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American steam locomotive efficiency- the effect of blastpipe size and superheat levels.

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Posted by Dreyfusshudson on Friday, March 30, 2012 8:31 AM

Dear Neil,

 

Thanks-the story of devices and systems aimed at improving economy or reliability not living up to their potential or being more trouble than they were worth seems to be a recurring theme.

I know very little about German steam developments other than part of a book on their attempts at turbines which I translated a few years back. Do you know if, how much and where good data is available? I found a reference to what was said to be the magnum opus on the subject, but it's not in libraries in this country. I am rather assuming that the Grunewald test plant and all its data succumbed to the RAF/USAF? Interestingly, Himmler lived in this suburb, as did Planck and Bonhoeffer in the same street about a mile away. Didn't even know there was asteam museum in Neuenmarkt- how remote can you get?

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Posted by Neil Robertson on Saturday, March 31, 2012 3:59 PM

Dear Hudhon-Dreyfoss,

I find this question easier to answer for Austria than for Germany. The 'standard' works for Austria, covering the complete steam era are:

'Locomotivebau in Alt-Osterreich 1837-1918' by Karl Golsdorf

and

'Die Ara nach Golsdorf' by Adolph Giesel-Gieslingen

Publisher Verlag Slezak (don't know if still in print).

The first has many wonderful photos of ancient locos with outside Stephenson gear, which was standard before being replaced by Heusinger gear about 1900 (Heusinger the same or very similar to Walchaerts and invented slightly later).

I don't have any corresponding German books' only more recent stuff. Both the German turbine locos were war casualties. Much data that may have come from Grunewald is given in current publications. I have one about Class 03 10. There is given in this the Merkbuch for the class, with all the weights and measures. More interesting is the Leistungsdiagramm. This gives for a boiler evaporation rate of 57 kg/(m2 h) for the speed range 40 to 130 kph curves for drawbar PS, drawbar pull, specific steam consumption and specific coal consumption (both drawbar values) and cut-off used. PS and drawbar pull alone are given in the 0 to 40 kph range. For example, cut-off falls from about 48% at 40 kph to 20% at 130 kph. Maximum drawbar pull was about 1460 PS at 64 kph. Minimum steam consumption was about 8 kg/(IPSh) from 40 to 60 kph, and minimum coal consumption about 1.1 kg/(IPSh) from 40 to 70 kph, with a footnote to the latter that is not given.There was also a train weight, gradient table, gradients from 1 in 1000 to 1 in 40. For example, a 3.10 was rated for 540 tonnes at 60 kph on a 1 in 140 and for 300 tonnes at 130 kph on a 1 in 1000. So, plenty documentation exists.

Most notable, I thought, were the high mileages in the 1950s. For example, at Dortmund shed in 1953, montly mileages were often over 20000 km. In July 1953 No. 03 1043 ran 28460 km. And this was with the original boilers of 1940, made without certain alloy elements which were reserved for war use. 

 

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Posted by Dreyfusshudson on Tuesday, April 3, 2012 12:38 PM

Many thanks- so much to catch up on!- the liestungsdiagramm of the 03-10 of particular interest.

Hoping some of this stuff will turn up in the UK libraries.

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Posted by Neil Robertson on Wednesday, April 11, 2012 2:22 PM

Giesel in his Anatomy of the Steam Engine does describe a couple of American blastpipes that are little known in Europe. There was the Sweeny blastpipe, of which he gives a photograph of the newer variant of 1899. It was star shaped with six elongated openings. It allowed a good mixing of steam and boiler gases already at a low height above the blastpipe. Its effective cross-section could be changed through a central plate (a bit like the Lemaitre). He says it was occasionally used. Only the Norfolk & Western used it consistently on their large Mallet compound engines for slow mountain service into the 1950s.

The Kiesel blastpipe was rather similar. A drawing is given. Here there were six narrow radially placed openings. He says from tests on the Wabash in July 1933 that it can be calculated that the Kiesel blastpipe gave the same backpressure as a conventional blastpipe of 19% smaller diameter. The main disadvantages were that the narrow openings soon became partially blocked with Olkohle (probably tar) and the exhaust beat down on the driver's sight at high speed (bit like Lemaitre again). The NYC tested the Kiesel blastpipe in 1937 with a chimney of 686 mm diameter, as used in Pennsylvanian pacifics but rejected it because of the visibility problem.

