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Old 08-10-2008, 08:43 PM   #1
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Gasoline-altitude hybrid?

One of the observations on the ill-fated FL trip that I thought counter-intuitive was that the mileage was better in hills than on flat ground.

I had assumed that opening it up on level ground would provide the best mileage. But in areas where there were hills, as long as the hills weren't so steep that it downshifted, mileage was improved. This was done on cruise control so the speed was constant in both cases. I interpreted that to be that the engine increased power going up hill. The higher power increased the thermal efficiency of the engine (RPM was constant) The increased power at the improved marginal efficiency was stored as potential energy (increased altitude) which was then recovered on the downhill side.
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Old 08-11-2008, 07:13 AM   #2
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I believe your analyzation is correct.
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Old 08-11-2008, 07:19 AM   #3
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ECU controlled cruise controls look more efficient than trying to keep a steady speed yourself in hilly country, because I think they go into DFCO instantly on a downhill and control lockup more effectively.
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Old 08-11-2008, 07:25 PM   #4
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ECU controlled cruise controls look more efficient than trying to keep a steady speed yourself in hilly country, because I think they go into DFCO instantly on a downhill and control lockup more effectively.
Oh this is killing me... I'm terrible with acronyms, somebody please tell me what the hell DFCO stands for... Is it dowhnill fuel cut off (That's a WAG on my part, I'll be impressed if I get it right).

Anyway, I've noticed a similar behavior in my auto CRX w/o cruise control. Basically in my case, a low (up)hill puts just enough load on the drivetrain that the trans isn't constantly hunting around for the "best" setting, locking, unlocking, etc. I recently got 46 mph, loaded, while gaining 2000 ft altitude over 250 miles. The next week, I only got 43 mpg unloaded, while losing 6000 ft over 150 miles (and at 5-10 mph lower speed!). The only differences I could find were that the transmission was doing a lot more shifting, locking, unlocking, etc. on the downhill run, and that I was running a few hundred rpm lower on that run (2900 vs 3200 or therabouts). Go figure....
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Old 08-11-2008, 07:52 PM   #5
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Driving back from Seattle to California through Oregon this weekend, I noticed the same thing via the trip computer which displays instant MPG. We climbed most hills in 5th or 4th gear depending on speed then coasted down hill in either neutral or 5th gear depending on the grade. On a 6 or 7% down grade I had to use 5th gear in order to maintain a constant 65-75mph. Fuel injectors were off and the trip computer was showing 99.9mpg. On 4 and 5% down hill grades I coasted in neutral to maintain a constant 65-75mph. Even though the engine was idling the trip computer still showed 99.9mpg on the down grade.
If you imagine climbing and then coasting down a perfectly symetrical hill. The volvo 850 would climb the hill at between 13 and 17mpg then coast down the hill at 99 mpg. The average MPG for the whole hill is between 56 and 58mpg. This is in a car that normally gets 31mpg at 70mph on perfectly flat ground. Mountain driving is the perfect opportunity to pulse and glide.
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Old 08-11-2008, 08:11 PM   #6
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It's like P&G at constant speed. But it only works in high gear. On the Grand Caravan, dropping from 4th sends mpg from 17-18 to 12-13 and there was no way to recover that on the backside. I have seen no indication of DFCO.
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Old 08-11-2008, 08:12 PM   #7
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Quote:
Originally Posted by Hal9000 View Post
Oh this is killing me... I'm terrible with acronyms, somebody please tell me what the hell DFCO stands for... Is it dowhnill fuel cut off (That's a WAG on my part, I'll be impressed if I get it right).

Anyway, I've noticed a similar behavior in my auto CRX w/o cruise control. Basically in my case, a low (up)hill puts just enough load on the drivetrain that the trans isn't constantly hunting around for the "best" setting, locking, unlocking, etc. I recently got 46 mph, loaded, while gaining 2000 ft altitude over 250 miles. The next week, I only got 43 mpg unloaded, while losing 6000 ft over 150 miles (and at 5-10 mph lower speed!). The only differences I could find were that the transmission was doing a lot more shifting, locking, unlocking, etc. on the downhill run, and that I was running a few hundred rpm lower on that run (2900 vs 3200 or therabouts). Go figure....
Its Decelleration Fuel Cut Off.

-Jay
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Old 08-11-2008, 10:34 PM   #8
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Its Decelleration Fuel Cut Off.

-Jay
Ahh, gotcha. Well, at least I was close.
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Old 08-12-2008, 10:51 AM   #9
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Quote:
Originally Posted by Hal9000 View Post
I recently got 46 mph, loaded, while gaining 2000 ft altitude over 250 miles. The next week, I only got 43 mpg unloaded, while losing 6000 ft over 150 miles (and at 5-10 mph lower speed!). The only differences I could find were that the transmission was doing a lot more shifting, locking, unlocking, etc. on the downhill run, and that I was running a few hundred rpm lower on that run (2900 vs 3200 or therabouts). Go figure....
That's amazing. We're talking about a HUGE difference in the amount of work done. I just can't make sense of it, even if I assume everything was optimal on the way up and terrible on the way down, no DFCO, and large fuel amount measurement error on both ends...
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Old 08-13-2008, 08:58 AM   #10
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I found the same thing going from IL to CT through eastern PA mountains 5 round trips going both directions in 2 different cars (02 buick century, 88 toyota cressida. both auto with DFCO). My theory is that the difference from level cruising fuel to go down DFCO is greater than the extra it takes to go up.... in other words, the mile-long DFCO pays off.
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