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Why do engines with high compression ratios need fuel with better antiknock qualities?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

In an internal-combustion engine, engines with high compression ratios require fuel with better antiknock qualities to prevent knocking, a phenomenon where the air-fuel mixture spontaneously ignites prematurely due to intense pressure and heat.1

In a properly functioning engine, the fuel charge should burn smoothly as the flame front moves across the combustion chamber. However, at higher compression ratios, the increased pressure can cause parts of the fuel charge to ignite uncontrollably ahead of the flame front. This creates high-frequency pressure waves that vibrate engine components, resulting in an audible knock.1

To mitigate this, fuel with a higher octane number—a measure of a fuel's ability to resist such premature ignition—is used. While increasing the compression ratio is a primary method for improving an engine's theoretical efficiency, the fuel's antiknock quality acts as a limiting factor in engine design.1234