Is the apparent benefit of P&G simply poor accounting of when energy is used?

Discussion in 'Gen 2 Prius Technical Discussion' started by mad-dog-one, Feb 22, 2010.

  1. mad-dog-one

    mad-dog-one Prius Enthusiast

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    I understand how to use repeating cycles of Pulse (to 45 MPH) and Glide (gradually back to 30 MPH) (P&G) to inflate MPG readings on the FDC relative to a constant velocity of 40 MPH. I also understand that the FDC is imperfect and that P&G is the holly grail of Prius for some. Nevertheless, P&G efficiency defies Newtonian laws governing conservation of momentum and energy. The best bouncing ball never returns to the height at which it was dropped.

    My concern is that the FDC mileage information may not be telling the whole story. The assumptions of my theory are:

    (1) the ICE is running near maximum efficiently at 40 MPH,
    (2) all of the electric power stored in the battery, ultimately comes from the ICE,
    (3) no mechanical or electrical system is 100% efficient and
    (4) you don’t get something for nothing.

    It seems to me that the power requirement for oscillating cycles of acceleration followed by gradual decelerating glides, like the bouncing ball, will be subject to a net efficiency loss when compared to a constant velocity. Could it be that P&G gives an artificially inflated FDC mileage reading because it consumes more battery energy, than steady velocity, and the net loss of battery energy is poorly integrated into the FDC MPG readings or is replenished later. Ultimately, the ICE will require fuel to replenish battery energy that was depleted during P&G and this can be reflected by poorer mileage during steady state velocity because the ICE will be required to run longer. P&G; what am I missing? With friction and inefficiency, the pulse has got to cost more than the return from the glide.
     
  2. Politburo

    Politburo Active Member

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    Someone with better technical knowledge will come along, but I think it has to do with where the Prius engine operates most efficiently. I don't think it's at the lowest RPM such as you would use for constant velocity. ETA: And obviously, as you state, battery power can only be less efficient.

    I don't think it's a measurement issue.

    Here's a good link: Driving technique: exploring 'Pulse and Glide' - MetroMPG.com
     
  3. qbee42

    qbee42 My other car is a boat

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    No, it actually works. What you are missing is that the Prius ICE is actually too large for the required work. Like all cars, the ICE in the Prius is sized to allow for acceleration and hill climbing. On the Prius the ICE is less oversized, but it is still larger than needed for steady state driving.

    P&G allows the ICE to run at its optimum level while storing the excess energy as kinetic energy. The ICE then shuts off and the stored kinetic energy is used to overcome drag. The cycle is repeated to keep the average speed at an acceptable level.

    Overall, P&G does not introduce many extra losses. The only additional loss is aerodynamic at the higher end of the speed curve, where the car's speed exceeds the average speed. This is offset somewhat by the lower end of the speed cycle, but since aerodynamic drag is non-linear, the effects do not cancel.

    Edit: Properly done, P&G does not use battery power. P&G is a gas-only kinetic energy operation. If the MGs come into play you are wasting energy.

    Tom
     
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  4. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Yes, it works. People in Japan has gone further than 1,000 miles in one tank.
     
  5. miscrms

    miscrms Plug Envious Member

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    Its important to note that P&G is not a Prius thing. Its an ICE thing (well at least gasoline ICE). You can P&G in almost any car. Its related to the engine operating much more efficiently under heavier load and with a more open throttle than when its lightly loaded as in most constant speed cruising cases.

    The Prius just tends to be particularly good at P&G, since it can completely shut the engine off during glide.
     
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  6. mad-dog-one

    mad-dog-one Prius Enthusiast

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    Thank you Priuschat group for the quick answers. I see a consensus that my assumption about the ICE achieving near maximum efficiency at a steady 40 MPH is flawed. Thats a whole lot better than chalenging Newtonian physics. :)
     
  7. ksstathead

    ksstathead Active Member

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    We'll let Newton live for another day.:)

    Another aspect to keep in mind: at constant speed of 40, the ICE is not even on all the time, much less at a constant rpm. This means conversion losses while the ICE charges the HV repeatedly, then shuts down. Better to put in into kinetic energy and glide.

    Further, the HP to move gen3 at 70ish mph is around 20 HP, a small fraction of the gen3 ICE HP. So the HP to move it at 40 mph is really a tiny fraction of its capability, hence the P&G benefit as Tom and others explained so well above.
     
  8. miscrms

    miscrms Plug Envious Member

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    [​IMG]

    I'm an EE, not a ME, so I may get some of the details wrong, but I think this image does a pretty good job of telling the general story. This data is for a Geo Metro 1L engine. The vertical axis is power output, which you can also think of as load or throttle position. The horizontal axis is rpms. The values on the contour map are BSFC, which is basically an inverse measure of efficiency, IE how much fuel are you burning vs. how much power are you generating. The lower the number, the better your efficiency. This graph basically shows that this engine is most efficient when run at medium rpms, at medium loads. It becomes less efficienct at very high or very low rpms, and at very high or very low loads.

    From this you can get a sense for why P&G works. Cruising at 40mph, rpms are low as is engine loading, requiring light throttle and low power output to maintain speed. This can easily put engine efficiency off by a factor of 2 or more from its peak value. Instead of operating the engine consistently at this low efficiency state when cruising, P&G attempts to alternate between running the engine at its most efficient (~1/2 throttle/load at medium rpms) and at idle (or in the Prius case off), resulting in better overall efficiency. The overall amount of work done is about the same for both P&G and steady cruising (technically a little higher for the P&G case I suppose since you are accelerating), but the P&G case will end up using less fuel by operating the engine with better average efficiency.

    Rob
     
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