as far as i know, a steam engine throttle is nothing more than a valve controlling the size of an opening. it doesn't regulate the pressure (i.e. limit the pressure). it controls the pressure only to the extent that it limits the flow and depends a lot on the consumption of steam by the cylinders
reducing cutoff more efficiently uses steam by allowing the lesser amount of steam to expand. in other words, when cutoff is reduced by half, there may only be a ~~20% reduction in power from the steam.
(the other advantage of decreasing cutoff is reducing the consumption of steam which means conserving water, fuel and manpower).
my questions are:
does a reduction in cutoff without any change to the throttle result in an increase in steam chest pressure (since less steam is being drawn into the cylinders)?
can reducing cutoff w/o a throttle change result in an increase in power (i.e. tractive effort) due to higher steam chest pressure?
and does this mean that if cutoff were reduced should there be a corresponding reduction in the throttle to limit to the increase in steam chest pressure to maintain a desired speed?
greg - Philadelphia & Reading / Reading
At what throttle setting? If severely pinched, just cracked, your pressure swings would be severe with each admittance event. If substantially wider, there'd be less amplitude swings because more refill flow would be pushing in behind the steam being admitted.
I don't have a *great* understanding of this (still learning -- see below) so forgive me if I'm telling you something you know. The issue, as I understand it, is not just that early cutoff makes more efficient use of the expansion of steam, but that at higher piston speeds admitting steam for too much of the travel becomes counterproductive.
I understand it to be a bit like the transmission in your car. From a snippet I read (one of the Stauffer NYC books), high throttle at advanced cutoff can be really rough on the equiment. Like trying to start your car from rest in 4th gear. (I believe NYC had reverser positions indicated on the speedometer.)
If no one here can answer your questions, can I suggest this book:
https://www.amazon.com/Steam-Locomotives-Really-Popular-Science/dp/0198607822/ref=sr_1_1
I'm only part-way through it, and it's an excellent book with a lot of information to absorb for into my little pea-brain... which is why I can't answer your question. :)
Aaron
Short cutoff doesn't generate more power. The more live steam, the greater the power. Short cutoff is more efficient, as it uses more of the energy of the steam, thus using less steam, less water, and less fuel. But power is actually lower. To continue the car analogy, it takes power to get the car moving to speed, but once at a speed, it takes less power to maintain that.
It's also the reason for many compound locos having starting vales that ran both sets of cylinders on live steam, instead of using live steam in the high pressure cylinders and then extracting the still abundant energy of that exhaust steam in the low pressure cylinders. It's much more efficient to use as much enrgy out of a given volume of steam as possible, but you get a lot more power by using more live steam as opposed to extracting more energy by allowing more expansion.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregc as far as i know, a steam engine throttle is nothing more than a valve controlling the size of an opening. it doesn't regulate the pressure (i.e. limit the pressure). it controls the pressure only to the extent that it limits the flow and depends a lot on the consumption of steam by the cylinders reducing cutoff more efficiently uses steam by allowing the lesser amount of steam to expand. in other words, when cutoff is reduced by half, there may only be a ~~20% reduction in power from the steam. (the other advantage of decreasing cutoff is reducing the consumption of steam which means conserving water, fuel and manpower). my questions are: does a reduction in cutoff without any change to the throttle result in an increase in steam chest pressure (since less steam is being drawn into the cylinders)? can reducing cutoff w/o a throttle change result in an increase in power (i.e. tractive effort) due to higher steam chest pressure? and does this mean that if cutoff were reduced should there be a corresponding reduction in the throttle to limit to the increase in steam chest pressure to maintain a desired speed?
Steam chest pressure is not going to exceed boiler pressure in any event, steam chest pressure will equalize to boiler pressure quickly at all but the lowest throttle settings.
Until you run out of boiler pressure, than both drop......
Cutoff is simply valve duration timing, similar to the variable cam shaft in your modern car.
Wide open for max power, reduced flow for crusing.
Throttle and cut off need to be within reasonable limits of each other for smooth and safe operation.
Sheldon
Basically the throttle controls the rate of steam flow into the cylinders, while the cutoff controls the duration of the steam flow. So when starting out the cutoff will be wide open to allow steam flow into the cylinders for the entirety of the piston stroke. Once up to speed, the cutoff is reduced to allow steam into the cylinders only for a very short period of time, to maintain speed while minimizing steam use.
Mark P.
Website: http://www.thecbandqinwyoming.comVideos: https://www.youtube.com/user/mabrunton
of course steam chest pressure can't exceed boiler pressure
my question is does steam chest pressure increase when cutoff is reduced because consumption is less?
what would the impact of higher steam chest pressure be on tractive effort
Semmens provide more of an overview and history than provide a complete technical description of steam engines
"A steam loco throttle is nothing more than a valve controling the size of an opening." - Essentially correct. Its a few openings actually. But youve got the idea.
"it controls the pressure only to the extent that it limits the flow and depends a lot on the consumption of steam by the cylinders" - Essentially correct. If the cyclinders are taking more than a wide open throttle/boiler can deliver, yes that pipe down to cyclinders will be lower pressure than a loco wide open throttle that has a boiler that can keep up. (Thus..... the term "Super Power" locomotive came to be. Locos with boilers that could keep up at wide open throttle.)
But know this - the pipe from throttle to cylinders will ALWAYS be lower pressure than the boiler even at wide open throttle, and no matter what the cylinders are doing. If it wasnt lower, steam would never leave the boiler in the first place. Like electricity takes the path of least resistance, steam takes the path of lower pressure.
Only two times is steam a higher pressure than the boiler: 1) when injecting water using an injector (venturi effect), 2) the compression stroke of the main piston.
"reducing cutoff more efficiently uses steam by allowing the lesser amount of steam to expand." - 1/2 correct. The whole idea behind cutoff was efficiency. Steam locos could only travel so far before needing a refuel. It wasnt beneficial for railroads to have to stop a fast express freight then start it again. If they could stretch those stops out, well that = better operation, less fuel costs, less tie ups on the line etc etc. You get the picture. Steam expands no matter what the cutoff is. Thats the nature of steam to begin with. So cutoff is efficiency based.
