Catalyst Monitoring
CATALYST MONITORING
GENERAL DESCRIPTION
Catalyst monitoring is accomplished by monitoring its oxygen storage capability. The engine closed loop feedback control system generates lambda (air/fuel ratio) oscillations in the exhaust gas. These oscillations and damped by the oxygen storage activity of the catalyst. The amplitude of the lambda oscillations downstream of the catalyst indicates the available storage capability.
In order to determine catalyst efficiency the amplitude ratio of the signal oscillations of the up- and downstream lambda sensors are compared. This information is evaluated separately at various engine load and speed ranges. If there is an indication of a lowered storage capability in a certain number of operating ranges a defective catalyst is recognized.
MONITORING STRUCTURE
The catalyst monitoring functions are activated above a catalyst temperature of 380 °C.
The catalyst monitoring system will be interrupted if the delivered mixture ratio from the evaporative system is [<] 15.
The check will be performed in closed loop operation and only in a speed/load map window of n = 1,000 to 2,600 rpm and TL = 0.7 to 2.8
For further information refer to the flow chart.
Catalyst monitoring structure
1. Computation of the amplitude ratio
The first step is computation of the amplitude of the signal oscillations of the lambda sensors up- and downstream of the catalyst. This is accomplished by extracting the oscillating signal component, computing it's absolute value and averaging over time. The quotient of downstream amplitude value divided by upstream amplitude value is called the amplitude ratio (AR). The AR is the basic information necessary for catalyst monitoring. It is computed continuously over certain engine load and speed ranges. The paths for both signals are identical. Thus variations which, for example, result in an increase of the control frequency affect both signal paths identically and are therefore compensated by the division.
2. Postprocessing
The actual AR is compared with a stored limit value which varies based upon 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 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 an FTP cycle.
3. Fault evaluation
The accumulated information regarding the amplitude ratio becomes more and more accurate as different load and speed ranges are encountered during a trip. If the amplitude ratio is greater than 0.63 to 0.65 above a certain predetermined value a fault is detected and an internal fault flag will be set. If the fault is detected again during the next driving cycle (DCY ) the MIL will be illuminated.
4. Check of monitoring conditions
The monitoring strategy is based on detection of relevant oscillations of the downstream sensor signal during normal lambda control. It is also necessary to check the driving conditions for exceptions where regular lambda control is not possible, for example during fuel cut-off. During such periods, and for a certain time thereafter, the computations of the amplitude values and postprocessing is halted. Thus distortion of the monitored information is avoided.
Block diagram of system operation
