![Catalytic-converter-simulation-velocity-streamlines](https://upload.wikimedia.org/wikipedia/commons/2/26/Catalytic-converter-simulation-velocity-streamlines.jpg)
A catalytic converter is an
exhaust emission control device that converts toxic gases and
pollutant
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
s in
exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
from an
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal c ...
into less-toxic pollutants by
catalyzing a
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
reaction
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
* Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and m ...
. Catalytic converters are usually used with internal combustion engines fueled by
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
or
diesel
Diesel may refer to:
* Diesel engine, an internal combustion engine where ignition is caused by compression
* Diesel fuel, a liquid fuel used in diesel engines
* Diesel locomotive, a railway locomotive in which the prime mover is a diesel engin ...
, including
lean-burn
Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1 (by mass). The air / fuel ratio needed to stoichiometrically combust gasoline, by cont ...
engines, and sometimes on
kerosene
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
heaters and stoves.
The first widespread introduction of catalytic converters was in the United States
automobile
A car or automobile is a motor vehicle with wheels. Most definitions of ''cars'' say that they run primarily on roads, seat one to eight people, have four wheels, and mainly transport people instead of goods.
The year 1886 is regarde ...
market. To comply with the
U.S. Environmental Protection Agency
The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
's stricter regulation of exhaust emissions, most gasoline-powered vehicles starting with the 1975
model year
The model year (sometimes abbreviated "MY") is a method of describing the version of a product which has been produced over multiple years. The model year may or may not be the same as the calendar year in which the product was manufactured.
...
are equipped with catalytic converters.
These "two-way" converters combine
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
with
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(CO) and
unburned hydrocarbon
Unburned hydrocarbons (UHCs) are the hydrocarbons emitted after petroleum is burned in an engine.
When unburned fuel is emitted from a combustor, the emission is caused by fuel "avoiding" the flame zones. For example, in piston engines, some of t ...
s (HC) to produce carbon dioxide (CO
2) and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(H
2O). Although two-way converters on gasoline engines were rendered obsolete in 1981 by "three-way" converters that also reduce
oxides of nitrogen Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:
Charge-neutral
*Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide
*Nitrogen dioxide (), nitrogen(IV) oxide
* Nitrogen trioxide (), or n ...
();
they are still used on lean-burn engines to oxidize particulate matter and hydrocarbon emissions (including Diesel engines, which typically use lean combustion), as three-way-converters require
fuel-rich or
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
combustion to successfully reduce .
Although catalytic converters are most commonly applied to
exhaust system
An exhaust system is used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall syste ...
s in automobiles, they are also used on
electrical generator
In electricity generation, a generator is a device that converts motive power (mechanical energy) or fuel-based power ( chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, g ...
s,
forklifts, mining equipment,
truck
A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...
s,
bus
A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
es,
locomotives,
motorcycle
A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising ...
s, and on ships. They are even used on some
wood stoves to control emissions. This is usually in response to
government
A government is the system or group of people governing an organized community, generally a state.
In the case of its broad associative definition, government normally consists of legislature, executive, and judiciary. Government is ...
regulation
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
, either through environmental regulation or through health and safety regulations.
History
Catalytic converter prototypes were first designed in France at the end of the 19th century, when only a few thousand "oil cars" were on the roads; these prototypes had inert clay-based materials coated with
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
,
rhodium
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...
, and
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
and sealed into a double metallic cylinder. A few decades later, a catalytic converter was patented by
Eugene Houdry
Eugène Jules Houdry (Domont, France, April 18, 1892 – Upper Darby, Pennsylvania, July 18, 1962) was a mechanical engineer who graduated from École Nationale Supérieure d'Arts et Métiers in 1911.
Houdry served as a lieutenant in a tank com ...
, a French mechanical engineer. Houdry was an expert in catalytic oil refining, having invented the catalytic cracking process that all modern refining is based on today.
Houdry moved to the United States in 1930 to live near the refineries in the Philadelphia area and develop his catalytic refining process. When the results of early studies of
smog in Los Angeles were published, Houdry became concerned about the role of smokestack exhaust and automobile exhaust in
air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different typ ...
and founded a company called Oxy-Catalyst. Houdry first developed catalytic converters for
smokestacks, called "cats" for short, and later developed catalytic converters for warehouse forklifts that used low grade, unleaded gasoline. In the mid-1950s, he began research to develop catalytic converters for
gasoline engines
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic com ...
used on cars and was awarded United States Patent
2,742,437 for his work.
