Anti-knock control function

P07.61-0398-06

Shown on Engine 119

A16 Knock sensors

B2/5 Hot film mass air flow sensor

B6/1 Camshaft Hall sensor

B11/4 Coolant temperature sensor

B37 Accelerator pedal sensor

L5 Crankshaft position sensor

N3/10 ME-SFI control unit

T1/1-
T1/8 Ignition coils, cylinders 1 to 8

Task

Ensuring knock-free operation of the engine with different fuels in all operating conditions from idling speed up to maximum speed of engine, from an engine load of more than approx. 40%. For this purpose, if uncontrolled combustion (knocking) occurs, the ignition angle at the relevant cylinder or cylinders is retarded.

Design/function

The anti-knock control (AKC) function is integrated in the ME-SFI control unit. The range of functions has been enlarged compared to previous versions.

Input signals:

 Engine speed (crankshaft position sensor)

 Camshaft position (camshaft Hall sensor)

 Knock identification:

Engine 104, 111, 112, 113, 119, 120 by means of knock sensors

Engine 137 by ionic current measurement

 Engine temperature (coolant temperature sensor)

 Engine load (hot film air flow sensor or pressure sensor on supercharged engines)

 Accelerator pedal position (accelerator pedal sensor)

 Precontrol by means of automatic RON switch (internal)

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Output signals:

 Cylinder specific ignition angle correction on the calculated map value of the ME control unit.

 Ignition angle emergency mode if fault identified.

Basic function

The anti-knock control is active from a coolant temperature of 70 °C. If knocking is detected at a cylinder above an engine load of approx. 40%, its ignition angle is retarded from the next ignition by about 2.3° crank angle (e.g. M119) or by about 3.0° crank angle (e.g. M120). If knocking continues to be detected, the ignition angle is continuously retarded in steps of 2.3° crank angle (e.g. M119) until the maximum retardation is reached (approx. 10 to 15° crank angle, depending on engine speed). In addition, the automatic RON switch influences the maximum retarding of the ignition timing - the higher the selected RON stage, the less is the maximum retarding of the ignition timing.

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

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Adaptation of anti-knock control

The adaptation of the anti-knock control is active from 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/speed range).

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

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

Faults in the case of knocking detection
If no signals are received from the knock sensors or from the ionic current measurement, the ignition angle of all the 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 the HHT, 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 which has been 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 which is carried out by means of the HHT/STAR DIAGNOSIS is at first active.