West Somerset Railway General Discussion

Hmmm - I think we might be in the realms of a perpetuum mobile here... (Since for comparable trains, the work done is the same, broadly the energy expended must be the same, all else being equal. The smaller-wheeled engine does more strokes per unit time to do a given distance, but each stroke needs to exert less force and uses less steam. The two effects balance out so that you use comparable amounts of steam. Likewise in .terms of load on the structure of the locomotive, the smaller-wheeled loco does more strokes, i.e. more cyclic repetition, but each impulse is smaller in force that the structure of the loco needs to absorb).

Tom

 

1 in 52 of course.

PH

 

That is far too simplistic an approach. The amount of steam used per wheel revolution cannot be constant across all 3 as the period of admission will be shorter the smaller the wheel size (assuming all other factors are constant).

As a more practical yard stick 3850 was reckoned to be amongst the most economical locos to operate on the WSR and 2857 is similarly regarded on the SVR.

 

Hear hear Jeff! Probably my favorite loco on the railway. Never had a bad trip on her in 20 years or so!

 

At the risk of entering the 'how to suck eggs' area. Ultimately a locomotive is dependent on its ability to boil water which is inter alia a function of fuel characteristics, grate area and evaporative areas, both saturated and superheated, within the boiler.

Boiler statistics.
45xx grate area 16.6 sq. ft. Total evaporation (tubes and firebox) 1114 sq. ft, superheater 94 sq.ft
Cylinders 17" X 24"

51xx grate area 20.35 sq.ft. Total evaporation 1265 sq.ft (tubes and firebox), superheater 82 sq.ft.
Cylinders 18" X 30"
(Nos 4160-79 Total evap 1110 sq.ft, S/H 122 sq.ft. BP increased to 225 psi.)

There were minor variations on some Class members.

With their larger grate and larger evap areas the large Prairies have a greater capacity to produce steam; they also have significantly larger cylinders, ( a greater facility to use the steam)

Observation on the WSR suggests at 25 mph, RFO and 40% a 51xx develops c. 800-850 EDHP , a small Prairie 550-600.

Of the various large Prairies that have run on the WSR, 4160 with its two row superheater, and particularly in the period when the valves were set at 225 psi, was significantly the most powerful and anecdotally (backed up by daily coal consumptions ie tipper buckets) the most economical.

9351 (former 5193) in its tender form, when hauling eight coaches is, because of its tender, effectively hauling nine coaches compared with the 0thers remaining in their tank engine format. It is noticeable that with 'eight on' 9351's speed is dropping very rapidly at the top of the 1/65 up to Washford compared with 4160 or the other Prairies that have visited.

Regarding the various exaggerated claims being made for the Ivatt/Riddles 2-6-0s and 2-6-2 tanks: there have been several visitors to the WSR and observations suggest that their maximum power output is similar to the small 45xx, 55xx Prairies; if anything slightly less although no doubt they have a higher overall thermal efficiency.

I would be very surprised if after the first initial movement,(and perhaps not even then when the TE is at a maximum) a small Prairie (despite the smaller coupled wheels) could accelerate a seven coach train quicker than a 51xx, certainly not over the first half mile.

Michael Rowe

ps I note the following comment from Jeff Price. Of course some 'crews' can obtain more from a specific loco than others; it is perhaps the importance of the 'human touch' that has brought the steam locomotive a special cachet. But Jeff ultimately even the very best cannot beat the rules of Thermodynamics and particularly the Rankine Cycle. It is perfectly reasonable for an arm chair observer to note and evaluate I suggest.

 

All this is dependent on how the driver uses the steam that the fireman provides, a few more variable that on train observers have to estimate

Jeff Price

 

I thought it had three rows having been built for moving heavy trains around London. I'm happy to be corrected. I also thought that the Rankine cycle was applicable to closed loop systems such as nuclear circuits. Perhaps the good people over at Hinkley could knock up a newbuild in the workshops during their lunch breaks and we could put the theory to the test.