Catalytic converters were further developed by a series of engineers including
Carl D. Keith Carl Donald Keith (May 29, 1920 – November 9, 2008) was a chemist who was inventor of the three-way catalytic converter, which has played a dramatic role in reducing pollution from motor vehicles since their introduction in the mid-1970s.
Keith w ...
,
John J. Mooney, Antonio Eleazar, and Phillip Messina at
Engelhard
Engelhard Corporation was an American ''Fortune'' 500 company headquartered in Iselin, New Jersey, United States. It is credited with developing the first production catalytic converter. In 2006, the German chemical manufacturer BASF bough ...
Corporation, creating the first production catalytic converter in 1973.
The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the
U.S. Environmental Protection Agency
The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
's new regulation of exhaust emissions, most gasoline-powered vehicles starting with the 1975
model year
The model year (sometimes abbreviated "MY") is a method of describing the version of a product which has been produced over multiple years. The model year may or may not be the same as the calendar year in which the product was manufactured.
...
are equipped with catalytic converters. These "two-way" converters combined
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
with
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(CO) and
unburned hydrocarbon
Unburned hydrocarbons (UHCs) are the hydrocarbons emitted after petroleum is burned in an engine.
When unburned fuel is emitted from a combustor, the emission is caused by fuel "avoiding" the flame zones. For example, in piston engines, some of t ...
s (HC, chemical compounds in fuel of the form C
mH
n) to produce
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
(CO
2) and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(H
2O).
These stringent emission control regulations forced the removal of the
antiknock agent
An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known as gasoline or petrol, when used in high comp ...
tetraethyl lead
Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...
from automotive gasoline, to reduce lead in the air. Lead is a
catalyst poison and would effectively destroy a catalytic converter by coating the catalyst's surface. Requiring the removal of lead allowed the use of catalytic converters to meet the other emission standards in the regulations.
William C. Pfefferle developed a catalytic combustor for
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directio ...
s in the early 1970s, allowing combustion without significant formation of
nitrogen oxides and carbon monoxide.
[Robert N. Carter, Lance L. Smith, Hasan Karim, Marco Castaldi, Shah Etemad, George Muench, R. Samuel Boorse, Paul Menacherry and William C. Pfefferle (1998).]
Catalytic Combustion Technology Development for Gas Turbine Engine Applications
. ''MRS Proceedings'', 549, 93 doi:10.1557/PROC-549-93[Worthy, Sharon.]
Connecticut chemist receives award for cleaner air technology
. ''Bio-Medicine''. 23 June 2003. Retrieved 11 December 2012.
Construction
![Pot catalytique vue de la structure](https://upload.wikimedia.org/wikipedia/commons/b/b2/Pot_catalytique_vue_de_la_structure.jpg)
The catalytic converter's construction is as follows:
# The
catalyst support or
substrate. For automotive catalytic converters, the core is usually a
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
monolith
A monolith is a geological feature consisting of a single massive stone or rock, such as some mountains. For instance, Savandurga mountain is a monolith mountain in India. Erosion usually exposes the geological formations, which are often ma ...
that has a
honeycomb structure
Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb structures ca ...
(commonly square, not hexagonal). (Prior to the mid 1980s, the catalyst material was deposited on a
packed bed of alumina pellets in early GM applications.) Metallic foil monoliths made of
Kanthal (FeCrAl)
are used in applications where particularly high heat resistance is required.
The substrate is structured to produce a large
surface area. The
cordierite
Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-temperat ...
ceramic substrate used in most catalytic converters was invented by
Rodney Bagley,
Irwin Lachman, and
Ronald Lewis at
Corning Glass
Corning Incorporated is an American multinational technology company that specializes in specialty glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The co ...
, for which they were inducted into the
National Inventors Hall of Fame
The National Inventors Hall of Fame (NIHF) is an American not-for-profit organization, founded in 1973, which recognizes individual engineers and inventors who hold a U.S. patent of significant technology. Besides the Hall of Fame, it also oper ...
in 2002.
