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08-10-2008, 08:43 PM
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#1
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Registered Member
Join Date: May 2008
Posts: 85
Country: United States
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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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08-11-2008, 07:13 AM
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#2
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Registered Member
Join Date: Apr 2008
Posts: 6,624
Country: United States
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I believe your analyzation is correct.
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08-11-2008, 07:19 AM
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#3
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Registered Member
Join Date: Feb 2008
Posts: 1,652
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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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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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08-11-2008, 07:25 PM
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#4
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Registered Member
Join Date: Jul 2008
Posts: 101
Country: United States
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Quote:
Originally Posted by RoadWarrior
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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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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08-11-2008, 07:52 PM
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#5
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Senior Member
Join Date: Aug 2012
Posts: 261
Country: United States
Location: The slums of Beverly Hills
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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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08-11-2008, 08:11 PM
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#6
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Registered Member
Join Date: May 2008
Posts: 85
Country: United States
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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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08-11-2008, 08:12 PM
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#7
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Site Team / Moderator
Join Date: Sep 2008
Location: Northern Virginia
Posts: 4,742
Country: United States
Location: Northern Virginia
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Quote:
Originally Posted by Hal9000
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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Its Decelleration Fuel Cut Off.
-Jay
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08-11-2008, 10:34 PM
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#8
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Registered Member
Join Date: Jul 2008
Posts: 101
Country: United States
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Quote:
Originally Posted by Jay2TheRescue
Its Decelleration Fuel Cut Off.
-Jay
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Ahh, gotcha. Well, at least I was close.
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08-12-2008, 10:51 AM
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#9
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Registered Member
Join Date: Apr 2008
Posts: 6,624
Country: United States
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Quote:
Originally Posted by Hal9000
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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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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08-13-2008, 08:58 AM
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#10
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Registered Member
Join Date: Nov 2007
Posts: 742
Country: United States
Location: Columbus, IN, USA
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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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1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
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