motor octane number

Motor octane number (MON) is a laboratory measure of how much a petrol fuel resists spontaneous ignition when compressed and heated inside an engine cylinder. It is determined by running a standardized single-cylinder test engine at 900 rpm under controlled conditions of heat, pressure, and fuel mixture composition, then comparing the fuel's knock behavior to reference fuels. A fuel that knocks at lower compression ratios receives a lower MON; one that resists knocking at high compression gets a higher number. The scale runs from 0 to 100, with isooctane (2,2,4-trimethylpentane) defined as 100 and normal heptane as 0.

MON differs from the more familiar research octane number (RON), which is measured at 600 rpm under milder thermal conditions. Because MON testing applies higher engine speed and temperature, it is generally 8 to 10 points lower than RON for the same fuel. Fuel pumps display a single number on the nozzle, typically the antiknock index, which is the average of RON and MON: (RON + MON) / 2. This compromise figure better predicts real-world knock resistance than either alone, especially during hard acceleration and high-load driving.

A fuel with a 95 RON and 85 MON rating, for example, would display 90 at the pump. The difference between RON and MON reveals sensitivity to engine operating conditions. High sensitivity fuels perform well in gentle driving but lose octane value under severe stress; low sensitivity fuels maintain more consistent knock resistance across a wider range of driving patterns. Refineries adjust MON by selecting base crude stocks, controlling aromatic and olefin content, and dosing organic octane boosters such as ethanol or methylcyclopentadienyl manganese tricarbonyl (MMT).

Why MON matters in practice

Modern engines with knock sensors can tolerate slightly lower octane fuels than carbureted engines of earlier decades, because electronic timing adjusts ignition advance to suppress knock. However, sustained knocking causes piston damage, bearing wear, and efficiency loss. Turbocharged and supercharged engines are particularly sensitive because they generate higher intake pressures, driving up in-cylinder temperature. A naturally aspirated engine rated for 87 MON fuel may require 91 MON when turbocharged. Engines tuned for high compression ratios demand higher MON; conversely, older low-compression engines run cleanly on 85 MON or lower.

Octane boosters work by raising the temperature at which fuel molecules ignite, competing for the combustion event rather than allowing random autoignition. Ethanol, widely blended into pump fuels, raises both RON and MON but adds hygroscopic properties that can corrode fuel system components if water contamination occurs. MMT has been phased out in many regions due to environmental and catalyst-poisoning concerns. The choice of booster and blend ratio is determined by local regulations, refinery feedstock availability, and engine population requirements in each market.