Definition
In a reciprocating engine, the ratio between the volume of a cylinder when the piston is at the bottom of its stroke and the volume remaining when the piston is at the top of its stroke. It expresses how much the fuel-air mixture is squeezed before ignition.
Plain English
A number that tells you how much the air and fuel inside a cylinder get squeezed before being lit. A ratio of 8.5:1 means the mixture is compressed into a space 8.5 times smaller than it started.
Context Anchor
Seen in aircraft engine descriptions, maintenance manuals, and discussions of engine power, fuel requirements, and abnormal combustion.
Derivation
From Latin 'comprimere', to press together. The term simply describes the ratio of pressed-together to original volume — a direct, mechanical description of what the piston does to the mixture.
Why Pilots Care
Affects engine power output, fuel efficiency, and detonation risk, influencing fuel selection and operating limits.
Grounding Statement
As the piston moves upward, the same fuel-air mixture is forced into a smaller space, and the compression ratio describes how large that squeeze is.
Intuition Check
Compression ratio is not the actual pressure reading in the cylinder. It is a volume comparison: how much space there is before the squeeze compared with after the squeeze.
Example Sentence 1
The Lycoming O-360 has a compression ratio of 8.5:1, which is why it requires 100LL avgas rather than a lower-octane fuel.
Example Sentence 2
During the pre-purchase inspection the mechanic verified the compression ratio matched the type certificate data sheet.