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Compression Ratio Explained

Omkar Phatak
Compression ratio is one of the most important and fundamental parameters, when evaluating the performance of an engine or its design. Reading this story will help you understand the significance of this ratio.
Compression is squeezing something so that it occupies a smaller volume. Ratio is the proportion between two quantities or numerical values. The basic unit of an engine assembly is a combustion chamber and piston combination.
This ratio is a proportion which denotes the degree of compression achieved, in an internal or external combustion engine, when the driving pistons compress the air-fuel mixture in every engine stroke.
It is a ratio of air-fuel mixture volume, when a piston is at the end of its up-stroke, to the air-fuel mixture volume at the end of its down stroke, in a combustion chamber. It is a measure of the air-fuel mixture compression and density achieved, in every engine stroke.
The combustion chamber is a cylinder, which is fed with a mixture of air and vaporized combustion fuel. A piston is placed inside the combustion chamber and it makes up and down movements, according to pressure applied. The air-fuel mixture, fed inside the combustion chamber, is ignited, when you turn on the ignition of the car.
The explosive combustion makes the piston move up and every time it comes down, it compresses the air-fuel mixture again, igniting it in the process. The thrust provided by the combustion of fuel is transformed into the movement of car crankshaft, which ultimately drives the wheels.
So, better the compression ratio, better is the density of the air-fuel mixture, achieved during each compression and better is the efficiency of the engine in burning fuel.

Static Compression Ratio

Technically, there are two main types of this ratio: 'Static' and 'Dynamic'. Static compression ratio is the absolute ratio between volume of the combustion chamber, when it's at the extreme end of the up stroke, to combustion chamber' volume, when it is at the extreme of its down stroke.
Dynamic ratio is a more realistic value, which takes into consideration, that the combustion chamber is not entirely sealed and the intake valve opening causes modification in the volume of air, which is trapped inside the chamber.

Calculation Formula

The formula, which enables the calculation of this parameter is the following:

Compression Ratio (CR) = (π/4 b2s + V)/V (Clearance Volume)
  • 'b' is cylinder bore diameter
  • 's' is the length of a piston stroke
  • 'V' is the minimum clearance volume left after piston reaches the extreme end of its down stroke.
This static ratio equation can enable you to calculate the ratio value in any case, where the required variables are known.

For a Petrol Engine

Petrol engines are widely used in most countries. The static compression ratio of a petrol engine never exceeds 10:1, as a higher value can cause a knocking phenomenon, when low-octane fuel is used. A turbocharged gasoline engine is always built with a ratio of 9:1 or lower than that.
Speed racing motorcycle engines are built with a ratio in excess of 12:1, as they use very high-octane fuel, which doesn't cause knocking problems.

For a Diesel Engine

Diesel engines are different from gasoline/petrol engines in one aspect. They do not require a separate mechanism to ignite fuel at the piston's down stroke. Diesel ignites on its own when it's compressed. So, diesel engines have inherently high compression ratios in excess of 14:1. Most diesel engines have ratios between 14:1 and 16:1.
If you are a student, interested in automotive design engineering, this ratio is something which you will be thinking about a lot, when designing your own engine.