Octane number and rating of SI fuels

Octane number

The octane number (ON) is the percentage by volume of iso-octane (2,2,4-trimethylpentane) in a blend with n-heptane that gives the same knocking behaviour as the fuel under test in a standard engine. Iso-octane (very knock-resistant) is given ON = 100 and n-heptane (very knock-prone) ON = 0. A fuel of ON 90 behaves like a mixture of 90% iso-octane and 10% n-heptane.

Rating method

The fuel is run in a standard variable-compression-ratio CFR engine. The compression ratio is raised until a standard knock intensity is shown by a knockmeter. A blend of reference fuels is then prepared that gives the same knock at the same compression ratio. The blend percentage is the ON.

MON is usually 8-10 units lower than RON. The anti-knock index is $(\text{RON} + \text{MON})/2$.

Improving anti-knock quality

• Blend high-octane components: aromatics, iso-paraffins, ethanol, MTBE.
• Catalytic reforming and alkylation in the refinery.
• Anti-knock additives: tetra-ethyl lead (TEL) with ethylene dibromide scavenger (now banned in most countries because of lead pollution); modern fuels use oxygenates and aromatics.

Highest useful compression ratio (HUCR)

It is the highest compression ratio at which a fuel can be used in a given engine, at the given speed and with the mixture and spark timing set for best power, without detectable knock. A higher HUCR gives higher efficiency.

Performance number (PN)

Used for fuels better than iso-octane (ON above 100). It is the ratio of the knock-limited power of the fuel to that of iso-octane, multiplied by 100. For example, a fuel with PN 130 allows 30% more knock-limited power than iso-octane. Approximately, $\text{ON} = 100 + (\text{PN} - 100)/3$.