Catalyst Monitoring
Catalyst monitoring structure
Block diagram of system operation
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 amplitude ratio of the signal oscillations of upstream and downstream lambda sensors are compared. This information is evaluated separately in different engine load and speed ranges. If there is an indication of low storage capability in a certain number of operating ranges, a defective catalyst is recognized.
1. Computation of the amplitude ratio
The first step is the computation of the amplitude of the signal oscillations of the lambda sensor upstream versus downstream of the catalyst. This is accomplished by extracting the oscillating signal component, computing the absolute value and averaging over time. The quotient of downstream amplitude value divided by upstream amplitude value is called amplitude ratio (AR). This AR is the basic information necessary for catalyst monitoring. It is computed continuously over a certain engine and speed range. The signal paths for both sensor signals are identical. Thus variations like e.g. an increase of the control frequency affect both signal paths in the same way and are compensated by the division.
2. Postprocessing
The actual amplitude ratio is compared with a limit value according to the load and speed range the engine is operating in. The result of this comparison, the difference of both values, is accumulated separately for each range. Thus, even short time periods of driving in a certain range yield additional information.
By using separate load and speed ranges in combination with the accumulation of information a monitoring result can be obtained during a FTP cycle.
3. Fault evaluation
The accumulated information about the amplitude ratio becomes more and more reliable as different load and speed ranges are used during a driving cycle. If the amplitude ratio is > fixed map values a fault is detected and an internal fault flag will be set. If the fault is detected again in the next trip the MIL will be illuminated.
4. 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.