When fuel mixed with air during the ignition lag period (from the time injection begins until the fuel is ignited) reaches ignition temperature, the mixture is combusted in one burst. The pressure in the combustion chamber at this time rises as the quantity of the air-fuel mixture increases. If a large amount of air-fuel mixture is created during the ignition lag period, the pressure in the combustion chamber will rise rapidly. The pressure waves resulting from fuel ignition vibrate the cylinder walls and engine components, which generates noise. The generated noise is called “knocking”. To some extent, knocking is unavoidable in engines that use a self-ignition system.
