What is a Three-Way Catalyst?

If you own or have recently purchased a rich-burn, natural-gas engine, you also need a three-way catalyst or TWC to comply with EPA air quality regulations. So - what is a three-way catalyst, what does it do and why do you need it?

A TWC is an emissions control product used to dramatically reduce nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and volatile organic compounds (VOCs) in rich-burn, spark-ignited engines.

It uses a flow-through ceramic substrate coated with a precious metal catalyst, to perform three simultaneous chemical reactions:

• reduction of nitrogen oxides

• oxidation of carbon monoxide

• oxidation of unburnt hydrocarbons and volatile organic compounds

The result of using a TWC is that toxic emissions from the engine are changed into nitrogen, carbon dioxide and water vapor as they pass through the three-way catalyst.

A three-way catalyst is different from a diesel particulate filter or a diesel oxidation catalyst in that it only works on natural gas, propane and liquefied petroleum gas engines (not diesel). The engine must also be "rich-burn" in order to use a TWC, which means that it operates using an air-to-fuel ratio of slightly more fuel to oxygen. A three-way catalyst (also known as a three-way converter) can be designed and manufactured to clamp-on to the exhaust after the muffler, as a muffler replacement or as a three-way converter silencer.

Why do you need a TWC?

• compliance with EPA NSPS air quality regulations and avoid costly fines

• provides a safer working/living environment through reduced emissions

• lessens the impact on human health by reducing toxic air pollutants, particularly in the immediate area that a spark-ignited engine is operating

• provides a cleaner environment by dramatically reducing engine emissions

To find out if your natural gas engine requires a three-way catalyst, details are contained in EPA's New Source Performance Standards, click here.