# The washcoat. A washcoat is a carrier for the catalytic materials and is used to disperse the materials over a large surface area.
Aluminum oxide,
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolub ...
,
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
, or a mixture of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
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 increases the surface area compared to the smooth surface of the bare substrate.
#
Ceria
Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial produ ...
or
ceria-zirconia. These oxides are mainly added as oxygen storage promoters.
# The catalyst itself is most often a mix of
precious metals
Precious metals are rare, naturally occurring metallic chemical elements of high economic value.
Chemically, the precious metals tend to be less reactive than most elements (see noble metal). They are usually ductile and have a high lu ...
, mostly from the
platinum group
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...
.
Platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
is the most active catalyst and is widely used, but is not suitable for all applications because of unwanted additional reactions and high cost.
Palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
and
rhodium
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...
are two other precious metals used. Rhodium is used as a
reduction catalyst, palladium is used as an
oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
catalyst, and platinum is used both for reduction and oxidation.
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...
,
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
,
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
, and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
are also used, although each has limitations. Nickel is not legal for use in the European Union because of its reaction with carbon monoxide into toxic
nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
.
Copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
can be used everywhere except
Japan.
Upon failure, a catalytic converter can be recycled into
scrap. The precious metals inside the converter, including platinum, palladium, and rhodium, are extracted.
Placement of catalytic converters
Catalytic converters require a temperature of to operate effectively. Therefore, they are placed as close to the engine as possible, or one or more smaller catalytic converters (known as "pre-cats") are placed immediately after the exhaust manifold.
Types
Two-way
A 2-way (or "oxidation", sometimes called an "oxi-cat") catalytic converter has two simultaneous tasks:
#
Oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of
carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
to
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
: 2 CO + O
2 → 2 CO
2
# Oxidation of
hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...
(unburnt and partially burned fuel) to carbon dioxide and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
: C
xH
2x+2 +
3x+1)/2O
2 → x CO
2 + (x+1) H
2O (a combustion reaction)
This type of catalytic converter is widely used on
diesel engine
The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-ca ...
s 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 Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:
Charge-neutral
*Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide
*Nitrogen dioxide (), nitrogen(IV) oxide
* Nitrogen trioxide (), or n ...
, they were superseded by three-way converters.
Three-way
Three-way catalytic converters have the additional advantage of controlling the emission of
nitric oxide (NO) and
nitrogen dioxide (NO2) (both together abbreviated with
and not to be confused with
nitrous oxide (N2O)).
species are precursors to
acid rain and
smog.
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 in effect require three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are typically contained in a common housing; however, in some instances, they may be housed separately. A three-way catalytic converter has three simultaneous tasks:
''
Reduction of nitrogen oxides to
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(N
2)''
*
*
*
*
''
Oxidation
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of carbon, hydrocarbons, and carbon monoxide to carbon dioxide''
*
*
*
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
point. For gasoline combustion, this ratio is between 14.6 and 14.8 parts air to one part fuel, by weight. The ratio for
autogas
Autogas or LPG is liquefied petroleum gas (LPG) used as a fuel in internal combustion engines in vehicles as well as in stationary applications such as generators. It is a mixture of propane and butane.
Autogas is widely used as a "green" ...
(or
liquefied petroleum gas LPG),
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
, and
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...
fuels can vary significantly for each, notably so with
oxygenate
Oxygenated chemical compounds contain oxygen as a part of their chemical structure. The term usually refers to oxygenated chemical compounds added to fuels. Oxygenates are usually employed as gasoline additives to reduce carbon monoxide and soot ...
d or alcohol based fuels, with
e85
E85 is an abbreviation typically referring to an ethanol fuel blend of 85% ethanol fuel and 15% gasoline or other hydrocarbon by volume.
In the United States, the exact ratio of fuel ethanol to hydrocarbon may vary according to ASTM 5798 that ...
requiring approximately 34% more fuel, requiring modified fuel system tuning and components when using those fuels. In general, engines fitted with 3-way catalytic converters are equipped with a
computerized closed-loop feedback fuel injection system using one or more
oxygen sensors, though early in the deployment of three-way converters,
carburetors equipped with feedback mixture control were used.
Three-way converters are effective when the engine is operated within a narrow band of air-fuel ratios near the stoichiometric point.
