Misfire Detection

Misfire Detection

Legal requirements












California exhaust emissions laws require the monitoring of combustion misfiring, as the latter leads to an increase in noxious emissions and to TWC damage. A distinction must be made between emission relevant misfiring after start-up and during the journey and misfiring which is damaging to the TWC. Emission relevant faults exist if the limits of the FTP emissions test have been exceeded 1.5 times.
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 threshold.
Misfiring damaging to the TWC is present if the temperature of the TWC reaches over 1000 ° C.
The drawing shows what misfire rate (in %) - as a function of the load/speed range - allows the temperature of the TWC to climb to approx. 1000 °C.

Emission relevant misfiring
To detect emission relevant faults, the misfires are counted within a cycle of 1000 crankshaft revolutions. The first 1000 crankshaft revolutions after the engine has been started are "emission relevant after start-up." After this, the misfires become "emission relevant during the journey". At the end of the cycle, the counter is reset to zero.

A fault is stored:
- The first time the threshold is exceeded in the case of emission relevant misfiring after start-up
- The fourth time the threshold is exceeded in the case of emission relevant misfiring during the journey.

TWC damaging misfiring
To detect TWC damaging faults, the misfires are counted within a cycle of 200 crankshaft revolutions. A fault is stored as soon as a given threshold - dependent upon the load/speed range - is exceeded (not simply at the end of a cycle), and the Check Engine MIL (Malfunction Indicator Light) flashes for the duration that TWC damaging misfiring is present.

Fuel injection cutoff
If TWC damaging misfiring occurs over two consecutive cycles, up to two fuel injectors can be cut off.

Detection of misfiring
To detect misfiring, the rough running of the engine is evaluated. To this aim, the flywheel (1 crankshaft revolution) is divided into three segments. The rpm/crankshaft position sensor measures the period of time which passes until the segment has moved past the rpm/crankshaft position sensor. If combustion misfiring occurs, the drive torque is reduced briefly. This leads to an interruption in the rpm and therefore to a longer segment time, or to negative angular acceleration. The rough running of the engine is proportional to the change in angular acceleration. At constant rpm or constant acceleration, rough running equals zero.
Marked changes in rpm also occur during dynamic engine operation, and these must be distinguished from combustion misfiring. It is possible to distinguish between them, as the change in rpm extends over more revolutions than in the case of combustion misfiring.