 

I can only offer a practical comment. 4160 is much easier to fire over the WSR that 5542 with 7 on. What do I mean by that? Well most firemen will endeavour to keep their engine close to the mark when the driver is using steam. Much easier to achieve this on the large prairie irrespective of who is driving. Which suggests to me there is more of a margin available which sort of supports some of what Maunsell907 says.
It is a bit hard to think about all the thermodynamics while firing though. Jeff also makes a good point about the loco being fully warmed up. The performance of all loco's straight off shed is nowhere near the maximum even if the gauge says you have maximum pressure IMHO.

 

An interesting item has appeared on the 4160ltd website. Not to do with loco performance BTW

http://www.4160ltd.com/news/

 

Having checked: I quote verbatim from RCTS GWR Locos Part nine, "In the last Lot, Nos 4160-79 of 1948-9, boilers with a new superheater arrangement still in two rows were fitted"

Regarding Rankine, yes commonly applied wrt power station turbines but is not each complete loco cylinder/piston cycle not a discrete Rankine Cycle ? The classic Indicator diagram in effect constitutes a Rankine Cycle ?

There are various papers but an approachable one is "Steam Locomotives as a sub set of Rankine Cycle heat engine" by Alan Robinson published "Trains" 21/9/2012. Robinson approaches the subject as an "open loop Rankine Cycle"

Michael,Rowe

 

You may well be right Mr Maunsell, I always thought it was 3 rows. 4160 was very much the "GTI" version compared to 4110. Mr Aldfort will have cleaned out the smokebox a few more times that I have so perhaps he can remember.

As the steam is exhausted to atmosphere with a considerable energy loss, I don't think that the Rankine cycle applies as steam locos (with a handful of exceptions) are an open loop rather than a closed one. Our former CME is a clever chap and during one memorable lunchtime debate on the laws of physics, he described it as a Carnot cycle.

 

The Carnot Cycle was surely the first formulation of what is now regarded as the second law of Themodynamics. We are going round in circles ! Perhaps we are adding to the measure of disorder enshrined in the Third Law.

Michael Rowe

 

[QUOTE="Snifter, post: 1661477, member: 25479]

"I thought it had three rows having been built for moving heavy trains around London. I'm happy to be corrected. "

I was not aware of the London requirement. I have been checking. Of the 4160-79 Lot: Number 4160 was allocated new of course to Barry MPD; in total nine of them were sent new to South Wales depots. The rest went to allocations such as Exeter, Kiddetminster, Tyseley etc. None were allocated within a hundred miles of London as far as I can ascertain.

Are you thinking of the 61xx Class designed with the PDN suburban, including outer, services in mind ?

Michael Rowe

 

Sadly I cannot remember if it's two rows or three rows of super heaters but there is nothing I can find written to indicate 4160 was modified from the standard of two rows so if it has 3 rows it will presumably have been a modification in preservation.

 

Super.

So the 5101 class, 61xx class and 41xx class are the same basic cylinder/wheel combination but the 61xx has a higher boiler pressure.

Please can you tell us which class have 2 row super-heaters and which have 3 row super-heaters

I am certain that 4160 has a 3 row super heater boiler but 4110 has a 2 row super-heater boiler

Unfortunately the 4160 Ltd web site has little technical information see http://www.4160ltd.com/news/

Jeff

 

And I thought quantum mechanics was difficult!

 

Not to do with loco performance!

 

Hi Jeff,

Have a look at 4110 next time you are in Minehead, a pint says it's a single row.

Mr Aldfort,

Alternatively, it may have had a replacement boiler in service and the London angle came from one of our traction inspectors. Apparently, a regular duty was moving empty coaching stock hence the need for some extra "oomph".

 

When we first restored 4160 at MD, I understood 4160 at its last works visit, received a boiler to 61xx spec for 225psi pressure.It also had a non41xx standard Molesco superheater.On entry into service, it ran at 225 psi for a period,certainly had some punch!!! It was later downrated to 200psi to extend boiler life.Possibly this can be confirmed from MD boiler records.