Catalytic "Converter"...What does it convert?
04-04-2002, 03:41 PM
#3
Just something off the top of my head here. Bear with the generalizations and vague overviews.
How Your Catalytic Converter Works
There are millions of cars on the road that are potential sources of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.
To help solve air pollution problems, cities, states and the federal government have created clean-air laws. Many laws have been enacted to restrict the amount of pollution that automobiles produce. To keep up with these laws, automakers have made many refinements to automotive engines and fuel systems. In a major effort to reduce automobile emissions further, automakers 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.
Pollutants Produced by a Car Engine
In order to reduce emissions, modern cars have been designed to carefully control the amount of fuel they burn. The goal is to keep the air-to-fuel ratio very close to the "stoichiometric" point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. [For gasoline, the stoichiometric ratio is about 14.7 to 1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be "lean" (an air-to-fuel ratio higher than 14.7); and other times the mixture can be "rich" (an air-to-fuel ratio lower than 14.7).]
The main emissions of a car engine are:
· Nitrogen gas: Air is 78 percent nitrogen gas, and most of this passes right through the car engine.
· Carbon Dioxide: This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.
· Water vapor: This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.
These emissions are mostly benign (although carbon dioxide emissions are believed to contribute to global warming). But because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines:
· Carbon monoxide: A poisonous gas that is colorless and odorless.
· Hydrocarbons or volatile organic compounds (VOC's): Produced mostly from unburned fuel that evaporates. Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground level ozone, a major component of smog.
· Oxides of nitrogen: Contributes to smog and acid rain, and also causes irritation to human mucus membranes.
These are the three main regulated emissions, and also the ones that catalytic converters are designed to reduce.
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, VOC's and nitrogen oxide molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization 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 the 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 nitrogen oxide emissions. When such molecules come in contact with 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.
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.
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. 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 insure that the engine is running at close to the stoichiometric point, while also making sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and carbon monoxide.
Other Ways to Reduce Pollution
The catalytic converter does a great job of reducing pollution, but there is room for improvement. One of the catalytic converter's biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.
One simple solution to this problem is to move the catalytic converter closer to the engine. This means that hotter exhaust gases reach the converter and it heats up faster, but this may also reduce the life of the converter by exposing it to extremely high temperatures. Most carmakers position the converter under the front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm it.
Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people won't wait a few minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.
How Your Catalytic Converter Works
There are millions of cars on the road that are potential sources of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.
To help solve air pollution problems, cities, states and the federal government have created clean-air laws. Many laws have been enacted to restrict the amount of pollution that automobiles produce. To keep up with these laws, automakers have made many refinements to automotive engines and fuel systems. In a major effort to reduce automobile emissions further, automakers 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.
Pollutants Produced by a Car Engine
In order to reduce emissions, modern cars have been designed to carefully control the amount of fuel they burn. The goal is to keep the air-to-fuel ratio very close to the "stoichiometric" point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. [For gasoline, the stoichiometric ratio is about 14.7 to 1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be "lean" (an air-to-fuel ratio higher than 14.7); and other times the mixture can be "rich" (an air-to-fuel ratio lower than 14.7).]
The main emissions of a car engine are:
· Nitrogen gas: Air is 78 percent nitrogen gas, and most of this passes right through the car engine.
· Carbon Dioxide: This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.
· Water vapor: This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.
These emissions are mostly benign (although carbon dioxide emissions are believed to contribute to global warming). But because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines:
· Carbon monoxide: A poisonous gas that is colorless and odorless.
· Hydrocarbons or volatile organic compounds (VOC's): Produced mostly from unburned fuel that evaporates. Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground level ozone, a major component of smog.
· Oxides of nitrogen: Contributes to smog and acid rain, and also causes irritation to human mucus membranes.
These are the three main regulated emissions, and also the ones that catalytic converters are designed to reduce.
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, VOC's and nitrogen oxide molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization 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 the 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 nitrogen oxide emissions. When such molecules come in contact with 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.
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.
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. 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 insure that the engine is running at close to the stoichiometric point, while also making sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and carbon monoxide.
Other Ways to Reduce Pollution
The catalytic converter does a great job of reducing pollution, but there is room for improvement. One of the catalytic converter's biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.
One simple solution to this problem is to move the catalytic converter closer to the engine. This means that hotter exhaust gases reach the converter and it heats up faster, but this may also reduce the life of the converter by exposing it to extremely high temperatures. Most carmakers position the converter under the front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm it.
Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people won't wait a few minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.
04-04-2002, 03:49 PM
#6
Greg, I quite don't know how to break it to you. If I said I did type it I would be lying. If I said I didn't type it, I'm obligated to tell you where I got the information. I will leave the above for you to ponder
Originally posted by Greg's2kGLE
Thanks Jeff. Damn, did you type all that out?
Thanks Jeff. Damn, did you type all that out?
