----
A 'catalytic converter' (colloquially, "cat" or "catcon") is a device used to reduce the toxicity of emissions from an
internal combustion engine. First widely introduced on
series-production automobiles in the
US market for the
1975 model year to comply with tightening
EPA regulations on auto exhaust, 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, and other engine-equipped machines. A catalytic converter provides an environment for a
chemical reaction wherein toxic combustion by-products are converted to less-toxic substances.
Functions
Three-way catalytic converters
A three-way catalytic converter has three simultaneous tasks:
#
Reduction of
nitrogen oxides to
nitrogen and
oxygen: 2NO
x → xO
2 + N
2
#
Oxidation of (toxic)
carbon monoxide to harmless
carbon dioxide: 2CO + O
2 → 2CO
2
# Oxidation of unburnt carcinogenic
hydrocarbons (HC) to carbon dioxide and
water: 2C
xH
y + (2x+y/2)O
2 → 2xCO
2 + yH
2O
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the
stoichiometric point. This is between 14.8 and 14.9 parts air to 1 part fuel, by weight, for
gasoline (the ratio for
LPG,
natural gas and
ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels). When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NO
x is favoured, at the expense of CO and HC oxidation. If an engine could be held at the strict stoichiometric point for the fuel used, it is theoretically possible to reach 100% conversion efficiencies.
Since 1981, three-way catalytic converters have been at the heart of vehicle emission control systems in North American roadgoing vehicles, and have been used on "Large
Spark Ignition" engines since 2001 in California, and from 2004 in the other 49 states. LSI engines are used in
forklifts, aerial boom lifts, ice resurfacing machines and construction equipment. The converters used in these are three-way types designed to reduce combined NO
x+HC
emissions from 12
gram/
BHP-hour to 3 gram/BHP-hour or less, per the
United States Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHP-hour of NO
x+HC emissions is mandated in 2007 (note: NO
x is the industry standard short form for
nitric oxide (NO) and
nitrogen dioxide (NO
2) both of which are smog precursors. HC is the industry short form for hydrocarbons). The EPA intends to introduce emissions rules for stationary Spark Ignition engines, to take effect in January 2008.
Two-way catalytic converters
A two-way catalytic converter has two simultaneous tasks:
#
Oxidation of
carbon monoxide to
carbon dioxide: 2CO + O
2 → 2CO
2
# Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and
water: 2C
xH
y + (2x+y/2)O
2 → 2xCO
2 + yH
2O
This type of catalytic converter is widely used on
diesel engines to reduce
hydrocarbon and
carbon monoxide emissions. They also were used on spark ignition (
gasoline) engines in USA market automobiles up until 1981, when they were replaced by three-way converters due to regulatory changes requiring reductions on
NOx emissions.
Curiously, the regulations regarding hydrocarbons vary according to the engine regulated, as well as the jurisdiction. In some cases, "non-
methane hydrocarbons" are regulated, while in other cases, "total hydrocarbons" are regulated. Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is more difficult to break down in a catalytic converter, so in effect a "non-methane hydrocarbon" standard can be considered to be looser. Since methane is a
greenhouse gas, more interest is rising in how to eliminate emissions of it.
Catalyst poisoning and deactivation
Catalytic converters become ineffective in the presence of
lead due to
catalyst poisoning. Therefore, vehicles equipped with catalytic converters must only be run on unleaded gasoline, and it is this fact, as much as concerns about the possibly harmful effects of lead emissions, which caused the end of pump-available
leaded gasoline in countries where catalytic converters have been in common use for many years. Leaded "race only" fuel is still used for non-catalyst vehicles in some countries.
Catalyst poisoning occurs when a substance in the engine exhaust coats the surface of the catalyst, preventing further exhaust access to the catalytic materials. Poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time to raise exhaust gas temperature, which may cause liquefaction or sublimation of the catalyst poison. Common catalyst poisons are lead,
sulfur,
zinc,
manganese,
silicon and
phosphorus.
Zinc, phosphorus and sulfur originate from
lubricant antiwear additives such as
ZDDP; sulfur and manganese primarily originate from fuel impurities or from additives such as
Methylcyclopentadienyl Manganese Tricarbonyl (MMT), respectively.
Silicon poisoning in automotive applications is the result of engine damage, such as a faulty
cylinder head gasket or cracked casting, admitting silicate-containing
coolant into the combustion chamber. In stationary engines silicon poisoning is more often caused by the use of methane
landfill gas as a fuel.
