Definition
Air added to the hot exhaust gases in a turbine engine combustion chamber to lower their temperature before they reach the turbine section. Only a small portion of the air entering the combustor is mixed directly with fuel for burning; the remainder is introduced downstream as dilution air to cool the combustion gases to a level the turbine blades can safely tolerate.
Plain English
Extra air mixed into the hot gases inside a jet engine to cool them down before they hit the turbine blades, so the blades don't melt.
Context Anchor
Seen in turbine engine combustion section discussions, especially when explaining how the engine controls the temperature of gases before they reach the turbine.
Derivation
Dilution comes from the Latin diluere, meaning to wash away or thin out. In this context the air thins out (cools) the very hot combustion gases by mixing cooler air into them.
Why Pilots Care
Controls turbine inlet temperature to protect engine components, maintain reliability, and prevent overheating failures.
Analogy
It is like adding cool water to very hot water so the final mixture is still warm, but not dangerously hot.
Grounding Statement
Inside the engine, the fire is hot enough to damage parts, so cooler air is mixed in before the gas reaches the turbine blades.
Intuition Check
Dilution air is not air for breathing and it is not contaminated air. It is cooling and mixing air used inside the engine.
Example Sentence 1
Roughly three-quarters of the air entering the combustor bypasses the flame zone and serves as dilution air to cool the gases before the turbine.
Example Sentence 2
Engine designers adjust dilution air flow to keep turbine inlet temperatures within safe limits during high-power operation.