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The three-way catalyst has been successfully
adopted to gasoline engine to reduce
pollutant emission. The engine is generally
operated at the stoichiometric air:fuel
ratio (A:F, about 14.7) by using a lambda
sensor in order to maximize the efficiency
of catalytic conversion from CO, Cm Hn , and
NOx into CO2 ,H2 O, and N2. It has been
reported that the catalytic activity
gradually decreases under the exposure to
high temperature or chemical poisons such as
lead or lubricant derived phosphorus.
However, the catalyst degradation is hardly
detectable unless a malfunction indicator is
in-stalled. Therefore, the monitoring of
catalyst degrada-tion is important in order
to maintain low emissions of CO, HC, and NOx
. Many researchers have studied the
catalyst-monitor-ing algorithm. Typically,
two A:F sensors were placed before and after
the catalyst and their signals were compared
in order to check the changes of A:F due to
the catalytic reactions. A pair of lambda
sensors or wide range A:F sensors have been
suggested to be installed for this purpose.
The lambda sensor measures electromotive
forces between the exhaust and the air
across a stabilized zirconia tube. This
potentiometric sensor shows an abrupt signal
change at the stoichiometric point of A:F
because the concentration of oxygen in the
exhaust changes extensively around this
point after the equili-bration of gas phase
over the Pt catalyst attached to the sensor.
This feature makes it easy for one to
control the engine at the stoichiometric
point. The cost of lambda sensor is fairly
low owing to successful mass production and
the sensor stability has also been
confi-rmed. On such background, the
two-lambda sensor method has been proposed
to detect the deterioration of catalyst.
However, limitation of this method has been
pointed out: When the HC conversion
efficiency goes down to less than 70%, both
of the sensors pro-duce the similar sensor
signals in the fuel-lean or the fuel-rich
regimes leading only to a small difference
of EMF in between.
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