Exhaust Gas Recirculation? 1

[phrostbit3]

I was just reading up on anti-lag systems the other day and after the site (

http://www.rallycars.com/Cars/bangbang.html)

mentioned all the disadvantages of anti-lag, they mentioned something called Exhaust Gas Recirculation is being developed as a new anti-lag technique. After searching Google all I could find about Exhaust Gas Recirculation was that it was being used to recirculate exhaust gas back into the engine's intake so as to reduce NOX emissions. However, this doesn't explain why it is being developed as a new anti-lag technique...Does anyone know how this works or does anyone know how this COULD work so as to reduce turbo lag?
Any comments would be great.


- Olek

 

[strokersix]

Since EGR is injecting inert exhaust gas back into the inlet manifold, the total volume of exhaust gases will be greater, keeping exhaust pressure and turbo speed up.

Mike

 

[Andy330hp]

I'm not sure I understand how recirculating a given mass creates additional mass. What goes in equals what comes out. If total intake air is not increased, then how is exhaust flow increased? If anything, it seems like it is delaying peak exhaust flow by diverting it back through the engine. Is this delay what they are looking for? Maybe by timing the EGR opening/closing, you can "save" the exhaust pulse for a few tenths to get the turbo going just before throttle is reapplied. This would be helpful for shifts, but not for large decels or off-the-line.

Also, inert gasses produce cooler combustion, hence less volume even with the same mass. I'm not sure which is more important though.

Please elaborate!!!

 

[strokersix]

OK, here goes:

For the sake of discussion let's ignore flame speed, heat transfer, pumping work, squish effects, etc. Assume that the engine produces power in direct proportion to the amount of fresh air and fuel mixture ingested. With that assumption, adding inert exhaust gasses will increase the total mass of gas in the cylinder, total mass exiting the exhaust port, and therefore more exhaust flow available to drive the turbine. Of course the EGR must be taken downstream of the turbine.

This effect is one benefit of EGR on naturally aspirated spark ignited engines. At low loads there is little mass in the cylinder and squish effects are less, causing less turbulence and incomplete combustion. Adding inert exhaust increases squish and turbulence which can have a benefit of improved fuel economy.

Someone please correct me if this logic is flawed!

Mike

 

[phrostbit3]

Thanks for the posts guys. Mike, your logic seems to be sound but I thought of another way that EGR could be used for earlier spool up time: exhaust gas downstream of the turbo could be taken and recirculated back into the exhaust coming out of the exhaust port, thereby adding much more exhaust to what is already coming out of the engine. I'm not sure if this is a good theory it's just one way I thought of right after the reading the info on the site.


- Olek

 

[Retracnic]

phrostbit3,

Recirculation of exhaust gas from gases taken downstream of the turbo would not be very effective. Turbos work by converting the heat energy of the exhaust gas into mechanical motion. As the gasses leave the exhaust port, they are in the neighborhood of 1800 F. On most cars, the gas manages to shed the bulk of it's heat energy before reaching the tail pipe. EGTs downstream of the turbo are often less than half of the upstream temps.

Some benefit might be seen from the simple act of adding mass to exhaust charge, but the overall energy potential of the charge would change very little.

Good Luck

Bryan Carter

 

[Marquis]

Adding EGR WAS once common practice to reduce Nox emissions-by reducing peak cylinder combustion temperatures. However it is now common practice in the industry to use it to increase fuel economy ( it obviously still benefits Nox emissions). It was not used in the old days to increase fuel economy because in the old Pre catalyst days, cars were allowed to run lean and weren’t required to run stoichiometric 14.7:1 AFR.
Bascially the EGR or residual exhaust gases are being used now to dilute the fresh intake charge at PART load stoichiometric operation. The EGR does NOT increase squish, neither does it increase turbulence. It improves fuel ecconomy at part load by reducing pumping losses. To further understand this consider this:
An engine requires, say 20 Nm for a drive condition to carry out a task. The car with no EGR is running at throttle position A, the car WITH EGR is running at throttle position A+20 degrees . Therefore the car WITH EGR is running less throttled with less pumping losses because the throttle is more open but because the EGR is an inert gas it produces the same torque (20Nm in this case).
There are two types of EGR often used, internal and external.
Internal EGR uses the engines cam overlap to induce it, while external EGR uses external piping from the exhaust to feed the intake plenum. The only major difference is external EGR is cooler and hence better for Nox, perhaps worse for HC emissions, and quite often adversely prone to transient effects and distribution-may be less controllable.

