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Old 11-29-2011, 03:26 PM   #11 (permalink)
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Colorado - '17 Chevrolet Colorado 4x4 LT
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Originally Posted by Diesel_Dave View Post
Originally Posted by UFO View Post
... when the CEL is lit, the ECM will retard the timing to avoid the creation of NOx and mileage will decrease.
Bingo! there's your reason for FE getting worse by removing EGR.

EGR wil improve FE at the same NOx level, however, if all the other engine settings are the same, I have never, ever seen FE improve with EGR on a diesel.

There's lots of research out there on this, here's one research paper:

Note that the legend is incorrect (it says brake thermal efficiency when it should say brake specific fuel consumption)
Figure III is the one you want to look at. Note that higher EGR always increases BSFC.
That is an interesting paper. I do wish they had provided enough excess air to burn the fuel completely even at higher levels of EGR; that would seem more fair. Of course, that might have adversely impacted the NOx production, I cannot divine the intent of the author in that regard. Then again, the test engine is not equipped with turbo, so extra air is hard to come by.

The paper's conclusions seem simple enough given the data, and that's why I want to demonstrate a measurable fuel efficiency gain on the TDI. This same type of EGR defeat has been done for the US model of the Jeep Liberty CRD (Cherokee in Europe), and although my wife drives one, I have not tested its fuel efficiency. Others who installed that circuit claim a 10% improvement in mileage, but it is undocumented.

I can believe it makes a significant difference though, as for some reason the US model CRD controls a huge amount of EGR. I pulled the MAP sensor when I first bought it, and with 30k miles it was completely encrusted with oil/EGR accretion. That Jeep had EGR defeat installed around 45k miles, and now has 95k miles. Probably time to check the MAP sensor again, although when it gets gunked up, engine operation is noticeably affected.

I'm not coasting, I'm shifting slowly.
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