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Old 01-13-2019, 03:48 PM   #1 (permalink)
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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.


Last edited by serialk11r; 01-13-2019 at 04:25 PM..
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Old 01-14-2019, 08:58 AM   #2 (permalink)
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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
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Old 01-15-2019, 12:52 AM   #3 (permalink)
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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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Old 06-18-2020, 01:33 PM   #4 (permalink)
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Bumping an old post after I thought about it some more recently.

I'm starting to like increasing EGR more than lean burn. Wideband O2 controllers are not all that expensive, but you need to change them out for emissions testing, you have to richen a bunch of parts of the fuel map, and you're only going to get maybe 10-15% dilution before you have ignition problems.

The beauty of EGR is it's basically a better warm air intake. The hot exhaust brings the temperature of the intake air up, reducing its density quite a lot more, but you don't need the hardware of a WAI to make it happen. The higher heat capacity ratio of the exhaust gas makes it less likely to knock compared to WAI at the same temperature, so you can go hotter. At 20% exhaust dilution (which is the limit for the usual spark ignition engine found by Toyota as well as some academic research), you're looking at on the order of 100K increase in temperature, which is around a 25% drop in air density.

So if we think about some older engines (intake VVT only or non-i VTEC) that don't really utilize significant internal EGR, boosting EGR dilution by 15% via an external tube could bring the MAP up from say 40kPa to 60kPa, or 50kPa to 75kPa, which is a massive difference.

After some thinking I realized manufacturers probably decided to avoid large volume EGR to prevent heat soak for aluminum intake manifolds, which can be mitigated by spraying the inside of the manifold with paint as thermal insulation.

If you have a car with external EGR controlled by the ECU, this is definitely the first place to look for mpg, IMO. Ignition advance does need to be increased quite a bit which can take some time, but a rough guideline would be to copy the value from the cell that's x% lower in load if you're using x% exhaust dilution.

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