He describes the extensible stacks used by the ATSF as a curiosity. Operated by compressed air they could be raised 915 mm. On open stretches the chimney top was 5.8 m above the rails and improved both the drivers sight and the draught for the fire. One wonders if drivers always remembered to retract them at the right time. In the War, a few Austrian engines, which had a generous height clearance, lost their chimneys when they moved off their home territory.

These examples go some way to dispel the idea that American engineers were always conservative in their choice of blastpipe.    

 

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Posted by Dreyfusshudson on Thursday, April 12, 2012 4:41 AM

Dear Neil,

Folk on this forum pointed me to RP Johnson's (Baldwin) comprehensive book  'The steam Locomotive' (1942) (available on library loan  in this country and at the NRM, a must read). This has a chapter on Front Ends, which begins 'Probably no part of the locomotive has been subject to more experiments and investigations than the 'front end', which is the right perespective, I think.  He goes on to observe that the results were contradictory and difficult to explain. The chapter includes results on the Kiesel nozzle.

It does seem to me that Chapelon discovered something important in this area with his Kylchap set up, though his ideas as to the basis of his invention, and those theorising on the subject  to this day seem to me wide of the mark.  

The reason Chapelon's Compounds achieved such high outputs is that the Kylchap set up allowed specific evaporation rates in excess of 1000lbs/sqft/hr to be obtained i.e. very high front end limit; similar results can be obtained with simpler devices but at higher back pressure. That the hand fired coal rates these efforts required were quite impracticable, and hopelessly uneconomic was overlooked.

 

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Posted by Neil Robertson on Friday, April 27, 2012 10:54 AM

The Churchward jumper ring was an early variable blastpipe that catered for a wider range of outputs than a simple blastpipe. An old book I have just got by Rutherford says the blastpipe area on the Kings was 23.8 in2 increased to 33.7 in2 with the ring open. The author says this allowed the Kings to produce transitory outputs far over their continuous rating, for example on Saunderton bank.

Another type to have this device was the single chimney A4. These reached up to 2500 IHP and my feeling is this would have not been possible without the increase in blastpipe area.

After the War, poorer maintenance led to these rings being deactivated/removed in both the A4s and Kings. Perhaps this was one reason for both types failing to reproduce their preWar performance.

To restore matters the Kings eventually received higher superheat and a double chimney with a relatively modest blastpipe area of 25.14 in2.

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Posted by Dreyfusshudson on Wednesday, May 2, 2012 11:03 AM

Dear Neil,

Many thanks. In a separate correspondence I am engaged in, I have been trying to work out the mysteries of the GW jumper cap. I have the GW original drawing and the details of the size of blastpipe cap and weights of jumper ring fitted to different classes- I hadn’t appreciated almost every original GW type had them, except their higher superheat designs from the late 1940s on. My understanding is this change led to carbonisation problems, hence they were abandoned. This will have also been a significant issue on the high superheat A4s.

Thinking about it, these are basically yes/no devices, which lift when the exhaust pressure on the inner cap surface exceeds the weight of the cap (20-25 lb). I think of draught as being the consequence of a momentum differential across the chimney choke.

Dp =  Mi – Mo/A                                                                                               

Where Dp is smokebox vacuum.

Mi is chimney inlet momentum.

Mo is chimney outlet momentum.

A is the choke area

Obviously, when the cap rises, there will be a sharp drop in inlet momentum, and special circumstances apart, the outlet momentum will drop also, so there will be a sharp decrease in smokebox vacuum, hence draught will be less. Not a recipe for going faster.

I think the logic of Rutherford and others, which up to now I accepted, is that the lower backpressure when the cap rises will allow a more efficient engine, hence power will rise. Well, maybe initially, but you can only use steam more efficiently when you’ve got some, and if the draughting disappears, pretty soon you won’t have.

A person whose knowledge of UK history is enormous, who I trust, says that the rationale for jumper caps, certainly on the LNER, was economy. If I’m right, this is not about more efficient engine working, but by putting an enforced draughting limit on crews, beyond which they couldn’t burn coal any faster. Devious, huh? Supporting this, earlier GW types do seem to hit a brick wall in terms of sustained power. The original Castles, for example, this is about 1450IHP, though you can get a bit more short term.  The claims, especially by Nock for power outputs of the original Kings climbing Saunderton bank border on the absurd. The highest powers I can find are three runs delivering 1850-1900ihp for 5 minutes. Sustained powers for the original Kings seem to hit a brick wall about 1600IHP; the three best runs I can find give 1680-1720IHP; trouble analysing old logs is you don’t know which way the wind was blowing which can have a significant impact on the estimate.

