Definition
A numerical measure of an aviation gasoline's resistance to detonation (uncontrolled, explosive combustion) inside an engine cylinder. Aviation fuels are rated by two numbers — a lean-mixture rating and a rich-mixture rating — which together describe how much pressure and temperature the fuel can withstand before it detonates rather than burning smoothly. Higher numbers mean greater resistance to knock.
Plain English
A score that tells you how well a fuel resists exploding instead of burning evenly when it's squeezed and heated inside the engine. The higher the score, the harder it is to make the fuel misbehave.
Context Anchor
Seen in aircraft fuel requirements, fuel grades, engine operating limits, fuel placards, and discussions of aviation gasoline such as 100LL.
Derivation
From 'anti-' (against) and 'knock,' the sharp metallic sound an engine makes when fuel detonates instead of burning smoothly. The rating measures how strongly the fuel resists that knocking behaviour.
Why Pilots Care
Using fuel below the engine's required rating risks detonation, power loss, and possible engine damage.
Grounding Statement
During high-power operation, such as takeoff, the engine needs fuel that will burn smoothly rather than erupting too fast inside the cylinders.
Intuition Check
Antiknock rating is not a measure of how much fuel is in the tank or how much energy the fuel contains. It is a measure of the fuel’s resistance to damaging abnormal burning in the engine.
Example Sentence 1
The mechanic confirmed that the new fuel met the antiknock rating required for the turbocharged engine.
Example Sentence 2
An engine requiring a high antiknock rating can safely use higher manifold pressure without detonation.