Monitoring of Combustion Misfire
Combustion Misfire
Misfiring during the combustion of one or more cylinders increases the emission considerably. A misfire rate of 2% is sufficient in the example to reach a high HC value and to exceed the emission threshold.
If the threshold is exceeded after two consecutive trips and emissions have increased 1.5 times, the Malfunction Indicator Lamp (MIL) must come on. If the misfiring rate is above a certain specification that could lead to permanent damage to the Three Way Catalytic Converter, the MIL will come on immediately and must blink.
The illustration shows a rpm/engine load map of a gasoline engine where the temperature of the Three Way Catalytic Converter (TWC) exceeds 1000°C and with that causing possibly permanent damage to the TWC.
To monitor combustion misfire, the engine vibration is used as a measuring value, i.e. every misfire causes a negative rpm change that is interpreted as a running problem. To record engine speed, an inductive speed sensor is mounted to the crank case that uses a ring gear mounted to the flywheel.
For a 6-cylinder engine there are 3 cycles per crankshaft revolution. At each cycle the speed of the crankshaft is increased. The Engine Control Module measures the turning speed (time) of the crankshaft via the received signals of the speed sensors from one cycle to the next and recognizes whether the crankshaft is accelerated at the right moment or not.
Together with the Hall sensor signal and the reference signal, the ECM recognizes top dead center of cylinder 1 during starting of the engine and assigns the individual cycles to the appropriate cylinder. This way the ECM can assign a misfire causing a slowdown of the crankshaft to the appropriate cylinder.
For a clear statement, whether a misfire-free operation or an operation with misfiring is apparent, a threshold value for each rpm/load point has to be available. This threshold has to be low enough to recognize singular misfire, but far enough removed from the engine specific "running characteristics" to avoid malfunction recognition.
The adaptation happens during coasting of the engine (load, no load operation). As the combustion cycles have no influence a constant speed signal is recorded that adapts to the existing speed values in the ECM.
When driving, the following operational conditions lead to a black-out of the monitoring:
- Under - or overstepping of a rpm limit (stalling)
- High rpm jumps (shifting)
- Time factor when starting engine (cold start)
- Below the load threshold (driving resistance)
- Recognition of poor road (pot hole)
- Engine at idle (as from program version 14)
Storing, recognition and fault correction of an emission relevant combustion misfire
Emission relevant combustion misfire occurs for the first time:
- Fault recognition takes place, i.e. one or more cylinders with an above average misfire contribution are recorded as fault causing cylinders.
- Fault segment tl/n is recorded.
- Engine temperature is recorded.
Assumed fault is not detected:
With every engine start an area window tI/n is opened. If this area window coincides with the fault window during misfire-free operation and the engine temperature is within the area committed to memory, the fault will be erased from memory.
Suspected fault is detected:
If an emission relevant fault is recognized during the second trip, the fault is finally detected. The Malfunction Indicator Lamp (MIL) is switched on.
TWC damaging misfire rate:
When TWC damaging misfire occurs, the MIL will light up immediately and starts to blink. When leaving the load/rpm range, the MIL changes. The blinking MIL changes to a steadily lighted MIL.
Fault correction:
The MIL may be switched off after three successive warm up cycles when the area window coincides with the fault window and the operation is without misfire and the engine temperature corresponds with the memory area.