cylinder geometry for scotch yoke

Discussion in 'General Engine Discussion' started by Anatol, Sep 15, 2018.

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  1. Sep 15, 2018 #1

    Anatol

    Anatol

    Anatol

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    I'm drawing up a design for a 2 cylinder unaflow scotch yoke engine, about 1" bore. My goal is efficiency and simplicity. I know there are good argument against the scotch yoke, but it reduces moving parts substantially, and with good design of the yoke "follower" (?) I think wear can be minimized. I'm thinking that in the scotch yoke horizontally opposed format, the gas-spring effect of compressing residual exhaust gas would protect the yoke from shock. But I do not want steal excess energy from the system.
    I'd like advice about piston/cylinder geometry. I'm imagining a long throw would be good in a scotch yoke, as it might extend expansion time and reduce required valve speed. But it might also result in higher pressure at TDC. I guess it *could* be higher pressure than inlet steam pressure? Any comments gladly received, thanks.
     
  2. Sep 16, 2018 #2

    lennardhme

    lennardhme

    lennardhme

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    Hi Anatol,
    Just assembling the goodies for a tandem scotch yoke engine myself. I am using & modifying plans from MEWS in the UK. I am enlarging the guide slot & putting a bearing on the follower pin to reduce wear.
    I think [but not sure without looking it up] that a uniflow engine is single acting? As scotch yoke engines are primarily used for heavyish loads, & are best suited to slow revs, I'm thinking that twin double acting cylinders will be kinder on the [follower] drive pin assembly.
    High pressure at TDC would mean the timing is askew, & also I think another good reason for twin double acting engines, which make the whole thing run smoother.
    Look forward to your build if you post it.
    cheers, Lennard.
     
  3. Sep 16, 2018 #3

    Anatol

    Anatol

    Anatol

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    Hi Lennard

    "putting a bearing on the follower pin to reduce wear."

    that's my plan too, but I will put a square block on the outside of the bearing, with a rectangular slot in the yoke.

    " uniflow engine is single acting?"

    usually. I've seen a DA design though.

    "As scotch yoke engines are primarily used for heavyish loads, & are best suited to slow revs, I'm thinking that twin double acting cylinders will be kinder on the [follower] drive pin assembly."

    yeah but - a two (single acting) cyl scotch yoke is essentially a DA cylinder - with a gap in the middle
     

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