Catalytic converter

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Catalytic converter on an eight-year-old Dodge Ram Van

A catalytic converter (colloquially, "cat" or "catcon") is a device used to reduce the toxicity of exhaust emissions from an internal combustion engine. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustioncarbon monoxide, unburned hydrocarbons, and oxides of nitrogen are converted to less-toxic or inert substances such as carbon dioxide, hydrogen, nitrogen and oxygen.[1]

First widely introduced on series-production automobiles in the United States market for the 1975 model year to comply with tightening U.S. Environmental Protection Agency regulations on auto exhaust emissions, catalytic converters are still most commonly used in motor vehicleexhaust systems. Catalytic converters are also used on generator sets, forklifts, mining equipment, trucks, buses, trains, airplanes and other engine-equipped machines.

Contents
[hide]
  • 1History
  • 2Construction
  • 3Types
  • 3.1Two-way
  • 3.2Three-way
  • 3.2.1Oxygen storage
  • 3.2.2Unwanted reactions
  • 3.3For diesel engines
  • 3.4For lean-burn engines
  • 4Installation
  • 5Damage
  • 5.1Poisoning
  • 5.2Meltdown
  • 6Regulations
  • 7Negative aspects
  • 7.1Warm-up period
  • 7.2Environmental impact
  • 8Theft
  • 9Diagnostics
  • 9.1Temperature sensors
  • 9.2Oxygen sensors
  • 9.3NOx sensors
  • 10See also
  • 11References
  • 12External links
  • 13Patents

[edit] History

The catalytic converter was invented by Eugene Houdry, a French mechanical engineer and expert in catalytic oil refining[2] who lived in the U.S. Around 1950, when the results of early studies of smog in Los Angeles were published, Houdry became concerned about the role of automobile exhaust in air pollution and founded a special company, Oxy-Catalyst, to develop catalytic converters for gasoline engines — an idea ahead of its time for which he was awarded a patent (US2742437). Widespread adoption had to wait until the extremely effective anti-knock agenttetraethyl lead was eliminated from most gasoline over environmental concerns, for the lead would spoil the converter by forming a coating on the catalyst's surface, effectively disabling it.[3]

The catalytic converter was further developed by John J. Mooney and Carl D. Keith at the Engelhard Corporation,[4] creating the first production catalytic converter in 1973.[5]

[edit] Construction

Metal-core converter

Ceramic-core converter

The catalytic converter consists of several components:

  1. The catalyst core, or substrate. For automotive catalytic converters, the core is often a ceramic monolith with a honeycomb structure, but metallic foil monoliths made of FeCrAl were introduced in the 1990's and are used by some automotive manufacturers.[citation needed] The honeycomb geometry provides a high surface area to support the catalyst washcoat, and therefore is often called a "catalyst support".[citation needed] The cordierite ceramic substrate used in most catalytic converters was invented by Rodney Bagley, Irwin Lachman and Ronald Lewis at Corning Glass, for which they were inducted into the National Inventors Hall of Fame in 2002.[citation needed]
  2. The washcoat. A washcoat is a carrier for the catalytic materials and is used to disperse the materials over a high surface area. Aluminum oxide, Titanium dioxide, Silicon dioxide, or a mixture of silica and alumina can be used. The catalytic materials are suspended in the washcoat prior to applying to the core. Washcoat materials are selected to form a rough, irregular surface, which greatly increases the surface area compared to the smooth surface of the bare substrate. This maximizes the catalytically active surface available to react with the engine exhaust.
  3. The catalyst itself is most often a precious metal. Platinum is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions[vague] and high cost. Palladium and rhodium are two other precious metals used. Rhodium is used as a reduction catalyst, palladium is used as an oxidation catalysts, and platinum is used both for reduction and oxidation. Cerium, iron, manganese and nickel are also used, although each has its own limitations. Nickel is not legal for use in the European Union (because of its reaction with carbon monoxide). Copper can be used everywhere except North America,[clarification needed] where its use is illegal because of the formation of dioxin.

[edit] Types

[edit] Two-way

A two-way (or "oxidation") catalytic converter has two simultaneous tasks:

  1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
  2. Oxidation of hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2] O2 → xCO2 + (x+1) H2O (a combustion reaction)

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 American- and Canadian-market automobiles until 1981. Because of their inability to control oxides of nitrogen, they were superseded by three-way converters.

