Catalyst Monitoring: General Description
The Catalyst Monitor is based on determining the oxygen storage capacity. The nonlinear correlation between conversion efficiency and storage capacity has been shown in various investigations. The catalyst is diagnosed by comparing its storage capacity against the storage capacity of a borderline catalyst.
The oxygen storage capacity can be determined by one of the following two methods:
- 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 capacity significantly above the borderline catalyst, the catalyst is determined to be good; otherwise the monitor will be restarted after the next fuel cut-off.
- Oxygen filling (active test)
- First, the engine is operated with a low A/F ratio to remove any oxygen from the catalyst.
- Then, the engine is operated with a high A/F ratio. The O
xygen S
torage C
apacity (OSC) is calculated out of the oxygen mass stored in the catalyst as follows:
OSC = air mass flow * lean mixture (-1) * dt
- The engine 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 capacity to the calibrated threshold of a borderline catalyst.
System overview: