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Torque vs speed

  1. Nov 4, 2009 #1
    Hi guys
    What is the relationship between torque and speed( angular speed or in rpm ). I think they should vary linealy................because consider a fan ........ i give the fan a certain force(torque) it starts moving with some speed..........then if i increase the torque it's speed would definitely increase............
    Also torque = I x Alpha where alpha is angular acc.
    alpha = dw/dt...........so this shows a direct relation ship
    but if i am correct why in engine torque vs rpm curves ........... torque first increase and then decreases...........how is this possible...............as to increase engine rpm engine can only do this by developing more torque...........then why such curves...............ya i know here you would suggest me the formula hp = t x rpm / 5252...........which shows inverse relationship but then what about above formula t = i x alpha.....................plz tell me where i am wrong or if i lack a certain concept..............plz explain how those engine curves are plotted then.......
    I appreciate any quick and logical responses
  2. jcsd
  3. Nov 4, 2009 #2

    Thus if T is konstant, you can integrate the equation to obtain the linear relation. However when T is not constant the relation is not linear.
  4. Nov 4, 2009 #3


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    Jesus man simmer down with the periods and write in complete sentences :cry: Reading your post is like reading a partial stream of consciousness.

    I'm not sure how your examples are useful or relevant. The fact is that torque times angular speed gives you power. An engine dyno is charted by measuring an engine's torque versus angular speed. Horsepower is calculated from the two measured values.
  5. Nov 4, 2009 #4
    I really wish people wouldn't do that either. It huurts my eyes to read a conversation style post.

    OP Just as defunc's formula shows.
    Torque controls angular acceleration, just like force controls linear acceleration.
    So to talk about speeds and torques is largely pointless.

    Peoople get confused about torque all the time.
    Would you ever say that 'What force do I need to go a certain speed'?

    In the ideal world this quiestion would be meaningless becuase of Newtons 1st law. In the real world the force required to maintain a speed is the force = to the forces acting to decelerate the object.

    To your next question which basically is. Why do RPM continue to rise even after peak torque?

    The most basic answer to this is that engines produce more than enough torque even at minimum torque to accelerate themselves. After peak torque the engine is still accelerating, just at a slower rate.

    The torque output is not only enough to accerate the internal comopnents but the entire car as well (which is kind of hte point). This is why the engine will rev up and down very quickly when not in gear, but slowly when draggin a load.

    So much like the linear case. In the ideal world, it requires no torque to maintain an engine speed. In the reasl world the minimum torque reqied is the torque to overcome losses.
    Last edited: Nov 4, 2009
  6. Nov 4, 2009 #5
    thnx chris
    when i thought of torque rpm curves i thought of the total torque but on graphs it is the new magnitude of torque that engine has to provide to maintain or accelerate. However, total torque would surely be more as it would be additive.
    i got it now!
  7. Nov 4, 2009 #6
    Power (watts) = torque (newtons) times angular speed (radians per second)
    Bob S
  8. Nov 6, 2009 #7


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    Torque (newton metres)

    Ahem... :smile:
  9. Nov 6, 2009 #8
    Sorry- I meant N-m. Bob S
  10. Nov 6, 2009 #9

    D H

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    Point of correction: This expression is not valid in general. It is valid in the special case that angular momentum is parallel to angular velocity.
  11. Nov 9, 2009 #10
    oh, I am sad, I thoght this was another HP vs torque vs speed (of car) vs any other variable we could install

    thought I'd get to argue (scratch that...heated discussion) with xxchrisxx like the last time
    just kidding

  12. Nov 9, 2009 #11
    I honestly don't remember that one.
  13. Nov 9, 2009 #12
    I'm sure that was clear to everyone...
  14. Nov 10, 2009 #13
    The torque/horsepower curve shows you the torque AFTER you take into account how much is needed to spin the engine. All losses, friction, internal, external, accesories, pumping, cooling, etc. are taken into account, and the torque number that you get is the NET torque. You can do whatever you want with this torque. You can accelerate the car, go up a hill, do dounts, plow a field...whatever, and you don't have to worry that "to increase engine rpm engine can only do this by developing more torque", because that's already taken care of.

    You are right in saying that if you want an engine to spin faster, you have to give it more torque. This is mainly due to due to friction, windage losses, etc. However, that has nothing to do with what the torque/HP curve is showing you. The curve is simply saying....if the enigne is spinning at a certain rpm, you have a certain amount of torque to play with. If it says 200lb-ft, you have 200lb-ft. Even if it takes 10000lb-ft to spin the engine (which it most certainly doesn't), you don't care, because that's already accounted for.
  15. Nov 11, 2009 #14
    Thread is closed. Thanx
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