Uphill Pulse and Glide???? Waste of time?

Discussion in 'Gen 2 Prius Main Forum' started by Santiago, Dec 12, 2008.

  1. Santiago

    Santiago New Member

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    90% of my driving is either uphill or downhill. Pulse and glide works great on level or downhill but it seems almost questionable at best or futile with any kind of uphill grade. Does anyone have any pro comments or am I wasting my time trying to do this? Going uphill seems to more than wipe out any gains in MPG from going downhill and using pulse and glide.:confused:
     
  2. Bob64

    Bob64 Sapphire of the Blue Sky

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    I haven't done any tests, but I feel its best if you - depending on the hill - build up speed before the climb, and have the engine on, but shedding speed, all the way up in one pulse, if possible. If others have some better techniques that arn't pulled out from my gut feeling, then please share :)
     
  3. efusco

    efusco Moderator Emeritus
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    The serious hypermilers on CleanMPG may have a different POV...but I tend to not P&G on uphill climbs.

    What I do is anticipate the hills as much as possible...I may start accelerating at 2300 or even 3000rpm just prior to the start of the climb in an effort to build enough momentum that I can climb the hill while staying at/below 3000rpm if at all possible...since above 3000rpm efficiency starts to drop fairly precipitously.

    A lot depends upon how much speed you're willing/able to lose to maintain that RPM...some of the long steep hills here in the Ozarks require me to push 4000-4500rpm to maintain the highway speeds necessary for normal traffic flow.
     
  4. andyprius

    andyprius Senior Member

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    You just can't get around topography and overall speed. Do an average of something more than 50% downhill, keep speed between 30-40 MPH and stress NO energy use on the MFD and I guarantee a green battery. Coming back the other direction where it's more uphill and the opposite occurs, a low charge rate and 2-4 purple bars (about) Exteme conservative driving on the drive back will not ever produce a green battery, but will drive the guy behind you crazy.
     
  5. JimboK

    JimboK One owner, low mileage

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    I do P&G only on the gentlest of hills, where my typical pulse rate (as determined by RPM and fuel injector timing) still allows acceleration. Otherwise, in hilly terrain I'm doing more of what could be called terrain driving: ICE-on going up, glide going down. And as the others suggest, I anticipate hills and build momentum to carry me up the next one when I can.
     
  6. JSH

    JSH Senior Member

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    I have been successful with pulse and glide on hills.

    For example the 65 mile drive to my former employer was remote state highway with little traffic. With the cruise control set to 65 I got 45 mpg.

    If I allowed my speed to gradually drop to 55 mph going uphill and glide or lightly regen downhill to a max of 75 mph I got 55 mpg.
     
  7. efusco

    efusco Moderator Emeritus
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    I think that's a bit different than what I consider pulse and glide, but it's equivalent to what I do...maintain a steady/efficient RPM uphill letting speed bleed off as tolerated. Downhill I'll glide/hyperstealth as fast as conditions allow. It's clearly better than maintaining steady speed.
     
  8. Celtic Blue

    Celtic Blue New Member

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    Yep, that's a different beast. I almost never get to do the classic pulse and glide because around here is mostly hills. When I have had the opportunity for true pulse and glide I've been astounded at the mileage.

    I call the hill driving mentioned previously "roller coastering" and it is what I practice when I can (except for traffic and signals of course.) I only regen to keep the speed down to a reasonable level, then go back into a glide before the crest where I hit my minimum speed.

    There is probably some speed/pitch/distance combination where it makes more sense to regen early rather than suffer the aero losses from higher speed. Obviously, if one ran flat out glide on a long downhill, energy that could be stored in the battery for the coming climb would instead all be wasted in aero drag. On the other hand regenning the whole way down to maintain a cruise speed needlessly converts potential energy into charge with its attendant losses. When you throw in engine efficiency and vehicle speeds on the climb it becomes a rather complex problem computationally. Picking a number out of the air I suspect the tradeoff generally becomes unfavorable above 65 mph...but I wish it was 70 or 75...
     
