Confused on EGR

[vristang]

I have read several posts here that seem to contradict what I have learned in school. How does utilization of EGR increase fuel economy? I understand that at part load there is less oxygen thus less fuel assuming all is well. As an example, if it takes 55% throttle to maintain 70mph without EGR, wouldn't you need more throttle (maybe 56%) to maintain that speed with EGR, since the motor is operating at a lower thermal efficiency. More throttle results in more fuel. I would think that this would counteract the benefit of fuel economy.
What I have learned is that the EGR decreases the combustion temps by using inert (less O2) exhaust gasses. This is only done to control NOx.
When I eliminated my EGR (for packaging reasons) I did not experience a decrease in fuel economy. At least not that was noticeable.
If you think you can straighten me out, please speak up!!!

 

[Zenix]

I'd say your right, but the gain/loss of fuel economy or power would be so small it probably wouldn't be worth much of anything. Besides, doesn't the ECU freak out if you just take off the EGR?

 

[jrichker]

The EGR system has a vacuum source (line from the intake manifold), a computer operated electrical vacuum regulator, the EGR valve and the passages in the heads and intake manifold that route exhaust gas to the EGR valve. The computer uses RPM, Load. and some other factors to tell the vacuum regulator to pass vacuum to open the EGR valve. The EGR sensor tells the computer how far the EGR valve is open. Then computer adjusts the signal sent to the EGR vacuum regulator to hold, increase or decrease the vacuum.

The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine to ping.

Here's a book that will get you started with how the Ford electronic engine control or "computer" works.

Ford Fuel Injection & Electronic Engine Control 1988-1993 by Charles Probst :ISBN 0-8376-0301-3.

It's about $20 from Borders.com see http://www.amazon.com/ . Select boo...very good, and I found it to be very helpful.

 

[vristang]

Thanks for the book reference. I love finding new literature!

Still have basically the same question though.

HOW does EGR improve fuel economy?

 

[chobracobra]

The addition of exhaust gas drops combustion temperature,

how can adding burnt hot gasses lower combustion temps?

 

[Paul Perreca]

if the difference is so minute, whats the big deal, leave it on! lol
Paul Perreca

 

[The Shape]

how can adding burnt hot gasses lower combustion temps?

Because the exhaust gas is still cooler than the combustion temps. Plus it is cooled further by the coolant that flows through the EGR spacer. Lowering the coolant temps in the combustion chamber this way allows the computer to dial in more timing. Removing the EGR without a custom chip to let the computer know has resulted in pinging for some people.

 

[stock 92 gt]

i would just assume to leave it on.....but if you plan on buying a bigger TB and also some other things that require the computer to be twEECed than dont worry about the EGR. or am i wrong?

 

[Michael Yount]

By injecting a small amount of exhaust into the manifold, a like amount of air/oxygen is displaced. This inert gas in the combustion chamber results in cooler temperatures during combustion. Because the combustion temps are cooler, the computer can add more timing advance and lean the fuel mixture out without risking detonation. If you were to hook up a wide band O2 sensor and look at a/f ratios during times when the egr is working, you'd see mixtures leaner than stoichiometric - 15, 16, 17:1. That's how fuel is saved - without having to worry about detonation as much - timing is advanced and fuel is pulled to run a leaner mixture.

That's why when egr systems fail, detonation is frequently a symptom that shows up. The computer keeps adding timing and pulling fuel, but without the inert exhaust to cool the chamber, combustion temps rise, and detonation often occurs.

 

[Farva]

that was good info michael i understand it better now.

 

[chobracobra]

i still dont understand how re-using exhaust gasses lowers temps and is better than sucking in, plain cool fresh air.

 

[Michael Yount]

When you dilute the air/fuel mixture with exhaust (which is inert) -- you have less air/fuel mixture to combust. Less air fuel mixture combusting means less pressure and therefore less temperature.

Do a google on 'exhaust gas recirculation'. There are 10's of thousands of explanations. Because temps are reduced, NOx emissions are also reduced.

 

[Michael Yount]

By the way - if you simply let more cool, fresh air in (oxygen comes as a part of it), and you put the same amount of fuel in, the mixure becomes leaner. All things equal, the leaner the mixture, the hotter it is (more NOx created; greater chance of detonation). The richer the mixture the cooler it is.

But you don't want to cool the mixture by making it richer - that wastes fuel, and creates more unburned hydrocarbons and more carbon monoxide.

EGR allows NOx be reduced, and less fuel to be used.

