P2 Locomotive Company and related matters

Possibly the bit missed is the receiver. The exhaust from the HP cylinders is not fed directly to the corresponding LP ones but goes to a receiver for all such exhaust, which tends, within reason, to dampen out fluctuations of the pressure to the LP cylinders.

 

It's a good point about the receiver, but if you could make constructive use of the pressure pulsations, rather than just damping them out...

 

Already been done and called Manorbier Castle.

 

yep, that occurred to me a few weeks ago: for all the flak poor old Collett gets about his streamlining there is quite a resemblance to Porta's Argentina, and also, give or take some smoke deflectors and economisers, also Wardale's 5AT.

 

The French did have some Woolf compounds which are continuous expansion machines with transfer passages between the high pressure and low pressure cylinders but no receiver. These were mainly tandem compounds.

The Roentgen/Wolff receiver compound is thermodynamically superior and this is the type that most people are familiar with. The cranks for the high pressure and low pressure pair of cylinders are set at 180 degrees. In the early days there was much confusion concerning the best way to drive a compound. Gresley was quite close to getting it right and should have trusted his instincts but instead he took advice from Professor Dalby. He did introduce a very good valve gear arrangement, derived from the von Borries system, which made independent adjustments of the low and high pressure cut offs possible and totally avoided the use of internal eccentrics.
It was Gresley's friend, and you might say collaborator, Andre Chapelon who worked it out.

The high pressure cylinder has to work at a long cut off. The easy way to view this is to see the hp and lp cylinders as one but to note that the hp volume is approximately 50% of that of the lp (it is actually less than that but this should serve for illustration) and only 33% of the volume of the whole. Working the hp at a short cut off will deliver insufficient steam to the receiver for the lp cylinders to make use of. The hp and lp cylinders should deliver equal power. Using a short cut off on the hp leaves the lp cylinders acting as little more than condensers.

 

Thanks for the reply, I've had to do some reading on Woolf compounds. These were originally used on beam engines, where both the HP and the LP cylinders were on the same rod (i.e a tandem compound), and the steam exhausted directly from the HP cylinder into the LP. However steam has inertia, so at high rpm it would have lost power. Allowing the steam to expand into a receiver also gives a loss of usable pressure, in other words it would be thermodynamically irreversible, and so not as efficient as it could be. A receiver will also have thermal losses and take up valuable space on the locomotive.
The suggestion I was thinking of would be to arrange the "continuous expansion" principle with an overlap between the change of direction of the HP and LP pistons, so that the LP changes direction first. This is clearly easier to arrange on a locomotive than on a beam engine. During the overlap period the same "charge" of steam would be expanding in both the HP and LP cylinders simultaneously. This could be arranged with a three cylinder triple expansion engine (as proposed by Porta), but I think it would only provide the correct effect when running in one direction. A triple expansion machine using conventional boiler pressure and exhausting to atmosphere would also in my opinion be excessively complicated. Controlling the cutoffs would be a nightmare. The other way would seem to be the four cylinder engine with the cranks set at 45/135 degrees (i.e Lord Nelson style) so HP cyl 1 discharges into LP cyl 1 when running forward, and HP cyl 1 discharges into LP cyl 2 when running in reverse.

From your analysis I don't understand how you can run with the HP on long cutoff at low power outputs - you would have part close the regulator. Also I've never really understood why the hp and lp cylinders should deliver equal power. Thermodynamically, though, I think there would be some sense in having the same temperature drop between the HP and intermediate pressure steam, and between the IP steam and exhaust.

 

Practically the only advantage of compounding is the use of larger cut-offs on both the HP and LP cylinders.
They can work with say 50% c.o. while a non-compound would need 25% c.o. with the corresponding
throttling losses in the port openings. Not my idea, Chapelon says so in his 1938 book.
Kind regards
Jos Koopmans

 

Thanks Jos,

It sounds like as soon as valvegear could be made to run efficiently at short cut off on a simple engine, the compound was basically obsolete.
It also sounds like no one has really looked at a 4-cyl compound with the LP cranks displaced by 45 degrees from the HPs, let alone built one.

