Catalytic Converters

A catalytic converter is a device used to reduce the emissions from an internal combustion engine. more...

Most commonly used in an automobile's exhaust system, catalytic converters are now commonly used on generator sets, forklifts, mining equipment, trucks, buses, trains, and other machines that have engines to provide an environment for a chemical reaction where unburned hydrocarbons are more completely combusted. Automobile converters use platinum or palladium and rhodium as catalysts. Hence the combustion (redox) process continues, but outside the engine's combustion chamber, so no useful energy is extracted. The catalytic converter was invented at Trinity College (Connecticut)

Purpose and function of catalytic converters

A three-way catalytic converter has three simultaneous tasks:

1. oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ → 2CO₂
2. reduction of nitrogen oxides to nitrogen: NO_x → O₂ + N₂
3. oxidation of hydrocarbons (unburnt fuel) to carbon dioxide and water: C_xH_y + nO₂ → xCO₂ + mH₂O

These three reactions are most balanced at the stoichiometric point, which is the mid-point between rich and lean operation. The amount of oxygen to fuel in the engine is in a ratio for the most complete combustion. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. The above two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured. When there is more fuel than oxygen (stoichiometrically), then the engine is running rich. The reduction of NO_x is favoured.

Catalytic converters are now standard fit in North America on "Large Spark Ignition" engines. LSI engines are used in forklifts, aerial boom lifts, ice resurfacing machines, and construction equipment. The converters used in these are three-way types designed to reduce combined NO_x+HC emissions from 12 gram/BHPhour to 3 gram/BHPhour or less, as per the Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHPhour of NO_x+HC emissions is mandated in 2007 (note: NO_x is the industry standard short form for nitric oxide (NO) and nitrogen dioxide (NO₂) both of which are smog precursors. HC is the industry short form for hydrocarbons).

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