Anti-knock control - function (Engine 113) - GF15.12-P-4024BK

ENGINE 113.967 in MODEL 219 

ENGINE 113.964 in MODEL 164.1 

ENGINE 113.971 in MODEL 251 

Fig 1: Anti-Knock Control Function Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The task of the anti-knock control 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 control unit.

Input signals: 

• Recognition of knocking over knocking sensors
• Engine speed from crankshaft position sensor
• Camshaft position (ignition circuit identification) by means of camshaft Hall sensor
• Engine temperature from coolant temperature sensor
• Air mass from hot film MAF sensor
• Engine load by means of accelerator pedal sensor
• Presetting (programming) by means of STAR DIAGNOSIS.

Output signals: 

• Ignition angle correction for specific cylinder to the map value calculated by the ME control unit
• The emergency running ignition angle for a detected fault on the knock sensors.

The anti-knock control is active from engine start. If knocking is detected at a cylinder, the electronic mapped ignition angle of this cylinder is retarded from the next ignition. If knocking continues the ignition angle will be adjusted step by step towards "retard" until the maximum retarding of the ignition timing depending in the engine speed is achieved.

If knocking-free combustion exists, the retardation is reduced in stages after a few ignitions (depending on engine speed) until the original performance map value is achieved or knocking once again occurs.

If the anti-knock control specifies retardation of the ignition angle for a lengthy period, the engine control system reduces the engine power output as a protection of the engine.

Faults if knocking detected 

If no signals are received from the knock sensor, the ignition angle of all cylinders is retarded by as much as a 15° crank angle for safety reasons, depending on engine speed and temperature. The anti-knock control and the adaptation are blocked and a fault is stored in the fault memory.

Adaptation of anti-knock control 

The adaptation of the anti-knock control is active as of a coolant temperature of approx. 50°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 angles so that each cylinder is immediately operated with the optimal ignition angle.

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

A default setting for the RON correction can be programmed using the STAR DIAGNOSIS.

The ignition angle corrections of the overheating and pinging protection, intake air temperature correction and anti-knock control are added together on the basis of the performance map.