1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Torque exerted by cranksahft on load

  1. Feb 1, 2005 #1
    I am in need of help regarding a problem which asks for the torque exerted by the crankshaft on the load.

    Initialy I am given that a gasoline engine operating at 2500 rev/min takes in energy 7.89 X 10^3 J and exhausts 4.58 X 10^3 J for each revolution of the crankshaft.

    I found the mechanical power output of the engine to be 185 horsepower.

    This was found by:

    ((7.89X10^3J - 4.58X10^3J) * 2500 rev * .738 ft lb/s) / (60s * 550 ft lb/s)

    I converted the 185 hp to ft lb/s by multiplying it by 550. Then I converted the lb to newtons and the ft to meters so I have Newton Meter/s. I am not sure if I am doing this correctly, or if I am, how to get rid of the seconds?

    Please help!!!


  2. jcsd
  3. Feb 2, 2005 #2
    Ok lets look at the units here. I'm going to use the FLT system (Force, Length, Time).

    A Joule is FL
    Your unit of power is FL/T
    And of course your unit of time is T

    So the units for you equation are

    ((FL - FL) * (1) * (FL/T)) / (T * (FL/T)) = FL/T

    The equation basically equals FL/T because horsepower is just another way of expressing ft lb/s.

    Because you only multiplied by revolutions, which are unitless (thats why its just a 1) I think you'll find that your units cancel out properly and you are only left with FL/T like you are supposed to.

    I checked your math myself and I got 185 hp as well.

    Another way of doing it would be to divided your rpm (units of 1/T because revolutions are unitless) by 60s to begin with and that would get your extra seconds out of the denominator so your equation would look like this

    ((FL - FL) * (1/T)(T/T) * (FL/T)) / FL/T

    And you will find it equals FL/T like it should.

    I'm pretty sure you used the right method but got confused by the units. If I'm wrong, I apologize, I'm sure someone else will correct me.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook