Apparently, to extend the life of the catalytic converter, Mazda throws gas at it for cooling purposes. I assume the gas is not burned in the cat otherwise temperatures would go up (and running rich means oxygen deprivation - I think the air pump only runs on initial startup), so would you not then have a hydrocarbon emissions problem?

 

Apparent to whom?

From:

Rotary Engine Illustrated

 

I think it's a ratio issue. There's air in there anyhow. The air pump really stokes the flame to a degree that would be quite unacceptable longterm. Extra enrichment does reduce the temps at a point and yes, I think it would be cleaner temporarily if it were leaner (Before it starts rapid cat deterioration).

Paul.

 

It's not a cat issue so much as a ratio issue, like Paul says.

Imagine the fuel/air ratio as a graph - at the peak of efficiency the temps are over 2000* and just too hot, would physically melt the cat ceramic, so we have to run on the 'slope' of the graph, away from peak efficiency.

Rotaries can't ski on the 'too lean' side of the graph, so we end up enriching the mixture until the efficiency is gone and the cat is happy.

Sounds like a 'green' solution, if I ever heard one - burn more gas, less efficiently, so it comes out 'cleaner'!! !!

S

 

Yep, and that's precisely how Mazda got around needing a cat in the first place to pass emissions standards. But escalating gas prices necessitated going with a cat.

 

well actually there wasn't a cat on a rotary engine until the early 80's RX-7.

 

Yeah, we meant the same thing. Of course there weren't any cat's at all before the mid- 70's.

 

hmm... this is an interesting thread.. I'm glad I stumbed upon it... and yeah, I'm sure "this has all happened before and will happen again" too (For my bellow BSG fans )

Anyway, I've recently leaned out my mixture considerably. I've gone from 11.2 AFR WOT all the way up to about 12.2 WOT.

I take it I should be watching my CAT temp and if I see it going over 2000 then its a sure sign I'm eating my CAT "faster" than normal?

 

73 for most piston engines in the USA

 

my CAT routinely operates between 1600-1700 on the highway - is that normal?

 

1970 - 72 Early 70's

1973 - 76 MID 70's

1977 - 79 Late 70's

But I'll throw in a year one way or the other.

 

LOL

 

RX-8 Frontier - you make two points. The renesis is more efficient than previous generations (because of side ports?) and that the air pump kicks in on startup to burn the excess fuel in order to quickly get the cat up to temperature. Neither answers the question of how a rich mixture cools the cat.

Mazmart (Paul) - How can there be air in there if the A/F ratio is less than stoichiometric? The air pump only operates for a minute after startup - not really relevant to steady state operation. You seem to imply that it's obvious that a rich mixture is effective in cooling the cat. Can you explain why?

The only way I can see it is this. There is insufficient air to convert the excess gasoline into CO2 and H20 (if there were you would essentially have combustion which would produce heat). Therefore the gasoline either cools by evaporating (not likely much liquid gas coming out the exhaust) or by blowing over the cat as an inert gas. In both cases you would be emitting hydrocarbons.

 

Okay, delmeister a basic Cat converter How it works:

Modern cats contain two sections, the reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts 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. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

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. For example:

2CO + O2 => 2CO2.

Got that so far?

Okay, now since cat converters only work at pre-determined temperatures. If the cat is hotter, it no longer converts... if the cat is colder it no longer converts. You could dump pure gas into a cat converter and it would do next to nothing if it was not hot enough or the correct mixture (which we will get to in a second).

So placement from the exhaust port has a major influence on the temp that a cat runs at.

For example in the non turbo FC (86-91 RX-7) there were multiple cats (three on the 86-88 models). This reduced emissions at all temps of the engine. The cat nearest to the exhaust manifold heated the fastest and started working the soonest... then the middle cat came online (and the first cat got too hot to rip apart those molecules), and finally the main cat got up to temp and started working (as the first two got too hot to work any longer ripping apart molecules). Again at the time the main cat was up to temp to start processing the above listed formulas the first two cats were past their operating heat temp range and no longer were effective at converting at all (no matter how much fuel was dumped into them directly).

So cats need to be at a particular temp to work. Too hot or too cold and they don't work- so the formula for how close or how far away they are from the exhaust port determines on when and how much they work just as much as the mixture hitting them does.

Now about air pumps: air pumps inject air into the exhaust stream after the exhaust port... this is that O2 that we mentioned above needed by the 2nd stage of the cat. Now once a modern engine is up to operating temps, the amount of O2 in the exhaust can be directly controlled by the ECU/PCM monitoring the O2 sensors both on front and aft of the main cat and can generally control the combustion event to help or hinder that O2 into the exhaust stream... if the cat requires more O2 to work than the ECU/PCM can control through the engine operating then the air pump comes online to dump more O2 into the exhaust for the cat.

