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Old 05-11-2008, 10:14 PM   #1 (permalink)
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What's better on hills: DWL or Pulse up, Glide down?

"Hammering it up hills is way less efficient than coasting up it and getting back to 55 going down"

Well, this is the part that really confuses me, because lots of people talk about DWL, but I'm not sure it always makes sense. Let's say I can climb a grade at a steady 30 mph, with a small, steady throttle.

Let's say I can climb the same grade, at 70% throttle, which causes my speed to go from 30 to 45, as I travel up the grade. Of course, let's assume that there's no stop sign right over the crest of the hill. In other words, let's assume that my 45 mph momentum as I crest the hill will be fully used. (And assume constant top gear in both scenarios.)

For various reasons I've become convinced that B is more efficient than A. With a large throttle opening, I do a better job of converting fuel into kinetic energy, and then I can use that kinetic energy for a nice long EOC after I crest the hill.

I've become convinced that the essence of effective P&G is to consistently avoid small throttle openings. Trouble is, this seems contrary to DWL doctrine.

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Old 05-11-2008, 10:16 PM   #2 (permalink)
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Quote:
Originally Posted by monroe74 View Post
"Hammering it up hills is way less efficient than coasting up it and getting back to 55 going down"

Well, this is the part that really confuses me, because lots of people talk about DWL, but I'm not sure it always makes sense. Let's say I can climb a grade at a steady 30 mph, with a small, steady throttle.

Let's say I can climb the same grade, at 70% throttle, which causes my speed to go from 30 to 45, as I travel up the grade. Of course, let's assume that there's no stop sign right over the crest of the hill. In other words, let's assume that my 45 mph momentum as I crest the hill will be fully used. (And assume constant top gear in both scenarios.)

For various reasons I've become convinced that B is more efficient than A. With a large throttle opening, I do a better job of converting fuel into kinetic energy, and then I can use that kinetic energy for a nice long EOC after I crest the hill.

I've become convinced that the essence of effective P&G is to consistently avoid small throttle openings. Trouble is, this seems contrary to DWL doctrine.


I've been pondering this for a while, somebody who lives around hills may have a better Idea. Should this become a new thread?
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Old 05-11-2008, 10:20 PM   #3 (permalink)
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Sure - it's good new thread material.

Pulse up and glide down makes the most sense at lower speeds or on steep hills.

At higher speeds where aero losses make a glide on the downside not worthwhile (or if the downgrade isn't steep enough), then I'd say driving with load will win out.
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Old 05-11-2008, 10:57 PM   #4 (permalink)
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Driving with load doesn't always mean keep a steady throttle up a hill, it's keep a steady throttle, and if it's big enough letting off the gas and bleeding off speed until you get to the top, then gaining speed down.

If your idling up the hill and hammering down it, it'd be more efficient than the other IMO
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Old 05-11-2008, 11:14 PM   #5 (permalink)
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If you know you're not going to have to brake on the downside, you're best bet is probably 80% throttle in top gear. Much more than that, you'll be in open loop mode (rich).
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Old 05-11-2008, 11:14 PM   #6 (permalink)
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I've found DWL to be the most efficient for me, but I drive in mountains, not hills. I keep a constant RPM going up the hill/mtn, when possible, and don't worry about the speed dropping off. Often this opens the throttle more anyways. If you zoom up and coast down, then you win up breaking a maximum or minimum speed limit by a lot....
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Old 05-11-2008, 11:33 PM   #7 (permalink)
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I live in an area that is basically a drumlin field, which means we have many many hills about 150 to 200 ft tall. The roads cut through them abit but it I think the longest perfectly flat stretch of road on my commute is about 2 miles. But most of the time you are going up and down.
So I pulse and glide with the hills which keeps me with traffic if there is any, and anyone behind me probably has no idea I'm coasting alot of the time. On a few hills I am speeding 25km/h over coasting in neutral. Also with my neon, 50% throttle will get me up the hills at 100km/h and then I coast down to 80km/h and then gas it up the next hill. For me the hills allow me to pulse and glide while maintaining speeds between 80 and 100km/h which is reasonable to do in light traffic.
I'd think slowing as you climb a hill is a bit counter productive as it keeps your engine in the inefficient low throttle opening zone for longer. The gravitational energy you have gained is equal at the top of the hill wether you idle up or use %50 throttle. In high drag vehicle I'd guess you want to always stay slow but for a streamline car probably you want to stay at 50 to 70% throttle all the way up and then coast down.
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Old 05-12-2008, 12:04 AM   #8 (permalink)
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Thanks, these are very helpful comments, and they make sense to me.

What still bugs me is this statement: "... you will back off the accelerator and lose speed ... It's far more efficient than pressing the accelerator more and more to maintain speed on the way up a hill and then releasing it down the other side."

(From here, item #55: http://www.ecomodder.com/forum/EM-hy...ecodriving.php)

I think the problem with that statement is it encourages people to use a small throttle opening, in a situation when a large throttle opening might be more efficient.

I realize people have pointed out some important exceptions, where the larger throttle, and higher speed, isn't a good idea. Like if you end up having to stop prematurely, after you crest the hill. Or if you're building up so much speed (e.g., over 50 mph) that aero resistance starts to be a real problem. Or maybe there's so much speed that it's unsafe or illegal.

But if conditions like that are not present, then I think #55 is perhaps a little misleading. Unless there's some other aspect of this that I'm not grasping.
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Old 05-12-2008, 12:20 AM   #9 (permalink)
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The most important exception is: if you're not going to glide, engine off, down the back side of the hill, then the least fuel will be consumed overall if you DWL up the front side.

Remember: best BSFC doesn't necessarily equal lowest overall fuel consumption, it means most power produced per unit of fuel consumed.
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Old 05-12-2008, 12:21 AM   #10 (permalink)
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Quote:
Originally Posted by monroe74 View Post
What still bugs me is this statement: "... you will back off the accelerator and lose speed ... It's far more efficient than pressing the accelerator more and more to maintain speed on the way up a hill and then releasing it down the other side."

(From here, item #55: http://www.ecomodder.com/forum/EM-hy...ecodriving.php)
That statement applies to traversing hills but not valleys. That is you climb first and coast second. Steep declines will accelerate the car providing free energy. Every slope has a point where gravity = Drag at a certain MPH. If you approach the decline slow, the hill will accelerate you more MPH until you hit the equilibrium and you waste less energy on the climbing side.

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