Exhaust Catalyst

The catalytic converter for cleaning up exhaust gas was invented by Eugene Houdry (1956), probably best known for inventing a process for cracking crude oil.

Two-Way Catalyst

Three-Way Catalyst

A pollution control device which reduces all three noxious substances: HC, CO, and NOx.


A catalytic converter is known to have two distinct acoustic effects:

Reactive Effect Acoustic wave reflections caused by cross-sectional area changes within the catalyst can and the catalyst brick.

Resistive Effect Acoustic wave dissipation caused by viscous losses. The resistive effect is frequency-dependent, that is, the catalytic converter acoustic transmission loss due to the resistive effect would increase with increasing frequency. It emanates from the multi-tubular brick of catalyst material. Other engine exhaust system components have resistive elements, such as the absorptive packing in a muffler.

Close Coupled Catalyst

A catalyst that is positioned close to the exhaust manifold to reduce the amount of heat lost from the exhaust gases before they reach the catalyst.

Conversion Efficiency

The efficiency with which undesirable exhaust gases are reduced to acceptable levels or are converted to desirable gases.

Diesel Oxidation Catalyst

Catalyst promoting oxidation processes in diesel exhaust. Usually designed to reduce emissions of the organic fraction of diesel particulates, gas-phase hydrocarbons, and carbon monoxide.

Geometric Surface Area

The total channel surface area per unit of substrate volume.

Intumescent Mat

Ceramic fibre mat which irreversibly expands after exposure to high temperature. Usually contains vermiculite. Intumescent mats are used in the canning of catalytic converters and diesel filters to hold the ceramic substrate inside the steel canister.

Lean NOx Catalyst

Catalyst designed to reduce nitrogen oxides from diesel or spark-ignited engine exhaust gases under net oxidizing conditions (in the presence of excessive amounts of oxygen).

Metallic Catalyst

A metal corrugate for the substrate of the catalytic converter. Typically, metal catalysts result in lower backpressure.

Nitrogen Dioxide

A major contributor to photochemical smog and acid rain, and is irritating to the eyes, respiratory system and skin.

Nitrogen Monoxide

Toxic by inhalation and irritating to eyes and skin.


Generic term for all compounds of nitrogen and oxygen.

Open Frontal Area

Total substrate cross-section area which is available for the flow of gas. Often expressed as a percentage of the total substrate cross-section (sometimes called the substrate void fraction).

Oxygen Storage Capacity

A capacity of the catalyst washcoat to store oxygen at lean and to release it at rich condition. Typically provided by cerium oxide, which oscillates between an oxidized and reduced state, depending on the exhaust gas chemistry. The oxygen storage capacity is an important component of three-way catalysts, used to extend the catalyst window.

Selective Catalytic Reduction

Used to describe the catalytic reduction of NOx in diesel exhaust or flue gases by nitrogen containing compounds, such as ammonia or urea.

Thermal Management

Bauer et al showed that the influence of the catalytic converter′s surface emissivity on the temperature of the component is greater than the influence of the temperature of the cat surface. Thus, when assessing a component′s thermal load, the surface temperature of the catalytic converter is not the only appropriate measure.

Wash Coat

Generic term for the combined support material and catalytic material deposited onto the substrate surface.

Historical Notes

  1. 1836 the phrase catalysed processes was coined by Jons Jakob Berzelius.
  2. 1956 catalytic converter for cleaning up exhaust gas was invented by Eugene Houdry.
  3. 1973 first production catalytic converter developed by John J. Mooney and Carl D. Keith at the Engelhard Corporation.
  4. 1981 three-way catalytic converters used in vehicle emission control systems.


Hans Bauer, H., G. Haldenwanger, Peter Hirth, Rolf Brück, "Thermal Management of Close Coupled Catalysts" SAE 991231

See also: Aftertreatment, Catalyst, Closed-Loop Fuel Control, Exhaust, Exhaust Catalyst Thermal Management, Exhaust Downpipe, Exhaust Gas Oxygen, Exhaust Gases, Exhaust System, Intumescent Mat, Oxygen Sensor.

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Subjects: Engines