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Old 01-03-2013, 01:44 AM   #1 (permalink)
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Hills=good mpg

Given the choice of a road over a hill and a flat road, I choose the hill because I believe it gives better gas mileage (and it's more fun of course).

First off, from a theoretical standpoint, if both roads are the same distance, are strait, and have no net elevation change, then fuel usage should be identical. Any extra energy that is used in getting up the hill is recovered when going down the hill; both paths require an equal amount of work to traverse.

In real life this isn't the case, though. Gasoline engines are most efficient (power output per unit energy consumed) at high load, but are held back by friction in the engine. In other words, working an engine hard is good, but you don't want to rev your engine too much due to friction losses.

This to me suggests that the most efficient operating rpm is wherever the engine has the most torque. That would probably be around 4500rpm for my car, and probably closer to 3000 or 4000 for most other cars. At this point in the power curve, the engine is producing the most power per rpm, thus is most efficient in terms of power output per unit energy.

On a flat road, the engine will be in top gear and will be turning (hopefully) no more than 3000 rpm, which will be quite a bit less than the torque peak on most engines. On a hill however, you will be in a lower gear (at least most of the time) and typically be working the engine harder and operating much closer to the torque peak of the engine. This means the engine is getting its work done more efficiently, even though it is working harder.

On the way down is when the benefits will really come because you can just coast in or out of gear, engine on or off. Most modern fuel injected engines will not use any fuel at all when coasting in gear, and will use very little if you just let it idle.



I got to thinking about this after getting 26 mpg on a camping trip over a mountain pass and back. I typically get 24 under similar driving conditions, expect without another person and a (tiny) trunk full of camping gear.




Let me know what you guys think.

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Old 01-03-2013, 01:53 AM   #2 (permalink)
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It's true. Rolling hills provide a means for pulse and glide without annoying people behind you. As long as your engine load doesn't go too high on the uphill so that it switches to open loop (rich default fuel mapping).

Neutral coasting (or EOC if you are brave) helps more than DFCO on the down hills, unless of course the hill is too steep to do this safely because DFCO robs energy as the car has to turn the "dead" engine as you coast - more energy than the energy required if the engine was idling in neutral.
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Old 01-03-2013, 02:13 AM   #3 (permalink)
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Quote:
Originally Posted by mechman600 View Post
It's true. Rolling hills provide a means for pulse and glide without annoying people behind you. As long as your engine load doesn't go too high on the uphill so that it switches to open loop (rich default fuel mapping).

Neutral coasting (or EOC if you are brave) helps more than DFCO on the down hills, unless of course the hill is too steep to do this safely because DFCO robs energy as the car has to turn the "dead" engine as you coast - more energy than the energy required if the engine was idling in neutral.
Yah, nuetral is best, but DFCO is better than needing to use the brakes; you have to retain some form of safe speed. I alternate between 5th, 4th, and 3rd before going to my brakes.
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Old 01-03-2013, 02:42 PM   #4 (permalink)
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Road A is a straight line.
Road B road goes up and down.
Road A, being a straight line, is less of a distance than the constant elevation change of road B. Your computer will show you better mpg's on Road A, but you most likely burn less gas on road B because it is a shorter route.

That is my take on it anyways. If you can EOC road B might be worth it.
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Old 01-03-2013, 03:05 PM   #5 (permalink)
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Old 01-03-2013, 03:21 PM   #6 (permalink)
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True. Even in my Odyssey with an autotragic transmission. I just have to be more careful to avoid it downshifting.

One of my common drives is a two hour stretch of highway, half flat and half rolling hills. Consistently, I get 30+ mpg in the rolling hills half, and struggle to reach 28 in the flat part. Hills provide natural p&g even if you're not trying.

