As a further 'aside' -- something I'm sure was tried at some point, somewhere, on a locomotive equipped with Valve Pilot was the further development of the servo coupling with road speed to keep the locomotive 'on the edge' of permissible road speed in servo while maintaining the 'best' efficiency. This requires only providing a kind of 'aperture priority' for the speed-recorder needle while coordinating the cutoff via its cam arrangement. I believe there were Flaman arrangements in France that performed this, as the French much more highly valued 'getting to the speed limit fast and then staying there without even going a km/h over' for their own political reasons than any American road would (prior to 1950/51, by which time no one really cared about speed control of reciprocating locomotives at high speed any more)
gregci believe TE is directly proportional to cylinder pressure. with a pressure regulating throttle, an engineer could presumably keep the TE within that range and avoid wheel slip by setting the throttle to a specific cylinder pressure.
This is true, except
1) the concern is not with boiler pressure, or steamline pressure, but MEP and peak pressure in the cylinder under given conditions, and
2) no throttling (except with modern methods -- they exist, but are expensive and may be 'tetchy' to calibrate and maintain --) will do this with throttled admission steam, particularly over the wide range of demand that in an engine without superheater dampering will lead to dramatically high superheat at high cyclic.
There are infinitely better methods of adjusting TE (or more precisely, wheelrim peak torque) to just 'short' of actual slip under any experienced road conditions. One such method is to use a laterally-acting air-over-hydraulic caliper on the driver wheelrims, in a method analogous to braked traction control on automobiles. This has the added advantage that relatively little energy of actual expansion in the steam is 'wasted' through friction (as it is in a comparable arrangement using a high-speed IC engine through a transmission) or heat loss due to delayed expansion. On the replica T1 5550 the same functionality is achieved with fast-acting mechanisms in the intermediate rods of the independent driver brake.
Meanwhile, if you have not had the chance to watch one of the UP steam engines up close, you would benefit from observing testing of the power reverse, more particularly the speed with which the gear on a locomotive like 3985 can be transitioned from full forward to full reverse. It would take comparatively little instrumentation to control this to any arbitrary position in its range, e.g. with LVDTs, and then to develop control valving that would settle it at that position with minimal shock or overshoot. This allows you to dynamically control admission into the cylinder with far more precision than any throttle far upstream of the admission ports could possible produce, and tailor MEP over a given range of piston excursion (which is much more important than just "MEP" over a whole stroke, for reasons I think you'll quickly see) with any desired real-world precision.
rrinkerHmm, I need to upload the video I took when I visited the Shreveport Water Works museum,
In the meantime, the Kempton Park triple expansion Engine should do. Fascinating to watch.
Regards, Ed
Overmodirst you have to provide a reason why 'maintaining the desired steam pressure set by the engineer' is important.
gregctractive effort (TE) needs to be sufficiently high to exceed train resistance and grades yet be less than MAX TE above which slip occurs. i believe TE is directly proportional to cylinder pressure. with a pressure regulating throttle, an engineer could presumably keep the TE within that range and avoid wheel slip by setting the throttle to a specific cylinder pressure.
i believe TE is directly proportional to cylinder pressure.
with a pressure regulating throttle, an engineer could presumably keep the TE within that range and avoid wheel slip by setting the throttle to a specific cylinder pressure.
greg - Philadelphia & Reading / Reading
gregci believe the answer is no?
The question isn't adequately stated. First you have to provide a reason why 'maintaining the desired steam pressure set by the engineer' is important. Then review the autonomic cutoff control system of the early '20s to see if that qualifies by your definition (it used back pressure as the control signal). Note that it is both easier and cleaner to implement this as a function of cutoff than one of throttle opening, particularly as all modern 'wisdom' involves getting the throttle as open as possible, as quickly as possible, and then doing speed control with maximum thermodynamic efficiency on the valve drive.
