Knock & Octane
The uninvited explosion that has capped petrol-engine compression ratios for a century — and the fuel-rating system invented to tame it.
That metallic ping under hard acceleration is the sound of the fuel-air charge exploding on its own, ahead of the flame — and it is the single phenomenon that has bounded petrol-engine efficiency since the beginning.
The end gas explodes
In an SI engine the flame races outward from the spark, consuming the charge. But the unburned mixture ahead of the flame — the end gas — is being compressed and heated by the advancing flame and the rising cylinder pressure. If the end gas gets hot enough for long enough, it can auto-ignite all at once before the flame reaches it — a violent, premature explosion that collides with the intended flame front. That collision generates a sharp pressure spike and the characteristic metallic knock (or pinging). Mild knock is just noise and a small efficiency loss; severe, sustained knock hammers the pistons and can destroy an engine in seconds. Because higher compression raises the end-gas temperature, knock is what caps the compression ratio of an SI engine — and since efficiency rises with compression ratio, knock directly caps SI efficiency.
Knock is not 'the fuel burning faster'. The normal flame propagates smoothly at tens of metres per second; knock is a separate auto-ignition event that detonates the remaining end gas almost instantaneously, generating a pressure wave that rings the combustion chamber. That is why prolonged heavy knock is destructive — it isn't a stronger version of normal combustion, it's an additional, violent explosion superimposed on it. Modern engines use knock sensors (piezo-accelerometers) to detect it and retard the spark timing instantly to suppress it, then advance timing back when it's safe — a continuous optimisation that lets them operate right at the knock limit for maximum efficiency.
Octane: the resistance rating
The octane rating quantifies a fuel's resistance to knock. It is measured by running the fuel in a special variable-compression test engine and comparing its knock behaviour to reference blends of iso-octane (100, knock-resistant) and n-heptane (0, knock-prone). RON (Research Octane Number) and MON (Motor Octane Number) are two test conditions; the pump number in many markets is an average ((R+M)/2). Higher-octane fuel resists auto-ignition, allowing higher compression ratio or more turbo boost — which is why high-performance and turbocharged engines specify premium fuel. Critically, higher octane does not add energy (a litre of 98 RON holds the same energy as 91 RON); it simply allows a higher-performance engine design to run without knocking.
- eta(10) = 1 − 1/10^0.4 = 1 − 1/2.512 = 0.602
- eta(11) = 1 − 1/11^0.4 = 1 − 1/2.612 = 0.617
- Relative gain = (0.617 − 0.602)/0.602 = 0.025 = 2.5% (ideal; real-world less)
- (R+M)/2 = (95 + 87)/2 = 182/2 = 91
- This is the 'anti-knock index' (AKI) shown on pumps in North America.
Check your understanding
- Knock = auto-ignition of the end gas ahead of the flame, generating a destructive pressure spike
- Knock caps SI compression ratio (and thus efficiency); higher-octane fuel resists it and permits higher r or boost
- Octane (RON/MON, pump AKI = (R+M)/2) measures knock resistance, not energy content
- Knock sensors let modern engines retard timing to suppress knock, then advance back to the limit for efficiency