How A Catalytic Converter Works

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A catalytic converter is a device that chemically changes the harmful components of Internal Combustion engine exhaust into harmless gasses. A converter is made up of several parts.

1.  SUBSTRATE- serves as the internal structure to which the catalyst washcoat and noble metals are deposited upon. It is through this substrate that the engine exhaust flows through, or comes in contact with, to have a chemical reaction.

PELLET TYPE - small ceramic pellets are treated with washcoat and noble metals.They are held in place inside the converter body in a cage. The exhaust gas passes over them. There could be a couple of hundred thousand pellets in the average converter. The main disadvantage of this converter is the high exhaust back pressure.pellet-type.jpgCERAMIC MONOLITH TYPE - Also known as the honeycomb type. This ceramic type substrate has thousands of small channels through which the exhaust gas flows. This piece of ceramic is treated with the washcoat and noble metals.ceramic-monolith-type.jpgMETALLIC FOIL SUBSTRATE - This type of substrate uses wound or stacked metal foil on to which the washcoat and noble metals are deposited. The exhuast gas flows through this substrate. This type has many advantages over ceramic, such as, being able to be made into shapes that cannot be easily done with ceramic.metallic-foil-substrate.jpgCERAMIC FOAM - This type of substrate is composed of ceramic solution on foam. It is then fired in a kiln to develop the mutli-pore design seen in this photo. This substrate is then treated with washcoat and noble metals. As the exhaust gasses flow through it, the catalytic reaction takes place.ceramic-foam.jpgIn addition, there are some other substrate designs that use screen, metal foam, wire wound elements, and even ceramic fiber elements. These types are currently not used in automotive applications.

2. WASHCOAT - This is a chemical formulation that is comprised of high surface area alumina and other compounds, such as, cerium, zirconium, nickel, barium and lanthanum. Its purpose is to extend the physical surface area of the substrate, and to act as a surface onto which the noble metals are deposited. It also acts to store the excess oxygen in the system so it can be used when needed.washcoat.jpg3. NOBLE METALS - These are the commonly used elements; platinum, palladium and rhodium, and are deposited onto the washcoated substrate. These metals are what serve to cause the catalytic reaction. The definition of a catalyst is: a material which increases the rate of a chemical reaction while it is treated during the noble metal and washcoat process, it passes through an oven process which removes excess water and calcines in the substrate. This means it removes all traces of the chemical carriers which are used to deposit the noble metals during the preparation of the catalyst.noble-metals.jpg4. SHELL - This is the stainless steel enclosure which the substrate is contained in.

5. INNERAM - This is a ceramic fiber based material which is wrapped around the ceramic substrate. It acts as a vibration cushion, as a seal to prevent exhaust gas from by-passing the substrate, and as an insulator to keep the heat from the substrate from transferring into the shell. Some types of inneram can also be made of stainless wire or silica materials. Generally, as a rule, metallic type substrates do not use inneram.

6. HEAT SHIELD - This is used to prevent the heat of the catalyst from causing any damage or fires to the under body or components of the vehicle.

7.END PIPES - The stainless steel tubes used to connect the catalytic converter to the vehicles’ exhaust system.end-pipes.jpg

How Does A Catalytic Converter Work

As the exhaust flows through the substrate inside the converter housing, the catalytic reaction takes place. As the pollutants like NO2 (nitrogen oxide), HC (hydro carbon) and CO (carbon monoxide) pass through the substrate they are converted into less harmful compounds such as CO2 (carbon dioxide), H2O (water) and N2 (nitrogen). This conversion starts at temperatures around 2500 Celsius.How Catalytic Converters Work

Rules Regarding the Development of Aftermarket Catalytic Converters

Rules for the development of converters that satisfy Federal EPA requirements are set forth in the Federal Register. Briefly, the rules state that the converter manufacturer must submit two catalyst samples to an independent laboratory, who will accumulate 25,000 miles or its equivalent, on each of the catalysts. The catalysts are then tested for emissions performance and must convert a minimum of 70% Hydrocarbons and Carbon Monoxide and 30% NOX on a worst case vehicle. A worst case vehicle is defined as having the largest engine displacement and heaviest test vehicle weight that the converter manufacturer intends to cover with the catalyst.

Currently, the United States EPA requires the aftermarket converter manufacturer to warranty that the converter will not cause the MIl (check engine light) to illuminate within the 25,000 mile required warranty period of the substrate that has met all the prior EPA requirements.

*Information obtained from the Catco Solutions Guide