Monitoring of Combustion Misfire
Monitoring of Combustion Misfire
Combustion misfiring at one or more cylinders increases emission values considerably.
If the misfire rate lies above the threshold value which exceeds the increase in emissions by 1.5 times the permissible limit value during two successive journeys, the Check Engine MIL must be switched on.
A misfire rate of slightly above 2% is sufficient in the example shown here to reach a high HC value and so to exceed the diagnostic threshold.
If the misfire rate is above a threshold value that could lead to permanent damage to the TWC, the Check Engine MIL will come on immediately and must flash.
The illustration shows an example of misfire rates in the rpm/engine load map of a gasoline engine, in which the temperature of the TWC exceeds 1000 ° C and permanent damage to the TWC may occur.
In order to monitor combustion misfire, the DME control module uses the rough running of the engine as a measuring variable, i.e. each misfire leads to a negative change in rpm which is interpreted as rough running.
The engine rpm is recorded by an inductive sensor located above a ring gear on the flywheel.
In a 6-cylinder engine, three power strokes take place per crankshaft revolution. With each stroke, the crankshaft is accelerated. Using the input signals from the speed sender, the DME control module measures the revolutionary speed (time) of the crankshaft from one stroke to the next and can therefore determine whether or not the crankshaft is accelerating at the appropriate point. In conjunction with the camshaft and crankshaft position sensor signals, the DME control module can recognize cylinder 1 ignition TDC when the engine is started and can therefore allocate the individual strokes to the respective cylinder. In this way, in the event of misfiring the DME control module can assign the negative acceleration of the crankshaft to the appropriate cylinder.
To obtain a clear statement as to whether operation is free from misfiring or misfiring is occurring, a rough running threshold value must be available for each rpm/engine load point. This threshold must be low enough to recognize single misfires, but far enough removed from the "engine specific" running characteristics to avoid fault detection.
When driving, the following operating conditions lead to a black-out of the monitoring:
- Dropping below or exceeding an rpm limit (stalling)
- High rpm jumps (shifting)
- Time factor after starting engine (cold start)
- Falling below a load threshold (driving resistance)
- Recognition of poor road (pot hole)
- Idle speed
The rough running threshold is adapted when the engine is decelerating (coasting/inertia fuel shutoff). Since the combustion strokes have no influence here, a uniform rpm signal is obtained which adapts the existing rpm data in the computer of the DME control module.
Storing, confirmation and fault healing of an emission relevant combustion misfire
Emission relevant fault occurs for the first time
- Fault entry takes place, i.e. one or more cylinders with an above average misfire contribution are recorded as fault-causing cylinders.
- Fault window tl/n is recorded.
- Engine temperature is recorded.
Suspected fault is not confirmed
With every engine start an area window tl/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 confirmed
If an emission relevant fault is recognized during the second trip, final entry of the fault takes place.
The Malfunction Indicator Lamp (MIL) is switched on.
TWC damaging misfire rate
When TWC damaging misfire occurs, the MIL lights up immediately and starts to flash. When the vehicle leaves this load/rpm range, the status of the Check Engine MIL changes. The flashing light stays on continuously.
Fault healing
The Check Engine MIL may be switched off after three successive warm-up cycles when the area window coincides with the fault window, operation is without misfire and the engine temperature corresponds with the area committed to memory.