The catalytic converter is placed upstream of the muffler, close to the engine, and it changes environmentally damaging pollutants in the exhaust stream into more benign gases. Under extreme heat, precious metals in a honeycomb structure catalyze the chemical reactions; they make today’s vehicles dramatically cleaner than older ones.
Catalytic converters can be both causes as well as victims of check engine lights. A failing “cat” can trigger a check engine light and/or produce an unpleasant exhaust odour, but converter failure is often caused due to abnormal combustion and rough engine operation, sometimes accompanied by a blinking check engine light. Due to its precious metals content, converters are expensive to replace and often targeted by thieves.
Cutaway of metal-core converter
Ceramic core converter
The catalytic converter’s construction is as follows:The catalyst support or substrate.
- The catalyst support or substrate.
The catalyst support or substrate for automotive catalytic converters, the core is generally a ceramic monolith that has a honeycomb structure (commonly square, not hexagonal). (Prior to the mid-1980s, the catalyst material was deposited on a packed bed of pellets, mostly in early GM applications.) Metallic foil monoliths made of Kanthal (FeCrAl) are used where particularly high heat resistance is required. The substrate is structured to design a large surface area. The cordierite ceramic substrate used in most of the catalytic converters 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 year, 2002.
- The wash coat.
A wash coat is a carrier for the catalytic materials and is used for dispersing the materials over a large surface area. Aluminium oxide, titanium dioxide, silicon dioxide, or a mixture of silica and alumina can be used for this. The catalytic materials are suspended in the wash coat to apply to the core. Wash coat materials are selected to form a rough, irregular surface, which largely increases the surface area as compared to the smooth surface of the bare substrate. This in turn increases and maximises the catalytically active surface available to react with the engine exhaust. The coat must retain its surface area and prevent sintering of the catalytic metal particles even during high temperatures (1000 °C).
- Ceria or ceria-zirconia.
These oxides are mostly added as oxygen storage promoters.
- The catalyst itself is mostly a mix of precious metal. Platinum is the most active catalyst and is widely used, but is not for all applications because of unwanted additional reactions and high cost. Palladium and rhodium are two other precious metals which are used. Rhodium is used as a reduction catalyst, palladium is used as an oxidation catalyst, and platinum is used for both reduction and oxidation. Cerium, iron, manganese, and nickel are used too, although each has limitations. Nickel is not legal to be used in the European Union because of its reaction with carbon monoxide into toxic nickel tetracarbonyl. Copper can be used everywhere except for Japan.
Upon failure, a catalytic converter can be recycled into a scrap. The precious metals inside the converter, including platinum, palladium, and rhodium, are all extracted.
Placement of catalytic converters
Catalytic converters need a temperature of 800 degrees Fahrenheit (426 °C) to efficiently convert harmful exhaust gases into inert gases, such as carbon dioxide and water vapor. Therefore, the first catalytic converters were placed close by the engine, to ensure fast heating. However, such placement can cause a lot of problems. One of these will be vapour lock
As an alternative, catalytic converters were moved towards a third of the way back from the engine, and were then placed underneath the vehicle.
A two-way (or “oxidation”, sometimes called an “oxi-cat”) catalytic converter has two simultaneous jobs:
- Oxidation of carbon monoxide to carbon dioxide: 2 CO + O2 → 2 CO2
- Oxidation of hydrocarbons (unburnt and partially burned fuel) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O (a combustion reaction)
This type of catalytic converter is mostly used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were used on gasoline engines in American- and Canadian-market automobiles too until 1981. Because of their inability to control oxides of nitrogen, they had to be superseded by three-way converters.
Three-way catalytic converters (TWC) have an additional advantage of controlling the emission of nitric oxide (NO) and nitrogen dioxide (NO2) (both together abbreviated with NOxand not to be confused with nitrous oxide (N2O)), which are precursors to acid rain as well as smog.