Think of a garden hose. At one end its attached to your house via a spiggot (a valve like a throttle). At the other end of hose is a sprinkler. And sprinklers have that little adjustment (which is essentially their 'cutoff') that attenuates the time allowed for water to shoot at that little prong that moves the sprinkler head around.
Say that it takes full pressure (wide open throttle) to make your sprinkler work. But at the sprinkler you dont want it flying around like a top because of that much pressure. So you dial down the 'cutoff'.
Its the same principle as "its takes more energy to get something moving than to keep it going". You can use full water pressure to turn the head, but limited cutoff so it only moves so far at a time.
Starting a train = lever in the corner! When you get up to speed you can ease the reverser lever back. Just how far depends on how good your ear is and how worn the seat of your pants are.
"does a reduction in cutoff without any change to the throttle result in an increase in steam chest pressure (since less steam is being drawn into the cylinders)?" - Not in the chest, but the pipe from throttle to valve cylinder in chest will back up slightly.
"can reducing cutoff w/o a throttle change result in an increase in power (i.e. tractive effort) due to higher steam chest pressure?" No, see above.
"and does this mean that if cutoff were reduced should there be a corresponding reduction in the throttle to limit to the increase in steam chest pressure to maintain a desired speed?" - This is strickly an operations based question. As in what is going on at that moment. There are many reasons to open/close a throttle, and the same for the cutoff. those reasons change from moment to moment depending on grade, slack, danger ahead, signal change, etc etc.
Normally, you get to speed under full forward or as such a time in the upward gaining of speed that you sense your loco doesnt need a full amount of steam. If shes in a place where she can hold her own without adjusting the throttle then you ease up on the cutoff till she has just what she needs to keep moving at the desired speed.
"what would the impact of higher steam chest pressure be on tractive effort." - well if the math of your loco is on, and she didnt slip, the TE would probably go up.
But you dont raise chest pressure. You raise boiler pressure and figure valve and main piston size and travel.
PMR
PM RailfanNot in the chest, but the pipe from throttle to valve cylinder in chest will back up slightly.
what does "back up slightly" mean?
my understanding is that the steam chest is the part of the cylinder that the cylinder valve slides in. it's pressure fluctuates as the valve open/closes.
with throttle constant, doesn't the steam chest pressure depend on the amount of steam "consumed" by the cylinders? wouldn't it increase if the cylinders drew less steam because of reduced cutoff?
255
"my understanding is that the steam chest is the part of the cylinder that the cylinder valve slides in. " - The steam chest is the part of the loco that encompasses both cylinder sets, each side.
The upper cylinder is called the 'valve' cylinder and the lower is the 'piston' cylinder.
"it's pressure fluctuates as the valve open/closes." - If i read that right.... the 'piston' cyclinder pressure fluctuates as the 'valve opens and closes'. Im not sure im following you on this sentence.
"with throttle constant, doesn't the steam chest pressure depend on the amount of steam "consumed" by the cylinders?" Well if nothing is changing, then yes you could measure it that way i guess. There is a measurable pressure at constant throttle vs constant rotation of wheels. Slow up the cycle, the pressure goes up. Quicken the cycle, the pressure drops. Thats is, no throttle change.
"wouldn't it increase if the cylinders drew less steam because of reduced cutoff?" Not neccessarily. If you were already using more steam than was passing the throttle, the cutoff would only bring you back up to pressure. That or ease off throttle because your going faster than youve given it throttle to go.
PM Railfan gregc it's pressure fluctuates as the valve open/closes.
gregc it's pressure fluctuates as the valve open/closes.
Part of the thing to remember about the throttle is that it restricts mass flow - pulsating mass flow. That means that the "pressure" effect at part opening is an effective pressure drop at times during the admission. Look up 'wire-drawing' for an older explanation of the effect.
This is related to cutoff, since cutoff also controls effective mass flow. It may help to think of cutoff as metering a given mass of steam (at whatever pressure profile describes its flow) which then expands for the remainder of the contained stroke -- the superheat in the contained steam keeping the water from condensing to liquid phase at effective pressure at any point in that expansion. Incidentally only about .007" of the wall metal cycles temperature along with the steam, which is why superheat has a fighting chance against wall condensation ... and why jacketing and Wardale-style insulation work, by keeping the wall metal just the other side of that .007 at a higher temperature so there is less heat exchange as the steam (which is a pretty good insulator even at these pressures) expands and cools.
"Conventional wisdom" for many years is to reduce any sources of flow restriction or drag in the inlet tract, all the way to the valves -- so the idea is to get the throttle open as wide as possible, as early as possible, and then 'drive on the reverser'. This is lovely from a thermodynamic perspective, but often lousy in real-world locomotives, where anything shorter than about 15% (in older engines, 25% or more) produces little advantage as the torque peaks and condensation losses become severe near the end of the stroke. In true high-pressure engines this becomes more severe as the peak pressure stays high but mep gets lower and lower; even with a relatively high mechanical factor of adhesion, this can produce both low- and high-speed slipping. There was a pretty good discussion of this with respect to the British Caprotti on the Duke of Gloucester, a gear capable of 3-5% cutoff with all the necessary mechanical precision -- the advantage of that precision then being in timing and duration rather than in 'overlap' of admission and cutoff resulting in the analogue of short 'shutter speed'.
In any case, the cutoff at highest speed will not be short, any more than the highest vehicle speed in fast cars is in the highest overdrive. This seems counterintuitive until you realize how demanded horsepower rises with speed. Interestingly, across a very wide range of historical examples, 'fastest' cutoff has been in a very short range, in the low 40% range, with some of the effects of compression control taking on what I consider to be dramatically heightened effect.
