A knock sensor senses vibrations that occur during engine knock and allows the car's power train control module to reduce ignition timing to prevent knocking. The sensor is located on the engine block, cylinder head or intake manifold and is a piezoelectric material that creates a small charge when vibrated. Engine knock leads to engine damage and is caused primarily by poor fuel, deposits in engine or incorrect spark plugs.
During normal combustion, the spark plug ignites the fuel mixture and the flame front progresses smoothly from the plug until the fuel mixture is completely burned. During engine knock, however, pockets of spontaneous explosions occur away from the spark plug, producing shock waves that rapidly increase cylinder pressure, causing interference to the engine cycle and catastrophic damage to the pistons and head gasket. Reducing the ignition timing causes the spark plug to fire later, reducing cylinder pressure and preventing knock.
Low-quality and low-octane fuels lead to high cylinder pressure and combustion chamber temperatures, causing fuel to ignite early. Higher-octane fuels burn more slowly and resist detonation due to extreme cylinder conditions. Deposits in the cylinder walls also raise cylinder pressure by taking up space normally used by the fuel mixture. Additionally, incorrect spark plugs may improperly burn the fuel mixture, causing deposits to form in the cylinders and increased operating temperatures.