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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 11-18-2013, 04:56 PM   #11 (permalink)
EcoModding Lurker
 
Join Date: Nov 2013
Location: lakewood, co, usa
Posts: 53

subey - '99 Subaru Impreza Outback Sport

rav4 - '07 Toyota Rav4
Thanks: 3
Thanked 1 Time in 1 Post
Quote:
Originally Posted by MetroMPG View Post
Constant speed is less fuel-efficient than DWL because at a constant speed:

- More fuel is burned ascending.
Sure, but let say you slow down your constant speed target just a little bit, so the time to destination is the same with either method. Then you will use more on the climb, but use less the rest of the time.
Quote:
Originally Posted by MetroMPG View Post
- While descending, the energy invested in the climb is then "wasted" through increased engine braking.
If the hill is steep enuf you can coast. If it is not steep enuf to coast you will waste more energy by increasing your speed due to due to aero drag, velocity squared.
Quote:
Originally Posted by MetroMPG View Post
One thing to keep in mind is that while target driving, you are typically exceeding the average (constant) speed by the bottom of the descent while still maintaining the target consumption.

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 11-18-2013, 05:00 PM   #12 (permalink)
.
 
Join Date: Dec 2011
Location: The Berkshires, Massachussetts
Posts: 885

Ruby Sparks - '01 Honda Insight
Team Honda
90 day: 64.27 mpg (US)
Thanks: 111
Thanked 362 Times in 202 Posts
Think of it this way. If you bleed speed going up your trading kinetic energy (speed) for potential energy (Height with gravity). Going down down hill, you want to trade your potential energy for kinetic energy and regain your original speed. That is the most efficient way.

Say your going 55mph and you want to get to the top of a hill. Driving with load allows you to use the least amount of energy to climb that hill. You crest the top at 40mph, and use your throttle to accelerate downhill. It's more efficient to accelerate WITH gravity.

Now if you keep a steady 55mph up that entire hill you use more fuel because energy consumption is a function of speed (and something else can't remember). Ignoring engine load, aerodynamics and rolling resistance, going 40 mph up a hill requires less energy than, going 55mph, than going 70mph. The faster you go up hill the more energy it requires.

Now your just fighting gravity which is pointless. Think back on how much more power you have to use to gain 5mph uphill. That same power (generalizing without physics equations here) will get you 10mph acceleration on the flat, and 15mph going downhill. The more you fight it the worse it gets.

If you use you power with gravity, you can convert that 15mph going downhill to potential energy going back up the next hill. This is why pumping on a swing swings you faster and higher. I like to imagine as rolling marbles.
__________________
I try to be helpful. I'm not an expert.
  Reply With Quote
Old 11-18-2013, 05:13 PM   #13 (permalink)
EcoModding Lurker
 
Join Date: Nov 2013
Location: lakewood, co, usa
Posts: 53

subey - '99 Subaru Impreza Outback Sport

rav4 - '07 Toyota Rav4
Thanks: 3
Thanked 1 Time in 1 Post
Quote:
Originally Posted by sheepdog 44 View Post
Think of it this way. If you bleed speed going up your trading kinetic energy (speed) for potential energy (Height with gravity). Going down down hill, you want to trade your potential energy for kinetic energy and regain your original speed. That is the most efficient way.
I think that is the answer, and DWL is just a way to get there. Minimize the kinetic energy at the top of the hill so you can convert the potential energy to KE without going too fast.
Quote:
Originally Posted by sheepdog 44 View Post
Say your going 55mph and you want to get to the top of a hill. Driving with load allows you to use the least amount of energy to climb that hill. You crest the top at 40mph, and use your throttle to accelerate downhill. It's more efficient to accelerate WITH gravity.

Now if you keep a steady 55mph up that entire hill you use more fuel because energy consumption is a function of speed (and something else can't remember). Ignoring engine load, aerodynamics and rolling resistance, going 40 mph up a hill requires less energy than, going 55mph, than going 70mph. The faster you go up hill the more energy it requires.
Not true. If you ignore the engine, aero, and rolling resistance it takes the same amount of energy to get up the hill not matter what the speed. The change in potential energy is mass*gravity*height. It takes more power to get up the hill faster, but not more energy.

