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

Automobile: power losses

  1. Jan 11, 2010 #1
    Hello, i would like to know in what ways power is loss from a car engine to the wheels and what are the percentages of the power losses that occur.
    Do the power losses vary with engine speeds?
    Also, how much power does the electric motor starter consume?

  2. jcsd
  3. Jan 11, 2010 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    power varies with RPM...so the power loss thru parasitic drag og transmission/differential would also vary. the amount is engine/ trans/diff dependent.
    starter motor takes minimal electric power to turn over engine and this is replaced via alternator
  4. Jan 11, 2010 #3
    I know that power loss due to air drag is proportional to the square of vehicle's speed.
    I have read somewhere that for a gasoline engine, "of the 25 percent crankshaft power, about 4 percent is used up by accessories, 9 percent by friction and slippage in the mechanical systems (transmission and differential), leaving only about 12 percent of the fuel energy to be delivered to the wheels".
    How does the power loss associated with trans/diff vary with engine speeds? Will it simply increase?

  5. Jan 11, 2010 #4
    Asking how much powerloss comes from an engine is like asking what colour are pencil crayons?

    The power output and losses of an engine and transmission system, will not only vary between types (can vary a great deal) but also between units of the same typre due to manufacturing tolerances.

    12% seems a very low estimate, even for poor engines.
  6. Jan 13, 2010 #5
    Proportional to the cube of the vehicle's speed.

    As has been said, power lost in the drivetrain is not so easy to figure out. There's windage losses (the gears, engine swinging through the oil/ oil mist) that will obviously rise with rpm, but not necessarily load. There's frictional losses in bearings and gears that will rise with rpm and load.

    Each gear, each bearing, each moving part takes up a certain small percentage of the total power. The more gears (such as in 4wd and rear-wheel drive cars), the more power lost. An automatic transmission usually gives more power loss.

    Good luck figuring out each individual component. The 12% mentioned is an easier way to estimate it, although it's probably being very optimistic. I've heard around 15% of power for front-wheel drive cars...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook