Smooth Engine Running Analysis Function
Smooth Engine Running Analysis Function
Shown on Engine 119
A1 Instrument cluster
B2/5 Hot film mass air flow sensor
B6/1 Camshaft Hall sensor
B24/7 Body acceleration sensor (up to 5/96)
L5 Crankshaft position sensor
N3/10 ME-SFI control unit
X11/4 Data link connector
Y62 Injectors (LH-SFI, HFM-SFI, PEC [LH, HFM, PMS])
CAN Data bus
Engine running is constantly monitored in order to protect the catalytic converter from excessive thermal stresses if combustion misfiring occurs.
If the smooth engine running analysis in the ME-SFI control unit detects combustion misfiring, the appropriate injector or injectors are no longer actuated after a certain number of misfirings
Smooth engine running signal
A Engine running without misfiring, acceleration values within tolerance range (up to approx. 3 m/s2 )
B Engine running with misfiring, acceleration values too large
C Misfiring counter, start when misfiring detected
What is detected is combustion misfiring caused by:
- Ignition system
- Injection system
- Engine mechanics (e.g. fresh air, valve seat, valve springs, defective shift rocker arms on Engine 113 with cylinder shutoff)
Design/function
For analyzing smooth engine running, the signal supplied by the crankshaft position sensor is analyzed using a sophisticated mathematical method to determine whether exact time synchronism exists between the individual combustions.
Each individual combustion must produce a characteristic acceleration at the flywheel. If combustion misfiring exists, the flywheel mass rotates slightly more slowly until the next ignition (short variations in torque).
The following factors are taken into account:
- Engine speed, engine load
- Detection of road bumps
- Synchronization of injection and firing order
- Fuel reserve signal
- Sensor rotor adaptation performed
- Sensitivity (adjustable with HHT/STAR DIAGNOSIS)
- 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. Recognition of uneven road surface
Up to 5/96 the body acceleration sensor is installed. As of 6/96 the signal is supplied over the CAN by the vehicle dynamic systems on the basis of the wheel speed (uniform running).
On poor road surfaces the smooth engine running analysis is set more sensitive. Vibrations transmitted through the drive train may have a negative influence on the analysis at certain engine speeds and load states.
3. Synchronizing fuel injection and firing order
The cylinder recognition is required for storing and -assessment as well as the selective cylinder shutoff.
4. Fuel reserve signal
If combustion misfiring has been detected, a fault is stored if the fuel tank level is too low. Combustion misses due to fuel starvation are detected via this additional information.
5. Sensor rotor adaptation
The smooth engine running analysis is not active until sensor rotor adaptation has been completed. In a deceleration phase (uniform smooth running) the smooth engine running analysis stores the particular smooth running properties of the engine. These are required as a comparison parameter for detecting misfiring.
6. Misfiring counters
Any misfiring detected is entered in the fault memory in the following counters:
- Total of all misfiring
- Misfiring at specific cylinder
- Misfiring at "emission limit"
- Misfire, damaging to TWC
Note: The smooth engine running analysis replaces the measurement of the combustion period and combustion voltage.