Removal of sulfur from a catalyst surface by running heated exhaust gases over the catalyst surface is often successful; however, removal of lead deposits in this manner is usually not possible because of lead's high boiling point. In particularly bad cases of catalyst poisoning by lead, the catalytic converter can actually become completely plugged with lead residue.
A variety of conditions may cause the catalyst to overheat (heat deactivation) and potentially to melt down. Some factors that can cause this are:
★ lubricating oil in the exhaust system (caused by engine wear, or by damaged rings or valves)
★ an engine misfire or ignition failure (causing unburnt fuel to enter the exhaust)
★ a cracked exhaust valve (again, causing unburnt fuel in the exhaust)
Overly rich fuel mixtures are not usually a problem - there is too little unused oxygen for the exothermic reaction to be large enough to cause damage. A slightly lean of stoichiometric mix is far more dangerous, as the oxygen level is elevated, allowing a very large exotherm, and many engine manufacturers design "rich excursions" as a catalyst protection measure in the engine control software. In the early days of
catalyst-equipped cars, (primarily in the USA) before the advent of sophisticated
engine management systems, it was necessary for fuel/air mixtures to be significantly richer than had hitherto been the case to allow the catalyst to work effectively. This contributed to the very poor fuel consumption figures achieved by such cars.
Engine misfires can overheat and destroy the converter as the excessive amounts of unburned fuel are broken down within it, especially when the engine is under heavy loads. Vehicles equipped with
OBD-II diagnostic systems are designed to alert the driver of a misfire condition, along with other malfunctions, using the Malfunction Indicator Lamp or "Check Engine" light. If the misfire and engine load can produce heating severe enough to cause catalyst damage, the MIL will flash until the misfire or engine load is reduced.
Technical details
Cross section of a Metal-core Converter
The catalytic converter consists of several components:
# The core, or substrate. In modern catalytic converters, this is most often a
ceramic honeycomb, however
stainless steel foil honeycombs are also used. The purpose of the core is to "support the catalyst" and therefore it is often called a "catalyst support". The ceramic substrate 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.
# The washcoat. In an effort to make converters more efficient, a washcoat is utilized, most often a mixture of
silica and
alumina. The washcoat, when added to the core, forms a rough, irregular surface which has a far greater surface area than the flat core surfaces, which is desirable to give the converter core a larger surface area, and therefore more places for active precious metal sites. The catalyst is added to the washcoat (in suspension) before application to the core.
# The catalyst itself is most often a precious metal.
Platinum is the most active catalyst and is widely used. However, it is not suitable for all applications because of unwanted additional reactions and/or cost.
Palladium and
rhodium are two other precious metals that are used. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidization catalyst.
Cerium,
iron,
manganese and
nickel are also used, though each has its own limitations. Nickel is not legal for use in the European Union (
due to reaction with carbon monoxide). While
copper can be used, its use is illegal in North America due to the formation of
dioxin.
Rich burn spark ignition engines
Catalytic converters are used on spark ignition (
gasoline;
liquified petroleum gas (LPG);
flexible fuel vehicles burning varying blends of
E85 and gasoline;
compressed natural gas (CNG)) engines; and compression ignition (
diesel) engines.
For spark ignition engines, the most commonly used catalytic converter is the three-way converter, which works best used on engines equipped with
closed-loop feedback fuel mixture control employing one or more
oxygen (lambda) sensor. While a 3-way catalyst can be used in an open-loop system (and has been for years in the non-road engine market), NO
x conversions tend to be less than stellar - and since World emissions regulations are primarily aimed at NO
x reduction, open loop fuel systems are obsolete in many jurisdictions. Many engines (including Ford, General Motors, Subaru, Nissan, etc.) go open loop under heavy load. This has two effects - the extra fuel cools (and lubricates) the combustion chamber, which extends engine life, while cooling the exhaust gases which extends catalytic converter life. To keep the air fuel ratio at stoichiometric (14.7:1 for gasoline), closed loop fuel systems are either
fuel injection or a
carburetor equipped for feedback mixture control. Within that band, conversions are very high, sometimes approaching 100%. However, outside of that band, conversions tend to fall off very rapidly (see
bell curve). Two-way converters have been abandoned on rich burn spark ignition engines in most jurisdictions, due to an inability to control NO
x.
A three-way catalyst reduces emissions of CO (
carbon monoxide), HC (
hydrocarbons), and NO
x (
nitrogen oxides) simultaneously when the oxygen level of the exhaust gas stream is below 1.0%, though performance is best at below 0.5% O
2. Unwanted reactions, such as the formation of H
2S (
hydrogen sulfide) and NH
3 (
ammonia), can occur in the three-way catalyst. Formation of each can be limited by modifications to the washcoat and precious metals used. It is, however, difficult to eliminate these side products entirely.
