higher compression ratio, a V-shaped engine is designed that has a compact design and offers a high compression ratio.

Read More: Different types of Piston Engines

How To Improve Engine Compression Ratio

Follow the below-given methods to achieve a higher compression ratio:

  1. Change the flat-topped piston to a high-compression that curves upward to result in a higher CR. However, the high compression of the air-fuel mixture generates more heat. Due to this reason, the fuel starts to burn naturally (before the spark plug produces the spark), which leads to knocking and decreases the performance of the engine. Because of this, the latest engines can only use high octane fuel, as the fuel having low octane ratings like 92 is easily knocked down.
  2. Supercharging: This increases the charge in proportion to the speed but puts a direct load on the engine like an air conditioner pulley. At low speeds, the boost effect is also not visible.
  3. Turbocharging: It provides maximum power when the turbo exceeds 3,000 rpm, but at lower than 3000 rpm, the turbo reduces the engine speed because it runs by exhaust gases. It is known as turbo lag. To prepare for the highly efficient engine compression that occurs when the turbo is fully operational, the engine must have a low compression ratio (i.e., 8:1) which further removes the power before the turbo is up. In general, this increases the fuel consumption in turbocharged cars.

How to calculate static compression ratio

First of all, you need to find the clearance volume and the displacement volume to calculate the compression ratio. The values of the clearance volume and displacement volume help you to calculate the ratio of combustion chamber volume to cylinder volume at the top (before compression) and bottom (after compression) of the piston stroke.

Displacement volume is the amount of the fuel-air mixture that is shifted when the piston pushes downward.

The clearance volume is the amount (or area) of the air-fuel mixture remaining when the piston is at TDC. The following formula is used to calculate the compression ratio:

CR = (Clearance Volume + Displacement Volume) / Clearance Volume

Consider the engine clearance volume is 30 and the displacement volume is 6, then the compression ratio is:

                            CR = (30 + 6) / 6 = 6:1

The ratio is 6:1. This is a low CR, indicating insufficient power produced by the piston cycle.

Suppose you measure the static compression ratio and find that the piston and combustor volumes are low. In that case, you can take the vehicle to a professional auto mechanic to determine the reason for the IC engine’s low compression ratio.

Read More: Working and types of Combustion Chambers

Compression ratio of Petrol Engine

The compression ratio of the four-stroke petrol engine is given below:

compression ratio of petrol engine

  1. As everyone knows, the petrol engine sucks the fuel-air mixture during the intake stroke. During the compression stroke, the air-fuel mixture is compressed in order for this mixture to mix and burn properly. The gasoline engine needs the correct compression ratio for the air-fuel mixture to burn air-fuel mixture properly and provide better thermal efficiency.
  2. During the compression stroke, the pressure and temperature of the air-fuel mixture in the cylinder increase, causing complete or normal combustion of the fuel when the spark plug fires, which improves the fuel economy and avoids the engine misfiring.
  3. A petrol engine with sufficient CR offers balanced power and speed.
  4. The latest petrol engines typically have a compression ratio of 10.0:1 to 13.5:1. The CR of an engine with a knock sensor is typically greater than 11.1:1 and close to 14.0:1 (typically for high octane fuel and direct fuel injection), but the CR of a petrol engine without a knock sensor is typically 8.0:1 to 10.5:1.

Read More: Types and working of Petrol Engine

Compression ratio of Diesel Engine

compression ratio of diesel engine

  1. The diesel engines don’t have a spark plug to combust the air-fuel mixture. Therefore, they require high CR to properly combust the air-fuel mixture. So, the fuel combustion relies entirely on air compression during the compression stroke of the diesel cycle.
  2. High compression ratio diesel engines highly compress the air so that the temperature of the compressed air must be increased to the point where the auto-ignition temperature of the fuel is injected, which ensures complete or proper fuel combustion.
  3. The diesel engines have more compression ratio than the petrol engines.
  4. Diesel engines produce high power due to the high CR of diesel engines. As you know, the higher the CR, the higher the thermal efficiency or output power.
  5. High CR diesel engines offer excellent fuel economy due to the increased thermal efficiency provided by high compression combustion.
  6. Normally, the compression ratio of the diesel engine is from 18:1 to 23:1, which varies according to the engine design and the nature of the application.

Read More: Working and types of Diesel Engines

How to increase the compression ratio?

The engine power increases by increasing the compression ratio (CR).

The high CR permits the engine to obtain maximum energy from the combustion process due to higher thermal efficiency.

As the compression ratio increases, the piston travels higher inside the cylinder, which increases the expansion force, leading to more motive power.

Higher CR = Higher octane

FAQ Section

Does increasing compression ratio increase power?

An increase in the CR increases the engine thermal efficiency. At higher CR, the engine has the ability to get maximum energy from the given mass of the air-fuel mixture. As the compression ratio increases, the engine produces more power.

What is the Compression ratio of the diesel engine?

The petrol engine uses a spark plug to ignite the air-fuel mixture, while the diesel engine doesn’t contain a spark plug. Therefore, the petrol engine has a lower compression ratio than the diesel engine.

The compression ratio of the petrol engine is 8:1 to 12:1, while the compression ratio of the diesel engine is 18:1 to 23:1.  

What is the compression ratio of the petrol engine

The compression ratio of the petrol engine is 8:1 to 12:1.

Read More
  1. Different types of IC Engines
  2. Different types of Engines
  3. Types of SI Engines

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