Keep in mind also that there were engines, some quite sophisticated in thermodynamic design, that featured two throttles, one in the dome and one at the header. Among the stated benefits was reduction of priming effects (and carryover) by limiting the steam mass flow (via nominal pressure reduction at a given demand, partially at the 'expense' of water rate) while keeping the throttle action at the front end proportional. This seems to me to be precisely what you're calling for a throttle design to do, the one difference being that it isn't 'load-following' like a version of cruise control designed to 'follow' only steam throttle pressure. (Let me mention that there are unintended consequence issues when you set a locomotive up this way, particularly requiring that you install pop safeties on the superheater header as the elements can now act as a 'separately fired pressure vessel' when both throttles are closed or nearly so, even in the presence of recirculating arrangements of the kind that made superheater dampers obsolete on large American power)
gregcwas there ever a throttle that acutally regulated steam pressure, that is, only allow steam flow to maintain some desired steam pressure set by the engineer
i belive the answer is no?
The problem with 'centrifugal governors' on reciprocating steam locomotives is that they have to be spinning at a reasonable rpm to have any useful sensitivity, and require careful gearing under driving torque to be reasonably precise and free from 'porpoising' control effects. Note that the effect needs to be like that of a good PID control scheme, which I won't lecture you about because that's one of your specialties. You would need to pair a centrifugal arrangement with something sensibly like a Wagner fluidic throttle (see patent, 1912) or with a trip-mechanism valve gear (like Corliss) to be able to use it meaningfully in the service you propose.
Keep in mind that any modern locomotive uses a severe amount of fixed cutoff, and absent either a 'pilot injection' function in, say, Franklin or British Caprotti poppet valves or the presence of a good riding cutoff, you're limited to some form of starting port (which is either dimensioned proportional to throttle opening or given separate valves like a good drifting valve) for reasonable starting and slow-speed action.
Multiple-poppet throttles are not well adapted for slow-speed control except insofar as they open sequentially to their reasonably clean flow state -- this tending to 'quantize' the steam admission, and throwing the engineman back on careful winding of the reverse and not falling for the old wives' panacea of getting the throttle open ASAP and winding up the reverse to suit to manage slow speed. A good air or steam actuator could (theoretically) do this, but it would require at least the sort of precision involved in 'correct' use of the Q2 anti-slip computer ... something that was never realized in practice, where the actuation was unsuitably 'bang-bang' at too long an interval, and stiction impossibly present as built. Keep in mind that in many cases, the effect is masked by long steam passages that tend to 'average out' opening and closing transitions to an extent, but the quantized flow remains a concern during real-world starting.
Note that the 'correct' answer to the question you asked is not in the 'dome' or header) throttle at all, which meters the amount of mass flow, but in the four little Wagner throttle arrangement Porta proposed for the LP engine of the ACE3000. Here a good proportional actuator would control the fluidics in full servo, which the Wagner throttles were well understood to provide, and the flow of steam empirically adjusted to provide good torque (in part through the starting 'Weiss' ports)
The Franklin Precision throttle on the T1s has the apparent drawback that it is not proportional in closing to the way it opens, and would need to be slightly redesigned to be precise in both directions over its range of actuation. Of course the T1 suffers fundamentally from having only one front-end throttle for both engines, with no practical location to fit double throttles (and actuators) and with its short stroke would continue to be severely hampered in starting if 'modulated' to avoid low-speed slipping. (The 'correct' answer is to fit the four Wagner throttles above, and operate them as 'trim' to adjust starting torque slightly to match the characteristics (and break any resonance effects) of the 'other' engine (by convention almost certainly the rear one on a typically proportioned duplex). While this does introduce slightly greater wiredrawing loss, the existing ports and passages of Franklin type A gear already pose much greater relative effect, which is factored into the low-speed efficiency of the poppet valve system and is likely of only minor effect on the high-speed mass flow (compared to other systems of valves and passage design).
centrifugal regulators
Centrifugal governors were invented by Christiaan Huygensand used to regulate the distance and pressure between millstones in windmills in the 17th century.[1][2] In 1788, James Watt adapted one to control his steam engine where it regulates the admission of steam into the cylinder(s)[3], a development that proved so important he is sometimes called the inventor. Centrifugal governors' widest use was on steam engines during the Steam Age in the 19th century. They are also found on stationary internal combustion engines and variously fueled turbines, and in some modern striking clocks.
