Bluebell Motive Power

What does the future look like for 80064? She hasn't run since the late 80s(?) And I don't recall haven't read anything in BN or heard any kind of speculation what her owners plan to do.

 

Actual tractive effort is not constant but varies throughout the wheel revolution. The general formula for the average T.E. is derived from the work done in the cylinder, which is force x distance. The force is the area of the piston x the steam pressure = πd²/4 x P (d = cyl dia & P = Pressure) and the distance travelled is the stroke (L) Thus, work done is πd²/4 x P x L. In one wheel revolution the piston will move back and forth so the total distance travelled by the piston is 2 x L and in doing this the loco will move one wheel revolution, which is π x D (where D is the wheel diameter.) Thus, the force at the rail is:
(πd²/4 x P x 2 x L)/ (π x D) Simplifying this we get P x l x d²/(2 x D). This is for one cylinder so, for multiple cylinders we get P x l x d² x N/(2 x D) whee N is the no of cylinders. That's how the basic formula is derived. Where does the 85% come from? The formula quoted assumes that full pressure is available for the full stroke of the piston and, in reality, this is not the case. What we really need is the average ( or mean effective) pressure and thus the average work done. In full gear, steam is cut of at about 75% of stroke but the steam in the cylinder continues to expand at a gradually reducing pressure , exact rate of reduction being dependent on lots of variables, such as clearance volume and heat losses to name but two. In the absence of true values, the M.E.P. is usually taken as 85% of the boiler pressure but this isn't cast in tablets of stone. For example The Midland Railway and Hunslet Engine Co, both used 75% in deriving the tractive efforts of their locos.

In terms of locos doing the impossible, the quoted T.E. is rated on the loco as designed. Cylinders are bored out and wheel tyres are turned down as they wear. For example, a Black 5 has a specified T.E. of 25,455 lbf but bore out the cylinders by ½" and turn the tyres down to the minimum of 69" and you get an actual average T.E. of 28,017 lbf which is 10% more. This doesn't make it more powerful, though, because, at the end of the day, it is the amount of steam used that counts.

 

[pedant]Of course it makes it more powerful[/pedant]. But whether the theoretical extra power which would be available if the cylinders were being filled with an unlimited amount of steam at boiler pressure is any use to you is quite another matter. You've got to get that theoretically available power to the drawbar which involves adhesion, steam delivery, friction and goodness knows what else.

 

Sort of depends on your view of where the power comes from - the ultimate source of power is being able to produce a given volume of steam at a given temperature and pressure at a given rate. You can bore out the cylinders all you like, but you haven't changed any of the fundamental boiler characteristics (grate area, heating surface etc) one iota!

Tom

 

But the boiler is only part of the story. Power is evaporation divided by specific steam consumption. For years locomotives became larger as evaporation was being driven (frequently in a less than satisfactory manner) while the impact of the SSC was ignored. So 4000 hp can be achieved from a grate area of some 40 sq. ft. because of improvements in cylinder and exhaust system design and using coal of a lower quality than that used in the UK. Given that only one UK steam locomotive had the potential to reach 4000 hp, and this from a grate area of 36 sq. ft. approx. why do we need 50 sq. ft. area grates in the UK? If you cast a glance at US locomotives it is hard to escape the conclusion that few engaged in the design of locomotives knew what they were doing - so much weight delivering so little power. And so much of that weight carried on undriven wheels. Allegheny anyone?

 

Sounds like the same designers moved on to working in the American Automotive industry..... how do they get such little power out of such large engines?

 

Well its a complicated little beast our steam engine, and we can chop logic endlessly.
The grate converts chemical energy to heat energy.
The boiler converts that energy to steam and acts as a considerable reservoir
steam goes off to the cylinders where its converted onto mechanical energy
wheels interact with the rails to get (limited adhesion)
and there are truly horrendous efficiency losses all along the way
So there's a maximum short term power output and a maximum long term power output (because of the reservoir effect), they both vary with speed, there are limitations of adhesion on how much power is actually deliverable to the drawbar and it goes on...

Because power too is only part of the story. This big American V8s only delivered low peak power outputs by european standards, but they did so over very wide rev ranges and at much lower rpms. Those were virtues *for the machines their customers wanted*.

 

The storage capacity of the boiler is negligable when you compare it with the amount of energy being released from the grate in the firebox. The steam locomotive is very complicated which explains why there are so few (if any) really good ones. We do new builds of third rate locomotives that were lost to preservation. All well and good. They fill a gap and feed the nostalgia hunger. The truth is that none of our preserved or recreated engines are anywhere near as good as they should have been. A further truth is that very few want to do something about it. Far too many of the posters on this forum believe that improvements on what they so jealously cherish are impossible, or if not impossible negligable. The data that proves otherwise is freely available. But, of course how could anything invented in Britain be improved or developed by anyone from abroad. It must all be lies and deception.
Ref Lima. All you need to know is the weight of the locomotive and the maximum sustainable power output of the same. If it cannot maintain 40 hp per ton you know the answer.

 

That's a bit extreme. I'm perfectly happy to potter about at weekends on Victorian engines that presumably by your standards are outdated and poor designs. I'm also perfectly happy to get whisked around the country in 140mph electric trains when I need to get somewhere. What I can't see the point of is bemoaning Victorian technology for being, well, Victorian. If you want to new-build improved steam engines, then be my guest - but you still won't meet a combination of thermal efficiency, manpower costs, emissions standards, capability, capital cost and a range of other factors that all-round beats modern non-steam traction - so what's the point? It all feels a bit like taking a historic Mk 1 Spitfire, replacing the Merlin engine with a Griffon, replacing the browning machine guns with 20mm cannon, stressing the airframe to pull higher G and then being surprised when in your first air battle, some hotshot in a desert 10,000 miles away shoots you down by remote control...

