Does an egr help that much?

[mwebb]

average peak combustion pressure occurs at about 15 degrees after TDC
pressure does NOT increase after this time

so to answer the question :

nothing causes pressure to increase after combustion because pressure decreases after combustion
============================
now then
the current VW TDi (diesel) system does use a throttle body type flap
near the center of the exhaust to increase exhaust pressure to assist EGR at various times as determined by the ECM
and
the current VW TDi does have a throttle body type flap in the intake , again it is controlled by the ECM and part of it's function is to assist with EGR

both of these "throttle body" type flaps are drive by wire , there are no "throttle cables" or any mechanical connections to them , the ECM has control using little motors with position sensors on the flaps.

the EGR gas flows from a higher pressure area to a lower pressure area as assisted by the ECM

Originally Posted by George Tyler

the other thing, what causes the pressure to increase after combustion? The heat produced. Until combustion the pressure is higher, a similar amount of heat is produced, but as that heat is heating an increased mass of gas the temperature rise is reduced.

[mwebb]

EGR does not have any effect on Air fuel ratio
EGR does NOT make the system richer
EGR does NOT make the system leaner

EGR dilutes the air fuel / mixture with an INert gas , dilution does not change the ratio of air to fuel in the Air Fuel Mixture

because EGR is INert it does not combine with the air Fuel mixture
EGR takes up space
EGR reduces the available DISPLACEMENT of the engine but not the physical size .

[old jupiter]

Those who are still making up their minds can Google this and find mechanics and service bulletins stating the contrary.

[roflwaffle]

Originally Posted by old jupiter

The Wikipedia article (Mr. Taylor's link, above) cites two effects as gains, that are actually net losses. "Reduced Throttling Losses," comes because the throttle must be opened a little more to make up for lost power. Put your foot to the floor and you can REALLY reduce throttling losses, but are you saving fuel? And "Reduced Heat Rejection," again should have been noted as a measure of the fact that your heat engine has a cooler fire inside it with EGR. What we'd like to do is make the MOST heat, and get the most out of that heat, with the smallest amount of fuel, emissions aside.

Mechanic, I'm not picking on anyone, and anyone reading Mr. Webb's posts can see he's a smart guy.

At the same average speed more throttle (via P&G or taller gearing) on most vehicles will reduce fuel consumption. The problem with higher peak combustion temps is that engines aren't perfect containers of heat, so the more heat that's made, the more we loose. Depending on the materials increasing combustion temps may not yield any gains.

Originally Posted by mwebb

[COLOR="Blue"]EGR does not have any effect on Air fuel ratio
EGR does NOT make the system richer
EGR does NOT make the system leaner

It depends on the system. If something is carb'ed with no feedback or post mixture feedback (eg a smog pump), then EGR will make the system a bit richer. If the system has O2 feedback then you're right, whatever regulates the A/F mixture will cut back on fuel to keep it at stoich.

[mechman600]

Originally Posted by tjts1

Disabling EGR was worth about 1mpg improvement on my old volvo 240. It also improved throttle response at part load cruise. Definitely worth disabling on some old cars where the EGR was a simple ON/OFF system. The only way to really find out if it helps or hurts your FE is if you try it.

Just the answer I was looking for! I just bought an '84 240 for $400. I was trying to figure out my EGR plumbing, and with some help from an online parts schematic, I realized that someone has removed/disabled it. I was wondering about the effect it would have and whether I should try to get an EGR valve working, but thanks to you, I don't have to!

Funny that the last emission test from this past June had driving NOx at 925 PPM and the max allowable is 2087 PPM.

[user removed]

Originally Posted by old jupiter

The Wikipedia article (Mr. Taylor's link, above) cites two effects as gains, that are actually net losses. "Reduced Throttling Losses," comes because the throttle must be opened a little more to make up for lost power. Put your foot to the floor and you can REALLY reduce throttling losses, but are you saving fuel? And "Reduced Heat Rejection," again should have been noted as a measure of the fact that your heat engine has a cooler fire inside it with EGR. What we'd like to do is make the MOST heat, and get the most out of that heat, with the smallest amount of fuel, emissions aside.

Mechanic, I'm not picking on anyone, and anyone reading Mr. Webb's posts can see he's a smart guy.

Well, you seem to believe what many people did in the 60s and 70s when EGR first came on the scene. Get rid of it an your car will run better. It didn't work then and today with the much more sophisticated systems it won't work now.
One point you completely missed was compression ratios. The statement about WOT not increasing efficiency is not what we are discussing. The early egr systems were atrocious. I know how hard it was to start a 1973 Chevrolet, worked for a dealership when they were new. The only way the car was even drivable was to let it run for a few minutes. If you started it and put it in gear it died, over an over.
Compression ratios were as low as 8 to 1 in the era. No matter what you did with the EGR the compression ratio was the problem. EGR allowed advanced timing which helped some but with a compression ratio barely better than my 37 Ford Flatheads 7to1 power was not an option.
EGR is a principle component of emission controls. Applied properly EGR allows much higher compression ratios. Today Mazda's SKYACTIV engines are running 14 to1 on regular gas, impossible to do on any non EGR equipped engine in current production, and very close to diesel CRs.
Disabling any EGR system is also illegal, and recent legislation allows for prosecution of individuals who are responsible for disabling their emission controls on cars, the fine is $2500 per offense.
It used to be that they went after businesses that disabled systems.
In the early 1990s when 3 way cats and oxygen sensors became the norm, manufacturers were able to raise compression ratios and increase power, even on cheap gas and with fully functional emission systems. Fuel injection was a principle component of this advancement, with precise delivery of fuel.
Today after two decades of refinement you see power levels than have never been reached by production vehicles before. The Honda S2000 produces 245 horsepower from 122 cubic inches without supercharging. These levels of power have no historical context prior to 1975.
Modern systems also allow for ignition timing control with spark advance to the best point for ignition regardless of the fuel quality. Put cheap gas in and the system retards the timing, put premium is and the system advances the timing. The engine will run better with premium as long as it was designed to run better with premium. If not it's a waste of money.
In all of this progress engines became cleaner. The average engine in the 60s was tired at 100k. Today engines can easily pass 200k with regular maintenance. Oil in my Maxima stays clean and the level does not fall, even though it has 140k on the odometer. I just had the EGR cleaned to restore function.
I would seriously consider trying to understand the emission systems before you do the same thing people did 40 years ago, removing or disabling those systems will not make your car run better AS LONG AS THE SYSTEMS ARE WORKING PROPERLY. Individual re engineering of those same systems can cost you dearly, if enforcement every decides to enforce current laws.
To many people decide they are better engineers than the ones that originally built your car. They even start to re engineer before they even know whether the existing systems are functioning properly.

regards
Mech

[TheEnemy]

Mech covered it pretty good, but one more thing to touch up on.

NOX pollution is created by high combustion temperatures, the creation of NOX absorbs energy from the combustion. NOX is one of the primary pollutants that cause smog, and is not good for your health or for the plants arround you. NOX = BAD.

High compression makes higher combustion temperatures, which is why during the early years of smog control compression ratios were droped to the lowest spec I have seen was 7:1

Like any devleoping technology the early systems were junk, now they are highly effective and efficient.

[E4ODnut]

Just to add fuel to the fire.

I don't doubt that newer EGR technology will both enhance economy and reduce emissions. But. If you have ever disassembled a fairly high mileage late 80s or early 90s engine, even with good maintenence, the intake tract is disgusting. What does this do for economy and emissions in the later stages of engine life? I really don't know, but I doubt that it's good.

Ever try clean one of these engines up for rebuild?

Not fun.

[UFO]

Originally Posted by E4ODnut

Just to add fuel to the fire.

I don't doubt that newer EGR technology will both enhance economy and reduce emissions. But. If you have ever disassembled a fairly high mileage late 80s or early 90s engine, even with good maintenence, the intake tract is disgusting. What does this do for economy and emissions in the later stages of engine life? I really don't know, but I doubt that it's good.

Ever try clean one of these engines up for rebuild?

Not fun.

You think the gas engines with EGR are disgusting, the diesels are far worse. They have higher compression for more EGR flow, and they get more oil blowby that coats everything. I've seen completely plugged intakes on cars with less than 100k miles.

[tjts1]

EGR is a cheap poorly conceived bandaid for emissions. Most newer gasoline engines don't use external EGR. They manipulate valve overlap and timing to achieve the same effect.