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What is a catalytic converter and what does it do?

Jan 23, 2018

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The cat sits around a third of the way down the exhaust system and resembles a small metal chamber that receives exhaust gasses and changes the chemical nature of them to reduce the volume of nasty emissions fresh from the exhaust manifold. Within the cat housing is a ceramic-based honeycomb structure that is lined with extremely precious metals, with each metal having a specific job in emission-reduction.

There are three main emissions produced by car engines: nitrogen gas (N2), carbon dioxide (CO2) and water vapour (H2O). The catalytic converter however is mostly used to tackle the smaller, more-harmful products that are produced due to the naturally-imperfect combustion process of the IC engine. These are carbon monoxide, hydrocarbons and nitrogen oxides. Most cats these days are therefore called three-way catalytic converters due to the three main types of emission that they manage to tackle.

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An exhaust system from an old Chevy, showing the placement of the catalytic converter

A ‘catalyst’ is a substance that accelerates a chemical reaction, and within a catalytic converter, there are two types of catalyst. The first is a reduction catalyst which uses platinum and rhodium within the honeycomb to reduce NOx emissions. NOx is produced by nitrogen oxide and nitrogen dioxide within the exhaust gasses. The nitrogen gasses come into contact with the catalyst metals which rip the nitrogen atoms out of the molecules which in-turn releases cleaner oxygen to continue down the exhaust system.

The second catalyst type is an oxidisation catalyst which uses Platinum and Palladium to complete the job. These catalysts oxidise or burn the carbon monoxide and hydrocarbons lingering within the incoming gas, helping reduce the amount of smog produced by evaporated, unburnt fuel.

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The final stage of emission control comes in the shape of an O2 sensor found just upstream from the cat. The sensor relays back to the ECU how much oxygen is found within the exhaust gasses, with the on-board computer then able to adjust the air/fuel ratio to allow the engine to run as close to the Stoichiometric point as possible.

This is the point at which – theoretically – all of the fuel entering the combustion chamber will use all of the oxygen provided to complete the combustion process. Not only does this last stage help the engine’s overall efficiency, it also allows the engine to provide the cat with enough oxygen to effectively complete the oxidisation process with the second catalyst.