Note that there is another wire-drawing effect once you get to the physical valves. One reason long-lap long-travel piston valves remain competitive with 'poppets' as long as they do is that the steam edge opens a comparatively large area to steam comparatively quickly, as the ports can extend around the periphery of the valve except for the bridges, and proper flow-shaping of the volume in the passage 'away' from the axis to the cylinder can assist in early effective flow to the piston head (which is of great importance at high cyclic rpm and short cutoff, where admission is measured in milliseconds). Here no amount of 'reserve capacity' in the steam chest improves admission, but proper compression management does because it can provide reasonably 'hot' steam at admission pressure right down the port to the swept volume at the moment the valve opens to steam, effectively eliminating the effect of dead space in the ports and passages and any great gas-cutting when the valves are still effectively 'shrouded' and would otherwise require high gas speed at small net opening -- not a recipe for avoidance of wire-drawing!
gregcdoes a reduction in cutoff without any change to the throttle result in an increase in steam chest pressure (since less steam is being drawn into the cylinders)?
So shorter cutoff results in 'less steam leaving the valve chest' per stroke, and hence the likelihood that both the average and 'peak' pressure in the chest will be closer to boiler pressure (or effective pressure with the throttle partway shut, which I'll get back to later.) With the throttle open and a properly-designed inlet tract, the steam-chest pressure will be higher in proportion to the lower mass flow going into the cylinders, but ideally this will not matter significantly as there will be adequate boiler supply to keep the chest pressure high at any point in the stroke.
(Incidentally you now see part of why 'underboilered' locomotives run out of 'horsepower' as they run out of the ability to convert water to steam -- they are plenty fast, and plenty economical, until the demanded mass flow exceeds what the boiler can source from its overcritical water (note that this need not concern firing rate yet!) and the pressure begins to fall.
What matters to "engine" power is mep, and this is only incidentally concerned with nominally higher starting pressure. If that pressure falls 'too quickly' across admission, then developed mep will be lower, and that isn't anything you can gauge by nominal chest pressure from an instantaneously-reading gauge or indicator.
Two peripheral observations: This is where the glorious promise of the French ethyl-ether bottoming cycle for compound ship engines comes apart: you might think with that colossal vapor pressure at comparably low temperature it would be a nifty working fluid, but take your rubber bible and see why the pressure disappears nearly as soon as it has to do any practical expansive work...
And if cutoff is short, the excursion of mep that does the expansive work will be across a comparatively short range of piston travel, which is then related to wheelrim torque through the piston rod/crosshead and main rod. Higher initial pressure means a variety of increased machine losses, and perhaps peak torque assisted by the mechanical advantage of the rod throw. This provides greater possibility of slip four times per revolution on a 2-cylinder DA, followed by fairly sharp fall in torque resulting in shock when adhesion is restored ... assuming the slip doesn't propagate around to the next torque peak, which especially with precise and low-mass poppet valves can give you rapidly increasing spin with perhaps no easy way to control it.
Something people easily forget is that a conventional throttle, even a modern multiple-poppet front-end throttle, is not an easy thing to open and close, or to adjust precisely. The situation is worse if you use different throttles on different engines of a locomotive (as some of the naive proposed for the 5550 T1 'betterment' program) unless you have expedient fine-adjustment methods built in; this is not a particularly difficult design exercise, but to my knowledge even the Franklin Precision air servo throttle control did not have the combination of rapid excursion and fine settling precision necessary for good effective throttle control of slip. And the usual cutoff control is an 'average', set by lever or wheel position, and incapable of being worked longer and shorter 'in time' with the engine, let alone in appropriate phase with it.
The 'correct' conventional approach is what is mirrored in the Valve Pilot operation: the chosen cam relates 'road speed' to some ideal or empirical degree of cutoff, and the most efficient operation of the engine is then to 'match needles' on the Valve Pilot's instrument, something relatively easily done.
Now, one of the theoretical things that might be necessary with a T1 experiencing true high-speed slipping is the analogue of the Australian starting method in slippery conditions: if there is a limit to how short the cutoff can be set without torque-peak slip induction, then the throttle might be closed to reduce the admission pressure at peak flow. Of course the 'correct' answer here is precisely what the PRR crews learned to do: sliding-pressure fire the engine so that you trade water rate for slip at starting or when high-speed slip becomes evident; there's no guarantee that attempting to induce wire-drawing without modulation will even help much (other than reduce the power of the engine to the point it no longer slips... which is not that different from just slowing down out of the critical speed range. But that is a more sophisticated combination of effects than your question is concerned with.
"how could you be using more steam than was passing the throttle?" - Your clipping along at say 40mph, which lets say equals 1/2 throttle and your running a mild cutoff. Your pressure is steady, and under load. Everything nominal.
You hit a section of track thats downhill. Youll pick up speed without ever adjusting throttle - gravity + momentum. So now the cylinders are working a pinch faster than they just were on level track.
Pressure will drop from throttle to valve cylinder because cylinders are taking more steam because they are cycling faster. They wont bring it to a vacuum, but more steam (volumetrically) will be passing the throttle than your original amount was on level track.
And only for a second as the first thing youd do going downhill is close the throttle to a pinch (or a bit more) and grab a little train brake. Now, thats little to no steam, but cylinders are cylcing cuz your doing 45mph now - downhill.
Thats one way.
The other way is you arent running a super powered locomotive and your boiler isnt keeping up with the tremendous amount of steam your feeding the cylinders. Which is the whole reason super power locos were designed.
"and i'm suggesting that the mean effective pressure (MEP) can be higher if cutoff is reduced while keeping the throttle constant" - you could say that, yes.
"isn't 'bring you back up in pressure' another way of saying pressure would increase?" - yes. However, "bring you back up to" is meant to infer that instead of pressure just rising, your rising from a lower point than normal instead of starting at normal then raising pressure. But its technically still "rising'.
PM Railfan gregc - "and i'm suggesting that the mean effective pressure (MEP) can be higher if cutoff is reduced while keeping the throttle constant" you could say that, yes.
gregc - "and i'm suggesting that the mean effective pressure (MEP) can be higher if cutoff is reduced while keeping the throttle constant"
you could say that, yes.
sounds like an affirmation of my quesiton
The single biggest difference between IC and external combustion is how the heat makes pressure across the length of the stroke.