I don;t think they usually botheres on stationary engines either. They did implement some of the more complex types of valve gear which were far more efficient than the typical types used on steam locos (and a whole lot less durable, and more complex mechanically, which is why they were at best experimetns on steam locos). In a stationary setting, the issue of the wear and tear of the constantly changing load on the loco wasn't as critical, and access for maintenance was also not much of an issue. Hmm, I need to upload the video I took when I visited the Shreveport Water Works museum, it actually provided water to the city of Shreveport with steam power up into the 1980's. Now it's a museum anad you can walk around the machinery, and they have one they rigged to turn with an electric motor so you can see all the motion of the pistons and valve gear.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
something that can automatically regulate (maintain) pressure would be one less (complicated) thing for an engineer to deal with ... even more so if boiler pressure varies.
i think it would be comparable on diesels to selecting the horsepower.
even for stationary engines, the desired pressure (force) would vary with load in order to maintain speed. For a stationary engine you want a speed regulating throttle.
I wonder how really practical a pressure regulating throttle would be? After all the object is not to regulat the pressure in the cylinders, the object is to match the speed of the train to operation. The speed of the train is very dependent on a multitude of "external" factors. In addition the boiler pressure probably varies as the water temperature changes and the steam is used.
Would the engineer have to keep adjusting the pressure regulator so much that it would make the concept really not viable?
I could see it for a stationary engine.
Dave H. Painted side goes up. My website : wnbranch.com
tractive effort (TE) needs to be sufficiently high to exceed train resistance and grades yet be less than MAX TE above which slip occurs.
gmpullmanTheoretically, the delivery pipe pressure downstream of the throttle could drop immediately when encountering wheel slip. The engineer needs to react just as quickly in closing the throttle. ... Any automatic device would probably have to be bypassed, adding even more complexity to the steam delivery system, in order to handle unusual events as slippery rail or other train handling anomalies.
if slip should occur because there's a particularly slick section of track, i agree that cylinder pressure would rapidly drop as the number of cylinder/exhaust cycles spikes. The engineer may need to reduce the pressure setting until the wheel get past the slick track, again, keeping it high enough to exceed train resistance.
presumably a pressure regulating throttle would reduce the occurrance of slip and automatically increase the flow of steam as the train speed and cylinder cycles increase. Of course train resistance increases with speed and the engineer may need to increase the pressure setting.
Your query brings up some interesting points on throttle development.
The throttle, in simplest terms is a regulator, where in England the term is even used to describe the throttle. Remove the hand-operated linkage and replace it with a piston operated valve and you have the automatic regulator.
As Selector points out the common location for the double-seat throttle was in the steam dome. With development of the superheater it became evident that the steam regulation would be better controlled if it were closer to the point of use (cylinders) and rather than regulating the wet steam into the superheater it was preferred to regulate the steam after the superheater.
Railroad_S_Damper by Edmund, on Flickr
Early "front-end" throttles were single-seated and had small pilot valves to assist with the initial opening of the valve. These throttles also had a balancing piston. Engineers generally did not like the "tempermental" operation of these throttles as they eliminated the "feel" of the control, especially while attempting to adjust the steam flow while drifting.
