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Torque Acting On Camshaft

  1. Jun 4, 2007 #1
    Let us assume all things being equal other than the weight of the camshafts themselves. Is more torque required to rotate the camshaft of a large displacement V8 car engine than a small displacement inline 4 cylinder car engine?
     
  2. jcsd
  3. Jun 4, 2007 #2
    aren't camshafts balanced? are you asking us to consider the weight or not consider the weight? cause the one with the bigger moment of inertia and more weight would be the one that requires the most torque. probably lobe height would contribute to moment of intertia but that might be negligible unless you have crazy lobes
     
  4. Jun 4, 2007 #3
    Yes, I am asking you to consider the weight of the camshafts with respect to the amount of torque needed to rotate them. Heavier camshaft vs. lighter camshaft: which one requires more torque to turn?
     
  5. Jun 4, 2007 #4
    :confused: obviously the heavier one
     
  6. Jun 4, 2007 #5

    FredGarvin

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    Whichever cam has the higher moment of inertia will require the more torque to get it up to speed at a given acceleration rate.
     
  7. Jun 4, 2007 #6
    i was going to put that but he already said all things being equal
     
  8. Jun 4, 2007 #7

    brewnog

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    As Fred has said, all other things being equal, the camshaft with the higher inertia will need more torque.

    However, this torque is insignificant compared with that required to open the valves. This isn't dependant on engine capacity as such, but a larger engine (or a higher rated engine) will generally require more force to open the valves.

    There's not nearly enough information about the two engines (large displacement V8/small displacement I4) to make a comparison here, you'd need design details for both.
     
  9. Jun 4, 2007 #8
    cause of the springs? and for the exhaust valves cause of the pressure in the cylinder after the combustion stroke?
     
  10. Jun 4, 2007 #9

    brewnog

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    Partly because of the springs, but also because of rocker ratio, cam lift rate, quietening ramps, cam timing, number of valves, number of camshafts, and gas flow (both exhaust and inlet).

    Hey, when did I become a science advisor?
     
  11. Jun 4, 2007 #10

    FredGarvin

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    CONGRATS!!!! I didn't notice either until now. Well deserved.
     
  12. Jun 4, 2007 #11

    berkeman

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    Yep, congrats brewnog! It just happened recently, and is definitely well deserved. There may be a couple other similar things in the works as well..... :smile:
     
  13. Jun 4, 2007 #12
    Ok fellas, settle down!

    Here are the specs:

    5.7 L DOHC V8 32 valves (4 valves per cylinder) vs. 1.8 L SOHC inline 4 16 valves (4 valves per cylinder).

    Which engine requires more torque to crank its camshaft?
     
    Last edited: Jun 4, 2007
  14. Jun 5, 2007 #13

    brewnog

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    You'll need more than that trial_user. As in my previous post, you'll need information about lift profiles, valve springs, cam timing and rocker ratio to get a reasonable estimate, as well as information about deadening ramps and gas flow (and possibly some other stuff) to work it out accurately. Bear in mind also that your V8 has four camshafts compared with the I4's one.
     
  15. Jun 5, 2007 #14

    Q_Goest

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    Torque needed to increase rotational velocity, or accelerate the camshaft, increases with an increase in moment of inertia as the others have stated, but there are other factors that add to the amount of torque needed to rotate a cam shaft.

    Journal friction is one obvious one. The larger cam shaft will in general require more torque to overcome friction in the journals.

    There's also a torque on each cam lobe equal to the normal force on the cam lobe times the distance from the center. It takes energy to open the valve as the cam rotates. That energy should all come back out as the valve closes, and the spring returns the energy to the cam only if there is no valve friction and no inertia. But of course, there's some valve friction, and valves have inertia. Thus, more energy is needed to open the valves than comes back out as the valves close. The more valves you have, the more energy is lost due to valve friction as well and the more torque you need to rotate the cam at constant speed.

    The energy needed to operate the valves comes out as heat due to friction in a number of different areas. I don't suppose these are very significant, given the huge amount of heat in other areas in the engine, but if you want to look at a cam in detail, it would help to visualize all the different areas where friction interacts with the cam and energy is converted to heat.
     
  16. Jun 5, 2007 #15
    brewnog, I don't have the specifics on these engines other than what I have given. But I don't need an accurate answer just a general answer will suffice.
     
  17. Jun 5, 2007 #16
    all anyone can do is guess, i pick </does eni meni mini mo> the v8, if you want a better answer do what brewneg said
     
  18. Jun 6, 2007 #17

    brewnog

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    Eenie meenie mynee mo, I guess the inline 4 cylinder.

    There you go, two different guesses, it's the best answer you'll get given that information.
     
  19. Jun 6, 2007 #18

    Mech_Engineer

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    Because the I-4's camshaft is pushing more valves than lone cam in the V-8, I suspect the I-4's cam wil take more torque to spin than a single cam in the V-8; however, it will probably take more torque to spin ALL of the four cams in the V-8 than the single cam in the I-4.
     
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