"leaning" with EGR or air? - Fuelly Forums

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Old 07-03-2008, 07:34 PM   #1
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"leaning" with EGR or air?

With all of the talk about "lean burn", a/f ratios, keeping the throttle open to increase dynamic compression and reduce pumping losses... what are the advantages and disadvantages to using extra exhaust gas vs. extra air to dilute/lean the mixture.

Leaning mixes (with air) means they will burn hotter and can preignite or misfire.

What about simply diluting with high EGR? It burns cooler and since exhaust gas is inert (non reactive) you really wouldn't be changing the a/f ratio. Disadvantage- I could see how that non-homogeneous EGR/AF mixtures may be hard to fire off with a single spark plug.

How about a throttleless gasoline engine that varies speed and power output by increasing or decreasing the proportion of exhaust gasses that the engine is breathing? Compression would always be at max and there would be no vacuum for the pistons to pull against during the intake stroke.

The reason why I bring this up is because I have been running my 87 civic with and without my EGR vacuum line hooked up and I seem to be getting better mpg with EGR.

Maybe even more EGR could be better... anyone ever try increasing EGR?
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Old 07-03-2008, 08:07 PM   #2
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You can probably find more information on this if you keep searching on the net. I have seen a fair amount of research on this subject and purchased a few SAE studies. There is an optimum EGR ratio for best mileage. It is usually limited by ones ability to light the mixture. This has been an avenue I have thought about some. I put a garden hose up the tailpipe of my Saturn and fed the other end into the air cleaner. Of course this was unregulated but it did not take much to kill the engine. When I did this I was looking for a drop in the Scangauge gph because at that time I thought the Scangauge was looking at injector pulse width. I now believe the Scangauge is calculating fuel burn from a speed density airflow calculation so my conclusion of higher fuel flow was flawed.

I have not noticed much ignition retard on my Saturn with hot intake air. I have never had any pinging running quite hot intake air. I have guessed that EGR is one of the strategies used is to help control detonation. Mind you it is only a guess but that might be one reason Saturns and some other GM cars respond well to hot intake air. Many GM cars use a stepper motor controlled EGR valve and some of those engines do very well in cruise. Maybe there is too much speculation in my thoughts and some one knowledgeable of GM's programs can shoot holes in my theory.

One of the things mentioned about hydrogen addition to the air stream is that it makes it possible to ignite higher ratios of EGR. If I was a little more motivated I could test this idea fairly quickly. I think EGR variation is why some cars respond so much more favorably to hydrogen that others even without playing with the oxygen sensor.

Sorry about all the speculation but I think there is some relevance.
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Old 07-03-2008, 10:51 PM   #3
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I maybe wrong but on my engine the EGR is controlled with a water heat valve, this basically opens a vacuum feed plunger, it allow the Exhaust gas back into the intake via a pipe to anover heating engine, the idea is the engines cools down with less oxygen, the fuel remains the same but it cools the engine down as its sprayed in.

I am currently trying to convert my EGR system into a steam boiler so I can feed steam into my air intake, it turns into Hydrogen and Oxygen at 1000 degrees Celsius, it sounds to good to be true I know, its still in the shed.
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Old 07-04-2008, 05:30 AM   #4
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Originally EGR was used to reduce peak combustion chamber temperatures which dramatically reduced NOX emissions.

Peak combustion temperatures are directly associated with power. The higher the peak temp the greater the "leverage: of the pressure differential that pushes the piston down and produces horsepower and torque.

Peak combustion chamber temperatures also cause preignition.

EGR also allows timing to be advanced beyond what would normally produce preignition, which increases peak pressure as long as that peak doesn't occur before the piston reaches TDC, or you get knocking.

EGR controls generally do not allow EGR at idle, since combustion pressure is already low due to restricting the intake air well below a "cylinder filling event" . Eger is generally introduced at greater cylinder filling events.

A cylinder filling event occurs when the cylinder recieves the maximum amount of air and fuel due to low throttle restriction. This can happen at smaller throttle openings when the load on the engine is high enough.

Adding more EGR will directly reduce combustion pressure, which could be offset somewhat by more advanced timing, but there is a point where your power produced drops off fairly rapidly, then you get into misfiring.

Think of it like this. Instead of restricting the incoming air, control the engine by restricting the exhaust. That is in essence controlling the EGR by increasing exhaust backpressure.

I am sure the idea has been tested thoroughly, since restricting the exhaust is a fairly easy method of testing.

regards
gary
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Old 07-04-2008, 06:16 AM   #5
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Originally Posted by R.I.D.E. View Post
Peak combustion temperatures are directly associated with power. The higher the peak temp the greater the "leverage: of the pressure differential that pushes the piston down and produces horsepower and torque.
Just being nitpicky here, but can we mentally file that as "the closer to the peak for the fuel(s) in use" because you can get a greater expansion ratio out of some fuels that run cooler, and gasoline is rather thermodynamically inefficient in that regard. Just wanna clarify that a bit, peak power on gasoline = hot, but peak power on something else might be not as hot as gasoline, but still the peak of which it is capable. Ergo, sticking something else in there and getting a cooler motor is not necessarily bad, it's not more heat = more power, it's optimum heat for what you put in = more power.

Having a "working fluid" rather than atmospheric Nitrogen in there also modifies the thermodynamics equation that defines optimum efficiency. While it might be true to say that in an engine running pure air and atomised droplet gasoline at 20C, that the point of peak heat production is the point of peak power production, adding other stuff, such that a lower peak temperature is seen is not necessarily detrimental. Retuning the air/gas ratio to lower heat gets less than optimum power, but keeping that ratio the same and adding extra stuff that makes the temperature peak less than the peak on gas/air alone does not necessarily mean less than optimum power, or less power than with gas/air alone. But that depends on getting a smaller SHC rise with temperature with one non-fuel gas than another.

However, unless there is a state change or reaction involved that has the input at higher SHC than output per unit mass, then essentially the bulk of any efficiency gain comes from i) pumping loss reduction ii) heat transfer into mixture before combustion or iii) reduction of heat lost to endothermic processes (production of NOx)
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Old 07-04-2008, 06:46 AM   #6
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Old 07-04-2008, 11:41 AM   #7
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Just run the egr gases past the tip of the fuel injectors.
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Old 07-04-2008, 12:15 PM   #8
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I have a half-assed plan for a new kind of induction system a bit like that... the exhaust gases drive and time fuel atomisation. Really need an old carbed car to mess around with it on. Pulse width would be modified by something a bit like a PCV valve with variable spring tension.
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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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Old 07-04-2008, 03:01 PM   #9
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The only real reusable force in the exhaust gas is its velocity and heat, the picture if the DEI is making the rounds on the internet but what the guy has accidentally made is the Rankine system used in power plants, the only problem is there is no fission or heat source.

If they can put fission material in a sub or boat why not miniaturise it into a car
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Old 07-04-2008, 04:08 PM   #10
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Quote:
Originally Posted by RoadWarrior View Post
I have a half-assed plan for a new kind of induction system a bit like that... the exhaust gases drive and time fuel atomisation. Really need an old carbed car to mess around with it on. Pulse width would be modified by something a bit like a PCV valve with variable spring tension.
I have a 650 Demon carb i'm doing mods too on a 351c. I was going to do the mods to the 750 edelbrock. The idle curcuit has easy access. But I tryed EVERYTHING too get that carb to work!
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