EGR Anti-lag?

[spdracer22]

I have heard mention of "EGR Anti-lag" systems being utilized on WRC rally cars, but I cannot find any details about how they work.

The only thing I have read is that they recirculate exhaust gasses back to the turbo manifold through a 'cat-like' device to burn residual fuel and increase manifold pressure.

This sounds plausible, but it raises questions. For instance, how is reversion through this device prevented? Is there really enough unburnt fuel available to increase exhaust gas pressure by a significant amount?

Any help is greatly appreciated.

 

[ivymike]

it seems unlikely that pressure will increase in the direction of flow in a continuous flow-through device (assuming you don't encounter a shock).

That leaves one to wonder what you could be describing. Since you're calling it an "egr" system, you must be feeding it exhaust gas. If you're taking that exhaust gas through a catalyst, then wherever it comes out must be at noticeably lower pressure than where it comes in. If you put it back into the same manifold, then you're not going to have any flow. You'd need to take it from a high pressure point and return it to a low pressure point... but where will you be able to find both of those upstream of a turbo?

You could have any quantity of extra fuel that you want, really, by overfueling the cylinders and/or adding fuel to the exhaust manifold. You won't be able to burn that fuel without also having excess air, which you would typically not have. If you add air you must pressurize it first, since all of this is happening upstream of the turbo (at higher-than-ambient pressure).

 

[spdracer22]

Exactly. And, that's where I am confused. I just don't see it working in the way I have seen it described.

What I have seen - and it works very well - is connecting the charge pipe (post compressor) to the turbo manifold and controlling the flow with a valve of some sort (electronic, pneumatic, etc). When the anti-lag system is engaged, this valve opens and feeds compressed air directly into the manifold, feeding the extra fuel.

Given the low AFRs of most performance engines, especially on initial drop-throttle, there is a definite excess of fuel in the exhaust stream. Combining air injection with a high temperature element in the turbo manifold might do the trick, but it's not really a recirculation system.

Having done a little bit more research, some people are using egr valves to do what I just described above. Maybe this is where the egr reference came from...?

 

[WGJ]

Do you mean this:

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

http://www.madabout-kitcars.com/kitcar/kb.php?aid=458

http://video.google.co.uk/videosear...ult_group&ct=title&resnum=10&ved=0CDMQqwQwCQ#

Bill

 

[spdracer22]

Specifically this statement from the rallycars.com link:

"Starting in 2002 new anti-lag techniques, such as Exhaust Gas Recirculation (EGR), are slowly overtaking the method described above as they are kinder on the engine's mechanical parts."

I know how the more conventional/common forms of anti-lag work, but this 'egr' system is new to me, and it's the details of this specific setup that I am trying to find out.

 

[JSteve2]

The EGR is thrown in at the end as a competitor of the bang-bang. The bang-bang essentially transfers combustion energy directly to the turbine wheel during low power operation (although in operation it seems very crude).

EGR itself is not helpful during low load operation. However, EGR can be used like a fluid fly wheel - high EGR flow rate maintained at low power and EGR flow rate briefly reduced at power request which can reduce the turbo lag. The effect can be enhanced with a variable geometry turbocharger.

You would probably not want to put the bang-bang and EGR on the same system, as some of the EGR hardware (coolers and valve) would probably not make very good combustion chambers, and the turbo boost effect would be reduced. That is, if you ever want to put the bang-bang on anything.

 

[spdracer22]

Thanks, JSteve. That makes perfect sense. That was the answer I was looking for.

 

[alpa38]

All that makes sense.
When throttle is closed the dump valve is open, so the compressor virtually does not consume power.
The intake pressure is very low.
If at that moment EGR is open the engine starts sucking still hot gas from the exhaust. The compression work will heat them again, the cooling system does not dissipate all the created heat. This should create some gas energy that may help the turbine in spinning. As gearboxes are very fast this process should not last more than few 0.1 seconds.
I understand (is that correct ?) it's a way of using the car's energy to keep the turbo spinning.

 

[alpa38]

I ment "it's a way of using the fly wheel's energy to keep the turbo spinning".

 

[JSteve2]

That's kind of right - but you can also keep a high mass flow going with EGR at idle. There are many schemes that could be used, and they depend upon the levers available in the system and the creativity of the controls team. US Patent 6,178,749 at Figs. 4a, 4b, and 4c conveys one general way to do it that.

I do not: advocate that patent, say that it works as shown, have knowledge if that patent is valid or not, claim any affiliation or unaffiliation, or any of that stuff, and all other caveats I can think of are thrown in here as well. It just happens to have 3 pictures that fit the discussion.

 

[alpa38]

I think the mentioned patent solved a radically different problem related to the usual management strategy of the EGR valve.