Pulse and Glide on Interstates? Efficiant highway driving?

Discussion in 'Gen 3 Prius Main Forum' started by cary1952, Aug 5, 2013.

  1. cary1952

    cary1952 Member

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    Other than coasting down hills, is there an efficient manor of pulse and gliding on flat freeways? Also What is the difference in MPG between cruising at 65, 70 and 75 mph on the interstate? What is an efficient way to drive the interstates?

    Would like to hear from you hypermiler experts.
     
  2. ksstathead

    ksstathead Active Member

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    P&G is mostly out since at high speeds there is so much wind resistance that the glide time is too short down to a reasonable low end speed. So you end up accelerating all that mass too much for the benefit. Now when you exit or when you see a slowdown ahead, glide for sure, trying not to use the brakes.

    You also want to watch your tire pressure. Door placard is a minimum, with mpg benefits as you increase toward max sidewall on the tire. Trade off is stiffer ride, but you also generally get better handling with the higher pressures along with (often) longer tire life.

    Don't overdo it on climate control, but don't swelter either since the batteries swelter with you and don't perform as well. (though not usually much of a factor on flat interstates that are not congested).

    This thread has speed vs mpg graph:
    Graphics of average fuel economy vs speed | PriusChat
     
  3. tumbleweed

    tumbleweed Senior Member

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    Keep your tires properly inflated for good mileage, set the cruise control for somewhere near the speed limit and accept the mileage for what it is.

    I think it's best to have a little consideration for other drivers and maintain a steady speed in traffic, at least where you can. I try not to be one of those Prius drivers who think they own the road and can continuously speed up and slow down with little regard for others just because it might gain a couple of mpg.
     
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  4. ksstathead

    ksstathead Active Member

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    tumbleweed, I too avoid holding up traffic. Not all roads are congested.
     
  5. tumbleweed

    tumbleweed Senior Member

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    Yes I understand that, my comment was intended to be general in nature and not aimed at you. :)
     
  6. KyleT

    KyleT Junior Member

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    The best way to achieve high MPG when driving on freeway/highway is to keep your car at the constant speed. Let the cruise control does the job. It is designed to optimize the fuel consumption of your car.

    "Pulse and Glide" theory is nothing but BS. It was invented by someone doesn't have basic understanding of science and engineering. Remember, your car will loose more energy when it is driven at higher speed due to the wind resistance. Furthermore, acceleration is always the worst part of the fuel consumption. When you let your car glide, it will regain far less energy than it wastes at the higher speed...
     
  7. g4_power

    g4_power Junior Member

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    P&G doesn't work well in high speed unless you're gliding down a slope. At high speed, the car has to overcome wind resistance and ICE drag. At speed over 40, the ICE will continue to turn even when there's no fuel going into it. I get superb mpg when I P&G below 40 mph but on the freeways, I don't see any benefit in doing it.

    PS: P&G is not B.S. I disagree with the poster who made that statement. I'm a bicycle rider and I P&G all the time on my bicycle
     
  8. GasperG

    GasperG Senior Member

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    Pulse and glide in general is not bullshit.
    General for the Prius it can be used when you need less than 8kW power for achieving cruising speed, that power is needed around 55 mph on the flat road.
    But apparently Prius hybrid system is so efficient, that there is no real benefit in P&G even at lover speeds, this was documented by Bob Wilson:
    Efficient driving for a 1,000 mile tank | PriusChat
     
  9. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    WOW! The hundreds of posts about P&G are wrong. And the high mpg folks get must be fake. This is real news.
     
  10. macman408

    macman408 Electron Guidance Counselor

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    KyleT isn't *entirely* wrong...

    ...just mostly.

    He is correct that it takes more useful energy to do P&G than it does to just accelerate to the desired speed and stay there, assuming that the average speed is the same in both cases.

    He is wrong in that P&G can still be more efficient. First, many people I know who do P&G don't maintain the same average speed that they would if they were just maintaining a given speed (usually the speed limit, +/- a few mph). Instead, they accelerate to maybe 5 mph over the speed limit, and then glide down to maybe as much as 10 mph below, if traffic allows. This generally gives a lower average speed, and thus less aerodynamic losses.

    But that's not even really what P&G is targeting. The real savings comes because you're using the engine in a more efficient RPM range. Say that accelerating to and maintaining a given speed requires 100 arbitrary units of energy, and using P&G for the same average speed requires 110 units of energy. This energy must be generated by the gasoline engine. If P&G allows it to be generated at 30% efficiency (thus burning gasoline containing 367 units of energy to produce 110 units of usable energy), and maintaining speed means that the engine is only running at 25% efficiency (thus burning gasoline containing 400 units of energy to produce 100 usable units of energy), you can see that the P&G method use less fuel, and thus be more efficient.
     
  11. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    So you are saying that its primarily average speed that counts due to aerodynamic losses. If you use P&G to provide that average speed there is a minor improvement in mpg for a given average speed. What would be some practical mpg values for P&G vs constant speed for various average speeds?
     
  12. g4_power

    g4_power Junior Member

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    Obviously, P&G is something that works. That's how NASA send spacecrafts to Mars and other destinations in space. Typically, the rocket engine(s) would burn hundreds of thousands of gallons of fuel in matter of minutes and then let the spacecraft glide for millions of miles in space. I don't think I'd need to explain why it could glide that far.

