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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 03-10-2012, 06:06 AM   #31 (permalink)
Master EcoModder
 
IamIan's Avatar
 
Join Date: Dec 2010
Location: RI
Posts: 692
Thanks: 371
Thanked 227 Times in 140 Posts
Just my 2 bits.

As others have indicated a specific number will vary depending on conditions.

If you wanted a specific number ... you would have to define the specific BSFC for the engine in question and all the other driving context conditions ... can be done if you are that interested ... but different vehicles will have different numbers under the same conditions ... and the same vehicle will have different numbers under different conditions.

As a crude General Rule ~10 Seconds is a safe point ... but I think learning the methodology itself is a better method than trying to pick one specific number.

Properly used engine off coasting is an FE asset ... over used it is a FE negative... I'm not just referring to too short of a engine off period... too long of an engine off period is also a FE negative.

- - - -

I usually recommend those interested first learn Conservation of Momentum , and driving awareness ... leaving engine off coasting for later development.

- - - -

Then if they still want to go even further ... I recommend the BSFC chart for their engine ... and to make an effort to operate the engine more often in it's higher efficiency points.

If current driving conditions warrant it using a engine off coasting event can be an effective method of avoiding those less efficient parts of the BSFC.

An additional Benefit of Engine off coasting beyond just avoiding low BSFC points , is that it also avoids pumping losses ... something the Engine has to spend energy/fuel for even at it's best BSFC.

- - - - - - -

The too long Engine Off Coast aspect is a factor of the exponential influence of aerodynamics ... meaning it is more aerodynamically energy efficient to travel at a steady average speed than it is to achieve the same average speed while fluctuating the speed up and down... no matter what average speed you want to travel at.

So there is a point where further engine off coasting will actually produce worse FE and lower MPG.

Although I suspect traffic conditions would often make it unsafe to fluctuate one's speed that much before you reached the FE loss point.

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 03-10-2012, 11:35 AM   #32 (permalink)
Master EcoModder
 
Join Date: Jul 2008
Location: Private
Posts: 282
Thanks: 2
Thanked 73 Times in 47 Posts
Quote:
Originally Posted by IamIan View Post
So there is a point where further engine off coasting will actually produce worse FE and lower MPG.
Traveling any distance and burning no fuel is always going to be more efficient than traveling the same distance with the engine running.

It is called "paralysis through analysis".

Last edited by moorecomp; 03-10-2012 at 01:24 PM..
  Reply With Quote
Old 03-10-2012, 02:17 PM   #33 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,850

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 51.9 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
90 day: 24.12 mpg (US)
Thanks: 197
Thanked 1,653 Times in 870 Posts
Quote:
Originally Posted by IamIan View Post
Just my 2 bits.
The too long Engine Off Coast aspect is a factor of the exponential influence of aerodynamics ... meaning it is more aerodynamically energy efficient to travel at a steady average speed than it is to achieve the same average speed while fluctuating the speed up and down... no matter what average speed you want to travel at.

So there is a point where further engine off coasting will actually produce worse FE and lower MPG.

Although I suspect traffic conditions would often make it unsafe to fluctuate one's speed that much before you reached the FE loss point.
Quote:
Originally Posted by moorecomp View Post
Traveling any distance and burning no fuel is always going to be more efficient than traveling the same distance with the engine running.

It is called "paralysis through analysis".
But there will be a point of diminishing returns, which is what I think IamIan is getting at. Accelerating to 70mph and coasting to, say, 20 and then accelerating back up to 70, repeatedly--would this be as efficient as simply maintaining 45-50 mph?
  Reply With Quote
The Following User Says Thank You to Vman455 For This Useful Post:
IamIan (03-10-2012)
Old 03-10-2012, 06:53 PM   #34 (permalink)
Master EcoModder
 
Join Date: Jul 2008
Location: Private
Posts: 282
Thanks: 2
Thanked 73 Times in 47 Posts
Yes, proven many times by countless P&G,ers.
  Reply With Quote
Old 03-10-2012, 08:06 PM   #35 (permalink)
Master EcoModder
 
IamIan's Avatar
 
Join Date: Dec 2010
Location: RI
Posts: 692
Thanks: 371
Thanked 227 Times in 140 Posts
Quote:
Originally Posted by moorecomp View Post
Traveling any distance and burning no fuel is always going to be more efficient than traveling the same distance with the engine running.

