What Ifs, and Locos that never were.

Just a 'thank you' for posting this superb web link, I've not seen that site before. Andy

 

On another thread, we have been discussing Mr Webb, including his interest in compounding.

So here is another exploration in compounding, this time from Mr Deeley of the Midland. But this particular example failed to get the guard's green flag.

The design was for a 2-4-4-2 compound tank engine, with 4 outside cylinders - 2 HP cylinders at the back under the bunker, and 2 LP cylinders in the conventional position at the front. The engine appears to have Crampton-style steam-pipes descending from just in front of the dome (obviously a pre-superheat design), but the steam has to travel a long way to reach the rear HP cylinders, and then even further from the HP exhaust to reach the front LP cylinders. The two sets of 4-coupled wheels are suggestive of Webb influence. The wheels were only 5-ft diameter, so possibly the loco was intended for freight rather than suburban passenger service.

The boiler appears to be that of the Midland compound 4-4-0, with 28 sq ft of grate. The lengthy side-tanks, extending to the front of the smoke-box, were also seen on the Deeley 0-6-4T "flat-irons".

 

Surely more sense to have the HP'S under the heaviest bit...

 

There's a painting of this and some discussion in Robin Barnes "Locos That Never Were".
The picture is reproduced here: http://www.steamindex.com/people/deeley.htm


Sent from my Pixel 3a using Tapatalk

 

Never seen this site before either. The Londonderry & Lough Swilly Rly. looked to have some pretty serious kit... HC 4-8-0 No.12 looks the business!

Richard.

 

Thank you very much for this interesting information and the diagram bluetrain! The cylinder arrangement reminds me of the american Pennsylvania Q1:


Knut

 

I'd have thought having the cylinders back to back in the middle would have been better for the steam circuit.

 

One obvious reason for having them at the ends would be to allow all the driving wheels to be coupled, yet they are shown as two separate sets of four.

 

HP cylinders at the back was always the convention with Mallets and Meyers, for the simple reason that the HP steam and exhaust pipes don't need to be so big as the LP equivalents (well, the HP exhaust is the LP steam pipe), so it's easier to do a reasonably proportioned long HP steam circuit. I wondered if this design was intended to be a Mallet, but the diagram doesn't hint at that. Cylinders at the end does allow for a shorter fixed wheelbase, which for an 8-coupled tank engine probably matters. @240P15 is right to liken it to the Pennsylvania Q1 Duplex, which was also uncoupled, and famously unsuccessful in part due to the inadvisability of having cylinders next to the firebox, although in this case that cylinder location takes them away from the firebox. As far as I know the only coupled Duplexes were also probably the most succesful, the PLM/SNCF 151A 2-10-2, which from the outside looked like a 2-4-6-2 but had inside coupling rods between the two groups of coupled wheels, where the HP cylinders (outside, between the coupled wheels) would have got in the way of conventional connecting rods.

 

Thanks for your added information and the link to another interesting web site 8126

Very sad that none of the PLM 151A survived into preservation..

Knut

 

Thanks for the interesting painting and article.

Regarding the proposed Deeley 4-6-0, I wonder if Deeley considered more than one version. The article describes a design with Inside HP / Outside LP, but Dr. Tuplin's book shows a version (attached) with cylinders the other way round - Outside HP / Inside LP in the classic De Glehn configuration. Tuplin's version also appears to have only inside valve gear.

A domed taper boiler is shown, but disguised by parallel boiler cladding. Compared with the contemporary GWR Star-class, the Deeley design had a narrower boiler barrel but longer firebox & grate.

 

Wouldn't it be surprising if he didn't?

 

That's really interesting, I'd never heard about the Deeley 4-6-0 designs. Considering how firmly the Midland held onto the compound 4-4-0 for years to come, I somehow hadn't been expecting him to be so early on the draw. It looks like a good boiler layout too, none of the shallow firebox mistakes that bedevilled so many early 4-6-0s.

Dare I say it, but the position of those cylinders is almost like a Thompson Pacific, but 35 years early. If you look at a classic De Glehn layout engine, say a Nord Atlantic or the Chapelon Pacifics (SNCF 231E) the HP cylinders are alongside the trailing bogie wheels, like the GWR 4-cylinder 4-6-0s that copied the layout. I wonder if Deeley had realised the limitation of his 3-cylinder compounds, and the usual De Glehn layout, namely that it makes for a wide engine. With the Midland loading gauge being fairly tight, he may have decided he couldn't push outside LP cylinder diameters any further, especially as his 4-6-0 schemes gave up the advantage of the 4-4-0s in tucking the connecting rods inside the coupling rods. Admittedly 21" LP cylinders would be no bigger than those eventually fitted to the Crabs, but they came rather later.

 

Bear in mind that in a compound/triple expansion machine, the power output is basically what you would get if you ran the engine on HP steam into the LP cylinder only - if that makes sense

 

Thermodynamics tells you that you can’t get something for nothing! It doesn’t matter how many stages you expand the steam through, you can only expand it once.

The real killer thermodynamically is that you start with water in the liquid state and get rid of it as a gas, so you lose the latent heat of evaporation. No one ever really successfully solved that problem on a locomotive without introducing a host of other problems.

Tom

 

Wasn't that what the Beyer-Ljungstrom locomotive was trying, unsuccessfully, to achieve?

 

I believe so. You run into problems of needing to provide a draft, condensers and so on - a lot of equipment needing to fit in a constrained space and possibly a hostile atmosphere. Power station and ships machinery tends to be far more efficient, but doesn't have those same constraints (and also tends to work for long periods at constant output).

I suspect an MIC on thermodynamics would have an audience of somewhere less than one, but looking at the relevant energy diagrams is illuminating, in particular how much energy is needed to go from water at room temperature to steam at 1 atm / 100°C, all of which is thrown away before you even start to think about how much useful work can you get out of the steam by heating / compressing it.

Tom

 

Well, perhaps an audience of one. I keep meaning to sit down with the Spirax Sarco website steam tables (having mislaid my paper tables years ago) and do some worked examples of combinations of parameters just to explore some of my thoughts in various areas. I fully expect to discover I'm wrong about several things, and there's always the danger of going down an alley of incorrect assumptions. Unfortunately, working from home appears to take up almost as much as my time as working from work.

 

I think that you'd get many more than an audience of one if it was explained in simple terms. People don't learn about steam and combustion these days.

 

My degrees were in physics but I never grasped thermodynamics very well. We have had a bit of discussion before now but I don't remember on what thread.