Diesel cat or gas engine cat any difference?

[cj036.0]

This sounds like a dumb question but is there any difference between a diesel cat and a gas engine cat?

 

[DaveBen]

Absolutely. In a gas motor the cat changes the exhaust gasses to "non polluting" gasses. But I doubt that sulfuric acid is non poluting - acid rain. In a diesel it is more of a smoke and soot trap and it burns off the smoke and soot particles at a very high temperature and releases no smoke or soot.

Dave

 

[95_stroker]

Dave,

Do you know if the catalyst used in the cat is different between a Diesel and Gas powered engine? I think its not, but thats just a gut feeling and I dont have any stats to back it up.

 

[Hoss 350]

This sounds like a dumb question but is there any difference between a diesel cat and a gas engine cat?

Not a dumb question at all. I've wondered the same thing myself, to the point to where I looked into it.

To answer your question, no, there is really no real difference. They are both a platinum-based catalyst, the primary function of which is to do exactly what Dave said, which is to take harmful, polluting gasses and convert them to "harmless" gasses. Most of their function revolves around turning hydrocarbon emissions into carbon dioxide by basically adding a catalyst to them to “complete” the burn. An OBDII gas engine, running properly with a catalyst will literally emit almost nothing but CO2 at the exhaust (under the current light-duty standards).

In a diesel engine, their function is the same. To take hydrocarbon emissions and convert them to CO2.

What Dave described as the function of a diesel cat is actually the function of what is known as a particulate filter, which is a device that will be added to diesel engines starting with the 2007 model year, based on new EPA regulations regarding particulate emissions. Catalytic converters do not actively reduce particulate or soot emissions (ie, this is not their primary purpose) although the high operating temperatures inside the catalyst probably do help to burn the particles up to an extent. The fins inside the catalytic converter are simply WAY too big to catch any particulate from a diesel engine. Typical soot emissions from a diesel engine are in the range of 2 to 10 microns in size. The holes in the catalyst are in the range of 1/16th of an inch, so, depending on which conversion is right (because I am having trouble with conversions today…) the holes are either 15,875 times bigger than the particles, or 1,587 times bigger than the particles. Either way, they catalyst really isn’t stopping a whole lot of the particulate in the exhaust stream.

Anyway, that is beside the point. The cats function in more or less the same fashion from gas to diesel. However, like anything else, you have to have the right cat for your application, so a gasoline engine cat will not work properly on a diesel engine (usually because the gasoline cat will be too small and restrictive) just like a gasoline muffler will not work on your diesel engine.

 

[powerboatr]

 

[DaveBen]

Good Job Hoss. I couldn't have said it better, and I didn't.

Dave

 

[Hoss 350]

I have to qualify what I said earlier, because I guess I realized after the fact that I am not completely sure of one thing, and that is whether the cats on diesels are 3-way cats like most modern gassers, or not. I guess I need to do a little research. If they are, then they have two different catalysts in them. One of platinum, and one of rodium, or some other similar metal. The platinum one recuces hydrocarbon emissions (which photo-react to create ozone and smog) and the other reduces NOx, by taking the nitrogen molecules out of the mix, and then binding them together to create pure oxygen (O1 and O2) and pure nitrogen (usually N2, since N1 doesn’t survive by itself for very long). I THINK that diesel cats do the same thing, but I realized that I may be wrong about this. Diesels may only have a platinum catalyst to reduce hydrocarbon emissions. I guess I am not sure. Does anybody know if diesels have 3-way cats on them?

The reason I ask is because running with a 3-way is a little more complicated than just running the exhaust through it. The O2 sensor on a gasser keeps the air/fuel ratio close to stochimetric, but also purposely runs over and under (the rich/lean cycle) in order for the catalyst to work properly. IIRC, one part of the catalyst likes to work better rich, and one likes to work better lean, so the computer constantly runs back and forth to keep both sides operating well. With a non-stochiometric engine (like a diesel) I am not sure how you would get this to operate properly, as DI engines run (at times ) VERY lean, and (at times) VERY rich depending on throttle and load. The “mix” is not modulated by a carburetor or a fuel injection system, it is modulated by how much fuel is dumped into the cylinder, which is modulated by your right foot.

So, I can tell you one thing. If diesels DO have 3-way cats, they don’t work nearly as well as they do on a gasser for one side or the other. I will check into this and see what I find out. You have got me thinking now…

 

[Hoss 350]

Okay, okay...

I admit it....

I was WRONG!

After my first post, I got to thinking about what I had said, and then realized that I was probably wrong, and needed to eat some crow. Now, I am sure of it.

The final answer to the original question is...

YES, there IS a difference between a present day diesel catalyst and a present day gasser catalyst. My confusion came from a mix-up between 2-way and 3-way catalysts. I had assumed that diesels had 3-way catalysts, since the new regs require 3-way catalysts, but then realized that a 3-way catalyst only works when the engine is moving back and forth across stochicmetric. So, I began to question my assumption yesterday, and have since found out that I am WRONG.

A 3-way catalyst, as discussed, reduces both hydrocarbon/CO emissions, and also reduces NOx emissions. The oxidizing part of the catalyst (HC/CO) likes a lean burn (ie, more oxygen than fuel) and the NOx part of the catalyst likes a rich burn. So, the fuel management system on new vehicles works the ratio back and forth from rich to lean so both can do their jobs.

After I made my fallacious statement yesterday, I realized that a 3-way catalyst would not work on a diesel, since they work through direct-injection (ie, non-stochiometric) combustion, so they are (from the factory) ALWAYS in a lean-burn situation, meaning the NOx side of the 3-way catalyst would almost never be accomplishing anything. So, I looked it up, and found out that before 1990 or so, diesel and gasser cats were identical. Both had 2-way catalysts. However, once they put O2 sensors on gassers, they converted to 3-way catalysts, while diesels stuck with 2-way, since there was no way to make a 3-way cat work on a diesel. So, the cat on a diesel only reduces HC and CO, but does noting for NOx. One method of NOx reduction that they are trying out is EGR valves, which recycle cooled, inert exhaust gasses through the intake to reduce combustion temps and therefore, NOx. The other option is urea injection, where they inject anhydrous of ammonia into the exhaust pipe, which breaks down the NOx and creates inert harmless exhaust gasses.

So, I retract my original statement and apologize for my mistake. There is a difference between them.

 

[powerboatr]

well now
its still darn good reading.
using urea....
so our tailpipes might smell like pee or chicken pee since a ton of urea comes from chicken processing farms.

so i burn turkey oil or french fryer oil and add urea to teh mix
that would be one smelly tailpipe

but still your pesistence at finding the answer is admirable.

 

[whatabudro]

-popcorn -popcorn -popcorn