Catalytic Converter Design
Passenger
vehicle manufacturers have made tremendous progress in
reducing emissions since the
introduction of the first automotive catalytic converter
in the mid-1970s. Early converters, called "two-way" converters,
burned a percentage of the unused hydrocarbons (HC) and
carbon monoxide (CO) produced by the relatively inefficient,
low
compression engines of the day.
Two-way (oxidizing) converters burn HC and CO molecules
with the assistance of a precious-metals catalyst. This process "converts" these
harmful gasses into water vapor and carbon dioxide (CO2).
It's important to understand that two-way converters are
most effective when used with engines that have a lean air/fuel
mix because this condition provides ample oxygen to "burn" the
pollutants.
Three-way converters use two catalyst processes — reduction
and oxidation —- and a sophisticated oxygen storage/engine
control system to convert three harmful gasses - HC, CO and
oxides of nitrogen (NOx). This is not an easy task: the catalyst
chemistry required to clean up NOx is most effective with
a rich air/fuel mix, whereas HC and CO reduction are most
effective with a lean air/fuel bias. To operate properly,
therefore, a three-way converter first must convert NOx (with
a rich air/fuel bias), then HC and CO (with a lean bias).
Older three-way converters, called "three-way with
air" or "three-way plus oxidation," perform
this process by introducing additional oxygen between the
reduction and oxidation stages to create a lean condition
for the oxidation catalyst. (These converters are easily
identified by their air tube.)
Modern three-way units, found on most vehicles manufactured
since the late 1980s, rely on an advanced catalyst chemistry
that stores and releases oxygen on a single substrate, and
an oxygen monitoring and control system (utilizing one or
more O2 sensors) that causes the engine to oscillate between
lean and rich conditions. This oscillation, combined with
the oxygen storage and release on the catalyst surface, enables
the unit to convert all three harmful gasses with the same
catalyst brick.
Today's "three-way" OBD II converters are just
the last step in a highly sophisticated emissions control
process. The chief component of this system is the engine
itself, which, when operating properly, is significantly
more efficient — and environmentally friendly — than
its 1970s and 80s counterparts. Other leading components
and systems playing important roles in reducing emissions
in today's vehicles are engine sensors/controls, improved
combustion chamber design and electronic fuel injection technology,
each of which enhance the efficiency of the combustion process,
resulting in fewer unburned pollutants.
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