Page 75 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Robert Bosch GmbH. 57

oxygen sensors which are arranged upstream and downstream of the converter respectively.
Such methods are known in the prior art. If the converting power determined in this manner is
not sufficient, measures are taken for heating the converter until a sufficient converting power
is reached. If the heating measures do not lead to a sufficiently high converting power of the
catalyst within a predetermined time interval, a diagnostic function is started in order to
determine whether the converter is damaged. In this way, a damaged converter is not heated
unnecessarily strongly and measures can be taken to limit the damage and to warn the driver.
On the other hand, if the catalyst has sufficient converting power, the heating is stopped or not
even started.

In DE4337793 (1995) a two-point regulation method is used. The signal 11 of the upstream
probe is used as the first airhe1 regulation signal from rich to lean and vice versa with a
certain frequency. When the frequency of the downstream signal is fixed and certain operating
conditions of the engine are set (e.g. idling), then a two-point engine aidfuel ratio control is
taken as the actual control signal with the downstream signal considered as a first control
signal. The frequency f of this regulation is averaged and compared to a threshold fiequency
value f,. If averaged Pf, then the catalytic converter is considered as aged and should be
replaced.

The method of DE4408504 (1995) uses a specially built gas component concentration sensor
for monitoring the efficiency of catalytic converters. The sensor comprises a solid oxygen-ion
conducting electrolyte having two measuring electrodes. The electrolyte consists of ZrOz
which is stabilized with Y20.1. The first measuring electrode catalyzes the equilibrium setting
of the exhaust gas, whereas the second electrode does not. A heating device controls the
temperature of the electrodes. The catalytic activity of the electrodes can be adjusted via
temperature by arranging them in different temperature regions of the electrolyte. The frst
measuring electrode consists of platinum (Pt) or a platinum alloy with rhodium or palladium.
The second measuring electrode consists of gold and/or silver, a platinum alloy with gold
and/or silver or platinum-bismuth.

The method of DE19623335 (1997) comprises the following steps:
I ) determining an actual output signal value of the downstream oxygen sensor
2) estimating an expected output signal value of the downstream oxygen sensor by means of
the measured output signal of the upstream oxygen sensor and the measured quantity of
aidfuel mixture introduced in the combustion chamber of the engine. The expected values
correspond to a model catalytic converter that still has a sufficient conversion ability
3) calculating the sum (or integral) of differences of the determined actual and expected output
signals of the downstream oxygen sensor during a time period
4) inhibiting the summation of step 3 when
a) the oxygen filling degree of the converter is higher than a predetermined maximum
value or the oxygen filling degree of the converter is lower than a predetermined
minimum value. or