nhrand.
Many thanks for the additional info.
I have to say that i find the statement that 'There is very little difference in the handling... compared to a normal piston valve locomotive' very surprising in view of the fact that they go on to note that the system allows use of very short cut offs. As noted in an earlier response, a K4 would have to be driven hard in 30+% cut off to deliver even 2200-2500HP at speed. At 11% (nominal) cut off a T1 would deliver 3600HP at the same speeds- chalk and cheese if ever there was such. What were they trying to say?
The steam flow to the cylinders at a given cut off hence power is significantly increased not only by the Franklin gear, but also the fact that at equal TE, there is much greater flow into four cylinders. And, the cylinder capacity of the T1 is of course much greater,which increases steam flow further. All of which accounts for the massive difference in the power vs cut off characterstics of the T1 and K4
So, it seems to me, someone who had spent a lifetime driving K4s would need to drastically modify their driving technique for the T1.
It is also interesting that they say maximum cut off should be used on starting and then 'gradually' reduced. This would lead to very high wheelrim powers when accelerating, more so than on a PV 4-8-4 because of the enhanced steam flow from the Franklin gear and four cylinders at a given cut off, and if driven this way, it is perhaps not surprising they tended to slip.
It seems to me that the PRR operating information meant that the crews had a lot to figure out for themselves about these very different beasts- this based on their own test plant data.
Sorry BTW to all for the delays in replying- the automatic e:mail notification of inputs hasn't been working
Timz
By time course I mean the passing times at all points along the route.
Selector- The 40% cut off I quote was on a test train when they were trying hard to deliver maximum power. I would think this was uncommon, but there is very little detailed timing of the running of US passenger trains that would allow an assessment of how much power was normally being developed, hence what the cut offs were. The K4s were pretty out of date and underpowered by the 1930s (sorry PRR fans), so much so that they sometimes had to be double headed. So I would guess single headed they would need to be worked pretty hard. At 30-35% cut off they would develop 2200-2600 cylinder HP at 80mph. Power in this range will allow you to run at 75-80mph with an 800 (US) ton train on level track, though would take some time to get there.
As for the 15% cut off question, that's what the valve gear allows, and if you want to reduce power, shortening cut off is one thing you can do. Some UK designs were able to be worked in short cut offs like this, but many suffered from rough riding worked this way, and so if reduced power was required, it was more common to stick with ca. 25% cut off and reduce the steam chest pressure (Throttle). Working this way leads to a small loss of efficiency.
Many North American designs I suspect needed to be worked in longer cut offs, since two cylinders were pretty universal, and at maximum permissible tractive efforts (determined by adhesion considerations) you need longer cut offs to get the requisite steam flow and power with two cylinders.
From the very limited data there is, I suspect 4000HP (perhaps 55000lbs/hr evaporation with a feedwater heater) is about as much as was normally developed. The NYC report on Niagara testing said that 4000HP was 'above average working'- about 30% cut off for a Niagara at 80mph. 4000HP is sufficient for a big 4-8-4 to cruise at 90-95mph on level/undulating track with 1000US tons. With a grate of 100 sqft, the evaporation rate required to achieve this would be not too taxing.
There are some details of a couple of runs with ATSF 2900s working uphill from Dodge City to La Junta with 900-1000 ton trains. The gradient, though very gentle, needs about 1000HP to overcome. 2921 began at about 4000hp, rising to 4500hp for the last 100 miles. This gained half an hour on the schedule. 2909 made a similar time, but gained an hour on its schedule. Working was less uniform, but after Syracuse had a couple of spells at 5000HP for 5 to 10 minutes. With the massive cylinder capacity and long lap of the 2900s, this will have required no more than about 30% cut off. Given the time gains, I suspect that this was exceptional working, showing what could be done, not the norm, but there is just not the data to be sure.
Some very high powers were also developed by 3776 climbing single handed to Cajon on test, though I think trains on this section were often assisted.
1949 PRR DESCRIPTION AND OPERATION OF FRANKLIN TYPE A ON T1
The Pennsylvania Railroad's 1949 Machinery Examinations For Locomotive Fireman had several pages and a number of large fold-out plates describing the "Franklin System of Steam Distribution - Type A - Class T1 Locomotives".
An excerpt from the operating information may be of interest to forum readers.
" Instructions for Operation "
"There is very little difference in the handling of a locomotive equipped with the Franklin System of Steam Distribution as compared with a conventional piston valve locomotive.
This system allows the use of very short cut-offs (approximately 10 per cent). For most economical operation at speed, the throttle shoiuld be kept wide open, and the locomotive should be operated with as short a cut-off as possible. The shortest running cut-off is mid-gear (marked on reverse gear plate Mid-gear Forward and Mid-gear Backward).
