Thermal efficiency, ex-P2 Locomotive Company and related matters

I've recently become intrigued by the N&W Y6-b 2-8-8-2 compound Mallet.
You could increase the front LP engine unit to 3 cylinders. i.e 2 x H.P's and 3 L.P's (or an LP turbine).
Add a H.P. water tube boiler, superheat and reheat.....

 

Well, up to a point - by the 1940s the USA was building giant locos that required mechanical stokers that were, when compared with hand firing, very inefficient. It was an entirely logical financial decision, but indicated that they clearly were more concerned with capital cost of construction and the running costs of manpower (one big loco being cheaper to build and operate than two smaller locos of equivalent combined power) that they were with absolute thermal efficiency.

Tom

 

The railways did care about thermal efficiency, they changed to Diesels and electrics.

 

There is more, far more, to overall traction cost than thermal efficiency. Whenever true costing exercises have been carried out the results for the various types all lie in a very similar field.
For steam, reducing the specific steam consumption in conjunction with fully developing the exhaust system allows the achievement of a very impressive power output. I mention, rather too frequently I suspect, the target of obtaining a constant, maintainable output of 40ihp per ton of locomotive weight. This dates back to the 1930s and if you don't believe that even better can be achieved today we can stick with the old established figure.
You admit to admiring the N & W Y6b, a worthy choice for many reasons. Chapelon rode behind the class, it was very good at delivering the work that was expected of it. But he proposed a three cylinder low pressure front engine unit. This would have delivered improvements in terms of power and balancing. No doubt a Y6c would have benefitted from all of the other proven Chapelon modifications. Think about it, run a few numbers. What conclusion do you come to?

 

I conclude that we might have a new thread; Newbuild - Advanced Steam Improvement or Retrograde Authenticity ?

 

1) Steam will never match the availability of modern traction.
2) If you boil water at a little over 200 deg C and exhaust it to atmosphere at a little over 100 deg C, the second law of thermodynamics tells us that, regardless of the details of the machinery, the thermal efficiency will be pathetic.
3) This thread has drifted too far from the P2

 

Just out of interest, have you ever measured the temperature of the exhaust gases leaving a diesel or petrol engine? Or the coolant temperature?

 

Ah, but very little latent heat in a ICE exhaust - though the heat rejected via the radiators must be significant.?

 

I would like to join in some further discussion of the overall costs of steam versus diesel, thermal efficiencies, actual improvements in steam locomotive design that happened in some countries, improvements that could have happened here but didn't, improvements that could even now be made in a new build, etc. But I agree that it doesn't belong in this P2 thread.

I will temporarily continue it here, in the hope and expectation that the mods will find a new home for this post of mine and a few of the preceding ones.

On the thermodynamics: the essence of a steam engine is the phase change from water to steam, so that you need little energy to get the working fluid into the boiler as water and you get a lot of energy from it when it comes out of the boiler as steam. (This applies to all steam engines, whether locomotive, marine or stationary, whether having condensers or exhausting to atmosphere.) So I find it ironic that the thermal efficiency improves greatly with superheating, which makes the thermodynamic cycle a bit more like one with a perfect gas and no phase change.

On USA practice: I recall seeing one exhibit (a photograph or model, I forget which) in the Smithsonian captioned with words to the effect that it represented the ultimate development of the steam locomotive. In terms of size that was true, but certainly not in some other respects.

 

Essentially because it takes far more energy to take water at its boiling point to steam at the same temperature than it does to heat water from room temperature to boiling (without phase change) or to heat steam at its vapourisation temperature to a higher temperature.

The consequence of that is that you want to take the smallest possible volume of water through the phase transition - so for a given volume of steam to charge the cylinders, you want it to contain the smallest possible number of mols of water - which is what superheating does. In effect, heating steam is a much better use of a given amount of energy than boiling water.

Tom

 

Am I misunderstanding your statement? The big problem with turning water into steam is that it requires a lot of energy to do so but you cannot get that energy back in a conventional steam loco and it is lost up the chimney. The energy involved in turning it from steam at 0psi to 225psi and some of which can be recovered, is small by comparison

 

I think you're agreeing, from a different perspective. Turning water into steam is a very convenient way of turning heat into mechanical energy, which is why steam engines were developed long before other kinds of heat engine, but the efficiency is poor unless you add such features as superheating, compounding and condensing.

I agree with that explanation. It still seems to me ironic that the phase transition, which made early steam engines practical and remains an essential feature, is also a cause of poor efficiency.

 

On point 1) Interesting statement. So modern steam (2nd edging on 3rd generation) cannot be an improvement on the N & W which was considered to have "dieselised without buying diesels". Thinking rather than knee jerk conventionalism requires here.
On point 2) It is not that simple and others on the forum are giving you more than enough to chew on. Chapelon made an illuminating observation with respect to the Rankine cycle in that he did not find it to be able predict what could be achieved in practice. And thermal efficiency is only one part of the overall cost calculations.
On point 3) I do agree but it is in the nature of threads to stray somewhat.

What you should have come up with were the results of some basic calculations. The Y6b weighs some 611,500lb. or 273 imp. tons or 277.37 metric tons. Based on the established attainment the proposed Y6c would maintain in excess of 11,000ihp.

It would offer an availability equal to that of the contemporary diesel electrics.

 

For point 1) I was referring to 21st century diesel electrics, not "contemporary" ones, which had a lot more scope for development.
For point 2), the comments on thermodynamics are very interesting and give an insight into why power stations operate at such high pressures. If you look into the numbers it actually takes less energy to boil a pound of water at 2500psi than it does at 250psi.

 

The one link back to the P2 with this thread is David Elliott's redesign of the cylinder block to separate inlet and exhaust steam passages to reduce the transfer of heat that the original No. 2001 suffered from.

Foxy