Exhaust Catalyst

The exhaust catalyst or 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

• 2CO + O₂ -> 2CO₂
• C_xH_2x+2 + [(3x+1)/2]O₂ -> xCO₂ + (x+1)H₂O

Three-Way Catalyst

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

• 2NO_x -> xO₂ + N₂
• 2CO + O₂ -> 2CO₂
• C_xH_2x+2 + [(3x+1)/2]O₂ -> xCO₂ + (x+1)H₂O

Acoustics – 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.

Engine Combustion

Fundamentally, combustion is a chemical reaction between a fuel and an oxidizing agent that produces heat and light.

Example: the combustion of methane is represented as CH₄(g) + 2 O₂(g) = CO₂(g) + 2 H₂O().

The vigorous and exothermic reaction that takes place between certain substances, particularly organic compounds, and oxygen.

For the cumbustion of a hydrocarbon in air we have the simple equation:

Fuel + Air → Heat + Water + Carbon dioxide + Nitrogen

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 NO_x 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.

NO_x – 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 NO_x 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.

The rate at which the catalyst heats up is an important factor in the control of the start-up emissions of an engine. A number of measures have been used to improve this:

• Moving the catalyst closer to the exhaust valve outlets – close coupled.
• Insulation of the exhaust manifold.
• Insulation of the downpipe.
• Insulation of the catalyst.
  1. Burch et al looked at vacuum insulation of a catalyst.
• Heat storage to pre-warm the catalyst.
  1. Burch et al looked at a phase change material as a heat store for a catalyst and managed to maintain the temperature of the catalyst for 17 hours compared to 25 minutes for an untreated catalyst.
• Electrical heating of the catalyst prior to engine start.

An exhaust catalyst runs at a high temperature internally and has a high surface temperature that also needs to be accounted for when packaging.

Bauer et al showed that All measures intended to reduce heat output, should be applied to all elements of the exhaust system (manifold, cones, catalytic converter), as otherwise the component’s thermal load is unchanged.

• 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.
• Despite a rising surface temperature, by reducing the radiation emissivity of the converter mantle, the heat output of the catalytic converter falls.
• Reducing heat loss by using a mantle material with a lower emissivity supplements the insulation on the exhaust system.
• Austenitic mantle material clearly displays a lower emissivity than a ferrite mantle.
• In the positioning of converter and component here, the distance from each other is of practically no importance, as long as a minimum distance is maintained.
• Simple and effective control of the component temperature can be achieved by reducing the emissivity of the component.

Wash Coat – generic term for the combined support material and catalytic material deposited onto the substrate surface.

Reference

1. Steve D. Burch, Thomas F. Potter, Matthew A. Keyser, “Reducing Cold-Start Emissions by Catalytic Converter Thermal Management”, SAE 950409
2. Hans Bauer, H., G. Haldenwanger, Peter Hirth, Rolf Brück, “Thermal Management of Close Coupled Catalysts” SAE 991231