Test Group 1BMXV05.4LEV

1. Catalyst Monitoring General Description -  Catalyst monitoring is based on monitoring its oxygen storage capability. Engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in exhaust gas. These oscillations are dampened by oxygen storage activity of catalyst. Amplitude of remaining lambda oscillations downstream of catalyst indicates storage capability. To determine catalyst efficiency, oscillation of upstream sensor is needed to calculate oxygen-in and-output (catalyst) by engine air mass and lambda-deviation. Downstream sensor signal for a threshold catalyst then is derived from this basic value. Any time real sensor signal oscillation (downstream) corresponds to model, a defective catalyst is recognized. See Fig 1.
   Fig 1: Catalyst Monitoring Structure (Test Group 1BMXV05.4LEV)

   Courtesy of BMW OF NORTH AMERICA, INC.
2. Computation Of Amplitude Ratio -  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 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. Thus, variations like an increase of control frequency affect both signal paths in the same way and are compensated by the division.
3. Postprocessing -  Actual amplitude ratio is compared with a limit value according to load and speed range engine is operating in. The result of this comparison, difference of both values, is accumulated separately for each range. 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 an FTP cycle.
4. Fault Evaluation -  Accumulated information about the 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 fixed map values, a fault is detected and an internal fault flag will be set. If fault is detected again, in next trip the MIL will be illuminated. See Fig 2.
   Fig 2: Block Diagram Of System Operation (Test Group 1BMXV05.4LEV)

   Courtesy of BMW OF NORTH AMERICA, INC.
5. 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 (e.g. fuel cutoff). During such periods, and for a certain time afterward, the computations of amplitude values and post-processing is halted. Thus, a distortion of monitoring information is avoided.
6. Electrically Heated Catalyst Monitoring -  Heating of electrically heated catalyst is only conducted at every engine start when numerous switch-on conditions are met and no faults are stored for following ranges:
   • CAN ECM.
   • CAN instrument cluster.
   • Output stage injection valve.
   • Misfire (catalyst damage).
   • Output stage secondary air injection pump.
   • Output stage secondary air injection valve.
   • Coolant temperature sensor.
   • Crankshaft sensor/reference point.

Adjustment and diagnostic for the heater of the electrically heated catalyst is conducted by a separate control unit directly communicating with ECM. If heating element is supplied by electrical current, it begins to glow. The heat generated by this process is lead to a small catalyst located directly behind heating element. Heating-time is independent from input parameters and always has the same duration.

Heating current is compared as a diagnostic criteria (1.5 x standard) to a defined current threshold. If heating current decreases defined threshold for a defined duration, a relevant fault is stored and MIL is illuminated during next driving cycle. In addition to 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 storage in case of a fault. Both exhaust systems are separately checked.