Go Back   EcoModder Forum > EcoModding > Hypermiling / EcoDriver's Ed
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 01-03-2013, 12:44 AM   #1 (permalink)
EcoModding Lurker
 
Join Date: Jan 2013
Location: Oregon, US
Posts: 43

ZoomFreakingZoom - '88 Mazda RX7 Convertible
90 day: 19.02 mpg (US)

Geo - '96 Geo Prizm Base
90 day: 32.42 mpg (US)
Thanks: 2
Thanked 2 Times in 2 Posts
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.

__________________
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
  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 01-03-2013, 12:53 AM   #2 (permalink)
Master EcoModder
 
mechman600's Avatar
 
Join Date: Jul 2008
Location: Langley, BC
Posts: 1,228

Fusion - '16 Ford Fusion Hybrid SE
Thanks: 190
Thanked 275 Times in 168 Posts
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.
  Reply With Quote
Old 01-03-2013, 01:13 AM   #3 (permalink)
EcoModding Lurker
 
Join Date: Jan 2013
Location: Oregon, US
Posts: 43

ZoomFreakingZoom - '88 Mazda RX7 Convertible
90 day: 19.02 mpg (US)

Geo - '96 Geo Prizm Base
90 day: 32.42 mpg (US)
Thanks: 2
Thanked 2 Times in 2 Posts
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.
__________________
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
  Reply With Quote
Old 01-03-2013, 01:42 PM   #4 (permalink)
EcoModding Apprentice
 
Join Date: May 2012
Location: Indiana, US
Posts: 133

TheCav - '04 Chevy Cavalier LS
Thanks: 13
Thanked 24 Times in 19 Posts
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.
  Reply With Quote
The Following User Says Thank You to YeahPete For This Useful Post:
Xist (07-27-2016)
Old 01-03-2013, 02:05 PM   #5 (permalink)
(:
 
Frank Lee's Avatar
 
Join Date: Jan 2008
Location: up north
Posts: 12,762

Blue - '93 Ford Tempo
Last 3: 27.29 mpg (US)

F150 - '94 Ford F150 XLT 4x4
90 day: 18.5 mpg (US)

Sport Coupe - '92 Ford Tempo GL
Last 3: 69.62 mpg (US)

ShWing! - '82 honda gold wing Interstate
90 day: 33.65 mpg (US)

Moon Unit - '98 Mercury Sable LX Wagon
90 day: 21.24 mpg (US)
Thanks: 1,585
Thanked 3,555 Times in 2,218 Posts
"COZX2" claimed to top 100 mpg out in the mountains...
__________________


  Reply With Quote
Old 01-03-2013, 02:21 PM   #6 (permalink)
Hypermiler
 
PaleMelanesian's Avatar
 
Join Date: Dec 2007
Location: Texas
Posts: 2,321

PaleCivic (retired) - '96 Honda Civic DX Sedan
90 day: 69.2 mpg (US)

PaleFit - '09 Honda Fit Sport
Team Honda
Wagons
90 day: 44.06 mpg (US)
Thanks: 611
Thanked 433 Times in 283 Posts
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.
__________________



11-mile commute: 100 mpg - - - Tank: 90.2 mpg / 1191 miles
  Reply With Quote
The Following User Says Thank You to PaleMelanesian For This Useful Post:
serialk11r (01-06-2013)
Old 01-03-2013, 03:46 PM   #7 (permalink)
Master EcoModder
 
mcrews's Avatar
 
Join Date: Oct 2009
Location: Texas
Posts: 3,523

The Q Sold - '02 Infiniti Q45 Sport
90 day: 23.08 mpg (US)

blackie - '14 nissan altima sv
Thanks: 2,203
Thanked 663 Times in 478 Posts
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!
__________________
MetroMPG: "Get the MPG gauge - it turns driving into a fuel & money saving game."

ECO MODS PERFORMED:
First: ScangaugeII
http://ecomodder.com/forum/showthrea...eii-23306.html

Second: Grille Block
http://ecomodder.com/forum/showthrea...e-10912-2.html

Third: Full underbelly pan
http://ecomodder.com/forum/showthrea...q45-11402.html

Fourth: rear skirts and 30.4mpg on trip!
http://ecomodder.com/forum/showthrea...tml#post247938
  Reply With Quote
Old 01-03-2013, 03:49 PM   #8 (permalink)
radioranger
 
Join Date: Dec 2011
Location: Canton CT
Posts: 442
Thanks: 140
Thanked 44 Times in 33 Posts
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.
  Reply With Quote
Old 01-06-2013, 01:51 AM   #9 (permalink)
Chief Cook & Bottlewasher
 
Join Date: Jun 2012
Location: California
Posts: 76

Xterra - '13 Nissan Xterra Pro-4X
90 day: 19.03 mpg (US)
Thanks: 11
Thanked 25 Times in 13 Posts
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.
  Reply With Quote
Old 01-06-2013, 02:15 AM   #10 (permalink)
EcoModding Lurker
 
Join Date: Jan 2013
Location: Oregon, US
Posts: 43

ZoomFreakingZoom - '88 Mazda RX7 Convertible
90 day: 19.02 mpg (US)

Geo - '96 Geo Prizm Base
90 day: 32.42 mpg (US)
Thanks: 2
Thanked 2 Times in 2 Posts
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.

__________________
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
  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com