Misfire Monitoring
3 MISFIRE MONITORING3.1 General description
The method of engine misfire detection is based on evaluating the engine speed fluctuations.
One crankshaft segment can be assigned to the combustion event of one cylinder. A combustion event that has not occurred or has not occurred completely leads to a drop in engine speed and engine torque compared to normal operation. As a result, the segment times of the crankshaft increase. The segment times are measured using a sensor at the crankshaft and the engine speed fluctuations are determined. The following figure illustrates the example of misfire due to missing spark.
3.2 Monitoring strategy
To detect a misfire, the engine speed fluctuations are assessed. The number of misfire events is counted and compared against a defined threshold value.
3.3 Enable conditions
- After engine start: the number of combustion events exceeds a defined threshold (2 crankshaft revolutions)
- (2 crankshaft revolutions)
- Engine speed within a defined range
- Engine torque ≥ threshold value (taking the positive torque condition according to the regulations into account)
- Intake air temperature > threshold value (this enable condition is disabled)
- Engine temperature at engine start > threshold value (this enable condition is disabled)
- Deceleration fuel cut-off not active
- Rough road condition not detected
3.4 Malfunction criteria
The crankshaft segment times are measured and these measurements are used to calculate the engine roughness values. If the engine roughness value is greater than a calculated threshold value, a misfire fault is detected. The threshold value for the engine roughness consists of a base value determined by means of geardependent characteristic engine-speed and load maps. According to the currently selected gear, the corresponding characteristic map is used (this applies to manual and automatic transmissions). During the engine warm-up a correction value, which depends on the engine temperature, is added to the base value.
If misfiring is present in more than one cylinder (e. g. a combustion and a misfire alternately), due to the respective engine roughness criterion, the detection capability based on the engine roughness value can decrease compared to single misfiring cylinders. For this reason the periodicity of the engine roughness values is used as additional information. The engine roughness values are filtered and a new multiple filter value is generated. In this way, multiple misfire events can be detected reliably.
The adaptation of the sensor wheel tolerances and the characteristic engine behavior are interpreted for defined engine-speed and load ranges, if no misfire has been detected. The robustness of the misfire monitor increases with the adaptation progress. The adaptation values are stored in a non-volatile memory and are included in the calculation of the engine roughness.
If the enable conditions are fulfilled, two counters are incremented when misfiring has been detected. This serves to distinguish between emission relevant misfiring and catalyst-damaging misfiring.
During intervals of 1,000 crankshaft revolutions the detected misfire events are counted for each cylinder. If the total sum of the detected misfire events is greater than a defined threshold, the fault "emission relevant misfire" is detected.
During intervals of 200 crankshaft revolutions the detected misfire events are counted and weighted for each cylinder. The weighting factor is determined by a characteristic map which is dependent on engine load and speed. If the total sum of the detected misfire events is greater than a defined threshold, the fault "catalyst-damaging misfire" is detected.
After each interval, the misfire counters are reset.
3.5 Flow chart
The flowchart represents one combustion event.