Catalytic Converters - Technology & How they work

Catalytic converter

Construction

A catalytic converter consists of a stainless-steel casing that houses a metallic (metalith) or ceramic (monolith) substrate or core. This substrate has a very large number of fine channels running through it along its entire length (like a honeycomb), which increase the active surface area available to support the catalyst, which explains why it is also often called a "catalyst support".

The surface of the substrate is coated with a highly porous "washcoat" (which forms a rough, irregular surface that gives the substrate an even larger surface area) to which precious metals (platinum, palladium and rhodium) are added in suspension. It is these precious metals that are the actual catalysts that initiate the chemical reactions that reduce the levels of pollutants in the exhaust gases.

Three-way catalytic converter

For a three-way catalytic converter to function to full effect, it requires a specific composition of exhaust gases. Precisely the amount of oxygen as is required to oxidise the hydrocarbons and the carbon monoxide in the exhaust gases must be released in the combustion process. This is the case when 1 part fuel is mixed with 14.7 parts air (by weight) and combusted in the engine. This is known as the "stoichiometric point" (lambda = 1).

To produce this mixture, the lambda sensor screwed into the exhaust pipe between the engine and the cat measures the residual oxygen content in the exhaust gases. The engine management system uses these measurements to compute and output the control pulses to keep the air-fuel ratio at the optimum point.

The cat used in spark-ignition engines performs all three reactions (reduction of nitrogen oxides to nitrogen and oxygen, oxidation of carbon monoxide to carbon dioxide and oxidation of unburnt hydrocarbons to carbon dioxide and water) simultaneously. That's why it's called a three-way catalytic converter.

Oxidation catalytic converter

Diesel engines function as a matter of principle with a leaner mixture, i.e. a higher air-fuel ratio, and their exhaust therefore contains a high level of oxygen. The catalytic converter fitted to diesel engines oxidises carbon monoxide (CO) to form carbon dioxide (CO2) and hydrocarbons (HC) to form carbon dioxide (CO2) and water (H2O).
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