It is called "paralysis through analysis".
Not 'paralysis through analysis' at all ... there is a point of diminishing returns... and past a certain point it is less efficient... the point is that it can be useful to know how it works in order to maximize benefits and minimize disadvantages.

It takes energy to get moving from a stop ... you already burned the fuel to get the kinetic energy you are consuming during the coasting portion ... you consume that invested kinetic energy to over come aerodynamic losses and to over come rolling resistance losses ... there is no traveling a distance without spending energy / fuel to do so ... there is no free lunch... having spent the the fuel/energy at a prior point doesn't change this , you still had to spend it.

It is more aerodynamically efficient to achieve a given average speed by a steady speed than it is to achieve the same average speed by fluctuating speed... like it or not ... that's just how aerodynamics works... when it comes to aerodynamics there is a bigger hit to going faster than there is a benefit to going slower... and yes aerodynamics has been tested over and over and over again ... this relationship is well known ... it is just the way it.

Example of concept:
Quote:
Say you have a Vehicle with a Cd of 0.25 ... and a Frontal Area of 20 Ft^2 ... Say for example you want an average speed of ~50 MPH over 50 Miles.

Looking at the aerodynamics.

If you achieve this average speed of 50 MPH via a steady speed of 50 MPH for 1 hour your wind resistance is about ~3.37 kwh of energy consumed... 50 miles covered.

If you achieve this same average speed by traveling for 30 minutes at 25 MPH ... covering 12.5 Miles , consuming ~0.210 kwh of energy ... and the 2nd 30 minutes at 75 MPH ... covering 37.5 Miles , consuming ~5.68 kwh... over the same 1 hour time period you have the same average speed of 50 MPH ... but due to the exponential effect speed has on aerodynamic losses ... your fluctuating speed method used ~5.89 kwh to cover the the same 50 miles at the same 50 MPH average speed.

The fluctuating speed method to cover the same distance at the same average speed consumed ~2.52 kwh more energy due to the exponential effects of aerodynamic losses.

The larger the speed variation the larger the penalty to the speed varying method ... the smaller the speed variation the smaller the penalty ... but it is always an aerodynamic penalty.
As I already wrote ... there are pros of the engine off method ... maximizing time / fuel spent in better / best BSFC ... and avoiding unnecessary pumping losses... but there are cons as well... there is a point of diminishing returns ... and a point where it breaks even ... and yes ... even a point where taken too far it is a net lower efficiency , and lower over all FE.

Last edited by IamIan; 03-10-2012 at 09:36 PM.. Reason: typo
  Reply With Quote
Old 03-10-2012, 10:38 PM   #36 (permalink)
EcoModding Lurker
 
Join Date: Mar 2012
Location: san antonio, TX
Posts: 12
Thanks: 4
Thanked 0 Times in 0 Posts
I see weather here is pretty good in the winter. Im going to try it out some this tank as my tank only netted 43 mpg this time around. I coasted from 65 down to 55 today once and it did take some effort to get back up the hill maybe negative idk but i coasted a good 30 secs or so.
  Reply With Quote
Old 03-11-2012, 12:29 AM   #37 (permalink)
Pishtaco
 
SentraSE-R's Avatar
 
Join Date: Jun 2009
Location: Bay Area, California
Posts: 1,485

Mean Green Toaster Machine - '06 Scion xB
Team Toyota
90 day: 48.92 mpg (US)
Thanks: 56
Thanked 285 Times in 181 Posts
You can't produce numbers like these engine on.

__________________
Darrell

Boycotting Exxon since 1989, BP since 2010
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac? George Carlin
Mean Green Toaster Machine
49.5 mpg avg over 53,000 miles. 176% of '08 EPA
Best flat drive 94.5 mpg for 10.1 mi
Longest tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg
  Reply With Quote
Old 03-11-2012, 07:19 AM   #38 (permalink)
Master EcoModder
 
IamIan's Avatar
 
Join Date: Dec 2010
Location: RI
Posts: 692
Thanks: 371
Thanked 227 Times in 140 Posts
Quote:
Originally Posted by shockj2000 View Post
I see weather here is pretty good in the winter. Im going to try it out some this tank as my tank only netted 43 mpg this time around. I coasted from 65 down to 55 today once and it did take some effort to get back up the hill maybe negative idk but i coasted a good 30 secs or so.
Just keep in mind the break even point isn't a simple static point for all vehicles ... or even the same vehicle under all conditions... and the concept is at the same average speed.

