FAQ: How does a Catalytic Converter work?

[Unlimited04]

Ok, since there seems to be some confusion on how a Catalytic Converter works, and why its important I figured I'd do a write up explaining the engineering behind it.

What is a Catalytic Converter ("cat")?
Its an emissions control device that is designed to remove Carbon Monoxide (CO) from the engine exhaust and convert it to Carbon Dioxide (CO2). Carbon Monoxide is one of the gasses responsible for the 'greenhouse effect', believed to be responsible to global warming, but thats a topic for a different thread.

Carbon Monoxide:
Carbon Monoxide has one carbon triple bonded to one oxygen molecule, and it looks like this:

This triple bond is typically very hard to break. CO becomes a problem because it can react with a series of other contaminents in the atmosphere to reduce the quality of the air we breath everyday.

Why do Jeep's make CO?
CO is a byproduct of our engines due to incomplete combustions (burning fuel), and since the typical gasoline engines are only around 25% efficient, there is a lot of CO produced. Multiply that by millions of cars, trucks and engines in the world, and you can see how we get trillions of tons CO produced every year.

The important part: How a Cat works
The catalytic converter works on a principal of a catalyized reaction - meaning an intermediate step is used to convert something to something else. For example - if you want to bring a box full of tools up to the 15th floor of a building, you could climb the stairs carrying the bucket, and it would take a lot of effort. However, if you take the elevator it takes a lot less effort, and you get there faster. In this example, the elevator is a catalyist that helps you get to the 15th floor.

The Cat in a vehicle converts Carbon Monoxide to Carbon Dioxide. Why do you want this to happen? Well Carbon Monoxide is a pretty nasty gas, but carbon dioxide is much better. Plants can use CO2 for energy, and they spit out oxygen when they're done...oxygen we can breath.

The details: Cats typically contain a Platinum. Platinum has a unique property of allowing CO to bind to its surface, and then when an oxygen molecule comes close to that CO molecule, it coverts them to CO2.

CO + * <--> CO(ad)
O2 + 2* <--> O2, (ad) --> 2 O(ad)
O(ad) + CO (ad) <--> CO2 +2*
* denotes a free adsorption site on the surface of Platinum.

The key to this reaction is it must occur above 500 Kelvin (440 degrees F), and this reaction only occurs on the (111) plane of Platinum. This is why catalytic converters are always mounted in close proximity to the engine's exhaust manifold - where exhaust gasses are the hottest directly from the combustion chamber. This is also why they have a honeycomb inside - it increases the surface area, resulting in a significantly increased number of reactions that can occur.

Why is all this important?
If the cat is not mounted where it can operate at 450+ degrees F, its essentially useless and turns into a restriction on the exhaust system. If its operating properly, the heat and high velocity of exhaust gasses will keep exhaust flowing and will reduce CO emissions in the process. In fact, an catalytic converter that is properly placed and sized for the engine will present very little resistance to exhaust flow. A correctly installed cat will NOT reduce horsepower or torque for 95% of Jeep street use, and it is required by the government in all 50 states and most industrial countries overseas!

Sources:
http://en.wikipedia.org/wiki/Catalytic_converter
http://en.wikipedia.org/wiki/Carbon_monoxide
Basic Solid State Chemistry
The Physics and Chemistry of Materials

[spdcat01TJ]

It is a proven fact that as you state, " most internal combustion engines are only 25% efficient". Therefore any process which improves that efficiency will result in more power. If you can put more air, and fuel through an engine, it is physically impossible for it not to increase power output unless it is running out the tailpipe without burning. That is the whole basis with superchargers, and turbochargers. You can't tell me a supercharger on your 4.0 will not increase it's power level. I remember JP Magazines dyno tests on Intake systems, and not one of them failed to show some improvement in horsepower. Whether it is worth the cost is another question, but any quality air intake system is going to improve performance in some way. In order to get the maximum benefit from a cold air kit, 1. it should have a source of cooler air than underhood, and 2. The fuel and exhaust system must be able to handle the increase in airflow.
And from your own wikipedia source; "Carbon monoxide, though thought of as a pollutant today, has always been present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the earth's mantle. Carbon monoxide contents of volcanic gases vary from less than 0.01% to as much as 2% depending on the volcano.[citation needed] It also occurs naturally in bushfires. Because natural sources of carbon monoxide are so variable from year to year, it is extremely difficult to accurately measure natural emissions of the gas." Maybe we should work harder to control those pesky volcano's and bushfires!

[Unlimited04]

Originally Posted by spdcat01TJ

It is a proven fact that as you state, " most internal combustion engines are only 25% efficient". Therefore any process which improves that efficiency will result in more power. If you can put more air, and fuel through an engine, it is physically impossible for it not to increase power output unless it is running out the tailpipe without burning. That is the whole basis with superchargers, and turbochargers. You can't tell me a supercharger on your 4.0 will not increase it's power level. I remember JP Magazines dyno tests on Intake systems, and not one of them failed to show some improvement in horsepower. Whether it is worth the cost is another question, but any quality air intake system is going to improve performance in some way. In order to get the maximum benefit from a cold air kit, 1. it should have a source of cooler air than underhood, and 2. The fuel and exhaust system must be able to handle the increase in airflow.
And from your own wikipedia source; "Carbon monoxide, though thought of as a pollutant today, has always been present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the earth's mantle. Carbon monoxide contents of volcanic gases vary from less than 0.01% to as much as 2% depending on the volcano.[citation needed] It also occurs naturally in bushfires. Because natural sources of carbon monoxide are so variable from year to year, it is extremely difficult to accurately measure natural emissions of the gas." Maybe we should work harder to control those pesky volcano's and bushfires!

wrong thread? what does any of this have to do with how catalytic converters work?

[Cutlass327]

The newer cats also control NOx (oxides of nitrogen) and also unburnt Hydrocarbons (HC). That is the purpose of the 3-way catalyst. That is also why the ECU switches back and forth between rich and lean - to properly keep the cat charged with sufficient O2, and to also allow the ECU to keep an eye on the O2 sensor for proper operation.