Knocks can also occur due to overheating of the engine and poor quality of air-fuel mixture.
Knocking generates oscillations with characteristic frequencies. The purpose of a knock sensor is to detect these frequencies and transmit these signals to the engine control system, where the control system evaluates the signal and identifies the extent of knocking. The engine control system then regulates the combustion process (such as advancing the ignition timing by a few degrees) to eliminate knocking.
Knock sensors are generally broadband knock sensor. It covers a frequency spectrum, from 3 to 20 with an intrinsic resonance of over 30 kHz. The knock sensors are placed at appropriate positions in an engine block. Multiple sensors are used in a multicylinder engine (such as 2 sensors in 6 cylinder engine).
Working:
Knock sensors work based on piezoceramic ring that converts vibrations into electric signals using a superimposed seismic mass.
Sensor sensitivity is measured in mV/g or pC/g and is practically constant over a wide frequency range. The transmission behaviour of the sensor can be varied by the choice of seismic mass.
The resonance frequency can be increased by reducing the seismic mass.
In some cases, knock sensors are also used in CI engines to control the fuel injection timing and the function of injection nozzles.
Types of Knock sensors:
The most commonly used knock sensor is the above mentioned piezoelectric sensor which converts vibrations into electrical voltage. A typical voltage signal is between 300 millivolts to 500 millivolts. The voltage signal is transmitted to the control system which in turn regulates the ignition timing and air-fuel mixture.
An acoustic sensor is often used in vehicles to detect knocking. It employs a small microphone positioned against a cylinder block and detects vibrations. The vibrations are converted into electrical voltage by the microphone and sent to the control unit.
An ion sensor is used in some top end models such as BMW, Ferrari and Mercedes Benz. This type doesn't employ any sensor. It uses spark plugs to detect knocking. By applying an electric current across a couple of spark plugs during the ignition process, a change in the conduction of voltage can imply the status of combustion.