Page 236 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

2 18 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

The United States Environmental Protection Agency

In [20] two general methods are presented for correlating the response differences of the
oxygen sensors located upstream and downstream of the catalytic converter. They are the
phase difference method and the integrated difference method.
sensors is Two variations of the phase difference method are identified:
1) The average time delay between threshold crossings of the upstream and downstream
measured, indicating a general relationship between the average delay time and the
esciency level
2) The ratio of downstream threshold crossings to the upstream crossings is measured

Two variations of the integrated difference method are also identified:
1) the average area difference under the rich swing of the curve between the response of the
upstream sensor and the response of the downstream sensor is identified, based on
integration over a specified time interval regardless of the number of rich excursions of the
sensor output signals
2) the average difference method is determined in a mathematical manner, based on each rich
excursion of the sensor output signals
The integrated difference method appears to be better suited for on-board diagnostics and
provides better correlation to efficiency with less data scatter than the phase difference method.

In the method of US5228335 (1993) the following steps are considered:
1) detecting the oxygen content of the exhaust gases upstream and downstream of the catalytic
converter by means of two oxygen sensors
2) determining the upper and lower limit of the upstream oxygen output signal during a
specified time interval, calculating the difference between the upper and lower limits,
normalizing the difference between the upper and lower limits and calculating a specified
percentage of the normalized difference to thereby establish a floating lower integration
bound
3) determining the upper and lower limit of the downstream oxygen output signal during a
specified time interval, calculating the difference between the upper and lower limits,
normalizing the difference between the upper and lower limits and calculating a specified
percentage of the normalized difference to thereby establish a floating lower integration
bound
4) integrating the area between the oxygen content output signal and an established lower
integration bound for a specific time interval and for both output signals
5) calculating the integrated area difference between the upstream and downstream output
signals
6) comparing the integrated area difference with an established area value and in response
thereto determining and indicating a level of catalytic converter deterioration