The myth of pulse and glide

Discussion in 'Fuel Economy' started by Chuck, Jul 11, 2009.

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Has pulse and glide worked for you?

  1. Yes

    80.8%
  2. No

    17.3%
  3. What is P&G?

    1.9%
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  1. Chuck

    Chuck just the messenger

    [​IMG] So, in our example of a 20 MPG pulse and 100 MPG glide, how far must you travel during the glide to get 60 MPG total? Five times the pulse distance!

    [fimg=LEFT]http://www.cleanmpg.com/photos/data/501/Pulse_Glide_graph.JPG[/fimg]Bill Wood - SQUARESPACE - July 8, 2009

    Commentable article....why is it working for so many of us then? --Ed.

    The math behind "pulse and glide" seems intuitive enough. At least that's what I thought. Until I started calculating my total MPG for trips to and from work. On the way to work I have gotten 60 MPG; on the way back I have gotten close to 40 MPG - that's 50 MPG total, right? I did the math, and my MPG to and from work is only 48 in that case.

    How does this apply to pulse and glide? Pulse and glide is a technique where you accelerate moderately to an upper target speed such as 45 MPH, then lay off the accelerator and glide down to a target speed such as 35 MPH, possibly putting the car in neutral during the glide to minimze fuel use. Then you repeat. The idea is that the pulse MPG and glide MPG should average out to some pretty good gas mileage.... [rm]http://ffh.squarespace.com/blog/2009/7/8/the-myth-of-pulse-and-glide.html[/rm]
     
  2. AlmightyEngineer

    AlmightyEngineer Engineering First!

    Without getting lost in the semantics, this is the goal of efficient driving but two pieces of information are needed:

    * Brake Specific Fuel Consumption(BSFC) - this is the amount of fuel burned to produce a given amount of mechanical or shaft energy
    * drag reduction - this is the sum of the rolling, aerodynamic and what Ken@Japan calls, 'heart beat' power ... the energy that is consumed by the electronics with the car regardless of what it is doing

    If you plot the drag force product with the velocity, you get an energy curve, the least amount of energy needed to sustain any given speed:

    [​IMG]

    Now you'll notice in some cases, the plotted performance exceeds the theory. This isn't "cold fusion" but upon closer inspection we find:

    1. tires fully inflated - in the first Prius marathon, one report claimed 50% over maximum side wall pressure and nearly bald tires.
    2. warm temperatures - generally above 80F. So far, no one has reported attempting a maximum, density altitude test, say Wyoming or Colorado during the hottest days.
    3. aerodynamic modifications - starting with bumper air inlet blocks and proceeding to wheel well covers and stripping off all external objects or if on public roads, putting them in a least drag position.
    4. minimum weight - take out everything that can be removed.
    5. lubricants - using a lower viscosity engine and transmission oil to minimize mixing losses and other properties that reduce thin film friction without leading to wear that shortens the vehicle life.
     
  3. diamondlarry

    diamondlarry Super MPG Man/god :D

    I left the following response to Mr. Wood's article:
     
  4. jimepting

    jimepting Well-Known Member

    Fascinating chart - to an engineer. But, I could stand a little more explanation. Where does it come from? Is the "fixed" a fixed load. What is NHW11 and NHW20? I get the impression that the curves are theoretical and the data points are measurements - correct? What is What is the "hack?" Anyway, would you explain the history and the relevance of the graph a bit more.

    I assume this data is all from one car and one driver. Correct?
     
  5. Jough96Accord

    Jough96Accord 1996 Honda Accord 2.2l 5spd

    Same for me. I would not try to pull a long glide if it was not at least somewhat going downhill. There are spots on my commute where I can glide for nearly 3 miles. Glide for 3 miles, how can that not save gas?
     
  6. vtec-e

    vtec-e Celtic MPG Warrior

    My recent attempts at p&g seemed to give me great results but my tanks weren't much better. My difficulty in dialling in my scangauge was giving me optimistic results. The 1:5 ratio, for me at least, is hard to do on the flat unless i plant the accelerator. This results in a net waste of fuel. Not to mention being harder on the clutch. It wouldn't last long if i pulsed like that every 20 seconds over an hour, twice a day, four days a week....
    I'll keep trying anyway! It works for some so why not me eh?!

    ollie
     
  7. fusion210

    fusion210 Well-Known Member

    Easy guys. I think the title is getting people slightly riled up.

    He's not saying it doesn't work. The only message is that you won't instantly get the average of how much gas you're using in a pulse and how much gas you're saving in a glide as if it was 50/50.

    Have you ever tried to explain pulse and glide to someone and had this happen? In my case it's getting around 30mpg in a pulse, 180mpg in a glide. The person I'm talking to usually combines 30mpg with 180mpg, getting 210mpg and then divide that by two. I must be getting 105mpg! But no, it's somewhere between 60-70mpg at freeway speeds.

    Perhaps it should have been just titled "A misconception of pulse and glide."
     
