Catalyst Monitoring

1 CATALYST MONITORING
There is one diagnostic function which is used for the monitoring of the catalyst efficiency. This is based on measuring the oxygen within the catalyst determined by at least two oxygen sensors. The oxygen storage capacity correlates with the NMHC or NOx emissions. Depending on which one of these two emission components reaches its OBD threshold first, the relevant threshold corresponds to either the NMHC or the NOx emissions.

The catalyst system may consist of two separated catalyst bricks, with the oxygen sensors mounted before and after the primary catalyst.

The diagnostic routine is executed once per driving cycle.

1.1 Active measurement of OSC

1.1.1 General description
The catalyst monitor is based on the determination of the oxygen storage capacity (OSC). The correlation between the conversion efficiency and the OSC has been investigated on catalysts with various characteristics specifically concerning stages of aging correlated to exhaust emissions (HC/NOx). Therefore, the catalyst is diagnosed by comparing its storage capability against the storage capability of a borderline catalyst.
The oxygen storage capacity (OSC) can be determined by the following method:

1. Determination of the Oxygen storage state (active test)
For the purpose of monitoring, the ECM cycles the A/F ratio by commanding a rich and a lean fuel mixture as follows.
- First, a rich A/F ratio is commanded by the ECM until a minimum of oxygen has been removed (cumulated rich gas > threshold - This threshold is derived from the Oxygen Storage Capacity of the borderline catalyst for the current operating point taken from the corresponding look-up table and typically multiplied with a factor between 1.3 and 2.5).
- Then, the catalyst is operated with a lean A/F ratio commanded by the ECM and the Oxygen Storage Capacity is calculated from the oxygen mass stored in the catalyst as follows:

- The catalyst is operated in this mode until the oxygen stored in the catalyst exceeds a calibrated limit or the downstream oxygen sensor indicates that the catalyst is completely saturated with oxygen (typically 450-550 mV), or when the measured OSC value is higher than the x-fold (x =1.5-2.5) Oxygen Storage Capacity of the borderline catalyst for the current operating point.
- The catalyst is then diagnosed by comparing its oxygen storage capacity to the calibrated threshold of a borderline catalyst. A normalized OSC value is calculated from the quotient of the actual OSC value and borderline OSC value.
- After each completion of a measuring cycle a cycle counter is incremented
- Measuring cycles are run until the cycle counter reaches a certain number (typically 3 to 5 without EWMA and 1 with EWMA), then the diagnostic is finished.
- Calculation of the normalized mean value from the single measurements
- Comparison of the filtered normalized value with the fault threshold

The following picture shows an example of an OSC measurement:

1.1.2 Monitoring function description
According to the operating principle described above the following main parts of the monitor can be distinguished:
- Check of monitoring conditions for active test
- Lambda request (interface to lambda controller)
- Mixture enrichment in order to remove any stored oxygen
- Measurement of oxygen storage capacity (OSC) by lean A/F ratio operation
- Processing
- Fault detection

1.2 Monitoring of front catalyst and catalyst system

1.2.1 General description
The catalyst monitor is based on the determination of the oxygen storage capacity (OSC). The correlation between the conversion efficiency and the OSC has been investigated on catalysts with various characteristics, specifically concerning stages of aging correlated to exhaust emissions (HC/NOx). Therefore, the catalyst is monitored by comparing its storage capability against the storage capability of a borderline catalyst.

In this special application, the exhaust gas system consists of a close coupled catalyst (primary catalyst) and an underfloor catalyst (main catalyst).

This diagnostic function determines the OSC of the primary catalyst and the OSC of the catalyst system, (close coupled and underfloor catalyst), and depending on the results, a malfunction of the cat system is detected.

The diagnosis only depends on the measured OSC of the close coupled catalyst, if the oxygen storage capacity of the close coupled catalyst is greater then a first threshold. In this case the OSC of the catalyst system will not be measured.

If the oxygen storage capacity of the close couple catalyst is smaller than a first threshold, the OSC of the catalyst system will also be measured.

