Page 283 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Chapter 2.4

Volkswagen AG

In the method of DE2351828 (1975), two temperature sensors are installed close to one
another downstream of the catalytic converter. The first temperature sensor has a catalytic
layer on its surface whereas the second sensor is inactive. In case of failure of the catalytic
converter, hydrocarbons and CO will pass through the catalytic converter without being
treated. These pollutants react then with the catalytic layer of the first sensor, increase the
temperature on the surface and the heat produced changes the electric resistance of the sensor.
The difference in the electric resistance of the catalytic and the non-catalytic temperature
sensors gives then an indication of the condition of the catalytic converter.

In the method of DE4100241 (1991) the catalytic converter has the form of a honeycomb.
Inside the honeycomb and in its longitudinal direction, there is a hole which allows a small
portion of untreated exhaust gases to pass through (fig. 117). Two non-catalytic temperature
sensors are installed downstream of the catalytic converter. One of the sensors is aligned with
the hole and measures the temperature of the untreated gases, whereas the other sensor
measures the temperature of the treated gases. When the difference of the two temperatures
becomes lower than a certain value then the catalytic converter is considered as deteriorated.

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