What is Catalytic converter

A catalytic converter in an automobile's exhaust system provides an environment for a chemical reaction where unburned hydrocarbons completely combust, using platinum and rhodium as catalysts. Hence the combustion (redox) process continues, but outside the engine's combustion chamber, so no useful energy is extracted. Toxic car gases such as unburned hydrocarbon (UHC) and carbon monoxide (CO) would not exist if the fuel to energy conversion in the engine were perfect.

A three-way catalytic converter has three simultaneous tasks:

1. oxidation of carbon monoxide to carbon dioxi
2. reduction of nitrogen oxides to nitrogen
3. oxidation of hydrocarbons (unburnt fuel) to carbon dioxide and water

These three reactions are most balanced at the stoichiometric point, where there is a balanced amount of oxygen to fuel in the engine. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing conditions. The above two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured. When there is more fuel than oxygen (stoichiometrically), then the engine is running rich. The reduction of NO_x is favoured.

Catalytic converters become ineffective in the presence of lead, and the introduction of catalytic converters triggered the end of leaded gasoline.

Catalytic converters are now standard fit in North America 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 NO_x+HC emissions from 12 gram/BHPhour to 3 gram/BHPhour or less, as per the Environmental Protection Agency (EPA) 2004 regulations. A further drop to 2 gram/BHPhour of NO_x+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). Resourc form Wiki Pedia.

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How Catalytic Converters Work

There are millions of cars on the road in the United States, and each one is potentially a source of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.

To solve those problems, cities, states and the federal government create clean-air laws, and many laws have been enacted that restrict the amount of pollution that cars can produce. To keep up with these laws, automakers have made many refinements to car engines and fuel systems. To help reduce the emissions further, they have developed an interesting device called a catalytic converter, which treats the exhaust before it leaves the car and removes a lot of the pollution.

In this article, you will learn which pollutants are produced by an engine and why, and how a catalytic converter deals with each of these pollutants. Catalytic converters are amazingly simple devices, so it is incredible to see how big an impact they have!

How Catalytic Converters Reduce Pollution

Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive).

There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.

The Reduction Catalyst. The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N₂ + O₂ or 2NO₂ => N₂ + 2O₂

The Oxidization Catalyst. The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O₂ => 2CO₂

The Control System. The third stage is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO. For more infomation about catalytic converters how to work and lots more information.

Fuels and Society C: 2. GM's Decision for the Catalytic Converter

People have been concerned about air pollution for a long time. Even non-industrial cities like ancient Rome had pollution from wood smoke, and coal smoke has been a well known part of the famous London fogs. The switch away from coal to relatively cleaner oil and gas in the 1940s and 50s helped with stationary sources like big power plants in the U.S. and Europe.

But mobile source – automobiles – were also creating smog problems that were noticeable after WWII. The concerns led to the Clean Air Act of 1970 which required 90 percent reductions in auto exhaust. The mandatory reduction was controversial, and had not yet been approved by Congress, when on January 14, 1970, GM president Ed Cole told a Society of Automotive Engineers conference that the “pollution free” car was possible if two conditions were met:

• Cars used a new devices called a catalytic converter;
• Lead was taken out of gasoline

Catalytic converters would greatly reduce carbon monoxide, nitrogen oxides and hydrocarbons (unburned fuel). The converters would do nothing to lower lead emissions, but their use made leaded gasoline impossible since lead would deactivate the main catalytic element, platinum. Unleaded gasoline would be necessary.

There was an irony in Cole’s speech that was widely acknowledged at the time. After all, the company that had created leaded gasoline was now announcing its demise. Aware that this moment might come, GM had pulled out of the leaded gasoline business only a few years before, in 1962, when it sold its interests in Ethyl Corp. to a small paper company in Richmond, Va.

GM’s proactive position on lead and catalytic converters should be seen in the context of the many pressures on the auto industry in the 1960s and 70s. Biting critiques by consumer advocates like Ralph Nader, anti-trust lawsuits by the federal government, Congressional investigations and a growing environmental movement all combined to convince GM that the time had come for change.

Catalytic converters were not introduced to reduce lead, as is sometimes suggested. It was the drive to reduce nitrogen oxides and CO that forced the converter – and ended TEL in U.S. gasoline. Even if there had been no public health issue with lead, the converters would still have needed unleaded gasoline. But new public health research did indicate serious problems and this was used as an added justification for eliminating leaded gasoline.