Total conversion efficiency falls very rapidly when the engine is operated outside of this band. Slightly lean of stoichiometric, the exhaust gases from the engine contain excess oxygen, the production of by the engine increases, and the efficiency of the catalyst at reducing falls off rapidly. However, the conversion of HC and CO is very efficient due to the available oxygen, oxidizing to H
2O and CO
2. Slightly rich of stoichiometric, the production of CO and unburnt HC by the engine starts to increase dramatically, available oxygen decreases, and the efficiency of the catalyst for oxidizing CO and HC decreases significantly, especially as stored oxygen becomes depleted. However, the efficiency of the catalyst at reducing is good, and the production of by the engine decreases. To maintain catalyst efficiency, the air:fuel ratio must stay close to stoichiometric and not remain rich or lean for too long.
Closed-loop engine control systems are used for effective operation of three-way catalytic converters because of this continuous rich-lean balance required for effective reduction and HC+CO oxidation. The control system allows the catalyst to release oxygen during slightly rich operating conditions, which oxidizes CO and HC under conditions that also favor the reduction of NOx. Before the stored oxygen is depleted, the control system shifts the air:fuel ratio to become slightly lean, improving HC and CO oxidation while storing additional oxygen in the catalyst material, at a small penalty in reduction efficiency. Then the air:fuel mixture is brought back to slightly rich, at a small penalty in CO and HC oxidation efficiency, and the cycle repeats. Efficiency is improved when this oscillation around the stoichiometric point is small and carefully controlled.
Closed-loop control under light to moderate load is accomplished by using one or more
oxygen sensors in the exhaust system. When oxygen is detected by the sensor, the air:fuel ratio is lean of stoichiometric, and when oxygen is not detected, it is rich. The control system adjusts the rate of fuel being injected into the engine based on this signal to keep the air:fuel ratio near the stoichiometric point in order to maximize the catalyst conversion efficiency. The control algorithm is also affected by the time delay between the adjustment of the fuel flow rate and the sensing of the changed air:fuel ratio by the sensor, as well as the
sigmoidal response of the oxygen sensors. Typical control systems are designed to rapidly sweep the air:fuel ratio such that it oscillates slightly around the stoichiometric point, staying near the optimal efficiency point while managing the levels of stored oxygen and unburnt HC.
Closed loop control is often not used during high load/maximum power operation, when an increase in emissions is permitted and a rich mixture is commanded to increase power and prevent exhaust gas temperature from exceeding design limits. This presents a challenge for control system and catalyst design. During such operations, large amounts of unburnt HC are produced by the engine, well beyond the capacity of the catalyst to release oxygen. The surface of the catalyst quickly becomes saturated with HC. When returning to lower power output and leaner air:fuel ratios, the control system must prevent excessive oxygen from reaching the catalyst too quickly, as this will rapidly burn the HC in the already hot catalyst, potentially exceeding the design temperature limit of the catalyst. Excessive catalyst temperature can prematurely age the catalyst, reducing its efficiency before reaching its design lifetime. Excessive catalyst temperature can also be caused by cylinder misfire, which continuously flows unburnt HC combined with oxygen to the hot catalyst, burning in the catalyst and increasing its temperature.
Unwanted reactions
Unwanted reactions result in the formation of
hydrogen sulfide and
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, which poison catalysts.
Nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
or
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
is sometimes added to the washcoat to limit hydrogen-sulfide emissions. Sulfur-free or low-sulfur fuels eliminate or minimize problems with hydrogen sulfide.
Diesel engines
For compression-ignition (i.e.,
diesel
Diesel may refer to:
* Diesel engine, an internal combustion engine where ignition is caused by compression
* Diesel fuel, a liquid fuel used in diesel engines
* Diesel locomotive, a railway locomotive in which the prime mover is a diesel engin ...
) engines, the most commonly used catalytic converter is the ''diesel oxidation catalyst'' (DOC). DOCs contain
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
and/or
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
supported on alumina. This catalyst converts
particulate matter (PM), hydrocarbons, and carbon monoxide to carbon dioxide and water. These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates. These catalysts are ineffective for , so emissions from diesel engines are controlled by
exhaust gas recirculation
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide () emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust ...
(EGR).
