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Old 09-14-2008, 07:22 PM   #1
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Question Extended uphill on highway: when to start acceleration and how fast?

My hypermiling habits on the highway usually involves crusing at 47mph on level road and coasting in gear on downhills to induce DFCO until 40mph. At 40mph, I'll begin to slowly accelerate back up to 47mph and repeat the process over again...or should I accelerate quickly to 47mph?

The issue that bothers me occurs when I'm going downhill that turns into a pretty long or steep uphill. It seems to me that if I've limited myself to 47mph, I'm going to be on that uphill section for longer periods of time, thus making the engine work harder and burn more fuel. Should I:

1. continue to accelerate slowly up the hill at 40mph, steadily reaching 47mph to set up for the next downhill coast
2. accelerate swifty right at the bottom of the downhill from 40mph to a speed greater than 47mph uphill to minimize time on the uphill but maximize the time spent coasting downhill
3. start acceleration earlier during a downhill (thus cutting off DFCO earlier) and accelerate to a speed greater than 47mph and continue to accelerate or maintain speed up the hill

I was wondering if anyone had any experience in this. Thanks!
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Old 09-14-2008, 07:55 PM   #2
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Steep uphill i use what ever gear keeps me at 2000rpm, no matter how fast i am going. Rolling hills i eoc uphill if traffic allows, until i start to lose to much momentum, then pop start again to top of hill then eoc from top to bottom. Thats what i try to do. Add some drafting when possible and there you have it.
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Old 09-14-2008, 08:15 PM   #3
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I try to gain momentum going down (up to 50-58mph) and go up the hill while gradually releasing throttle pressure so I maintain at least 40 at the top of the hill. Really long hills are tough, you have to burn some gas.
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Old 09-15-2008, 08:34 AM   #4
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Allow the vehicle speed to gradually SLOW as you climb, use momentum for as much of the elevation increase as possible. If this is a frequent hill, like one on a daily commute, you'll be able to determine how much you're willing to slow by the summit. Then try holding different accelerator pedal positions to see what provides the combination of momentum and applied power to reach that summit at that self-imposed lower speed limit. This is a bit different that Z man's in that the accelerator is held constant through the climb rather than being backed off (implying excess at the start). Keep holding the pedal at that position (or back off further) as you start down the other side and kinetic, momentum energy replaces the static, altitude energy and your speed rises again.
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Old 09-15-2008, 08:41 AM   #5
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You do 47mph on the highway? I really hope you only do that on multi-lane roads.
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Old 09-15-2008, 09:58 AM   #6
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Quote:
Originally Posted by Lug_Nut View Post
Allow the vehicle speed to gradually SLOW as you climb, use momentum for as much of the elevation increase as possible. If this is a frequent hill, like one on a daily commute, you'll be able to determine how much you're willing to slow by the summit. Then try holding different accelerator pedal positions to see what provides the combination of momentum and applied power to reach that summit at that self-imposed lower speed limit. This is a bit different that Z man's in that the accelerator is held constant through the climb rather than being backed off (implying excess at the start). Keep holding the pedal at that position (or back off further) as you start down the other side and kinetic, momentum energy replaces the static, altitude energy and your speed rises again.
It seems that if I do this, I'll reach the summit at 40mph...but then I won't have any speed to coast down the next hill! I was thinking maybe it would be more effective to just speed up that hill as fast as you can, even accelerating to greater than 47mph in my case (like 55 maybe) at the bottom of the hill. At these higher speeds, there would be enough energy stored up to help move the car up the hill faster and use less gas, right?

But then there's consideration for wind resistance at higher speeds. It seems to me that I've got to strike a balance between the high speed air resistance and the length of time on the uphill.
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Old 09-16-2008, 05:48 AM   #7
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It's not the length of time during the climb, it's the amount of fuel used during that climb. Longer time is frequently more than offset by less fuel. Saving 10% of the time but using 15% more fuel to do that (e.g.: a 16.5% faster rate for 10% less time) is still more fuel use.
And note that I specifically mentioned "how much you are willing to slow by the summit" as the determinant of the amount of accelerator to apply on the climb. If you don't want 40 mph, and prefer 47 mph, then you'll be able to use that same constant pedal position technique for the climb, but just pressed a bit further down.
Do not speed up to build momentum for the climb. Continue at your desired speed on the flat before the uphill. Once on the hill THEN use the deeper accelerator to the position that will produce your desired speed at the summit.
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Old 09-16-2008, 06:05 AM   #8
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there aren't many hills in central florida, but...

on a recent trip from orlando to tampa i achieved my best ever trip(46.9mpg) via pulse and glide on overpasses and some drafting.