The best sustained powers I can find for jumper cap A4s are about 2150IHP. This will be a not dissimilar steam rate to the best sustained powers from original Kings, which were hopelessly inefficient on account of their Churchwardian superheat.  The Kylchap raises the sustained A4 limit to about 2600IHP, in common with other UK Pacifics. The limit here is the fireman. With the kind of firing the French Compounds were given on test, this could be pushed up quite a bit more, since the draughting now allows it.

All I need to do now is work out at what steam rate the jumper cap actually rises. It may be then that it will be back to the drawing board!

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Posted by Neil Robertson on Thursday, May 3, 2012 2:59 PM

Dear Dreyfuss-Hudson,

 

 

 

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Posted by Neil Robertson on Thursday, May 3, 2012 3:22 PM

Dear Dreyfuss-Hudson,

Thank you for your informative reply. The idea that the jumper ring was mainly about economy may make some sense. The South Devon banks are very steep but short and Dainton and Hemerdon can be climbed in less than 4 minutes while the very steep part of Rattery takes less than 7 minutes. The jumper ring opening at the bottom increases power temporarily, while keeping ejected coal down (Reed says LMS types ejected 30% of their coal on the last four miles of the climb to Shap). By the top, boiler pressure is down, which means that the safety valves do not blow when the regulator is shut for then descent, another economy.

Rutherford gives details of tests with various levels of superheat on Castles in 1946. At 240 rpm with the standard superheat, IHP was 1500. The highest superheat boilers gave 1720 IHP (essentially the same as a low superheat King) it says). Summarising Ell says:

'... an increase of 14.5%, and it may be noted that this extra power is obtained with a normal rate of combustion and with a normal boiler efficiency. Hence it is obtained at a far less cost than for a similar increase by methods requiring a higher draught, where the extra power is obtained always at the expense of lower boiler efficiency by reason of excessive loss of unburnt fuel.' 

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Posted by Dreyfusshudson on Sunday, May 6, 2012 4:09 PM

Dear Neil,

Ell's comments on Castle superheat refer to what he thought would happen, not actual test results. 2, 3 and 4 row superheater Castles were tested subsequently. Only the results on the 3 row Castle were published. The original report contained some nonsensical  efficiency diagrams which Ell later corrected, although the efficiency claims for this Castle remained unchanged, even though I am pretty certain they are significantly overstated, for reasons I think I understand. Part of the benefit of raising the superheat was lost by narrowing the blastpipe.

I think from a driver's perspective, a 3 row would scarcely be any better than a 2 row. Worked in the same way i.e. same cut off, power would not increase. What would happen is that steam hence coal consumption would go down by several percent. Enough for a crew to throw their caps in the air? I'm not sure.

With a four row superheater and double chimney, you do get a step change in performance at a given cut off, and the better efficiency and higher darughting limit (no jumper cap!) do allow outputs in a different league to the original two row single chimney types.

I am beginning to think that, whilst increasing superheat is surely by far the best way to improve engine efficiency, the benefit in practice may not be as large as test plant results suggest, for the simple reason that it takes a significant amount of time for equilibrium superheat to be reached, and if the road is undulating, or with frequent speed restrictions, the equilibrium value may not be reached. It's one thing climbing 5000' from Topock to Wampai Az in 120 miles, another working the Midland main line from St Pancras to Leicester, constant undulations with necessary variations in steam rate, four separate summits in 100 miles.

My suppostion is that lower superheat classes get closer to their equilibrium value sooner than higher superheat types do. See Wardale for some interesting on the road test results on superheat.

  

 

 

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Posted by BerkshireHathawayHobo on Sunday, May 6, 2012 4:43 PM

The Problem with Steam Locos was not so much the cost of fuel (After all N&W and the Laccawanna owned huge coal mines) but the cost of a army of craftsmen to fix and maintain the locomotives

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Posted by Dreyfusshudson on Monday, May 7, 2012 9:09 AM

Yes, I agree that all sorts of economic factors other than thermal efficiency killed the steam locomotive. This discussion about efficiency is really about the era when steam was the only game in town, and you had to make the best of it.

