bore/stroke and crank rules of thumb (steam)

Discussion in 'General Engine Discussion' started by Anatol, Apr 21, 2018.

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  1. Apr 21, 2018 #1

    Anatol

    Anatol

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    Hi
    I'm designing a 2cyl single acting uniflow engine
    I'm using an IC style bearing I cylinder head approach (no cross slides)

    I have the idea that a 'square' bore/stroke ratio is good in terms of minimal surface area. Any advice?

    Does anyone have a rule of thumb regarding crank length (in proportion to bore/stroke)? Clearly a too long crank would mean the corned would pass through too widen angle to be mechanically efficient. OTOH, I'm thinking con rod should pass through 30 degrees, no more than 45 deg. Anyone got anything more specific?

    thanks!
     
  2. Apr 21, 2018 #2

    Hopper

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    Many steam engines have a stroke about twice the bore. Long stroke with small bore tends toward a lower-revving engine with more torque, which suits steam engines. Crank "length" aka throw has to be half the stroke you want. Con rod angle is determined by the length of the connecting rod. The longer the rod, the lesser the angle.
    You might be best off to find an existing design -- plenty of free plans on the internet -- and base your own design on that.
     
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  3. Apr 21, 2018 #3

    Mechanicboy

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    Steam machines have longer stroke length to utilize steam expansion while the slider valve is closed. Timing is determined by the length of the slide and setting of eccentrics to make steam consumption economical and minimal water consumption. Expansion time can be from 40 to 60 percent of stroke length.
     
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  4. Apr 21, 2018 #4

    Anatol

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    Thanks Hopper and Mehanicboy (I've been to Bergen, long ago!)

    > Crank "length" aka throw has to be half the stroke you want

    ok, that's obvious when put that way.

    >Con rod angle is determined by the length of the connecting rod. The longer the rod, the lesser the angle.

    right, and less angle means smaller sideways forces. So greater efficiency. But what is the sweet spot? double the crank length would seem to be a minimum... 3x crank length? 4x?

    >Expansion time can be from 40 to 60 percent of stroke length.

    you mean inlet is open for half the ~driving stroke? That seems like a number for low pressure steam.
     
  5. Apr 21, 2018 #5

    Mechanicboy

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    With closed valve is the time it will be expanded until the exhaust is open. See the diagram from A to B in diagram is expansion.

    expansion steam.jpg
     
  6. Apr 22, 2018 #6

    Jasonb

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    If you want an example then Westbury's "Spartan" high speed uniflow engine has 7/8" bore, 3/4" stroke and conrod ctrs of 1.5". This would give max conrod angle of about 14degrees

    This was most likely intended to run from a flash steam boiler
     
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  7. Apr 23, 2018 #7

    Anatol

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    Thanks for those figures. That's an interesting little engine. Do you have one?
    I saw online someone said 3/4 x 3/4. That is the 'square' relationship I was thinking of.
    Regarding conrod angle, my gut feeling of 15 deg either side was pretty close :)

    This is for a high speed, high pressure engine. I guess, in a lower speed higher torque engine you could increase the angle, especially with a flywheel to drag the conrod 'over the hump'?

    Newbie question - the bore figure is measured from top of cylinder TDC top top of cylinder BDC. ie actual bore of the cylinder is longer to accommodate the depth of the piston. ?
    thanks
     
  8. Apr 23, 2018 #8

    Anatol

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    thanks, I know. But I want to know how to determine duration valve is open, ie between TDC and cutoff - presumably dependent on cylinder volume and steam pressure.
    thx
     
  9. Apr 23, 2018 #9

    Charles Lamont

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    Sort of.

    The 'stroke' is the distance the piston moves.

    Yes, the cylinder bore has to be longer for the piston (and the end clearance and the locating shoulders on the cover{s}).

    The swept volume is the area of the bore multiplied by the stroke.

    The clearance volume is the remaining enclosed space at top dead, incuding the ports and passages.
     
  10. Apr 23, 2018 #10

    Charles Lamont

    Charles Lamont

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    The cut-off has nothing to do with the cylinder volume. With a simple, eccentric operated, fixed cut-off slide valve, the cut-off is often dependent on the limitations of the mechanism. If the cut-off is earlier than about 60% of the stroke, it either results in premature release and compression events, or the
    amount valve opening to steam has to be restricted.

    You are asking the right questions, but I suggest you need to do some reading. We can give you some suggestions.
     
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  11. Apr 23, 2018 #11

    Anatol

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    Thankyou Charles for you two replies and your encouragement :).

    >The cut-off has nothing to do with the cylinder volume.

    surely the goal is to inlet the right amount of steam for expansion in the remainder of the stroke - to generate optimum power with least steam consumption?
    So inlet duration should be determined by steampressure and valve aperture, in relation to cylinder volume?

    >With a simple, eccentric operated, fixed cut-off slide valve, the cut-off is often dependent on the limitations of the mechanism.

    I'm beginning to understand some of the challenges of valve timing. Siimple mechanisms, like eccentrics, cams and swashplates provide adequate but not ideal timing.
    Most valves (ie like slide valves), are never full on/full off, but come on and off gradually. Its all very complex. Which explains the more sophisticated valve linkages. (Corliss etc)

    Correct me if I'm wrong, but broadly, the ideal valve arrangement is to open inlet full from TDC to ~ 60 deg or 30%, and open exhaust for the full return stroke. right?

    I've seen some specs where inlet opens 10 deg before TDC. And closes 60 deg after. (But degrees aint % - 100% stroke is 108 deg, right?)

    > If the cut-off is earlier than about 60% of the stroke, it either results in premature release and compression events, or the amount valve opening to steam has to be restricted.

    could you unpack that?
    What do you mean by premature release?

    thanks!
     

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