Definition
In a reciprocating engine, the ratio of the volume inside the cylinder when the piston is at the bottom of its stroke to the volume inside the cylinder when the piston is at the top of its stroke. It expresses how much the fuel-air mixture is squeezed before ignition.
Plain English
It's a number that tells you how much the air and fuel inside a cylinder gets squeezed before the spark plug fires. A compression ratio of 8.5:1 means the mixture is squeezed into a space about one-eighth of its starting size.
Context Anchor
Seen in engine specifications, maintenance manuals, powerplant training, and discussions of engine efficiency, fuel requirements, and abnormal combustion.
Derivation
From Latin 'comprimere', to press together. The word 'ratio' comes from Latin for 'reckoning' or 'calculation' — a comparison between two quantities. So literally: the calculation of how much something is pressed together.
Why Pilots Care
Higher compression ratios increase power and efficiency but raise the risk of detonation, requiring higher-octane fuel and careful mixture management.
Analogy
Think of a syringe partly filled with air. If you push the plunger in and trap the air in a much smaller space, you have compressed it. Compression ratio compares the starting space to the smaller squeezed space.
Grounding Statement
A higher compression ratio means the engine squeezes the mixture into a smaller space before it burns.
Intuition Check
Compression ratio is not a pressure reading. It is a comparison of two volumes: the cylinder space before squeezing and the cylinder space after squeezing.
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 overhaul the shop measured the compression ratio to confirm it still matched the manufacturer's specifications.