Function Descriptions, Model/Equipment Combinations

Shown On Engine 119:

Engine running is constantly monitored in order to protect the catalytic converter from excessive thermal stressed during combustion misfiring.

If the smooth running analysis in the ME-SFI control module detects combustion misfiring, the relevant injector or injectors are no longer operated after a certain number of misfirings.

Smooth Engine Running Signal:

The system detects combustion misfiring caused by:
- Ignition system
- Fuel injection system
- Engine mechanical components (e.g. unmetered air, valve seats, valve springs, faulty shift rocker arms in the case of engine 113, 137 fitted with cylinder shutoff)

Design/function
For the smooth engine running analysis, the signal supplied by the crankshaft position sensor is exactly analyzed by means of a comprehensive mathematical method to ensure time synchronization between the individual combustions. Each individual combustion must produce a characteristic acceleration at the flywheel mass. If combustion misfiring occurs the flywheel mass rotates slightly more slowly (short variations in torque) until the next ignition.

The following factors are taken into account:
- Engine speed, engine load
- Recognition of poor road surfaces
- Synchronization of fuel injection and firing order
- Fuel reserve signal
- Sensor rotor adaptation
- Sensitivity (can be set with HHT)
- Engine 113.960 with cylinder shutoff: Cylinder shutoff ON or OFF

1. Engine speed, engine load
A comparative value for the time between two combustions is formed from these parameters.

2. Detecting uneven road surfaces
Up to 5/96 the body acceleration sensor was installed. As of 6/96 the signal is supplied over the CAN from the vehicle dynamic systems on the basis of the wheel speed (uniform running). If the vehicle is driving on a poor road surface, the smooth running analysis is set less sensitive. Vibration influences transmitted through the power train can have a negative effect on the analysis at certain engine speeds and loads.

3. Synchronizing fuel injection and firing order
Cylinder recognition is necessary for fault storage and analysis as well as for selective cylinder shutoff.

4. Fuel reserve signal
If combustion misfirings are detected, a fault is stored if the level in the fuel tank is too low. This additional information makes it possible to detect combustion misfiring caused by fuel deficiency. In the case of engine 112, 113 an entry is made in the fault freeze frame data.

5. Sensor rotor adaptation
The smooth engine analysis is not active until sensor rotor adaptation has been performed. The smooth running analysis stores the particular smooth running properties of the engine in a deceleration phase (uniform smooth running). This is required as a comparative parameter for detecting misfiring.

6. Misfiring counters
Misfirings detected are entered in the following counters in the fault memory;
- Total of all misfirings
- Cylinder-specific misfirings
- "Emission limit" misfirings
- "TWC-harmful" misfirings


NOTE: The smooth engine running analysis replaces the measurement of combustion period and combustion voltage.


Body acceleration sensor position/task/design/function
- Detecting poor road surfaces, up to 6/96

CAN data bus, function
- Fuel reserve signal in the case of CAN - compatible instrument cluster
- Uneven road surface signal