The myth of pulse and glide

Discussion in 'Gen 3 Prius Fuel Economy' started by WPWoodJr, Jul 9, 2009.

  1. WPWoodJr

    WPWoodJr New Member

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    Here's an article on the myth of pulse and glide. You may not be getting as great an MPG boost from P&G as you thought.
     
  2. justlurkin

    justlurkin Señor Member

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    The thing with the Prius hybrid system is that if you take your foot completely off the pedal, the hybrid system will induce some regenerative drag to slow the car down-- One can see in the Prius energy display screen regeneration occurs when you take your foot off the gas pedal, and this will limit your glide distance.

    So in order to maximize pulse-and-glide in the Prius, you don't actually "glide" with your foot off the gas pedal. You keep a slight pressure on the gas pedal, just enough to prevent the hybrid system from inducing the regenerative drag, so you can coast as far as possible.

    This is easily done on the Prius because we have an energy-flow display that tells you if regen is occuring. Not sure if that sort of display is available on your Ford Fusion or if Ford programmed the FFH's hybrid system to induce a similar regen drag. I'd be interested to hear from you how it's implemented in the FFH though!
     
  3. a priori

    a priori Canonus Curiosus

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    I've posted a response to Bill on his blog.

    It is an interesting exercise to go through the math, but it does a bit of damage to the reality of things.

    Until the combination of wind resisitance, tire traction and braking equal or exceed the power of the HSD system to accelerate the, a Pulse and glide will result in more net forward motion than net "stopping effect." What this means is that more energy goes into the acceleration than the glide, so the car willl glide for a longer distance, and it still will end up with greater kinetic energy then at its starting point.

    Proof? Cars get better gas mileage than double the FE used during accelration. How else do I explain all of these cars getting such high mileage?
     
  4. msirach

    msirach Member

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    You must be a rookie of Pulse and Glide if you can't realize a true benefit. It is a proven technique.


     
  5. timberwolf

    timberwolf New Member

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    I consider this nothing more than an advert by the author for his FFH blog.
     
  6. fuzzy1

    fuzzy1 Senior Member

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    The blog author is just performing a simple math exercise based on some assumptions grabbed out of thin air.

    He makes no reference to real-world results. The top ranks of the CleanMPG fuel logs are filled by folks who have mastered it.

    He also makes no reference to the real-world reasoning behind P&G. To really understand why it can work, one needs to understand an engine BFSC map, which graphs fuel consumption vs. torque and RPM. It is most effective in traditional American pre-hybrids: oversized Otto-cycle (fixed valve timing) engines geared for performance over efficiency, causing normal cruise operation to be very far out of the engine's most efficient operating area. It also helps to use an engine monitor (e.g. ScanGauge), because random guesses about what sort of pulse to use often produce awful results.

    Modern hybrids cruise with the engine operating much closer to peak efficiency, so they have less to gain from P&G. I suspect that the best hybrid P&G results are occurring at lower speeds because they are not tuned for their best efficiency at those speeds. Without P&G they still get wonderful results at those low speeds due to the much lower air resistance and their ability to reduce friction by halting the spinning of the ICE.
     
  7. Fred_H

    Fred_H Misoversimplifier

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  8. WPWoodJr

    WPWoodJr New Member

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    You're correct that a good glide will be longer in distance than the pulse, and I wasn't disputing that. I'm also not disputing P&G as a technique.

    The interesting thing is the easy misconception that the average MPG during P&G is higher than it really is. To get 60 mpg in my example of a 20 mpg pulse and 100 mpg glide, your glide needs to be 5 times the length of the pulse - I believe that is counter-intuitive to most people.
     
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  9. WPWoodJr

    WPWoodJr New Member

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    In the FFH, there are three types of regen - regen from braking, regen from the ICE when it is providing more power than the car needs, and regen from taking your foot off the pedal. Regen from braking is displayed on the battery SOC display on the dash as 3 arrows in a circle; regen from the ICE or lifting your foot from the pedal is displayed as an upwards-pointing arrow. If the FFH has nav there is also a large power-flow diagram similar to the Prius but it is harder to read - no color-coding and the arrows that indicate power flow direction are small and hard to see while driving; so I don't use this much.

    On the FFH its hard to keep the balance between regen and acceleration during a glide. Its much easier to put it in neutral for the glide, but then you lose all regen capability so I don't use neutral often. I find where I drive that I usually either need to slow down, so I let it regen, or I need to keep the glide going longer, so I apply a little EV power.
     