Since 1981, “three-way” (oxidation-reduction) catalytic converters have been used in vehicle emission control systems in countries like the United States and Canada; many other countries have also adopted stringent vehicle emission regulations that in effect need three-way converters on gasoline-powered vehicles. The reduction and oxidation catalysts are mostly contained in a common housing; however, in some instances, they might be housed separately. A three-way catalytic converter has three simultaneous jobs:
Reduction of nitrogen oxides to nitrogen (N2)
- 2 CO + 2 NO → 2 CO2 + N2
- hydrocarbon + NO → CO2 + H2O + N2
- 2 H2 + 2 NO → 2 H2O + N2
Oxidation of carbon monoxide to carbon dioxide
- 2 CO + O2 → 2 CO2
Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water, in addition towards the above NO reaction
- hydrocarbon + O2 → H2O + CO2
These three reactions occur most efficiently when the catalytic converter gets exhaust from an engine running slightly above the stoichiometric point. For gasoline combustion, this ratio is 14.6- 14.8 parts air to one part fuel, by weight. The ratio for auto gas (or liquefied petroleum gas LPG), natural gas, and ethanol fuels is slightly different for each, needing modified fuel system settings when using those fuels. In general, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system using one or more than one oxygen sensors, though early in the deployment of three-way converters, carburetors which are equipped with feedback mixture control were used.
Three-way converters are effective when the engine is functioned within a narrow band of air-fuel ratios near the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen). Conversion efficiency falls quite rapidly when the engine is operated outside of this band. Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NOx is not favoured. Under rich conditions, the excess fuel consumes the available oxygen prior to the catalyst, leaving only oxygen stored in the catalyst available for the function of oxidation.
Closed-loop systems are important for effective operation of three-way catalytic converters because of the continuous balancing required for effective NOx reduction and HC oxidation. The control system must stop the NOx reduction catalyst from becoming fully oxidized, yet replenish the oxygen storage material so that its function as an oxidation catalyst is maintained.
Three-way catalytic converters can store oxygen from the exhaust gas stream, generally when the air–fuel ratio goes lean. When there is no sufficient oxygen available from the exhaust stream, the stored oxygen is released and consumed. A lack of sufficient oxygen occurs when oxygen derived from NOx reduction is unavailable or when certain manoeuvres such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.
Unwanted reactions can take place in the three-way catalyst, such as the formation of odoriferous hydrogen Sulphide and ammonia. Forming of each can be limited through modifications to the wash coat and precious metals used. It is difficult to completely eliminate these by-products. Sulphur-free or low-Sulphur fuels eliminate or reduce the hydrogen sulphide.
For instance, when control of hydrogen-Sulphide emissions is desired, nickel or manganese is added to the wash coat. Both substances act in order to block the absorption of sulphur by the wash coat. Hydrogen Sulphide forms when the wash coat has absorbed Sulphur during a low-temperature part of the operating cycle, which is then released during the high-temperature part of the cycle takes place and the Sulphur combines with HC.
For compression-ignition (i.e., diesel) engines, the mostly used catalytic converter is the diesel oxidation catalyst (DOC). DOCs contain palladium, platinum, and aluminium oxide, all of which catalytically oxidize the hydrocarbons and carbon monoxide with oxygen to make carbon dioxide and water.
2 CO + O2 → 2 CO2
CxH2x+2 + [(3x+1)/2] O2 → x CO2 + (x+1) H2O
These converters usually operate at 90 percent efficiency, virtually eliminating diesel odour and helping reduce visible particulates (soot). These catalysts are not active for NOxreduction because any reductant present would first be reacting with the high concentration of O2 in diesel exhaust gas.
Reduction in NOx emissions from compression-ignition engines has earlier been addressed by the addition of exhaust gas to incoming air charge, called exhaust gas recirculation (EGR). In 2010, most light-duty diesel manufacturers in the U.S. added catalytic systems to their vehicles in order to meet new federal emissions requirements. There are two techniques that are developed for the catalytic reduction of NOx emissions under lean exhaust conditions: selective catalytic reduction (SCR) and lean NOx trap or NOxadsorber. Instead of precious metal-containing NOx absorbers, most manufacturers selected base-metal SCR systems that make use of a reagent such as ammonia to reduce the NOx into nitrogen. Ammonia is supplied to the catalyst system through the injection of urea into the exhaust, which then undergoes thermal decomposition and hydrolysis into ammonia. One trademark product of urea solution, also called as Diesel Exhaust Fluid (DEF), is AdBlue.