In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles to meet federal emissions requirements. Two techniques have been developed for the catalytic reduction of emissions under lean exhaust conditions,
selective catalytic reduction
Selective catalytic reduction (SCR) is a means of converting nitrogen oxides, also referred to as with the aid of a catalyst into diatomic nitrogen (), and water (). A reductant, typically anhydrous ammonia (), aqueous ammonia (), or a urea () ...
(SCR) and the
adsorber.
Instead of precious metal-containing absorbers, most manufacturers selected base-metal SCR systems that use a
reagent such as
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
to reduce the into
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and water. Ammonia is supplied to the catalyst system by the injection of
urea
Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid.
Urea serves an important ...
into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. The urea solution is also referred to as
diesel exhaust fluid
Diesel exhaust fluid (DEF; also known as AUS 32 and marketed as AdBlue) is a liquid used to reduce the amount of air pollution created by a diesel engine. Specifically, DEF is an aqueous urea solution made with 32.5% urea and 67.5% deionized w ...
(DEF).
Diesel exhaust
Diesel exhaust is the gaseous exhaust produced by a diesel type of internal combustion engine, plus any contained particulates. Its composition may vary with the fuel type or rate of consumption, or speed of engine operation (e.g., idling or at ...
contains relatively high levels of particulate matter. Catalytic converters remove only 20–40% of PM so particulates are cleaned up by a soot trap or
diesel particulate filter
A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.Tom Nash (May 2003) "Diesels: The Smoke is clearing", ''Motor '' Vol.199 No. 5, p. 54, Hearst Business Publ ...
(DPF). In the U.S., all on-road light, medium, and heavy-duty diesel-powered vehicles built after 1 January 2007, are subject to diesel particulate emission limits, and so are equipped with a 2-way catalytic converter and a diesel particulate filter. As long as the engine was manufactured before 1 January 2007, the vehicle is not required to have the DPF system. This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.
Lean-burn spark-ignition engines
For
lean-burn
Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1 (by mass). The air / fuel ratio needed to stoichiometrically combust gasoline, by cont ...
spark-ignition engines, an oxidation catalyst is used in the same manner as in a diesel engine. Emissions from lean burn spark ignition engines are very similar to emissions from a diesel compression ignition engine.
Installation
Many vehicles have a close-coupled catalytic converter located near the engine's
exhaust manifold. The converter heats up quickly, due to its exposure to the very hot exhaust gases, enabling it to reduce undesirable emissions during the engine warm-up period. This is achieved by burning off the excess hydrocarbons which result from the extra-rich mixture required for a cold start.
When catalytic converters were first introduced, most vehicles used
carburetors that provided a relatively rich
air-fuel ratio. Oxygen (O
2) levels in the exhaust stream were therefore generally insufficient for the catalytic reaction to occur efficiently. Most designs of the time therefore included
secondary air injection
Secondary air injection (commonly known as air injection) is a vehicle emissions control strategy introduced in 1966, wherein fresh air is injected into the exhaust stream to allow for a fuller secondary combustion of exhaust gases.
Development
...
, which injected air into the exhaust stream. This increased the available oxygen, allowing the catalyst to function as intended.
Some three-way catalytic converter systems have air injection systems with the air injected between the first ( reduction) and second (HC and CO oxidation) stages of the converter. As in two-way converters, this injected air provides oxygen for the oxidation reactions. An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period. This causes unburned fuel to ignite in the exhaust tract, thereby preventing it reaching the catalytic converter at all. This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or
operating temperature.
Most newer vehicles have
electronic fuel injection
Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a mani ...
systems, and do not require air injection systems in their exhausts. Instead, they provide a precisely controlled air-fuel mixture that quickly and continually cycles between lean and rich combustion.
Oxygen sensors monitor the exhaust oxygen content before and after the catalytic converter, and the
engine control unit uses this information to adjust the fuel injection so as to prevent the first ( reduction) catalyst from becoming oxygen-loaded, while simultaneously ensuring the second (HC and CO oxidation) catalyst is sufficiently oxygen-saturated.