It’s maybe worth bearing in mind that in this country, the decision to get rid of steam was only taken in 1955. One plan was to import diesels with proven technology from the US; politics killed this idea, so local manufacturers produced a lot of underpowered and initially not very reliable diesels. So, in the summer of 1960, the world still was pretty much in order, most everything steam hauled, shiny passenger locomotives. Even in 1961, with a bit of myopia and optimism you could believe things were ok, 15 years after the corresponding era in the US. The consequence of this is that there is a whole generation of folk in their 60s, 70s and 80s over here with time and money that remember how it used to be. The consequence of this is that between now and the end of September, there will be more or less a steam express train every day on our main lines, with 60-80 year old 4-6-0s and Pacifics with heavy loads mixing it with high frequency modern trains, often travelling at 125mph. In addition there are at least a dozen tourist lines that need 3-4 locomotives in steam to run their daily services, and a host of smaller ones. Scarcely believable, certainly not foreseen.

So, as you can imagine there are numerous small businesses and a veritable army of volunteers to whom steam traction is still a live issue. How does it feel living in this time warp? Well, I can think of a lot worse things! Imagine if your daily commute from Glenview, Il was enlivened a dozen times a year by a Hudson plus a reasonable facsimile of the Afternoon Hiawatha racing through at track speed. Sure you’d get covered in coal char, but is it that bad?

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Posted by Neil Robertson on Monday, May 7, 2012 11:35 AM

Dear Dreyfuss-Hudson,

I guess at 60 I come into the category you describe. After 20 years in Austria I find there is much more steam here. Passengers on diesel trains are mostly in the 20 to 40 age bracket, while on steam turns it's nearly all 60 plus year old men. Comfort is also likely a factor: the old 2nd class Mark 1s were more spacious. The current standard class is nearly all high density commuter-type stock. I believe the HST 2nds started off with 72 seats but are now 84 seaters. So anyone over 5 ft 7 in is cramped. In the Mark 1s everyone had a good view out both sides, now anyone in an aisle seat has nowhere to look. Few travel in 1st class. On steam few travel in 2nd, Premier Dining Class is the most popular (£200) and there is sometimes Pullman above that.

Coming back to more technical matters, Brian Reed wrote in his book commemorating 150 years of British steam that superheat attained in daily work was never high in Britain. Anglo-Scottish turns rarely reached 620 F band the average throughout a run was usually only 550 F. Maybe this is why the low superheat GWR types did not stand out as very inefficient in relation to the others.

For efficiency, the boiler effeiciency and superheat seem the most important factors. And unfortunately the work against each other. The boiler efficiency peaked at a fraction of the maximum output and fell in a straight line thereafter. Superheat on the other hand was low at low outputs and then rose significantly. Cylinder efficiency peaked at a fairly low average cylinder pressure and then rose a little more than linearly with average cylinder pressure.

The Kychap was about the only unambiguous improvement: better draughting and increased power.

BR perhaps made a mistake with some of the power ratings: the King was more 7MT (on account of its adhesive weight), the Castle 6MT and the Britannia nearer 8P6F.

The reputation of the GWR types was made more on short steep banks. Last Sunday I saw an ancient Castle 4-6-0 (the 76 year old Earl of Mount Edgecombe) come through Bristol Parkway. It had at least maintained the 14 minute allowance from Temple Meads with a 12 coach train on a very wet morning when adhesion on the 4 miles of 1 in 75 might have been a problem for some more powerful Pacifics. But this does not shed any light on the jumper ring as this is one of the engines with a double blastpipe. I recall the heaviest turn in the 1980s was the 12 coach Plymouth to Edinburgh, which was always down to about 30 mph on the curved part of the bank. But it was baulked to 20 mph at the foot of the bank by the weak bridge over Stapleton Road, closed for many years. The Castle was probably doing 45/50 mph at that point.

 

 

   

 

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Posted by Dreyfusshudson on Wednesday, May 9, 2012 6:02 AM

Dear Neil,

I agree there is a sweet spot for efficiency, since as steam rate increases, boiler efficiency decreases and engine efficiency increases due to superheat. Boiler efficiency starts to deteriorate significantly due to increased coal loss above about 600lbs/sqft/hr. The reason for initiating this thread in part was for me to understand what steam rates were typically required to maintain schedules in the US, and it seems it was about 500-600lbs/sqft/hr. In the UK the smaller 4-6-0s were steamed at 600-700lbs/sqft/hr. Whether this ‘glass ceiling’ was a consequence of crews not wanting to waste coal, or a desire to keep boiler maintenance costs down I do not know. Maximum outputs above this level, as quoted for US, UK and French types, were in practice irrelevant.

