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A CAT is only one part of a vehicle’s exhaust emission control equipment. The vehicle’s ECU controls the air/fuel mixture being injected into the engine by constantly monitoring the exhaust gases using an oxygen, or Lambda sensor.
Signals from the Lambda sensor are fed to the ECU which responds accordingly. This process is critical to the operation of the CAT. The CAT is normally mounted as close as possible to the exhaust manifold of the engine in order to help heat up the CAT as quickly as possible.
A CAT is made up of a monolith (also known as the substrate or brick). The monolith is made up of ceramic material or stainless steel and is constructed in a honeycomb, through which the exhaust gases pass. In order to increase the surface area of the monolith honeycomb, a rough Washcoat is applied.
This multiplies the surface area to that of approximately two football pitches. The Washcoat is then coated with a fine layer of precious metals, normally a mixture of Platinum and Rhodium although Palladium is also used. It is these precious metals that act as the catalyst and convert the poisonous gases to harmless substances.
Platinum or Palladium accelerate the oxidation of hydrocarbons and carbon
monoxide, while
Rhodium reduces the oxides of nitrogen. Rhodium is also used as a light-off
promoter. As a general rule there are only between 1 - 2 grams of precious
metals in every catalytic converter.
The monolith is encased in a thick stainless steel case, designed to withstand corrosion and the thermal and mechanical stresses imposed on the CAT.
Ceramic monoliths are fragile and sensitive to stress, so are held in place within the stainless casing by a protective thermal blanket that forms a seal around the substrate to ensure all gases pass over the substrate. Metal’Cat from Arvin Replacement Products have metal monoliths which are welded to the casing, giving a far more durable product.
The catalyst starts operating once the monolith has attained a temperature of 250 - 270OC, the temperature (commonly known as light-off) which a car will normally reach from cold start within a few minutes. Under normal operating conditions the catalyst maintains a temperature of between 400 - 600OC.