Front end throttles required longer linkage and compensating levers to accommodate expansion of the boiler during temperature fluctuations. Some had the linkage under the jacketing, others exposed to the elements.
throttle_link by Edmund, on Flickr
Later developments brought about the "multiple front-end throttle" using poppet valves and an operating cam. These still used a small pilot valve to initially admit steam to the steam chest and cylinders to balance the pressure before admitting more steam as the locomotive began to move. Engineers began to get a "feel" for these newer types of throttles. They required a little more "finesse" and a better understanding of the operating characteristics than a plain-seated throttle would. Anything that reduces the tactile feel of control between the power supply and the resulting application to the wheels is going to add a layer of uncertainty to the operator.
Throttle by Edmund, on Flickr
This throttle became part of the superheater header so that the entire unit could be removed for maintenance or replacement. I've heard of several instances of the poppets sticking open. Perhaps getting proper lubrication to the valve guides contributed to this.
I'll have to do a little more digging as I seem to recall the Pennsy using a Franklin Precision air throttle front end throttle on the T1s which might be a design that comes close to your pressure-regulator question.
I recently had a conversation with a gentleman concerning steam throttles and "automatic" control in the present-day. From what I gather there is increasing concern over operating steam locomotives under new regulations for Positive Train Control. One of the requirements of PTC, besides penalty brake applications, is a requirement that the throttle has to be shut or otherwise disengaged from the drive system. Not so difficult on a diesel-electric but to retro-fit a steam locomotive with an automatic closing throttle (and/or reverser) may prove difficult.
Interesting. —
{edit} In mulling over automatic pressure control of steam delivery I also wonder what effect wheel slippage would have in regards to the design constraints of such an arrangement. Theoretically, the delivery pipe pressure downstream of the throttle could drop immediately when encountering wheel slip. The engineer needs to react just as quickly in closing the throttle. Cases were reported while testing the PRR's Q1 where the train had stalled due to difficulties in attempting to stop wheel slipping of the rear engine. Any automatic device would probably have to be bypassed, adding even more complexity to the steam delivery system, in order to handle unusual events as slippery rail or other train handling anomalies.
Good Luck, Ed
Yes, throttles are often cracked open, just enough to lift the valve off its seat. This is done to get couplers to close, to move short distances/at slow speeds with limited tonnages. Out on the road, pulling normally, the throttles are left wide open in most instances and only the cut-off limits the admission of steam. Remember, 'putting the bar in the corner' means shoving the reverser all the way forward...or bringing it all the way back. That is where yard work and 'straining' moves are done. You want longer admission, but controlled by limiting how much via the cracked throttle. In videos, you'll see the engineer jerk several times on the throttle to reverse onto a string of cars for coupling, but he never pulls the throttle wide open when doing that.
Also, your first statement, about the valve in the steam dome, was by no means universal for steam. In fact, pretty much all steamers erected after 1910-ish (don't have the right dates in my head) had what are known as 'front end throttle's, meaning they were in the smoke box, forward of the flue sheet, and directly linked to the super-heater. Meant long linkages. If you look at the fold-out of the Big Boy in last months' magazine special edition, you will see a representation of the long linkage all the way forward.
my understanding is that "steam regulator" or throttle is simply a valve that allows steam flow from the steam dome into the pipes leading to the cylinders. It neither regulates (maintain constant) flow nor pressure.
was there ever a throttle that acutally regulated steam pressure, that is, only allow steam flow to maintain some desired steam pressure set by the engineer
it seems tricky to start a locomotive (especially with a lot of tonnage) w/o slipping. The throttle needs to be opened to build up steam pressure, but must be limited once that pressure is sufficent to produce enough tractive effort to start moving the train to avoid exceeding max TE and slipping.
Once the train beings moving, the throttle needs to be opened more to match the consumption of steam by the cylinders to maintain cylinder pressure and TE. The problem at low speeds is that after cutoff and before the end of the cylinder cycle, there is a longish period of time where steam pressure can build.
at very slow speeds (wheel rotation taking many seconds) it seems that the throttle needs to be backed off when cutoff occurs until the cycle completes
an adjustable steam pressure regulator seems likes an obvious solution.