EGR function

[GTRS Fiero]

The EGR is supposed to recycle unburnt fuel in the exhaust.  Unburnt fuel is only a small portion of the exhaust gases.  The EGR sits on the crossover pipe.  The crossover pipe is much larger than the EGR, so flows more gauses.  The EGR sits more or less on the top edge of the crossover pipe, so even with some vacuum, only pulls a small portion of the gases back through the combustion chamber.  The EGR  is located between the front and rear exhaust manifolds.  The front manifold has a more direct path out of the echaust than by the EGR, and even if it got by the EGR, there is no guarantee that it would go through the EGR.

So, of the exhaust gases, the 50% from the front 3 cylinders will never be processed by the EGR.  We can assume that exhaust pressure is about equal to the intake vacuum.  Of the 50% from the rear 3 cylinders, the EGR can pick up at most 16%. (.25"/1.5")  So, only 8% (the half for the rear 3 cylinders) of the total exhaust gases could be processed by the EGR at most.

Of those 8%, how much of that would possibly be unburnt fuel?  Is there something about the EGR that attracts unburnt fuel?  Certainly, much of the gases that flow through the EGR into the intake must necessarily be just exhaust.

If we figure that a huge number like 10% of the exhaust is unburnt fuel, and that 8% of that makes its way into the EGR, that is less than 1%.  So, 98% of what flows through the EGR into the intake is non-combustible gases.  98% of the unburnt fuel still flows out the exhaust.  But the bumbers are worse than that.  Only half of that 10% goes by the EGR, so only .4%.  So, 99.6% of what flows back into your engine is unburnt, and 99.6% of the unburnt fuel flows out the exhaust.

What am I missing?

[Fierofool]

Yep!  What you just said.   ......I think! 

[GTRS Fiero]

How does that help anything?

[TopNotch]

From Wikipedia:

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. This dilutes the O2 in the incoming air stream and provides gases inert to combustion to act as absorbents of combustion heat to reduce peak in-cylinder temperatures. NOx is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure. Another primary benefit of external EGR valves on a spark ignition engine is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses.

Nothing to do with unburnt fuel.

[GTRS Fiero]

Um, everything to do with unburnt fuel, supposedly.

From autoblog:

In the 1960's, the automotive industry experienced a renaissance with the invention and introduction of the exhaust gas recirculation (EGR) system. The intent of this system, which was launched in 1966, was to find a way to reduce the particulate matter or “smog” that would flow from vehicle exhaust pipes. Smog is essentially nitric oxides, or unburnt fuel in the form of NoX. The EGR system is able to take unburnt fuel that comes from the exhaust manifold and recirculate the molecules into the intake manifold. This allows the engine to consume less "new fuel" which increases fuel economy and also significantly reduces vehicle emissions. The component that distributes these unburnt fuel molecules to the intake is known as the EGR tube.

[Fierofool]

How does that help anything?

I didn't understand what you wrote and couldn't dispell any of your data, so I just agreed. 

[TopNotch]

The autoblog quote contradicts itself. Nitric oxide is "burnt", that is, oxidized, nitrogen. And they're calling it "unburnt fuel".

[GTRS Fiero]

The autoblog quote contradicts itself. Nitric oxide is "burnt", that is, oxidized, nitrogen. And they're calling it "unburnt fuel".

I am not disagreeing; I merely seek understanding of things like this:

If the EGR Valve Sticks Open:
This will cause a continuous flow of exhaust gases into the intake manifold. You'll notice one or more of the following symptoms:

A rough idle upon starting the engine (that is, when the engine is cold) and sometimes at a stop light or while looking for a spot in a parking lot (that is, at low engine speeds in a warmed-up engine).
Stalling when the engine idles.
Increase in fuel consumption.
A slight—or strong—fuel odor while operating the vehicle, because of the increase in hydrocarbons leaving the tailpipe (see the next symptom).
Your car fails the emissions test. When the engine is running at low RPM, lower temperatures in the combustion chambers prevent all the fuel from burning, so the flow of unburned hydrocarbon gases coming out of the tailpipe increases significantly.
The Check Engine light (or Malfunction Indicator Light, MIL, depending on your model) illuminates on your dashboard.

From https://axleaddict.com/auto-repair/Bad-EGR-Valve-Symptoms-and-What-to-Do-About-Them

[GTRS Fiero]

How does that help anything?

I didn't understand what you wrote and couldn't dispell any of your data, so I just agreed.

I wasn't being rude.  I just do not understand how exhaust gas in the combustion chamber helps anything.

[TopNotch]

I just do not understand how exhaust gas in the combustion chamber helps anything.

If nothing else, it helps the air to be cleaner.

[GTRS Fiero]

I was thinking more in terms of the engine itself.  It would seem that an engine that is breathing exhaust fumes would run less well.