For example, when control of H
2S (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. H
2S 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 "lean burn" spark ignition engines (e.g. compressed natural gas, or compressed natural gas with diesel fuel pilot injection), an oxidation catalyst is used in the same manner as in a compression ignition engine.
Recently, many systems have used a pre-catalyst in the system to reduce startup emissions and burn off hydrocarbons from the extra-rich mixture used in a cold engine. Upstream and downstream parts are now often separated in the system to provide an optimum temperature and space for extra oxygen sensors. The converter needs to be placed close enough to the engine to quickly reach operating temperature but far enough away to avoid heat damage.
Early three-way catalytic converters utilized an air tube between the first part of the converter (the NO
x part) and the second part, which is virtually unchanged from earlier two-way catalytic converters. This tube was fed by either an air pump (derived from the earlier
A.I.R. systems) or by a
Pulse Air system. The extra oxygen was used to offset the less precise control of earlier systems by providing the oxygen for the catalyst's oxidizing reaction. The first section was still prone to difficulties on lean conditions with too much oxygen for the NO
x reduction to be complete, but the second section always had oxygen available. These systems also commonly included an upstream air injector, either a modified A.I.R. system or another opening in the manifold, to add oxygen into the system to burn the extra-rich mixture used in a cold engine and to allow the additional burning to happen as close to the converter as possible to heat it up to operating temperature quickly.
Newer systems use several techniques to avoid the air tubes. They provide a constantly varying mixture that quickly cycles lean and rich mixtures to keep the first catalyst (NO
x reduction) from becoming oxygen loaded and the second catalyst (CO oxidization) sufficiently oxidized, which is less of a concern due to the oxygen created in the first section. They also utilize several oxygen sensors to monitor the exhaust, at least one before the catalytic converter for each bank of cylinders, and one after the converter. Newer systems also often have several units mounted along the pipe to provide different functions rather than one monolithic system.
Diesel engines
For compression ignition (i.e.,
Diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst. The catalyst uses excess O
2 (oxygen) in the exhaust gas stream to oxidize CO (Carbon Monoxide) to CO
2 (Carbon Dioxide) and HC (hydrocarbons) to H
2O (water) and CO
2. These converters often reach 90% effectiveness, virtually eliminating diesel odor and helping to reduce visible
particulates (
soot), however they are incapable of reducing NO
x as chemical reactions always occur in the simplest possible way, and the existing O
2 in the exhaust gas stream would react first.
To reduce NO
x on a compression ignition engine it is necessary to change the exhaust gas - two main technologies are used for this -
selective catalytic reduction (SCR) and NOx (NO
x) traps (or
NOx Adsorbers).
Another issue for diesel engines is particulate (soot). This can be controlled by a soot trap or
diesel particulate filter (DPF), as catalytic converters are unable to affect elemental carbon (however they will remove up to 90% of the soluble organic fraction). A clogging soot filter creates a lot of back pressure decreasing engine performance. However, once clogged, the filter goes through a regeneration cycle where diesel fuel is injected directly into the exhaust stream and the soot is burned off. After the soot has been burned off the regeneration cycle stops and injection of diesel fuel stops. This regeneration cycle should not affect performance of the engine.
All major diesel engine manufacturers in the USA (Ford, Caterpillar, Cummins, Volvo, MMC) starting January 1, 2007 are required to have a catalytic converter and a soot filter inline, as per new EPA legislation. http://www.epa.gov/otaq/highway-diesel/regs/2007-heavy-duty-highway.htm
Oxygen storage in three-way converters
In order to oxidize CO and HC, the catalytic converter also has the capability of storing the 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. This happens either when oxygen derived from NO
x reduction is unavailable or certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to compensate.
Note that diesel catalysts do not use this feature as there is sufficient O2 in the exhaust gas stream to handle the CO & HC reductions needed.
Regulations
Emissions regulations vary considerably from jurisdiction to jurisdiction, as do what engines are regulated. In
North America any spark ignition engine of over 19
kW (25
hp) power output built later than
January 1,
2004 probably has a three-way catalytic converter installed. In
Japan a similar set of regulations came into effect
January 1,
2007, while the
European Union has not yet enacted analogous regulations. 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.
