How can all that soot in my tailpipe
 

be good for the environment? I can smell burning oil and see the soot so how does the RX-8 gets such fairly good ratings for emissions?

Maybe the same way when the say it's 238BHP and gets 18/24 mileage.... :)

The soot is from the oil the rotary burns.

I understand that but doesn't some of the soot go into the air? Is that what our lungs will look like if we keep driving the car :eek:

I bet the soot is from the car running rich to keep the cat cooler. Hell, even on my CR125 that I run with 28:1 oil mix, there's not much soot at all when it's jetting is right... and not much smoke either.

So I doubt the soot is from the oil... because it's not burning that much.

My bet is that it's mixture related, and Canzoomer's already shown that the 8 runs rich on the top in stock form.

Then you need to speed up, I drive too fast for the exhaust to reach me! :D

Well, a bit is from the oil. Most is from the excess fuel above 6000 RPM. Oh, and don't ask me how they can get away with it. I think it's govt. bungling. Forcing manuf. to ensure the cat. survives 120,000 mi is silly. Just make us change it, say every 80,000 mi, with a CEL.

Absolutely. That is such a stupid rule on the cat... I like your solution much better.

The Rotary is actually a pretty clean running motor. Back in the 70's it had GM and several other manufacturers looking at it pretty seriously because it was one of the only power plants that could meet the emmision requirements of the time without a cat. My other 6 speed is an F250 SD. Now that generates some soot. Funny though, the fuel economy is just about the same :)

Not that I know anything, but the How Stuff Works website says the following:

"There are some challenges in designing a rotary engine:
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.
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3. Typically, it is more difficult (but not impossible) to make a rotary engine meet U.S. emissions regulations."

Anyone know why this is the case, or why How Things Works is wrong (if they are)?

BTW, the URL for that page is http://auto.howstuffworks.com/rotary-engine5.htm

I was referring to early 70's emission standards. With today's technologies (i.e. FI & Cats etc.) it is quite a different story.

Ah. I missed the 70's reference. Although I'm still curious in general as to what makes an engine more or less emmissions friendly. They burn the same fuel. If all cars used the same cat/exhaust system, why would one engine be better than another?

Reading the posts in the tech section last night, I learned about the RENESIS's "wasted spark." I doubt that's a major reason the RENESIS is cleaner, but the point was that having a second spark in the chamber burns more of the gas that might otherwise go unburned and actually gives the car a bit more power.

That soot is nasty and will ruin any cloth used to clean up the tail pipe. Keep your good cloth away from it. Is there a chemical that removes it?

The previous 13B engine had a wasted spark as it only had three coils - the trailing plugs in the two rotors both sparked at the same time, therefore one of them was "wasted". The current engine doesn't do this, as it can spark each plug separately. It has four coils (you can see them just under the rubber intake tube that exits the air cleaner box).

Emissions are not simple. The rotary is bad for generating HC, but that is easily cleaned up with cat. converter tech. High compression engines start to get bad numbers with NOx, but now we finally have cats. that can deal with that too.

Appearantly, we just can't have the cats. that last long enough doing the above.

The problem the rotary has is the shape of the combustion area - it's rather long and thin. Because of this, there will be unburned or incompletely burned fuel in the "nooks and crannies" (corners for example). Hence two spark plugs. Optimum design would ask for a circular or even spherical shape, provided you could get enough compression (a drawback with "hemis"). Again, combustion chamber shape is not a simple subject, and views have changed radically over the past 10 years or so.

Allowable emissions have become so low now, this stuff is getting very hard to understand. Oh no, they're using "the calculus"! ;)

What does a Cat cost? It seems like Mazda had major headaches trying to extend the life of the catalytic converter to meet U.S. law, when simply swapping it out when the time to replace it might have made more sense. I know that people with 8-year old cars balk at paying a lot for repairs, but sometimes swapping something out makes more sense than trying to extend it forever.

The OEM RX-8 cat is $1800.
A Good aftermarket "universal" cat is $230.
Somewhere between the two lies the solution.

Mazda are well known for overcharging for replacement cats. They should be less then $500 retail.

OK. So it sounds like the emmissions problem primarily lies with unburnt fuel during combustion. Doesn't that imply that running rich would yeild more emmissions that need to be cleaned up by the cat.? If Mazda changed the ECU, forcing it to run rich in order to keep the cat cool (?), then isn't it forcing the cat to scrub more gases in exchange? Wouldn't that eventually deplete the cats catalysts? Or is there more than enough platinum, etc. to last? Or do I not understand the chemical reactions occuring in the cat?

If you don't supply enough O2 to actually burn the HC then nothing happens (except you get smoot out the tailpipe - and HC). The excess fuel would lower the temp. of the exhaust gas (a rich mixture is seen as a lower exhaust gas temp. - as you lean it out the exhaust gas gets hotter - this is often used as a tuning tool) so the cat. wouldn't get heated up as much.

The catalyst only causes or enhances the reaction between the HC and the O2. It isn't "used up" except for a small amount lost due to bonding failure (for lack of a better term). Remember, we're talking about too much fuel above 6000 RPM. I doubt any smog test goes there.

I wonder if a solution would be a second cat. that is added in parallel with the first above 6000 RPM so each one only had to do half the work meaning they would only get half as hot? Guess that was judged too expensive.

You guys do realize there are two different standards applied to OEM and aftermarket cats right? The OEM cats will always perform their catylast function better than the aftermarket solution because the government does not require the aftermarket to meet requirements such as LEV, ULEV, PZEV etc. (those are wholistic interpretations of emmissions anyways, not just exhaust). From what I can recall from a lecture given by a Johnson Controls (OEM cat supplier) engineer at school last year (mech engr program), the different standards are mainly based on bringing down the price of a cat for consumers. So when the cat fails (which is inevitable given it's service), there's Johnny Aftermarket with a model that uses less precious metal and requires substantially less cost to the supplier in terms of R&D, manufacturing, and materials, but doesn't perform nearly as well as the OEM piece. But who really wants to fork out $1800 at 50K miles, JUST because of emissions equipment standards?

So by requiring the higher performing cats to last longer and longer, the government (CARB, specifically) is requiring that vehicles run with lower emissions over the life of the vehicle. I think the goal is to all but eliminate the need for replacement cats and have the OEM gear last the life of the car. But until that day arrives (and 120K is getting closer), aftermarket gear gets a lower standard, does not perform as well, and costs much less - all okay by me.

As for how they function, that's best left to industry people, but from what I recall, cats fail by eventual bonding of particulates to the catylast applied over the substrate, and eventually the gas passes through without much reaction. Either that or they mechanically fail (ceramics break apart and block flow). Apart from failure, I'm sure there are specifics to the rotary engine that best mate certain cats with them, due to the different rxns occuring at different temperature intervals, and the higher heat output by the rotary.

I'm not ranting to defend or disagree with anyone, but at least for me, I didn't have a good idea of all that was involved with most automotive emissions gear (technically and politically) until I went to the aforementioned lecture.

Now where's my straight pipe? ;)

Tx guys. Fascinating subject I never thought much about until now.