In the IC engine, the heat is developed entirely within the closed volume connecting with the cylinder, and the heat rise producing effective pressure occurs after the valves are effectively closed. In fact when pressure-charging an IC engine the purpose is to get higher density of oxygen (and then stoich fuel) into the limited volume for the limited time available, and useful heat is actually (and often somewhat lavishly!) removed from the charge via intercooling. The two issues of peak firing pressure and EGT limit the power output per cylinder far more than any MEP concern.
Meanwhile with external combustion, you 'could' have MEP at essentially throttle pressure (for reasonably low cyclic) all the way to exhaust release and likely relatively high for a bit thereafter merely by keeping the inlet valve open -- in fact there is a weird approximation on limited-cutoff engines by fitting slot ports (Porta calls them 'Weiss' ports because he liked to recognize who he thought were people with key ideas) which are small ports in parallel that open to steam earlier but that can build up high equilibrium pressure given time... which will stay high, and exert "starting TE" type pressure on a piston, as long as the piston speed stays relatively slow.
Of course this also implies that nearly a whole swept volume's worth of high-pressure steam, still with considerable superheat, will blow to exhaust ... and yes, there are momentum effects in the steam just as there are in partial-vacuum ramcharging or tuned ports in IC; some very interesting ones in PRR Q2 testing... and very heavy loads applied by the piston to the motionwork and parts of the suspension and frame will be joined by heroic periodic induced draft if the front end is any good... leading to all sorts of effect from fire throwing to increased nozzle gas cutting, assuming the boiler can source that volume... here indistinguishable from mass-flow because at a reference pressure throughout... without pressure drop in parts of its structure. A version of this could be seen in the pre-planetary-transmission PRR external steam-turbine (6200) if the throttle were opened too briskly while there was still considerable slip in the turbine... I forget the record number of staybolts pulled out, but it was distressing...
Instead of going for Brobdingnagian horsepower ratings, though, we find that metering admission mass precisely gives you that 'volume' of steam that just gets done expanding to the desired exhaust conditions to produce draft in your front end ... this being an average back pressure in the low 20s in the low '20s (see the fascinating experiments into automatic cutoff control servo-based using that as a control signal!) dropping to as low as 5 or so in late steam planning. (Naturally you will need both extended entrainment area and better combustion-gas flow shaping at zone of entrainment to make that low a back-pressure work with the volumes of steam and combustion gas needed at high speed, but we can revisit that later...)
Now we can easily trade throttle pressure against water rate for quite a range within the constraints of EC displacement and stroke (a point that Tuplin never quite got tired of advocating). Quite a bit of the nominal case for high (above about 300psi) throttle pressure evaporates when additional costs involved with boiler construction and maintenance are factored in (and modern water treatment is not). But there have been interesting examples where sliding-pressure firing can produce interesting performance 'at lower required peak horsepower'.
IDEALLY a certain amount of pilot injection technique, both during the admission period and (for reasons like those justifying the N&W 'booster valve) part of following expansion, can be valuable, just as they are for good compounds. There were some interesting discussions in late French practice, some probably at least partially mythical, regarding the best way to optimize steam distribution.
But we now come to the other end of the engine discussion, which involves how to get rid of the mass in the exhaust. IC engines with trunk pistons have little concern in this regard, but large DA engines with SHM or radial valve gear trap a certain amount of steam at exhaust cutoff, which is then compressed (and compression-heated) to what may be a substantial counterpressure, many psi above peak boiler pressure (see some of the Okadee literature for an inkling of just how much). Some of this is recognized as essential in overcoming inertial force on the running gear; as I noted, I consider some of it essential to get good effective mass flow at high cyclic. But if the necessary mass is retained in a larger-bore cylinder with low port-to-passage ratio that attempts to minimize physical 'dead space'... you can get some ridiculously high rates of increase in both pressure and temperature.
Jay Carter (in SACA) handled this with proportional 'overpressure' valving at the cylinder head that meters the surplus into an insulated reservoir space, then uses this to keep pressure up in the early part of the physical return stroke. There is little reason not to apply this to a modern (small-bore, longer-stroke) steam locomotive...
The idea that mass flow would increase as a locomotive starts accelerating downgrade with constant (full) throttle is an interesting thought experiment. Naturally the acceleration in such a case will still 'balance' as in the Davis formula, but now because there is more 'contribution' from the steam since gravity has taken over some of the work. (You could likely figure out just how much if you knew the machine losses for gravity and thrust respectively and integrated them across the profile, but... in practice the engineman would wind back the cutoff to keep the engine from going over timetable speed, and save water, rather than use the 'larger' amount of steam going down the inlet tract thanks to the higher swept volume at the higher achieved speed...)
Now the idea that MEP in the cylinders would increase if gravity is doing some of the work is interesting to contemplate, except that the portion of 'contribution' up to cutoff is proportional to stroke, not number of strokes per unit time, and the amount of expansion in the cylinder is proportional to geometry, not power extracted. So yes, the steam might have higher temperature (by the amount of work 'not extracted' by expansion) and hence the indicated MEP a tad higher... except this is heat and pressure that get promptly blown out the exhaust tract (or in residual form contribute to compression issues) if it is not 'equilibrated' into producing further acceleration. (Note that something artificial eiukd have to be done to make the train in the thought experiment accelerate 'only' at the proportional gravitational rate with the throttle and cutoff unaltered...
A more interesting discussion arises out of how you practically DO handle an engine that 'turns downhill' and you don't want to have accelerate. That gets into the realm of drifting using sniffing air vs. steam, working steam downhill vs. using a special 'drifting throttle' with a pressure regulator (set not coincidentally around 15psi) vs. using a bypass arrangement like Wagner or Nicolai or Trofimov valves.
LastspikemikeWhy would there be a pressure difference on each side of the throttle?
why would a fluid move between two volumes if they are at the same pressure?
of course pressures will equalize in a closed system and as they do, fluid flow will decrease
but while in motion a steam engine is not a closed system. steam is constantly exiting thru the exhaust ports of the cylinder and out the stack.