Aerodynamic drag changes with wind speed , air pressure, temperature, humidity, etc.

Different Engines have different BSFC , different pumping losses, etc.

Different driving contexts consume different amounts of energy ... all other things being equal slower is less energy than faster ... energy conservation is more efficient than energy conversion ... etc.

For example of the 75MPH and 25MPH example of aerodynamic concept of that example vehicle 0.25 Cd and 20 Ft^2 ... while it increased the Aerodynamic drag energy consumed by ~2.5 kwh ... that would only be a negative to net FE if that ~2.5 kwh is larger than the gains from reduced Pumping losses and improved % of time at better or best BSFC.

I am not trying to propose a specific set thing ... like 75 to 25 is always too much of a speed variation ... that is not my point ... my point is the reality of the pros and cons ... and it is better to understand how things actually work than a hard set rule that will be wrong in some contexts.

I am referring to the known scientifically confirmed pros and cons of the method ... the break even point will not always be a 50 MPH speed variation ... it will not always be 25 MPH speed variation ... etc.

- - - - - -

For Example:
If the steady state vehicle in the previous example achieved 50 MPG ( so the math is easy at 1 gallon consumed ) and averaged of 28% ICE efficiency ... from average ~36 kwh / gallon gasoline... using ~10.1 kwh per gallon of the chemical energy of the gasoline.

Even with the additional ~2.5 kwh of aerodynamic losses would not result in a net lower FE if the speed varying method can achieve an average ICE efficiency of at least ~35% ... including the efficiency benefits of both time and fuel at better ICE BSFC points and reduced time spent paying for pumping losses.

At that point there would be no net negative FE effect.

But if the increased average ICE BSFC only went from 28% to 32% efficiency ... that is not a large enough gain to offset the aerodynamic penalt in that example.

In different context with different aerodynamic CdA, different conditions, etc... the break even point moves ... it is not static.

- - - - - - - -

Quote:
Originally Posted by SentraSE-R View Post
You can't produce numbers like these engine on.
Can't produce those numbers if engine is just off either... use 0 fuel/energy travel 0 Miles... no free lunch.

It isn't the engine off itself that achieved those numbers ... it is how the engine was operated when it was on to push the vehicle , by converting fuel chemical energy into vehicle kinetic energy , and the context of the conditions to achieve whatever average speed you had over that distance.

Last edited by IamIan; 03-11-2012 at 07:38 AM..
  Reply With Quote
The Following User Says Thank You to IamIan For This Useful Post:
shockj2000 (03-12-2012)
Old 03-12-2012, 12:06 AM   #39 (permalink)
EcoModding Lurker
 
Join Date: Mar 2012
Location: san antonio, TX
Posts: 12
Thanks: 4
Thanked 0 Times in 0 Posts
Simply I understand that what your saying is everything has to be factored in but the equation is kinda over my head. My guess is that if i make sure i still have enough momentum before i hit the next hill from the last coast and don't gas it too hard i'll be ok. Or if my fuel mileage goes up, maybe im doing something right lol. I bought a vacuum gage. just waiting on that to come in. I'm unsure what i'll be looking for there as well when using it.
  Reply With Quote
Old 03-12-2012, 08:49 AM   #40 (permalink)
Rat Racer
 
Fat Charlie's Avatar
 
Join Date: May 2011
Location: Route 16
Posts: 4,150

Al the Third, year four - '13 Honda Fit Base
Team Honda
90 day: 42.9 mpg (US)
Thanks: 1,784
Thanked 1,919 Times in 1,246 Posts
The equation is simple: terrain dictates. What works best at one spot today might not be the best in the same spot tomorrow. I find that EOC from 70 to 20 works wonders for me... when there's a tollbooth in my future. Except that one of them is best hit from 60. Usually. Only one of my four tollbooth glides has a clearly defined start point, and even that doesn't have a clearly defined starting speed.

A good rule is to never go faster than you have to, and experiment to find out if starting a particular glide from a higher speed pays off by getting you farther enough to matter. Ain't hypermiling fun?

__________________

Quote:
Originally Posted by sheepdog44 View Post
Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com