Malfunction criteria:
The exhaust gas aftertreatment system is recorded defective, if the OSC of the complete catalyst system is smaller than a second threshold.
Based on the following hardware configurations and the corresponding diagnostic concepts the following methods are applied to receive a diagnostic result:

1. System with total volume catalyst monitoring (ULEV II/Bin5, 1 brick or 2 bricks):
- Determination of a normalized mean overall catalyst diagnostic value, system without EWMA
- Determination of a normalized, EWMA filtered overall catalyst diagnostic value, system with EWMA
- Determination of a normalized mean total volume catalyst diagnostic value,
- Generation of the product from the primary catalyst diagnostic value and the total volume diagnostic value and comparison of the result with a fault threshold

Depending on the calibration of the look-up tables the following methods are applied:

In case the measurement of the primary cat delivers a very positive result (value exceeds a threshold), the main cat is not measured, which means that in this case only the primary cat factor is compared with a fault threshold.

Primary cat factor > threshold --> Pass Result
However, measuring of the main cat is simulated for the IUMPR creation.

In case the result of the primary cat measurement remains below the threshold, the additional measurement of the total volume catalyst monitoring is activated.

Processing:
After the measurement of the OSC, the OSC value is normalized to the OSC value of the borderline catalyst, which is taken from a map, depending on the exhaust gas mass flow and catalyst temperature.

The final diagnostic result is calculated by averaging several normalized OSC values and comparing the result to the threshold. The measurement of OSC can be carried out consecutively or stepwise.

1.2.2 Secondary parameters
All secondary parameters listed below have to be fulfilled for a calibrated duration:
- O2 Sensors ready for operation
- Catalyst temperature in calibrated range
- Maximum permissible catalyst temperature change < threshold
- Axial temperature distribution of catalyst within range (the catalyst temperatures are modeled in several layers)
- In case the lowest temperature of all the layers falls below a threshold for a certain time (cool down time), this temperature must be exceeded for another time period (re-heatup time) before the diagnostic is re-enabled (radial warming after cooling-down must be ensured)
- The low-pass filtered minimum temperature > threshold (ensures thorough radial warming in warm-up phases even after cooling down caused by overrun fuel cut-off phases)
- Delta exhaust mass flow < threshold
- Exhaust gas mass flow in calibrated range
- Deviation of actual lambda from setpoint lambda < threshold
- Lambda controller not at minimum or maximum
- Requested lambda value for diagnostic routine is achieved
- Engine speed in calibrated range
- Engine load in calibrated range
- SAS not active
- Canister purge flow is low

1.2.3 In-Use Monitor Performance Ratio
The incrementing of the numerator, the denominator and the ratio calculation for the catalyst monitoring is executed by the IUMPR kernel function. Like all monitors, for which a standardized track and report of the in-use performance is required, the catalyst monitor reports to the IUMPR kernel function via status flags - see the description of the IUMPR kernel function.

Incrementing the numerator
In determining the numerator we must differentiate between the exhaust systems and the diagnostic strategies.

For the system with the total volume catalyst monitoring as well as for the system with partial volume catalyst monitoring with only two oxygen sensors available for the diagnostic concept (one before the overall catalyst volume and one after the overall catalyst volume), only the OSC of the overall catalyst system can be measured.
This is why the PASS result is not faster than a FAIL result for the catalyst diagnostic. Hence, this is a symmetric diagnostic and the numerator can be incremented after the required number of diagnostic cycles.

For the PASS result, the measured primary catalyst OSC only needs to exceed a first threshold.
For the FAIL result, additionally to the primary catalyst OSC measurement the OSC measurement of the overall catalyst volume is activated. Hence the time required for the FAIL result is longer than the time required for the PASS result and is actually prolonged by the time necessary for the overall catalyst volume measurement. Therefore, in case a PASS result for the primary catalyst is achieved, the numerator is not incremented before the time, that would have been necessary for the FAIL result of the total volume catalyst monitor has passed after the primary catalyst PASS result.
This time period is calculated such that the 3-fold rich mixture charge of the borderline catalyst and the 1.65-fold lean mixture charge of the borderline catalyst into the actual catalyst would have occurred with a lambda adjustment of a fictive conditioning phase and an oxygen charge phase, based on the subsequent exhaust gas mass flow.

Hence, the numerator for the measurement of the primary catalyst cannot be incremented before the above described time period has passed.

When measuring both the primary and the overall catalyst volumes the numerator is incremented immediately after the diagnostic has finished.

Incrementing the denominator
The denominator is incremented when the monitor is not inhibited due to stored faults and the general denominator conditions have been fulfilled for the current driving cycle.