RON

RON stands for research octane number, a measure of how much a petrol can be compressed before it spontaneously ignites. The higher the RON, the more resistant the fuel is to knock, or detonation, which occurs when the air-fuel mixture ignites prematurely inside the cylinder rather than at the spark plug's intended moment. A fuel with a RON of 95 can withstand higher compression than one rated at 91 before it self-ignites.

RON is determined by comparing a test fuel against pure iso-octane (assigned a value of 100) and heptane (assigned a value of 0) in a single-cylinder engine operated under controlled conditions. The test engine's compression ratio is progressively increased until knock begins, and the fuel's performance is expressed as a percentage. A petrol rated 98 RON behaves like a blend containing 98 percent iso-octane and 2 percent heptane in terms of knock resistance.

Why RON matters in engine design

Modern petrol engines are designed to operate at specific compression ratios. A high-compression engine (such as 12:1 or higher) demands high-RON fuel; running it on low-RON petrol will cause knock, which damages pistons and valves through violent pressure spikes and heat stress. Conversely, a low-compression engine (around 8.5:1) runs adequately on 91 RON fuel. Engine manufacturers specify minimum RON in the owner's handbook because octane rating directly affects how hard the engine can be pushed before detonation occurs.

RON is distinct from Motor Octane Number (MON), which is measured at higher engine speeds and temperatures and typically runs 8 to 10 points lower than RON. Many jurisdictions display both values on the pump; the average is sometimes labeled as (R+M)/2 or AKI (Anti-Knock Index) in North America. A fuel pump in Europe might show 95 RON with no MON; in North America the same position often shows 87 AKI.

The octane rating is boosted through additives such as alkylates, aromatics, and organic compounds like ethanol, or historically through lead compounds like tetraethyl lead (TEL). Ethanol-blended petrol (often marked as E10, containing 10 percent ethanol) typically has a higher RON than straight petrol because ethanol itself has a RON of around 110. Oxygenates and aromatic hydrocarbons increase octane rating by changing how the fuel burns, but additives are regulated for emissions and environmental reasons.