Page 84 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

66 Methods for Monitoring and Diagnosing the Eficiency of Catalytic Converters

In fig. 31a an example of artificial frequency modulation of the engine airhe1 ratio is
presented. The artificial frequency modulation is initiated at a high frequency at the beginning
of the interrogation period and progressively reduced to below the normal limit cycle
frequency of the closed-loop feedback system at termination of the interrogation period.
During the interrogation period the amplitude of the oscillation is set higher than the amplitude
of a normal limit cycle. For an efficient catalytic converter, the downstream sensor does not
detect any modulation breakthrough until the AIE frequency has reached some low value. For
faulty catalytic converters this breakthrough appears at a much higher air/fLel ratio fiequency.
“Breakthrough is defined herein to mean that the oxygen storage capacity of the catalytic
converter has been exceeded, and the modulation signals pass through the converter.

A/F

+Test Interval 4

t

Time To breakthrough

Fig. 3 1 (from EP04663 1 1)