Octane Number
Definition and meaning of Octane Number in chemistry.
Octane number is a measure of a gasoline's resistance to engine knock, defined as the percentage of isooctane in a standard isooctane/n-heptane mixture that produces the same knocking behavior in a test engine. Higher octane numbers indicate greater resistance to premature ignition under heat and compression.
In more detail
Engine knock (pinging) occurs when fuel ignites spontaneously before the spark plug fires, causing damaging shock waves and reduced efficiency. The octane rating is determined by comparing a fuel sample's knock resistance to reference blends of isooctane (C8H18), which resists knock well, and n-heptane (C7H16), which knocks easily. The scale ranges from 0 to 100-plus, where 100 represents pure isooctane. Different engines require different octane levels depending on their compression ratio; higher-compression engines need higher-octane fuel to prevent detonation.
Key facts
| Field | Physical Chemistry |
|---|---|
| Reference Fuel | Isooctane (2,2,4-trimethylpentane), C8H18 |
| Standard Scale | 0-100+; 100=isooctane, 0=n-heptane |
| Common US Ratings | Regular 87, Mid-grade 89, Premium 91-93 |
Regular unleaded gasoline typically has an octane rating of 87, premium unleaded is rated at 91-93, and racing fuel can exceed 100 octane. A high-performance car with a compression ratio above 10:1 requires premium fuel to prevent knock, while a typical sedan with a 9:1 compression ratio runs safely on regular 87-octane fuel.
Frequently asked questions
What causes engine knock?
Knock occurs when fuel ignites from heat and compression before the spark plug fires, creating shock waves that damage engine components and reduce power output.
Why do high-performance engines require premium fuel?
Higher compression ratios generate more heat and pressure, which will ignite low-octane fuel prematurely; premium fuel with better knock resistance prevents this detonation.