Diesel engine regulations are similarly varied, with some jurisdictions focusing on NO
x (Nitric Oxide and Nitrogen Dioxide) emissions and others focusing on particulate (soot) emissions. This can cause problems for the engine manufacturers as it may not be economical to design an engine to meet two sets of regulations.
Note that no jurisdiction has specific legislation mandating the use of catalytic converters, however with spark ignition engines a catalytic converter is usually the only practical way to meet regulatory requirements.
An important issue is that fuel quality varies widely from place to place, even within jurisdictions, as do the regulations covering fuel quality. In North America, Europe, Japan, and Hong Kong both gasoline and diesel fuel are highly regulated and there are campaigns under way to regulate CNG and LPG as well. In most of Asia and Africa this is not true - in some places
sulfur content of the fuel can reach 20,000 parts per million (2%). Any sulfur in the fuel may be oxidized to SO
2 (
sulfur dioxide) or even SO
3 (
sulfur trioxide) in the
combustion chamber. If sulfur passes over a catalyst it may be further oxidized in the catalyst, i.e. (SO
2 may be further oxidized to SO
3). Sulfur oxides are precursors to
sulfuric acid, a major component of
acid rain. While it is possible to add substances like
vanadium to the catalyst wash coat to combat sulfur oxide formation, this will reduce the effectiveness of the catalyst—the best solution is further refinement of the fuel at the refinery to remove the sulfur. Regulations in Japan, Europe and —by 2007— North America tightly restrict the amount of sulfur permitted in motor fuels. However, the expense is such that this is not practical in many developing countries. As a result cities in these countries with high levels of vehicular traffic suffer damage to buildings due to acid rain eating away the stone/woodwork, and acid rain has deleterious effects on the local ecosystem.
Regulatory agencies
The agencies charged with regulating engine emissions vary from jurisdiction to jurisdiction, even in the same country. For example, in the
United States, overall responsibility belongs to the
United States Environmental Protection Agency (EPA), but due to special requirements of the
State of California, emissions in California are regulated by the
Air Resources Board. In Texas, the
Texas Railroad Commission is responsible for regulating emissions from
LPG fueled rich burn engines (but not gasoline fueled rich burn engines).
★
California Air Resources Board - California, United States (most sources)
★
Environment Canada - Canada (most sources)
★
Environmental Protection Agency - United States (most sources)
★
Texas Railroad Commission - Texas, United States (LPG fueled engines only)
★
Transport Canada - Canada (trains and ships)
Criticisms
Catalytic converters have proven to be reliable devices and have been successful in reducing noxious tailpipe emissions. However, they may have some adverse environmental impacts in use:
★ The requirement for a rich burn engine to run at the
stoichiometric point means it uses more fuel than a "
lean burn" engine running at a mixture of 20:1 or less. This increases the amount of
fossil fuel consumed and the carbon dioxide emissions of the vehicle. However NOx control on lean burn engines is problematic at best, and many lean burn engine manufacturers are considering rich burn variations. Another solution is to increase the amount of biofuels used - if 100% biofuel was used the engines would be CO
2 neutral.
★ Catalytic converters are "estimated" to account for 50% of total
nitrous oxide (dinitrogen oxide, 'laughing gas') emissions to atmosphere. While N
2O emissions in these concentrations are not harmful to human health, it is a potent greenhouse gas, accounting for around 7% of the overall greenhouse effect despite its small concentration in the atmosphere. The California Air Resources Board is investigating this issue, and will introduce legislation to address it if necessary.
★ The manufacturing of catalytic converters requires
palladium and/or
platinum; a portion of the world supply of these
precious metals is produced near the Russian city of
Norilsk (about 15%), with significant negative environmental effects due to the lack of environmental protection legislation.
[1]
It can be argued that catalytic converters have reduced toxic emissions and
smog at the expense of increased
greenhouse gases, however anyone making this argument should consider the California Air Resources Board reports on improvements in Air Quality that have been achieved over the last 30 years.
Catalytic converter theft
Due to the use of precious metals including
platinum, which is worth up to $1,200 an ounce;
palladium, which can fetch $320 an ounce; and
rhodium, which can go for up to $6,000 an ounce on the market, catalytic converter theft is on the rise. (Note, however, that the loading of precious metals in a converter is low, and seldom over $50 per converter at 2007 spot prices). The problem is especially common among mid-90s to present
Toyota sport-utility vehicles and trucks, due to their high ground clearance and bolt on catalytic converters, which are easy to remove. Welded-in converters are also at risk of theft from SUVs and trucks, for they can be removed within five minutes by means of a battery powered
reciprocating saw.
