How Does the EGR Work, Anyway?

[Stealth TDI]

Hi,

I already know that the EGR’s purpose is to introduce exhaust gases into the intake manifold for further combustion to reduce NOX emissions. But how does it actually work? I have two main questions/comments:

1) Many have wondered if a bypassed EGR contributed to an unnamed crippled NB due to higher EGTs. I can’t understand how this can be. Wouldn’t a properly functioning EGR that is pumping hot exhaust gases into the intake manifold RAISE EGTs? I don’t see how bypassing the EGR will increase EGTs if hot exhaust gases are left out of the equation. Can someone explain?

2) At what point are the exhaust gases added to the intake? It would seem that introducing them anywhere post-turbo (charged air) would restrict the low-pressure exhaust gases from entering the high-pressure intake manifold. Also, what good is the intercooler if the air will just be reheated by exhaust gases anyway? Do the recirculated exhaust gases go through the intercooler to be cooled, too? If so, could that be the source of oily residue that “elephant-hosees” are still finding in the intercooler?

These questions may seem a little dumb to some. I MAY be the only one analytical enough to actually ASK these questions. But I’m sure I’m not the only one that ponders the answers.

Thanks In Advance,


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Scott's Jetta TDI Page and TDIClub.com Stuff
98 Jetta TDI w/90k, 802-mi/55-mpg (best)

 

[GoFaster]

1. The exhaust dilutes the air/fuel mixture under part load conditions (i.e. most of the time), which reduces the peak combustion temperature under most operating conditions, and that's what reduces the NOx.

Having thought about it some more, I don't think a bypassed EGR would contribute to future engine problems. The EGR only operates at part load. The peak combustion temperatures are lower at part load than at full load anyway, even without the EGR. (Since most driving is done at part load - at least for the EPA test! - the fact that it spews NOx at full load doesn't have a big effect on total emissions.) So at full load, whether the EGR is connected or not, the heat load on the engine is the same, since even if the EGR is connected, it's off under those conditions anyway. And since the heat load is lower at part load anyway, so what?

2. The exhaust is added to the high-pressure air as it goes into the intake manifold, after the turbo compressor and after the intercooler, the last thing before the engine itself. The exhaust doesn't go through the intercooler. The exhaust is coming from the high-pressure side of the exhaust manifold, before it goes out through the turbo.

Although the exhaust does raise the temperature of the intake air, it still reduces peak combustion temperatures even though the intake temp is higher. The newer cars have a cooler for the exhaust gases before they go into the intake manifold, to reduce the intake temperature even further.

The system operates as follows. For a given fuel quantity, the ECU calculates the optimum air quantity. If the measured amount of air going into the engine exceeds the desired amount, the ECU commands the EGR valve to open. The exhaust gases displace some of the air which would otherwise be drawn in, thus acting to reduce the amount of air going into the engine. If the measured amount of air is less than optimum, the ECU commands the EGR valve to close, and it may cut back the amount of fuel to be injected in order to avoid producing black smoke.

Brian P.
'96 Passat TDI

 

[mickey]

The EGR can only function when the pressure inside the intake is lower than the exhaust backpressure. That means that it can only be open when there is little or no positive boost. GoFaster has it right: Disabling the EGR doesn't affect engine operation during high load/high boost since the valve would be closed anyway.

-mickey

 

[SkyPup]

See this thread for more disucssion of EGR

http://forums.tdiclub.com/NonCGI/Forum4/HTML/001779.html

 

[Stealth TDI]

Hi,

Thanks for the info. I didn't know the EGR performs any cooling. I thought more heat would result in a more complete combustion. I also didn't know the EGR is liquid-cooled. I guess I learn something everyday.

Thanks Again,

Scott

 

[christi]

NOx emissions occur in a diesel due to the extreme excess of oxygen that is present at anything less then full load.

This does not happen on a petrol car as the intake system is throttled to reduce the amount of air (oxygen) that the engine can ingest at part load.

The extreme quantities of oxygen result in very complete (and therefore clean) combustion of the diesel fuel, but also combustion (oxidisation) of nitrogen. Nitrogen is the main constituent of ordinary, breathable air (I believe). The Nitrogen burns to make NOx.

This does not occur at full power, as there is not an excess of oxygen at full power, all the oxygen is being used to burn diesel, and therefore none is left to oxidise Nitrogen.

For this reason a diesel will not spew NOx at full power.

The ECU on the TDI knows how much fuel it has injected, and it has a map that tells it how much air is needed to burn that much fuel.

The ECU modulates the EGR valve to introduce exhaust gasses instead of air. It increases the modulation until the Mass AirFlow sensor tells it that the correct airflow is present for that much fuel.

At full power the map will tell the ECU that it needs as much air as it can get, just to burn all the diesel without smoke, so the EGR valve will stay firmly shut.

It does not matter that exhaust gases for EGR do not flow through the intercooler. The intercooler is only needed for the top 10% of power, when the EGR is closed anyway. If you never demand more than 90% of power, then you don't need an intercooler.

The oily residue in the intercooler is from the crank case breather system introducing an oily mist into the intake system, and I guess any oil that gets past the oil seals in the turbo.

This is the problem with EGR. When that oily mist gets to the inlet manifold and then mixes with exhaust gasses, it mixes with any particles of soot in the exhaust gases and makes an oily black slime. This then gets baked onto the inlet ports of the cylinder head; bad news

Also the particles of soot are slightly abrasive and wear out the engine faster

Also the reduction in oxygen levels at part throttle actually result in a less clean burn of the diesel, resulting in more particulate emissions

Now the last part of the riddle. In order to burn Nitrogen to form NOx, you need high termperatures, high compression, and an excess of oxygen.

For some wierd reason, introducing hot exhaust gases, instead of cold air actually does reduce temperatures inside the cylinder. In fact enough to be below the temperature required to burn nitrogen. It is definitely true, even if I can't understand how.

Maybe hot exhaust gases are more compressible than cold air, effectively reducing the compression, or something...

The latest TDIs actually have coolant hoses wrapped around the EGR system to cool down the exhaust gases a bit before they are ingested into the motor, apparently this reduces NOx emissions even more than the older system.

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1996 Passat Tdi estate (wagon (variant))
See my Peugeot / Passat site

 

[Powder Hound]

The reason EGR works to cool the EGT is that by diluting the intake charge, the combustion process cannot be as efficient as if the intake charge were clean outside air. The process proceeds more slowly and will produce much lower peak temperatures.

This is basic chemistry: dilute the reagents and a reaction proceeds much more slowly.

WOT conditions shut down the EGR system because the assumption is that you need the power for emergency conditions such as passing that truck up a hill with oncoming traffic quickly closing the gap. Under these conditions, most systems also shut down the a/c compressor and alternator if possible.

Gas engines have been saddled with EGR systems for at least 25 years (at least in the US of A). Their NOx problems are at least as problematic as diesel NOx problems, even if it is only because there are so many more of them. A gas engine has hotter peak EGTs therefore can produce lots of NOx. Lower compression ratios and lower intake charges due to the throttle plate helps this situation, though, as does the catalytic convertors they have had for a long time.

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"I love the smell of [burning diesel] in the morning. It smells like ... victory!"

 

[christi]

European gassers don't have EGR, just three way cats.

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1996 Passat Tdi estate (wagon (variant))
See my Peugeot / Passat site