Using DFCO to increase your MPG

[Loren]

Valid point, Steve. We're probably seeing most of our gains in MPG simply from changing our driving habits. DFCO is a good excuse to do it, but we wouldn't be using THAT much more fuel if the engine were idling at all times.

Let's say I was driving conservatively and went 400 miles on 10 gallons of gas. (a reasonable assumption for me, I usually do much better than that in city driving) My average speed for this tank of city driving is 30 mph, so total driving time is 13.3 hours.

Let's say that instead of clutching in and letting the engine idle, I spend 20% of my driving time coasting in DFCO. How much of a difference is it making? 13.3 hours x 20% = 2.66 hours at .2 gph... call it .5 gallons.

400 miles / 10 gallons = 40 mpg
400 miles / 9.5 gallons = 42.1 mpg

Wait... 42.1/40 = 5.2% improvement!

So, yeah... the bulk of our improvements (easily 10-20% over EPA without trying hard) come from driving style changes. But, I'd say that there's a solid 3-5% or more to be gained from making good use of the DFCO feature in city driving. And for people who practice the "pulse and glide" technique, DFCO can improve their highway mpg, too.

No, it's not magic. No, it's not "new" technology. But, now that people know about it, they have developed repeatable driving techniques to take advantage of it.

Footnote: Is it possible to spend 2.6 hours of a tank of gas in DFCO? I think so. Using myself again: My typical trip is maybe 20 miles round trip. If I don't hit at least 20 stops on that trip, I'm stunned. So, for each of those stops, I'm doing a full 20 seconds or so of DFCO coasting. Call it 7 minutes for every 20 miles. 20 of those trips on a 400 mile tank gives us 2:20 just from traffic lights. That's not including the amount of pulse & glide that I do. I'd say for any given mile that I spend cruising at speed, I'm DFCO coasting at least 10 seconds. (if my cruising speed is 45, that's 10 seconds out of every minute and a half, not at all unreasonable) This might only apply to half the miles traveled (the rest of the miles are spent accelerating or decelerating due to stops), so there's 200 miles x 10 seconds each for another 30 minutes of DFCO. I think my estimates here are very conservative for when I'm seriously "hypermiling", and they've added up to 2:50 of DFCO time.

While I completely get what you're saying, Steve... you can't deny that DFCO absolutely CAN make a difference in fuel economy for the driver to takes complete advantage of it.

And it all goes hand in hand. If you're maximizing your DFCO time, you ARE driving conservatively.

[sbergman27]

Originally Posted by Loren

Footnote: Is it possible to spend 2.6 hours of a tank of gas in DFCO?

Actually, that's not the right question. You must spend that amount of *extra* time, over and above what would have been the case had you never even heard of DFCO, in order for the changed driving technique to result in that much improved fuel economy. And modern DFCO is designed so that people *don't* need to know about it to benefit from it. (I think the RPM cutoff on the Yaris is 1100 rpm?)

As mentioned in my previous post, this is about people having an accurate understanding of what is going on. And appealing to the fact that efficient driving practice involves a lot of interacting factors is really irrelevant to that point. Because the benefit specifically attributed to conscious use of DFCO *can* be separated out, and is relatively small in the grand scheme of things.

That doesn't make DFCO bad or insignificant. It just defines its place in the landscape better.

-Steve