Quote:
Originally Posted by sheepdog 44 View Post
Now your just fighting gravity which is pointless. Think back on how much more power you have to use to gain 5mph uphill. That same power (generalizing without physics equations here) will get you 10mph acceleration on the flat, and 15mph going downhill. The more you fight it the worse it gets.

If you use you power with gravity, you can convert that 15mph going downhill to potential energy going back up the next hill. This is why pumping on a swing swings you faster and higher. I like to imagine as rolling marbles.
  Reply With Quote
Old 11-18-2013, 05:16 PM   #14 (permalink)
.
 
Join Date: Dec 2011
Location: The Berkshires, Massachussetts
Posts: 885

Ruby Sparks - '01 Honda Insight
Team Honda
90 day: 64.27 mpg (US)
Thanks: 111
Thanked 362 Times in 202 Posts
I would suggest you get a scangauge to make it clearer in your daily driving.

One of your misconceptions is that driving slower (and therefore longer) going uphill wastes more fuel than keeping a constant and faster speed. Such is not the case. You are in fact easing the throttle to gain more mpg as you use momentum to crest the hill.

For extended hill climbs you do want to maintain a minimum mph speed so you keep your engine and transmission in an efficient zone. For example, 40mph would be good. A constant 5mph going up a long hill is terrible! For smaller hills you can bleed more speed.

Your second misconception is that the increased aerodynamic resistance going downhill is ENOUGH to NEGATE the benefits of the increased speed and kinetic energy. That is not the case until you get past 75-80mph depending on how aerodynamic your car is. Cresting a hill at 40mph, then reasonably accelerating downhill to 65mph doesn't increase aerodynamic resistance enough to be unbeneficial.
__________________
I try to be helpful. I'm not an expert.
  Reply With Quote
The Following User Says Thank You to sheepdog 44 For This Useful Post:
niky (11-19-2013)
Old 11-18-2013, 05:26 PM   #15 (permalink)
EcoModding Lurker
 
Join Date: Nov 2013
Location: lakewood, co, usa
Posts: 53

subey - '99 Subaru Impreza Outback Sport

rav4 - '07 Toyota Rav4
Thanks: 3
Thanked 1 Time in 1 Post
Quote:
Originally Posted by sheepdog 44 View Post
I would suggest you get a scangauge to make it clearer in your daily driving.

One of your misconceptions is that driving slower (and therefore longer) going uphill wastes more fuel than keeping a constant and faster speed. Such is not the case. You are in fact easing the throttle to gain more mpg as you use momentum to crest the hill.
I don't have that misconception. I agree that easing the throttle will gain more MPG. It will under constant speed method too.
Quote:
Originally Posted by sheepdog 44 View Post
For extended hill climbs you do want to maintain a minimum mph speed so you keep your engine and transmission in an efficient zone. For example, 40mph would be good. A constant 5mph going up a long hill is terrible! For smaller hills you can bleed more speed.
On any highway with traffic 40 MPH would be obnoxious. In my Sube that would be lugging. In my rav4 w AT it would shift down to 3rd
Quote:
Originally Posted by sheepdog 44 View Post
Your second misconception is that the increased aerodynamic resistance going downhill is ENOUGH to NEGATE the benefits of the increased speed and kinetic energy. That is not the case until you get past 75-80mph depending on how aerodynamic your car is. Cresting a hill at 40mph, then reasonably accelerating downhill to 65mph doesn't increase aerodynamic resistance enough to be unbeneficial.
I fail to see your point. Energy is lost due to aero drag proportional to velocity squared. It makes more sense to coast and let gravity do it's thing.
  Reply With Quote
Old 11-18-2013, 05:28 PM   #16 (permalink)
Batman Junior
 
MetroMPG's Avatar
 
Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 21,641

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 53.78 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 61.98 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 66.29 mpg (US)
Thanks: 3,394
Thanked 6,201 Times in 3,212 Posts
Quote:
Originally Posted by bikenfool View Post
Sure, but let say you slow down your constant speed target just a little bit, so the time to destination is the same with either method. Then you will use more on the climb, but use less the rest of the time.
I think you can still beat constant speed (cruise control style) driving with DWL, and still have the same time to destination. The lower average ascending speed of DWL is offset by the higher than average speed by the bottom of the descent. (I'm assuming that in the constant speed approach, there is higher manifold vacuum to hold speed on the downhill part.)

