brillo

On of the issues I keep reading about with direct fuel injection/lean burn engines is that the lean causes the exhaust gas to be cooler, therefore, the Cat isn’t as effective in cleaning up the emissions.

Ok, so could you just warm the Cat up with a heater and get the same effect? Put some kinda heating element like a stove around it? So long as the catalyst gets hot, shouldn’t it work? I know this is more complicated than normal, but doesn’t seem to me to be that difficult or expensive.

Am I missing something here?

zoom44

why not just use a precat?

wedge357

I thought the reason the 8 has such a lousy gas mileage is because the ECU dumps extra fuel into the engine in order to cool the cat converter and therefore prolong the life of the cat?

brillo

this is a general question, not rx8 specific, but the idea came up wen i was reading about the various rotary engine types mazda has worked on. In my Jack Yamaguchi book, it mentions the DISC I and DISC II and that as a result of the ultra lean burn, the exhaust temps are critically lowered which was detrimental to the catalyst systems ability to react.

Pre cats would be another area to explore, not sure why we don't have those either. Prolly due to the fact that the current rx8 exhaust temps are so high they would cook them literally. As far as a DI engine, not sure if pre-cats would solve the problem or not.

lurch519

the leaner a gasoline engine runs, the hotter the exhaust is. thats why when u run lean, you experience detonation, and in piston engines can burn through the head gasket and other very important and expensive to replace parts.

Chrissss

Running lean usually RAISES the combustion and exhaust gas temperatures, which is why you start melting things when using FI without an appropriate increase in fuel to retain stoic A/F ratio's.

Chris...

brillo

I think that running a lean burning conventional engine and running a lean burning direct injection stratifed charge engine is diffient in how the engines effect the exahust gas temps.

Since the DISC engine localizes the combusion just by the spark plug, there is a whole order of magnitude less fuel in the combusion chamber, and this affects the temps.

I'm still working this out as I agree, when I think lean, I think hot not colder.

zoom44

read this story from autoweek http://66.102.7.104/search?q=cache:z...ient=firefox-a

zoom44

from this article http://www.daimlerchrysler.com/Proje...-0-0-0,00.html

Rasputin

I'm very surprised by this statement as the only operation mode where an engine would struggle to heat up the cat to the minimum effective temperature would be on cold starts. In this mode most engines runs with significant enrichment which inhibits the exothermal (= heat inducing) oxydation. Some strategies exist to accelerate the catalyst warm up :
- spark retard (to put more heat in the exhaust) ;
- extra air to enhance exothermal HC oxydation (Secondary Air Pump).

I'm sure that either of these techniques would work on a DI gasoline engine as it is not going to run in lean burn mode until properly warmed up.

The main issue with lean burn (= excess of air, and therefore oxygen) operations is the increased NOx emmissions. Indeed, it is very difficult to reduce (= take oxygen out) NOx in an excess of oxygen! So car manufacturers use a NOx trap which actually chemically stock NOx (don't ask me how, I don't know yet). This NOx trap needs to be regenerated very frequently. When the trap is emptied of the NOx it contains, the PCM schedule a temporary rich mode in order to effectively conduct NOx reduction in the catalyst. All these change in operating modes happens as you drive, sometimes in steady state. The torque variations due to these operating changes are compensated by spark advance variations and electronic throttle control.
Note that a lot of existing DI gasoline engine actually work in stoich mode, not lean burn, to avoid to use a NOx trap and a complex torque-based engine management. The gain in fuel economy from these engines is essentially down to the increased compression ratio. This is however very relative as an Honda Accord is as economical as the Audi A3 2.0 FSI (the normally aspirated 150 hp version that is sold over here in Europe).

Hope it helps,

Fabrice

zoom44

the bits i posted above it mentions that onstart up idle DI exhaust can be as little as 200 which cats are not designed for. i think that is where the "lean exhaust is cooler" is coming from

Rasputin

That's coldish indeed. But probably a very lean burn operating mode.
I think a cat needs 300°C to work effeciently. Maybe less these days.

brillo

back on topic for a second, lets assume for the moment that a DI engine creates exhaust gases that are 200 degrees when cruising. could you just heat the cat to get the desired effect of scrubbing the gases? I don't see why it matters if the gas is hot, or the cat, b/c it seems to me that its just a matter of getting the cat medium hot enough to work

zoom44

well you probably could but what they are trying to do is make new cat mediums that will work in a broader temp range from 200c to over 1000c.

Rival

#1 - pre cat is just an extra cat to help with emissions
#2 - The reason rotarys destroy there cats so fast is because the oil eats it
#3 - The reason they dump gas in at a cold start is to heat up the cat to operate corectly and the o2 sensor does not start reading until it is heated. The faster you can heat them up the better the emissions will be.
#4 - An oven will not work. do you know how hot those catalitic converters get. Plus how would you get power to an oven and how much weight would this add???

Rotarys will never have great gas milage