Damage
Catalyst poisoning Catalyst poisoning refers to the partial or total deactivation of a catalyst by a chemical compound. Poisoning refers specifically to chemical deactivation, rather than other mechanisms of catalyst degradation such as thermal decomposition or physic ...
occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, so that they cannot contact and react with the exhaust. The most notable contaminant is
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
, so vehicles equipped with catalytic converters can run only on
unleaded
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic com ...
fuel. Other common catalyst poisons include
sulfur,
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
(originating primarily from the gasoline additive
MMT), and
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
, which can enter the exhaust stream if the engine has a leak that allows
coolant
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosi ...
into the combustion chamber.
Phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
is another catalyst contaminant. Although phosphorus is no longer used in gasoline, it (and
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
, another low-level catalyst contaminant) was widely used in engine oil
antiwear additives such as
zinc dithiophosphate
Zinc dialkyldithiophosphates (often referred to as ZDDP) are a family of coordination compounds developed in the 1940s that feature zinc bound to the anion of a dialkyldithiophosphoric salt (e.g., ammonium diethyl dithiophosphate). These uncharge ...
(ZDDP). Beginning in 2004, a limit of phosphorus concentration in engine oils was adopted in the
API
An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...
SM and
ILSAC GF-4 specifications.
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 vaporize or sublimate 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.
Any condition that causes abnormally high levels of unburned hydrocarbons (raw or partially burnt fuel or oils) 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. These conditions include failure of the upstream components of the exhaust system (manifold/header assembly and associated clamps susceptible to rust/corrosion and/or fatigue e.g. the exhaust manifold splintering after repeated heat cycling), ignition system e.g. coil packs and/or primary ignition components (e.g. distributor cap, wires, ignition coil and spark plugs) and/or damaged fuel system components (fuel injectors, fuel pressure regulator, and associated sensors). Oil and/or coolant leaks, perhaps caused by a head gasket leak, can also cause high unburned hydrocarbons.
Regulations
Emissions regulations vary considerably from jurisdiction to jurisdiction. Most automobile spark-ignition engines in North America have been fitted with catalytic converters since 1975,
and the technology used in non-automotive applications is generally based on automotive technology. In many jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its direct and immediate replacement. Nevertheless, some vehicle owners remove or "gut" the catalytic converter on their vehicle.
In such cases, the converter may be replaced by a welded-in section of ordinary pipe or a flanged "test pipe", ostensibly meant to check if the converter is clogged by comparing how the engine runs with and without the converter. This facilitates temporary reinstallation of the converter in order to pass an emission test.
In the United States, it is a violation of Section 203(a)(3)(A) of the 1990 amended
Clean Air Act for a vehicle repair shop to remove a converter from a vehicle, or cause a converter to be removed from a vehicle, except in order to replace it with another converter,
[Sale and Use of Aftermarket Catalytic Converters]
US Environmental Protection Agency, US Federal Register Volume 51 and Section 203(a)(3)(B) makes it illegal for any person to sell or to install any part that would bypass, defeat, or render inoperative any emission control system, device, or design element. 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.
Effect on exhaust flow
Faulty catalytic converters as well as undamaged early types of converters can restrict the flow of exhaust, which negatively affects vehicle performance and fuel economy.
Modern catalytic converters do not significantly restrict exhaust flow. A 2006 test on a 1999 Honda Civic, for example, showed that removing the stock catalytic converter netted only a 3% increase in maximum horsepower; a new metallic core converter only cost the car 1% horsepower, compared to no converter.
Dangers
Carburetors on pre-1981 vehicles without feedback fuel-air mixture control could easily provide too much fuel to the engine, which could cause the catalytic converter to overheat and potentially ignite flammable materials under the car.
Warm-up period
Vehicles fitted with catalytic converters emit most of their total pollution during the first five minutes of engine operation; for example, before the catalytic converter has warmed up sufficiently to be fully effective.
[Catalytic converters](_blank)
nsls.bnl.gov
In the early 2000s it became common to place the catalyst converter right next to the exhaust manifold, close to the engine, for much quicker warm-up.
In 1995,
Alpina
Alpina Burkard Bovensiepen GmbH & Co. KG is an automobile manufacturing company based in Buchloe, in the Ostallgäu district of Bavaria, Germany that develops and sells high-performance versions of BMW cars.
Alpina works closely with BMW an ...
introduced an electrically heated catalyst. Called "E-KAT," it was used in Alpina's B12 5,7 E-KAT based on the BMW
750i.
