COMPRESSION RATIO

in an internal-combustion engine, degree to which the fuel mixture is compressed before ignition. It is defined as the maximum volume of the combustion chamber (with the piston farthest out) divided by the volume with the piston in the full-compression position. A compression ratio of six means that the mixture is compressed to one-sixth its original volume by the action of the piston in the cylinder. The maximum possible ratio based on cylinder dimensions may not be achieved if the intake valve closes after the piston begins its compression stroke. A high ratio promotes efficiency but may cause engine knock.