04-04-2002, 03:53 PM
#7
Senior Member
Thread Starter
Join Date: Mar 2001
Posts: 3,373
Originally posted by Jeff92se
Greg, I quite don't know how to break it to you. If I said I did type it I would be lying. If I said I didn't type it, I'm obligated to tell you where I got the information. I will leave the above for you to ponder
Greg, I quite don't know how to break it to you. If I said I did type it I would be lying. If I said I didn't type it, I'm obligated to tell you where I got the information. I will leave the above for you to ponder
04-04-2002, 04:34 PM
#11
What does a catalytic converter do?
Here are the reactions that the catalytic converter catalyzes:
Reduction: NO2 --> N2 + O2
Oxidation: CO + 1/2 O2 --> CO2
Hydrocarbons + O2 --> H2O + CO2
The catalyst is usually a mixture of Aluminum, Silica and Copper Oxides. The oxidation catalyst can be that, as well as Cerium Oxide. The oxides form a crystalline structure that allows oxygen to either be trapped inside, or released by it (since O molecules are part of the single cell). Thus, the reduction catalyst is first, and the oxygen it releases is used by the oxidation catalyst to create CO2 and H2O from CO and HC's. I am a chemical engineer, and one of the professors in our department (who I am working for) is a nationally-known expert in the field of automotive catalysis. She is currently working with the DOT, DOE and GM motors to build a better, cheper catalyst. She is also working with the Army to build a better catalyst for the water-gas shift reaction. Here is her website if anyone wants to check it out.
http://ase.tufts.edu/chemical/facult...hanopoulos.asp
Reduction: NO2 --> N2 + O2
Oxidation: CO + 1/2 O2 --> CO2
Hydrocarbons + O2 --> H2O + CO2
The catalyst is usually a mixture of Aluminum, Silica and Copper Oxides. The oxidation catalyst can be that, as well as Cerium Oxide. The oxides form a crystalline structure that allows oxygen to either be trapped inside, or released by it (since O molecules are part of the single cell). Thus, the reduction catalyst is first, and the oxygen it releases is used by the oxidation catalyst to create CO2 and H2O from CO and HC's. I am a chemical engineer, and one of the professors in our department (who I am working for) is a nationally-known expert in the field of automotive catalysis. She is currently working with the DOT, DOE and GM motors to build a better, cheper catalyst. She is also working with the Army to build a better catalyst for the water-gas shift reaction. Here is her website if anyone wants to check it out.
http://ase.tufts.edu/chemical/facult...hanopoulos.asp
04-04-2002, 07:27 PM
#12
Senior Member
Join Date: Apr 2001
Posts: 689
Originally posted by Jeff92se
Copy / paste. But the question is not how but where hehe. Greg, it took a total of about 20 seconds on Yahoo search to find it. That was my point. hehe
Copy / paste. But the question is not how but where
hehe. Greg, it took a total of about 20 seconds on Yahoo search to find it. That was my point. hehe
04-04-2002, 07:41 PM
#13
Here's something more simple:
A catalyst speeds up a reation. In other words, during cold starts where you engine pollutes more, a car without the catalyst will emit pollutants longer than an engine with. It speeds up the reaction and it creates heat, making the engine warm up quicker, which reduces emissions.
A catalyst speeds up a reation. In other words, during cold starts where you engine pollutes more, a car without the catalyst will emit pollutants longer than an engine with. It speeds up the reaction and it creates heat, making the engine warm up quicker, which reduces emissions.
04-04-2002, 11:53 PM
#15
Originally posted by ScreamingVQ
Here's something more simple:
A catalyst speeds up a reation. In other words, during cold starts where you engine pollutes more, a car without the catalyst will emit pollutants longer than an engine with. It speeds up the reaction and it creates heat, making the engine warm up quicker, which reduces emissions.
Here's something more simple:
A catalyst speeds up a reation. In other words, during cold starts where you engine pollutes more, a car without the catalyst will emit pollutants longer than an engine with. It speeds up the reaction and it creates heat, making the engine warm up quicker, which reduces emissions.
04-05-2002, 08:36 AM
#16
Senior Member
Join Date: Jul 2001
Posts: 7,530
04-05-2002, 09:30 AM
#18
Originally posted by Mizeree_X
The catalyst does NOT heat the engine at all. It is so downstream from the combustion chamber and the heat of reaction is much less than that of the heat of combustion. The cat is heated by the hot exhaust gases coming out of the engine. Also, to clarify, when the car is initially started, the cat is cold and so the kinetics are slower, thus causing a lower conversion of NOx and CO to N2 and CO2. The proper operating temperature for a cat is 350C, and this is when the efficiency of the converter approaches 100% for the reactions is catalyzes. Reducing emissions has little to do with the engine temperature -- it is dependent on the air/fuel ratio. When the car is cold the ECU is in a fuel-rich mode. This combined with a cold cat temperature cause 80% of all hydrocarbon release to be in the first 2 minutes of driving. To alleviate this, companies (Nissan included) put pre-cats further towards the engine block -- the reasoning being that these pre-cats would heat to 350C much faster and so convert pollutants much quicker than the standard cat. Then, when the main cat was heated, it would perform most of the catalytic duty. This is why you need to drive your car with an aftermarket y-pipe installed for 20 min to pass emissions -- no pre-cats means low catalytic activity until the cat has warmed up. Hope this clears up any confusion. If anyone wants a good article on this, I can send one.
