Overview
This document contains information on:
General
Function requirements
Function
General
Knocking is a sudden, uncontrolled combustion of fuel in spark-ignition engines.
Knocking combustion takes place considerably more quickly than normal combustion. This subjects engine component parts to greater stress.
Knock control ensures that the uncontrolled combustion of fuel is prevented under all operating conditions. This also applies for differing fuel grades.
Knocking combustion is detected by knock sensors that are attached to the crankcase.
Function requirements
Engine management ON (circuit 87M)
Engine in operation
Coolant temperature > 30 °C
Engine load > 34%
Function
Knock control
When the knock sensors detect knocking, the ignition angle of the affected cylinder is adjusted towards "retarded."
If a knock sensor fails, the ignition angles of all cylinders (depending on the engine speed and coolant temperature) are adjusted towards "retarded" by a maximum of 12° CKA BTDC. The knock control and the adaptation of the knock control are deactivated. A fault is stored in the fault memory.
The knock control takes the requirements of torque coordination into account.
The following sensors are used for knock control:
Knock sensors (knock signals)
Camshaft Hall sensors (rpm signals from the camshaft/camshafts)
Coolant temperature sensor (coolant temperature)
Pressure and temperature sensor downstream of exhaust gas turbocharger (pressure and temperature downstream of exhaust gas turbocharger)
Accelerator pedal sensor (accelerator pedal position and accelerator pedal movement)
Crankshaft Hall sensor (engine speed, position of the crankshaft)
RON default setting (Research Octane Number)
RON correction
The task of the Research Octane Number correction (RON correction) is to control the maximum retard position of the ignition angle in advance, through which the knock control is relieved. The higher the RON correction level, the smaller the maximum retard position of the ignition angle.
The following RON correction options are available:
Permanently through correction programming
Temporary, until the engine is switched off, through the automatic RON correction
If the automatic RON correction detects that the engine tends to knock frequently, the RON correction saves the following information and subsequently determines the RON correction level:
Values for the adaptation of knock control
Ignition angles of all cylinders
Number of control interventions in particular engine load ranges and engine speed ranges
Depending on the engine’s tendency to knock, the RON correction adjusts the correction level by one or multiple levels. This automatic RON correction level is retained until the engine is switched off.
Adaptation
Adaptation also has the following function requirements:
Coolant temperature > 75 °C
To optimize the knock control for specific cylinders, the following values are continually stored:
Ignition angle correction
Engine speed
Engine load
These stored adaptation values are taken into account when determining the individual ignition angles.
Function schematics | |||
Function schematic for knock control | Engine 282 in model 118, 177, 247 | ||
Engine 260 in model 118, 177, 247Engine 264 in model 167 | |||
Engine 254, 264 in model 213, 238, 257 as of model year 2021Engine 256 in model 167Engine 256 in model 213, 238 as of model year 2021 | |||
Engine 176, 177 in model 167 | |||
Engine 139 in model 118, 177 | |||
Additional basic functions | |||
Ignition angle setting, basic function | Engine 139, 176, 256, 260, 264, 282 | ||
Control units | |||
Combustion engine control unit, basic function | Engine 139, 176, 177, 254, 256, 260, 264, 282 | ||
Component parts | |||
Knock sensor, basic function | Engine 139, 176, 177, 254, 256, 260, 264, 282 |