Operation of an engine with knocking combustion over a prolonged period can lead to serious damage. Knocking tendency is increased by:
The compression ratio can also reach excessively high values due to deposits or production-related scatter.
On engines without knock control, these unfavourable 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:
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 unit.
The knock sensor is a piezo-electric structure-borne noise microphone. It picks up the structure-borne noise and converts it into voltage signals.
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 unit. In the case of fault, the engine is protected by constant retard setting of the ignition timing.
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 of the knock control system includes following checks:
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.