How does a EGR really work, scientifically - Fuelly Forums

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Old 02-20-2009, 05:30 PM   #1
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How does a EGR really work, scientifically

I understand what it does and why. but I don't really understand how it really work on a technical scientific level. It seems like a really bad idea dumping exhaust gas into the intake charge. I haven't found any info that explains it in a why I can understand or its really basic.
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Old 02-20-2009, 05:36 PM   #2
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well theres always some unburtnt fuel in the exhaust especially with the cars back in pre 1975 with thier non changeing A/F ratio there was lots of unburnt fuel leaving in the exhaust, this way with an egr it can allow some of that unburnt fuel along with fresh stuff to be reburned, less fuel wasted, less fuel in the atmosphere. one of the many clean air emmisions crap mandated in 75.

then it just kinda hung around even with fuel injected cuz the computers were changing the AF ratio but not very fast/ accurately.

some cars dont have a EGR valve nowadays since the computers and OBD2 are so much better and accurate.
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Old 02-20-2009, 06:02 PM   #3
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ok, putting the CO back in the intake to burn it completely makes sense.

you'd think they make better cats. heat is energy anything you do (turn on the fan, valve timing to reduce dynamic compression) you going to lose heat to reduce nox. If there was an adaptive learning egt sensor so the absolute minimum exhaust gas was dumped in.
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Old 02-20-2009, 07:05 PM   #4
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The purpose of EGR is to reduce the combustion temperature to reduce the production of NOx, which is generated when combustion temps are too high. NOx creates smog which the EPA and some other folks don't like too much.

Exhaust gas ( in theory) contains very little oxygen. When reintroduced into the intake manifold, the exhaust gas takes up some of the volume of the next charge of incoming air - reducing the amount of total oxygen available to oxidise (burn) the fuel. Less oxygen - less NOx.

That's basically the idea.
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Old 02-20-2009, 07:20 PM   #5
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EGR was first used in 1969 I believe.

The purpose is to reduce peak combustion temperatures from 3500 degrees to about 3200 degrees to drastically reduce NOX (nitrous oxide) emission levels. NOX is a major contributor to the smog that those who are old enough may remember as a yellow haze around most major cities in those days.

It also allows for more advanced timing without spark knock, as many people who disconnected the EGR systems learned quickly.

The negative side is peak combustion temperatures also equate to peak pressure and more power. When unleaded fuel came along, compression ratios were reduced which exacerbated the lower combustion pressure levels and power dropped off dramatically.

The advent of the oxygen sensor and 3 way catalyst with their ability to control emissions much better allowed engines to be tuned more for power again. Nissan incorporated them in theri Z car in September of 1980 as a 1981model, and at that time all the Z cars were legal in Califirnia, whereas before they required two different emissions setups.

The first 240 Z, built in November of 1969, had EGR.

The first fuel injected 280 Z in 1975, as well as the 76 model did not have EGR systems, but they were used in 1978.

Air pumps and air injection were used to burn unburned fuel in those days. Nissan used air injection until they switched to fuel injection in 1975 on the Z car.

That was one of the reasons I especially like the 75-76 Z cars, no EGR, no air injection. Only PCV valves. You never had a tar build up in the intake manifold from the mixing of EGR and crankcase vapors from the PCV.

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Old 02-20-2009, 07:22 PM   #6
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Beat me to it Jadziasman LOL.

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Old 02-20-2009, 08:57 PM   #7
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Quote:
Originally Posted by jadziasman View Post
The purpose of EGR is to reduce the combustion temperature to reduce the production of NOx, which is generated when combustion temps are too high. NOx creates smog which the EPA and some other folks don't like too much.

Exhaust gas ( in theory) contains very little oxygen. When reintroduced into the intake manifold, the exhaust gas takes up some of the volume of the next charge of incoming air - reducing the amount of total oxygen available to oxidise (burn) the fuel. Less oxygen - less NOx.

That's basically the idea.
less oxygen as in less to even covert to oxides of nitrogen? but also that the total btu's are less because of the lower VE which burns less fuel at the same afr?



Quote:
Originally Posted by R.I.D.E. View Post
EGR was first used in 1969 I believe.

The purpose is to reduce peak combustion temperatures from 3500 degrees to about 3200 degrees to drastically reduce NOX (nitrous oxide) emission levels. NOX is a major contributor to the smog that those who are old enough may remember as a yellow haze around most major cities in those days.

It also allows for more advanced timing without spark knock, as many people who disconnected the EGR systems learned quickly.

The negative side is peak combustion temperatures also equate to peak pressure and more power. When unleaded fuel came along, compression ratios were reduced which exacerbated the lower combustion pressure levels and power dropped off dramatically.

The advent of the oxygen sensor and 3 way catalyst with their ability to control emissions much better allowed engines to be tuned more for power again. Nissan incorporated them in theri Z car in September of 1980 as a 1981model, and at that time all the Z cars were legal in Califirnia, whereas before they required two different emissions setups.

The first 240 Z, built in November of 1969, had EGR.

The first fuel injected 280 Z in 1975, as well as the 76 model did not have EGR systems, but they were used in 1978.

Air pumps and air injection were used to burn unburned fuel in those days. Nissan used air injection until they switched to fuel injection in 1975 on the Z car.

That was one of the reasons I especially like the 75-76 Z cars, no EGR, no air injection. Only PCV valves. You never had a tar build up in the intake manifold from the mixing of EGR and crankcase vapors from the PCV.

regards
gary
great info


advacing timing, doesn't it not mean you have to advance timing to make up for the less dense charger. not to make more power then you would without it and leaving the timing alone?
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Old 02-20-2009, 09:57 PM   #8
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I always wondered if EGR improves or hurts mpg?
I've found plenty of anecdotal evidence that goes both ways.
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Old 02-21-2009, 04:10 AM   #9
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Quote:
Originally Posted by DRW View Post
I always wondered if EGR improves or hurts mpg?
I've found plenty of anecdotal evidence that goes both ways.
Would it be difficult to experiment and find out how it affects your car?
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Old 02-21-2009, 06:23 AM   #10
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Forgot another reason for EGR:

When the EGR system is active, intake manifold vacuum drops given the same rpm and throttle position. Less manifold vacuum = less pumping losses = better cruising mileage.

One of my many projects that I haven't gotten to yet is actually to build an engine that is throttled almost entirely on the amount of exhaust gas that is recirculated through the engine so it has almost no intake vacuum at most throttle openings. Should heat up to full temp pretty quick too!
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