Catalytic converter
Catalytic converter on a Dodge Ram Van.
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 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: 2NOx → xO2 + N2
- Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
- Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
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 NOx is favoured, at the expense of CO and HC oxidation. If an engine could be operated with infinitesimally small oscillations[citation needed] about the 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" (LSI) engines since 2001 in California, and from 2004 in the other 49 states.[citation needed] LSI engines are used in forklifts, aerial boom lifts, ice resurfacing machines and construction equipment. The converters used in those types of machines are three-way types, and are designed to reduce combined NOx+HC emissions from 12 gram/BHP-hour to 3 gram/BHP-hour or less, as mandated by the United States Environmental Protection Agency's (EPA) 2004 regulations.[citation needed] A further drop to 2 gram/BHP-hour of NOx+HC emissions is mandated in 2007[citation needed] (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). The EPA intends to introduce emissions rules for stationary spark ignition engines, to take effect in January 2008.[1]
Two-way catalytic converters
A two-way catalytic converter has two simultaneous tasks:
- Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2
- Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
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 up until 1981, when they were replaced by three-way converters due to regulatory changes requiring reductions on NOx emissions. Reduction of the NOx emissions requires an additional step. Platinum catalysis can be used.
Instead of catalysis, for diesel engines a true reactant, ammonia pyrolyzed in situ from urea, can be used to reduce the NOx into nitrogen, see AdBlue.
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 not toxic, but 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, interest is rising in how to eliminate emissions of it.
Catalyst poisoning and deactivation
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. This is because if a 'leaded' fuel (a fuel with lead content) is burnt there will be a residue of lead which can damage the catalytic converter. Other common catalyst poisons include manganese primarily from the gasoline additive MMT, and silicon 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 sublime the catalytic 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. In particularly bad cases of catalytic lead poisoning, the catalytic converter may actually become completely plugged with lead residue[citation needed].
Any condition that causes abnormally high levels of unburned hydrocarbons 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.
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.
Conventional 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 converts the three main pollutants of concern — CO, HC, and NOx— to less-toxic substances. The control of NOx involves a reduction process that releases oxygen and the control of CO and HC involves an oxidation process that consumes oxygen. Therefore, a 3-way converter contains two catalyst-coated stages: The first catalyst stage encountered by the exhaust is for reduction of NOx, which produces oxygen employed by the second stage to oxidize CO and HC. 3-way converters work most efficiently with exhaust from engines operated on a stoichiometric air-fuel mixture. Generally, such engines are equipped with closed-loop feedback fuel mixture control employing one or more oxygen (lambda) sensors. While a 3-way catalyst can be used in an open-loop system, NOx reduction efficiency is low. Since NOx emissions are now regulated throughout the world, open-loop fuel systems are obsolete in many jurisdictions. Closed-loop maintenance of the stoichiometric air-fuel ratio is most often attained by means of an engine management system with computer-controlled fuel injection, though early in the deployment of 3-way converters, carburetors equipped for feedback mixture control were used during the transition to fuel injection. Within a narrow ratio band surrounding stoichiometry, conversion of all three pollutants is very complete, sometimes approaching 100%. However, outside of that band, conversion efficiency falls off very rapidly. Two-way (or oxidation) converters act only to control CO and HC, and have therefore been abandoned on conventional spark ignition engines in most jurisdictions due to an inability to control NOx.
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, such as the formation of H2S (hydrogen sulfide) and NH3 (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 H2S (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. 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.
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.
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 either an air pump or by an aspirator. 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.
Most 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.
[ 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). 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 United States Environmental Protection Agency (EPA) regulation.[2]
[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 NOx 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 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.
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 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, 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 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).
Criticisms
Environmental impact
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.[citation needed]
- 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, with significant negative environmental effects.
Catalytic converter theft
Due to the exterior 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 with a battery powered reciprocating saw.[3] [4]The saw removal of the converter can often inadvertently damage the car’s wiring or fuel line resulting in dangerous consequences. Rises in metal costs inside the United States during recent years have lead to a large increase in theft incidents of the converter, encouraged in part by a resale price for converters that approaches two hundred dollars.[5]
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 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 two-way catalytic converter. Note that modern catalytic converters are not as susceptible to temperature damage with many modern three-way platinum based converters able to handle temperatures of 900°C sustained.[citation needed] 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.[citation needed]
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 a delay before this happens - usually five 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 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.
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 O2 levels. The on-board computer makes comparisons between 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 leaded racing fuel in an on-road vehicle.
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 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.