  8. cpeter38

    cpeter38 Right Lane Dweller

    This is the meat of my response:
    The focus of the article should be different.
    The point should be that a properly applied P&G will deliver VERY SIGNIFICANT fuel economy benefits. As an example, I am averaging approximately 60 MPG in my car by using P&G (a Ford Contour SVT that nearly qualifies for the cash for clunkers bill). I've actually had serious debates about whether or not I should try to go below the 1/8th mark on this tank because I am very close to 1000 miles on this tank (~850 miles).
     
  9. JusBringIt

    JusBringIt Be Inspired

    Fusion210 has it exactly right.

    The author just wanted to explain the misconception people have of using pulse and glide.

    For reference, my pulse is at about 17mpg, my glides are at 9999mpg. I avg out to 55mpg doing this. A simple calculation will show you that my pulses are less than 1/3 of the distance of my glides on average.

    All he is saying is Time is NOT a factor in the equation of why pulse and glide works, it's distance.
     
  10. Or perhaps it should have been just titled, "A misconception about how MPG is calculated." I'm afraid that too many people think that if you get 20mpg for a 100 mile stretch of road, and then get 100mpg for a second 100 mile stretch of road, that you have averaged 60 MPG for the trip. In reality, you have used 5 gallons for the first stretch, and 1 gallon for the second stretch. This means for the 200 mile trip, you used 6 gallons of gas and averaged 33.3 MPG.

    Cheers,
    Bryan
     
  11. JusBringIt

    JusBringIt Be Inspired

    All the more reasons for it to be calculated as gallons per mile
     
  12. iamian

    iamian Well-Known Member

    As others have stated Pulse and Glide isn't 'magic'.

    The article seems to clearly describe the limits of the Glide that is needed to make P&G work... if anything it has a hint of bias against P&G in the way it is presented ... but all of its math and such is correct.

    It ends up saying exactly what any P&G practitioner already knows ... the glide should be significantly longer than the pulse... the greater the difference the better the results... if the glide to pulse ratio is not over a minimum level P&G ends up being a net MPG hindrance.

    As with just about anything... the specific details and conditions will end up determining where the break even points are and on either side of that point ... it leans one way or the other ... for a benefit one way ... or as a hindrance the other way.

    My personal point of view is ... learn your common routes ... practice techniques ... and learn what works best for you given the specific conditions you have to deal with... if a different technique comes along try it... if you have trouble mastering a given technique yourself look too see if you can get some pointers from someone else who has already masters that specific technique you are struggling with... no body walks the day they are born ... and conditions will vary from route to route... not all techniques work as well from vehicle to vehicle.
     
  13. Chuck

    Chuck just the messenger

    As several of our members have said, the title was provocative and misleading.

    For P&G to save gas, you have to glide far enough to recoup the gas used in the pulse - no charts or technobabble needed

    To the AlmightyEngineer: we know the NHW11 is the Prius I, the NHW20 is the Prius II, and the ZVW30 is the Prius III....you might impress us more by articulating in plain English. ;)
     
    Last edited: Jul 11, 2009
  14. Chuck

    Chuck just the messenger

    For Honda hybrid drivers, the battery pack is another consideration on pulse and glide.

    On my Insight, I will lose a bar on the SoC every time or two I cut the ignition.

    For me, I need at least a 0.1 mile and probably more like a 0.25 glide for P&G to benefit me.

    It's not only the length of the glide, but how you pulse (amount of throttle, upshifting, etc)
     
  15. 99LeCouch

    99LeCouch Well-Known Member

    My glides have to be pretty long to recoup the difference since my car uses so much gas to accelerate at a brisk clip. 5-6 mpg or lower is common at city speed pulses at 10 LOD and ~1700-2000 RPM, and it doesn't glide well in neutral unless there's a downslope. Can't FAS since it's not flat-towable. Owner's manual warns of significant transmission damage if flat-towed.

    Where I know there's a downhill coming up, I'll pulse a little bit. Otherwise it's DWL combined with moderate acceleration to get the car into TCC lockup sooner.
     
  16. Chuck

    Chuck just the messenger

    Forgive me, but I don't always have the time to study all the articles posted, but take note of this:
    Effective pulse and glide always means gliding several times longer than the pulse.

    Typically I'm in a residential section and pulse 2-10 seconds, then glide 30-60 seconds.

    The ratio of pulse time vs glide time is going to vary among members and even on driving conditions, but if pulse time remotely equals glide time, it's poorly executed!
     
    Last edited: Jul 11, 2009
  17. GaryG

    GaryG Well-Known Member

    The author is a new owner of the FFH and a member here. He explained to me that the math of calculating MPG was the point of the article and not that there is any question weather P&G works. Based on a 84mpg segment he got out of his FFH, I would say he is a quick learner of P&G.

    I would like to say the title of the article got your attention and caused you to post. The good part was there are people here that do not fully understand how to do P&G and this article may improve their understanding and MPG now. Good job Bill!

