cylinder geometry for scotch yoke

[Anatol]

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.

 

[lennardhme]

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.

 

[Anatol]

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