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Catalytic Converter

Catalytic Converter

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 U.S. 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.

History

The catalytic converter was invented by Eugene Houdry, a French mechanical engineer and expert in catalytic oil refining who lived in the United States. About 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 tetra-ethyl lead was eliminated from most gasoline over environmental concerns, as the agent would "poison" the converter by forming a coating on the catalyst's surface, effectively disabling it.

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.

Construction

The catalytic converter consists of several components:

  1. The core, or substrate. The core is often a ceramic honeycomb in modern catalytic converters, but stainless steel foil honeycombs are used, too. The honey-comb surface increases the amount of surface area available to support the catalyst, and therefore 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.
  2. The washcoat. A washcoat is used to make converters more efficient, often as 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 do, which then gives 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 being applied to the core.
  3. The catalyst itself is most often a precious metal. Platinum is the most active catalyst and is widely used. It is not suitable for all applications, however, because of unwanted additional reactions and/or cost. Palladium and rhodium are two other precious metals 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, although 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.

 


Ceramic-core converter


Metal-core converter

 

 

Types

Two-way

A two-way catalytic converter has two simultaneous tasks:

  1. Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
  2. Oxidation of unburnt 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 spark ignition (gasoline) engines in USA market automobiles through 1981, when the two-way converter's inability to control NOx led to its supersession by three-way converters.

Three-way

Since 1981, three-way catalytic converters have been used in vehicle emission control systems in North America and many other countries on roadgoing 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 is between 14.6 and 14.8 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. Generally, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system employing one or more oxygen sensors, though early in the deployment of 3-way converters, carburetors equipped for feedback mixture control were used. While a 3-way catalyst can be used in an open-loop system, NOx reduction efficiency is low. Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is nearly complete. However, outside of that band, conversion efficiency falls off very rapidly. 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 NOx is favoured, at the expense of CO and HC oxidation.

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. This happens 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 compensate.

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.

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. This uses excess O2 (oxygen) in the exhaust gas stream to oxidize CO (carbon monoxide) to CO2 (carbon dioxide) and HC (hydrocarbons) to H2O (water) and CO2. These converters often reach 90% efficiency, virtually eliminating diesel odor and helping to reduce visible particulates (soot), however they are incapable of reducing NOx as chemical reactions always occur in the simplest possible way, and the existing O2 in the exhaust gas stream would react first.

To reduce NOx on a compression ignition engine, the chemical composition of the exhaust must first be changed. Two main techniques are used: selective catalytic reduction (SCR) and NOx (NOx) traps (or NOx Absorbers).

Diesel engine 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 will remove up to 90% of the soluble organic fraction, so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). In the United States, all on-road heavy-duty vehicles powered by diesel and built after 1 January 2007 must be equipped with a catalytic converter and a diesel particulate filter.

Instead of catalysis, a reagent such as ammonia pyrolyzed in situ from urea, is sometimes used to reduce the NOx into nitrogen.

For lean-burn engines

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

Installation

Many vehicles have a pre-catalyst located close to the engine's exhaust manifold. This 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 in a cold engine.

Many three-way catalytic converters utilize an air injection tube between the first (NOx reduction) and second (HC and CO oxidation) biscuits of the converter. This tube is fed by a secondary air injection system. The injected air provides oxygen for the catalyst's oxidizing reaction. These systems also sometimes include an upstream air injector to admit oxygen to the exhaust system before it reaches the catalytic converter. This precleans the extra-rich exhaust from a cold engine, and helps bring the catalytic converter quickly up to operating temperature.

Some newer systems do not employ air injection. Instead, they provide a constantly varying mixture that quickly and continually cycles between lean and rich to keep the first catalyst (NOx reduction) from becoming oxygen loaded, and to keep the second catalyst (CO oxidization) sufficiently oxygen-saturated. 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. Some systems contain the reduction and oxidation functions separately rather than in a common housing.

Damage

Poisoning

Catalyst poisoning occurs when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, encapsulating the catalyst so that it cannot contact and treat the exhaust. The most notable contaminant is lead, so vehicles equipped with catalytic converters can only be run on unleaded gasoline. Other common catalyst poisons include manganese 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 ZDDP. Beginning in 2006, a rapid phaseout of ZDDP in engine oils was begun.

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 sublimate the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is usually not possible due to lead's high boiling point.

Meltdown

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

Emissions regulations vary considerably from jurisdiction to jurisdiction. In North America 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 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 NOx (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.

Regulations of fuel quality vary across jurisdictions. In North America, Europe, Japan, and Hong Kong, gasoline and diesel fuel are highly regulated, and CNG and LPG are being reviewed for regulation, too. 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 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 like vanadium to the catalyst wash coat 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 make it not practical 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 produces deleterious effects on the local ecosystem.

Negative aspects

Some early converter designs created a great deal of restriction to 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 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

Many Catalytic converters have a long warm-up time, of up to thirty minutes.

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 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.
  • Although catalytic converters are effective at removing hydrocarbons and other harmful emissions, most of exhaust gas leaving the engine through a catalytic converter is carbon dioxide (CO2), 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. Environmental Protection Agency (EPA) has stated catalytic converters are a significant and growing cause of global warming, due to their release of nitrous oxide (N2O), a greenhouse gas over 300 times more potent than carbon dioxide.
  • Catalytic converter production requires palladium and/or platinum; part of the world supply of these precious metals is produced near the Russian city of Norilsk, where the industry (among others) has caused Norilsk to be added to Time Magazine's list of most polluted places.

Theft

Due to 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, due to 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 United States during recent years have led to a large increase in theft incidents of the converter, which can then cost as much as $1000 to replace.

Diagnostics

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 2-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 give the same output, the computer recognizes the catalytic converter is not functioning or removed, and will operate a 'check engine' light and retard engine performance. Simple "oxygen sensor simulators" have been developed to circumvent this by simulating the change across the catalytic converter with plans and pre-assembled devices available on the internet, though these are not legal for on-road use. 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 converter, or a NOx adsorber 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 utilized for the same reasons and in the same manner as an oxygen sensor - the only difference is the substance being monitored.

SAMPLE SUBMISSION REFERENCE - http://en.wikipedia.org/wiki/Catalytic_converter