Managing EGR for MPG

[omgwtfbyobbq]

Eh... It makes the average intake/cylinder temperature higher because exhaust is hotter, but it reduces the peak cylinder temperature because the intake charge contains less oxygen per volume. If too much exhaust gas/heat is present, the fuel may combust before the spark fires.

[kps]

Originally Posted by rh77

OMG, I just looked it up -- it turns out that it's the main component in Cancer cells, people use it on their yards, and it's even been found in bottled drinks!

That's why I stick to ones with the legal minimum of that nasty dihydrogen monoxide and a maximum of that nice environmentally-friendly ethanol.

[Sludgy]

Originally Posted by kps

That's why I stick to ones with the legal minimum of that nasty dihydrogen monoxide and a maximum of that nice environmentally-friendly ethanol.

<URP> I'd rather have a bottle in front of me than a frontal lobotomy. <URP>

[rh77]

Originally Posted by Sludgy

<URP> I'd rather have a bottle in front of me than a frontal lobotomy. <URP>

An old member here added dihydrogen monoxide in an experiment to improve FE. We never heard from him again...

Also, the darn stuff comes out of the exhaust pipe already!



RH77

[JanGeo]

you are forgetting a couple of things - first the gasses will have cooled at the tail pipe and on the way back in the hose to the intake, second is the load is light when vacuum is high and that's when you need more exhost recirculated and that means low exhost temps anyway because you are not burning a lot of fuel - plus less O2 would inject less fuel and you will loose power that will require more throttle and more air and lower pumping losses . . . exactly what you want.

[omgwtfbyobbq]

Check this out.

Further, in both two and four stroke cycle experiments spark ignition was possible with high levels of internal EGR but was not possible with an equivalent amount of air or N2, even with compensation for EGR's effect on charge heating.

The point being that EGR has limits with every engine and fuel. If it didn't we could ideally route as much exhaust as is needed to the engine in order to eliminate pumping losses and achieve diesel efficiency with gasoline engines. But we can't because we can only heat the intake charge so much or we risk pinging, which is why there's no EGR on idle, such a small quantity of fuel heats up much quicker with the hot exhaust gasses and is more prone to combusting before it's supposed to. It's also why EGR coolers (pg.7) are being considered. They allow a cooler intake charge that reduces the octane requirements needed to run an engine, effectively reducing one of the limits to the percentage of EGR, the other being spark retardation. Essentially, diesel efficiency and characteristics from a low octane gasoline powered SI engine.

[MetroMPG]

Originally Posted by omgwtfbyobbq

But we can't because we can only heat the intake charge so much or we risk pinging, which is why there's no EGR on idle

High EGR also leads to mushy/delayed throttle response, which is another reason it would be avoided at idle - reasons of driveability.

---

Another question I've been mulling over: if EGR were "manually" introduced to my engine, how quickly would it respond with correct fuel metering? Initially, would it not lead to an over-rich condition because the engine is pumping fuel based on readings from the MAP and TPS?

Would an O2 sensor respond quickly enough that this would even be a feasible experiment?

Does it even have enough "influence" on fuel metering to preserve stoich A/F mixtures at 20 or 25% EGR?

I don't know enough about O2 sensors.

I suppose the same question could be asked in terms of driving at high altitudes: "At 9,800 feet, for example, there's about 2/3 of the oxygen in the air than at sea level." - source

Even if you were driving at 4500 feet, there may be 15% less o2 available than at sea level. Modern engines must be able to adjust to that, or we'd all be spewing soot driving through some of the passes in the Rockies.

[DRW]

Thanks to the inspiration I get from this site, I dug into the ECU disassembly for my car and found an interesting section. The problem is, I'm not sure what to do with it? Here's a copy/paste, have a look:

;************************************************* *******
;
; EGR solenoid duty cycle
; as a function of rpm(column) and airVolume(row)
;Value of $00 to $80 will produce 0 to 100% duty cycle
;************************************************* *******
t_egrDutyFact
#ifdef E931
RPM 500 1000 1500 2000 2500 3000 3500 4000
Load
1. $10 .byte $00, $00, $00, $00, $00, $00, $00, $00
2. $20 .byte $00, $00, $00, $5b, $56, $4c, $4d, $00
3. $30 .byte $00, $00, $00, $5b, $56, $4c, $4d, $00
4. $40 .byte $00, $00, $00, $5b, $56, $60, $5c, $00
5. $50 .byte $00, $00, $60, $76, $6c, $5f, $5b, $00
6. $60 .byte $80, $80, $80, $7d, $6e, $5f, $60, $00
7. $70 .byte $80, $80, $80, $80, $76, $68, $61, $00
8. $80 .byte $80, $80, $80, $80, $80, $60, $61, $00
9. $90 .byte $00, $80, $80, $80, $80, $78, $6e, $00



;************************************************* ********
;
; EGR solenoid duty cycle as a function of ECT
;
; Value of $00 to $80 will produce
; 0 to 100% duty cycle
;
;
;************************************************* ********
t_egrDuty .byte $80, $80, $5b, $4f, $00, $00, $00, $00

ECT degreesF- 186 176 125 95 69 46 19 3 ->
************************************************** ********

As you can see, this is where EGR is handled. I know where this section is located in the code, so I can change any of the bytes to adjust the duty cycle.

I've searched for good articles about EGR, but mostly I seem to find basic info about what it does. As you might have guessed from the disassembly, I'm looking for reasons why EGR might be useful or harmful to FE when increased/decreased as the engine sees different loads and different temperatures. If needed I can also make adjustments to the fuel and/or ignition timing maps to optimize the EGR effect. But like I said, where do I start? If anyone out there has run across some detailed tech info about EGR, let me know. I'll keep searching too. If the code isn't clear, just ask.
Thanks!

Dave W.

[rh77]

This is all just a guess...

Since ECT regulates the EGR as a function of emissions/warmup, that part should probably be left alone.

I'd reccommend "baby-steps" in introducing small amounts of EGR in low-RPM, low-load situations and see how driveability and FE react.

For example, in the $40 column of load, at 2000 RPM, add a $4f (from the engine temp byte); see how that works, and do the same for $30 and $20. Since it's an experiment, document your findings at each change, and see what happens.

I would then keep adding a small amount of EGR at lower load and RPM, if it works, and maybe until it's no longer running properly or efficiently.

I couldn't tell from the data, but is there a smaller amount than $4f for EGR duty (such as $3x)? You may look into that and add even smaller amounts if the larger one is too much. Good luck!

RH77

[cfg83]

DRW -

No intelligent statements, but here is what the data looks like in 3D :



CarloSW2