Functional description

These unwanted combustion processes are referred to as knocking (or pinging) and are the consequence of spontaneous combustions of mixture that has not yet encountered the flame front. Normal combustion and compression by the piston cause pressure and temperature increases that lead to spontaneous combustion of the mixture that is not yet burning. Here, flame speeds of more than 2000 m/s occur, whereas normal combustions involve flame speeds of around 30 m/s.

Fig 1: Identifying Knock Sensor

Courtesy of BMW OF NORTH AMERICA, INC.

Causes of knocking can be:

• Poor fuel grade (RON/MON)
• High cylinder filling
• High intake air and engine temperature
• Increased compression ratio (e.g. deposits)

If the knocking persists for a longer period, the pressure waves and thermal load can cause mechanical damage on the cylinder head gasket, on the piston and in the area of the valves. The characteristic vibrations of knocking combustions are picked up by the knock sensor, converted into electrical signals and sent to the engine control system. In the engine control system, the signals are processed in such a way that they can be assigned to the corresponding cylinder.