COMPRESSION RATIO

In internal combustion engines the compression-ratio may be defined as the ratio of the maximum cylinder volume when
the piston is at its outermost position (BDC) to the minimum cylinder volume (the clearance volume) with the piston at its innermost position (TDC) - that is, the sum of the swept and clearance volumes divided by the clearance volume.

Mathematically,

Compression Ratio=Swept Volume+clearance volume/clearance volume

Swept volume is the volume between TDC (Top Dead Centre) to BDC (Top Dead Centre) of cylinder and clearance volume is the remaining volume at the top of the piston.

Compression-ratio is a very important parameter for measuring engine performance.

Let's make it more simple,

Look at the diagram below,

Here total volume is 14 and clearnace volume is 1 so compression ratio becomes 14:1

Petrol engines have compression-ratios of the order of 7:1 to 10:1; but, to produce self-ignition of the charge, diesel engines usually double these figures and may have values of between 14:1 and 24:1 for naturally aspirated (depression-induced filling) types, depending on the design.