What Ifs, and Locos that never were.

You could add 232 U1 to that list also.

Incidentally, I understood French coal to be quite satisfactory from a thermal point of you, but exceedingly dusty, hence the use of briquettes. The real problem though was its scarcity and expense.

PH

 

Outside admission piston valves below the cylinders on some Atbaras as built were a failure (though not necessarily due to their location) and soon replaced by slide valves but later with the fitting of superheaters piston valves under the cylinders were fitted as standard to the double framed locos and these, of the semi-plug type, were quite satisfactory.

 

AIUI all the Dean/Churchward inside cylinder GWR 4-4-0s and 4-2-2-s had the valves under the cylinders, slide valves or (smallish) piston valves. When Churchward told Holcroft to draft out a series of inside cylinder locomotives with 10" piston valves over the cylinders he couldn't make it work.

 

I don't understand that. According to my understanding the scissors gear was a variant of Walschaerts, with the quadrature component of valve motion obtained from the opposite crosshead, thus obviating the need for any eccentrics.

 

As @242A1 says, this is the same basic arrangement as on, er, 242A1, so the outside steam chests would have been above.

To look at the question another way, where else could the inside steam chest reasonably go and what effect would that have? The cylinder configuration is basically the same as used by Gresley and Bulleid for their three-cylinder classes and I don't believe anyone ever designed anything bigger than a 2-4-0 that only had a single carrying axle up front and yet drove on the leading axle. Gresley had the steam chest alongside the middle cylinder, so it could be in plane with the outside steam chests to facilitate the conjugated valve gear. The problem there is that it's very restrictive on the valve diameter; the Gresleys have valve diameters in inverse proportion to the cylinder diameter and this is a very powerful Smith-type compound, so that inside cylinder is pretty large, making that arrangement a non-starter. Bulleid had the steam chests above and offset to one side, with the steam chests sandwiched between the top of the cylinder and the underside of the smokebox - remember that the taper is entirely on the underside of the Pacific boiler barrels. Bulleid managed bigger valves than Gresley, but again, this is a big machine with a large boiler and cylinders, there's not going to be a lot of room for enormous steam chests, which to Chapelon were absolutely non-negotiable.

Underneath the inside cylinder there's plenty of room - assuming it's the same as 242A1 there will actually be two separate valve assemblies, probably sharing steam chest volume. In addition, this being a compound, the exhaust from this cylinder is actually going to the outside steam chests, so the exhaust path isn't nearly as tortuous as it was on some of the more infamous UK classes with piston valves underneath. The only remaining challenge is getting big enough high pressure steam pipes to the inside steam chest and those at least can be the smallest in the whole arrangement.

 

I suspect the problems with valves below the cylinders was more due to restrictions in the steam and exhaust ways to and from the valves. There is not much room between the cylinders and the frames. Separate pipework might have been a solution.

An interesting comment about Holcroft because he managed to get 10" valves above the cylinders on the E1s and D1s.

 

The original piston valves were 6.5", the later standard semi-plug spool valves, still outside admission, were 8".

 

The advantage of valves underneath the cylinders comes in when the second coupled axle is the driven one, the reason being, in order for the connecting rod to clear the leading coupled axle, the cylinders must be raised up, placing the valves on top would cause them to start encroaching into the space occupied by the boiler and smokebox (not such an issue on the outside where you are coming up the side of the barrel) so placing them under the cylinders allows for a larger diameter boiler to go on top of them whilst still fitting within the loading gauge.

 

Interesting indeed. I got the statement from Holcroft's book, where he said it encroached too much on the smokebox.
The plan was apparently for 2-6-0s and 4-4-0s. I wonder (uninformed speculation) whether the 2-6-0 was a particular problem? the drawings I have seem to show the cylinders on the Aberdare (which drives on the second wheelset) inclined rather more steeply than the 4-4-0s.

 

Holcroft did say he'd worked out the design for the inside framed inside cylinders 2-6-0 but Churchward wouldn't accept the single slide bars necessary to get the cylinders low enough.

 

That - and GJC had a bit of a dislike of inclined cylinders on principle after that feature being blamed for some of the problems on the Kruegers. I have read of several other design proposals the GJC vetoed as they included this feature. A slight (2.5 inch I believe) offset was apparently acceptable and appeared on several classes but not inclined cylinders.

One of GJC's strengths was that he learned from his mistakes - but perhaps he took a dislike of this particular feature a bit far.

Mike

 

Its striking, now I look at it, just how few inside cylinder locomotives Churchward introduced in the end. If you exclude the updated versions of Dean/Churchward 4-4-0s and their relatives the Aberdares I can only off hand think of the 3600 2-4-2s, which were very early on. All those had inclined cylinders I believe. The objection to outside cylinders might have been that it required different castings for L & R cylinder/saddle assemblies.

 

I think it would be harder to find designers who did not have certain hang ups than those who did.

PH

 

Several designs with 'underslung' slide valves also incorporated valves which (naturally) dropped away from their faces when steam was shut off in order to reduce wear.

 

And one man's hangup is another one's rational design decision.

 

Upside down valves under inside cylinders was one of the features Dean copied from Stroudley.

https://catalog.hathitrust.org/Record/012303959

Pages 574-576 attached.

The exhaust passage is quite direct - up between the two cylinders to a blastpipe bolted to the cylinder casting. Probably not possible with cylinders larger than 18" dia. This is of course the arrangement on 3440 - and the large slide valves and direct steam passages was probably a factor in the Cities' reputation as flyers.

 

The 3001s and Armstrongs had the same arrangement with 20" cylinders.

 

Another flight of fancy, BR Std. 2-8-4T

 

Perhaps not quite a beautiful swan, but perhaps a bit better?

 

That handrail may need to be lowered a bit!