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Clarification on some Variables

  1. Jun 5, 2015 #1
    I'm looking back at a previous thread here, in hopes of an answer regarding acceleration of engine RPM, and I have found something that might have answered it, the first post on this page:

    https://www.physicsforums.com/threads/accelerator-pedal-and-rpm.760572/page-4

    (I suggest you don't go exploring this thread, it seems to go in circles.)

    However, I'm a little hazy on some variables and what they represent.

    First off, I understand that the variable I is a mass moment of inertia, but what of, exactly? The crankshaft? How would I go about calculating this? If I remember right from my physics class, shape is a factor in doing this: could I just assume the crankshaft is a solid cylinder (even though it's a bit more complex than one) and get a close approximation?

    And secondly, the variable alpha. Does that represent the acceleration of engine RPM itself, which can be directly seen on the tachometer; or is it of another component that is connected to the engine, which needs some math to find the actual RPM acceleration?
     
  2. jcsd
  3. Jun 6, 2015 #2

    Simon Bridge

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    It is the mass moment of inertia of the "crankshaft assembly"
    You can work out what that is in the usual way - draw a diagram, identify the elements, apply the definition.
    You can also measure it by conducting an experiment on the assembled engine.

    You can use simplified models but it is impossible to tell how close an approximation this will be without looking at your crankshaft ... they are not normally very close to a cylinder. You can usually just look up the moments of inertia for all the components and add them up.

    The term alpha is the angular acceleration of the crankshaft assembly ... the equation calls for radians per second rather than rpm though.

    Have you seen:
    http://www.hotrod.com/how-to/engine/ctrp-0803-inertia/
    ... there seems to be quite a lot of lit on how to find the mass moments of inertia of different crankshafts.
     
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