GF07.61-P-4031MIS ME-SFI Knock Control Function
GF07.61-P-4031MIS ME-SFI Knock Control Function
- as of model year 09
Function requirements for knock control, general points
^ Circuit 87M ON (engine control ON)
^ Engine running
Knock control, general points
The knock control is integrated in the ME-SFI [ME] control unit (N3/10) and its purpose is to ensure knock-free engine operation in all operating conditions and when different fuel grades are used. In case of uncontrolled combustion (knocking), the ignition angle of the ignition coil (T1) for the related cylinder is adjusted towards "retard".
The ME-SFI [ME] control unit (N3/10) manages the knock control according to the following sensors and signals:
- Knock sensor 1, on the right (A16/1)
- Knock sensor 2, on the left (A16/2)
- Hot film MAF sensor (B2/5), engine load
- Intake air temperature sensor (B2/5b1), intake air temperature
- LH and RH intake camshaft Hall sensor (B6/4, B6/5), intake camshaft positions (ignition circuit detection)
- LH and RH exhaust camshaft Hall sensor (B6/6, B6/7), exhaust camshaft positions (ignition circuit detection)
- Coolant temperature sensor (B11/4)
- Pressure sensor (B28), intake manifold air pressure
- 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 the Diagnosis Assistance System (DAS) (except code (494) USA version)
The requirements of the engine torque-requirement 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 adjustment
^ Function sequence for RON correction
^ Function sequence for knock control adaptation
^ Function sequence for knock identification error
Function sequence for ignition angle adjustment
Knock control is enabled in the following operating conditions:
- Coolant temperature > 40 °C
- Engine load > 40%
According to its input signals, the ME-SFI [ME] control unit calculates the cylinder-specific ignition angle correction .
If knocking is detected in a cylinder by the knock sensors, the ignition angle for this cylinder is adjusted towards "retard" (depending on engine speed) by about 3° CKA (Crank Angle) at the next ignition. If knocking still occurs, the ignition angle is adjusted towards "retard" by about another 3° CKA , and so on until the maximum adjustment is reached (about 10 to 15° CKA).
If there is no knocking, after a few ignitions the retard adjustment is reversed in 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 with the DAS through the correction programming (except code (494) USA version).
Knock control is also fitted out with an automatic RON switch. This detects a persistent high knocking tendency 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.
Depending on the threshold which has been exceeded, the RON stage is adjusted by one or several stages. The regulated RON stage remains until stopping of the engine.
The higher the RON stage selected, the lower the maximum retardation of the ignition angle.
Function sequence for knock control adaptation
Adaptation by the knock control becomes active starting from a coolant temperature of about 75 °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 speed).
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 11° 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.
