What is Compression ratio of I.C. engine?

Compression ratio in an internal combustion engine is defined as the ratio of the volume of the cylinder (when the piston is nearest with the head of the cylinder) and the combustion chamber (when the piston farthest out). Higher the ratio, more the air is compressed in the cylinder. In engines, the air is tightly compressed as it gives a powerful explosion from the air-fuel mixture which in turn makes more fuel to get used. Automotive engineers use this technique to design engines with full fuel efficiency.