i always pulse up hills(overpasses) and glide down them(engine on).
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Old 09-16-2008, 09:25 AM   #9
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Most of the roads I drive are not very well maintained or what you would consider good for high speed travel so I normally drive at about 50 mph, coast downhill to whatever speed it gains up to and start to accelerate up the next hill as soon as I reach the bottom of the downhill grade before I have lost any momentum. I still try to maintain about 50 mph on the uphill as well so that I don't cause a rolling road block. With this technique on my '88 Escort I am averaging almost 45 mpg and the car was old EPA rated at 42 hwy. 33 city and 37 combined city/hwy. Just guessing I would say 30-40% of my driving is city and even on the highways that I travel most out of the city have several stop signs and stop lights.
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Old 09-16-2008, 10:58 AM   #10
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Quote:
Originally Posted by FLAteam View Post
My hypermiling habits on the highway usually involves crusing at 47mph on level road and coasting in gear on downhills to induce DFCO until 40mph. At 40mph, I'll begin to slowly accelerate back up to 47mph and repeat the process over again...or should I accelerate quickly to 47mph?

The issue that bothers me occurs when I'm going downhill that turns into a pretty long or steep uphill. It seems to me that if I've limited myself to 47mph, I'm going to be on that uphill section for longer periods of time, thus making the engine work harder and burn more fuel. Should I:

1. continue to accelerate slowly up the hill at 40mph, steadily reaching 47mph to set up for the next downhill coast
2. accelerate swifty right at the bottom of the downhill from 40mph to a speed greater than 47mph uphill to minimize time on the uphill but maximize the time spent coasting downhill
3. start acceleration earlier during a downhill (thus cutting off DFCO earlier) and accelerate to a speed greater than 47mph and continue to accelerate or maintain speed up the hill

I was wondering if anyone had any experience in this. Thanks!
The engine will not be working harder at the slower speed--it will only be working longer. I think there are very few circumstances where it's the most efficient to accelerate up a hill! Generally you want to carry momentum into a hill and slowly decelerate up the hill. i don't think #2 is a good strategy. I think 3 has the right idea. But instead of gaining speed up the hill, you want to slowly lose speed going up the hill--as much speed as you are willing to lose given traffic behind you. I'd say you'd have to get going pretty darn slow before the lose speed strategy would be doing you in, although it depends on how long of a hill you're talking. Long, steady hills it's better to find a sweet spot and maintain that speed. So I would say you don't want to try to go more than 47mph going up the hill if you're doing less than 47mph coming into the hill.

In a six cylinder 3.8 liter Chrysler Town and Country AWD minivan with automatic transmission I averaged 17mpg going up a long 6 mile hill on the interstate. Most of the climb was at 50mph (relatively slow for a car with its gearing, weight and engine displacement--the gearing is taller than my Honda Civic VX). I went as slow as I could while keeping it in the top gear going up the hill--that was the key point, to prevent it from downshifting. I believe the car was at its "sweet spot" going up this hill at 50mph. I've done the hill at faster speeds and average about 13 or 14mpg. I did the same hill in a Prius at 90mph and completely drained the battery and averaged about 20mpg before the battery was pooped and maybe 15mpg thereafter. I'm fairly certain slower in the Town & Country would be worse FE (because it would downshift) and any faster and the forces of wind resistance and overcoming the hill are going to outweigh the faster speed. I think a good rule of thumb is to go up a hill as slow as you can without the car downshifting from the top gear. I used to wait too long after coasting down a hill before going on the gas at the bottom of the hill. For steep hills, pick a throttle position and don't change it for the duration of the hill. That means correctly predicting the right throttle position for the hill in order to get the best FE going up that hill. Too rich and you will burn too much gas and go faster up the hill than you perhaps needed to. Too light and you will create a moving road block or will lose too much momentum and end up at 15mph.

It's much more efficient to cut the coast a little short in favor of carrying
momentum into the hill. The logic for this is easy to see. The amount of fuel saved during the coast is very small in proportion to the amount of fuel used going up a hill. So while you will save, say, 25% less fuel by cutting the coast short, you will use a number of times less fuel going up the hill than what you lost by cutting the coast short. It's a balance, though--it doesn't pay to get on the gas too soon either. It all depends on the steepness of the hill you are about to go up, its length and your rate of deceleration coming into the hill. The steeper the hill the more speed you want to have before beginning it so you can scrub that speed slowly with a light foot on the gas while you go up the hill. Nothing worse than having to give it a lot of gas because you lost too much momentum! Hope that made sense and was helpful. I'm interested to hear anyone's rebuttals to any of the points I made as I don't pretend to have the science of hillclimbing down.
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