Knock sensor

Two knock sensors are employed depending on the engine.

The knock sensors detect oscillations in the structure-borne noise from the engine block. Combustion events accompanied by knock generate structure-borne noise characterized by a specific oscillation pattern; this is registered by the knock sensors for subsequent processing in the DME engine-management system's module. The DME digital engine electronic system's knock control can then subdue the combustion knock with countermeasures such as adjusting the ignition timing.

The tendency for combustion knock to occur is influenced by the following factors:

• Pressure
• Temperature
• Fuel-air mixture
• Fuel grade (research octane number/motor octane number).

Combustion with knock contrasts with normal combustion in that portions of the air-fuel mixture ignite suddenly and spontaneously. These ignition events occur before the flame front initiated by the ignition spark during normal combustion reaches the affected portions of the air-fuel mixture. During these events the flame front propagates at velocities exceeding 300 m/s, while the comparable figure for normal combustion is approximately 30 m/s.

The following graphic shows the engine N55 as example.

Fig 1: Identifying Knock Sensor

Courtesy of BMW OF NORTH AMERICA, INC.

When knock persists over an extended period, the pressure waves from this violent combustion of the air-fuel mixture can cause mechanical and thermal damage to the head gasket, the pistons and the cylinder head itself. The knock sensors register the characteristic oscillations from combustion knock and convert them into electrical signals for transmission to the DME digital engine electronics system module. Within the DME module the signals are processed according to firing order to allow correlation with specific individual cylinders.

The knock sensor detects structure-borne noise within a frequency range extending from 5 to roughly 20 kHz. Engine knock generally occurs within an approximate frequency range of 7 to 16 kHz. The DME digital engine electronics system selects the ideal processing frequency for detecting combustion knock with reference to the following factors:

• Engine speed
• Load
• Cylinder.
  Fig 2: Knock Sensor Characteristic Curve

  Courtesy of BMW OF NORTH AMERICA, INC.