Antiknock

Antiknock refers to the property of a fuel to resist premature ignition under the high pressures and temperatures inside a spark-ignited internal combustion engine. When a fuel-air mixture auto-ignites ahead of the spark, the resulting pressure waves can cause a characteristic knocking or pinging sound that reduces efficiency and can damage the engine. A fuel with higher anti-knock quality can be compressed more before ignition, enabling higher compression ratios and potentially greater efficiency.

Anti-knock quality is commonly quantified by octane rating, a relative scale comparing a fuel's resistance to knocking with reference hydrocarbons. The two main measures are the Research Octane Number and the Motor Octane Number, with the overall rating often reported as a single octane value in consumer fuels. A higher octane rating indicates greater resistance to detonation. In practice, octane ratings vary by country and by fuel grade, but premium fuels typically have higher octane ratings than regular grades.

Historically, anti-knock additives such as tetraethyl lead were used to raise octane ratings, enabling higher compression ratios. Due to environmental and health concerns, leaded gasoline has been phased out in most regions, replaced by other additives like ethanol, MTBE (where applicable), and various hydrocarbon blends. Modern gasoline formulations balance octane with emissions controls and fuel economy.

Modern engines incorporate knock sensors, advanced fuel-injection strategies, and precise control of ignition timing to manage knocking without excessively high octane fuels. In high-performance or turbocharged engines, higher-octane fuels are often required to exploit increased compression or boost. Regulations and market differences influence the availability and use of different octane grades worldwide.