 

No, not really. One should look at the countries that used compounds. Most had little coal supplies of their own which
made coal consumption a prime economical factor. Until the end proper compounds were the most economical, see f.i.
de Caso's designs.
Kind regards
Jos Koopmans

 

Try to obtain a copy of ISBN 0-906899-61-3 by John van Riemsdijk. It isn't perfect but within it are references to people whose works are well worth some study.

We are not very well served by readily available books but try ISBN 978-0-9547131-5-7.

Minimising losses due to incomplete expansion is a big part of this. In a simple expansion engine these losses increase considerably when cut off is beyond 30%. Things rapidly become worse beyond 50%. I rather admire Gresley Pacifics, short cut off working with full regulator, very responsive to slight changes in cut off setting, but you do see references to them working at 10% which if true means that you are looking at over expansion and subsequent condensation - another loss to be avoided.

Wall effects and leakage all tie in to the compounding scheme of things. What is the point in producing a quantity of steam if you are not going to make the best use of it?

 

I'm wondering whether this dialogue on compounding should go onto a new thread, it has lasted longer than I expected, and I don't think there is much chance of the P2 Locomotive Company wanting to build a compound ;-)

Thanks Jos for mentioning Marc de Caso, I hadn't heard of him and there are some good pictures on the internet.
Regarding short cut-off working, you shouldn't forget to include the cylinder clearance volumes in the calculations. I'm have heard that 71000 can run at 3% cut-off, but the clearance volume is somewhat larger than a typical piston valve engine. For both this and the Gresley Pacifics, I am sure that the steam will still be superheated at the exhaust.

 

Have a look at the work of Gaston du Bousquet. He produced an interesting range of locomotives, tandem compounds and other compounds based on the De Glehn type. One of his "Baltics is on display, sectioned rather like Ellerman Lines, in the museum at Mulhouse. Perhaps more interesting is his mixed traffic 4-6-0. This machine can maintain an output of 2000ihp and it doesn't weigh more than 70 tons. In 1939 there was an international demonstration of the Vitry Test Plant and one of these engines was used for the demonstration. At the time it was 26 years old but it still produced the cylinder horsepower. This man set the foundations for Chapelon to build on.

The impact of clearance volumes is frequently overlooked. 2001 had problems with clearance volumes, they were unequal and vastly inferior to what was being achieved on LNE piston valve fitted types. A Gresley Pacific when working within its normal cut off range (the valve gear is optimised around 25% cut off) with cylinder temperature stabilised to a working maxima will not suffer significant losses, if any, due to wall effect and condensation. An A4 has a clearance volume of 7.9%, 2002 ranged between 7.19 and 7.83% depending on factors such as front stoke or back stroke, inside cylinder or outside. 2001 ranged between 11.78 and 16.10 at first, being improved to 10.26 to 14.35 in 1935. A large clearance volume means that the 3% cut off mentioned is not so meaningful as it might first appear, but for an engine with small or modest clearance volumes a cut off of less than 10% is not recommended.

I did start to write a book in an effort to cover all this and more. Maybe I should dig it out and finish it.

 

I cannot encourage you enough to complete this book; there is an incomplete story of where steam could have been developed, the tale above of the du Bousquet locomotive speaks loud and clear.

Cheers,

Alan

 

Seconded.....

 

All in favour?
(Consider my hand to be up.)

 

I did suggest that to the Mods by way of a report: not complaining about any of the posts but just suggesting a separate thread.

 

And now the P2 has a cab.
https://www.facebook.com/photo.php?fbid=10153433851718871&set=oa.1654373524779604&type=3&theater

 

You can see the P2 WS update about the cab here. As can be seen the Trust has gone for the amended rear profile with the smaller cut-out to accommodate the addition of bucket seats, a detail missed by some illustrating what the finished loco will look like!


Foxy

 

Actually Steve, there are photos around of an American S160 2-8-0 being offloaded from a ship via a beam attached to the dome studs and a bracket on the front of the smokebox.

 

So, given that the P2's never made it in to BR days in their original form, does this mean the livery will be LNER Apple Green (I presume) in perpetuity?