In 1973, Ethyl Corp. sued the EPA and won a temporary victory when a federal court set aside the leaded gasoline phase-out regulations, saying EPA hadn’t demonstrated that lead was a public health hazard. This ruling was overturned in favor of the EPA in 1976 when a federal appeals court said while lead was not a certain danger, “awaiting certainty will often allow for only reactive not preventive regulation.”

Catalytic converters are now a standard part of a car’s exhaust system. They reduce three main types of emissions: hydrocarbons HC (unburned fuel); carbon monoxide CO; and nitrogen oxides NOx. The converter reduces the nitrogen oxides back to nitrogen and oxygen and oxidizes carbon monoxide and hydrocarbon emissions.

Why do catalytic converters go bad?

There are two ways a converter can fail, it can become clogged or it can be poisoned.

When catalytic converters fail they normally clog up with debris and block the flow of exhaust gas from getting out of the system. This will cause tremendous performance problems. In extreme cases it will prevent the vehicle from starting at all. Most of the time it just limits engine performance by choking the flow through the engine. So how do you check a catalytic converter without removing it from the car?? Sometimes an indication that a converter is clogged is that you don't go any faster when you push the gas pedal down. In addition there usually is a noticeable drop in fuel economy associated with a clogged catalytic converter. A totally clogged converter will cause the engine to die because of the increased backpressure.

There is no way for anyone to actually see a clog in a converter. Usually the only way to tell if a catalytic converter is clogged is to remove it and check the change in engine performance. When a mechanic suspects a clogged converter they may remove the O2 sensor from the exhaust pipe and see if there is a change in performance. A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Some engine oil additives or engine problems that cause the mixture or the temperature of the exhaust gases to change reduce the effectiveness and life of the catalytic converter. Leaded gasoline and the over-use of fuel Additive can shorten the life of a catalytic converter considerably. Even some gasket sealers and cements can poison a converter.

A catalytic converter can also fail because of certain other factors. A number of problems could occur to the catalytic converter as the result of an engine that is out of tune. Any time an engine is operating outside proper specifications, unnecessary wear and damage may be caused to the catalytic converter as well as the engine itself. The damage is often the result of an incorrect air/fuel mixture, incorrect timing, or misfiring spark plugs. Any of these conditions could lead to a catalytic converter failure or worse.

Fouled plugs can cause unburned fuel to overheat the converter and melt the catalyst to a solid mass. If the O2 Sensor is not functioning properly it will give the ECU incorrect readings of exhaust gasses. The faulty sensor can cause an excessively rich or excessively lean condition. If the mixture is too rich, the catalyst can melt down. If the mixture is too lean, the converter is unable to convert the hydrocarbons into safe elements.

Oil or antifreeze entering the exhaust system can block the air passages by creating heavy carbon soot that coats the catalyst. These heavy carbon deposits will cause two problems. First, the carbon deposits prevent the catalytic converter from reducing harmful emission in the exhaust flow. And second, the carbon deposits clog the pores in the ceramic catalyst and block exhaust flow, increasing backpressure and causing heat and exhaust to back up into the engine compartment.

Your engine may actually draw burnt exhaust gasses back into the combustion chamber and dilute the efficiency of the next burn cycle. The result is a loss of power and overheated engine components.

Catalytic converters can be physically damaged as well. The catalyst contained inside a catalytic converter is made from a lightweight, thin-walled, fragile material. It is protected by a dense, insulating mat. This mat holds the catalyst in place and provides moderate protection against damage. Broken support hangers can cause the converter to bounce around and the result can be breakage of the mat. Rocks or other road debris can also hit the converter, causing the internal mat to break also. Off road vehicles often suffer this type of converter failure. Once this mat starts to break up, it will collect in the smaller passages and clog the converter.

The catalytic converter should last the lifetime of the vehicle it is installed in. if it does fail, it is most often a symptom of another problem. This problem must be identified and repaired or the new converter will fail in the same manner. You can keep it running well by keeping the ignition system in top shape and to prevent any unburnt fuel from entering the catalytic converter.

Here is an important safety reminder: Do not park your car over tall grass or piles of dry leaves. Your cars perfectly running catalytic converter gets very hot… enough to start fires! Article by Vincent Ciulla, more tips about catalytic converters maintenance,

How Catalytic Converter Fail

A catalytic converter will rarely fail without a problem or malfunction occurring somewhere in the emission system in front of the converter. It is important to determine what caused the converter to fail, so that the problem can be fixed and to prevent a recurrence of the failure.