Diesel exhaust has relatively high levels of particulate matter (soot), consisting largely of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction, so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). Historically, a DPF has a cordierite or silicon carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. Contemporary DPFs can be manufactured through a variety of rare metals that provide a superior performance (at a greater expense). As the amount of soot trapped on the DPF increases, so will the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are needed to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration might also be limited to prevent extreme exotherms from damaging the trap during regeneration. In the U.S., all on-road light, medium and heavy-duty vehicles powered by diesel and built after January 1, 2007, must meet diesel particulate emission limits, which means that they effectively have to be equipped with a two-way catalytic converter and a diesel particulate filter. Note that this applies only to the diesel engine. As long as the engine was manufactured before January 1, 2007, the vehicle doesn’t need to have the DPF system. This led to an inventory run up by engine manufacturers in late 2006 so they could continue the selling of pre-DPF vehicles well into 2007. During the re-generation cycle, most systems need the engine to consume more fuel in a relatively short amount of time in order to generate the high temperatures necessary for the cycle to complete. This adversely affects the overall fuel economy of vehicles having DPF systems, especially in vehicles that are driven mostly in city conditions where frequent acceleration needs a larger amount of fuel to be burned and therefore has to be more soot to collect in the exhaust system.
Lean-burn spark-ignition engines
For lean-burn spark-ignition engines, an oxidation catalyst is used just in the same manner as in a diesel engine. Emissions from lean burn spark ignition engines are quite similar to emissions from a diesel compression ignition engine.
Catalyst poisoning takes place when the catalytic converter is exposed to exhaust containing substances that coat the working surfaces, so that they cannot contact and react with the exhaust. The most notable contaminant is lead, so vehicles equipped with catalytic converters can run their engine only on unleaded fuel. Other common catalyst poisons include sulphur, manganese (originating 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 one of the other catalyst contaminant. Although phosphorus is no longer used in gasoline, it (and zinc, another low-level catalyst contaminant) was widely used until recently in engine oil anti-wear additives such as zinc dithiophosphate (ZDDP). Beginning in the year 2004, a limit of phosphorus concentration in engine oils was adopted in the API SM and ILSAC GF-4 specifications.
Depending on the contaminant, catalyst poisoning can be reversed sometimes by running the engine under a very heavy load for an extended period of time. The increased exhaust temperature can occasionally vaporize or sublimate the contaminant, removing it from the catalytic surface. However, removal of lead deposits in this manner is generally not possible because of lead’s high boiling point.
Any condition that causes abnormally high levels of unburned hydrocarbons—raw or partially burnt fuel—to reach the converter will significantly elevate its temperature, bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and causing severe exhaust restriction. Usually the upstream components of the exhaust system (manifold/header assembly and associated clamps susceptible to rust/corrosion and/or fatigue for instance, the exhaust manifold splintering after repeated heat cycling), ignition system for e.g. coil packs and/or primary ignition components (e.g. distributor cap, wires, ignition coil and spark plugs) and/or damaged fuel system components (fuel injectors, fuel pressure regulator, and associated sensors) – since 2006 ethanol has been used mostly with fuel blends where fuel system components which are not ethanol compatible can damage a catalytic converter – this also includes use of a thicker oil viscosity not recommended by the manufacturer (especially with ZDDP content – this includes “high mileage” blends regardless if its conventional or synthetic oil), oil and/or coolant leaks (e.g. blown head gasket inclusive of engine overheating). Vehicles equipped with OBD-II diagnostic systems are made to alert the driver to a misfire condition by means of illuminating the “check engine” light on the dashboard, or flashing it if the current misfire conditions are severe enough to potentially damage the catalytic converter.