Part of the equalization involves the steam circuit design. In the years before the ASME 'restored' the locomotive-boiler code, there was some discussion whether a superheater needed its own separate safety-valve arrangement under Part 230 as a 'separately fired pressure vessel' -- the logic being that with good long-lap valves at mid at one end, and a tight throttle at the other, carryover 'trapped' in the elements might build up dangerous pressure. Now this is true of a dome throttle, but a modern front-end multiple is ahead of the superheater elements, so any pressure increase that might occur in them relieves back along the dry pipe to the vast reservoir of overcritical water 'under the dome' so no explosion is possible.
Carryover into the tract between the multiple throttle and the valve chests is certainly possible, but that's much more a bursting hazard in the liquid phase than it could ever be as a 'fired' vessel in the smokebox...
Something else that is fun is to see how much of the effect of very high boiler pressure is wasted with poor front-end design: it's fine to get it into the cylinder faster, but then you have to get it out again after it's volumetrically expanded...
Lastspikemike ... The cut off device is closed more then it is open.
...
The cut off device is closed more then it is open.
Not so. When lifting heavy tonnage, the reverser will be 'in the corner', meaning the valve is slower in action and leaves the inlet port open longer so that full steam can work longer against the piston face. When a steamer is skipping along at limited speed, that is when the valve covers the inlet longer and restricts admission so that the steam has to work longer in the piston's stroke.
i know a steam engine is far more complicated than my simple explanation below which explains the basics
when the throttle is first "cracked", with the locomotive standing still, steam flows thru the throttle raising pressure in one or both cylinders (depends on valve timing). Flow thru the throttle decreases as pressure rises. At some point, there is sufficient pressure to move the cylinder piston.
when the piston moves, cylinder volume increases which impacts the increase in pressure. In an ideal world the product of pressure and volume is constant, PV = C, Pressure decreases as volume increases. This is only true for a closed system (no change in steam mass).
Pressure remains constant if the flow thru the throttle is equal to the rate of change in volume (constant density). Pressure decreases if flow is less and increases if greater than the rate of change in volume.
And any change in pressure will affect the flow thru the throttle.
If the flow is more than enough to accommodate the increase in volume, pressure will increase, applying more force on the piston and increasing the change in volume.
If the throttle is open too wide, pressure will increase rapidly and excessive piston force can result in slip.
as speed increases, there is more consistent steam consumption. The need for greater steam flow necessitates the need for both less restriction between the boiler and steam chest (more open throttle) as well as a greater pressure difference between the boiler and steam chest. (I believe this is analogous to Ohm's Law where pressure is like voltage and steam flow like current).
if there is a need to increase speed, the throttle may be opened as speed and consumption increases. Depending on cutoff, cylinder ports will not be opened all the time and steam chest pressure will be allowed to recover (increase more so) when valves are not opened.
at higher speeds, when less force and acceleration are needed, cutoff can be reduced to both reduce consumption of steam and use it more efficiently (expansion). The reduction in steam consumption allows a higher steam chest pressure, higher starting cylinder pressure and possibly higher mean effective pressure. If this is the case, a reduction in both cutoff and throttle are necessary to maintain an MEP resulting in constant speed.
Of course, cutoff would likely be increased when going up a grade to increase MEP especially at lower speeds where steam consumption is lower and steam chest pressure is not as low as at higher speeds.
LastspikemikeThis thread seemed to me to be about what happens to pressure in the steam chest as the throttle is opened as contrasted to when the cut off is changed.
gregcwhen the throttle is first "cracked", with the locomotive standing still, steam flows thru the throttle raising pressure in one or both cylinders (depends on valve timing).
An American multiple throttle is located at the superheater header, and consists of a series of relatively flow-streamlined poppet valves that open sequentially via cam as the lever is moved. Since even this would be hard to move initially, the first poppet is purposely made very small, so there is less need to 'hog' the throttle open and then slam it most of the way shut for making smooth starts -- I trust you recognize what about a dome throttle makes that practice less critical in producing excess torque. The older Wagner throttle (crca 1912) was an early and very effective form of fluidic amplifier, using a relatively small coaxial control valve to regulate differential steam pressure on the main throttle valve, and inspection of that patent and its claims will go a long way toward explaining to you what happens as steam traverses a partially-open throttle valve into a 'draining' load.
Keep in mind that as you open the throttle, the steam enters a fairly long, fairly cold tract, probably with high speed and associated flow and reflection shocks, and will suffer wall condensation to a higher degree until the tract comes up to reasonable temperature. (I am uncertain the degree to which hot smokebox gas actually heats this tract; it may be considerable, but one notes that all the economizers like the Pielock that attempted to use smokebox-gas heat for steam superheating were more or less functional failures). So you don't have immediate boiler pressure 'through' the throttling orifice, and then again you don't have immediate boiler pressure evident at the valve chest. Further condensation then occurs in the ports and passages, and all this is the reason for cylinder-cock actuation, which of course lowers cylinder pressure while open. A gauge of this is the 0.85 formula allowance for pressure in calculating starting TE.
Now it is true that
Flow thru the throttle decreases as pressure rises.
At some point, there is sufficient pressure to move the cylinder piston. when the piston moves, cylinder volume increases which impacts the increase in pressure.
In a good engine with minimized dead space, only a very small volume exists around the head of the piston. Once this opens even slightly the 'throttling' effect of steam entering this volume drops greatly. Now look back along the passage, valve, and chest toward the throttle: you will comparatively shortly fill up this new volume with additional mass flow, and only the amount of 'steam' that does this will pass the throttle. (Remember that condensation in the newly-opened volume, plus the exhaust through the open cocks, account for some of the flow). Very quickly we can expect the steam pressure against the whole of the head of the piston to reasonably equilibrate (this because superheated steam is a relatively good insulator and there is little nucleate condensation in it yet).
Think for a moment about what happens if the throttling orifice is not open far enough for the 'necessary' mass flow -- the pressure 'available' will not be at full boiler pressure, and may steadily fall below the fixed loss (decreasing as things warm up) until things balance at some lower nominal pressure. Likewise if the throttle is cracked open past the loss rate, the pressure will rise until, again, balance across the throttle orifice is achieved.
