[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.

[warsw]

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[xJoshxx]

Originally Posted by warsw

Good write up. Thanks. Maybe you can answer a question for me. Why does my 04 Rubicon have three cats? I have never seen this setup on any other vehicle I have owned.

California Emissions? i know certain WJ's have 4 of these

[Unlimited04]

Originally Posted by warsw

Good write up. Thanks. Maybe you can answer a question for me. Why does my 04 Rubicon have three cats? I have never seen this setup on any other vehicle I have owned.

thats just the way Jeep designed it. My guess is the two mini-cats need to be heated quickly and function at very hot temps, so they are positioned close to the head. The 3rd (or secondary) probably doesn't run as hot, and can be positioned further away.

As a side note - before you remove your cat thinking you're going to gain some horsepower or mileage or something...you'll want to read the Factory Service Manual for your model and determine the functions of the cat converter, and what effect its removal on the PCM will be. Aside from the technical implications, it is illegal to remove a cat converter from a vehicle originally equipped with one.

Here's a good discussion:
http://www.jeepforum.com/forum/f9/did-exaust-mod-now-poor-mpg-help-897940/

So even if you want to run illegally without a cat because you don't have emissions testing in your area, you'll still need to read the FSM for your year & model and read how the O2 sensors determine cat health and A/F ratio, and make sure you are not giving yourself a host of problems.

The simple answer is to keep a functional cat on your vehicle if it originally came with one.

[warsw]

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[USAFRecon]

And on a related note but off topic kinda, I have an 01. So I have the 2 after the exhaust manifold, what's that doohicky before the muffler?

BTW nice writeup.

[Unlimited04]

Originally Posted by USAFSahara

And on a related note but off topic kinda, I have an 01. So I have the 2 after the exhaust manifold, what's that doohicky before the muffler?

BTW nice writeup.

newer 4.0L's have 3 cats. 2 mini cats off the 2 cast exhaust manifolds and one on the t-case skid plate. the 2 closest to the exhaust manifold need to run HOT, hence they are located where they are.

[skeeterbeater]

Originally Posted by xJoshxx

California Emissions? i know certain WJ's have 4 of these

i forget what the catalist is in cats (not platinum) but there is only one mine in the world that has it... and GM owns it. so gm uses the best for them selves and sells the rest to everyone else to pay for the mining. that is why GM cars only need one cat when other manufacturers have three or four. also why GM cats are worth more at the shredder.

[2003RubiBlue]

[QUOTE=leviroberts;9084607]i forget what the catalist is in cats (not platinum) but there is only one mine in the world that has it... and GM owns it. so gm uses the best for them selves and sells the rest to everyone else to pay for the mining. that is why GM cars only need one cat when other manufacturers have three or four. also why GM cats are worth more at the shredder.

I think it palladium, which is a platinum metal family member. GM does not own the mine, but they may be in trouble for making deals. Its the only mine in the US but there are others in the world. I really don't think there is a difference on the quality of the cats, im sure they are all manufactured by the same companies for the major auto manufactures in the US. Heres a link to the story on their deal.

[skeeterbeater]

Originally Posted by 2003RubiBlue

I think it palladium, which is a platinum metal family member. GM does not own the mine, but they may be in trouble for making deals. Its the only mine in the US but there are others in the world. I really don't think there is a difference on the quality of the cats, im sure they are all manufactured by the same companies for the major auto manufactures in the US. Heres a link to the story on their deal.

good to know

[Barcus]

Originally Posted by skeeterbeater

i forget what the catalist is in cats (not platinum) but there is only one mine in the world that has it... and GM owns it. so gm uses the best for them selves and sells the rest to everyone else to pay for the mining. that is why GM cars only need one cat when other manufacturers have three or four. also why GM cats are worth more at the shredder.

hate to bring up an old thread but my 05 GMC Sierra had 4 cats on it.

[bruteboy]

platinum,palladium and rhodium are the most common materials in a cat.
there are two basic types of catalyst.
reduction catalyst and oxidation catalyst.
a three way cat has a reduction catalyst in the front(inlet) part of the cat and an oxidation catalyst in the rear(outlet)part of the cat.
some vehicles use a reduction cat and a seperate oxidation cat.
the cats installed close to the exhaust manifold on the jeeps with three cats are the reduction cats and the third cat is the oxidation cat.

the reduction catalyst uses carbon monoxide(CO) to reduce oxides of nitrogen(NOX).(nitric oxide=no,nitric di-oxide=no2)so it needs to be upstream of the oxidation catalyst to ensure sufficient CO supply.
the oxidation catalyst uses oxygen to reduce hydrocarbons(HC)and carbon monoxide(CO)