Catalyst Monitoring
Catalyst monitoring structure
Block diagram of system operation
Tables of OBD Requirements for Catalyst System Monitoring
GENERAL DESCRIPTION
Catalyst monitoring is based on monitoring the oxygen storage capability.
The engine closed loop feedback control generates lambda (air/fuel ratio) oscillations in the exhaust gas. These oscillations are damped by the oxygen storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream the catalyst indicates the storage capability.
MONITORING PROCEDURE
The conversion efficiency of both catalyst systems (cyl. 1 - 3 and cyl. 4 - 6) are separately monitored.
In order to determine the efficiency the lean and rich time periods of the oxygen sensors up- and downstream will be compared.
The smaller of the two ratios determined is used for further processing.
The cycle counter is incremented by 1 after each cycle.
If the counter reaches a predetermined number of checks (235 lambda controller cycles) and the sum of all ratios determined is greater than a fixed value a fault will be stored in the computer memory.
The monitoring procedure is then terminated until the engine is shut-off and started again.
MONITORING STRUCTURE
The catalyst monitoring functions are activated if the following conditions are fulfilled;
- closed loop operation
- coolant temperature > 70.5 °C
- vehicle speed between 5 and 120 km/h
- catalyst temperature between 353 and 628°C
- limited dynamics
throttle angle change < 8.4°
engine speed change < 382 rpm
mean lambda value change < 6%
TEMPERATURE MODEL
The statistical exhaust temperature as a function of air mass is stored in the ECM. For stationary operation there are the following values:
airmass: 10-250kg/h
exh. temperature: 119 - 777 °C
The thermal inertia of the catalyst is represented by means of a floating average.
The catalyst temperature is calculated every 1 second.