In an ideal world the product of pressure and volume is constant, PV = C
Pressure decreases as volume increases. This is only true for a closed system (no change in steam mass).
Pressure remains constant if the flow thru the throttle is equal to the rate of change in volume (constant density)
... If the flow is more than enough to accommodate the increase in volume, pressure will increase, applying more force on the piston and increasing the change in volume.
Now you will see this cut all sorts of fascinating didoes at very high cyclic, but we aren't anywhere near there yet.
So far there is no mystery. Likewise it is evident that pressure and 'mass flow' remain proportional as the effective cylinder volume increases: only the areas of head and piston 'matter' in the pressure calculation, and those do not change with volume (the hoop stress in the cylinder certainly does, but we give few craps about that). So if there is no cutoff, you eventually wind up with a cylinder full of "near boiler pressure" steam (depending as you noted on the percentage of throttle opening vs. losses and volumetric expansion) at the moment of release ... at which point you have a huge mass flow of still high-pressure steam blowing down into what is the effective flow restriction of the exhaust tract. That, like discharging vs. charging a capacitor, is determined only by the flow characteristics of the tract as the steam edge exposes port area, with a few exceptions like jumper caps or variable nozzles that were uncommon and a bit 'tetchy' to maintain in practice...
In any case a colossal waste of the energy expensively put in the steam. So instead, we cut off admission at some percentage of stroke, at which point flow through the throttle quickly equilibrates to near-zero at high pressure, and the trapped volume of steam does its business. (Note that the rate of cutoff is influenced by increasing shrouding between the steam edge and the port, the opposite of steam inlet to 'infinite volume' at the moment of admission -- this is the counterpart of what happens at the exhaust)
as speed increases, there is more consistent steam consumption.
The need for greater steam flow necessitates the need for both less restriction between the boiler and steam chest (more open throttle) as well as a greater pressure difference between the boiler and steam chest. (I believe this is analogous to Ohm's Law where pressure is like voltage and steam flow like current).
if there is a need to increase speed, the throttle may be opened as speed and consumption increases.
Depending on cutoff, cylinder ports will not be opened all the time and steam chest pressure will be allowed to recover (increase more so) when valves are not opened.
At higher speeds, when less force and acceleration are needed, cutoff can be reduced to both reduce consumption of steam and use it more efficiently (expansion). The reduction in steam consumption allows a higher steam chest pressure, higher starting cylinder pressure and possibly higher mean effective pressure.
If this is the case, a reduction in both cutoff and throttle are necessary to maintain an MEP resulting in constant speed.
LastspikemikeDisregarding leakage the system is closed until the locomotive begins to move. Theoretically, the steam only "leaks" out of the cylinder after the steam chest is again a closed system. How much can the pressure in the steam chest actually drop during the time the valve is open?
The concerns with steam-chest pressure are, I think, much more relevant in the range of speeds that the time the long-lap valve is closed to steam cause pressure effects in the steam tract. I think of this as analogous to some of the effects with proportional relieving and recovery of compression.
...does the overlap between the two cylinders (among the three?) effectively mean there is constant flow through the throttle into the steam chest and up the exhaust. Just posing the question answers it, never a constant flow, always a series of discrete lumps of steam.
One way to look at this is to examine a torque diagram like those in Wardale's Red Devil book to see where the little transition at admission cutoff comes, then compare this to indicated pressure (which also reflects tract and passage pressure to within a small amount). But it is complicated because the effective branch point to the cylinders occurs downstream of the throttle but appreciably upstream from the chest, and it is that whole volume and mass flow of steam that is accelerated/decelerated, then stopped and started, as the piston valves work.
This thread seemed to me to be about what happens to pressure in the steam chest as the throttle is opened as contrasted to when the cut off is changed.
Pressure transfers virtually instantly, speed of sound in the fluid actually, which is pretty damn fast.
The bangs you hear from unmuffled exhaust (or badly muffled as in a Harley V twin) are sound waves traveling down the exhaust pipe much, much faster than the gas flows.
For ICE piston engines using valves it is thought there is an rpm limit dictated by the speed air can move through the inlet and out the exhaust. At say 22,000 rpm, the maximum achieved by F1 V10 before the big rule changes to V6 turbo hybrids, engineers thought they might be getting close. Even then the gas flow is "particle" more than "wave" but wave effects have enormous effects.
Practical high cyclic is limited by a couple of other factors before pure steam kinetic effects become theoretically limiting -- balance effects and relative unsafety of precise valve-gear arrangements among them. But a major reason attributed to the better 'efficiency' of Lenz-type poppet valves at high speed is that very-high-pressure admission (resulting in shorter mass transfer and thrust risetime) can be balanced by earlier progressive release, asynchronous timing, and much longer duration -- you rapidly get into the range where turbines are much more effective 'users' of the high inlet pressure and required compounding staging than piston engines are, probably for anything about 130mph or above.
In theory a steam engine does not need the discrete volumes required by a piston engine.
What peak rpm is reached by a high speed steam locomotive bearing in mind only two cylinders and related valve mechanisms are involved in the majority of engines.
Roosen and Besler both thought high speed would require great reduction of reciprocating mass and great shortening of steam passages; they combined this with inherent low final-drive gearing to keep machinery speeds relatively low at high speed (which I think is a vital element about ~125mph). I have no results pro or con regarding any of the French motor-locomotive testing, although I have tried on and off (and many of the results are at best confused by strange French operating conditions and attention to design detail). Certainly PRR had firmly decided by 1944 that much of the coal future involved steam turbines, and by 1947 that it would involve Bowes drive for passenger speed.
Incidentally both the steam turbine and the gas turbine are of much more considerable antiquity than generally thought: the principal reason the steam turbine came first was as ship propulsion (where it is admirably suited, TIT is comparably low, and operation down to a low fraction of atmospheric pressure is easy) and gas turbines were impeded by temperature limitations for many decades. (There is a fascinating paper, I think in the SAE repository, on how different the world would have been if more-practical gas turbines had been adopted for early automobiles in place of tetchy and complex spark-ignited carbureted engines... you'll get a kick out of it if you haven't yet read it...)
