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.
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?
Depends on what you mean by “capability”. At midgear the cutoff is … maybe 1-2%? So some reverse-lever setting will produce 15% cutoff – but the engine likely won’t run smoothly. As I recall UP Spec Instr said (at some time or other) not to run their 4-8-4s at less than 35%.
(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
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.
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 w
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.
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.
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
You are correct on the lack of training. After reading seveal articals in the Keystone Mag. on T1, this consensus came out. Unlike how management views training of new equipment (think flight simulators for a Boeing 777), back then you learned on the job and because you can run a K4 you go run a T1. Some Engineers learned as they went and where very good handeling a T1, while other’s just run them hard and the rest be darned.
There is a post in Classic Trains forum “PRR Duplexes and Experimental Engines Discussion ( S1, S2, T1 etc.)” http://cs.trains.com/ctr/f/3/t/271182.aspx discussing different experimental steam engines, including Pennsy’s duplexes. Please feel free to check it out if you like. Your input and participation are always welcomed! Thank you for your attention.
This actually means something a bit different, which is well-established wisdom in some parts of the world like Australia. That it’s wisdom developed after the recognition of low- and high-speed slipping (and adjustment of things like FA and equalization to overcome the problem) on T1 locomotives should tell you there’s more to it.
Typical “thermodynamic expert” revealed wisdom is to get the throttle fully open as fast as possible and then ‘drive on the reverse’ – this produces theoretical minimum throttling loss/wiredrawing in the admission steam and micrometric cutoff control of developed power. The ‘advocated’ strategy is saying just the opposite: keep the reverse ‘down in the corner’ in the desired direction for a while, and drive on the throttle (which is the thing I think you’re missing in the criticism here).
The problem with early advance of the cutoff is the problem you note: much peakier torque (with conventional valve cutoff methods) resulting in more propensity to slip under any given adhesion conditions at a given developed horsepower (or average demanded torque, to use a different physical metric). Keeping the cutoff ‘long’ does two things: it keeps the average torque more normalized over the stroke, and it provides a vast sink of steam (at a given throttle opening) so that any slip once it starts to propagate becomes quickly self-limiting (in the same sort of way that the LP of a Mallet is self-limiting).
Problem #1 with Franklin-modified Lentz gear is tha
Jones 1945 -thanks for the tip off- a great set of postings that it will take me some time to digest. I think there are a couple of points i would like to add, but need time to engage brain fully on these rather different topics.
The data I have are times- to the nearest minute only-at Vandale, Warsaw, Plymouth (these two the stretch in question), with an overall elapsed time of 91 minutes for the 118 miles from Fort Wayne to Liverpool- an exceptional effort after a slow start to Vandale.
The first point is that you cannot make any meaningful claim to average speed when times are only to the nearest minute- there could easily be an error of a minute between two points, which has a big impact on the average speed.
The second point is that if you work out the power needed to achieve what was claimed, it works out at about 4500 cylinder horsepower for a quarter of an hour. This is way in excess of what they were able to achieve on the test plant with a much improved version of 5399- and then only for a short period. Steam rate would be in excess of 80000lbs/hr. So, we are in dreamland.
The log, such as it is, analyses more sensibly if you assume maximum power was around 3500HP- as on the earlier tests with the 1000ton test train when they were trying to push the loco to its limits. The average speed then works out at about 91mph on this stretch- remember it is ever so slightly up hill, so this is a great effort. It could be a bit better than this, but nowhere near 100mph.
I suspect a bit of judicious rounding up of the Warsaw and Plymouth times was made to support the claim. Franklin were clearly in ‘sales’ mode and this was great publicity. As I read the papers, the PRR folk were equally keen to believe and support the claim. Times had been very tough and they needed something to cheer.
I agree that, for the reasons you give, full gear and part regulator is the way to start.
My only point was ( apologies if I wasn’t clear) that if you drove a T1 in this fashion, in the way you would drive a K4, the wheel rim horsepower on an individual wheel would be much greater than on a K4. The load on the drivers is not that different, so other things being equal (they are not) you might expect more slippage. Also, because of the different characteristics of the valve gear, and the presence of four cylinders, you would also get more HP at each wheelrim than on a Niagara driven in identical fashion.
As to whether Duplexes, by their nature, slip more, I am quite unable to comment
I remember a Trains “Second Section” piece about a cab ride on the 20th Century Limited, where the locomotive got into a high speed slip and the correspondent was “guest engineer”. The correspondents first impulse was to back off on the throttle, but the engineer said “no” because it would cause slack action. The engineer cured the slip by spinning valve gear wheel to lengthen cut-off. One explanation was this would soften the impulses and thus relieve the slip.
You are welcome, Dreyfusshudson. Take your time, It’s not urgent. [tup]
I am a user of Trainz simulator, I understand that it is not a professional mechanical or automobile engineering program but it is the most realistic simulator available for trains simulation. I believe many of our forum members using this software like me. One of my projects is to simulate PRR S1 6-4-4-6 #6100 by using all actual figures of the real engines specification I gathered from books, forums, and internet.
Starting the engine (*1460 tons passenger stock behind ) with a maximum cut-off at full throttle caused the engine wheel slipping endlessly and stuck in the same spot. The only way to get the train moving forward is to adjust the cut-off to 26%-32%, full throttle, the S1 (in the computer) will start moving gradually and reach 60mph by increasing the cut-off from 26% to maximum cut-off. The engine can reach 105mph or above after 12-15 mins on level track by gradually decreasing the cut-off to 41% or less, depending on the gradient of the track.
In the realistic mode which allows derailment, my S1 derailed at every single sharp curve [(-D] and it is not easy to restart the train once she stopped in random places; sometimes I have to spend 5 mins to get her moving forward again by adjusting the throttle and % of cut-off.
