Catalytic Converter

Technical Option | Generic Example

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A catalytic converter is a technology employed to reduce end-of-pipe emissions from the internal combustion engine. The converter functions by way of a catalyst which triggers chemical reactions that convert forms of pollutants to less toxic substances.

There are two types of catalytic converter: two-way and three-way.

The two-way catalytic convertor has two simultaneous tasks:

  1. Oxidation of carbon monoxide to carbon dioxide
  2. Oxidation of unburned hydrocarbons to carbon dioxide and water

This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on gasoline engines in USA market automobiles until 1981. Due to their inability to control nitrous oxide NOx, they were superseded by three-way converters.

Since 1981, three-way catalytic converters have been used in vehicle emission control systems on road-going vehicles. A three-way catalytic converter has three simultaneous tasks:

  1. Reduction of nitrogen oxides to nitrogen and oxygen
  2. Oxidation of carbon monoxide to carbon dioxide
  3. Oxidation of unburned hydrocarbons to carbon dioxide and water


This measure is effectively implemented in Europe through (though no specific technology is defined) a series of EU directives whereby progressive stages (Euro standards) are introduced with increasingly stringent standards. Catalytic convertors are a principal technology driver for abatement within these Euro standards.


The reactions from a three-way catalytic converter occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point (i.e. the point between 14.6 and 14.8 parts air to 1 part fuel, by weight) for gasoline. The ratio for LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels. Generally, engines fitted with three-way catalytic converters are equipped with a computerised closed-loop feedback fuel injection system using one or more oxygen sensors. While a three-way catalyst can be used in an open-loop system, NOx reduction efficiency is low. Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is nearly complete. However, outside that band, conversion efficiency falls very rapidly. When there is more oxygen than required, the system is said to be running lean, and the system is in oxidising condition. In that case, the converter's two oxidising reactions (oxidation of CO and hydrocarbons) are favoured, at the expense of the reduction of NOx. When there is excessive fuel, the engine is running rich; the reduction of NOx is favoured, at the expense of CO and HC oxidation. Furthermore, the oxidation of CO and HC produces increased levels of CO2.

Costs & Benefits


Catalytic converters have proven to be reliable and effective in reducing tailpipe emissions. However, they may have some adverse environmental impacts in use:

  • The requirement for a rich burn engine to run at the stoichiometric point means it uses more fuel than a "lean burn" engine running at a mixture of 20:1 or less. This increases the amount of fossil fuel consumed and the carbon dioxide emissions of the vehicle. However, NOx control on lean burn engines is problematic.
  • Although catalytic converters are effective at removing hydrocarbons and other harmful emissions, they do not solve the fundamental problem created by burning a fossil fuel. In addition to water, the main combustion product in exhaust gas leaving the engine – through a catalytic converter or not – is carbon dioxide (CO2). Additionally, the U.S. Environmental Protection Agency (EPA) has stated catalytic converters are a significant and growing cause of global warming, due to their release of nitrous oxide (N2O), a greenhouse gas over 300 times more potent than carbon dioxide.
  • Catalytic converter production requires palladium or platinum; part of the world supply of these precious metals is produced near the Russian city of Norilsk, where the industry (among others) has caused Norilsk to be added to Time Magazine's list of most polluted places.


Catalytic converters aid the reduction of NOX emissions, as well as hydrocarbons (HC) and carbon monoxide (CO).

Evidence & Reference

Deconstruction of the operation of a catalytic converter


Modelling this Measure

In the case of the catalytic convertor, which is implemented through Euro standards (link), modelling this measure is achieved by examining the removal efficiency of a given standard for a given fuel type, and applying the revised (abated) emission factor to the activity going through that standard.

Examples of models which are currently used to model this measure include COPERT (link) and GAINS (link). Whilst adopting somewhat varied methodologies, in both cases the broad approach is to define disaggregated vehicle types, associated activity levels and to estimate emissions based upon the specific euro standard technology in place. The euro standard performance as an abatement technology is estimated in applied research and reformatted for use in the specific modelling framework.

Site Entry Created by Policy Measures Admin on Oct 08, 2010
Edited by J A Kelly

Reference This Source (2019). Catalytic Converter. Available: Last accessed: 19th February 2019

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