Converter Meltdown. This is an example of a converter meltdown. The converter was super-heated due to a raw fuel condition in the exhaust flow. The excess unburned fuel ignited when it struck the hot ceramic catalyst and drove the temperature far above the normal operating condition of the converter. The ceramic catalyst is unable to with stand the extremely high temperature and begins to melt. The ceramic collapses and the converter is destroyed. The melted ceramic may block the exhaust flow and cause additional damage to the engine. A converter glowing red-hot or evidence of heat discoloration confirm this situation.

The too-rich condition that led to this converter meltdown could be the result of a number of malfunctions including faulty oxygen sensor, an incorrect fuel mixture, worn spark plugs or plug wires, a faulty check valve, incorrect ignition timing, sticking float, faulty fuel injectors, a failed fuel pressure regulator or other ignition malfunctions.

An oxygen sensor failure can lead to incorrect readings of exhaust gasses. The faulty sensor can cause a too-rich or too-lean condition. Too rich and the catalyst can melt down. Too lean and the converter is unable to convert the hydrocarbons into safe elements and may not pass a state inspection.

Carbon Deposits. This is an example of a converter with carbon deposits in the ceramic catalyst. This is usually a result of oil or antifreeze entering the exhaust system or a too-rich fuel mixture. The heavy carbon deposit clogs the converter and reduces exhaust flow. This increases backpressure and causes the entire exhaust system to heat up. The heat backs up into the engine and causes additional problems.

These heavy carbon deposits create two problems. First, the carbon deposits prevent the catalytic converter from reducing harmful emission in the exhaust flow. And second, the carbon deposits clog the pores in the ceramic catalyst and block exhaust flow, increasing backpressure and causing heat and exhaust to back up into the engine compartment. Your engine may actually draw burnt exhaust gasses back into the combustion chamber and dilute the efficiency of the next burn cycle. The result is a loss of power and over-heated engine components. Possible causes are worn piston rings, faulty valve seals, failed gaskets or warped engine components.

Mild carbon deposits may do less damage to engine components but it may seriously affect the converter's ability to reduce harmful emissions. It could easily cause a vehicle to fail an emission test.

Catalyst Fracture. This is an example of a catalyst fracture. The ceramic became loose, cracked and began to break down. There is evidence of partial meltdown in this example due to overheating.

The ceramic catalyst inside a catalytic converter is made from a lightweight, thin-walled, fragile material. However, rock or road debris striking the converter or improper or broken exhaust system support can cause a catalyst fracture. Once the ceramic catalyst is fractured, the broken pieces become loose and rattle around and break up into smaller pieces. Flow is interrupted and backpressure in the exhaust system increases. This leads to heat build up and loss of power. Possible causes of a catalyst fracture are road debris striking the converter, loose or broken muffler hangers, potholes, jumping a curb or off-road driving.

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Motor Vehicle Emission Controls: Technologies

The number of vehicles on the roads is continually increasing - between 1970 and 1995 the number of vehicles almost doubled. It now stands at 29 million. With a large rise in traffic numbers, it becomes increasingly important to keep pollutant emissions to a minimum. There are presently a number of ways in which road traffic pollution can be reduced, including the use of technological solutions.

Catalytic Converters

Since January 1993, all new cars sold in the European Union (EU) have been fitted with a catalytic converter (CAT). This is made up of a very thin layer of platinum group metals on a honeycomb structure. The surface area of a typical 3-way CAT covers the equivalent of two football pitches. As exhaust gasses pass through the catalyst a chemical reaction occurs which converts carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) to less harmful compounds (water, nitrogen and carbon dioxide).

To work most effectively, a catalytic converter needs to reach an optimum temperature. It may not reach this in a short journey. Devises to pre-warm the catalyst are being developed which improve the overall performance of catalytic converters. The use of catalytic converters leads to a dramatic reduction in the emissions of CO, HC and NOx. However, they also result in an increase in CO2 emissions, which do not cause a problem for urban air quality, but may contribute to global warming. The efficiency of a CAT can be as high as 90%.

Conclusion

Whilst technical fixes, such as those outlined above, may provide cleaner air for the next 10 - 15 years, they do not represent a long term solution to transport related urban air pollution. They need to be combined with management schemes to reduce traffic in city centres, education to encourage the public to use their cars less, and the further development of alternative fuels that are not harmful to the environment.