  10. ksstathead

    ksstathead Active Member

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    ... and that's why P&G is primarily a low-speed technique, avoiding the worst of the wind resistance. It also helps to have LRR tires, tires at higher pressure, perfect alignment, etc.
     
  11. WPWoodJr

    WPWoodJr New Member

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    Guilty of being a rookie! Didn't mean to imply that P&G wasn't beneficial.
     
  12. a priori

    a priori Canonus Curiosus

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    That part is understood, Bill!

    There's the rub, because the title of the thread, regardless of your intention, allows most every reader of PriusChat to infer you have "found out" P&G and that it isn't anything more than a myth.

    You'll likely suffer a few slings and arrows for that one!
     
  13. justlurkin

    justlurkin Señor Member

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    Thanks for the description-- I think it's informative to see how it's implemented the FFH, considering the Prius and the FFH uses very similar drive systems.

    For a casual pulse-and-glider like me, I find the Prius energy flow display to be very helpful in maximizing glide distance. I don't know if it's the pedal setup in the Prius, but on highways I find it pretty easy to keep the gas pedal in the right position to prevent regen. It's only on really hilly or bumpy roads when I find it more challenging.

    I'll have to rent a Ford Fusion Hybrid one of these days and do a comparison to see if it's any harder to do than in the Prius. :cool:
     
  14. WPWoodJr

    WPWoodJr New Member

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    Maybe I should have called it "Mathematics behind natural inclination to overestimate mileage attained during pulse and glide" - that would have gotten huge interest :D
     
  15. ken1784

    ken1784 SuperMID designer

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    Are you saying the FFH consumes some fuel during the 100mpg gliding?
    Prius does not consume any fuel during gliding, therefore we see an infinity mpg then.

    [email protected]
     
  16. WPWoodJr

    WPWoodJr New Member

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    FFH consumes no fuel during glide unless going more than 47 mph, like Prius Gen III above 45. However, unless there is no A/C, no lights, no radio - basically no battery losses at all - you are consuming mpg because the battery is discharging and will need to be recharged at some point. So equivalent mpg even when gliding is not actually infinite.
     
  17. ken1784

    ken1784 SuperMID designer

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    OK, I understand what you're saying.
    But, you're saying between 20 and 150 glide MPG's which consume no fuel and consume some battery power.
    it's very hard to imagine how we consume battery power on such wide range of MPG numbers.
    Would you please explain how do you assume the battery power?
    Such as...
    20 MPG glide: (it must be a huge battery power:heavy A/C use???)
    80 MPG glide: (it may be moderate?)
    150 MPG glide: (It's very little battery consumption?)

    Anyway, I believe following chart is better to understand how the Pulse & Glide work.
    I assume the Glide part does not consume any fuel, therefore an infinity MPG.
    The "Glide %" means gliding distance percentage of total driving.

    [email protected]

    [​IMG]
     
  18. Celtic Blue

    Celtic Blue New Member

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    What myth? That's not pulse and glide. In P&G you extend the glide as long as possible with no gasoline consumption until the pulse. That's why in low speed situations where light timing and such are possible it's not too difficult to get 80-100 mpg.

    At any rate, I remember watching some PBS or other documentary about P&G without a hybrid drivetrain ~15 years or so ago. The engine was only operating briefly and efficiently which is what P&G does. (I don't recall what terminology they used, and it might have been a motorized bicycle or something like that but I still remember the central theme.)
     
  19. CharlesJ

    CharlesJ Member

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    How would the numbers stack up if that glide mph in reality is 9999mpg instead of that 100mpg? Or any number between 9999 and 100? Maybe that makes a huge difference? the dash readout only goes to 99.9 but, a scan gage will read 9999mpg, or can.
     
  20. hobbit

    hobbit Senior Member

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    Most folks are treating P&G as some sort of cookbook procedure,
    without understanding that it's a simplified version of the real
    purpose -- to run the engine as efficiently as possible when
    it's running, and let it shut down in between. The run times
    and off times and speed ranges don't have to be some exact
    regime; all you have to do is realize when conditions are most
    optimal for one or the other and adapt the right scenario to
    the traffic and terrain around you. It's madness to hit a rise
    and think "but it's time to glide!" and slow way down because of
    some rote notion that it wasn't time to light the engine yet.
    It's all about anticipation and understanding how to work the
    car best for what's coming up.
    .
    Don't think of it as "P&G" per se, think of it as efficient
    running conditions and judicious use of the one form of energy
    storage that requires no conversion to be used for the desired
    purpose -- momentum.
    .
    _H*
     
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