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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 combustion carbon monoxide,
unburned hydrocarbons, and oxides of nitrogen are converted to less-toxic or inert substances
such as carbon dioxide, hydrogen, nitrogen and oxygen.
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 vehicle
exhaust systems. Catalytic converters are also used on generator sets, forklifts, mining
equipment, trucks, buses, trains, airplanes and other engine-equipped machines.
o 3.1 Two-way
o 3.2 Three-way
3.2.1 Oxygen storage
3.2.2 Unwanted reactions
o 3.3 For diesel engines
o 3.4 For lean-burn engines
o 5.1 Poisoning
o 5.2 Meltdown
7 Negative aspects
o 7.1 Warm-up period
o 7.2 Environmental impact
o 9.1 Temperature sensors
o 9.2 Oxygen sensors
o 9.3 NOx sensors
10 See also
12 External links
The catalytic converter was invented by Eugene Houdry, a French mechanical engineer and
expert in catalytic oil refining 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
agent tetraethyl 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
The catalytic converter was further developed by John J. Mooney and Carl D. Keith at the
Engelhard Corporation, creating the first production catalytic converter in 1973.
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. 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.
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.
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.
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 computerized closed-loop feedback fuel 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.
 Oxygen storage
Three-way catalytic converters can store oxygen from the exhaust gas stream, usually when the
air-fuel ratio goes lean. 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.
 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.
 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 NOx adsorber. Instead of precious metal containing
NOx adsorbers, 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, 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.
 For lean-burn engines
For lean-burn, spark-ignition engines, an oxidation catalyst is used in the same manner as in a
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. 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.
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.
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.
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.
This section does not cite any references or sources. Please help improve this section
by adding citations to reliable sources. Unsourced material may be challenged and
removed. (March 2009)
Emissions regulations vary considerably from jurisdiction to jurisdiction. In North
America,[clarification needed] most spark-ignition engines of over 25 brake horsepower (19 kW)
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 beginning with Euro 1 regulations in 1992 and becoming progressively more
stringent in subsequent years. 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.
 Negative aspects
Some early converter designs greatly restricted the flow of exhaust, which negatively affected
vehicle performance, driveability, and fuel economy. 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. Removing a modern catalytic converter in new condition will only
slightly increase vehicle performance without retuning, but their removal or "gutting"
continues. 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. 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; 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.
 Warm-up period
Most of the pollution put out by a car occurs during the first five minutes before the catalytic
converter has warmed up sufficiently.
In 1999, BMW introduced the Electric Catalytic Convert, or "E-CAT", in their flagship E38
750iL sedan. Coils inside the catalytic converter assemblies are heated electrically just after
engine start, bringing the catalyst up to operating temperature much faster than traditional
catalytic converters can, providing cleaner cold starts and low emission vehicle (LEV)
 Environmental impact
Catalytic converters have proven to be reliable and effective in reducing noxious tailpipe
emissions. However, they may have some adverse environmental impacts in use:
The requirement for an internal combustion engine equipped with a three-way catalyst to
run at the stoichiometric point means it is less efficient than if it were operated lean.
Thus, there is an increases the amount of fossil fuel consumed and the carbon-dioxide
emissions from the vehicle. However, NOx control on lean-burn engines is problematic
and requires special lean NOx catalysts to meet U.S. emissions regulations.
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). Carbon dioxide
produced from fossil fuels is one of the greenhouse gases indicated by the
Intergovernmental Panel on Climate Change (IPCC) to be a "most likely" cause of global
warming. Additionally, the U.S. EPA has stated catalytic converters are a significant
and growing cause of global warming, because of their release of nitrous oxide (N2O), a
greenhouse gas over three hundred 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 Norilsk, Russia, where the industry (among
others) has caused Norilsk to be added to Time magazine's list of most-polluted places.
Because of the external location and the use of valuable precious metals including platinum,
palladium, and rhodium, converters are a target for thieves. The problem is especially common
among late-model Toyota trucks and SUVs, because of their high ground clearance and easily
removed bolt-on catalytic converters. Welded-in converters are also at risk of theft from SUVs
and trucks, as they can be easily removed. Theft removal of the converter can often
inadvertently damage the car's wiring or fuel line resulting in dangerous consequences. Rises in
metal costs in the U.S. during recent years have led to a large increase in theft incidents of the
converter, which can then cost as much as $1,000 to replace.
Various jurisdictions now legislate on-board diagnostics to monitor the function and condition of
the emissions-control system, including the catalytic converter. On-board diagnostic systems
take several forms.
 Temperature sensors
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 750 °C (1,380 °F).
More-recent catalytic-converter designs are not as susceptible to temperature damage and can
withstand sustained temperatures of 900 °C (1,650 °F). 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. For every
1% of CO in the exhaust gas stream, the exhaust gas temperature will rise by 100 °C.
