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

Power of engine

  1. Jul 4, 2010 #1
    How the specification of an engine power done. Why its said power and torque max at some speeds. I am searching for the answer for a long time. Can someone help me..
  2. jcsd
  3. Jul 5, 2010 #2

    Filip Larsen

    User Avatar
    Gold Member

    Welcome to PF!

    Are you asking in general or for a specific engine?

    Anyway, most engines which produces power by turning an axle are usually characterized by a relationship between angular speed and torque. If you search for "engine speed torque diagram" you should be able to find some useful explanations, like for instance [1].

    If such explanations do not help you understand then try explain what it is you don't understand. The best would be if you could point to some calculation (from a book or web) and say which part of it that eludes you.

    [1] http://lancet.mit.edu/motors/motors3.html
  4. Jul 5, 2010 #3

    jack action

    User Avatar
    Science Advisor
    Gold Member

    The engine gets its torque from the combustion of fuel and air. Power is the product of torque and rpm. In theory, the torque should be constant no matter the rpm of the engine, if the quantity of air and fuel is the same. But the air has to go trough the intake and exhaust runners to get in and out the cylinder, which size is greatly dependent on the cylinder bore. The cross-sectional area of these runners will determine the maximum quantity of air that can get in the cylinder. At a certain rpm, the runners will create such a restriction that there won't be enough air to fill the cylinder to give the same torque as the one from lower rpm.
  5. Jul 5, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    Are you asking why the specs are given at particular engine speeds?

    Power is generally quoted at the engine speed at which it peaks (or its rated power). This is the maximum power produced by the engine and is either limited by design (such as in the case of most automotive engines) or by a physical limitation on some component or system.

    Torque is usually quoted at the engine speed at which that peaks.
  6. Jul 7, 2010 #5
    Thank you.I will check it out
  7. Jul 7, 2010 #6
    This realy satisfies me . Thank you
  8. Jul 15, 2010 #7
    We should dismantle any thought here that has/could have moved towards the bigger is better theory when it comes to heads, intakes, valves etc.
    The runner's CC amount does have influence on cylinder filling, but, depending on your application your air velocities are more important than the runner area to a point, sure the higher the rpm range you want to operate at or the higher horsepower goal, you need larger runners. Too big is bad because of 'sluggish' flow happening, finding the right runner size is much needed. Camshaft events immensely dictate the cylinder filling action in correlation to the velocity of flow throughout the induction manifold and intake & exhaust ports. You could have great flowing heads, but, have a terrible running engine simply from a bad camshaft choice or slow air flow velocities in general.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook