Page 244 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

226 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters

The method attempts to prevent a false determination of the efficiency of the main catalytic
converter due to a phase opposition of the signals from the two banks of cylinders and it can
be better explained by means of fig. 97.

Fig. 97a shows the measured variation of oxygen content V2 in the exhaust gases downstream
of the main catalytic converter. In time periods TI and T3 there is a strong amplitude variation
of V2, whereas in time period T2 the amplitude variation becomes very low due to the
opposition in phase of the oxygen content in the exhaust gases of the left and right branch.
Signal V2 is then electrically treated in order to bring its average value to zero. Then the signal
is reestablished and filtered to obtain an indicator V2’, which represents a sliding average
amplitude of the original signal V2 (fig. 97b). Signal VZ’ is filtered by a low-pass filter with a
time constant C which varies in function of the instant value of output signal VZ or of the signal
V2’ (fig. 97c). The time constant C is longer than the output signal V2 and of a lower
amplitude, in order to obtain a final indicator Vz” which decreases more slowly in time than
VZ’ (fig. 97d). The indicator VT is then compared to a predetermined threshold E. if the
indicator VT is superior of E, this means that the amplitude of the output signal VZ is
excessive and the converter does not damp sufficiently the variations of oxygen in the exhaust
gases and consequently indicates that the catalytic converter is deteriorated.

Volvo AB

In the method of W09641071 (1996) each output signal of the upstream and downstream
oxygen sensors is used as feedback-control parameter of the engine airlfbel ratio. The
resultant signal fluctuations of the two sensors are analyzed to determine the time displacement
between fluctuations in each of the two signals. The oxygen buffer capacity is then determined
based on said time displacement.

0 Denso Corp.

The method of DE19646008 (1997) comprises the following steps:
1) calculating an amount of an exhaust gas component upstream of the catalytic converter
when the actual aidfuel ratio upstream of the converter differs from the stoichiometric one.
When the difference between the actual air/fbel ratio and a predefined upper limit of the
air/fbel ratio is positive (Am,,, fig. 98a) then this difference is integrated in time and the
integrated area is called a lean component (fig. 98b). When the difference between the
actual aidfuel ratio and a predefined lower limit of the aidfuel ratio is negative (AA/FR, fig.