Single valve compounds

[Ethelred]

As a naive autodidact I look at my compound and wonder about all that reversing linkage flapping about. I read that many builders spend as long on them as the entire rest of the engine. Surely I thought, you could put the valves in the middle and drive them off one artfully contrived linkage. It seems so obvious that there must be some good reason for not doing so. So that one gets parked in the nether regions of the grey matter and then I came across this:

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This is the engine from the steam launch Advance

http://media.wix.com/ugd/3c78c8_c2de096 ... 739b8a.pdf
http://www.steamboatassociation.org.uk/page-1854442

It seems to be a Russian engine from the 1860's.
It appears to have one central piston valve with reversing gear.
Damn clever them Ruskies, but how do they do it?
It obviously works OK as it's pushing a reasonably sized boat for the engine capacity, so why isn't everyone doing it, or am I missing something.
I also noticed in a recent discussion on here, a reference to the Halcyon class destroyers, some of which had a shared piston valve between two cylinders.

[TahoeSteam]

I'm pretty sure the only way to do it with one set of valve gear is to either be a steeple compound, a wells compound, or have the crank throws 180* opposed. The problem with the options is the difficulty to start on TDC or BDC, similar to a single, which is not ideal when it comes to maneuvering. The way the Halcyon class engines worked is they had essentially two compounds bolted together with their respective 180* crank assemblies 90* opposed.

[Ethelred]

I think you're right Wesley, it does look like the cranks are at 180*
It still seems like a good idea though, and the stopping on the centres is no more a problem than it is on a single or steeple, and all those folk running them seem to cope. In fact historically it appears most small launches were supplied with a single. Sure you can't use a simpling valve but in reality I've observed folk tend to apply boot to flywheel.

[TahoeSteam]

The 180* opposed would, in theory, make balancing much easier and more effective, especially if one took the time and care to make sure all reciprocating parts were equal in weight.