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I'm planning to purchase one of the new Accucraft UK Welshpool & Llanfair "Earl/Countess" engines when they become available next Spring. I remember reading somewhere that these engines were going to use slide valves, just like the prototype. But when I saw Tac Foley's spy photo of the engineering sample and the official photos that subsequently appeared on various dealers' web sites, it's clear that the eccentric crank on the Walschaerts valve gear is set ahead of the crank axle, that is, for inside admission. My understanding is that a slide valve is by definition outside admission, which should require the eccentric crank to be offset so that it will follow the crank axle -- see any Roundhouse engine with outside v.g. as an example.
As a check on my own sanity, I looked up some photos of other AC engines that use slide valves with Walschaerts gear (such as the K-27), and sure enough, the eccentric crank is set ahead of the crank axle. That pretty much rules out the possibility that the photos of the W&L engine show a misadjusted eccentric crank. So how do they do it? Is it the fact that the combination lever and the line of the valve spindle aren't offset? Does the AC design use some kind of novel slide valve? Does the fore-aft position of the eccentric crank really matter when the v.g. is scaled down?
I'd be interested in seeing what the valve gear experts or those having experience with the Accucraft setup might be willing to share.
Thanks.
As a check on my own sanity, I looked up some photos of other AC engines that use slide valves with Walschaerts gear (such as the K-27), and sure enough, the eccentric crank is set ahead of the crank axle. That pretty much rules out the possibility that the photos of the W&L engine show a misadjusted eccentric crank. So how do they do it? Is it the fact that the combination lever and the line of the valve spindle aren't offset? Does the AC design use some kind of novel slide valve? Does the fore-aft position of the eccentric crank really matter when the v.g. is scaled down?
I'd be interested in seeing what the valve gear experts or those having experience with the Accucraft setup might be willing to share.
Thanks.