Tom

 

So what are these locos delivering 100hp / sq. ft of grate? 40 - 50 hp / sq. ft seems to be about the limit in this country, (to be sustainably delivered: you can do more over short periods of say 5 - 10 minutes, by reducing water level).

I'd also ask what the coal consumption per horsepower-hour is?

Steam locomotive design is a compromise between many factors. Loading gauge is one limit; coal quality another; capital construction costs and crew costs also come into it.

To take the US as an example: with a generous loading gauge and long steady runs between towns (especially out west), and with cheap coal, engines became gigantic. Reducing crew costs was a more effective way to save money than reducing (relatively cheap) coal consumption. So - giant locos, grates too big to fire by hand, so the quid-pro-quo for having a crew of two on a locomotive twice the power of anything in Britain was to have fuel consumption maybe 25% higher than would be acceptable here.

To take France as an example: coal was expensive, so the designers built technologically complex locos and invested in crews to get the best out of them. The result was higher thermal efficiency than we are used to, but at the expense of much higher first-cost in construction, higher crew training costs, and almost certainly higher maintenance costs. The geography of France - with longer distances between similar-sized towns relative to Britain - was also more conducive to building locos optimised to run comparatively long periods at steady outputs, rather than the stop-start nature of a lot of British traffic.

So - two different countries with different geographies and cost base resulted in two different philosphies of construction. Neither is better than the other, and neither is better than the British system. The job of an engineer is to find the best compromise between many conflicting demands (and I could also mention workshop capacity, legislation ...); it is not to simply build the "biggest" on one particular measure in some kind of virtual game of locomotive top trumps...

Tom

 

I started this little divergence by saying that by boring out cylinders and reducing wheel diameter doesn't make it more powerful; 'it' being a Black 5. JimC pedantically said it wasn't quite right and that's true, but only within the output below that of the boiler. As usual, you jump on your bandwagon and start bleating on about how crap British locos are. It doesn't take a genius to realise that, if you throw away the boiler and replace it with a 21st century one, re-design the cylinders, valve gear and steam circuit, fit roller bearings and do a host of other things, it would be more efficient and more powerful. However, that is not what we are talking about and nobody in their right mind is even going to consider doing this. I am quite happy with keeping an existing Black 5 as it is, as I suspect are 99.5% of those with any interest in the subject.

 

So many posters on this thread criticise 242A1 but make his point for him inadvertently by coming forth with variations upon "we've always done it this way" or, perhaps worse, "not invented here". Modern exhaust systems, for example, could be applied to many surviving steam locomotives often with only a proportional sacrifice of efficiency if a traditional appearance was desired to be retained. Those who have done so are convinced but the reasons put forward by others for doing nothing, in addition to the two previously cited, include "the engines would not chuff as loud!

Adams, Chapelon, Giesl and Porta all wept.

PH

 

Yes, but .... we're a heritage business! No-one is actually in the business of finding a thermally-efficient way to move people from A to B, just so we can move them back again thirty minutes later. Indeed, as you have often made the point - most heritage railways have locomotives that are inherently too powerful for the job they need to do of moving 150 to 250 ton trains 5 to ten miles at no more than 25mph. So where is the incentive to make them even more powerful - especially if doing so detracts from the traditional appearance?

The charm of heritage railways is precisely because we are using antiquated machines to do a job they were designed for, when the modern world has moved on. So why actively work against what is our key selling point for what would be at most a marginal financial gain in the overall scheme of costs?

(For the avoidance of doubt: by "we" I mean the Heritage Railway industry in general, not the Bluebell in particular).

Tom

 

Spot on. Most heritage lines could exist quite happily with saturated 0-6-0 locos running at around 180psi. Running something as complicated as the Red Devil or with all the Chapelon mods would be quite counter productive.
To develop an earlier Spitfire analogy, I'm sure there are many improvements available to make one more efficient but if those improvements make it look and sound completely different to the original, who's going to pay to see one fly?

 

I'm envisaging something along the lines of the time-travelling steam engine at the end of the Back to the Future series. I'm sure there's an audience for it, but I'm not sure it's that great…

Simon

 

There's a fine line between making it easier to live with and keeping it as what people want to see, I suspect that's why many preservation modifications are kept to ones which are not outwardly visible, hopper ashpans and spark arrestors in the smokebox for instance.

Yes, you could build some 71000/Red Devil alike thing which is far more powerful/efficient than anything that has gone before, but where's the nostalgia in it ?, it serves no purpose as it a new design to the heritage market and steam is outdated technology to the commercial market regardless of technical improvements, that's why the 5AT is getting minimal support IMO, looks like Thunderbird 1 with a chimney, it's like saying Warwick Castle (the brick & mortar version) would be far more efficient as being a Fortress if it resembled Guantanamo Bay.

Thin tyres and bored out cylinders may help power output in short blasts, but not sustained output where the boiler is the deciding factor, I guess that's why Austerities can physically move 9/10 MK1's, but are unsuitable for some miles at 25MPH with that weight.

 

I agree with most of what's been said in the last few posts, insofar as it applies to heritage lines. But there is a case for design improvements in locomotives intended to run on the modern main line railway. That was the intention of the 5AT: a locomotive with a much improved power-to-weight ratio but still looking and sounding more or less like a conventional steam locomotive (not like a Leader, for instance). I disagree with Steve's saying that "nobody in their right mind is even going to consider" such a thing. Some people certainly did consider it, though not enough of them to make it viable. And there is a case for more modest improvements to existing preserved locomotives such as fitting them with Lempor or Giesl exhaust systems.