LastspikemikeMEF is the "halfway" pressure on the piston face.
And it is NOT a 'halfwit' pressure; it's the integral of the instantaneous pressure over the length of the stroke from admission to exhaust cutoff. That's a highly different thing because of the 'double salience' imposed by the rod geometry. If you wondered how an engine with an indicator diagram like, as Angus Sinclair so wittily noted, a small leg of mutton could show better over-the-road performance than one with a textbook-clean diagram -- here is part of your answer.
In brief, even just with reference to a good indicator diagram (which if you're lucky will actually have an indication of MEP calculation on it) ... look at the rectangular hyperbola from cutoff to exhaust release? the MEP of concern is the line that produces equal AREAS under that curve to either side of it, and on a good diagram you can almost read that off or extrapolate it to get a working number. The nice part is that this adds to the portion of MEP you read off the admission, which ought to be nearly linear (see below) to calculate your overall effective pressure...
Reducing the cutoff time will lower the MEF because the steam pressure is now stretched for a proportionally longer portion of the piston stroke.
Roughly speaking, cutting the cutoff time in half should cut the MEF roughly in half.
The MEP during expansion is instructive to study, and as Professor Loeb said to my father in medical terror school, I recommend it to your attention.
Throttle position should have no effect.
I looked a little at the superheater effect and much to my surprise the steam is superheated in expansion tubing so as the keep the pressure of the dry steam at boiler pressure.
Now, if you do as older practice did, and put the throttle (or "a" throttle) in the dome or dry pipe itself, you can have a separately-fired vessel overpressure issue in the elements, especially if there is any carryover into them which then is 'trapped, PEA-like, if the throttle is slammed closed. The National Board ESC was quite justified in being alarmed by the possibilities should the valves or valve gear actually be arranged so that less flow is permitted 'through' at mid than is being evolved in such a case -- and it can be surprising just how much that is. I'm sure "Blue Peter" rings a bell to some people here -- and that incident involved uncontrolled reverser excursion toward longest, not shortest, cutoff...
Adding heat can cause fluid flow without necessarily increasing pressure. At least, not by much.
Of course there must be a pressure difference for fluids to flow but a steam engine boiler system is a closed system until the exhaust valve opens.
Steam expands from the boiler through the throttle valves into the superheater...
... expands into the steam chest and expands into the cylinder when the cutoff-controlled valve opens where it is trapped inside a chamber of changing volume. Only there should there be any appreciable pressure drop.
On the other hand, once the steam edge closes admission, wherever the cutoff sets that to happen, you enter the happy world of actual intended expansion, where the effect of superheat and jacketing become more highly relevant (and the effect of open cylinder cocks, gland blow, and the like become more substantial influencers of subsequent EP and hence thrust, and make even numerically calculating the MEP integral a mess, but I digress) and the importance of using mass flow as an index of subsequent 'steam behavior' rather than pressure comes into its own. Part of the important thermodynamics that made Schmidt such a hero is in keeping the expanding steam, a two-phase material at part of the expansion 'when it does any work', closer to acting like an ideal gas ... and that, rather than 'packing more expansive enthalpy into the steam mass', is the principal purpose of the superheat. If you have a CRC 'rubber bible' or equivalent, you can look up the enthalpy added in superheat and see how dramatically small it is per degree rise compared to that in saturation under pressure -- Tuplin never failed to try to point this out, bless him.
LastspikemikeRoughly speaking, cutting the cut off time in half should cut the MEF roughly in half. Throttle position should have no effect.
ideally when steam expands, PV=C, illustrated below for 10, 40 and 80% cutoff. i calculate the average pressure (MEP) across the cylinder cylce as 33, 77 and 98%.
at 10%, for example, steam consumption is reduced by a factor of 10, but MEP is only reduced by a factor of 3.
LastspikemikeIf you could take the necessary pressure readings half would be higher than MEF and half would be lower.
isn't that the median?
Lastspikemike A "mean" is halfway by definition. ... ...
A "mean" is halfway by definition. ...
Ummm....no. A mean is a figure derived by adding the values in a data set, and then dividing that total by the number of data points, by count.
You're thinking of the 'median', as Greg has pointed out. That is the value which has an equal number of data points on either side of it.
LastspikemikeA "mean" is halfway by definition.
If you could take the necessary pressure readings half would be higher than MEF and half would be lower.
Meanwhile MEF measured someplace logical, for example at the key between the piston rod and the crosshead, is likewise an average over the stroke; we won't yet have converted it into wheelrim torque (which is really, I think, the 'figure of merit' that ought to matter here if the translation to 'force' is going to be made).
MEF isn't calculated that way anyway. It's a deduced number from data that are not in themselves pressure numbers.
As far as I know locomotive engineers have a boiler pressure gauge. There's no pressure reading taken from anywhere else in the steam system.
But there ARE a couple of places where additional pressure gauging is important, and many modern engines have it there. One of these, paradoxically, is in the steam-chest area, and it measures back pressure and compression, two things we haven't really taken up yet (because they matter after release). It is valuable to know the latter in particular, because with improper manipulation of valve gear (and it's not difficult to do it wrong!) the compression pressure can peak wildly higher than boiler pressure, with its risetime being related to the mass retained as release ends being heated as the resultant of those last long-travel inches to dead center, and "MEF" no longer being meaningful in determining the piston (counter)thrust change in that distance.
An analogy to a garden hose was used earlier in this thread. I suggest the boiler system is more like a household water system. Municipal line pressure is limited by a pressure reducer at the main service tap. This would be the throttle. The valves would be the taps.
The chief concern I have with it is that the nature of hydraulic flow in a water system doesn't fully mirror that for high-pressure steam. So you can't extend the analogy all the way to be a full predictor. There are more interesting analogues in the convection section during certain types of boiler explosion...
If the system is designed correctly there should be in practice an "infinite" supply of steam until the water or fuel runs out.
