Need for EGR?

[livingwater]

My understanding of the need for the EGR valve is as emission "fooler" only. I realize that introducing the exhaust gas will reduce NOX. Can't the ECU get emissions under control by simply using the feedback from the O2 sensor, along with it's other parameters? From looking at the system on a SBC, it looks like the gas is only introduced to one or two cylinders anyway, as it feeds into the side of the intake, and not into the throat of the throttle body. So, I'm not sure it's benefits outweigh the cost of the airflow interruption it produces by intruding into the manifold.

Thanks.

 

[Chumley]

"Can't the ECU get emissions under control by simply using the feedback from the O2 sensor, along with it's other parameters?"

Sure. Not all gasoline emission control systems use EGR.
Cleaner burning engines don't need it. It depends on the
combustion chamber design and the fuel injection system
as well as a few other factors.


"So, I'm not sure it's benefits outweigh the cost of the airflow interruption it produces by intruding into the manifold."

There might be a few engineers who disagree with you there!

Especially on diesel engines that run with excess air, EGR
is now used to reduce soot particulate concentrations in
the exhaust by two methods. Soot in the exhaust gas is
flammable and if given another chance, can reburn in the
presence of excess O2 to produce additional CO2 if rerouted
to the combustion chamber. Also during high power settings
rerouting soot through the combustion chamber gives it an
extra opportuniuty to load the engine oil rather than the
ambient air. Engine oils will be taking on additional
waste basket duties. Check out the new Euro diesel oil specs.

http://www.crcao.com/

Chumley

 

[Andy330hp]

Also, using EGR as a diluting agent reduces pumping work for higher efficiency.
Newer cars may not have egr VALVES, but they are still taking advantage of burnt exhaust gases, using VVT to generate "internal" egr.
Even on older cars, there's really no reason to get rid of EGR unless you're building a show car or something. It doesn't affect maximum power, increases mileage, reduces emissions, and if you maintain the EGR system (replace the valve when it goes bad instead of just complaining about it!) it won't hurt driveability. I suspect it MIGHT slightly accelerate wear in the head (guides, seats, etc), but I have absolutely no eveidence to support that. Other than that, it's a free lunch.

 

[patprimmer]

I think it also heats up the inlet manifold and carby a little (on older cars), possibly contributing to vapor lock in some circumstances.

Regards
pat

 

[spereira]

I don't think the ECU can control NOx emissions just by the O2 sensor which I believe changes the AF ratio. The EGR is able to reduce NOx emission because it reduces combustion temperature thus reducing N2 oxidation to NOx (I guess this could be achieved by increasing the amount of excess air but then it would also reduce combustion efficiency). I think this is why it also increases soot emission since combustion of heavy diesel/gasoline compounds becomes incomplete under lower temperatures. So, there is really a drawback with EGR: it decreases NOx emissions but increases soot (specially in diesel engines, not sure about gasoline though since it's lighter than diesel).
Also, at full throttle and under idle conditions it is closed so there's no soot rerouting to the intake, correct?
What I was told is that after some (long) miles it's advisable to clean the intake system since soot accumulation (together with oil coming from the CCV) can severely reduce intake cross sectional area reducing power delivery at full throtlle.

 

[Chumley]

"I don't think the ECU can control NOx emissions just by
the O2 sensor which I believe changes the AF ratio."

Sure it can. NOx emissions rise substantially on the lean
side of stoichiometric, and drop off on the rich side when
the CO and HC emissions increase. See this link:

http://www.sjmautotechnik.com/injector.html#osens

See the exhaust gas concentration charts at the above link
along with an explanation of the closed loop feedback
system. (is't for a pre OBDII Bosch K-Jet/lambda U.S.
emissions systems without EGR)

When the mixture is slightly rich, the reduction section
in a three way catalytic converter uses excess CO
to "reduce" the NOx back to N2 and then also produces
CO2. A properly functioning O2 sensor continuously alters
the A/F ratio from the lean side to the rich side of
stoichiometric several times a second. When the mixture is
lean the oxidation section in the CAT is burning up the
stored excess HC's because there is excess O2, when it's
rich, it's reducing the NOx. That's why an old slow or non
reacting O2 sensor will often send the NOx emissions over
the limit. The mixture stays lean for too long and the
reduction section of the catalyst gets saturated, then
dumps the NOx out the tailpipe to the waiting sniffer. It
also explains why some cars experience a lean hesitation
under mild acceleration when the O2 sensor is tired and
ready for the scrap can.

Chumley

 

[Andy330hp]

I was speaking purely about gasoline, not diesel. Sorry for any confusion

 

[spereira]

Chumley,
Very interesting the link you've posted, thks. It also explains a lot of other ECU functions!
After reading about the closed loop cycle of the O2 sensor I understand why an engine wouldn't need an EGR system but I guess catalysts for diesel and gasoline engines would work in different ways since we're talking about differnt level of pollutants. I know, because I've seen diesel engines running with the EGR off, that by turning off the EGR soot emissions decrease and from what I read this is because combustion temperature increases.
I was just saying that if EGR systems have such a drawback (and you really can't tell which is worst, soot or NOx since both are harmful to human health) why use them unless it's the simplest and most efficient way of controlling NOx emissions...

 

[livingwater]

Guys,

Thanks for continuing to throw things in on this. My wife has a '94 Suburban, and it's my first dealings with EGR, per se. My 84 Ford pickup had a Carter Feedback carb on the 300 -I6 engine. It had some kind of exhaust feedback into the carb itself. I suppose it was somewhat of a good thing from the factory (although Ford used that Carb for less than one production year). At about 111000 miles, it started giving me real problems. I rebuilt it, and meesed with it for a while, then finally put the other 1bbl offered that year (super basic carb, with only the electric choke and one vacuum line for connections). That improved the power and mileage tremendously. As a young test engineer with a growing family, I didn't have the time or facilities to do much for the old truck except keep it running.

My question here stems from desire and interest in improving performance (economically), and perhaps mileage in the Sub. The mileage thought is more sideline, torque at and below 3000 rpm is the primary design consideration. Economy would be an interesting benefit. Kind of like my day job objective of finding ways to make my airplane better, faster to assemble AND cheaper. The boss gets to pick two. I just don't see a lot of info on EGR, other than it adds a considerable amount to the price of an intake manifold.

I've also run across a link for an interesting ECU and ignition system. As I get further along in my project (past breathing and flow corrections and upgrades), I may look more at this system. See it at:

http://www.injectionlogic.com

Dman had asked soemthing about a WIndows based tuning system. This thing looks like quite the toy. Too bad my garage and wallet don't have room for an engine test stand....