Due to short cut-offs, the exhaust of a poppet valve locomotive is likely to be soft. Therefore, the depth of the fire must be regulated accordingly and should be as thin as possible.
When starting the train, it is nesessary to have the reverse gear in the maximum cut-off position (full gear). As soon as the train is under way, the cut-off should be shortened gradually.
Upon shutting off the throttle, move handle of three-way drifting control cock into drifting position.
During long drifting periods, move the reverse gear into the position indicated on the reverse gear dial by "Drifting". When running in one direction, never pass drifting position (located centrally between mid-gear forward and mid-gear backward on reverse gear indicator) toward the other direction.
When the engine is left by the crew, the reverse gear and the drifting control valve should be placed in drifting position."
The operting instructions continued with information on how to operate the T1 with only one unit in case of damage. Also included was information on lubrication such as, "The cam boxes are flooded with valve oil pumped through the distributor pipes in the cover plate." etc., etc.
Thanks for that, really appreciate it. Now, back to our OP's intentions for this thread...and I thank him for his forbearance.
The figure of around 15% is largely empirical and reflects the amount of admission needed to get the engine past the following dead center under load; it's got little to do with 'practical' economical running. UP and other roads don't like short cutoff for another important reason (similar to why they may not like using reverse rather than partial throttle when starting or running heavily loaded at slow speed) - the torque produced by a typical quartered two-cylinder simple becomes very peaky as cutoff is wound toward mid, and when the throttle is worked open quickly and things warm up so that steam pressure (net of superheat pressure gain) at the piston face is an appreciable high percentage of theoretical, the engine will become prone to slip with very high short-period acceleration at the periods of highest applied force through the main pins.
British Caprotti famously boasted that it offered precise events down to somewhere in the 3-5% cutoff range. This of course isn't a measure of economy, it's a measure of how precise the admission and cutoff can be timed when the gear is set up (and the example on Duke of Gloucester 71000 is, in fact, that precise). You would never use cutoff that short - the engine would stall. Note that at high speed, the steam mass flow requirement turns around at some point and then increases to where the exhaust characteristics balance practical admission flow -- on Mallard this was, famously, about 40% at 'record' speed, and you can almost take that neighborhood of cutoff setting as the one you'll observe producing practical top speed during testing.
There are a couple of meanings for 'company notch' depending on who's spewing the tale: the one I remember hearing from old Railroad Magazine stories was the 'down in the corner' position on a lever reverser, where the engine would be working steam like crazy but producing maximum torque to haul the longest train up the steepest grade at what would eventually be slow drag speed but never quite a stall. The 'college boy' version of what the company notch was is the reverser setting that provides the most efficient running (best water rate, least smoke, or whatever) and presumably saves the company money -- this is for example what a properly-set-up Valve Pilot device would show and then help maintain when running.
Thanks, timz. I have laboured, perhaps in error, under the impression that the figure of 15% was available in valve gear design because it was probably going to be useful at least some of the time, and that low it would have been near the 4 cycles per second (?) of the piston where the most horsepower is being produced on most steamers. But now that I think of it, there was reference to the 'company notch or position" for cut-off that the engineers were encouraged to use when up to speed, so what you say about the UP's policy makes sense.
selectorWhy do many steamers have cut-off capability all the way down to 15%?
(Edit: it was a 1951 Oregon Div Spec Instr, and it said not to work them at less than 33%, to avoid "hot main pins". Page 17 of
http://wx4.org/to/foam/maps/1-Ogle/1951-02-01UP_Oregon_SI_10_OCR-Ogle.pdf
Haven't seen that on any other UP division; lots of other UP Spec Instr on that site.)
DreyfusshudsonIt is not possible for 5399 to have averaged 100mph for 25 miles on end as claimed, nor does the time course data support this claim.
I'm way out of my depth, but I do have a question that we may get around to discussing, perhaps needing an entirely separate thread...but bear with me for one moment, please:
You mentioned that the cut-off for the K4 would be near 40%. This sounds high to me for a steamer at full throat near 70 mph. Is that figure realistic? Why do many steamers have cut-off capability all the way down to 15%? What effect would a mid-range cut-off impose on a steamer attempting to get up to track speed with 700-1200 tons behind it, assuming good traction, steaming, and about 4000 hp working for it?
You folk have been very helpful to me in the past, and I’m posting this in the belief that you will have some further useful commentary. Its aim is to throw some light on the development trajectory of latter day US steam.
I was recently sent copies of the PRR test reports on the Franklin valve K4, the T1, and Q2- something of a holy grail for me finding these. There are also snippets of information on the tests of the K5, which I see as the missing link in the PRR chain.