The catalyst does NOT heat the engine at all. It is so downstream from the combustion chamber and the heat of reaction is much less than that of the heat of combustion. The cat is heated by the hot exhaust gases coming out of the engine. Also, to clarify, when the car is initially started, the cat is cold and so the kinetics are slower, thus causing a lower conversion of NOx and CO to N2 and CO2. The proper operating temperature for a cat is 350C, and this is when the efficiency of the converter approaches 100% for the reactions is catalyzes. Reducing emissions has little to do with the engine temperature -- it is dependent on the air/fuel ratio. When the car is cold the ECU is in a fuel-rich mode. This combined with a cold cat temperature cause 80% of all hydrocarbon release to be in the first 2 minutes of driving. To alleviate this, companies (Nissan included) put pre-cats further towards the engine block -- the reasoning being that these pre-cats would heat to 350C much faster and so convert pollutants much quicker than the standard cat. Then, when the main cat was heated, it would perform most of the catalytic duty. This is why you need to drive your car with an aftermarket y-pipe installed for 20 min to pass emissions -- no pre-cats means low catalytic activity until the cat has warmed up. Hope this clears up any confusion. If anyone wants a good article on this, I can send one.
I had a feeling something was wrong with what I said.
06-07-2002, 01:12 PM
#20
Donating Maxima.org Member
Join Date: Aug 2001
Posts: 2,267
Originally posted by Nick97SE
What happens when you add a high-flow Catalytic Converter (like an RT Cat)? Does this let more pollution flow through the exhaust?
Thanks,
-Nick
What happens when you add a high-flow Catalytic Converter (like an RT Cat)? Does this let more pollution flow through the exhaust?
Thanks,
-Nick
Anyone going to answer Nick's question?
06-07-2002, 01:16 PM
#21
Donating Maxima.org Member
Join Date: Aug 2001
Posts: 2,267
Originally posted by Jeff92se
Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people won't wait a few minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.
Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people won't wait a few minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly.
Are there actually any products available to preheat the catalytic converter? I want to swap the y-pipe but also keep emissions down. If I can achieve that by preheating the catalytic converter, I'd be very interested.
Thanks,
06-07-2002, 02:02 PM
#22
Member
Join Date: Apr 2002
Posts: 139
Yes, the RT cat is just a good (at least they claim to be)
Originally posted by Nick97SE
What happens when you add a high-flow Catalytic Converter (like an RT Cat)? Does this let more pollution flow through the exhaust?
-Nick
What happens when you add a high-flow Catalytic Converter (like an RT Cat)? Does this let more pollution flow through the exhaust?
-Nick
In theory a high flow converter can be just as efficient (pollution wize) as normal OEM models. They usually do require more complex cores (higher honey comb densities) or more exotic catalysts (rhodium, pallidium, etc) to do it though and thus they cost more to manufacture. That's why they aren't used as OEM equipment. They may also have shorter lifespans due to thinner core walls but this is usualy only an issue if your engine is poorly maintained.
Random Techs website has some good information about this at:
http://www.randomtechnology.com/technical.html
06-07-2002, 09:16 PM
#23
Re: Yes, the RT cat is just a good (at least they claim to be)
Originally posted by Myrv
All replacement Catalytic converters must meet certain EPA standards. I'm not too sure though if these standards are as high as OEM equipment (I believe they are).
In theory a high flow converter can be just as efficient (pollution wize) as normal OEM models. They usually do require more complex cores (higher honey comb densities) or more exotic catalysts (rhodium, pallidium, etc) to do it though and thus they cost more to manufacture. That's why they aren't used as OEM equipment. They may also have shorter lifespans due to thinner core walls but this is usualy only an issue if your engine is poorly maintained.
Random Techs website has some good information about this at:
http://www.randomtechnology.com/technical.html
All replacement Catalytic converters must meet certain EPA standards. I'm not too sure though if these standards are as high as OEM equipment (I believe they are).
In theory a high flow converter can be just as efficient (pollution wize) as normal OEM models. They usually do require more complex cores (higher honey comb densities) or more exotic catalysts (rhodium, pallidium, etc) to do it though and thus they cost more to manufacture. That's why they aren't used as OEM equipment. They may also have shorter lifespans due to thinner core walls but this is usualy only an issue if your engine is poorly maintained.
Random Techs website has some good information about this at:
http://www.randomtechnology.com/technical.html
-Nick
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