Catalyst Monitoring

9.1.A Catalyst Monitoring for Test Groups:

Please note that all sections with a .A suffix refer to models below. All sections with a .B suffix refer to 745i and 745Li models. Sections with no suffix refer to all models.

2BMXV04.4LEV: 540i, 540i sport wagon
2BMXT04.4E53: X5 4.4i
2BMXT04.6XHP: X5 4.6is

9.1.1.A General description

Catalyst monitoring is based on monitoring it's oxygen storage capability.

The engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in the exhaust gas. These oscillations are dampened by the oxygen storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream of the catalyst indicates the storage capability.

In order to determine catalyst efficiency the oscillation of the upstream sensor is needed to calculate the "02-in and-output (catalyst)" by engine air mass and lambda-deviation. The downstream sensor signal for a "threshold catalyst" then is derived (model) from this basic value.

Any time the real sensor signal oscillation (downstream) corresponds to the model, a defective catalyst is recognized.





9.1.2.A Monitoring structure

9.1.3.A Monitoring Cycle

9.1.3.1.A Computation of the efficiency factor

First the signal of the lambda-controller is filtered and multiplied by the engine air mass. A value is now added which considers the downstream sensor layer. This result is representative of 02- load (OKB) into the catalyst.

The standard amplitude (NSA) of the downstream sensor is computed by averaging the measured signals.

The difference between NSA and OKB is integrated and divided continuously by a time range during which catalyst monitoring is active.

This factor (GW) is an indicator of the catalyst efficiency and it is determined continuously in a certain engine speed and engine load range within the time of monitoring.

9.1.3.2.A Fault evaluation

After time range of monitoring has elapsed the efficiency factor (GW) is compared with the threshold value. If GW > than the threshold value, a fault is detected and the MIL is illuminated after the next driving cycle.

9.1.3.3.A Check of monitoring conditions

The monitoring principle is based on the detection of relevant oscillations of the downstream sensor signal during regular lambda control. It is necessary to check the driving conditions for exceptions where no regular lambda control is possible, e.g. fuel cut-off. During such periods, and for a certain time afterwards, the computations of the amplitude values and the postprocessing is halted. Thus, a distortion of the monitoring information is avoided.





9.1.4.A Block diagram of system operation

9.1.B Catalyst Monitoring for Test Group 2BMXV04.4LEV Models: 745i, 745Li

9.1.1.B General description

The Catalyst Monitor is based on determining the oxygen storage capability. The (nonlinear) correlation between conversion efficiency and storage capability has been shown in various investigations. The catalyst is diagnosed by comparing its storage capability against the storage capability of a borderline catalyst.

The oxygen storage capability can be determined by one of the following two methods:

1. Oxygen reduction after fuel cut off (passive test)

During fuel cut off oxygen is stored in the catalyst. After fuel cut the catalyst is operated with a rich A/F ratio and the amount of removed oxygen is determined. If this passive test indicates an oxygen storage capability highly above the borderline catalyst, the catalyst is diagnosed without an error, otherwise the monitor will be restarted after the next fuel cut off.

2. Oxygen filling (active test)

- First, a mixture with a low A/F ratio is put through the catalyst until any oxygen has been removed.





- Then, the catalyst is operated with a high A/F ratio. The Oxygen Storage Capability (OSC) is calculated out of the oxygen mass stored in the catalyst as shown.

- The catalyst is operated in this mode until the oxygen stored in the catalyst exceeds a calibrated limit or the downstream oxygen sensor indicates the catalyst is completely saturated with oxygen.

- The catalyst is then diagnosed by comparing its oxygen storage capability to the calibrated threshold of a borderline catalyst.





System overview:

9.1.2.B Monitoring structure

According to the above described operating principle the following main parts of the monitor can be distinguished:

- Monitoring the amount of removed oxygen after fuel cut off

- Check of monitoring conditions for active test

- Lambda request (interface to lambda control)

- Mixture enrichment in order to remove any stored oxygen

- Measurement of oxygen storage capacity (OSC) by lean A/F ratio operation

- Fault detection





Monitor structure overview:

9.1.3.B Monitoring Cycle

9.1.3.1.B Oxygen Removal after fuel cut off

During fuel cut off the catalyst is completely filled with oxygen. If the amount of rich gas required to remove this oxygen exceeds a threshold, a good catalyst will be notified.

9.1.3.2.B Check of Monitoring Conditions for active test

The diagnostic principle uses the precise measurement of the A/F ratio fed into the catalyst. This requires verification if the engine runs in the acceptable operating conditions. If such operating conditions are not present any more, the monitor is terminated and restarted only some time later.

9.1.3.3.B Lambda Request

The requested A/F ratio (enrichment or enleanment) is commanded from the monitor through the mixture control.

9.1.3.4.B Mixture Enrichment

Low A/F ratio (X < 1) is applied in order to remove any stored oxygen from the catalyst.

9.1.3.5.B Measurement of Oxygen Storage Capacity (OSC)

Next, a high A/F ratio (X > 1) is fed into the catalyst. The oxygen mass absorbed by the catalyst is determined according to the signal of the upstream lambda sensor as follows:





This lean mixture is requested until:

- either the downstream lambda sensor indicates the catalyst to be completely saturated with oxygen,

or

- the stored oxygen mass (osc(t)) exceeds threshold OSC of the borderline catalyst.

9.1.3.6.B Fault Detection

The monitor finishes without an error, if the passive test detects a storage capability highly above the borderline limit. If the passive test does not meet the condition for a good catalyst, the active test is initiated and the OSC is calculated. The catalyst is diagnosed by comparing the determined OSC the against the borderline threshold.

Monitoring conditions for active test:

- no error on lambda sensors (signal, aging, heater)

- canister purge value < limit

- no error on EGR system (if available)

- modeled catalyst temperature within range

- misfire rate < limit

- regular A/F control (no fuel cut-off)

- engine air mass flow within range





9.1.4.B Block diagram of system operation