Details of Catalytic converter
A catalytic converter contains a ceramic element coated with a catalyst.
A catalyst is a substance that causes a chemical reaction in other elements without actually becoming part of the chemical change and without being used up or consumed in the process.
Catalytic converters may be pellet type (used in older systems) or monolithic type. Exhaust gases pass over this bed of catalyst material. In a monolithic-type converter, the exhaust gases pass through a honeycomb ceramic block.
The converter beads or ceramic block are coated with a thin coating of cerium, platinum, palladium, and/or rhodium and are held in a stainless steel container. These elements are used alone or in combination with each other to change the undesirable emissions into harmless compounds.
Since the late 1980s, vehicles have had a three-way converter (TWC) that treats all three controlled emission gases.
It oxidizes HC and CO by adding oxygen and reduces NOx by removing oxygen from the nitrogen oxides.
The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way catalytic converter has three simultaneous tasks:
Ø Reduction of nitrogen oxides to nitrogen (N2): 2 CO + 2 NO → 2 CO2 + N2
Ø hydrocarbon + NO → CO2 + H2O + N2; 2 H2 + 2 NO → 2 H2O + N2
Ø Oxidation of carbon monoxide to carbon dioxide: 2 CO + O2 → 2 CO2
Ø Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water, in addition to the above NO reaction hydrocarbon + O2 → H2O + CO2
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. For gasoline combustion, this ratio is between 14.6 and 14.8 parts air to one part fuel, by weight. The ratio for autogas (or liquefied petroleum gas LPG), natural gas, and ethanol fuels is slightly different for each, requiring modified fuel system settings when using those fuels