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CATEGORIES (articles) > Fuel System > Components > Workings of a Catalytic converter

Workings of a Catalytic converter


Catalytic converter on a Saab 9-5.

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 byproducts are converted to less-toxic gases. The catalytic converter was invented at Trinity College (Connecticut).


Functions


Three-way catalytic converters

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: CxHy + nO2 → xCO2 + mH2O
These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running at the stoichiometric point. This is 14.7 parts oxygen to 1 part fuel, by weight, for gasoline (the ratio for propane, 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 NOx 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 are also used on "Large Spark Ignition" engines. 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 NOx+HC emissions from 12 gram/BHP-hour to 3 gram/BHP-hour or less, per the Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHP-hour of NOx+HC emissions is mandated in 2007 (note: NOx is the industry standard short form for nitric oxide (NO) and nitrogen dioxide (NO2) both of which are smog precursors. HC is the industry short form for hydrocarbons).


Two-way catalytic converters

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: CxHy + O2 → xCO2 + mH2O
This type of catalytic converter is commonly used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They also were used on spark ignition (gasoline) engines in 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 what is regulated is "non-methane hydrocarbons" and in other cases the regulated substance is "total hydrocarbons". 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. However since methane is a greenhouse gas, more interest is rising in how to eliminate emissions of it.


Catalyst poisoning

Catalytic converters become ineffective in the presence of lead due to catalyst poisoning. The widespread use of catalytic converters caused the end of leaded gasoline. 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, silicone 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. Silicone poisoning is usually the result of engine damage, such as a faulty cylinder head gasket or cracked casting, admitting silicate-containing coolant into the combustion chamber.

Removal of sulfur from a catalyst surface by running heated exhaust gasses over the catalyst surface is often successful, however removal of lead deposits is often not possible because of its high boiling point. In particularly bad cases of catalyst poisoning by lead, the catalytic converter can actually become completely plugged with lead residue.

Any condition that increases the concentration of CO or HC reaching the catalyst can cause it to overheat and melt down, restricting the exhaust flow, rendering the converter useless for emission control purposes and creating an undercar fire hazard. Some such conditions are oil-burning engines, overly rich fuel mixtures and misfires.


Technical details

The catalytic converter consists of several components:

  1. 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".
  2. The washcoat. In an effort to make converters more efficient, a washcoat is utilized, most often a mixture of silicon and aluminum. 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.
  3. The catalyst itself, 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. Rhodium is the material that makes a three-way reaction possible, while palladium may be used as a substitute for platinum in three-way catalytic converters. Cerium, iron and nickel are also used, though each has its own limitations. Nickel is not legal for use in the European Union (due to nickel hydrate formation). While copper can be used, its use is illegal in North America due to the formation of dioxin.

Gasoline 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 should only be used on engines equipped with closed-loop feedback fuel mixture control employing an oxygen (lambda) sensor. Practically, this means either fuel injection or a carburetor equipped for feedback mixture control. This is because the three-way converter works best when the air-fuel ratio of the engine is kept within a certain very narrow range of the oxygen:fuel stoichiometry point which is 14.7:1 for gasoline. 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). There are also two-way converters available that were used in early carburetored cars.

A three-way catalyst reduces emissions of CO (carbon monoxide), HC (hydrocarbons), and NOx (nitrogen oxides) simultaneously when the oxygen level of the exhaust gas stream is below 1.0%, though performance is best at below 0.5% O2. Unwanted reactions can occur in the three-way catalyst such as the formation of H2S (hydrogen sulfide) and NH3 (ammonia). Formation of each can be limited by modifications to the washcoat/precious metals used. It is, however, difficult to eliminate these side products entirely.

For example, when control of H2S (hydrogen sulfide) emissions are desired, nickel or manganese is added to the washcoat - both substances act to block the adsorbtion of sulfur by the washcoat. H2S 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.

Early three-way catalytic converters utilized an air tube between the first part of the converter (the NOx 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 air injection reactor (AIR) 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 NOx reduction to be complete, but the second section always had oxygen available. These systems also commonly included an upstream air injector, either a modified AIR 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 (NOx) from becoming oxygen loaded and the second catalyst 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.

Recently, systems have used a separate early catalytic converter in the system to reduce startup emissions and burn off the hydrocarbons from the extra-rich mixture used in a cold engine. Also, the other parts are now often separated in the system to provide optimum temperature and provide space for extra oxygen sensors.


Diesel Engines

For compression ignition (i.e., Diesel) engines, the most commonly used catalytic converter is the diesel oxidation catalyst. The catalyst 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% effectiveness, 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 it is necessary to change the exhaust gas - two main technologies are used for this - selective catalytic reduction (SCR) and NOx (NOx) 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). However, DPFs can clog and lose their effectiveness with time and use.


Oxygen storage

In order to oxidize CO and HC, the catalytic converter also has the capability of storing the oxygen from the exhaust gas steam, 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.


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 will come 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.

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 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 oxized 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, 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 jursidiction to jurisdiction, even in the same country. For example in the United States overall responsibility belongs to the United States Environmental Protection Agency (EPA), however due to the special requirements of the State of California emissions in California are regulated by the Air Resources Board, and in Texas the Texas Railroad Commission is responsible for regulating emissions from LPG fueled rich burn engines (but not gasoline fueled rich burn engines).

  • 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 of catalytic converters

Catalytic converters have proven to be reliable devices and have been successful in reducing noxious tailpipe emissions. However, they have two adverse environmental impacts in use (ignoring the pollution caused in their manufacture, which would not exist were they not mandated):

  • The requirement for the engine to run at the stoichiometric point means fuel economy is not as good as that of a "lean burn" engine running at a mixture of 20:1 or weaker. This increases the rate at which fossil fuel resources are consumed and the carbon dioxide emissions of the vehicle.
  • Catalytic converters are estimated to account for 50% of total nitrous oxide (dinitrogen oxide, 'laughing gas') emissions to atmosphere. While N2O 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.
Therefore one conclusion is that catalysts have reduced toxic emissions and the incidence of smog at the expense of increased global warming.


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
  • NOx

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 the 2-Way catalytic converter of 750°Celsius. 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 O2 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. 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 2 rule (though temperature sensors are sometimes used for this). For OBD 2 a second oxygen sensor is fitted after the catalytic converter, and this sensor monitors the O2 levels, and 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

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 (though many SCR systems do not use a NOx 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.




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