Overview
This document contains information on:
General
Function requirements
Function
General
The smooth running control reduces any rough running in the rpm range from 750 to 2600 rpm. It also protects the catalytic converter from overheating.
Function requirements
Engine management ON (circuit 87M)
Engine in operation
Function
Smooth running control
In the event of combustion misfiring, unburnt fuel can find its way into the catalytic converter. When this fuel ignites in the catalytic converter, the catalytic converter may be damaged by thermal overload. In order to prevent unburnt fuel from getting into the catalytic converter, the smooth running control detects combustion misfiring and intervenes in the engine running.
Detecting combustion misfiring
In order to detect combustion misfiring, the combustion engine control unit evaluates the engine speed signal from the crankshaft Hall sensor.
Each combustion produces an acceleration at the multipole wheel on the crankshaft. If a combustion misfires, the multipole wheel turns measurably more slowly until the next combustion. The engine torque fluctuates.
Signal path for smooth running |
A | Engine running without combustion misfiring Acceleration values within tolerance range (up to approx. 3 m/s2) |
B | Engine running with combustion misfiring Acceleration values too great (more than 3 m/s2) |
C | Misfire counter starts with misfire detection |
a | Acceleration |
t | Time |
P07.61-3854-11
The following causes of combustion misfiring are detected:
Faulty ignition system
Faulty fuel injection system
Mechanical engine components (for example, valve seats, valve springs, unmetered air)
Fuel deficiency
Deactivating fuel injectors (spark-ignition engine)
When the smooth running control detects combustion misfires, the affected fuel injector is deactivated after a certain number of combustion misfires. In four-cylinder engines, a maximum of two fuel injectors are deactivated at the same time. In six-cylinder engines, a maximum of three fuel injectors are deactivated at the same time. In 8-cylinder engines, a maximum of four fuel injectors are deactivated at the same time.
Spark-ignition engines: counting and storing combustion misfires
The total number of combustion misfires detected is stored in the fault memory and subdivided into:
"Cylinder-specific" combustion misfires
"Emission limit" combustion misfires
"Catalytic converter damaging" combustion misfires
Adjusting the fuel injection quantity (diesel engines)
When the smooth running control detects combustion misfires, the combustion engine control unit adjusts the fuel injection quantity. To this end, the combustion engine control unit adjusts the following parameters:
The fuel pressure in the fuel distributor is regulated via the pressure regulating valve and the quantity control valve.
The injection duration is regulated via the actuation duration of the fuel injectors.
Influencing factors for smooth running control
The following factors affect smooth running control:
Engine speed and engine load
Coolant temperature
Road bumps
Synchronization of fuel injection and firing order
Engine smooth running characteristics
Engine speed and engine load
Using the engine speed and engine load, the combustion engine control unit calculates a reference value for the time between two power strokes.
Coolant temperature
The smooth running control permits more combustion misfires for a cold engine than for an engine at operating temperature.
Road bumps
Vibration influences on the drivetrain may have a negative effect on the evaluation of the engine running at certain engine speeds and load conditions. Therefore, the tolerance range for the smooth running control is increased on poor road surfaces.
In order to detect road bumps, the combustion engine control unit evaluates the wheel speed signal.
Synchronization of fuel injection and firing order
The cylinder recognition is required for the storage and assessment of faults. The cylinder recognition evaluates the signals from the Hall sensors of the crankshaft and the intake and exhaust camshafts. Using these signals, the cylinder recognition can determine the stroke sequence of the individual cylinders.
Engine smooth running characteristics
When the vehicle is in overrun mode and the engine is operating in a uniform, smooth manner, the combustion engine control unit stores the smooth running characteristics of the engine as reference values. The combustion engine control unit uses these reference values to detect combustion misfiring.
Once the reference value has been successfully stored, the tolerance range of the smooth running control is downsized. In this way, the smooth running control is adapted to the specific smooth running characteristics of the engine.
Function schematics | |||
Function schematic for smooth running control | Engine 608 in model 118, 177, 247Engine 654 in model 167Engine 654 in model 213, 238, 257 as of model year 2021 | ||
Engine 282 in model 118, 177, 247 | |||
Engine 254, 264 in model 213, 238, 257 as of model year 2021Engine 256 in model 213, 238 as of model year 2021Engine 260 in model 118, 177, 247Engine 264 in model 167 | |||
Engine 176, 177 in model 167 | |||
Engine 139 in model 118, 177 | |||
Control units | |||
Combustion engine control unit, basic function | Engine all (CAR) |