Definition
A hard, carbon-rich deposit that forms inside engine components when oil or fuel is exposed to high temperatures over time. In aircraft engines, coke commonly builds up on hot surfaces such as turbocharger bearings, oil passages, exhaust valve guides, and piston ring grooves, where it can restrict oil flow, impair cooling, and cause sticking or wear of moving parts.
Plain English
A hard, crusty residue left behind when oil or fuel gets baked onto hot engine parts. Over time it builds up in places that need to stay clean and can block oil flow or jam parts that should move freely.
Context Anchor
Seen in engine maintenance discussions, especially during inspections of areas exposed to high heat, oil leaks, or restricted oil flow.
Derivation
The word coke comes from a northern English term for the solid residue left after coal is heated without air. Mechanics borrowed the same word for the similar hard, blackened residue left when oil or fuel is cooked onto hot engine parts.
Why Pilots Care
Coke deposits are a sign that oil or fuel has been overheated, often due to shutdown procedures, oil quality, or cooling problems. Letting a turbocharged engine cool down before shutdown helps prevent oil from coking inside the bearing, which can shorten turbocharger life dramatically.
Analogy
It is like the hard black residue left on a pan after something oily has been overheated, except inside an engine it can interfere with moving parts and oil flow.
Intuition Check
Coke does not mean the soft drink here. In this context, it means a hard carbon deposit left by overheated oil or fuel.
Example Sentence 1
The mechanic found heavy coke buildup in the turbocharger bearing, likely caused by hot shutdowns without an adequate cool-down period.
Example Sentence 2
Heavy coke deposits on the combustion chamber walls can raise exhaust gas temperatures and reduce engine efficiency.