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

Engine 276.8 in model 205, 253 

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 (T11 to T16) for the related cylinder is adjusted towards "retard".

The ME-SFI [ME] control unit manages the knock control according to the following sensors and signals:

• Knock sensor 1 (A16/1)
• Knock sensor 2 (A16/2)
• 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)
• Charge air temperature sensor (B17/8)
• 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 presetting (research octane number) 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 no knocking 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. The regulated RON stage remains until stopping of the engine.

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.

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-2715-97FBB
	Overview of system components for gasoline injection and ignition system with direct injection		GF07.70-P-9998MMV