[edit] Three-way

Since 1981, three-way (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that effectively require three-way converters on gasoline-powered vehicles. A three-way catalytic converter has three simultaneous tasks:

  1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
  2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
  3. Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O

These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This point is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. The ratio for Autogas (or liquefied petroleum gas (LPG)), natural gas and ethanol fuels is each slightly different, requiring modified fuel system settings when using those fuels. Generally, engines fitted with 3-way catalytic converters are equipped with a computerizedclosed-loop feedbackfuel injection system using one or more oxygen sensors, though early in the deployment of three-way converters, carburetors equipped for feedback mixture control were used.

Three-way catalysts are effective when the engine is operated within a narrow band of air-fuel ratios near stoichiometry, such that the exhaust gas oscillates between rich (excess fuel) and lean (excess oxygen) conditions. However, conversion efficiency falls very rapidly when the engine is operated outside of that band of air-fuel ratios. Under lean engine operation, there is excess oxygen and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, thus only stored oxygen is available for the oxidation function. Closed-loop control systems are necessary because of the conflicting requirements for effective NOx reduction and HC oxidation. The control system must prevent the NOx reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material to maintain its function as an oxidation catalyst.

[edit] Oxygen storage

Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the air-fuel ratio goes lean.[6] When insufficient oxygen is available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NOx reduction is unavailable or certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.

[edit] Unwanted reactions

Unwanted reactions can occur in the three-way catalyst, such as the formation of odiferous hydrogen sulfide and ammonia. Formation of each can be limited by modifications to the washcoat and precious metals used. It is difficult to eliminate these byproducts entirely. Sulfur-free or low-sulfur fuels eliminate or reduce hydrogen sulfide.

For example, when control of hydrogen-sulfide emissions is desired, nickel or manganese is added to the washcoat. Both substances act to block the adsorption of sulfur by the washcoat. Hydrogen sulfide is formed when the washcoat has adsorbed sulfur during a low temperature part of the operating cycle, which is then released during the high-temperature part of the cycle and the sulfur combines with HC.

[edit] For diesel engines

For compression-ignition (i.e., diesel engines), the most-commonly-used catalytic converter is the Diesel Oxidation Catalyst (DOC). This catalyst uses O2 (oxygen) in the exhaust gas stream to convert CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping to reduce visible particulates (soot). These catalyst are not active for NOx reduction because any reductant present would react first with the high concentration of O2 in diesel exhaust gas.

Reduction in NOx emissions from compression-ignition engine has previously been addressed by the addition of exhaust gas to incoming air charge, known as exhaust gas recirculation (EGR). In 2010, most light-duty diesel manufactures in the U.S. added catalytic systems to their vehicles to meet new federal emissions requirements. There are two techniques that have been developed for the catalytic reduction of NOx emissions under lean exhaust condition - selective catalytic reduction (SCR) and the lean NOx trap or NOxadsorber. Instead of precious metal containing NOxadsorbers, most manufacturers selected base metal SCR system which use a reagent such as ammonia to reduce the NOx into nitrogen. Ammonia is supplied to the catalyst system by the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also referred to as Diesel Emission Fluid (DEF), is AdBlue.

Diesel exhaust contains relatively high levels of particulate matter (soot), consisting in large part of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction[citation needed], so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). A DPF consists of a cordierite substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road heavy-duty vehicles powered by diesel and built after January 1, 2007, must be equipped with a catalytic converter and a diesel particulate filter.[7]

[edit] For lean-burn engines

For lean-burn, spark-ignition engines, an oxidation catalyst is used in the same manner as in a diesel engine.

[edit] Installation

Many vehicles have a close-coupled catalysts located near the engine's exhaust manifold. This unit heats up quickly due to its proximity to the engine, and reduces cold-engine emissions by burning off hydrocarbons from the extra-rich mixture used to start a cold engine.

In the past, some three-way catalytic converter systems used an air-injection tube between the first (NOx reduction) and second (HC and CO oxidation) stages of the converter. This tube was part of a secondary air injection system. The injected air provided oxygen for the oxidation reactions. An upstream air injection point was also sometimes present to provide oxygen during engine warmup, which caused unburned fuel to ignite in the exhaust tract before reaching the catalytic converter. This cleaned up the exhaust and reduced the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature.

Most modern catalytic converter systems do not have air injection systems.[citation needed] Instead, they provide a constantly varying air-fuel mixture that quickly and continually cycles between lean and rich exhaust. Oxygen sensors are used to monitor the exhaust oxygen content before and after the catalytic converter and this information is used by the Electronic control unit to adjust the fuel injection so as to prevent the first (NOx reduction) catalyst from becoming oxygen-loaded while ensuring the second (HC and CO oxidization) catalyst is sufficiently oxygen-saturated. The reduction and oxidation catalysts are typically contained in a common housing, however in some instances they may be housed separately.

