EGR Exhaust Gas Temps

[Docengineheat]

Almost all of the diesel engines now being manufactured are equipped with exhaust gas recirculation to lower nitrous oxide(s) production. The system works by effectively lowering the charge air oxygen concentration, therefore lowering peak combustion temperature and nitrous oxide(s) production.

Here's the question; how much has this lowered the exhaust gas temperature, if at all? Or does it just lower the peak combustion temperature but leave the exhaust gas unchanged -- or maybe even higher?

I haven't been able to find the relevant data. Anybody know the answer?

 

[CDC]

There is an article of 1997 on this subject on

http://www.damek.kth.se/~borje/Artiklar/SAE 982476.pdf

For normally aspirated engines the temperature story also follows those lines, nevertheless EGR is not normally used at WOT when the temperatures are usually at their highest.

EGR is also good for economy at part load because it reduces the pumping losses caused by the vacuum in the manifold, something many technicians forget when they say that to remove the EGR is good for performance. As you can guess that's one of these myths again.

 

[CDC]

Note that the pumping losses apply only to gasoline engines as diesels do not have throttle valves.

 

[tbuelna]

docengineheat:

most engine manufacturers are now working to utilize "cooled" EGR. That is where the exhaust gas is passed through a heat exchanger prior to introduction back into the intake. Lowering your intial intake temperature will lower your final combustion temperature, all other things being equal.

EGR reduces peak combustion temperatures by acting as sort of an "inert" gas (ie. there is no oxygen present). It absorbs heat during the combustion process but does not contribute anything chemically. In practice, EGR rates are limited to about 10% maximum. Anything more than that and it begins to impact power output.

regards,
Terry

 

[Docengineheat]

Thanks. . .

I know about the cooled EGR systems. I was wondering about the temperature of the exhaust gas when it comes from the cylinder and before its cooled.

It was my impression that the way EGR works is by slowing down the rate of combustion by decreasing the intake charge oxygen concentration. Even with cooled EGR, the intake air will be warmer then without it, since you won't be able to cool exhaust gas below the temperature of the ambient air (unless, of course, you rig up some coolant system to do so).

It's been widely reported that diesel EGR decreases fuel mileage by 3-5%. Where does the missing energy go to? Into the coolant system? Out the exhaust pipe?

That's what I'm trying to figure out.

 

[CDC]

Well, if you read this long term test report then it appears that EGR can improve mileage:

http://fleetowner.com/ar/fleet_bowman_reports_egr/

It seems that the only thing that is certain is that the NOx is reduced, if that compensates the increase in particle count and ppm HC will have to be valued by environmental specialists. The matter seems to be far from resolved and should probably be seen as another compromise.
The more you read about it the more you wonder if this solution will still be around in 10 years time.

 

[Docengineheat]

I didn't get that from reading the post. . . .

"With nearly half a million miles on four Mack tractors equipped with cooled-EGR diesel engines, regional carrier D.M. Bowman reports that initial fuel economy numbers were 4% below non-EGR engines, but seem to be improving as the engines accumulate more miles."

This seems to be in line with what Steve Duley of "Schneider International" trucking company reported:

http://www.ttnews.com/chat/diesel/index.html

"Steve Duley: We see no advantages with the 2003 engines over the pre-October engines aside from them meeting the EPA requirements. Possibly driving responsiveness is slightly better, especially with the feel of the accelerator. The mileage per tank of diesel fuel is 3% to 5% worse. It is too early to tell on what the life-to-overhaul [cycle] will be. To date the dependability of the engines has been comparable to pre-October engines. But it is early."

Surely somebody out there knows what happens to the energy from the 3-5% efficiency drop? Does it go out in the exhaust, or is it absorbed by the cooling system?

 

[CDC]

Docengineheat

You're right, I misread that, probably because we express economy here in liters per 100 km, so if the number lowers then economy improves, stupid mistake of me. The Bosch book on Diesel engine Management draws a curve of fuel consumption that decreases, may it be minimally (1%)at about 20% EGR, at the same time the excess air factor, which started at about 3.7 in this sample, lowered also by 20% of course, while soot in g/kWh approximately drops 40%. NOx drops significantly (the major benefit of EGR, from about 5 to about 2 g/kWh. HC remains unchanged and CO goes up fdrom about 3.5 to just over 4 g/kWh.

From experience I can say that exhaust gas temperatures at medium load in passenger cars, running at about 3000 rpm (from a usable 4500 rpm) at half load will be around 400 degrees C before the turbo and at full load they reach around 750 degrees C before the turbo. This is also the max temperature we aim for when increasing the power output of passenger diesel cars above the OEM spec. Injection point is very important here and as a rule of thumb we can expect a deduction of 30 degree C per degree of additional advance from where the engines are normally (OEM) set. As with gasoline engines the A/F ratio will have a lot of influence on the correct injection point and with it the exhaust gas temperature. We do not touch the EGR program, maybe I should do some experiments but I believe that the long term tests of OEMs and fleet operators are the best test benches available. We do not pretend that our power increases reduce emissions but take care that visible emissions remain as they were. What can be noted on the dyno is that at part load, with the throttle in a fixed position, changing the injection point even a degree or two, will decrease/increase output. We program in real time and have a digital power reading. Normally we see a significant increase (5% or more), when the injection time is advanced under those conditions and at the same time have to assume that NOx will increase, otherwise OEMs wouldn't have done their job very well. The question is by how much, since the German TUV has approved various pre-programmed chip upgrades to diesel cars one can assume that the increase must be negligible. In theory and as per information from our clients (how reliable is that?) a reduction in fuel consumption at medium loads is achieved.

With regards to the 4% reduction in fuel economy noted by the fleet owners I can only see that going into the exhaust gases as unburned or too late burning fuel, if it would go into the cooling water the gas temperature would have increased and according to the various research documents available on the net you can see that it drops with increase of EGR.

 

[enginesrus]

I would say the reason economy goes. Is, 1. EGR reduces thermal efficiency. 2. EGR dilutes the contents of the combustion chamber. In other words it would do the opposite of what a nice Nitrous system will do for you.
Kinda like running a marithon breathing your old breath over and over again. Instead of a breath from a bottle of O2.