GxT Ferret 54 ENGINE DIAGNOSTIC CENTER - Manual (Page 82)

3 WAY CATALYTIC CONVERTER
The three-way catalyst uses the metals
Palladium, Platinum and Rhodium to cause
the reactions needed to lower emissions of
HC, CO and Oxides of Nitrogen. HC and CO
emissions are lowered by a chemical
reaction called ‘oxidation’ resulting in steam
(H2O) and Carbon dioxide (CO2). Emissions
of Oxides of Nitrogen (NOx) are lowered by
a chemical reaction called ‘reduction’ where
oxygen atoms are taken away and attached
to Carbon Monoxide (CO), resulting in
Carbon Dioxide and Nitrogen (N2).
The ‘oxidation’ of CO and HC works best
when the mixture is lean (air fuel ratio is
higher than the stoichiometric point), where
excess O2 is available. The ‘reduction’ of
NOx into N2 and CO2 works best when the
mixture is rich (air fuel ratio is lower than the
stoichiometric point), where O2 is absent and
CO is available to take up the Oxygen
separated from the NOx. When the mixture
is lean the catalytic converter absorbs some
excess oxygen. When the mixture is rich, the
oxygen is released in reactions with HC and
CO.
The Vehicle O2 sensor sends a signal to the
Engine Control Module (ECM) to indicate
whether the mixture is rich or lean. The ECM
then controls the fuel delivery in the opposite
direction. After the engine is warmed up, a
properly operating system will alternate
between rich and lean. During lean
conditions oxygen is stored; during rich it is
released.
If the rate at which the system changes
between rich and lean is too slow, the
catalyst runs out of stored oxygen, leaving
HC and CO not oxidized. If the rate is too
fast, not enough time is allowed for oxygen
to be stored, again leaving HC and CO
unoxidized in the following rich cycle. In
either case, HC and CO emissions will be
higher than expected. If the system is
running mostly rich, NOx will be lowered at
the expense of higher HC and CO. If it is
running lean, HC and CO will be lowered at
the cost of higher NOx.
This activity can be seen at the output of the
Vehicle Oxygen sensor as voltage changes
above and below the 0.45 volt line. The
Oxygen Sensor Test shows live voltage and
graphs the range of voltage swing around the
stoichiometric point (.45 volts), marked by a
'
λλλλλ
'. Crossings per second are shown and can
be compared to the specified number for the
type of vehicle being tested. The %DUTY
RICH indicates the portion of time the Vehicle
Oxygen Sensor is indicating rich.
If the Vehicle Oxygen Sensor is out of
calibration, the ECM will still try to operate
around the 0.45 volt level that corresponds
to the expected stoichiometric point. As an
example, if the Vehicle Oxygen Sensor
actually reads 0.55 volts at stoichiometry,
then the ECM would actually operate around
the lean side of stoichiometry. Similarly, a
sensor reading below the expected level will
cause the system to run on the rich side of
stoichiometry, again impairing the catalyst's
efficiency. An aging Vehicle Oxygen Sensor
can respond too slowly, which will slow the
crossings and widen the region around the
stoichiometric point. This will also impair the
catalyst's efficiency, as described above.
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