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Homework Help: Gas consumption to overcome drag force

  1. May 29, 2013 #1
    1. The problem statement, all variables and given/known data

    A car moving with speed [itex]v[/itex] is subject to a drag force [itex]F_{d}=0.5\rho A c_{d}v^{2}[/itex], where [itex]\rho[/itex] is the density of air ([itex]1.25 kg/m^{3}[/itex] at STP), A is the cross sectional area of the car, and [itex]c_{d}[/itex] is the drag coefficient.

    A Honda Civic has [itex]A=1.9 m^{2}[/itex] and [itex]c_{d}=0.36[/itex].

    Assuming gasoline contains 125 MJ/gallon of energy, and that the car's engine is 25% efficient, calculate the gasoline consumption (in miles per gallon) needed to overcome this drag force if the car is traveling 75 mi/h (=33.3 m/s). Hint, 1 mile = 1600 m.

    B. What is the power output of the engine needed to overcome this drag force at 33.3 m/s? Hint, 1 horsepower = 750 watts.

    2. Relevant equations


    3. The attempt at a solution

    From the given information, I know that the drag force is equal to 474 N (using MKS units), and the power needed to overcome the drag is approximately 15,800 W.

    From unit analysis, I have to somehow convert MJ/gal into mi/gal, which would mean multiplying by mi/MJ, but I am not sure how to get that.

    I know from the efficiency of the engine, that you can only extract a maximum of 31 MJ/gal of gas.
  2. jcsd
  3. May 29, 2013 #2
    If I might make a suggestion, try solving it symbolically first using, for instance:
    \eta = \frac{P_{out}}{P_{in}} \,, P_{in} = u Q
    - η is the efficiency of the car engine in converting fuel to mechanical work.
    - Pout is the mechanical power supplied by the engine.
    - Pin is the chemical power supplied by the fuel.
    - u is the energy density of the fuel.
    - Q is the volumetric flow rate of fuel into the engine.
  4. May 29, 2013 #3
    But how would the volumetric flow rate factor into miles/MJ? VFR would give me m^3 /s or gallons/s
  5. May 29, 2013 #4
    If you know the velocity of the car (assume it's constant), you know how far it travels in some period of time.
    If you know the flow rate of fuel, you know how much fuel it'll use in that same period of time.

    To clarify, can you use this to find an expression that relates the fuel flow rate [m3/s], the fuel consumption [m/m3] and car velocity [m/s]?

    You can use the dimensions of the units to give you a hint. Try coming up with a symbol for the fuel consumption and relate it to the fuel flow rate Q and car velocity v.
    Last edited: May 29, 2013
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