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Bike vs. Prius

Discussion in 'Environmental Discussion' started by TimBikes, Jun 29, 2006.

  1. TimBikes

    TimBikes New Member

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    I have a friend who is into biking even more than I. He also happens to be an engineer and threw together this analysis. It is far from complete, but it makes you think, as he says:

    "I was using my power meter to ride home recently, and I find I average about 250 watts to ride the 25 miles home in 1.5 hours. Round trip, that would be 250 watts * 1.5 hours * 2 = 750 Watt-hours, or 0.75 kWh.

    Using the gasoline engine alone, the Prius would burn about a gallon of gasoline. Every gallon of gas contains 37 kWh.

    So every time your run your Prius for2 hours, I could ride across the country for the same energy usage. 2 hours * 37 kWh / .75 kWh * 25 miles = 2500 miles.

    Makes you think, no?"
     
    #1 TimBikes, Jun 29, 2006
  2. hobbit

    hobbit Senior Member

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    That's good confirmation of my own observations, e.g. going at it
    reasonably hard on an elliptical also shows about 250W steady-state.
    I use this to try and illustrate to people what a 50 Wh "regen car"
    is -- you put someone else in the car to have them steer, and push
    the unpowered car vigorously around the parking deck for 12 minutes.
    I guess that same effort gets you about 3 miles on a bicycle?
    .
    _H*
     
    #2 hobbit, Jun 29, 2006
  3. priusenvy

    priusenvy Senior Member

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    You're comparing the energy OUTPUT of a human to the INPUT of the car.

    You should compare the energy input to the person in the form of food if you're going to compare it to the energy input of the car. From what I've found via a little Googling, the human body is about 25% efficient. So you could only go 625 miles if you consumed food with 37kWh of energy (about 32 kcal).
     
    #3 priusenvy, Jun 29, 2006
  4. tochatihu

    tochatihu Senior Member

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    I have to say that you are pretty lean and mean of you can output 250 watts all day long.

    On a long bike tour I ate as much as I could stand (est. 5000 kcal/day) and still lost 20 lbs over 3 months. Total distance 5000 km. Bike had heavy load and I crossed the Rockies a bunch of times. A 'flat' W->E trip might take a few less watts I guess.
     
    #4 tochatihu, Jun 29, 2006
  5. TimBikes

    TimBikes New Member

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    <div class='quotetop'>QUOTE(priusenvy @ Jun 29 2006, 02:23 AM) [snapback]278498[/snapback]</div>
    Actually I pointed this out to my friend and reworked the calculation a bit. You also have to account for the fossil fuel energy required to produce a calorie of food energy (about 10:1, depending on what you like to eat). And then the energy required to find, extract, refine, transport, and store gasoline. That math got a bit complicated in terms of knowing with any certainty the values to use. His calculation may be off but is elegant in it's simplicity and the visual impact of a person biking across country on the equivalent of a gallon of gas.

    As for the other comment on Watts, he's pretty lean and mean. His measures of power output are based on his bike's power meter. For me, I think I'm closer to 200 - 215 Watts (an educated guess). I understand Lance Armstrong can peak at nearly 500 Watts.
     
    #5 TimBikes, Jun 29, 2006
  6. priusenvy

    priusenvy Senior Member

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    <div class='quotetop'>QUOTE(TimBikes @ Jun 29 2006, 09:14 AM) [snapback]278619[/snapback]</div>
    I do not know, if you think about this a little, it's not that impressive and a rather obvious result.

    150lb person + 20lb bike / 250 watts is about 500lbs/hp

    A Prius has maybe what, 16x higher power to weight ratio? That's why it can carry five people and their luggage and still go up hills at 60mph. A powered vehicle with the power to weight ratio of a person on a bike could get incredible mileage but its performance would not be acceptable.

    Slap a 1/3hp gas engine in a lightweight vehicle built out of bicycle components and I'm sure you could go a few hundred miles on a gallon of gas. Engineering schools participate in these contests every year. Heck, in 2005 four of these vehicles got 1500mpg (1500mpg is correct, I did not type an extra zero) or higher, including one fielded by a high-school team.

    http://www.sae.org/students/sm2005results.pdf

    Just saw the 2006 results. A team from the University of British Columbia got over 3100mpg.

    http://www.sae.org/students/sm2006results.pdf
     
    #6 priusenvy, Jun 29, 2006
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