Emissions regulations varies considerably from jurisdiction to jurisdiction. Most of the automobile spark-ignition engines in North America have been fitted with catalytic converters since 1975, and the technology used in non-automotive applications is generally based on automotive technology.
Regulations for diesel engines are similarly varied, with some jurisdictions focusing towards NOx (nitric oxide and nitrogen dioxide) emissions and others focusing on particulate (soot) emissions. This regulatory diversity is quite challenging for manufacturers of engines, as it may not be economical to design an engine to meet two sets of regulations.
Regulations of fuel quality varies across jurisdictions. In North America, Europe, Japan, and Hong Kong, gasoline and diesel fuel are highly regulated, and compressed natural gas and LPG (autogas) are being reviewed for the purpose of regulation. In most part of Asia and Africa, the regulations are often lax: in some places Sulphur content of the fuel can go upto 20,000 parts per million (2%). Any Sulphur in the fuel can be oxidized to SO2 (Sulphur dioxide) or even SO3 (Sulphur trioxide) contained in the combustion chamber. If Sulphur passes over a catalyst, it may be further oxidized in the catalyst, i.e., SO2 might be further oxidized to SO3. Sulphur oxides are precursors to Sulphuric acid, a major component of acid rain. While it is possible to add substances such as vanadium to the catalyst wash coat to fight Sulphur-oxide formation, such addition will reduce the effectiveness of the catalyst. The most effective solution is to further refine fuel at the refinery to make ultra-low-Sulphur diesel. Regulations in countries like Japan, Europe, and North America tightly restrict the amount of Sulphur permitted in motor fuels. However, the direct financial expense of producing such clean fuel might make it impractical for use in developing countries. Because of this, cities in these countries with high levels of vehicular traffic suffer from acid rain, which damages stone and woodwork of buildings, poisons humans and other animals, and damages the city’s local ecosystems, at a very high financial cost.
Vehicles having catalytic converters emit most of their total pollution during the first five minutes of engine operation; for instance, before the catalytic converter has warmed up sufficiently to be fully effective.
In 1995, Alpina introduced an electrically heated catalyst known as “E-KAT,” it was used in Alpina’s B12 5,7 E-KAT based on the BMW 750i. Heating coils in the catalytic converter assemblies are electrified just after the engine is started, bringing the catalyst up to operating temperature very quickly to qualify the vehicle for low emission vehicle (LEV) designation. BMW later on introduced the same heated catalyst, developed jointly by Emitec, Alpina, and BMW, in its 750i in 1999.
Some vehicles contain a pre-cat, a small catalytic converter upstream of the main catalytic converter which heats up faster on vehicle start up, decreasing the emissions associated with cold starts. A pre-cat is mostly used by an auto manufacturer when trying to attain the Ultra Low Emissions Vehicle (ULEV) rating, such as on the Toyota MR2 Roadster.
Catalytic converters have proven to be reliable as well as effective in reducing noxious tailpipe emissions. However, they also have some shortcomings in their use, and also adverse environmental impacts in production:
- An engine havig a three-way catalyst must run at the stoichiometric point, which means more fuel is consumed as compared to a lean-burn engine. This means approximately 10% more of CO2 emissions from the vehicle.
- Catalytic converter production needs palladium or platinum; part of the world supply of these precious metals is produced near Norilsk, Russia, where the industry (among others) has caused Norilsk to be added to Time magazine’s list of most-polluted places.
- Pieces of catalytic converters, and the extreme heat of the converters themselves, can lead to wildfires, especially in dry areas
How does it work?
In a catalytic converter, the catalyst (in the form of platinum and palladium) is coated onto a ceramic honeycomb or ceramic beads that are placed in a muffler-like package attached to the exhaust pipe. The catalyst allows conversion of carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide as well as water. It also converts the nitrogen oxides back into nitrogen and into oxygen.
Causes of Catalytic Converter Failure
There are two ways in which a converter can fail:
- It can become clogged.
- It can become poisoned.