This says that superheat is primarily what matters for overall efficiency. Reed was a very knowledgeable writer, but I think his estimate of 320 deg C as a typical maximum in the UK is low, for some types at least- though you wouldn’t get above that with a Duchess or Princess on a run to Scotland. 60007 has a pyrometer on the footplate, top right of the backplate. There are some footplate runs on You Tube (put in 60007 footplate). One clip shows 60007 leaving Retford from a water stop, climbing the 5 miles to Markham with 500 tons working quite hard. Going more easily down the other side the pyrometer shows 340-350 deg C. Another clip has it climbing the 1% from Settle Junction to Helwith Bridge, and it shows 360-370 deg C after about 10 minutes I guess. These figures are about 20 deg C below what one might expect the equilibrium value to be, looking at the V2 test plant results (very similar boiler). I was talking to a friend yesterday who has spent a lot of time on the Duke footplate on the main line. This also has a pyrometer. He said it took about 15 minutes for it to reach a maximum equilibrium value of 400degC, when working hard. Having said this, we do treat our Pacifics in quite ungentlemanly fashion these days, often whipping these old iron horses more than old Jockey club rules allowed.

I agree some the UK power classifications were overstated.

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Posted by Neil Robertson on Tuesday, May 15, 2012 4:13 PM

Dear Dreyfuss-Hudson,

Yes, the A4 was by reputation the British pacific with the highest superheat annd this probably allowed it to nearly equal the output of the larger Pappercorn A1 and also make the fast non-stop runs from Edinburgh to London without the need to refuel.

Another European simple pacific which had apparently high superheat was the German class 10. Clay and Cliffe (West Coast Pacifics) in an international comparison of pacific tests put that maximum sustained output of the class 10 as: evaporation 39700 lbs/hr; IHP 3000;  steam rate 13.2 lb/ihp-hr. The official rating of the class 10 was 2500 PS.  The 3000 figure is in no way exceptional for an engine as large as the 10. The corresponding figures for a Duchess  were 42000 lb/hr, 2910 ihp and 14.5 lb/ihp-hr. Where the 10 apparently scored was in having a distinctly low steam rate at maximum output.  Clay and Cliffe say its minimum steam rate was 12.07 lb/ihp-hr, marginally lower than the BR8 at 12.2 lb/ihp-hr, but probably in the margin of error. In practice the 10s were overweight and restricted in the routes they could operate on but German limitations on axle load were greater than British. Clay and Cliffe say the 10s exceeded the 100000 mile per year target. That was probably true to begin with but 10 001 preserved at Neuenmarkt-Wirsberg ran a lifetime average of just under 60000 miles a year.

Clay and Cliffe also give test results for the US K4S. Piston valve in 1937: steam rate at maximum output 19.8 lbs/ihp-hr (3530 ihp @ 700000 lb/hr).  And with Franklin poppet valves: 16.4 lb/ihp-hr (4267 ihp @ 70000 lb/hr) with a minimum rate of 15.0 lb/ihp-hr (3190 ihp @ 47770 lb/hr). Of course, these were old engines and not representative of the best US types.    

  

 

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Posted by Dreyfusshudson on Thursday, May 17, 2012 11:56 AM

Dear Neil,

I'm a bit sceptical about the 12.1lbs/ihp-hr claim for the 01-10, not least because I think the 12.2 of the Duke is significantly overstated. The raw data from the Swindon tests is at the NRM, and if you look at this then a) the test bulletin takes a rosy view of what they actually measured and b) the cylinder powers they estimated were in any case probably about 5% high, meaning that I doubt what they actually got on test was better than 13lbs/ihp-hr, slightly but not significantly better than the Britannia - Caprotti brings very little in the way of efficiency itself.

The minimum specific steam consumption is determined by the fact that superheat, hence efficiency increases with steam rate; working against this is the fact that as steam rate increases, backpressure increases, and longer less efficient cut offs are needed to consume the steam. So the 01-10 would require very high superheat at lower steam rates than UK types for the claim to be true.

Alternatively, conventional designs lose a minimum of 5% efficiency through leakage of various kinds, often more, through valve and piston rings, stays etc. Now if the Germans were better in these respects, the claim would be more credible. Vorsprung durch Technik, as they say.

I have done some first estimates of the steam rates at which the GW jumper cap would lift, and it seems that this is less than the economic limit i.e. not in accord with my original postulate. I'm hoping to get an independent verificaiton of this; if correct, the question is how come the drop in inlet momentum due to the lifting of the jumper does not reduce draughting.

