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Old 02-16-2006, 08:12 PM   #1
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hill driving techniques

<img src="http://www.co.larimer.co.us/engineering/traffic/W7-1.gif" align="right" width="100">

i want to start a thread to talk about the finer points of driving efficiently in hills.

the first time you watch instant fuel consumption data on a trip computer or OBD gauge when transitioning from a flat road to an incline, you may be surprised at the magnitude of the mileage hit.

Quote:
to drive a typical car at 70 mph (115 kph) takes about 20 kW (27 hp). A barely noticeable 1% uphill gradient increases this by 5 kW (25%), with a corresponding increase in fuel consumption. - source
why? because now, not only is your engine working to overcome rolling & wind resistance, internal & driveline friction, but in addition it's also effectively lifting the weight of the vehicle up the slope.

techniques

the most commonly espoused method is to accelerate gently, on the flat, just before the hill. then let your speed fall off as you climb. the goal in this technique is to "drive for load": don't ask the engine to do any extra work, instead, bleed off momentum while going up.

but let's take this thread to another plane (pardon the pun)...

finer techniques

<ul><li> need to accelerate, and approaching a descent? save your strongest acceleration for the downhill.</li>

<li> i generally don't draft (i prefer good forward visibility), but the one place i break my own rule is on long up-hills on the freeway. if i get passed by someone at the right time, i'll speed up and tuck in behind for the ascent</li>

<li> big/steep descents offer the option to glide/coast in neutral with the engine off (or on - you decide). this is apparently illegal in some jurisdictions. (i've wondered why - was there once an epidemic of coasting crashes? or is it a throwback to the days of drum brakes and the risk of overheating them instead of using engine braking?)</li>

<li> the furthest i've ever coasted down a hill with my engine off was on the downhill side of a pass in the rockies on my way to vancouver in my old accord: something like 12 kilometers. i won't tell you how fast i got going (there was little traffic)</li>

<li> assuming you don't glide/coast, what's more efficient at a constant speed: driving 2 miles on a flat road? or driving 1 mile up a grade, and then 1 mile down an equal decline?</li></ul>

discuss amongst yourselves...
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Old 02-17-2006, 03:06 AM   #2
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Re: hill driving techniques

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Originally Posted by MetroMPG
<img src="http://www.co.larimer.co.us/engineering/traffic/W7-1.gif" align="right" width="100">

<li> assuming you don't glide/coast, what's more efficient at a constant speed: driving 2 miles on a flat road? or driving 1 mile up a grade, and then 1 mile down an equal decline?</li></ul>
Well it depends on the profile of the hill and your speed. At low enough speeds such that aero drag isnt a factor, and the grade going up is the same as the grade going down, and the grade is sufficient enough to allow your car to engine brake all of the way down (injectors off) without losing speed, then driving the hill will be more efficient than a flat road. The theory is that cruising at low speeds puts a low load on the engine which is not as efficiant as a higher load associated with climbing a grade. In the case just mentioned, as long as the fuel consumption is less than 2x hill climbing as opposed to flat cruising, and the downhill portion is driven with closed throttle, injectors off, you will use less fuel. No documentation to back this up, just my opinion.

At higher speeds, when areo drag kicks in, a flat level cruise may require enough load such that the engine is already operating in it's efficient range. In that case, flat vs hill is probably a wash. Again, no documentation to back this up, just my opinion.

Another thing to consider is hill profile. If you have to use your brakes during the downhill, you just threw out some of the energy that you already paid for on the way up the hill. I did some testing a few years ago with the Prius. I found a test loop, 4 miles I think. The elevation change from the lowest point to the highest point was 300 ft. If I went clockwise, I had a short very steep incline followed bu a very long, gradual decline. Going counterclockwise the profile was reversed. I went 4 times around the loop one way and then the other way. One way I got 80+ MPG. The other way I got 50+ MPG. Can you guess which was which? This one I have test data for. :-)
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Old 02-17-2006, 03:53 AM   #3
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crest

I usually slow down for the crest since you will gain more speed on the down side you can start saving fuel sooner and bleed off a little speed before gaining it back on the down side.

