Anti-knock control function - GF15.12-P-4024M

ENGINE 275.950 in MODEL 215, 220

ENGINE 275.951 in MODEL 230 up to Model Year 8

ENGINE 275.953 in MODEL 216, 221 up to Model Year 8

ENGINE 275.954 in MODEL 230 up to Model Year 8

ENGINE 275.980 in MODEL 215, 220

ENGINE 275.981 in MODEL 230 up to Model Year 8

ENGINE 275.982 in MODEL 216, 221 up to Model Year 8

ENGINE 285.950 in MODEL 240

ENGINE 285.980 in MODEL 240

Fig 1: Anti-Knock Control Function Diagram

Courtesy of MERCEDES-BENZ USA

The task of the anti-knock control AKC is to ensure knock-free operation of the engine with different fuels and in 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 cylinder or cylinders is retarded. The anti-knock control (AKC) function is integrated in the ME-SFI [ME] control unit (N3/10).

Input signals:

Engine speed (crankshaft position sensor)
Camshaft position (camshaft Hall sensor)
Recognition of knocking over 4 knocking sensors
Engine temperature (coolant temperature sensor)
Engine load (pressure sensor downstream of throttle valve actuator)
Accelerator pedal position (accelerator pedal sensor)
Precontrol by means of automatic RON switch (internal)
RON correction with STAR DIAGNOSIS.

Output signals:

Ignition angle correction for specific cylinder to the map value calculated by the alternator control unit
Ignition angle emergency mode if fault identified.

Anti-knock control is active from a coolant temperature of 70 °C. If cylinder knocking is recognized above an engine load of about 40%, its ignition angle is retarded by approx. 3.0° crank angle after the next ignition. If knocking continues to be detected, the ignition angle is continuously retarded in 3° crank angle increments until the maximum retardation is reached (approx. a 10 - 15° crank angle, depending on the engine speed). The automatic RON switch also influences the maximum retardation of ignition timing - the higher the selected RON stage, the less the maximum retardation of ignition timing.

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

Adaptation of anti-knock control

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

The stored adaptation value is taken into account in the output of the individual cylinder angles so that each cylinder is immediately operated with the optimal ignition angle.

During abrupt acceleration, no values are stored for about 2 seconds to avoid any incorrect adaptation values.

Faults in knocking detection

If no signals are received from the knock sensors or from ionic current measurement, the ignition angle of all cylinders is retarded for safety reasons by as much as approx. 11° crank angle depending on engine speed and temperature. The anti-knock control and the adaptation are deactivated and a fault is stored.

Automatic RON switch

In addition to the RON correction using STAR DIAGNOSIS, the anti-knock control features an automatic RON switch. This detects a persistent high knocking tendency of the engine and assists the anti-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 exceeded, the RON value is adjusted by one or several stages. The value is retained until the engine is switched off.

After engine start, only the RON correction performed via STAR DIAGNOSIS is active at first.

The engines are designed to run with RON 98 fuel. If only fuel with RON 91 is available, the RON correction must be carried out immediately using STAR DIAGNOSIS, otherwise engine damage is possible.

	Crankshaft position sensor, location/task/design/function		GF07.04-P-4116M
	Camshaft Hall sensor, location/task/design/function		GF07.04-P-4117M
	Knock sensors, location/task/design/function		GF15.12-P-4101M
	Coolant temperature sensor, location/task/design/function		GF07.04-P-5026M
	Pressure sensor downstream of throttle valve actuator, location/task/function		GF07.04-P-5049M
	Accelerator pedal sensor, location/task/design/function		GF30.20-P-4010M
	ME-SFI control unit, location/task/design/function		GF07.61-P-5000M
	ME-SFI synchronization of fuel injection and firing order, function		GF07.61-P-4009M