 

 

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Posted by Neil Robertson on Sunday, May 27, 2012 2:41 PM

Dear Dreyfuss-Hudson,

Sorry for the delayed reply but I was on holiday in Switzerland. There, there is still some steam on the public timetable. I was on a paddle steamer, Lotschberg, on Lake Brienz. This was built in 1914 and still has the original 2 cyl compound condensing engine: 550/950 x 1050 mm, 14 atm, 300 F 450 IPS. Not much but the boat can carry up to 800 passengers. And it is smoother with less vibration than any diesel. One steps off the quay and then almost on to the valley station of the Rothornbahn. This converted back to steam in the 1990s with new steam engines. It was open only to the half-way mark, but spectacular is almost an understatement about this railway. The engine was No 12 built in 1992. More powerful than the older ones and able to go up smoothly without much of the familiar jolt-jolt of steam cog. An unusual feature was the location of the water gauges. Both on the right hand firebox side outside the cab. At Interlaken there are two small operational metre gauge engines, one a 2 cyl and the other a 4 cyl with both adhesion and rack drives. There is also a standard gauge project: the frames and cylinders are now complete for a 2-10-0 4 cyl compound engine of 1917 known as an Elephant. These lasted till the 1950s and some I believe were lent to the Germans in the War.

Regarding the Class 10s the main way they were state of the art was in having all roller bearings, including for the connecting and coupling rods. This would not have increased their IHP but might have reduced the difference between IHP and DBHP.

Were all test results optimistic? Probably not. Clay and Cliffe also give figures for the original unfancied Merchant Navy class: 2480 IHP at 42000 lbs/hr sustained (16.9 lbs/IHP hr). This is probably an unvarnished figure given the enthusiam to rebuilt them. Another well known low rating was when an S3/6 was tested at Grunewald in the 1930s. The steam chest pressure was about 20 lbs/in2 below the rated boiler pressure, and these engines could sustain significantly more then the Wagner boiler rating of 57 kg/m2 hr (as could the 38s). The official raating of the S3/6 of 1830 PSi appears to give 13.55 lbs/PSi hr, but this figure was neither the lowest consumption nor the maximum sustained.

Neil  

 

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Posted by Dreyfusshudson on Monday, May 28, 2012 4:38 PM

Dear Neil,

I have a friend who has created a first principles model of the machinery resistance (those between the pistons and the wheel rims) which would include components which can be fitted with roller bearings. The model is in good agreement with the ‘good‘ Rugby data. For an MN, the value is about 150 HP at 70 mph. (The model needs some adaptation for US designs). Only 40% of this is due to Journal friction and Coupling rod losses, which says that you are working with about 60 HP with roller bearings. Total resistance between the cylinders and drawbar is about 540HP at this speed, so any benefit for roller bearings will be relatively small.

The Rugby MN bulletin is the only one from that testing station that I believe is in significant error. You can see this by looking at difference between the reported IHPs and those measured at the wheel rim by the Amsler dynamometer. The values are generally negative! If you add a sensible value of machinery resistance to the wheel rim powers, what you find is that the difference between the measured IHP and the IHP that was probably being developed, as estimated by this method, is dependent on cut off. At long cut offs, and low speeds, the values are in reasonable agreement. At high speeds, the measured IHPs are up to 15% too low, a problem I believe with measuring areas on ‘thin’ indicator diagrams. Other locomotives tested at this time also show this problem, though the bulletin results seem to have been massaged and are basically correct. Rugby beefed up its expertise on indicators in 1953 and this resolved the issue.

If the original MN cylinder efficiency was as bad as you say, then its drawbar efficiency would be poor. In fact,on full regulator,  it is just the same as the Duchess, confirming the hypothesis above. There was already a strong body of opinion that the class should be rebuilt, and the Rugby tests supported this. The results were however wrong! In good condition, the original and rebuilt locomotives have similar efficiency. The efficiency of the class was not as good as it might be a) because the thermic syphons fitted reduced superheat b) they did not have exhaust injectors and c) they were generally far too powerful for the work required (especially at the original 280 psi boiler pressure), which meant that they were habitually driven on the regulator/throttle often 100-120 psi only in the steam chest, with the valve gear set in a very long, inefficient cut off.   