Incidentally it is possible to cheat on draft, using characteristics of the locomotive slipstream to produce any degree of firing and also pull a partial vacuum on even a fairly large mass flow of exhaust steam. It remains to be seen precisely how effective this might be in practice, but it promises to be no less than interesting...
A Newcomen engine was an atmospheric vacuum engine.
Both types didn't rely on steam pressure to do any work. Trevithick was da man.
Superheaters run at boiler pressure.
Keep in mind that there is, of course, volumetric increase in the steam mass flow as it transits the superheater, and this can be dramatic at periods of highly excessive superheat (some of the figures I've gotten from operators who knew how to use their superheat gauges -- yes, there are such things on good modern engines -- definitely qualified as 'crazy high'). I leave the solution to the reader if any reader doesn't understand why this does not produce a "pressure increase" at the throttle. Astute readers will already understand; hopefully that includes you.
I had understood that there were some locomotives with superheaters that placed the throttle between the boiler and the superheater.
A number of engines are equipped with two throttles: one in the dome, and the second (usually a multiple type) as a front-end arrangement. One overt purpose is to provide a solid cutoff of steam when the engine's valve gear provides imperfect isolation at mid -- you may be familiar with 'nightwalking', which occurs where pressure builds up in the tract and something like the combination lever in Walschaerts develops enough valve motion to permit slow but continued creeping as pressure builds up past a leaking throttle poppet. Another is to allow -- in theory, at least -- access to parts of the steam circuit, including the superheater and front-end throttle, with the engine still at least partially in steam. Of course with a proper direct-steam system and proper gas blanketing it would be unnecessary... but that's yet another digression.
I have been looking to see if there are online minutes from the National Board ESC meetings that discussed the issue of superheaters as separately-fired pressure vessels lo! these many years ago now. One of the reasons for the ESC was that there were 'steam safety experts' who swore that the superheater on a Lima Super-Power pattern engine could constitute a separately-fired pressure vessel under certain conditions, just as it can on a locomotive with dome throttle -- they, too, didn't quite know where the throttle actually was on such a design as opposed to where they thought it to be.
looks to me that it's the average pressure (mean) during the cycle. see Method of Ordinates which is what i calculated in my plot as well as using calculus
1448
Since perhaps the whining English schoolboy claims to be resolved on creeping like snail reluctantly to knowledge, let's revisit those original questions:
Steam is always 'pushed' into the cylinder (more saltily 'mass flow is achieved through pressure differential) and it should be clear that EP, as an index of extra table work via piston thrust, will be lower than chest pressure if mass flow during admission is to take place. Now EP at the piston face is partially determined by flow through the passages, so it may not necessarily be 'higher' than chest pressure, but any (necessary!) tendency to flow reversal at any such time would be a Bad Thing for subsequent expansive work. So in all probability a situation in which chest pressure cycles higher than passage EP would be a poor thing to be in initially.
If the cutoff is shortened (we have amply seen by now the point I keep trying to make about using clearer terms for cutoff) with the throttle wide, the chest pressure will now peak a bit higher, closer to boiler pressure, since there is less mass flow into the cylinder and (assuming a radial gear) its interruption from admission to admission will be longer. I would expect this to be minimal in an actual locomotive design, but for looking at the OP's question it will bear consideration.
If the throttle is not fully open, the steam-chest pressure will tend to fall, as tract pressures back to the throttle will fall, during admission, but then start to rise again during the subsequent period of cutoff as pressure tries to equilibrate in the tract. The issue is whether there is adequate 'mass flow' into the chest volume during that period, and then adequate port and passage capacity to admit it, to give better net mass flow over admission to provide higher MEP (and hence higher work) over the ensuing stroke.
If there is 'inadequate steam' even at the reduced cutoff, the steam-chest pressure will cycle, but with decaying MEP in proportion to the decreasing tract mass flow, probably with a different point in the stroke where each successive MEP is achieved but now with some effective 'throttling' even during admission (as the swept volume increases but there is inadequate mass flow to hold EP up).
We would expect the effect to be roughly the same at x diameters down the tract from the throttle whether the lack of mass flow comes from inadequate boiler capacity, lower firing or firing efficiency, improper use of injection, or excessive throttle closure or wiredrawing effects at the throttle.
I still am unsure why the OP is concerned with the chest pressure, other than as a concern for understanding complex physics, as (including for the reasons I provided) it is incidental to how the locomotive would be practically driven. I would agree that it can be helpful to see indicated pressure there during testing, but there were relatively few attempts to provide a desirable variable valve and chest capacity historically (and some of those attempts were decidedly unsuccessful) and in normal design the valve and chest proportions are fixed in service. Even Wardale's proposal to use double piston valves to achieve different exhaust timing and duration from inlet did not change the valve proportions dynamically or alter the chest volume.
... can reducing cutoff w/o a throttle change result in an increase in power (i.e. tractive effort) due to higher steam chest pressure?
... and does this mean that if cutoff were reduced should there be a corresponding reduction in the throttle to limit to the increase in steam chest pressure to maintain a desired speed?
The catch here, as I noted, is that most throttles don't work at all like automotive 'accelerators'. Pre-Wagner dome throttles are particularly awful as, to get reasonable area through them, the force holding them shut even at turn-of-the-century boiler pressure is enormous, and as noted will remain 'evident' over the range that downstream tract pressure 'just the other side' remains higher than boiler pressure. The mechanical advantage necessary in a grapevine throttle, and some of the evolved reflex involved in having to slam such a throttle open but then bring it back without overshoot to near desired position will convince you that depending on fine adjustment other than at tricky starting is not much fun even if your haptic skills are high.
With a good proportional direction servo (both the Franklin Precision and Throttlemaster designs are capable of the physical position but might have needed better linear sensing precision, for gregc e.g. with LVDTs) you would have a somewhat better chance at the position, but you then have the fun that a multiple is almost certainly not quite linear in flow relative to angular cam position, probably worse and less predictable as the cams wear. This of course wasn't as much of an issue in the 'get it open to where you can drive on the reverser' era, but if you were experimenting with fine steam-chest cycling effects it would be an additional measuring and control concern.