The particular interest for me was whether the supposed benefits of the Franklin poppet valves were real. I have written a report on this, and what is below is a rather lengthy summary of what I discovered. The bottom line is that much of the hype about the poppet valves is unjustified. They did change performance characteristics, but, at that time, the leakage problems that had dogged such designs since they were introduced in the 19thcentury were not solved, and they provided no advance in engine efficiency. There were in addition well known mechanical issues on the T1s.
In addition to the test reports themselves, my additional sources are primarily a random selection of ‘Keystone’ articles, and several from Railway Mechanical Engineer, plus articles by distinguished US authors in the post steam era.. But some of you may have access to other primary sources.
Please do not interpret this as in any way a criticism of the T1 new build project. This is an amazing effort which deserves everyone’s full support.
I would be happy to forward the full report to anyone interested, though fear its primary use would be as a cure for insomnia.
Summary
· This paper analyses plant and road test data in Altoona Bulletins on the PRR K4 and Duplex T1 classes with Franklin A poppet valves, to see what benefits the valves gave. Comparators are the original K4, a 1930s modified K4 and a Walschaerts Duplex, the Q2.
· The context of this work was the belief in the late 1930s that US passenger trains of the future would need to run at 100mph, and load to 1000 US tons.
· A 1930s piston valve K4 was tested on a 1000 ton test train for the AAR. It developed about 2800IHP at 75-80mph in ca 40% cut off, a steam rate in the high 40000s lbs/hr. It could not reach 100mph on the relatively flat PRR main. It was noted that the steam rate achieved was some way short of the 65000lbs/hr achieved on the test plant, and that this could not be achieved without use of excessively long, inefficient cut offs.
· The resistance of the coaching stock was established on these tests, which showed that over 3000EDHP would be needed to move a 1000 ton train at 100mph on the level. With 2000HP for the locomotive, plus acceleration needs, this gave target cylinder power around 6000IHP.
· It was believed that poppet valves were to be preferred for high speed running and would use steam more efficiently in part because more is admitted at a given cut off, allowing more economical cut offs to be used. This also helps to deliver the enormous power required.
· Lentz poppet valves, initially with oscillating and then rotary cams had been tested in the UK, with Gresley taking the lead. Oscillating cams were unsuccessful, rotary cams on the D49 showed more promise, but showed no efficiency savings, and were quickly removed from P2 2001. Leakage was the suspected problem. The D49s would likely have been rebuilt had other priorities including WW2 not intervened.
· In the US, the Franklin valve company visited Lentz in London, and produced their own oscillating cam version, the Franklin A valve. The Franklin Valve company fitted K4 5399 with these valves and improved steam circuit, and it was road tested in a similar fashion to the standard K4. It produced about 3400IHP, and achieved speeds of 85-90mph. Later plant tests imply the steam rate was 55000+lbs/hr, much higher than the original K4, with cut offs again approaching 40%. That is, the higher power was mostly due to the higher steam flow the poppet valves allowed, in turn allowing boiler to be steamed harder.
· The increased performance of 5399 meant it was sufficiently powerful to eliminate wasteful double heading of K4s on premier trains. So although the boiler would be operating inefficiently at the high steam rates required, it was likely overall much more economical.
· Before being tested on the Altoona plant, 5399 was then fitted with a new sine wave superheater, which eliminated much of the serious pressure drop to the steam chest on earlier versions, and a much wider exhaust.
· It proved possible to steam this machine at 77000lbs/hr (1100lbs steam/ sqft grate/hr). It showed marked improvements in engine efficiency over the Standard K4 at a steam rate of 70000lbs/hr, with particularly marked advantages at high speed. This was taken as a resounding endorsement of the suitability of Franklin valves for a future high speed design, and was a critical factor in the decision to fit the duplex T1 with Franklin A valves.
· There was clearly a great sales push by the Franklin Valve company to recoup their development effort, but it reads like the PRR’s desire to gain a breakthrough technological lead made them more than willing partners.
