What is octane number and how does it relate to fuel quality?

Octane number measures a fuel's resistance to knock (premature ignition) in spark-ignition engines. Reference scale: n-heptane = 0 (knocks easily), iso-octane (2,2,4-trimethylpentane) = 100 (knock resistant). Two measurements: RON (Research Octane Number) - moderate conditions, and MON (Motor Octane Number) - severe conditions. Pump octane (USA) = (RON + MON)/2. Higher octane fuels can use higher compression ratios for better efficiency. Octane boosters: aromatics (high octane), branched paraffins (high octane), straight-chain paraffins (low octane), olefins (moderate). Refinery processes to increase octane: reforming, isomerization, alkylation. Previously tetraethyl lead was used as octane booster, now phased out.