 

[Michael Yount]

Here's a blurb which summarizes it by Henry Guzman:

"Exhaust gas recirculation (EGR) systems were introduced in the early '70s to reduce an exhaust emission that was not being cleaned by the other smog controls. Oxides of nitrogen (NOx) are formed when temperatures in the combustion chamber get too hot. At 2500 degrees Fahrenheit or hotter, the nitrogen and oxygen in the combustion chamber can chemically combine to form nitrous oxides, which, when combined with hydrocarbons (HCs) and the presence of sunlight, produces an ugly haze in our skies known commonly as smog.

How to reduce NOx NOx formation can be reduced by:

* Enriching the air fuel (A/F) mixture to reduce combustion temperatures. However, this increases HC and carbon monoxide (CO) emissions.
* Lowering the compression ratio and retarding ignition timing; but this leads to reduced performance and fuel economy.
* Recirculating some exhaust gases.

How EGR systems work The EGR valve recirculates exhaust into the intake stream. Exhaust gases have already combusted, so they do not burn again when they are recirculated. These gases displace some of the normal intake charge. This chemically slows and cools the combustion process by several hundred degrees, thus reducing NOx formation.

The design challenge The EGR system of today must precisely control the flow of recirculated exhaust. Too much flow will retard engine performance and cause a hesitation on acceleration. Too little flow will increase NOx and cause engine ping. A well-designed system will actually increase engine performance and economy. Why? As the combustion chamber temperature is reduced, engine detonation potential is also reduced. This factor enabled the software engineers to write a more aggressive timing advance curve into the spark timing program. If the EGR valve is not flowing, onboard diagnostics (OBD) systems will set a code and the power control module (PCM) will use a backup timing curve that has less advance to prevent engine ping. Less timing advance means less performance and economy. "

 

[89MustangGX]

Does this mean that the EGR actually increases performance?

Or does it decrease performance but as a byproduct more timing can be used which gains some back (or increases even?)?

 

[Stang8URMPRT]

Does this mean that the EGR actually increases performance?

Or does it decrease performance but as a byproduct more timing can be used which gains some back (or increases even?)?

The EGR does not fuction at WOT, so performance is not hurt.

 

[HISSIN50]

Does this mean that the EGR actually increases performance?

Or does it decrease performance but as a byproduct more timing can be used which gains some back (or increases even?)?

yeah, Nick is right (and Adam, i know you knew that). i would not say that it increases performance, but it sure does increase non-demand performance and economy.

 

[Michael Yount]

I would say it increases performance in the area of emissions reduction and fuel economy (you have to define what you mean by performance), without harming peak HP generation as it only functions during light load/part throttle conditions. It's one of the few win-win emission/fuel economy gadgets. The only downside is the clutter.

 

[vristang]

I finally got off my butt and did some research in my old textbooks. Here is the answer which was eluding me. The following is from Introduction to Internal Combustion Engines by Richard Stone.
“The part-load fuel consumption is reduced because EGR leads to a reduction in the throttling loss-to admit a given quantity of air the throttle has to be more fully open, thereby reducing the pressure drop (and the loss of work) across the throttle.”(p.155)
I still wonder if this increase in engine efficiency will translate to an increase in vehicle efficiency. Has anyone noticed a change in fuel consumption (better or worse) when switching from EGR to without, or vice-versa?

As mentioned by others, another advantage of EGR is that you can run more ignition timing. In fact fuel economy is again improved if the ignition timing is optimized for power on an EGR vehicle, when compared to non-EGR. I would assume that Ford did a fairly decent job with the spark advance.

Thanks for all the comments, and the book recommendations (I already ordered that and another Ford book from Amazon).

Chobracobra,
Think of the engine as an air pump. When the fuel is burned the combustion heats the air. When the air is heated it expands, forcing the piston down. If less air is in the combustion chamber then less work can be done when it is heated and expands.

Thanks for all the responses. EGR seems to be a little confusing and difficult to properly understand.

 

[89MustangGX]

yeah, Nick is right (and Adam, i know you knew that). i would not say that it increases performance, but it sure does increase non-demand performance and economy.

I would say it increases performance in the area of emissions reduction and fuel economy (you have to define what you mean by performance), without harming peak HP generation as it only functions during light load/part throttle conditions. It's one of the few win-win emission/fuel economy gadgets. The only downside is the clutter.

I know that it does not function at WOT, so my question is, how do you run more timing with it? Or is it that you can run more timing in the curve but total timing is the same?

That's what I'm really trying to ask.

If you compare two identical cars, one with and one without EGR -- will one be faster in a race or will you only see the difference in fuel economy and emissions (and the car show!)?

Hopefully that's a better way to ask ... ?