Catalytic converters

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					          Catalytic converters
A catalytic converter is a device used to reduce the
toxicity of emissions from an internal combustion
engine.
First widely introduced on production automobiles
in the US market for the 1975 model year to comply
with tightening regulations on auto exhaust,
catalytic converters are still most commonly used in
motor vehicle exhaust systems
                Catalytic converters
There are two types of catalytic converter: 3 way and 2 way
Three-way catalytic converters
A three-way catalytic converter has three tasks:

Reduction of nitrogen oxides to nitrogen and oxygen:
2NOx → xO2 + N2
Oxidation of carbon monoxide to carbon dioxide:
2CO + O2 → 2CO2
Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water:
2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
These three reactions occur most efficiently when the engine is running
slightly above the stoichiometric point. This is between 14.8 and 14.9
parts air to 1 part fuel, by weight, for gasoline (the ratio for LPG, natural
gas and ethanol fuels is slightly different, requiring modified fuel system
settings when using those fuels)
                Catalytic converters
Two-way catalytic converters
A two-way catalytic converter has two tasks:
Oxidation of carbon monoxide to carbon dioxide:
2CO + O2 → 2CO2
Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to
carbon dioxide and water:
2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
This type of catalytic converter is widely used on diesel engines to reduce
hydrocarbon and carbon monoxide emissions. They were also used on
spark ignition (gasoline) engines in USA market automobiles up until 1981,
when they were replaced by three-way converters due to regulatory
changes requiring reductions on NOx emissions.
                 Catalytic converters
The catalytic converter consists of several components:
The core, or substrate. In modern catalytic converters, this is most often a ceramic
honeycomb, however stainless steel foil honeycombs are also used. The purpose
of the core is to "support the catalyst“. The ceramic substrate was invented by
Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass.
The washcoat. In an effort to make converters more efficient, a washcoat is
utilized, most often a mixture of silica and alumina. The washcoat, when added to
the core, forms a rough, irregular surface which has a far greater surface area than
the flat core surface and therefore more places for active precious metal sites. The
catalyst is added to the washcoat (in suspension) before application to the core.
The catalyst itself is most often a precious metal. Platinum is the most active
catalyst and is widely used. However, it is not suitable for all applications because
of unwanted additional reactions and/or cost. Palladium and rhodium are two other
precious metals that are used. Platinum and rhodium are used as a reduction
catalyst, while platinum and palladium are used as an oxidization catalyst. Cerium,
iron, manganese and nickel are also used, though each has its own limitations.
Nickel is not legal for use in the European Union (due to reaction with carbon
monoxide). While copper can be used, its use is illegal in North America due to the
formation of dioxin.
                 Catalytic converters
           Construction of catalytic converter

                                                      Alumina or Silica coating
       Ceramic or Stainless Steel                     Of honeycomb with catalyst
       Honeycomb (lower Pressure drop)                Particles




Engine exhaust                                                       Clean exhaust


                                Catalytic converter
                 Catalytic converters
             Future improvements to catalytic converter
         Particulate solids removal        Higher activity catalysts


                                                    Nano-particle coatings




Engine exhaust                                                           Clean exhaust


                                      Catalytic converter
             Catalytic converters
In the future Catalytic converters will be:
        -      smaller
        -      lighter
        -      more efficient
        -      capable of nano particle removal
        -      required until hydrogen fuel becomes widely
               available.
See Taymark Engineering for more information on future
improvements

				
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posted:10/7/2011
language:English
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