Spent a number of happy holidays in Ringgenberg watching the steam paddle steamers on Lake Brienz, and riding them to the Reichenbach falls, where, as we know, Sherlock Holmes met his end fighting Moriarty- also listening to the exhaust chatter on the Brienzer Rothornbahn. One summer had a steam service on the main line between Interlaken and Brienz too! Hard to beat.

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Posted by Neil Robertson on Sunday, June 3, 2012 1:17 PM

Dear DreyfussHudson,

Thank you for the interesting information about the Merchant Navy tests.

I have just acquired an old book (1974) about the Bullied pacifics. In it there is an account of how the BR engineers initially tried to improve the Merchant Navy class. The Lemaitre blastpipe was considered non-optimal and it was felt a standard BR type single blastpipe would be better! They found that whatever they did that a single blastpipe limited the engine to about 31000 lb/hr. This is not really surprising as the slightly smaller V2 also had a single blastpipe limit of 31000 lb/hr at a relatively high coal consumption of 5760 lb/hr (Clay and Cliffe). The 'unsatisfactory' Lemaitre gave 39000 lb/hr for 1 hr and 42000 lb/hr for 20 minutes, which was not the absolute limit but thought to be near it. Perhaps rather surprisingly, given their preconceived ideas, they decided to retain the Lemaitre system even in the rebuilds. Far from showing the advantage of a single blastpipe, their tests showed the considerable increase in power available from a more advanced exhaust. Probably the superheat was a little higher too at the higher outputs.

Winkworth, who gives many timed runs of both original and rebuilt engines, says the coal consumption was not reduced by the rebuilding although the oil consumption was. But the performance, in terms of power and speed, of both was entirely up to class 8 standards, while keeping to quite modest size and weight limits, especially in the originals.

BR might have done worse than to make the rebuilt MNs the new standard 8P, rather than embarking on a completely new design, which would initially inevitably have teething problems, and meant needing to keep an additional set of spare parts.

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Posted by Dreyfusshudson on Monday, June 11, 2012 6:08 AM

Dear Neil,

Thanks for this. BR’s sole 8P Pacific 71000 was of course largely designed by Harrison of the LNER and Cocks from the SR, and so not surprisingly it has three cylinders! I always see this as a fight back of these factions against the dominance of the LMS mafia that ran the BR ‘Standard’ programme, seizing the opportunity presented by the loss of 46202 at Harrow in 1952. In essence basic boiler dimensions are those of an SR MN and LNER A2, and it shares their 74” drivers. The cylinders are 28” stroke in line with other Standards, and doubtless many other components were ‘Standard’- not that this guaranteed best practice was being followed! I can’t recollect why they went for Caprotti gear, but this was certainly seen as the future at the time. So I think of the Duke as basically a Caprotti A2. What the BR designers would not tolerate of course was the fancy continental exhausts espoused by the LNER and SR, so it finished up with a plain BR DC. At Swindon, it could, as a consequence not be steamed satisfactorily above 26000lbs/hr on the test plant, though with better firebed airflow on the road, they did make 30000lbs/hr, still very poor for a 48.6 sqft grate. The Kylchap now fitted has transformed things of course.

The defenders of the LMS/BR faith still think that the plain BR DC is wonderful of course, and that the Duke’s poor steaming was due to excessive spark loss due to the Caprotti gear.

To try to get to the bottom of this, I have had a go at applying turbulent jet theory to locomotive exhausts in the past, unsuccessful because it was not possible to estimate the momentum loss in the chimney (which determines draught) from simple dimensions. I’m having another go at this, having found that the University Illinois report on draughting their ¼ scale model boiler with a large variety of draughting devices is available in full on the internet.

https://www.ideals.illinois.edu/handle/2142/4435

 

www.ideals.illinois.edu/.../engineeringexperv00000i00274.pdf?...3

 

Whilst turbulent jet theory is very straightforward for an orifice discharging into a uniform, stationary pressure field, as soon as you go to more complex systems, you need to do experimental work to modify it appropriately, and the Illinois data may provide the information needed. However, this really is way outside my area of expertise, and I’m not sanguine I can come up with anything useful. Success would mean putting to bed the plain DC vs Kylchap argument once and for all.

The rebuilt MN did not appear until 1956, and by that time, BR had abandoned the idea of steam (a decade behind the times!), so could not have been the basis for a BR Class 8. Detailed engineering considerations apart, it would have been scarcely possible to improve on the LNER A1/A2. Really good to see A1 Tornado and A2 Blue Peter proving the point still.