  9. Bob64

    Bob64 Sapphire of the Blue Sky

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    Well... I have to add that I avoid using brakes when going downhill.... Especially since my uphill climbs tend to leave a rather large "buffer" zone between me in the car ahead of me, such that by the time I catch up to him, I'm matching his speed.

    It's my belief that regeneration is much less efficient then losing the energy to aerodynamic drag (the prius has a low drag coefficient anyway).

    Think about it:
    kinetic - > electrical -> chemical - > electrical -> mechanical is VERY inefficient

    I'd rather keep as much energy stored in kinetic momentum then force it though all those hoops. It also gives you time to gain a lead on that car thats tailgating you.... and a chance to start the next hill at a running start.

    One other thing... my hills typically leave me at less then 40mph. So I make sure my ICE is off, then drop it into neutral (to prevent ice startup when accelerating above 41mph), and allow gravity to take me to 55+ mph (and down again to normal speeds once I reach the next hill).
     
  10. efusco

    efusco Moderator Emeritus
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    +1--I aim for zero regen...never get there, but that's the target.
     
  11. Celtic Blue

    Celtic Blue New Member

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    Depends on the hill. I can start from 40 and be doing 75 in a 55 if I don't regen. I have to regen on a short hill that I start at 20 and quickly hit 40 on. (That is if there are no other vehicles...it is posted lower.) I time the regen to keep below 42 at the bottom then switch to glide on the flat, then give it some gas as I start a brief steep climb on the other side.

    Evan, the drag coefficient in the Prius is low, but it's not that low, about 12% less than my 240 (which nevertheless had 6% less aero drag than the Prius because of its smaller profile), and 27% less than my wife's previous vehicle. Reduced drivetrain losses, the slight pull of the electric in glide, and tire pressures/configuration contribute substantially to the long glides and the drag free feel. The aero losses do become considerable at highway speeds. If they were not then my mileage would be far better at 70 mph than it is. It's that velocity squared problem.

    In theory the ideal profile would be one where any required regen was done early so that the max speed target was not reached until the end. That should be the most efficient. Doing the opposite by hitting max speed earliest in glide will lead to greater aero loss and less total regen. Of course, doing it the ideal way is slower and less satisfying. It would make for an interesting scangauge experiment for one of the engineers.
     
  12. Bob64

    Bob64 Sapphire of the Blue Sky

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    Uhhh... I had thought the energy lost from regeneration was around 50% according to The new 2004 Toyota Prius : EV efficiency measurements

    Is there anyone here that is savvy enough in math or physics to attempt a comparison in the amount of energy lost through drag VS the amount of energy lost through regenerative losses?

    edit: http://www.gtechprosupport.com/support/AeroDragCalc.php
    this calculates horsepower lost at a given speed...

    frontal area: 24sq feet according to http://www.caranddriver.com/content/download/100711/1306461/version/1/file/CD02_PRIUS.pdf

    coefficient of drag: .26 according to wikipedia


    now if we can just figure out how much gas does it take to create one horsepower...
     
  13. donee

    donee New Member

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    Hi Bob,

    The Prius only looses about 20 % of energy regeneratng. Where it looses energy is if the battery is used for standing stop, pure EV accelleration. The full cycle of regeneration to pure ev accelleration is only a 40 % or so efficient. The Prius just does not have a large enough battery (low enough internal resistance), wires and motor for effienct pure EV accelleration - because its designed to use the engine for accelleration - that is, its a hybrid car.

    If you use the regenerated energy for lower power applications, such as maintaining speed the full cycle loop is going to be much better.
     
  14. Bob64

    Bob64 Sapphire of the Blue Sky

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    Ok, assuming your correct, the main question to be answered is:

    Is the energy lost to drag from accelerating to a higher top speed greater then 20% (energy lost through regeneration) starting from speeds of 40mph and accelerating to a speed of 60mph?
     
  15. Celtic Blue

    Celtic Blue New Member

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    Bob64,

    One needs two curves (or assumptions) to make the determination. The first curve is the regen cycle efficiency. Of course this efficiency is dependent on how one will use the recaptured power on average. The page you referenced said 54% for acceleration/deceleration. (I don't know if this is representative or not or if that includes rolling resistance and aero losses...I think it does.)