The up down slight slope is really a great way to save a little gs by running up hill at a maybe more efficent load on the engine then coasting down the incline in neutral you reduce the total number of revs on the motor thus saving gas. That is where the real savings come into play - less revolutions results in less friction losses.
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Old 02-17-2006, 06:05 AM   #4
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Quote:the furthest i've ever

Quote:
the furthest i've ever coasted down a hill with my engine off was on the downhill side of a pass in the rockies on my way to vancouver in my old accord: something like 12 kilometers. i won't tell you how fast i got going (there was little traffic)
When i leave the ski resort i go to there is a hill around 6miles long. I coast the whole thing and sometimes hit 70-80mph. It is a gradual hill. One thing that is interesting: My engine isn't fully warmed up when i go down this hill. But it is back down to C when I get to the bottom of it.


Normally I just try to get speed for the hill and kill it off as i make my way to the top.

If it is really big there isn't much to do. I just drive 45-50 and make my way to the top.

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Old 02-17-2006, 04:36 PM   #5
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Re: hill driving techniques

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Originally Posted by krousdb
I went 4 times around the loop one way and then the other way. One way I got 80+ MPG. The other way I got 50+ MPG. Can you guess which was which? This one I have test data for. :-)
knowing you're a consummate glider, i'd guess you're getting your best mileage when you "invest" energy for a relatively short duration on the steep incline, and "recoup" that investment over a longer duration on the other, less steep, side.
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Old 02-22-2006, 12:03 PM   #6
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hm my technique is punch it

hm my technique is punch it as fast as you can before you climb the grade (this is no cars in front of you) my climb is usually just less than a km so we are talking about speeds of 100-120 km up the hill. Then no more gas, when I'm at the top of the hill I noticed my speed is back down to 30-40 km/h and I haven't even use any gas at all

well to punch up to 100 km/h I'd stay in third gear and extract all that power from the ICE before I climb that hill. Then coast till your hearts content.
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Old 02-22-2006, 04:29 PM   #7
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engine braking

Hey I down shifted to 3rd the other day going down a hill and my MPG dropped to 66mpg doing 40mph if I left it in 5th it would be like 120-150mpg. Looks like the xB does NOT shut down the injectors when you back off the gas . . . at least not in 3rd gear.
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Old 02-22-2006, 04:34 PM   #8
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Re: hill driving techniques

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Originally Posted by MetroMPG
Quote:
Originally Posted by krousdb
I went 4 times around the loop one way and then the other way. One way I got 80+ MPG. The other way I got 50+ MPG. Can you guess which was which? This one I have test data for. :-)
knowing you're a consummate glider, i'd guess you're getting your best mileage when you "invest" energy for a relatively short duration on the steep incline, and "recoup" that investment over a longer duration on the other, less steep, side.
Bingo! This is teken into consideration during my commute. I choose the steep climbs and long gradual decline rather than vice versa. It also impacts the order in which I run my errands.

Have I ever mentioned that my wife thinks I am a nut? ;-)
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Old 02-22-2006, 04:39 PM   #9
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Re: engine braking

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Originally Posted by JanGeo
Looks like the xB does NOT shut down the injectors when you back off the gas . . . at least not in 3rd gear.
i bet it does. this question came up in at teamswift.net from another scangauge user.

we concluded that the scangauge is at fault. it doesn't recognize the 'injectors off while engine braking' state. the factory service manual for my car states that this IS a feature of my engine, yet the s/g never indicates that it's happening in the "instantaneous" (gauge) screen.

i would imagine 'injectors off' is a feature of all modern engines.

feel like asking the manufacturer? i've bugged him about a bunch of other things lately, so i'm reluctant to send yet another question. but he's usually pretty good at writing back.

his name is ron delong: ron.delong AT linear-logic DOT com
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Old 02-22-2006, 07:12 PM   #10
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Hummm

It may be that at higher rpm it factors in other things like deceleration - seems I have been seeing that sometimes affecting the mpg reading as well. Maybe the real test is to see if applying brakes affects the mpg reading more on a down hill if I decelerate that going at the same speed.

I may email him because I have found my SG running when the car has been parked and was wondering if my paging alarm system transmitter could be triggering it to wake up.
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