Page 57 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

Catalytic Converter Functionality Diagnosis by Means of Oxygen or AiriFuel Ratio Sensors 39

Control of engine airfluel ratio

The richness of the aidfuel mixture (i.e. ratio I) of an internal combustion engine is mostly
controlled by adjusting the duration of opening of each of the fuel injector valves. The
adjustment of the ratio h precisely enough to enable the catalytic converter to operate
effectively, converting all three CO, HC and NO, into harmless substances, is mostly achieved
by controlling the engine aidfuel ratio by means of a I-sensor (similar to this described in fig.
13) installed in the exhaust pipe of the engine upstream of the catalytic converter (closed-loop
COr?trOf) (fig. 12).
The h-sensor responds to changes in exhaust-gas composition resulting from deviations from
h=l in the intake mixture by producing an abrupt voltage jump in its output signal, a high
voltage corresponding to a rich intake mixture (h less than 1).
The voltage signal from the hsensor is transmitted to a proportional integral computer (fig.
12), which responds by changing the duration of opening of the fie1 injection valves, so as to
make the intake mixture richer or leaner, whichever may be required, i.e. the controlling
computer, responding to the output signal of the Lsensor, corrects the richness of the intake
mixture to bring it back to h=l. In practice, h oscillates at high frequency but with a low
amplitude, the effective h being the mean of the oscillations (conversion window).
A catalytic converter, for example a three-way catalytic converter, has an oxygen storage
function i.e. it absorbs and stores excess oxygen existing in the exhaust gases when the air/fiel
ratio of the aidfuel mixture fed into the engine cylinder becomes lean, and releases oxygen
when the aidfuel ratio of the air/ikel mixture fed into the engine cylinder becomes rich.
Accordingly, where the aidhe1 ratio is alternately changed on the rich side and the lean side of
the stoichiometric aidfuel ratio, since excess oxygen is absorbed and stored in the three-way
catalytic converter due to its oxygen storage hnction when the aidfuel mixture becomes lean,
NO, is reduced. Conversely, when the aidfuel mixture becomes rich, since the oxygen which
has been absorbed and stored in the catalytic converter is released, HC and CO are oxidized.
As a consequence, NO,, HC and CO can be purified at the same time.

In theory a properly operating catalytic converter promotes the complete reaction between free
oxygen in the exhaust stream resulting in the ideal products of combustion, i.e. COZ and water
vapor. As a result the exhaust stream downstream of the catalytic converter is composed
primarily of these products, and has a low concentration of reducing agents. The lack of
oxygen and low concentration of reducing agents in the exhaust stream leads to a decrease in
oxidationheduction activity at the noble metal electrode (e.g. Pt) of a downstream oxygen
sensor. In such conditions the output signal of the downstream sensor of a properly operating
catalytic converter under steady state operating conditions will tend to stabilize with low
fluctuations in the output response pattern (see US5228335 (1993)).

With continued use the catalytic converter becomes degraded by thermal aging, sintering and
crystallization of the active surface layer, and it is poisoned by such substances as S, Pb, P, Zn