Heating coils inside the catalytic converter assemblies are electrified just after the engine is started, bringing the catalyst up to operating temperature very quickly to qualify the vehicle for
low emission vehicle
A low-emission vehicle is a motor vehicle that emits relatively low levels of motor vehicle emissions. The term may be used in a general sense, but in some countries it is defined in air quality statutes.
Different groups of people ("go greens", " ...
(LEV) designation.
BMW later introduced the same heated catalyst, developed jointly by Emitec, Alpina, and BMW,
in its 750i in 1999.
Some vehicles contain a pre-cat, a small catalytic converter upstream of the main catalytic converter which heats up faster on vehicle start up, reducing the emissions associated with cold starts. A pre-cat is most commonly used by an auto manufacturer when trying to attain the Ultra Low Emissions Vehicle (ULEV) rating, such as on the
Toyota MR2
The Toyota MR2 is a line of two-seat, mid-engined, rear-wheel-drive sports cars manufactured in Japan and marketed globally by Toyota from 1984 until 2007 over three generations: W10 (1984–1989), W20 (1989–1999) and W30 (2000–2007). It ...
Roadster.
Environmental effect
Catalytic converters have proven to be reliable and effective in reducing noxious tailpipe emissions. However, they also have some shortcomings in use, and also adverse environmental effects in production:
*An engine equipped with a three-way catalyst must run at the
stoichiometric point, which means more fuel is consumed than in a
lean-burn
Lean-burn refers to the burning of fuel with an excess of air in an internal combustion engine. In lean-burn engines the air:fuel ratio may be as lean as 65:1 (by mass). The air / fuel ratio needed to stoichiometrically combust gasoline, by cont ...
engine. This means approximately 10% more CO
2 emissions from the vehicle.
*Catalytic converter production requires
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
or
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
; part of the world supply of these
precious metals is produced near
Norilsk
Norilsk ( rus, Нори́льск, p=nɐˈrʲilʲsk, ''Norílʹsk'') is a closed city in Krasnoyarsk Krai, Russia, located south of the western Taymyr Peninsula, around 90 km east of the Yenisey River and 1,500 km north of Krasnoyarsk. Norilsk ...
,
Russia
Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...
, where the industry (among others) has caused Norilsk to be added to ''Time'' magazine's list of most-polluted places.
*The extreme heat of the converters themselves can cause
wildfires
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identif ...
, especially in dry areas.
Theft
Because of the external location and the use of valuable precious metals including
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Pla ...
,
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
and
rhodium
Rhodium is a chemical element with the symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring i ...
, catalytic converters are a target for thieves. The problem is especially common among late-model
pickup trucks and truck-based
SUV
A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive.
There is no commonly agreed-upon definiti ...
s, because of their high ground clearance and easily removed bolt-on catalytic converters. Welded-on converters are also at risk of theft, as they can be easily cut off. The
Toyota Prius
The is a car built by Toyota which has a hybrid drivetrain, combining an internal combustion engine with an electric motor. Initially offered as a four-door sedan, it has been produced only as a five-door liftback since 2003.
In 2007, ...
catalytic converters are also targets for thieves.
Pipecutter
A pipecutter is a type of tool used by plumbers to cut pipe. Besides producing a clean cut, the tool is often a faster, cleaner, and more convenient way of cutting pipe than using a hacksaw, although this depends on the metal of the pipe.
Ther ...
s are often used to quietly remove the converter but other tools such as a portable
reciprocating saw
A reciprocating saw is a type of machine-powered saw in which the cutting action is achieved through a push-and-pull ("reciprocating") motion of the blade. The original trade name Sawzall is often used in the United States, where Milwaukee Electr ...
can damage other components of the car, such as the alternator, wiring or fuel lines, with potentially dangerous consequences. Rising metal prices in the U.S. during the 2000s commodities boom led to a significant increase in converter theft. A catalytic converter can cost more than $1,000 to replace, more if the vehicle is damaged during the theft.
From 2019–2020, thieves in the United Kingdom were targeting older-model
hybrid car
A hybrid vehicle is one that uses two or more distinct types of power, such as submarines that use diesel when surfaced and batteries when submerged. Other means to store energy include pressurized fluid in hydraulic hybrids.
The basic princip ...
s (such as
Toyota's hybrids) which have more precious metals than newer vehicles—sometimes worth more than the value of the car—leading to scarcity and long delays in replacing them.