 Oxygen sensors
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 O2 levels. The on-board computer makes
comparisons between the readings of the two sensors. If both sensors show the same output, the
computer recognizes that the catalytic converter is either not functioning or has been removed,
and will operate a "check engine" light and retard 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.
 NOx sensors
NOx sensors are extremely expensive and are generally only used when a compression-ignition
engine is fitted with a selective catalytic-reduction (SCR) converter, or a NOx absorber catalyst
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
Automobile emissions control Exhaust system
Catalysis NOx adsorbers
Cerium(III) oxide Roadway air dispersion modeling
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January 10, 2011.
2. ^ Csere, Csaba (January 1988). "10 Best Engineering Breakthroughs". Car and Driver 33
3. ^ Staff writer (undated). "Eugene Houdry". Chemical Heritage Foundation. Retrieved
January 7, 2011.
4. ^ (registration required) "Carl D. Keith, a Father of the Catalytic Converter, Dies at 88".
The New York Times. November 15, 2008.
5. ^[unreliable source?] Staff writer (undated). "Engelhard Corporation".
referenceforbusiness.com. Retrieved January 7, 2011.
6. ^ Brandt, Erich; Wang, Yanying; Grizzle, Jessy (2000). "Dynamic Modeling of a Three
Way Catalyst for SI Engine Exhaust Emission Control". IEEE Transactions on Control
Systems Technology 8 (5): 767–776. doi:10.1109/87.865850.
7. ^ "Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control
Requirements"PDF (123 KB)
8. ^ "Council Directive 91/441/EEC of 26 June 1991 amending Directive 70/220/EEC".
Official Journal L 242. http://eur-
Retrieved 17 May 2011.
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10. ^ a b Crutsinger, Martin (September 29, 1982). "Kits to Foil Auto Pollution Control Are
Selling Well". The Gainesville Sun.
11. ^ Ullman, Owen (June 14, 1976). "Catalytic Converter Still Controversial after Two
Years of Use". The Bulletin[clarification needed].
12. ^ a b Catalytic Converter Removal – Beat the Law – Import Tuner Magazine.
Importtuner.com (2007-02-26). Retrieved on 2011-01-09.
13. ^ "Some of Us Can Only Afford a Clunker". The Palm Beach Post. February 23, 1996.
14. ^ Tanner, Keith. Mazda MX-5 Miata. p. 120.
15. ^ Catalytic converters, nsls.bnl.gov
16. ^ Wright, Matthew. "What Exactly is a Catalytic Converter? The Science Behind Catlytic
Converters". About.com. http://autorepair.about.com/od/glossary/ss/how-
it_catalyti_3.htm. Retrieved 2009.
17. ^ Le Treut, H; Somerville, R.; Cubasch, U; Ding, Y; Mauritzen, C; Mokssit, A; Peterson,
T.; and Prather, M. (2007) (PDF). Historical Overview of Climate Change Science In:
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the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
(Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.;
and Miller, H.L., editors). Cambridge University Press. pp. 5, 10.
January 18, 2009.
18. ^ Wald, Matthew (May 29, 1998). "Autos' Converters Cut Smog But Add to Global
Warming". The New York Times. http://www.nytimes.com/1998/05/29/us/autos-
19. ^ Walsh, Bryan (undated (circa 2007)). "The World's Most Polluted Places — From
Lead in the Soil to Toxins in the Water and Radioactive Fallout in the Air, The
Blacksmith Institute Has Created a List of the World's Worst Ecological Disaster Areas".
html. Retrieved January 7, 2011.
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21. ^ Murr, Andrew (January 9, 2008). "An Exhausting New Crime — What Thieves Are
Stealing from Today's Cars". Newsweek. Retrieved January 7, 2011.
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— As Precious Metals Prices Soar, Catalytic Converters Are Targets for Thieves".
MSNBC. Retrieved January 7, 2011.
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24. ^ "Settlement Involves Illegal Emission Control 'Defeat Devices' Sold for Autos". June 1,
25. ^ "Check Engine Lights Come On for a Reason". Concord Monitor. January 12, 2003.
 External links
Howstuffworks: "How Catalytic Converters Work"
High Temperature Insulation Wool — Automotive Applications
Keith, C. D., et al., – U.S. Patent 3,441,381 – "Apparatus for purifying exhaust gases of
an internal combustion engine" – April 29, 1969
Lachman, I. M. et al., – U.S. Patent 3,885,977 – "Anisotropic Cordierite Monolith"
(Ceramic substrate) – November 5, 1973
Srinivasan Gopalakrishnan – GB 2397782 – "Process And Synthesizer For Molecular
Engineering Of Materials" – March 13, 2002
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