From this logic we can see where the anti lag thinking has come from.
In a turbo charged car we want to keep the turbo spinning fast and when the throttle is closed, the volume throughput of gases from the exhaust manifold isminimised-this leads to lag-when the throttle is opened again. I would say that the use of EGR allows you to have the throttle further open at part load then you would normally, and hence allow a higher throughput of exhaust gases to keep the turbo spinning, while overall the torque produced at that part load site would be the same, as the car with no EGR which is running a more closed throttle. It is obviously a transient effect, but then again, so is Turbo lag.

One more point, the addition of EGR does NOT increase mass of gas entering the cylinder ( this is misleading)- Not at full load. Infact if EGR were to be used at full load, it would lower the engines volumetric efficiency because there is EGR in the cylinder when there could be fresh charge instead. This is why it is only used during part load operation, where the engine becomes less throttled by using a more open throttle to compensate!

 

[patprimmer]

Thankyou Marquis
It now makes sense

Regards
pat

 

[ivymike]

It was not used in the old days to increase fuel economy because in the old Pre catalyst days, cars were allowed to run lean and weren’t required to run stoichiometric 14.7:1 AFR.

That's odd, I thought that in the old days cars always ran on the rich side of stoich to keep the in-cylinder temps down (hence all the stories about running lean -> "burnt valves).

I also wasn't aware that we are currently required to run @ stoich. Are you quite sure about that? It seems like there are lean-burn engines in current production automobiles (Honda vehicles, for example).

I know that stoich operation (or even a little bit rich) is the standard at full-load, but that doesn't really pertain to an EGR discussion...

 

[ShaunT]

Has anyone ever considered closing the wastegate for only an instant? Doing this would put all the exhaust into the turbo. Talk about EGR!

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[GregLocock]

Been done, oo, 15 years ago. Lotus Esprit Turbo Intercooled and Carlton had it. We called it overboost, great fun in dry weather. I think it ran the turbo unregulated for about 1 second before normal service was re-introduced.



Cheers

Greg Locock

 

[Retracnic]

ShaunT,

Has anyone ever considered closing the wastegate for only an instant?

If one makes use of a quality aftermarket external wastegate, along with electronic boost control, the wastegate does remain closed until the desired boost level is reached. OE suppliers tend to use internal wastegates, with conservatively weak spring pressures. This causes the wastegate to “flutter” and begin opening long before the desired boost level has been reached.

Marquis

I would say that the use of EGR allows you to have the throttle further open at part load then you would normally...

ALS (anti-lag system) uses either a solenoid to hold the throttle open, or an air bypass valve in the manifold to achieve a similar effect. Along with the throttle position, the ALS severely retards ignition timing to radically increase exhaust gas temperatures. The end result is that the ALS puts more energy into the exhaust than normally would be available at a given engine RPM. I think people are beginning to look at EGR as a strategy because ALS is not fit for street use and is it also greatly reduces the turbochargers service life.

Bryan Carter

 

[Marquis]

"That's odd, I thought that in the old days cars always ran on the rich side of stoich to keep the in-cylinder temps down (hence all the stories about running lean -> "burnt valves).

I also wasn't aware that we are currently required to run @ stoich. Are you quite sure about that? It seems like there are lean-burn engines in current production automobiles (Honda vehicles, for example).

I know that stoich operation (or even a little bit rich) is the standard at full-load, but that doesn't really pertain to an EGR discussion..."

At part load older cars ran lean, Porsche 944's ran at lambda=1.2-1.3, at full load you want maximum torque so they ran rich, say lambda =0.9

The Japanese market is different to most, where Nox regulations are less stringent. Alot of japanese market cars run lean on part load, Honda S2000s and mazdas, are not unknown to be optimised for part load fuel economy,say lambda =1.4

The compromise is that during the part load "emissions test operation window",the catalyst does converts most efficiently at stoich.
Richer then stoich, and HCs increase, leaner Noxs increase.
The US market regulations, such as stage4, are partculalry stringent on Nox, while the European market is more strict on HCs (hence you see places like Sweden taxing farmers on even how much HCs their Cows make!). Running stoich reachs a good compromise of both HCs and Nox.

Lean burn engines for world markets usually have lean Nox traps such as Zeolite converters-which periodically have to be purged by the engine during the normal running cycle. It's also common for world market lean running cars to have a large proportion of ceria in their normal 3 way catalysts to encourage oxidation. Lean burn world market cars are of low volume and quite specialised.



At WOT, cars , especially boosted ones run far richer then stoich, quite often for either component protection and/or Close coupled catalyst protection. Usually this is out of the emissions test window, but there are groups that want vehciles to run stoich even at WOT-provided the component protection criteria has not been contravened