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Old 11-22-2007, 10:34 AM   #1
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EGR Overrides and Strategies

Making a secondary EGR came up in another thread.... I happened to have this in a bookmark and would like to open a discussion....

EGR Strategies

IAT overrides
EGR tends to increase the chance of surge symptoms. The recycled exhaust gases inhibit combustion and increase sporadic partial misfires when the engine is operating under a variety of compromising circumstances.

When the air/fuel mixture is too lean or unstable, EGR operation can cause surge complaints. The injection nozzles on most new engines are placed in the intake manifold very close to the combustion chamber. This subjects the nozzle to very high temperatures that can cause vapor locking in the nozzle, which is prevented by the cooling effect of the intake air.

But when the IAT is very high, the nozzles can partially vapor lock and can increase the chance of engine surge symptoms. Under these conditions, EGR commands are reduced or eliminated. Most port fuel-injected engines will start reducing EGR flow when the IAT sensor indicates temperatures of 130?F to 150?F. This override strategy does not apply to throttle body injection.
So considering combustion is inherently unstable (at least, according to a colleague working on feedback controls for a swirl burner), what exactly does that mean?

But more, on the subject of hot air intakes -- I remember members saying that at certain high temperatures, their FE actually decreases. This could potentially be the reason why and perhaps worth investigation - EGR output vs. IAT input.

Thoughts on this or anything else in the article?

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Old 11-23-2007, 07:24 AM   #2
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IAT temp vs FE wall for a saturn S-car seems to be around 200f. I like to see around 170-180 for max FE. I tried a few tanks with the EGR unplugged and didn't really see any difference. The runs were high speed highway. However when cruising below 60mph the scangauge reported gains of 10-20%.

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Old 11-23-2007, 08:40 AM   #3
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I've played with the egr solenoid duty cycle map in my ecu in hopes of getting better FE. I get slight misfires at very light throttle opening, which is also when my lean burn code is at it's leanest. My car has port fuel injection. The injectors are about 4" away from the intake valves. I have a standard air intake, and temps stay within 20*F of ambient temps.

There's also the chance that the misfires on my car are due to the extremely small fuel injector duty cycle used at such light throttle, since it takes a certain ammount of time for the injector to open and close, and the duty cycle may be less than the open/close time, which would lead to inaccurate fuel metering. btw, I'm using Denso 660cc injectors, and a big injector needs a smaller duty cycle to supply a given ammount of fuel, plus the bigger injectors take longer to open and close, which makes it worse.

When I first felt these light throttle misfires I changed the EGR map to reduce EGR at light throttle in the affected rpm range, and it seemed to help. I still get misfires at very light throttle, but it's just a reminder I'd be better off coasting a little than trying to Drive With Load at such an inefficient load.
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Old 11-24-2007, 04:23 AM   #4
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Might try making these plugs?

commercial version:

If you are leaning things need plugs that can better fire a lean mix?
Leading the perpetually ignorant and uninformed into the light of scientific knowledge. Did I really say that?

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