Test Group WBMXV01.9M44
General Description - Catalyst monitoring is based on monitoring its oxygen storage capability. Engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in the exhaust gas. These oscillations are dampened by oxygen storage activity of catalyst. Amplitude of remaining lambda oscillations downstream of catalyst indicates storage capability. In order to determine catalyst efficiency, amplitude ratio of 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. See Fig 1-Fig 2 .
Computation Of Amplitude Ratio - The first step is the computation of amplitude of signal oscillations of lambda sensor upstream versus downstream of catalyst. This is accomplished by extracting oscillating signal component, computing absolute value and averaging over time. 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. Signal paths for both sensor signals are identical. Variations like an increase of control frequency affect both signal paths in the same way and are compensated by the division.
Post Processing - Actual amplitude ratio is compared with a limit value according to load and speed range engine is operating in. Result of this comparison, difference of both values, is accumulated separately for each range. Therefore, even short time periods of driving in a certain range yield additional information. By using separate load and speed ranges in combination with accumulation of information, a monitoring result can be obtained during an FTP cycle.
Fault Evaluation - Accumulated information about amplitude ratio becomes more and more reliable as different load and speed ranges are used during a driving cycle. If amplitude ratio is greater than a fixed map value, a fault is detected and an internal fault flag will be set. If fault is detected again in next trip, MIL will be illuminated.
Check Of Monitoring Conditions - Monitoring principle is based on detection of relevant oscillations of downstream sensor signal during regular lambda control. It is necessary to check driving conditions for exceptions where no regular lambda control is possible (such as fuel cut-off). During such periods, and for a certain time afterward, computations of amplitude values and post processing is halted. Therefore, a distortion of monitoring information is avoided.