Knock Control, Function - GF07.10-P-1015MNC

Engine 276.8 in model 172.4 

Function requirements for knock control, general points 

• Circuit 87M (Engine management ON)
• Engine runs

Knock control, general points 

The knock control is integrated in the ME-SFI [ME] control unit (N3/10) and has the task is to ensure knock-free engine operation under all operating conditions and when different fuel grades are used. In the case of uncontrolled combustion (knocking), the ignition angle of the ignition coil for cylinder 1 to 6 (T1/1 to T16) of the related cylinder is adjusted towards "retard".

The ME-SFI [ME] control unit controls the knock control dependent on the following signals:

• Knock sensors 1 and 2 (A16/1, A16/2), knock signal
• Left and right intake camshaft Hall sensors (B6/4, B6/5), intake camshaft positions (ignition circuit detection)
• Left and right exhaust camshaft Hall sensors (B6/6, B6/7), exhaust camshaft positions (ignition circuit detection)
• Coolant temperature sensor (B11/4), coolant temperature
• Charge air temperature sensor (B17/8), charge air temperature
• Pressure sensor downstream of throttle valve (B28/7), engine load
• Accelerator pedal sensor (B37), load request by driver
• Crankshaft Hall sensor (B70), engine speed and crankshaft position (ignition circuit detection)
• RON default setting (R  esearch O  ctane N  umber) with Xentry Diagnostics

The requirements of the torque coordination function are also taken into account.

Function sequence for knock control 

The function sequence is described in the following steps:

• Function sequence for ignition angle setting 
• Function sequence for RON correction 
• Function sequence for knock control adaptation 
• Function sequence for knock identification error 

Function sequence for ignition angle setting 

Knock control is enabled under the following operating conditions:

• Coolant temperature > 30°C
• Engine load > 34%

According to its input signals, the ME-SFI [ME] control unit computes the cylinder-specific ignition angle correction.

If knocking is detected in a cylinder by the knock sensors, its ignition angle for this cylinder is adjusted towards "retard" (dependent on engine speed) by about 3°CKA (C  rankshaft A  ngle). If knocking still occurs, the ignition angle is retarded by about another 3° crank angle and so on until the maximum shift value is reached (about a 15° crank angle).

If there is knock-free combustion, after a few ignitions the retarding the ignition timing is reversed in operation steps of about 0.75° crank angle, until the normal performance map value is reached, or until knocking occurs again.

Function sequence for RON correction 

The RON correction can be preset using Xentry Diagnostics by means of correction programming.

Knock control is also fitted out with an automatic RON switch. This detects a persistent high tendency to knock of the engine and assists the knock control by means of an appropriate pre-control.

This is done by evaluating the stored adaptation values, the ignition angle of all the cylinders and the number of control interventions in certain engine load and speed ranges.

The RON stage is adjusted by one or several stages depending on the threshold which has been exceeded. The regulated RON stage remains until stopping of the engine.

The higher the RON stage selected, the lower the maximum retarding the ignition timing of the ignition angle.

Function sequence for knock control adaptation 

Adaptation by the knock control becomes active starting from a coolant temperature of about 70°C.

The ignition angle corrections required due to combustion knocking are stored continuously for the specific cylinder, along with the related operating condition of the engine (engine load and engine RPM).

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

Function sequence for knock identification error 

For safety reasons, if one of the knocking sensor signals fails, the ignition angles of all cylinders is retarded by up to 12° crank angle (according to engine speed and coolant temperature). Knock control and the adaptation are deactivated and a fault is stored in the fault memory.

	Electrical function schematic for knock control		PE07.10-P-2715-97TAH
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-P-9998MNC