In 2021 a trend emerged in the
Democratic Republic of Congo
The Democratic Republic of the Congo (french: République démocratique du Congo (RDC), colloquially "La RDC" ), informally Congo-Kinshasa, DR Congo, the DRC, the DROC, or the Congo, and formerly and also colloquially Zaire, is a country in ...
where catalytic converters were stolen for use in drug production.
Diagnostics
Various jurisdictions now require
on-board diagnostics to monitor the function and condition of the emissions-control system, including the catalytic converter. Vehicles equipped with
OBD-II
On-board diagnostics (OBD) is a term referring to a vehicle's self-diagnostic and reporting capability. OBD systems give the vehicle owner or repair technician access to the status of the various vehicle sub-systems. The amount of diagnostic inf ...
diagnostic systems are designed to alert the driver to a misfire condition by means of illuminating the "check engine" light on the dashboard, or flashing it if the current misfire conditions are severe enough to potentially damage the catalytic converter.
On-board diagnostic systems take several forms.
Temperature
sensors are used for two purposes. The first is as a warning system, typically on two-way catalytic converters such as are still sometimes used on LPG forklifts. The function of the sensor is to warn of catalytic converter temperature above the safe limit of . Modern catalytic-converter designs are not as susceptible to temperature damage and can withstand sustained temperatures of . Temperature sensors are also used to monitor catalyst functioning: usually two sensors will be fitted, with one before the catalyst and one after to monitor the temperature rise over the catalytic-converter core.
The
oxygen sensor is the basis of the
closed-loop control system on a spark-ignited rich-burn engine; however, it is also used for diagnostics. In vehicles with
OBD II, a second oxygen sensor is fitted after the catalytic converter to monitor the O
2 levels. The O
2 levels are monitored to see the efficiency of the burn process. The on-board computer makes comparisons between the readings of the two sensors. The readings are taken by voltage measurements. If both sensors show the same output or the rear O
2 is "switching", the computer recognizes that the catalytic converter either is not functioning or has been removed, and will operate a malfunction indicator lamp and affect engine performance. Simple "oxygen sensor simulators" have been developed to circumvent this problem by simulating the change across the catalytic converter with plans and pre-assembled devices available on the Internet. Although these are not legal for on-road use, they have been used with mixed results. Similar devices apply an offset to the sensor signals, allowing the engine to run a more fuel-economical lean burn that may, however, damage the engine or the catalytic converter.
sensors are extremely expensive and are in general used only when a compression-ignition engine is fitted with a selective catalytic-reduction (SCR) converter, or a absorber in a feedback system. When fitted to an SCR system, there may be one or two sensors. When one sensor is fitted it will be pre-catalyst; when two are fitted, the second one will be post-catalyst. They are used for the same reasons and in the same manner as an oxygen sensor; the only difference is the substance being monitored.
See also
*
Catalytic heater A catalytic heater is a flameless Heater#Heating, heater which relies on Catalysis, catalyzed chemical reactions to break down molecules and produce califaction (heat). When the catalyst, natural gas, and oxygen combine together they ignite at a lo ...
*
Cerium(III) oxide
Cerium(III) oxide, also known as cerium oxide, cerium trioxide, cerium sesquioxide, cerous oxide or dicerium trioxide, is an oxide of the rare-earth metal cerium. It has chemical formula and is gold-yellow in color.
Applications Engine and exha ...
*
adsorber
*
Roadway air dispersion modeling
References
Further reading
*Keith, C. D., et al. : ''"Apparatus for purifying exhaust gases of an internal combustion engine". 29 April 1969''
*Lachman, I. M. et al. : ''"Anisotropic Cordierite Monolith" (Ceramic substrate). 5 November 1973''
*Charles H. Bailey. : ''"Combination muffler and catalytic converter having low backpressure". 13 June 1978''
*Charles H. Bailey. : '"Caseless monolithic catalytic converter". 10 February 1981
*Srinivasan Gopalakrishnan. : ''"Process And Synthesizer For Molecular Engineering of Materials". 13 March 2002''.
{{Authority control
Air pollution control systems
American inventions
Automotive technologies
Carbon monoxide
Catalysis
Exhaust systems
NOx control
Products introduced in 1975