Knock Sensor: Description and Operation

Knock Sensor System (KSS)
The ignition maps of the HFM-SFI are designed for optimal power. If under certain operating conditions, combustion knock occurs, the KSS control integrated into the engine control module identifies the knocking cylinder and retards the ignition timing.

If, for example, the knocking is due to fuel with a low octane rating, the mechanical oscillations produced in the knock sensor are converted to electrical signals and are sent to the engine control module (N3/4).

The engine control module processes the following information for this purpose:

- Knock sensor (KS) signals,
- Camshaft position, TDC cylinder no.1,
- Engine speed/crankshaft position/ignition circuit recognition,
- Engine coolant temperature (ECT),
- Intake air temperature (IAT).

The engine control module compares these incoming signals with nominal values stored in memory.

If discrepancies exist, then the ignition timing of the knocking cylinder is retarded 3° CKA (Crank Angle) at the next ignition cycle. If the cylinder continues to knock, the ignition timing is retarded an additional 3° CKA. The ignition will continue to retard under knock conditions until an ECT dependent maximum ignition retard adjustment is reached (e.g.: 10° CKA at 80-90 °C ECT). If the combustion knock ceases, then the ignition point of the affected cylinder is advanced in steps of 0.35° CKA until the nominal value is once again attained.

Should one of the following parts fail, the ignition timing is retarded for all cylinders depending on the engine coolant temperature:

- Knock sensor (KS),
- Knock sensor evaluation of KSS control integrated in engine control module,
- Camshaft position (CMP) sensor (L5/1).