Definition
Gasoline produced by the simple distillation of crude oil, without any cracking, reforming, or chemical processing to improve its anti-knock qualities. Straight-run gasoline has a low octane rating and is unsuitable for use in aircraft reciprocating engines without further refining or the addition of anti-knock compounds.
Plain English
Gasoline made just by heating crude oil and collecting the part that boils off in the gasoline range. Nothing else is done to it, so it has poor resistance to engine knock and cannot be used in aircraft engines as-is.
Context Anchor
Seen in aircraft fuel, engine, and maintenance discussions when explaining how gasoline is produced and why aviation gasoline must meet specific fuel standards.
Derivation
The name comes from the refining process: the gasoline is taken 'straight' from the distillation column in one run, with no further treatment. It is the raw cut of fuel before any upgrading.
Why Pilots Care
Aircraft piston engines run at high compression and need high-octane fuel to prevent detonation. Straight-run gasoline does not meet that requirement, which is why aviation gasoline is always refined further or blended with anti-knock additives.
Intuition Check
Do not read “straight-run” as meaning a special fuel for straight-line operation or normal engine running. Here it means gasoline taken directly from the first refining step, before later upgrading.
Example Sentence 1
Straight-run gasoline taken directly from the distillation tower has too low an octane rating to be used in a high-compression aircraft engine.
Example Sentence 2
Mechanics sometimes compare straight-run gasoline to finished avgas to show why additives and higher octane ratings are needed.