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Posted by Neil Robertson on Sunday, June 17, 2012 4:47 AM

Dear Dreyfuss-Hudson,

The comparison A2/MN/BR8 is interesting. Where they differed was in their piston strokes. The short stroke (24 in) MNs had the same piston speeds as an A4. This allowed a genuine 100+ mph engine in a relatively small package. The compromise was probably lower cylinder efficiency. With a short stroke, other things being equal, clearance volume is higher, reducing cylinder efficiency. In Britain and Germany, 26 in stroke was most frequently chosen. Golsdorf was an early European advocate of relatively long stroke (28.34 in) and later Austrian designs retained this. Probably Golsdorf saw this as a weight sving measure and he was working within a 14.5 mt axle load.

Laterly, there was some move toward longer stroke The German class 10 had 28.34 in. Cylinder efficiency was probably better but machine resistance increased. However, some American engines with 32 in stroke still easily reached 100 mph.

The main fault of the A1/A2s was their indifferent riding (A2s the poorest). Do you know if Tornado is any better in this respect? My understanding is that the A4/MN layout was better in having a lower rotational moment of inertia (MOI) about the central vertical axis. The A1/A2s had higher MOIs through the forward location of their middle cylinder, so the energy of the swaying motion that needed to be damped out was higher. Also their short rigid wheelbase would have likely made them less steady. This was inherited from Thompson on whom it was forced by his ackward cylinder layout.

 

Tags: Blast Pipe
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Posted by Dreyfusshudson on Monday, June 18, 2012 7:20 AM

Dear Neil,

As your data show, I don’t think that within limits, mechanical considerations such as wheel diameter:stroke (D:S)  ratio had much influence on ability to reach 100mph. The D:S ratios of British locos that reached 100mph lie in the range 4.15 (Britannia, 74” drivers, 28” stroke)-4.89 ( Scot, 81” drivers, 26” stroke). The MN and LNER Pacifics are at the top of this range, about 4.83, also the GW Castle. Only the LNER Pacifics and GW Castles were ever scheduled to run at 90mph on a daily basis.  In this D:S range however, the mean and maximum piston speeds vary by only 5 ft/sec at 90mph, about 15%. All types that succeeded in reaching 100mph had 74+” drivers, though none with smaller drivers could develop enough power to make 100mph except down very steep grades. The N&W J has a D:S ratio of 3.44 only, 70” drivers, 32” stroke, and one of these famously reached 110mph, though this was more a stunt than demonstration of daily capability. The speediest locomotives on a daily basis were the MILW F7s, with a ratio of 4.4, 84” drivers. It is said that the ATSF 2900s ‘regularly’ reached 100mph and their ratio is 3.93. So I don’t think D:S ratio in the range 4.2-4.9 is a critical consideration.

There was no apparent correlation between stroke and clearance volume on UK locomotives, CV being more of a design choice. The GW believed low CV was good, hence went for 6% with 26 and 28” stroke.  The LMS had 12% CV on the 28” stroke Duchess, BR standards with 28” stroke were also generally 10+%.  LNER/Bulleid went for about 8%, with 24-26” stroke. CV data on US locomotives is hard to come by.  

The idea that Low CV is good for efficiency is true at low speeds; at high speeds it is a positive handicap because of compression loops as the GW found on the King. At high speeds the effect of increased CV on engine efficiency is pretty small, certainly in the range 8-12%; remember also the HP cylinders of the French Compounds had CVs of 24+%! I think the myth of CV effects on engine efficiency at speed is one of those that will outlive us all.

I have no information about the riding of 60163, though on all the head on videos of it I have seen heading through curves at high speed it looks as steady as a rock, likewise 60532. I will ask a friend who might know. I did read somewhere that whilst the Peppercorn Pacifics were worse riding than the Gresley ones, the real culprit for the reputation was the state of ER track in the immediate post war period- it had deteriorated badly during the war. This highlighted the differences. There was 60 mph limit out to Hatfield for this reason well into the 1950s.

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Posted by Neil Robertson on Monday, September 10, 2012 12:37 PM

Hi Dreyfuss-Hudson,

Following my visit to the NRM last month, I have managed to obtain the two volumes of The Science of Railways: The Steam Locomotive by Dr Lee Towers and am working my way through these. These books formed the basis of a course given at the University of Leeds School of Continuing Education in the late 1990s. I spoke to the author who kindly sent me the books. He is a physicist and as that is also my own background I find his approach more systematic and meaningful than what I've read before, which has been written mostly by engineers,

Neil

 

 

 

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