[1],
Diagnostics
Various jurisdictions now legislate on-board diagnostics to monitor the effectiveness of the emissions control system, including the catalytic converter and such diagnostics are often included in aftermarket
retrofit kits as a matter of course, even if legislation does not directly require them.
On-board diagnostics take several forms, depending upon the legislation and the type of emissions control product being monitored, the three main types are:
★ temperature
★ oxygen
★ NO
x
Temperature sensors
Temperature
sensors are used for two purposes. The first is as a warning system, typically on obsolete 2-Way catalytic converters such as are still sometimes used on LPG forklifts. The function of the sensor is to warn of temperature excursions above the safe operating temperature of 750°
Celsius of the 2-Way catalytic converter. Note that modern catalytic converters are not as susceptible to temperature damage, many modern 3-Way platinum based converters can handle temperatures of 900°C sustained, while many modern 3-way palladium based converters can handle temperatures of 925°C sustained. Temperature sensors are also used to monitor catalyst functioning - usually two sensors will be fitted, 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 or "lambda sensor" is the basis of the
closed loop control system on a spark ignited rich burn engine, however it is also used for diagnostics. Oxygen sensors only work when at operating temperature, when they output a voltage based on the O
2 level in the exhaust gas to the computer. Typically a single wire oxygen sensor will take 3-5 minutes to reach operating temperature. The more expensive heated sensors (3 to 5 wires) can reach operating temperature in 1 minute.
The simplest sort of diagnostic an oxygen sensor can perform is related to the closed loop control system. If the system makes a change to the air-fuel ratio based on oxygen sensor readings, and the readings do not change the sensor will light an indicator on the instrument panel warning the operator that there is a problem with the vehicle. There is always a delay before this happens, usually 5 minutes of engine operation. Most systems do not store the state, so turning off the engine and turning it back on will reset the system, and if the error is transient (i.e. fuel filter is partially blocked) the light will not come back on. However, if the problem is recurring the light will come on as soon as the sensor reaches operating temperature and a manufacturer-defined driving pattern known as a drive-trace is completed. Until this procedure has finished, the diagnostic computer will set a parameter called a readiness monitor to "unready". The readiness monitor system was implemented in order to ensure that diagnostic computers would not falsely report working emissions systems in vehicles whose computer's error memory had recently been cleared. Such diagnostics have been factory fitted to automobiles since 1985 in North America and factory fitted to off-road Spark Ignition engines since 2004 (however such systems have been available as retrofit kits for off-road SI engines since 1997).
The second sort of diagnostic is more complex and is a result of the California
OBD-II rule (though temperature sensors are sometimes used for this). In vehicles with OBD-II, a second oxygen sensor is fitted after the catalytic converter to monitor the O
2 levels. The on-board computer makes comparisons to the readings of the two sensors. If both sensors give the same output, the catalytic converter is non-functioning, and must be replaced. It will also spot less serious damage to a catalytic converter, such as the use of racing fuel in an on-road vehicle. Lead is still legal in racing fuel, and use of as little as half a tank of leaded fuel will cause enough damage for the computer to notice, and warn the operator that the converter is not functioning properly.
NOx sensors
NO
x sensors are extremely expensive and are generally only used when a compression ignition engine is fitted with a Selective Catalytic Reduction Converter, or a NO
x Adsorber Catalyst in a feedback system (though many SCR systems do not use a NO
x sensor, but instead rely on the engine map being programmed into the Engine Control Unit or computer). 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 is post catalyst. They are utilized for the same reasons, and in the same manner as an Oxygen Sensor - the only difference is the substance being monitored.
See also
★
Exhaust system
★
Automobile emissions control
★
Diesel particulate filter
★
SCR
★
NOx Adsorbers
★
Roadway air dispersion modeling
★
Catalysis
Notes
1. [2]
External links
★
Johnson Matthey plc - The Inventors of the Catalytic Converter
★
Howstuffworks: "How Catalytic Converters Work"
★
Source for Electronic Catalytic Convertor Info
★
Detecting Clogged Converter
Patents
★ Keith, C. D., et al., -- -- ''"Apparatus for purifying exhaust gases of an internal combustion engine" -- April 29, 1969
★ Lachman, I. M. et al., -- -- ''"Anisotropic Cordierite Monolith" (Ceramic substrate) -- November 5, 1973''
★ Srinivasan Gopalakrishnan -- -- ''"Process And Synthesizer For Molecular Engineering Of Materials" -- March 13, 2002''
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