Now you asked how more fuel (as in a richer mixture) can cool a cat down, more fuel (with less O2) takes the cat out of the operating range needed for the conversion process, and lowers the cats temp because the conversion can no longer take place at the optimum chemical level. Remember for the 2nd stage to be effective, the mixture coming into it needs to be as close to 14.7:1 or higher if possible. This wasn't the case on the older systems that injected air into the middle of the cat, but since modern systems hardly ever inject air directly into the cat, newer systems rely on the PCM/ECU correcting the mixture and switching the air pump in and out as needed.

If you watch your cold start mixture levels at cold idle, they actually are very lean after the first minute (above 17.5:1 typically), while this is great for gas mileage concerns, it also helps heat that cat up with all that unburned O2 and HC. But if you ran a rich mixture (such as 11.7:1) the out of optimum range for the main cat would result in the cat taking much much longer to heat up, unless you could lean it out further with again the air pump (or moved the main cat closer to the exhaust port, but then you have exhaust temp issues to deal with- remember too hot and it stops working).

So this is a long way to answer your question, but I think you can see it is not just the mixture that influences the cats operating range. And the two (mixture and location) really is what makes the cat work at its best.

 

for me the burning question remains... what is the ACTUAL "acceptable" operating temperature range for the RX8 cat?

 

IceMark - Thank you very much for the considerable effort you put into your explanation - very informative. But I am working on the following assumptions:
- The air pump never turns on again after it shuts off during the startup period (I monitored it some time ago, and I believe that was the case).
- The mixture is always on the rich side during normal operation.
Ergo, there is never enough oxygen available to react with the excess gasoline (assuming the O2 release from NOx breakdown is insignificant). If there were sufficient oxygen available, the temperature of the cat would rise from the reaction. If the temperature rose too high, the cat would be ineffective and again you are into hydrocarbon emissions. In other words, if one speaks of excess gas and oxygen, you are either led into a temperature increasing or hydrocarbon emissions scenario.
How do you know the temperature of your cat (are your units in degrees F?)?

 

does the airpump not run during idle on the Renesis? It is my understanding that on the older peripheral exhaust port motors the actual idle mixture (coming out of the engine, before the airpump adds air) is closer to 12.5:1 - 13:1 and then the ACV directs air into the exhaust ports before the cat to lean it out.

"Cooling the cat by running rich" is not going to be during normal low load closed-loop operation, where the target AFR is 14.7 . If you look at the efficiency curves of a catalytic converter (the old factory training manual for the FC has a diagram), leaner mixtures = less hydrocarbons, more NOx , while richer mixtures = less NOx, more hydrocarbons. 14.7:1 has the "happy medium" between the two.

 

yep exactly...

Most states are concerned the most with HC output, but some (like CA and the 5 states) are concerned with HC and NO and CO/CO2. For the states that HC is the biggest issue you can run leaner, but at the cost of having more NO and CO.

As an interesting side note, there were mods or supermods on this board advocating the destruction of the 2nd stage brick in the cat converters, suggesting this free'd up power. Understanding that the two stages in the cat work together to reduce all types of emissions, ripping out that 2nd stage just cut out half the reaction, and only allowed a minimal flow increase (since rotary engine cats have been honey comb designs for as long as I can remember- and not pellet designs like you find on some Nissans and other cheaper and older cars) and of course cut out half the converters job. But if your state was only concerned with NO then that would be a perfectly acceptable way to go, but of course the HC and CO would be off the scale.

 

My understanding is that it does not come on again after the initial startup period.

I thought the target for the renesis was always less than that during normal operation -hence HC emissions without oxygen injection, or higher cat temperatures with injection.

 

Yes that is F. I have an OBD2 diagnostic program that monitors it. The RX8 has a catalyst temp sensor.

Is that operating temp normal?

edit: check my sig (far right) for how I monitor it.

 

Yeah I saw that a while back while reading your carputer thread. Nice job by the way. I just wondered how the system determines the temperature since I didn't think there was a temperature indicator in the cat.

 

There is a temp sensor by the CAT.

I forgot exactly what the operating range are, but it should be somewhere between 1400-1600f.

The 2nd air pump only comes on only when the CAT temp is really low + initial 30 seconds of startup. It recycles warm air back into the engine so the engine heats up faster.

 

2nd air pump? Where is it? Recycles air? So the air is used and then reused?