    The second curve will show the percent efficiency that is lost to aero drag (a slight simplification since some other factors are also speed dependent, just not as strongly.) The point at which increasing momentum versus regeneration becomes negative is the point at which drag percentage exceeds the EV cycle losses.

    So if 100-54%=46% is taken as the regen/EV loss number one need only know at what speed aero losses reach 46% of the total. I'll have to see if I can find that curve.
     
  16. Bob64

    Bob64 Sapphire of the Blue Sky

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    Wonder if Bob Wilson has a graph for this...
     
  17. Celtic Blue

    Celtic Blue New Member

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    Okay, I think I have values for the second curve. Take a look at the Prius Palm Mileage Simulator Prius Palm Mileage Simulator There is a table showing the percent aero losses versus speed based on the 2004 Prius.
    Speed, energy/mile, Percent aero losses
    40, 152.8 Wh, 29.8%
    45, 181.3 Wh, 31.9%
    50, 190.8 Wh, 37.3%
    55, 202.9 Wh, 42.5%
    60, 215.5 Wh, 47.6%
    65, 233.4 Wh, 51.6%
    70, 250.3 Wh, 55.8%
    75, 268.5 Wh, 59.7%
    80, 287.8 Wh, 63.4%

    The crossover point when comparing to regen/EV appears to be somewhere in the 60 mph range. The key is in determining what the real world electrical path efficiency would be. Wish I could provide it, but that is beyond my expertise. There are others who have a firmer grasp of the physics of all of this than I do.

    Seat of the pants is how I came up with the 65 mph figure earlier. Somewhere above that is where I estimated I'm losing efficiency by not regenning based on the feel of the vehicle. With a Scangauge watching amps, SOC and fuel consumption it should be possible to determine the optimum on a given hill (starting and ending at the same speed at the top each time.) I expect this to show a broad range of similar overall efficiency at highway speeds for the hill and begin to drop off noticeably starting around 70 mph or so as the max downhill speed.
     
  18. Celtic Blue

    Celtic Blue New Member

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    For anyone who tries to conduct such a test, I believe a long hill would be best as it would allow more dwell time at a given max bottom speed so that the regen vs. aero would be amplified relative to the acceleration and deceleration phases.
     
  19. Bob64

    Bob64 Sapphire of the Blue Sky

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    Hmmm... I just thought of something.... regenerating energy, even though it recaptures energy, still FORCES you to use gas earlier when you re-accelerate at the bottom of a hill. As opposed to using momentum, which may or may not need additional gas input, but always at a later time due to increased amounts of kinetic energy.

    That being said, another factor is pumping losses, if downhill speeds exceed 41mph, we all know the engine is forced to spin. Where as if we drop it into neutral and prevent the engine from pumping, additional energy may be saved in the form of momentum. Assuming of course, we don't need to hit the brakes/engage regenerative braking to maintain control of the vehicle.

    Wonder if anyone is willing to test this out in a hilly region with several passes back and forth, to calculate the average mpg of both techniques.
     
  20. Celtic Blue

    Celtic Blue New Member

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    Bob64,

    Yes, that is true. There is a trade off made between SOC and gasoline, at least at the bottom half of the hill climb. After that one might/should be pulling off some of the SOC to aid the climb depending on the throttle position and what the ECU thinks is best.

    It is also for that reason that I was suggesting the option that will eliminate the lower momentum (speed) concern: pick a target speed at the bottom, but do most (target all) of the regen earlier, releasing the vehicle into a glide nearer the bottom to pick up the remaining speed. In this scenario one achieves the same velocity but converts some of the would be aero losses into charge. Momentum going into the climb will be identical, but charge will be greater, so the result should be something like later/lower gasoline consumption and some extra charge at the top. What would be traded in this case is time for efficiency.

    (Part of the problem in testing could be in linearizing the SOC values in terms of kWh. I'm not sure if 1% at 60% charge is the same as 1% at 65% SOC for example.)

    Seems like fertile ground for the hypermilers to explore.
     
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