EGR Question

[BoomInTheTrunk]

I have an 81 300d and I was just wondering.
Just what does the EGR do?
Why was it put on theses vehicels?
Do you even need it?

[kerry]

Based on the theory that if you breathe in your own farts, they won't smell so bad the second time around.

[charmalu]

HAHHAHAHHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH
HARTY HAWHAWHAHAHA Now that is the best description I have ever heard.

Charlie

[pjc]

Good joke!

But seriously, it's on your 300D for the same reason it's on every other passenger vehicle sold in the US since the late 70's or early 80's: pollution control.

No engine is 100% efficient (although diesels are more efficient than petrol engines). The exhaust contains some unburned fuel and other incompletely oxidized compounds (CO, NO, etc.). The EGR valve feeds some of the exhaust back into the engine so it can undergo a second oxidation, resulting in fewer pollutants.

The downside in a diesel is that it also feeds back in the particulates, which can build up on the intake manifold and engine ports like saturated fats form plaque on the walls of your arteries and veins. It can also reduce engine power (slightly).

Many on this forum advocate disabling the EGR valve ("for test purposes only," of course). Just be honest with yourself: Whether you drive an '81 300D or an '07 Whatever, disabling the EGR increases pollution.

Originally Posted by pjc

disabling the EGR increases pollution.

...reduces fuel consumption, reduces engine wear, reduces smoke, increases power and means one less thing to break.

The EGR is an American abomination. None of the engines had it unless it was sold in N. America.

[Brian Carlton]

Originally Posted by pjc

No engine is 100% efficient (although diesels are more efficient than petrol engines). The exhaust contains some unburned fuel and other incompletely oxidized compounds (CO, NO, etc.). The EGR valve feeds some of the exhaust back into the engine so it can undergo a second oxidation, resulting in fewer pollutants.

The EGR valve does not exist to feed exhaust back into the cylinder for a second oxidation.

The purpose of EGR (exhaust gas recirculation) is to inject a small quantity of exhaust into the incoming charge to cool the combustion with the result being a lower level of nitrous oxides.

A diesel doesn't need much EGR since it's a much "colder" engine than it's gasoline bretheren and you'll not notice any increase in fuel economy or additional power from removing the EGR. However, the elimination of the EGR will result in no further buildup of sludge in the intake tract.

[kerry]

How do hot exhaust gases cool combustion?

[Brian Carlton]

Originally Posted by kerry

How do hot exhaust gases cool combustion?

The exhaust gas displaces some air. The gas provides an inert "filler" in the combustion chamber which causes the a reduction in the cylinder temperatures achieved during combustion. This temperature reduction results in lower NOx production. It's a cheap and effective system to keep the engine below the NOx limits prior to the introduction of the three way catalyst.

[ConnClark]

Originally Posted by kerry

How do hot exhaust gases cool combustion?

In a diesel when fuel is injected into a dense oxygen concentration the fuel burns quicker and has less time to disperse through the cylinder. The end result is fuel being burned in a small area which creates hot spots where NOx is formed.

What EGR does in a diesel is displace some of the oxygen as well as increase the mass of the intake charge. This slows the burn rate allowing the fuel to be more evenly distributed thus heating the cylinder charge more evenly. The water vapor and combustion products in the EGR increase the mass of the intake charge which requires more energy to heat than straight air, this also contributes to cooling. Additionally the lower oxygen content leaves less O2 to form NOx.

Note: on modern diesels they are using cooled EGR. This increases the effectiveness of the amount of EGR used meaning less EGR can be used. Also in some cases a little cooled EGR can actually get you a very slight increase in power. Unfortunately this small level does virtually nothing to keep NOx in check.