There actually is no “inspection port” for the consumer or mechanic to see an actual clog in a converter. Usually, the only way to tell if a catalytic converter is malfunctioning (plugged) is to remove it and check the change in engine’s performance. When a clogged converter is suspected, some mechanics are temporarily remove the O2 sensor from the exhaust pipe ahead of the catalytic converter and look for a change in performance.
A catalytic converter depends on receiving the proper mix of exhaust gases at the proper temperature. Any additives or malfunctions that lead to 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 certain fuel additives can decrease the life of a catalytic converter.
A catalytic converter can also fail because of:
- Bad exhaust valves on the engine
- Fouled plugs causing unburned fuel to overheat the converter
Sometimes you can tell that a converter is clogged because you don’t go any faster when you’re pushing the gas pedal. Also, there generally is a noticeable drop in gas mileage associated with a clogged catalytic converter. A partially clogged converter mostly acts like an engine governor, limiting the actual RPMs to a fast idle. A totally clogged converter makes the engine to quit after a few minutes because of all the increased exhaust back pressure.
How to Replace a Catalytic Converter
The catalytic converter, which is responsible for the cleaning up of a vehicle exhaust, is a key component of a vehicle emission control system. When this piece malfunctions, the car creates additional emissions, run more roughly, and have reduced fuel efficiency. While the cost of replacing a catalytic converter can be expensive, you can save money through doing it yourself with just a few hand tools and jack stands.
1.Park in a level place and jack the vehicle at all four wheels and support on jack stands.
Replacing your vehicle’s catalytic converter isn’t like replacing a tire — you’ll need to raise the entire vehicle off of the ground, rather than just one corner. It’s important to find a level spot to do this maintenance on your car. If your car is not stable, you will have a risk of serious injury or death if your jacks fail.
- If you have access to a professional-quality hydraulic lift and know how to use it safely, this is also an acceptable way to lift your car when replacing the catalytic converter.
2.Allow the vehicle’s exhaust to cool down.
If your vehicle hasn’t had an opportunity to cool down after running, its exhaust system can be hot. To reduce the risk of painful burns, give your vehicle a chance to properly cool down before working on it. Depending on your vehicle’s exhaust system, this will mostly be a matter of just a few minutes.
- To test the heat of the exhaust system, just put on a pair of heavy mechanic’s gloves and gently brush the exhaust tube with the back of your hand. If you can’t feel any heat, you might cautiously repeat this test without the glove.
3.Locate the catalytic converter.
Slide under the vehicle and find the tubes of the exhaust system, which should run all the way to your vehicle’s rear exhaust. The converter should not be terribly difficult to find — it will usually take the form of a rectangular or rounded “box” in the middle of your exhaust system. Some models can have a cylindrical shape.
- Check to see if the converter is bolted or welded to the rest of the exhaust system at its connection points. You might need to take it to an auto shop to have it repaired if it has already been replaced and welded back into position, rather than bolted. You can still replace a welded converter if you have the needed access to a sawzall (or similar tool) and a welding machine and know how to safely use both, but these advanced tools are beyond the mastery of most of the amateur mechanics.
4.Remove the O2 (oxygen) sensor from the catalytic converter.
Most modern catalytic converters are equipped with one or more oxygen sensors that monitor the efficiency of the car’s exhaust system continuously. If your catalytic converter has an attached oxygen sensor, then use an oxygen sensor socket and a ratchet wrench to disconnect it before proceeding.
- When you’re done, move the sensor out of your way so that it doesn’t interfere with the rest of the process.
5.If bolted, apply penetrating oil to the bolts.
Catalytic converters that are bolted in can occasionally have bolts that are rusted, partially-corroded, or “jammed” into their nuts. These bolts can be very tricky while being removed, so loosen them up by applying a penetrating oil (available from most auto stores). Allow the oil to seep into the bolts and lubricate them for several minutes before attempting to remove them.
6.Remove the bolts at the back end first, then in the front.
