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Oh I need some help.

  1. Nov 24, 2009 #1
    Hi guys

    I'm new here and needed some help with an idea i have. I am trying to work out how much torque/power a single piston engine will have, after i have designed it. I know how much force there wil be on the piston (in lds) at all positions of the piston. I gather i need to work out the leverage on the crack.

    Is there a web site i can read about how to work this out?

  2. jcsd
  3. Nov 24, 2009 #2
    Do you have the geometry and weights of the components?
  4. Nov 24, 2009 #3


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    Staff: Mentor

    Have you had basic trigonometry? What math classes have you had? What physics classes?
  5. Nov 24, 2009 #4
    I am studying with the open university at the moment only just touching on physics at the minute (studying a book called Energy and light). So i have basic maths skills but learning all the time.

    Also I have a modle drawn up in CAD (solidworks).

  6. Nov 24, 2009 #5
    Good good, well you can use the following method.

    You need to plot the gas pressure force vs crank angle. Then to find the inertia force of the piston going up and down, and plot that vs crank angle.

    You can combine these two fo find the force acting down the cylinder axis. The graph should look like this:

    You acn then use this figure to work out the torque, at the crankshaft using the folling equation.

    From that you get a graph of torque vs crank angle. Fin the average torque and work out power from that.
  7. Nov 24, 2009 #6
    This is great stuff.

    I can easily find out what presure is on the piston as it pushes down, on the up stroke it will be inertia from the fly wheel. Do I find out the presures for every 5 degrees or do i go for 10 degrees?

  8. Nov 24, 2009 #7
    Thats a judgement call, as with any other numerical simulation, the smaller the timestep the more accurate but time consuming it is.

    Only you can judge the level of accuracy required. Tbh I can't remember what I used, but just that I got to within 5% accuracy. I didnt have a plot of gas pressure force I had to construct one and had to estimate pumping losses.

    Note that at high engine speeds, the torque graph looks very funny, this is becuase inertia forces dominate and the gas pressure makes no difference. The average torque should always be the same though, if the gas pressures are constant (they arent in realisty, but i made that assumption)
  9. Nov 24, 2009 #8

    Thanks for the help, I may call back and need more guidance if thats ok. I will put my pressures into a excel spreadsheet like yours to start of with, This will take me a while.

  10. Nov 24, 2009 #9
    No problem bud.
  11. Nov 24, 2009 #10
    Hi chris

    So Ive got pressures for every 10 degrees from TDC (top dead centre). apart from the up stroke,

    How do i calculate the inertia of the fly wheel as that is what it will need to push the piston back up. Also will i need to include this in the pressures for the down stroke?

  12. Nov 24, 2009 #11
    You dont need to know the flywheel inertia.

    You can work this out directly from the mathematical equation determining the acceleration of the piston.
  13. Nov 24, 2009 #12
    Ok so where do i go from here?

  14. Nov 24, 2009 #13
    It's time to dig out the maths books, and do some horrid derivation i'm afraid.

    Or if you dont need to show derication, google piston acceleration. It'll give an equation in terms of crank angle and engine speed.
  15. Nov 24, 2009 #14

    Once again, thanks for the help.

    Ive found this website, Is this what i should be reading through and applying to my piston engine?

  16. Nov 24, 2009 #15


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    Staff: Mentor

    You found what website?
  17. Nov 25, 2009 #16
    Sorry I forgot to add the link.

    http://www.marinediesels.info/2_stroke_engine_parts/Other_info/piston_movement_2.htm" [Broken]

    Last edited by a moderator: May 4, 2017
  18. Nov 25, 2009 #17
    I've just noticed something from another thread, my constants were lumped together differently, so my equation is different to many that are out thre. Until I can figure out exaclty what I did (i've forgotten) and re done my derivations, please don't use my formula directly as it may very well be wrong.

    I seem to have added a half to the second part, which I can't remember why. The method is correct however.
  19. Jan 20, 2011 #18
    how did you determine the gas forces..mine looks a bit differ..can give me the formula...already pm you..please help me chris X(
    Last edited by a moderator: Jan 24, 2011
  20. Jan 20, 2011 #19
    What are you trying to achieve, and what have you done already?

    EDIT: It may be worth just starting a new thread on this.
  21. Jan 20, 2011 #20
    here is my F vs theta...

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