[edit] Damage

[edit] Poisoning

Catalyst poisoning occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, encapsulating the catalyst so that it cannot contact and treat the exhaust. The most-notable contaminant is lead, so vehicles equipped with catalytic converters can only be run on unleaded gasoline. Other common catalyst poisons include manganese (originating primarily from the gasoline additive MMT), and silicone, which can enter the exhaust stream if the engine has a leak, allowing coolant into the combustion chamber. Phosphorus is another catalyst contaminant. Although phosphorus is no longer used in gasoline, it (and zinc, another low-level catalyst contaminant) was until recently widely used in engine oil antiwear additives such as zinc dithiophosphate (ZDDP). Beginning in 2006, a rapid phaseout of ZDDP in engine oils began.[citation needed]

Depending on the contaminant, catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time. The increased exhaust temperature can sometimes liquefy or sublime the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is usually not possible because of lead's high boiling point.

[edit] Meltdown

Any condition that causes abnormally high levels of unburned hydrocarbons — raw or partially burnt fuel — to reach the converter will tend to significantly elevate its temperature, bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction. Vehicles equipped with OBD-II diagnostic systems are designed to alert the driver to a misfire condition by means of flashing the "check engine" light on the dashboard.

[edit] Regulations

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Emissions regulations vary considerably from jurisdiction to jurisdiction. In North America,[clarification needed] most spark-ignition engines of over 25 brake horsepower (19kW) output built after January 1, 2004, are equipped with three-way catalytic converters. In Japan, a similar set of regulations came into effect January 1, 2007, while the European Union has focused on regulations limiting pollutant output without specifying that any specific technology must be used[8] beginning with Euro 1 regulations in 1992 and becoming progressively more stringent in subsequent years.[9] Most automobile spark-ignition engines in North America have been fitted with catalytic converters since the mid-1970s, and the technology used in non-automotive applications is generally based on automotive technology.

Regulations for diesel engines are similarly varied, with some jurisdictions focusing on NOx (nitric oxide and nitrogen dioxide) emissions and others focusing on particulate (soot) emissions. This regulatory diversity is challenging for manufacturers of engines, as it may not be economical to design an engine to meet two sets of regulations.

Regulations of fuel quality vary across jurisdictions. In North America, Europe, Japan and Hong Kong, gasoline and diesel fuel are highly regulated, and compressed natural gas and LPG (Autogas) are being reviewed for regulation. In most of Asia and Africa, the regulations are often lax — in some places sulfur content of the fuel can reach 20,000 parts per million (2%). Any sulfur in the fuel can be oxidized to SO2 (sulfur dioxide) or even SO3 (sulfur trioxide) in the combustion chamber. If sulfur passes over a catalyst, it may be further oxidized in the catalyst, i.e., SO2 may be further oxidized to SO3. Sulfur oxides are precursors to sulfuric acid, a major component of acid rain. While it is possible to add substances such as vanadium to the catalyst washcoat to combat sulfur-oxide formation, such addition will reduce the effectiveness of the catalyst. The most effective solution is to further refine fuel at the refinery to produce ultra-low sulfur diesel. Regulations in Japan, Europe and North America tightly restrict the amount of sulfur permitted in motor fuels. However, the expense of producing such clean fuel makes it impractical for use in many developing countries. As a result, cities in these countries with high levels of vehicular traffic suffer from acid rain, which damages stone and woodwork of buildings and damages local ecosystems.

[edit] Negative aspects

Some early converter designs greatly restricted the flow of exhaust, which negatively affected vehicle performance, driveability, and fuel economy.[10] Because they were used with carburetors incapable of precise fuel-air mixture control, they could overheat and set fire to flammable materials under the car.[11] Removing a modern catalytic converter in new condition will only slightly increase vehicle performance without retuning,[12] but their removal or "gutting" continues.[10][13] The exhaust section where the converter was may be replaced with a welded-in section of straight pipe, or a flanged section of "test pipe" legal for off-road use that can then be replaced with a similarly fitted converter-choked section for legal on-road use, or emissions testing.[12] In the U.S. and many other jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its immediate replacement[citation needed]; vehicles without functioning catalytic converters generally fail emission inspections. The automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.[14]