· However, as a number of contemporary observers had suggested, analysis of the test plant results with modern techniques says:
o The increase in engine efficiency on the test plant over the Standard K4 was entirely due to higher steam chest pressure and wider exhaust, not the Franklin valves.
o The higher steam flow at a given cut off allowed by the Franklin valves ought to have given a further significant improvement in efficiency by reducing the very long cut offs needed to work at high steam rates. This does not appear, and analysis of the data says that, as with other poppet valve set ups of that era, steam leakage was occurring which negated any benefit that the lower cut off gave.
o The enhanced efficiency benefit of 5399 at very high speed was again not due to the poppet valves, but rather that 5399 was operating at much lower back pressure, thus lowering the over compression that occurred at high speeds on the original K4.
o In any event, 70000lbs/hr is quite impracticable in service from a 69 sqft grate.
o By inference, the superior performance of 5399 in road testing was almost entirely due to the fact that the greater steam flow at a given cut off given by poppet valves allowed 5399 to be steamed at higher rates than the standard K4. A similar result could have been achieved by raising the boiler pressure to 250 psi, and indeed Altoona testing of a 250psi K5 Pacific showed it had superior efficiency to all K4s tested, but changes to the boiler design on the K5 made it a temperamental beast.
o It is not possible for 5399 to have averaged 100mph for 25 miles on end as claimed, nor does the time course data support this claim.
· Based on the contemporary interpretation of the 5399 results, two prototypes of a futuristic high power high speed Passenger locomotive, the T1, were built, incorporating both Franklin A valves and also a four cylinder Duplex drive. Both features would enhance the flow of steam at a given cut off, hence increase power at that setting. Driven in the same cut off as a K4, a T1 would produce nearly twice the power at speed.
· Prototype 6110 was tested on the Altoona test plant. It proved able to meet the 6000+IHP at 100mph brief, although the boiler performance was deteriorating quickly at the high steam rates needed, with low efficiency and high levels of black smoke. Maximum service IHP would likely be have been no more than 5000-5500IHP.
· The high steam rates required needed a narrow blastpipe, acknowledged to be sub optimal.
· Even at 5000-5500IHP, and with a target load now reduced to 880 US tons, a T1 could have comfortably averaged 100mph over long stretches of the PRR Crestline to Chicago main, had the necessary investments in infrastructure been made. (They never were).
· At a more realistic 4000-4500IHP, major schedule improvements with plenty of 100mph running would still have been possible.
· Its higher pressure, superheat and large grate meant the T1 was far more economical in coal consumption than double headed K4s, hence a major step forward. Note however Altoona felt the need to test 6110 with reduced superheater area, reducing efficiency, because superheat was ‘somewhat higher than is desirable from a maintenance perspective’.
· The massively high powers achieved by 6110 were far higher than anything needed on PRR schedules of that era. The Altoona report acknowledges this in as much as it was recognised that at the very low cut offs needed in service at full boiler pressure (due to valve design and four cylinders) ‘the action of the valve was not entirely satisfactory’. For this reason, tests were done at 150-170 psi steam chest pressure to establish what was needed in practice, this negating the efficiency benefit of 300 psi boiler pressure. At these lower in service steam rates, too high superheat and back pressure would not have been problems.
· As with 5399, analysis of the results says that the engine was not as efficient as it ought to have been. Using test results on the Walschaerts Q2 and K4 as a comparator, it can be shown that the Franklin valves on the T1 were also leaking to a degree which significantly reduced overall engine efficiency, negating the benefit of shorter cut off, so contrary to even recent commentary, did not provide an efficiency benefit, even at full boiler pressure. This coupled with the fact that four valves and cylinders will inevitably leak more than two means that the T1 was likely no more efficient than a PV 4-8-4 working in longer cut off, and under typical service conditions, quite possibly worse.
· The combination of two radically new technologies, Duplex drive and Franklin valves led to a number of operational issues with the 50 T1s purchased on the back of the 6110 results. Development work was done to eliminate these problems, including a newly designed rotary cam Franklin B valve, but the T1s were made redundant before they were even built by the purchase of diesels, so it was all irrelevant. The leakage problem was not recognised, and whether it was a tractable one is not known, nor whether Franklin B solved this problem.
· Overall, one can say that the misinterpretation of results on 5399 meant the speed and economy logic for poppet valves on the T1 was quite spurious. Further, leakage of the poppet valves, which paralleled earlier UK experience with both oscillating and rotary cam valves, had not been solved at that time.
· More fundamentally, one may speculate that the creation of 5399 was a consequence of the failure of the K5, which could have matched 5399. This failure has been linked to deficiencies in its boiler design, consequent on the firmly held belief that, since greater evaporative capacity was needed the evaporative surface area of the K4 boiler had to be increased. Modern analysis says this is not so; heat transfer does not limit evaporation and power.
· The British persisted with poppet valves, and a class of about 30 4-6-0s was built with valves to a modified Caprotti design in the 1950s. They worked in service for a decade without adverse comment, the right kind of compliment! A Pacific with the same set up showed good efficiency when tested. It was suggested that the modified Caprotti valves might seat better (i.e. be more steam tight) than the Lentz arrangement because their weight is not carried on the valve spindle.
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