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| What
Does A Catalytic Converter Do? |
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| In
an effort to control automotive emissions, reduce carbon monoxide,
smog, and acid rain catalytic converters began appearing on cars in
the 1970's. Over the decades, automotive manufacturers have used many
methods to control exhaust emissions. Exhaust gas re-circulation,
containment systems for evaporative emission, on board engine management
systems, air pumps, are just some examples of their efforts, but the
single most effective way to reduce exhaust emissions has been the
catalytic converter. The catalytic converter has evolved over from
the decades to where most modern vehicles are equipped with a three-way
converter that effectively reduces harmful exhaust gases. |
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| The
catalytic converter usually looks like a muffler and is located underneath
the vehicle in the proximity of the passenger seat in most cars although
some cars have multiple converters. The purpose of a catalytic converter
is to convert harmful hydrocarbons, carbon monoxide, and nitrogen
oxides into harmless compounds. The catalysts inside the catalytic
converter convert carbon monoxide and hydrocarbons into carbon dioxide
and water, and nitrogen oxides back into nitrogen and oxygen. |
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| The
catalysts inside of the catalytic converter are precious metals such
as platinum, palladium, and rhodium coated on a ceramic honeycomb
structure or pellets. When exhaust contacts the catalysts a chemical
reaction takes place converting the harmful exhaust gases into harmless
compounds. Most modern converters contain two distinct catalysts,
the reduction catalyst and the oxidation catalyst. |
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| The
first stage, or reduction catalyst, consists of a ceramic honeycomb
coated with platinum and rhodium. In this section of the converter,
Nox emissions are converted to oxygen and nitrogen. The exhaust then
flows to the second stage, or oxidation catalyst. It is here where
unburned hydrocarbons and carbon monoxide are burned or oxidized by
passing them over a platinum and palladium honeycomb. The oxygen from
the first stage further aids this oxidizing. |
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| The
third stage of controlling emissions utilizes an engine management
system. Although every automotive manufacturer employs different methods
and components, most utilize an oxygen sensor mounted upstream of
the converter which transmits a voltage to an onboard computer. This
voltage reading varies with the amount of oxygen present in the exhaust
gas. The ideal air fuel ratio for gasoline is 14.7 to 1. If the ratio
of air is less than 14.7, then there will be fuel present in the exhaust
after combustion therefore creating a rich condition. A lean condition
occurs when the ratio of air to fuel exceeds 14.7 to 1. The engine
management system uses the voltage reading from the oxygen sensor
to make adjustments to the air to fuel ratio, which in turn will either
increase or decrease the amount of oxygen present in the exhaust gas
therefore controlling the chemical reactions in the catalytic converter. |
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