Quote:
If the hill is steep enuf you can coast.
True. And that's more efficient because now we're effectively talking about pulse & glide.
__________________
Latest mods: 3-cylinder Mitsubishi Mirage. EcoMods now in progress...
Ecodriving test:
Manual vs. automatic transmission MPG showdown



EcoModder
has launched a forum for the efficient new Mitsubishi Mirage
www.MetroMPG.com - fuel efficiency info for Geo Metro owners
www.ForkenSwift.com - electric car conversion on a beer budget
  Reply With Quote
Old 11-18-2013, 05:46 PM   #17 (permalink)
NightKnight
 
NachtRitter's Avatar
 
Join Date: Dec 2008
Location: Placerville, CA
Posts: 1,594

RippinRoo - '05 Subaru Legacy Wagon 2.5 GT
Subaru
90 day: 21.16 mpg (US)

Helga - '00 Volkswagen Jetta TDI
TEAM VW AUDI Group
Diesel
90 day: 53.91 mpg (US)

Olga - '03 Volkswagen Jetta Wagon
90 day: 46.24 mpg (US)
Thanks: 303
Thanked 310 Times in 186 Posts
I'm not sure I understand your definition of "constant speed". When I read "constant speed", I think of (let's say) 65mph on the flat, 65mph up the hill, and 65mph down the hill. If you are talking about a "target average" speed of 65mph for the trip, where you are under the average on the uphill and over the average on the downhill, then that sounds like it is compatible with DWL.

In my case, I also do Pulse & Glide... despite going faster than my average speed on the pulse, and gliding down below my average speed afterwards, I still have a target average speed.
  Reply With Quote
Old 11-18-2013, 11:48 PM   #18 (permalink)
Not banned yet
 
deejaaa's Avatar
 
Join Date: May 2010
Location: Texas Coast, close to Houston
Posts: 880

getto - '02 Volkswagen Jetta GLS TDI
90 day: 53.82 mpg (US)
Thanks: 413
Thanked 258 Times in 206 Posts
good explaining guys.
bikenfool, get a UG or SG and try it yourself.
__________________
FOR SALE: 02 TDI Jetta- 5 sp: Cooper CS5 at 50 psi, UltraGauge, ventectomy, rear air shocks, mufflerectomy, LED plate light, upper fiberglass grille block, front lower pan, front fairing.
93 Chevy CrewCab 6.5 TurboDiesel, GearVendor OverDrive, 4L80 w/ TCI controls. Now DailyDriver

  Reply With Quote
Old 11-19-2013, 01:59 PM   #19 (permalink)
Master EcoModder
 
Join Date: Aug 2010
Location: Philippines
Posts: 2,173
Thanks: 1,739
Thanked 587 Times in 401 Posts
In practice, reading off of instant meters on various cars, it's more efficient to pursue load or to use a pulse-and-glide with hills than to maintain a constant speed over them... even a slower speed.

Aero drag going downhill hurts. Gravity drag going uphill hurts more.

Tried it several ways when researching for a Shell eco-seminar. It's more effective to allow speeds to vary on the hills on my regular highway route than to go over them at a constant slow or fast speed.

It works. Never mind whether you think it does or doesn't, it works. Gravity > Aero.
  Reply With Quote
Old 11-19-2013, 07:52 PM   #20 (permalink)
Master EcoModder
 
JRMichler's Avatar
 
Join Date: Feb 2011
Location: Phillips, WI
Posts: 975

Nameless - '06 GMC Canyon
90 day: 35.74 mpg (US)
Thanks: 186
Thanked 403 Times in 258 Posts
Back when I drove "normal", I would maintain 55 MPH uphill and down. That resulted in WOT throttle uphill and throttle closed downhill. When I learned DWL, and after practice, I still average 55 MPH. My speed now drops to 50 MPH uphill and reaches 60 MPH downhill. My manifold pressure (MAP on the Scangauge) mostly stays between 7 and 10 PSI. The secret to DWL is keeping MAP within the smallest practical range.

Result is that I gained a full 10% MPG while driving at exactly the same average speed.

__________________
The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 33,021 miles 2016-2018 at 35.00 MPG.
  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com