Test Group YBMXVO5.4LEV
Catalyst & Adsorber Monitoring General Description - Catalyst monitoring is based on monitoring catalyst's oxygen storage capability. This strategy includes monitoring of the HC-Adsorber indirectly. 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 catalyst 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 over 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.
- Catalyst & Adsorber Monitoring Structure -
Catalyst and adsorber monitoring structure consists of 4 processes:
- 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). AR is the basic information necessary for catalyst monitoring. AR is computed continuously over a certain engine and speed ranges. Signal paths for both sensor signals are identical. Thus variations, like an increase of control frequency, affect both signal paths in the same way and are compensated by the division.
- Postprocessing - The actual AR 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.
- Fault Evaluation - The accumulated information about the AR becomes more and more reliable as different load and speed ranges are used during a driving cycle. If the AR is greater than 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.
- 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. See Fig 2.
- Electrically Heated Catalyst Monitoring Function & Diagnosis -
Heating of the electrically heated catalyst is only conducted at every engine start when numerous switch-on conditions are met and no faults are stored for the following ranges:
- CAN ECM
- Output stage secondary air injection pump
- CAN instrument cluster
- Output stage secondary air injection valve
- Output stage injection valve
- Coolant temperature sensor
- Misfire (Catalyst damage)
- Crankshaft sensor reference point
Adjustment and the diagnostic for the heater of the electrically heated catalyst is conducted by a separate control unit which directly communicates with the ECM. When the catalyst heating element is supplied by electrical current, it begins to glow. The heat generated by this process is let to a small catalyst located directly behind the heating element. The heating time is independent from the input parameters and always has the same duration. The heating current is compared as a diagnostic criteria (1.5 x standard) to a defined current threshold. If the heating current decreases the defined threshold for a defined duration a relevant fault is stored and the MIL is illuminated during the next drive cycle. Additionally to the above mentioned diagnostics, further electrical diagnostics and plausibility checks are conducted which cause a break-off or a lock of the heating cycle and a relevant fault stored. In case of a fault, both exhaust systems are separately checked.