    On earth, we have wind resistance and friction to deal with so gliding millions of miles is out of the question. However, it still does work well at low speed because wind resistance is small at low speed. Also, the ICE is completely turned off at speed below 40 mph. Ideally, you would want to able to glide at least the same distance than it took to pulse. ie: If you accelerate modestly for 500 feet and the mpg was 35, and then you glide for 500 feet, your final mpg would be 70 mpg.

    P&G does not work well at high speed due to much higher wind resistance and the ICE would be turning at speed above 40 mph. Adding engine friction and inertia will certainly decrease the gliding distance drastically.
     
  13. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    Sounds good but what are the min and max speeds. Can you really glide the same length as you accelerate?
     
  14. g4_power

    g4_power Junior Member

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    The message was to point out that P&G does work. I'm not here to tell you whether you should use it. We are all adults here, right?

    I live in a small town and travel often in the evenings. Traffic congestion isn't something I had to deal with often if I just drive around locally. The speed limits around here is 25/35/45. If the limit is 45, I would pulse to 50 and glide down to 35. I also put the Prius in neutral while gliding.
     
  15. Randy G.

    Randy G. Member

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    Man I'd hate to be the guy following you.
     
  16. g4_power

    g4_power Junior Member

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    It's a two lane road, if you were behind me, what's preventing you from changing lane? Again, traffic congestion isn't common around here in the evenings.
     
  17. tv4fish

    tv4fish Member

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    WELLLLLL - IMO - KyleT is not only "all wet" when it comes to Pulse and Glide, but also with his statement above ^^
    I know I'm not going to "achive high MPG" with the cruise on as I'm going up a hill.....................
     
  18. KyleT

    KyleT Junior Member

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    The serious flaw with the Pulse & Glide (P&G) theory is that it completely ignores the two most important factors which make its theory become worthless if they are brought into the equation. They are the enegy losses due to the wind resistance and vehicle’s acceleration.

    First, let's look at the wind resistance (or drag force):
    F (drag) = ½ (d)(C)(A)(V^2)

    Where:
    F (drag) is the drag force
    d is the density of the fluid. (In this case it's the air)
    C is the drag coefficient number
    A is the cross-sectional area
    V is the speed of the object relative to the fluid

    Since everything on the right side of the above equation is constant numbers except for the speed, I’d like to simplify it so that it will be easier to discuss:
    F (Drag) = K * V^2 [where K = ½ (d)(C)(A) = (constant number)]

    You can see the wind resistance increases as the square of the driving speed:
    - At 40 mph, the wind resistance or drag force will be F (Drag) = K * 1600
    - At 56.6 mph, the drag force will be double of that amount or F (Drag) = K * 3200
    - At 70.3 mph, the drag force will be triple double of that amount or F (Drag) = K * 4800

    In the pulse cycle, you will need to increase your car speed to be above the target one therefore it will consume more energy to overcome the wind resistance. The higher the speed you drive, the more drag you get. The end result, your car’s travel distance will suffer in the gliding cycle.

    Second, let's look at the force which the ICE need to generate to accelerate your car (Newton's 2nd law):
    F (accelerate) = (m)(a)

    Where:
    F (accelerate) is the acceleration force
    m is the mass of your car
    a is the the acceleration rate.

    You can look at the dashboard of you car, and see how much fuel your car consumes during acceleration. Your total MPG depends greatly on how you accelerate your car.

    Let's look at the graphs in the pictures below:
    Assumtions:
    -No head or tail wind.
    -Flat/level road
    -Target speed 65 mph and P&G +/- 15 mph
    -Constant acceleration during pulse cycle
    -Begining speed 50 mph and then accelerated to 80 mph and then glide back to 50 mph.
    -The car will be in the pulse cycle 2/3 of the target distance and then glide about 1/3 of it

    (Sorry - Pictures being edited and will be reposted)

    The conclusion is that the total energy your car needs to overcome wind resistance and acceleration always larger than the energy your car need to maintain at the constant speed.


    If you intend to practice P&G, you should be careful about a few things:

    -Watch out for cops. You could get a ticket for driving above or below the speed limit. One ticket would hurt you for a long time.
    -Learn to ignore people who could drive parallel with you and curse at you or point his/her index finger to his/her head and move it in circular motion.
    -If you live in a rough neigborhood, make sure you learn how to duck and have a good life insurance policy. You could be a victim of a drive-by shooting just because you want to save couple bucks in fuel.


    One of the beauties of the computer controlled cruise control combined with CVT is that it will try its best to optimize the fuel consumption of your car based on the road condition, wind resistance, friction caused by car’s internal mechanical parts and tires’ rolling resistance. Why don't you just let the car do the work and enjoy your trip without looking over your shoulders...
     
  19. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    So you are saying that P&G doesn't work at highway speeds (the topic of this thread). Might it be effective at lower speeds considering V^2?

    Folks seem to be getting some very high mpg with P&G at lower speeds but perhaps is just the lower speed that is improving the mpg and not the P&G.
     
  20. GasperG

    GasperG Senior Member

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    Ok, the theory behind P&G is that ICE has the best efficiency under high load. For instance do a calculation when pulse is 35% efficient and constant speed is only 25 % efficient (higher rpm and lower load).

    With Prius you have some problem at high speed P&G:
    - Glide at high speed must turn the ICE presenting an additional drag of around 2 kW (see PSD animation)
    - At higher speed ICE will have enough high load to be always on and working at max efficiency and best rpm

    Question is what happens at lover speeds
    - from the formula obviously P&G needs more energy for same average speed, but air resistance is much less at lover speeds where rolling resistance represents greater drag.
    - when using battery you obviously don't get max efficiency from gasoline, but question is if it's still better than P&G with ICE only
     
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