With a manual transmission it's not even close. Hills all the way, every time.
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Old 01-03-2013, 04:46 PM   #7 (permalink)
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The drive across the 'high plains' on hwy 40 thur AZ and NM are like that. Plus high elevation always seem to help.
But I did enjoy the "engine on coasting"!
It's fun comiong down the hill and having to worry about going tooo fast!
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Old 01-03-2013, 04:49 PM   #8 (permalink)
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On my way to work i go up a steep hill for three miles then a bunch of shorter hills with mostly downhill stretches, coming home i go north a bit and go up a steep hill for a good two miles then can coast a few mile and a half stretches down again, pretty ideal setup. steep climbs at lower speeds then long runs downhill coasting on gentle slopes are the best.
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Old 01-06-2013, 02:51 AM   #9 (permalink)
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Here are my thoughts:

If you think that the reason hills return higher MPG is because your engine is working at it's most efficient operating RPM (4500RPM for you, apparently) then if you're on a flat road why not shift from 5th to 4th or 3rd to get your RPMs to 4500? Would that give you best MPG? Of course not. Therefore THAT is not the reason you get better MPG in the hills. It's the natural P&G opportunities hills provide and, again, has nothing to do with the most efficient operating RPM of your engine.

Unless, of course, I'm wrong!

Quote:
Originally Posted by 13B_88FC View Post
Given the choice of a road over a hill and a flat road, I choose the hill because I believe it gives better gas mileage (and it's more fun of course).

First off, from a theoretical standpoint, if both roads are the same distance, are strait, and have no net elevation change, then fuel usage should be identical. Any extra energy that is used in getting up the hill is recovered when going down the hill; both paths require an equal amount of work to traverse.

In real life this isn't the case, though. Gasoline engines are most efficient (power output per unit energy consumed) at high load, but are held back by friction in the engine. In other words, working an engine hard is good, but you don't want to rev your engine too much due to friction losses.

This to me suggests that the most efficient operating rpm is wherever the engine has the most torque. That would probably be around 4500rpm for my car, and probably closer to 3000 or 4000 for most other cars. At this point in the power curve, the engine is producing the most power per rpm, thus is most efficient in terms of power output per unit energy.

On a flat road, the engine will be in top gear and will be turning (hopefully) no more than 3000 rpm, which will be quite a bit less than the torque peak on most engines. On a hill however, you will be in a lower gear (at least most of the time) and typically be working the engine harder and operating much closer to the torque peak of the engine. This means the engine is getting its work done more efficiently, even though it is working harder.

On the way down is when the benefits will really come because you can just coast in or out of gear, engine on or off. Most modern fuel injected engines will not use any fuel at all when coasting in gear, and will use very little if you just let it idle.



I got to thinking about this after getting 26 mpg on a camping trip over a mountain pass and back. I typically get 24 under similar driving conditions, expect without another person and a (tiny) trunk full of camping gear.




Let me know what you guys think.
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Old 01-06-2013, 03:15 AM   #10 (permalink)
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Quote:
Originally Posted by campisi View Post
Here are my thoughts:

If you think that the reason hills return higher MPG is because your engine is working at it's most efficient operating RPM (4500RPM for you, apparently) then if you're on a flat road why not shift from 5th to 4th or 3rd to get your RPMs to 4500? Would that give you best MPG? Of course not. Therefore THAT is not the reason you get better MPG in the hills. It's the natural P&G opportunities hills provide and, again, has nothing to do with the most efficient operating RPM of your engine.

Unless, of course, I'm wrong!
Simple. Because even though the engine is less efficient, it's still turning slower and thus is using less fuel. There are two types of efficiency that we're talking about here. Driving at the peak torque rpm is most efficient in terms of power per unit fuel, but driving at a lower rpm when that much power is not needed will give worse miles per gallon.

When I accelerate, I try to sandwich my torque peak between shifts, because that power is being put to use. Driving in a lower gear when cruising is just wasteful because you aren't putting that power to use.

The reason "pulse and glide" is efficient is because of what I've explained, so you contradict yourself by saying I'm wrong.

In summary, hills are more efficient because the engine is opperating more efficiently AND that power is being put to use.

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1988 rx7 convertible. Streetported, racing beat header and presilencer replacing all 3 catalytic converters. Atkins 6 port sleeves. K&N cone filter intake. 22mpg average, 27mpg max. Spreadsheet (updated regularly):
https://dl.dropbox.com/u/5310442/198...%20mileage.xls
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