GF15.12-P-4024CY Knock Control Function

GF15.12-P-4024CY Knock Control Function
ENGINE 272.911 in MODEL 204.0 up to Model Year 8
ENGINE 272.921 in MODEL 204.0 /2 up to Model Year 8
ENGINE 272.947 in MODEL 204.0 /2 up to Model Year 8
ENGINE 272.948 in MODEL 204.0 up to Model Year 8
ENGINE 272.961 in MODEL 204.0 /2 up to Model Year 8
ENGINE 272.971 in MODEL 204.0 up to Model Year 8

The task of the knock control (AKC) is to ensure knock-free operation of the engine with different fuel qualities and under all operating conditions (from idling speed up to maximum engine speed).
For this purpose, if uncontrolled combustion (knocking) occurs, the ignition angle at the relevant cylinders is retarded. The knock control is integrated in the ME-SFI [ME] control unit (N3/10).

Input signals:

^ Knock sensor signals
^ Engine speed
^ Position of crankshaft and camshaft (ignition circuit recognition)
^ Engine temperature
^ Air mass
^ Intake manifold air pressure
^ Intake air temperature
^ Engine load request from driver
^ Presetting of the RON switch using the DAS

Output signals:

^ Ignition angle correction for specific cylinder (to the map value calculated by the ME-SFI [ME] control unit)
^ Emergency mode ignition angle if knock sensor fault is detected

The knock control is active from a coolant temperature of 40°C. If cylinder knocking is recognized for an engine load of about 40%, its ignition angle is adjusted to retarded by about a 3.0° crank angle after the next ignition. If knocking continues to be detected, the ignition angle is continuously retarded in steps of a 3.0° crank angle (depending on the engine speed) until the maximum retardation is reached of an approx. 10 - 15° crank angle.
In addition, the automatic RON switch influences maximum retardation. The higher the RON stage selected, the less the maximum retardation.

If knocking-free combustion exists, the retardation is reduced in steps of approx. a 0.75° crank angle after a few ignitions (depending on the engine speed) until the normal performance map value is achieved, or knocking occurs again.

Adaptation of knock control
The adaptation of the knock control is active as of a coolant temperature of approx. 75°C.
Any corrections required to the ignition angle because of knocking combustion are constantly detected for the specific cylinder and stored together with the relevant operating state of the engine (engine load and speed).

When determining the individual ignition angles, the stored adaptation values are taken into account, so that the optimal ignition angle can be calculated immediately for each cylinder.
During an abrupt acceleration, no values are stored for about 2 seconds in order to avoid any incorrect adaptation values.

Errors in knock identification
In the event of failure of the knock sensor signal, for reasons of safety, the ignition angle for all cylinders (dependent on engine speed and temperature) is retarded by up to an 11° crankshaft angle. The knock control and the adaptation are deactivated and a fault is stored in the fault memory.

Automatic RON switch
In addition to the preselected RON correction using DAS, the knock control features an automatic RON switch. This detects a persistent high knocking tendency of the engine and assists the knock control by means of an appropriate pre-control.
This is done by analyzing the stored adaptation values, the ignition angles of all the cylinders and the number of control interventions in certain engine load and speed ranges.
Depending on the threshold which has been exceeded, the RON stage is adjusted by one or several stages. The regulated RON stage remains until the engine is switched off.