Ignition Failure Sensor: Description and Operation

MISFIRE MONITORING

GENERAL DESCRIPTION
Engine misfire detection is based on evaluating engine speed fluctuations. In order to detect misfiring in any cylinder, the torque of each cylinder is evaluated by measuring the time between two ignition events. This is also a measure for the mean value of the speed of this angular segment. This means, a change of engine torque results in a change of engine speed. Additionally, the influence of the load torque is be evaluated, such as the effects of different road conditions (pavement, pot holes etc.). If the mean engine speed is measured the influences caused by road surfaces are eliminated. This method consists of the following strategies:

- data acquisition, including adaptation of sensor wheel
- calculation of engine roughness
- comparison with threshold limits dependent on engine operating range
- extreme conditions, during which misfire detection is disabled for a short time
- fault processing, counting procedure of single misfire events

Misfire monitoring structure







1. Data acquisition
The duration of the crankshaft segment intervals are measured continuously for each combustion cycle.

2. Sensor wheel adaptation
Within a defined engine speed range and during fuel cut-off, the adaptation of the sensor wheel tolerance, instead of the misfire detection, is performed. With progressing adaptation the sensitivity of the misfire detection is increased.

The adaptation values are stored in nonvolatile memory and utilized for the calculation of the engine roughness.

3. Misfire detection
The following operating steps are performed for each measured segment corrected by the sensor wheel adaptation.

3.1 Calculation of the engine roughness
The engine roughness is determined from the differences of the segment duration. Several statistical methods are used to distinguish between normal changes of the segment duration and changes due to misfiring.

3.2 Detection of multiple misfiring
If several cylinders are misfiring (for example, alternating one combustion/one misfire event), the calculated engine roughness values may be so low that the threshold is not exceeded and therefore not detected. Based on this fact, the periodicity of the engine roughness value is used as additional information. The engine roughness value is filtered and a new multiple filter value is created. If this filter value increases due to multiple misfiring, the roughness threshold is decreased. By applying this strategy, multiple misfiring is detected reliably.

3.3 Calculation of the engine roughness threshold value
The engine roughness threshold value consists of a base value. which is determined from a load/speed dependent map. During warm-up a coolant temperature dependent correction value is added. In case of multiple misfiring the threshold is reduced by an adjustable factor. Without sufficient sensor wheel adaptation the engine roughness threshold is limited to a speed dependent minimum value. A change of the threshold towards a smaller value is limited by a variation constant.

4.0 Determination of misfiring
Misfire detection is performed by comparing the engine roughness threshold value with the actual engine roughness value.

4.1 Deactivation of misfire detection

Misfire detection is deactivated during the following extreme operating conditions:

- engine speed is [>] 7,200 rpm
- engine speed is [<] 520 rpm
- engine speed and load are changing rapidly:
load = TL [>] 0.9 ms/ignition to ignition
speed [>] 2,080 rpm/sec.

- during engine start
- during coasting conditions:
speed between 520 and 7,200 rpm and load below 0.9 and 1.9 ms

- if misfiring is detected monitoring is interrupted for 17 consecutive ignitions, to avoid false detection due to misfire-induced crankshaft oscillations. However, continuous misfire is detected reliably.
- Exceeding the wheel-speed fluctuation of approximately 1.6 to 5.0 rev/sec2 (dependent on vehicle speed ) when driving on a rough surface.

4.2 Statistics, fault processing
During an interval of 1,000 crankshaft revolutions the detected misfiring events are summed for each cylinder. If the sum of all cylinder misfire incidents exceeds a predetermined value, the fault code for emission relevant misfiring is temporarily stored. If only one cylinder is misfiring, a cylinder specific fault code is stored. If more than one cylinder is misfiring, the fault code for multiple misfiring is also stored.

within an interval of 200 crankshaft revolutions the detected number of misfiring events are weighted and calculated for each cylinder. The weighting factor is determined by a load/speed dependent map. If the sum of cylinder misfire incidents exceeds a predetermined value the fault code for indicating catalyst damage relevant misfiring is stored and the MIL is immediately illuminated (flashing). If the cylinder selective count exceeds a predetermined threshold the following countermeasures take place:

- the lambda closed loop system is switched to open-loop
- the cylinder selective fault code is stored. If more than one cylinder is misfiring, the fault code for multiple misfire is also stored
- the fuel supply to the respective cylinder is shut off ( up to a maximum of two cylinders)
- Resets after engine start

All misfire counters are reset after each interval (refer to flow chart).

4.3 Additional Information
With regard to low fuel capacity, misfiring will be detected as described previously and additionally stored under fault code P 1585.

Functional Flow Chart







Statistics: FAULT PROCESSING