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Homework Help: How do you calculate the average force on a piston?

  1. Jul 12, 2016 #1
    1. How do you calculate the average force on a piston in one stroke from TDC to BDC given the following Kg forces and their distances in mm from TDC?

    1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 7 mm 8 mm 9 mm 10 mm 12 mm 13 mm
    9.9 Kg 7.8 Kg 6.3 Kg 5.3 Kg 4.4 Kg 3.8 Kg 3.2 Kg 2.8 Kg 2.4 Kg 2.1 Kg 1.6 Kg 1.4 Kg

    14 mm 15 mm 16 mm
    1.3 Kg 1.1 Kg 1 Kg

    2. I am absolutely blank.

    3. Tried a sample average, but as the sample increases so the average drops.
  2. jcsd
  3. Jul 12, 2016 #2


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    Hello Kitson, :welcome:

    Surely you can't let the sample increase further than the BDC ?
    TDC is at 0, but what is the stroke length ?
    You could let excel fit a polynomial and integrate, but you still need the BDC position.

  4. Jul 12, 2016 #3
    Hey BVU
    Correct, any force still around at BDC is waste.
    BDC is 30 mm and the force is 0.2 Kg so insignificant. I did not put the last figures in because they are easy to include once I understand the solution.
    I'm old school, pensioner, specialized in rotor dynamics and balancing so very grey in this area.
    Total stroke is 31 mm.

  5. Jul 12, 2016 #4
    Here are the rest of the distances and forces:
    17 mm 18 mm 19 mm 20 mm 21 mm 22 mm 23 mm 24 mm 25 mm 26 mm 27 mm 28 mm 28 mm 29 mm 30 mm
    0.9 Kg 0.8 Kg 0.7 Kg 0.6 0.6 Kg 0.5 Kg 0.5 Kg 0.4 Kg 0.4 Kg 0.3 Kg 0.3 Kg 0.3 Kg 0.3 Kg 0.2 Kg 0.2 Kg
  6. Jul 12, 2016 #5
    Try statistical approach.

    Suppose x ∈ [a, b], and the interval [a, b] is partitioned into n subintervals, each of length ∆x = (b – a)/n, so that n = (b – a)/∆x. Then, the mean of f(x) is reasonably well approximated as
    $$\mu = \frac{\sum_{i=1}^{n}f(x_i)}{n}$$
  7. Jul 12, 2016 #6


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    I had fun going through this:

    A 6th order (bit much..) polynomial (the red line) gave
    y = -0.00000408x5 + 0.00038121x4 - 0.01390910x3 + 0.25371572x2 - 2.47577020x + 11.96556136

    Integrating from 0 to 31 gives the area under the red curve
    A = -0.00000408x6/6 + 0.00038121x5/5 - 0.01390910x4/4 + 0.25371572x3/3 - 2.47577020x2/2 + 11.96556136x with x = 31 that gives an average of 68.7/31 = 2.22 (if I didn't make any typos...)

    A simple average gives 2.05

    If you add an extra measurement at TDC of 12 kg, then a direct average gives 2.29.

    However, this gives too much weight to that TDC point (it is an estimate for 0-0.5 mm, the measured 10 kg is for 0.5 to 1.5 mm) so a better estimate is to give that point half the weight of the others:

    (sum of measurements + 12/2)/30.5 = (61.4+6)/30.5 = 2.21

    (close to the integral value)
  8. Jul 13, 2016 #7
    Hi Irene

    I like your suggestion, the last time I did stats was in the 1980s. I would be interested to see the result if you ran the figures. Con't stop thinking that an effective average would give a more accurate result. A bit like the rifle bullet accelerating down a barrel, versus a shotgun coasting down the barrel. The rifle having the slower burning propellant??
  9. Jul 13, 2016 #8
    KitsonSteam22, if you chose statistics, do not forget to calculate f(11), since it's omitted. Use linear approximation: f(11) = (f(10) + f(12) )/ 2

    There are also methods to find the integral by the points given: method of left(right) rectangles, method of trapezoids, Simpson's method and many others.
  10. Jul 13, 2016 #9
    Hey BvU

    Thanks, (Have a look at what I said to Irene's reply). There's defiantly weight in what you said about adding an extra measurement at TDC. In an engine, the firing occurs a couple of degrees before TDC (varies from manufacturer), a force is applied to the piston which cant go any where during the dwell angle, the potential energy then becomes kinetic energy as the piston clears the dwell angle (crank momentum). Maybe this is for somebody with more than the two brain cells that I have. I think you onto something I haven't read about.
    Back to the average, I think looking at an effective average might give a better result.

  11. Jul 13, 2016 #10


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    There are well established methods for analysing what happens in an engine .

    Can you tell us more generally what you are trying to understand or calculate ?
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