Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Heat Engines problem

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data
    When gasoline(density=.7297 g/cm^3) is burned, it gives off 5.00 times 10^4 J/g(its heat of combustion). If a car's engine is 25% efficient, how much gasoline per hour must it burn in order to develop an output of 50.0hp. 1hp=746 W


    2. Relevant equations
    P=W/t
    e=W/Qh

    3. The attempt at a solution
    I believed I was solving for time so what I did was I used P=W/t
    then I rearranged it to t=W/P and plugged 5.00 times 10^4 for w and 746 times 50 for P and it didn't get me the right answer because the answer is 10.7 kg/h
     
  2. jcsd
  3. Jan 29, 2012 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi ChunkymonkeyI! :smile:
    erm :redface: … what about your 25% efficiency? :wink:
     
  4. Jan 29, 2012 #3
     
    Last edited: Jan 29, 2012
  5. Jan 29, 2012 #4

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    aha!

    everything in an exam question is there for a reason!

    if you're totally stuck, just try either multiplying the answer by 0.25 (ie 25%), or dividing by it …

    50% chance you'll get full marks! :biggrin:

    in fact, the "car's engine is 25% efficient" means that the power output (to the wheels) is only 25% of the power input (from the gasoline) :wink:
     
  6. Jan 29, 2012 #5
    Lol but I want to learn how to do the steps can u please show me them it's bothering me that I can't solve this physics heat engine problem pleaaaaaaaaaaase
     
  7. Jan 29, 2012 #6

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    the power output (to the wheels) is only 25% of the power input (from the gasoline) …

    so put that into the equation :smile:
     
  8. Jan 29, 2012 #7
    What equation
     
  9. Jan 30, 2012 #8

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    (just got up :zzz: …)
    this one :smile:
     
  10. Jan 30, 2012 #9
     
  11. Jan 31, 2012 #10

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    find the energy from the gasoline,

    then multiply by 0.25 to get the energy output :smile:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook