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Old 02-01-2011, 12:20 PM   #1 (permalink)
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DWL - but no downhill assist?

Here is my understanding of DWL, please correct me if I am wrong: You gradually lose speed going uphill, but you regain it back (at a lesser cost) on the downhill as gravity assists acceleration.

But what if you had to climb a hill that did not have a downhill portion?

I have a hill to climb on my commute that I usually approach at 45mph or so, and peak at around 33-35mph or so. However, because there is no downhill portion I find myself having to accelerate back up to PSL, which I believe is 40, on flat ground.

Would I be better off just hitting the hill hard so that I don't have to re-accelerate back up to PSL?

I have no Scan Gauge, but I am hoping that there is a rule of thumb for this kind of terrain.

Thanks for the suggestions,

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Old 02-01-2011, 03:18 PM   #2 (permalink)
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You know the Hypermiling gods are against you if...
... You always have to drive uphill. Bummer

OK, seriously: Unless your commute is in an alternate, Escher universe, then you'll have to go down sometime.


What goes up, must come down. If your commute is uphill one way, then the return must be downhill. Then you can coast in neutral or coast in gear, depending on how steep the grade is. When neutral coasting your engine is at idle, so it uses some fuel, but less than during normal driving. When coasting in gear (DFCO) your engine mostly likely doesn't use any fuel, but also you won't coast as far.

So, if you have to use more fuel going one way because of an uphill, then you will have a chance to use less fuel on the way back.
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Old 02-01-2011, 03:48 PM   #3 (permalink)
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You're right. I use the same route on the way home and enjoy the benefits of the downhill. I DFCO for the first half, and then uppshift to coast in gear the rest of the way. (If I coast in gear the whole way, I'd go too far over PSL than I'm willing to do and would have to brake).

I guess what I meant to say is that I have a steep uphill followed by a flat. I figure there are two choices:

1. DWL up the hill, and re-accelerate up to PSL when I reach the flat. This is what I have been doing.
2. Give it more throttle before and during the hill, so that I crest it at a speed closer to my target and do not have to accelerate back to speed.

My gut tells me that the most efficient option has a lot to do with variables such as steepness, length of the hill, and BSFC to such an extent that there may be no
'rule of thumb.'
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Old 02-01-2011, 04:17 PM   #4 (permalink)
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If you can hold speed, or only decelerate a little up the hill without downshifting, it's likely more efficient in this case (better BSFC under more load at same rpms).
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Old 02-01-2011, 04:20 PM   #5 (permalink)
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Quote:
Originally Posted by Kodak View Post
I guess what I meant to say is that I have a steep uphill followed by a flat. I figure there are two choices:

1. DWL up the hill, and re-accelerate up to PSL when I reach the flat. This is what I have been doing.
2. Give it more throttle before and during the hill, so that I crest it at a speed closer to my target and do not have to accelerate back to speed.
I'd vote for option #1. Let your speed drop to the slowest that other drivers will allow, get into the highest gear (lowest engine rpm) that your engine and transmission will safely take and chug along, then floor it on the flat.
Option #2 will drown you in all the fuel that will go through the engine.
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Old 02-01-2011, 05:39 PM   #6 (permalink)
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#2. It's much more fuel-efficient to accelerate on level ground than to accelerate on an uphill grade. Accelerating up hills, my mpg is typically in the teens, below 20 mpg. Accelerating on flat land, I can double that, but I don't. For best BSFC, I still accelerate getting below 20 mpg. The difference is, I only need to accelerate for mere seconds on flats. When I accelerate going uphill, I may have to do it for minutes at a time.
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Old 02-02-2011, 09:49 PM   #7 (permalink)
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Ah okay. I read the thread about brisk vs slow acceleration, so I guess I'll deal with having to accelerate back up to speed and just get into high gear when I return to flat land.

I'll probably continue what I'm doing then. Most drivers are actually pretty cool with my speed dropping from around 45 to just below 35. The hill is fairly steep, but not too long, and I find that most drivers slow a bit too.

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