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!

What parts of the car are responsible for the greatest amounts of energy loss?

  1. Aug 28, 2012 #1
    I know air conditioners use a significant amount of energy, but turning the fan on high uses a fairly negligible amount. This started me thinking about the engine of the car.

    Where do most of the losses occur? I know any time you have an energy conversion or transfer, you will be losing some energy. Which of these are most significant? For example, is it heat loss in the combustion? Friction on the road? Internal friction?

    I don't know much about how engines work, but am curious to understand the energy involved.
  2. jcsd
  3. Aug 28, 2012 #2
    Losses in combustion process (heat, noise, unburnt fuel), losses in gear train, losses in drive shaft connections, losses in axle shafts/differentials, losses at the wheels.

    There are losses due to the power steering, the cooling, charging the battery, and on and on and on.
  4. Aug 28, 2012 #3


    User Avatar

    Staff: Mentor

    If you are referring to the energy of the fuel, a good rule of thumb to get you started is that a third goes out the tailpipe and a third goes out the radiator/engine case. Of the third that is turned into mechanical work, a third of that is lost in your transmission. Where it goes after that is highly variable.
    Last edited: Aug 28, 2012
  5. Aug 28, 2012 #4
    Yes, this is exactly what I was wondering about. Thanks!

    It sounds like the third that goes out your tailpipe would be in the form of heat and unburned fuel.

    The third that goes out the radiator/engine case would be purely heat?

    Then one ninth is lost to friction in the transmission? Is this the same for manual vs. automatic?

    So that leaves two ninths of the total energy in the fuel to do be converted into kinetic energy of the car. I assume much of this amount is still lost due to friction?
  6. Sep 1, 2012 #5
    You cannot use approximate numbers to add up like that. I have a engineering book from back when they actually taught such things in practical language. I could look up a listing that does give add-up percentages if I had a good reason.

    Where exactly are you going with this so someone might be motivated to give you a more detailed answer. There's also TONS of such information all over the Internet on this subject.
  7. Sep 3, 2012 #6
    The greatest amount of "loss" would be aerodynamic drag......but that depends entirely on the speed.
  8. Sep 6, 2012 #7
    Practically ALL of it is lost to friction, whether internal or external.
  9. Sep 6, 2012 #8
    99% of it is converted to lard in the passenger cell.
  10. Sep 6, 2012 #9


    User Avatar
    Gold Member

    Under-inflated tires increase rolling resistance, which results in energy-loss and increased fuel consumption. Just tossing that into the mix.
  11. Sep 6, 2012 #10
    I remember reading a few years ago that of the potential energy contained in the fuel, only about 16-18% is converted into the kinetic energy to actually propel a car. The rest is either left un-combusted going directly out the tailpipe, or is converted to heat and expelled/radiated from the engine. Some part of that potential energy was lost to mechanical friction from any of thousands of moving parts (in turn becoming just heat) in the engine, transmission, drivetrain, tires on the road, etc.

    And I think kinetic energy lost to things like wind friction were not part of the energy considered "lost" because it was first made kinetic and then lost again. So the overall efficiency of the internal combustion engine is pretty dismal. Factor in the energy expended to get that from the ground and into the car in the first place and it's even worse, but now I'm up on a soapbox.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook