Octane the Key to Engine ‘Knocking’

Question: What is octane?

Answer: Octane is a measure of how much compression gasoline can be subjected to before it spontaneously ignites. When gasoline ignites in the cylinder before it is ignited by the spark plug, it produces “knocking,” which can damage the engine. The higher the compression ratio of the engine, the higher the octane required.

When petroleum is refined into gasoline, it yields hydrocarbon molecules of various lengths, especially molecules with either seven or eight carbon atoms--called heptane and octane, respectively. Heptane ignites easily when compressed, but octane does not. A mixture of 87% octane and 13% heptane is said to have an octane of 87. The 87-octane gasoline you buy at the pump will have a different mixture of components, but will perform the same as that octane/heptane blend.

Octane can be increased easily by adding small quantities of tetraethyl lead, but the lead clogs catalytic converters and is a severe pollutant. Piston-engine airplanes are still permitted to use leaded gasoline.