Knock Control

Knock Control

Tasks of the knock control system
Operation of an engine with knocking combustion over a prolonged period can lead to serious damage. Knocking tendency is increased by:
- Increased compression ratio
- High cylinder charge
- Poor fuel quality (RON/MON)
- High intake air and engine temperatures

The compression ratio can also reach excessively high values due to deposits or production-related scatter.

On engines without knock control, these unfavorable influences must be taken into consideration in the ignition design by providing a safety distance to the knock limit. However, this results in unavoidable losses in efficiency in the upper load range.

The knock control can prevent knocking engine operation. For this purpose, if there is an imminent risk of knocking, the control retards the ignition timing of the corresponding cylinder(s) (cylinder-selective) as far as necessary. This makes it possible to adapt the ignition characteristic map to the consumption-optimum values without having to take the knock limit into consideration. A safety distance is no longer necessary.

The knock control system carries out all knock-related corrections to the ignition timing and enables perfect operation also with regular grade fuel (minimum RON 91).

The knock control provides:
- Protection against knocking damage also under unfavorable conditions
- High efficiency due to optimum utilization of the fuel quality and consideration of the relevant engine status
- Logistics advantages with regard to fuel availability
- Lower consumption and higher torque over the entire upper load range (corresponding to the fuel quality used).

Design of knock control system
The M54, the M52 and the M43 are equipped with a cylinder bank-selective, adaptive knock control system. A double knock sensor detects combustion knock. The sensor signals are evaluated in the DME control module.

The knock sensor is a piezo-electric structure-borne noise microphone. It picks up the structure-borne noise and converts it into voltage signals.

Function of the knock control system
If knocking occurs, the ignition is retarded for a certain number of working cycles and then gradually approaches the original value. The retard setting can be controlled individually for each cylinder bank (cylinder bank selective).

In the event of the knock sensor failing, a fault code is entered in the fault code memory of the DME control module. In the case of fault, the engine is protected by constant retard setting of the ignition timing.

Installation Locations/Conditions
The double knock sensor is secured by means of an 8 mm screw on the water jacket of the engine block between both cylinder banks. It is arranged such that each sensor monitors one cylinder bank.

Only screw locking compound may be used to lock the screws. Washers, spring washers or serrated lock washers must under no circumstances be used.

Self-diagnosis and emergency operation of the knock control system
Self-diagnosis of the knock control system includes following checks:
- Check for sensor signal interference/line break, plug connector defective etc.
- Self-test of entire evaluation circuit
- Check of basic engine noise level detected by the knock sensor

The knock control system is switched off if a fault is found during the course of one of these checks. The emergency program adopts the task of controlling the ignition timing. At the same time, a defect code is stored in the defect code memory. The emergency program ensures damage-free operation as from minimum RON 91. It depends on the engine load, speed and temperature.