Using an appropriately-sized wrench, begin loosening all of the bolts before you start removing them. Once all of the bolts are loosened (but still attached), just remove the “rear” bolts (those closest to the open end of the vehicle’s exhaust) before removing the “front” ones (those further away). Remove the converter after you’re done. You might have to support the exhaust once the converter is removed.
7.Alternatively, for welded-in converters, cut the converter out.
If your converter is welded into the rest of the exhaust system, rather than bolted in, the only way to remove it is to cut it out physically of the pipes it’s connected to. Most mechanics will use a sawzall or similar tool to do this. Cut along (or near) the existing weld lines and then remove the converter after it’s cut free.
- If you finish and the converter doesn’t seem to budge, you may want to use a hammer to knock it out of its place as long as you take care not to damage or rattle any other parts of the exhaust system (this can lead to harmful exhaust leaks down the road).
1.Always defer to any included instructions.
The instructions given in this article are written for general cases of catalytic converter installation. Because the exact part needed and the installation process can vary from vehicle to vehicle, the steps you’ll be requiring to replace your vehicle’s converter may be different than the ones here. When in doubt, always follow the instructions provided with your replacement part or consult the advice of a good mechanic. Converters have a specific direction in which they should flow and have an arrow for direction of exhaust flow.
2.Insert any gaskets supplied with the new catalytic converter.
Some converters, especially bolt-installed ones, will come with small and round gaskets that sit in the pipes connected to the converter to give the converter a snugger, more secure fit. If your replacement converter came with these gaskets, put them according to any provided instructions before proceeding.
3.Put the new catalytic converter in place.
Next, hold the catalytic converter in the position where it has to be installed. Double check to make sure it is pointed in the right direction (there should be an arrow indicating this) and that the correct side is facing downwards.
- Since it’s quite tricky to work on the converter with one hand while you hold it in place with another, for the next few steps, it can be useful to enlist a willing friend to hold the converter in place while you’re working or using a stand to hold it up in place.
4.Finger-tighten nuts on the bolts.
If your vehicle’s catalytic converter was bolted in and your replacement converter has bolt holes that match exactly to your exhaust system’s holes, installation is usually a cinch. To start, re-insert your bolts and use your hands to tighten them. This makes it easier to get all of the bolts to align correctly because the looseness will give you a small degree of “wiggle room” to make minor adjustments as needed.
5.Tighten down all the bolts.
Starting on the “front” end of the converter (the end further away from the vehicle’s exhaust), tighten the bolts with a suitable sized wrench. Proceed to the back end when you are finished tightening the bolts on the front.
- You would want your bolts to be very tight. Most exhaust leaks are caused by loose bolts, so making sure your bolts are extra tight can save you headaches in the future.
6.Alternatively, weld the converter into place.
If you need to weld your converter into place, the process is somewhat even more involved. You’ll need a professional-grade welding machine (such as a MIG welder) and the proper training and expertise needed to use one safely (or a friend who has these things). Don’t attempt to weld your converter back into place if you’re not a competent welder — you could damage your vehicle or might even hurt yourself.
- Weld your converter into place by carefully joining it to the
exhaust system pipes at either of the ends. Be sure to make a secure, air-tight
seal at each weld. If the pipes aren’t wide enough, you might need to heat them
up and flare them to make them fit. If your pipes don’t quite reach one end of
your converter, you might need to weld an additional extender pipe in.
Sometimes you have to do a partial weld then lower the exhaust just to finish
the top part of the weld.
- Be sure to allow your welds to cool to a safe temperature before you proceed.
7.Screw the oxygen sensor back into place.
If you originally removed one or more than one oxygen sensors to access your converter, replace them now. As you do, make sure that the attached wiring is secure and is not frayed or damaged — this can lead to inaccurate readings and even false “check engine” lights.
8.Double-check your work.
At this point, if you’ve done everything properly, you’re basically done. One last time, take the opportunity to make sure that the catalytic converter is connected correctly and there are no gaps or leaks at either of the connections or the oxygen sensor. If you bolted your converter in, make sure all of your bolts well tight. If you welded it in, make sure that your welds are sturdy and airtight.