Page 53 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Catalytic Converter Functionality Diagnosis by Means of Oxygen or Air/Fuel Ratio Sensors 35

A-probe and UEGO sensor operation principles

The principle of operation of the 5 probe is shown in fig. 13. The elongated thin-wall ceramic
thimble isolates the exhaust gas from the atmospheric air which is in contact with the interior
of the thimble. The interior and exterior surfaces of the thimble are coated with porous
platinum bands or strips that serve as catalytic surfaces as well as electrodes which are
connected by means of external circuitry.

The thimble is fabricated from zirconium oxide which contains a small amount of a lower
valent oxide, such as yttrium oxide. The dispersion of the trivalent yttria within the tetravalent
zirconia crystalline lattice results in valence ‘holes’ which allow oxygen ions to migrate
through the zirconia wall of the thimble but prevent such migration by oxygen atoms and
molecules or by any other kind of ion, atom or molecule.

The migration of oxygen ions through the zirconia thimble involves several processes. As the
porous platinum electrode is on the air side of the thimble, atmospheric oxygen molecules are
catalytically dissociated into oxygen atoms.

Each liberated oxygen atom gains two electrons and is thereby converted into an oxygen ion
which migrates through the thimble under certain conditions. Catalyzed reactions also occur at
the porous platinum electrode on the exhaust gas side of the thimble. Each oxygen ion that
emerges from the thimble loses the two extra electrons to be converted into an oxygen atom.
These ‘migrated’ oxygen atoms associate with each other to form diatomic molecules to
become part of C02 or water vapor molecules. The net effect of the migration of oxygen ions
through the thimble wall is the transport of electrons via the oxygen ions through the wall,
from the inner surface, and the return flow of electrons from the outer surface to the inner
surface via the external circuit. The net result in the circuit is the development of an
electromotive potential of about 900 mV.

In the temperature range met in automobile exhaust systems, the operation of the zirconia cell
involves oxidationheduction reactions on the porous platinum electrode on the exhaust gas
side of the thimble, mainly reactions of CO and H2 with exhaust gas 02. Therefore, voltage
generation in the outer circuit can only take place if reducing agents are present and available
for reaction with the oxygen ions that migrate through the cell thimble. Consequently, the
concentrations of the reducing agents must be greater than those needed for stoichiometric
reaction with exhaust gas oxygen.

The sensor output takes its high value (600-900 mV) when the exhaust gas mixture is rich and
its low value (under 150 mV) for lean exhaust gas. It exhibits, therefore, a ‘step function’
characteristic, which can be used to detect deviations from stoichiometry. However, sensor
output voltages for temperatures under 250 “C are low even in the presence of excess of
reducing agents in which case it is not possible to detect deviations from stoichiometry.