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Pulse and Glide at Constant Speed?

Discussion in 'Gen 3 Prius Fuel Economy' started by mmurphy, Aug 16, 2010.

  1. mmurphy

    mmurphy Junior Member

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    Two weeks ago we were driving across rolling hills in southern Ohio. I started to wonder, "What would be the perfect road for the Gen 3 Prius?" Is there a road shape that would allow the benefits of pulse and glide without the speeding up and slowing down?

    Let's assume no net elevation change on an interstate highway and a driving speed of 60 or 65 mph. To stay in the most fuel efficient engine ranges for the ICE, we'd want down-hill stretches where the power demand shown on the hsi would be in the bottom 1/4 of the total Echo range, and up-hill stretches where we could stay in the top 1/4 of the hsi Echo range (just below Power).

    Would the optimal ascent and descent angles be the same, or would the angle of decline be shallower that the assent angle? Is this idea completely crazy? Would mileage (mpg) be better on this “ideal†up and down road than mileage on a flat road at constant speed?
     
  2. qbee42

    qbee42 My other car is a boat

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    This isn't a crazy idea; it makes complete sense.

    The best angles for climbing and descending would depend on drag. Higher speed makes for more air resistance, and some road surfaces roll better than others.

    The best angle for the climb would be one that forces the ICE to run at its most efficient point, including drag. The best angle for the glide would be exactly steep enough to counter drag, allowing a steady speed with no power or braking.

    Tom
     
  3. ken1784

    ken1784 SuperMID designer

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    It's an interesting theme.
    The ideal situation can be seen at below 46mph threshold where Prius does complete ICE off downhill coasting.

    At 40mph, a 2% grade downhill will cause a steady speed with no power or braking.
    We know 20kW ICE output is the best efficient area, 220g/kWh.
    We can go a 6% grade uphill using 20kW ICE power at 40mph.

    So, please look for the 6% grade uphill and 2% grade downhill route for the best result.
    The opposite 2% up and 6% down route will cause a terrible result. :(

    Ken@Japan
     
    2 people like this.
  4. mmurphy

    mmurphy Junior Member

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    So, if I find a job with the perfect commute from home -- 6% up grades and 2% down, I can never come home from work? Ouch!!
     
  5. teeasal

    teeasal New Member

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    I average 65 mpg going to work, but 53 coming home, just because of the reverse of slope grades.

    I have an alternative coming home route that's pretty much flat except one short but steep uphill climb. I wonder if that's a better route for me.

    I think I'll try that and compare.
     
  6. kenoarto

    kenoarto Senior Member

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    oxyMORON
    ;-)
     
  7. EmuMessenger

    EmuMessenger New Member

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    Amazing numbers. I cannot do better than 45MPG.
     
  8. guinness_fr

    guinness_fr Junior Member

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    Are you sure that your work and home are at exactly the same elevation?

    I'm guessing that your home is at a higher elevation than your workplace.
     
  9. mmurphy

    mmurphy Junior Member

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    It might be interesting to try the experiment. Try both routes going in each direction. The best route may depend on direction, while elevation has an effect as well.
     
  10. fuzzy1

    fuzzy1 Senior Member

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    You just need multiple route choices, with routes optimized for different directions.
     
  11. austingreen

    austingreen Senior Member

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    Yes it is a crazy idea. It would be much cheaper to move closer to work or work closer to home, then change the landscape for the prius. Fewer miles, even at lower mpg will result in less g, which I assume you are trying to optimize.

    Constant speed is a much easier constraint to move than optimal hills for a specific car at a specified speed. Ken's math seems right for the speed. I would assume that taken round trip in the same route, a starting down hill and ending up hill with flat in between would give the driver the chance for best mileage for a given trip duration if (s)he can vary the speed. On flats I'm sure one of the hyper milers here can give you some optimal speeds and rates for acceleration and deceleration.

    Now a nav computer integrated with radar cruise control that optimizes speeds for your trip taking into account light timing and traffic is an interesting engineering problem.
     
  12. mmurphy

    mmurphy Junior Member

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    When you get some numbers, please share!

    I live about 10 miles from work. One route takes me through the Cuyahoga river valley, another is flatter. Unfortunately I don't have any mileage numbers, but I have found myself taking the flatter route to work and the valley route home. The valley route (going home) has a long shallow descent into the valley and a steeper ascent. I think I've come to prefer the valley route home because I like to see the cumulative (Trip A) tank mpg numbers tick up on that descent!

    Has anyone else ended up taking different paths on the two legs of a frequent round trip? Ken1784 suggests that optimal efficiency may be obtained with about a 2% down slope and about a 6% upslope. Could choice of paths be partly determined by efficiency?