    GaryG
     
  18. Chuck

    Chuck just the messenger

    I hope we did not flog you too much - Bill.

    Yes - if this enlightens it was a good article.
     
  19. AlmightyEngineer

    AlmightyEngineer Engineering First!

    Pulse and glide advocates seldom list:
    • speed range - the minimum and maximum values
    • constant speed equivalent - what the vehicle achieves as a reference
    This is the problem shared by the referenced article. The author claims a "15%" performance improvement but there is no reference to the minimum and maximum speed values that achieve this gain nor the reference speed. To show you what I mean, what is the performance of a given vehicle at:
    minimum:maximum speed -> MPG?
    30:30 -> ?
    29:31 -> ?
    28:32 -> ?

    25:35 -> ?
    20:40 -> ?
    15:45 -> ?
    10:50 -> ?
    05:55 -> ?
    00:60 -> ?
    Without a good model, pulse and glide leaves more questions than answers. But I've done some experiments to define these terms and measure the effects with my NHW11, 2003 Prius.

    25-43 P&G vs 33 miles per hour


    This speed range is the minimum and maximum that my Prius cruise control uses yet supports engine-off glide. Starting at 25 miles per hour, the cruise control "resume" consistently accelerates the car back to the set speed, 43 miles per hour. Then the cruise control is set to "coast" and when the car reaches 40 miles per hour, shifted into "N" for an engine off glide. Here is a graph of the speed profiles:
    [​IMG]

    So using data from the mileage display, we can plot the MPG for these runs:
    [​IMG]

    So in this case, a minimum speed of 25 miles per hour and maximum of 43 miles per hour gives a mileage improvement of 11%. This not quite the 15% reported by the article at the expense of a speed difference of 18 miles per hour from 25 to 43 miles per hour. I had the advantage of a road with no other traffic so I didn't attract the attention of "road rage" warriors or the police looking for intoxicated drivers.

    15 - 25 P&G vs 18 MPH

    My earlier studies had determined that the maximum range speed, best fuel efficiency, for my Prius is 18 miles per hour. But trying to measure MPG at these speeds directly requires very long distance and timed runs. To shorten the time and distance, I switched to measuring the energy consumed per meter. So here are the pulse and glide and steady speed energy charts:
    [​IMG]
    Steady speed:
    [​IMG]

    In this case, there is a net 14% savings by holding 18 miles per hour versus the P&G equivalent speed of 19 miles per hour between 15 to 25 miles per hour. The difference in efficiency between 18 and 19 miles per hour at a steady speed is ~1%, much less than the 14% measured loss of pulse and glide:
    [​IMG]

    WARM-UP PULSE AND GLIDE


    Until the engine coolant reaches 70C, my Prius burns more fuel in any gear than when in "N" including "P":
    [​IMG]
    So during the engine warm-up, I accelerate to neighborhood speeds, ~25-27 miles per hour, and then slip into "N" to coast down to about 18 miles per hour or so depending upon traffic. Usually there is little or no traffic on the neighborhood streets and it only takes about 1.5-2.0 miles to warm-up. But once the engine coolant reaches 70C, I revert to normal Prius efficiency driving. I'm typically seeing ~35 MPG versus ~25 MPG over ~1.5 miles of warm-up.

    Conclusion

    My 2003 Prius does not seem to perform exceptionally better using pulse and glide except during warm-up. Once it is warmed up, driving within a few simple, control law limits has returned reproducible, 'old' EPA and better mileage over 50,000 miles. Yes, I've done better than my signature 52 MPG and know how to reproduce results like this over any distance:
    [​IMG]

    It isn't that pulse and glide has no utility but I can't find it in practical driving or to max out the fuel efficiency with my 2003 Prius. Other vehicles with different architectures may do better with pulse and glide. But sometimes, the pulse and glide advocates describe 'terrain driving' and call it pulse and glide. Regardless, I noticed there seems to be some reluctance by P&G advocates to quantify the speed ranges but great enthusiasm for claiming results. That isn't science or engineering, it is advocacy.
     
  20. JusBringIt

    JusBringIt Be Inspired

    AlmightyEngineer:

    I would recommend you to DiamondLarry to learn about your prius as you have a lot to learn.

    52mpg in a prius is pathetic to consider "good results". I got 56mpg in busy chicago traffic LEARNING how to drive the prius.

    You have not begun to grasp the concept of pulse and glide much less the ability to utilize it efficiently. Different cars behave differently at different speeds, gears and rpm's etc. You need to find out what's right for your car.

    There is no real Universal consensus as to what speeds give you what results. I have a V6AT Dodge Avenger that gets better mileage on the highway than your prius and your poor 52mpg performance. I cannot apply pulse and glide in the same manner it is applied in a prius because the cars are vastly different.

    I'll actually recommend you find a clinic somewhere around you where one of our experienced members will be able to properly assist you in getting to know your car. Until you are able to pull 100+mpg segments with your Prius, you haven't begun saying much.

    Good luck, and hope you can break that 52mpg barrier on a daily basis.
     
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