How does a EGR really work, scientifically

[jadziasman]

EGR is strictly an emission control device in (most) naturally aspirated gasoline engines.

To reiterate - EGR reduces combustion temperatures by reducing the amount of elemental oxygen (02) in the intake air charge. At high combustion temperatures, the oxygen in the intake air combines with nitrogen to form NOx.

By adding "oxygen free" exhaust gas to the air charge, you reduce the ability to burn or oxidize the fuel since less oxygen is now available to burn the fuel. The combustion does not reach a very high temp because there is just not enough oxygen to reach it for a particular amount of fuel.

Supercharged engines run at high combustion temps because there's extra oxygen to burn the fuel due to compressed (dense) air being supplied. They need premium fuel and use more robust components (forged pistons, etc.). I don't know much NOx supercharged engines produce but NOx production is primarily a lean burn issue. Supercharged engines are calibrated to the rich side so the excess oxygen combines with the fuel to make HC CO2 and CO instead of producing NOx.

As gary said, the drawback is a loss of power but lean burn engines like the VX would not last as long as they do without EGR. High combustion temps would eventually wear out the piston ring grooves, compression rings, the exhaust valve guides and seats over years of use. Then the Civic D-series (with EGR) engines would not last until 350K miles plus they often make it to. Of course, some hypermilers drive more conservatively than grandmas and blind people so they never have to worry about it - LOL!

I don't know what the maximum amount of EGR in volume % is used in a VX. It would be interesting to find out how much. Any wild guesses? My guess is 15%.

[dkjones96]

http://en.wikipedia.org/wiki/Exhaust_gas_recirculation

The three reasons given for EGR:

A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:

* Reduced throttling losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
* Reduced heat rejection. Lowered peak combustion temperatures not only reduces NOx formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
* Reduced chemical dissociation. The lower peak temperatures result in more of the released energy remaining as sensible energy near TDC, rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.

It also decreases the efficiency of gasoline engines via at least one more mechanism:

* Reduced specific heat ratio. A lean intake charge has a higher specific heat ratio than an EGR mixture. A reduction of specific heat ratio reduces the amount of energy that can be extracted by the piston.

[VetteOwner]

Originally Posted by DRW

I always wondered if EGR improves or hurts mpg?
I've found plenty of anecdotal evidence that goes both ways.

well since i have disabled both of mine, the cars was plugged (someone put a BB in the vacum line so it LOOKED connected but wasnt) i thought it was odd, got it out hooked it up car started and idled fine then fell on its face when i gave it any gas. my guess is it broke and someone "fixed" it. that car doesnt have any emissions stuff on it as it all broke/clogged/ wore out.

my truck, no matter what i did/how i drove where i went it got 25mpg, thought it was odd so started asking about, someone said the egr if its failing can cause gas mileage issues, i physically removed the thing, made a metal plate and bolted that over the opening. instantly gained 2-3 mpg and it ran smoother(it used to buck and sputter trying to cruise at 30 in 4th gear, 3rd gear was to high and the shift light would come on, removed egr instantly fixed that problem)

i would replace them but not for $80 each if it does nothing performance or mpg wise...

[DRW]

So you're saying a broken EGR can hurt FE, so removing it improves FE.
Thanks for the input, I just realized the hot start stumbling issue I'm getting might be from too much EGR. :lightbulb: !
This is why I like this forum, there's good info bouncing around everywhere!

[imzjustplayin]

Originally Posted by DRW

So you're saying a broken EGR can hurt FE, so removing it improves FE.
Thanks for the input, I just realized the hot start stumbling issue I'm getting might be from too much EGR. :lightbulb: !
This is why I like this forum, there's good info bouncing around everywhere!

Of course, any car that is running improperly will get worse mileage.

[VetteOwner]

yea it can, allowing too much exhaust in the intake and the computer tries to compensate by making it overly rich.

[dkjones96]

Originally Posted by VetteOwner

yea it can, allowing too much exhaust in the intake and the computer tries to compensate by making it overly rich.

Not saying it won't because of that, but in a mass air EFI system the EGR is introduced after the sensor meaning EGR is unmetered, the ECU doesn't really care about it since it's supposed to be inert anyways. In a speed-density system the MAP sensor would read higher manifold pressures but the O2 would confirm that the current mixture was fine and it 'shouldn't' adjust it.

A malfunctioning EGR can cause reduced power which usually leads to more throttle and worse mileage. Especially when you go past 80% throttle where most ECUs go into fuel enrichment mode and the more egr in the intake the richer it'll run. Optimally you wouldn't have any vacuum pulling in EGR under heavy load but between intake restrictions and exhaust restrictions you will if it's sticking.

[GasSavers_Pete]

I understood there was a simple displacement factor involved to decrease fuel consumption.

As the EGR is taken from the exhaust and sent back to the inlet manifold the engine computer decreases the fresh air and fuel injected by a similar amount.

Pete.

[dkjones96]

As manifold vacuum falls the amount of air that is pulled in through the same throttle opening is decreased.