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serialk11r 01-13-2019 03:48 PM

EGR charge dilution target table tuning for mpg
I don't have a car right now, but there is a certain c. 2006 luxury gas guzzler I have in mind which would be really fun to ecomod because the gas mileage is so abysmal.

While newer engines in 2018-2019 are coming out with high volume cooled EGR, most engines before that (Prius being a notable exception) had hot EGR or internal EGR.

The usual drawback of increasing EGR is that combustion speed and stability are decreased. However, on older cars with no exhaust VVT, the % charge dilution is actually very small because the external EGR might only add a few %.

In this situation, there's two variables that would be very positively impacted by a slight increase in (hot) EGR:
- Charge temperature increases significantly because the exhaust is very hot, reducing pumping loss like a warm air intake
- As a result, the charge temperature at ignition is higher, which increases flame speed quite a lot in the absence of the dilution effect.

I imagine maximum flame speed is achieved at some non-zero EGR fraction due to this effect, otherwise knock from residual exhaust would not be a problem. If target EGR dilution is only say 2%, I imagine peak flame speed might actually go up with more EGR. It's been documented here for example that hot EGR gives better flame speed. I think the reason new cars use cooled EGR is because working fluid heat capacity ratio deteriorates as you raise the temperature, and you can put in more cooled EGR before the temperature is unacceptably high.

Has anyone with a car with external EGR messed with target EGR to see if it can boost mpg? Changing the dilution from say 2 to 4% doesn't sound like much, but it also raises the air temperature by ~20K which drops the charge density by over 10%, so it could be a substantial pumping loss savings. If you have, some shared tables would be nice :)

One idea I have is that adding EGR under load combined with lean burn could help increase combustion speed which is a problem with lean burn. Under load the residual exhaust fraction is quite low and external EGR is typically shut off (not the case for some of the new engines coming out right now), so maybe this "hot air intake" via EGR could help.

Daox 01-14-2019 08:58 AM

I know Autospeed did a series on EGR. He increased and decreased EGR flow on his Honda Insight.

AutoSpeed - EGR Comeback

AutoSpeed - Tweaking the EGR, Part 1

AutoSpeed - Tweaking the EGR, Part 2

serialk11r 01-15-2019 12:52 AM

Great link, thanks! 3% on an Insight is actually a huge improvement because the engine is so tiny already, and has lean burn mode. That said, because of its lean burn design, it probably can handle more EGR without stumbling than the typical turn of the millennium engine.

I did find anecdotes of people tuning big V8 cars for more EGR and getting fuel economy improvements, but not in a controlled experiment. If anyone else finds more articles, please share.

E.g. looking at this target EGR table kindly posted by someone who spent a lot of time hacking their AMV8/Ford EEC-6 ECU:

It's clear there's potential because the OEM added EGR as an afterthought to trim emissions a bit in certification conditions. I have a hard time imagining that EGR would be useless at 4000rpm, or above 50% load, and I have a hard time imagining 0.25% charge dilution is anywhere near the limit in the lower cells.

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