i hope it’s obvious that the pressure differential between two ends of a pipe are what cause a fluid to flow in the pipe (if the pressures were the same, nothing would flow). so the pressure at the cylinder intake is only every close to the boiler pressure when there is very little flow at very slow speeds.
early cutoff, “cuts off” the flow of steam into the cylinder, so that it will expand (not prevent it) as the cylinder moves expanding the volume of the steam. there is probably minimal cutoff (late in the cycle) when starting to maximize the pressure in the cylinder for as much of its’ cycle as possible.
of course yet another advantage of cutoff is to allow pressure to build in the steam chest outside the cylinder while the intake valve is closed, maximizing the volume of steam that enters the cylinder during the next cycle.
as an EE I see analogies to electron flow: pressure is voltage, pipe diameter and length is resistance and steam chest volume is capacitance.
I look at cutaway drawings of the valve chest and cylinders. I see the valve opening to the cylinder at full steam pressure (assuming the throttle is fully open) and the steam expands to fill the space inside the cylinder. Since the cylinder is closed off at the operating end by a movable piston the full steam pressure moves the piston. The exapansion ratio would be the change effected in the cylinder volume by pushing the piston as compared to the total volume of pressurized steam right back to the dome.
If a cut off stroke is used for the valve movement then at some point the full steam pressure is cut off and the steam remaining trapped by the closed valve and the piston face expands by a completely different ratio.
The former develops full power (torque actually) while the latter does not.
Thats what I observe from the drawing and I’m quite sure I am correct.
Just waiting for a logical and understandable rebuttal.
Incomprehensible prolix rebuttals will be included in the model railroading chapter of my new book: “Semantics for Pedantics” quite possibly following my chapter on judicial writing ( not to be confused with judicious writing for which there will be no room in my book of course).
I prefer not to confuse pressure with flow. They are related but not the same thing at all.
Chip, you really did it this time, I hope you are having fun and following this because you will learn a lot if you do.
I will give you the really simple practial description, that skips a lot of details.
The engineer has a throttle, which controls the volume of steam, and he has control of the cutoff, which is similar to valves/cam timing in your automobile engine.
By leaning how to use these two adjustments in concert with each other, the engineer does what would otherwise be considered magic, he moves thousands of tons with hot water vapor.
And then he has brakes when he needs to stop, do you know how the Westinghouse air brake works?
That will need to be another thread…
There is plenty of expert info just from Overmod alone, I will not muddy the water any unless I see a good point to be made.
Everyone in this hobby should understand at least the basics of these things, just my old fashioned opinion.
yes, but the steam chest pressure outside the cylinder is not necessarily at boiler pressure when at higher speeds. there’s the pressure drop between the boiler and steam chest and the ability to produce more steam.
And yes, while there continues to be flow into the steam chest, it may not equal the volume entering the cylinder at that time, resulting in the cylinder pressure being less than the steam chest pressure.
(i wish i could estimate both those pressures. I have tried to estimate the pressure on the output side of a throttle based on the area of the opening).
again yes. after the intake valve closes at some fraction of the cylinder cycle, as the cylinder moves, its volume increases resulting in a reduction in pressure (~~PV/T), but significant work continues to be performed with no additional steam. this results in the more efficient use of steam, but not maximal possible work output (if steam pressure can be sustained).
the art is recognizing that without cutoff, allowing as much steam into the cylinder as possible each cycle, results in a lower sustainable pressure and sub-optimal work from the cylinder. at “optimal” cutoff, maximal work output results from the cylinder.
i think(?) optimal obtains the maximum average cylinder pressure.
imagine trying to control a model that models this behavior, trying to find that cutoff setting while at the same time keeping track of boiler water
Keep in mind that a steam locomotive is a complex machine, not just a constrained projectile piston in a cylinder. Many of the apparent complexities in cutoff stem from this.
First, there is no ‘expansion’ in lastspikemike’s given example – the drop in pressure is trivial, corresponding to that in a ‘fireless cooker’ locomotive after the first ‘chuff’. This is due to phase-change expansion in the supercritical boiler water mass, which responds to attempted drop in pressure with massive expansion – the mechanism behind the rocket effect in boiler explosions, but I digress. So it is reasonably safe to assume that admission pressure – whatever that happens to be at a given time – is reasonably maintained at the end volume as it is at the moment admission pressure is re-established in the cylinder via the ports and dead space – a matter of milliseconds in a modern locomotive.
It then follows that the thrust of the piston is as lastspikemike suspects it to be, substituting only measured admission pressure for the approximation percentage in the canonical PLAN formula. And that this produces the highest thrust per stroke that this expander will produce.
Moreover, at the moment of early cutoff the ‘equalized’ admission-pressure equality is indeed broken, and the subsequent ‘expansion’ is proportional to the mass of steam contained in the cylinder at the moment of cutoff, expanding against the piston and falling in effective pressure both due to expansion in volume and work done against the piston to move it.
However, the piston is not driving a linear load (as in a Westing
I suspect that most of this is far beyond basic concepts. Think of a steam engine as a teakettle. Heat is applied, water boils and produces steam. When enough steam is produced it builds pressure and the tea kettle whistles to say it is hot enough. Plug the opening and eventually the steam pressure will build to wear it blows up the kettle. Forget who it was in England who made a steam driven pump to remove water from the coal mines which started the mechanical revolution resulting in primitive locomotives that replaced horse drawn rail carts. Everything since then is ways to produce higher efficiencies just like cars today compared to model T Fords
Hornby tried an electrically heated OO scale Pacific live steamer. I neariy bought one (sale price CAD$1,000.00. [symbols removed by moderator]) it was so pretty. Quite the technological tour de force.
The challenge you describe basically killed a really great idea. The steam control was remote and probably servo or stepper motor electric. In theory it should’ve worked and the model is still reasonably popular in a devastatingly restrained way, price wise. Operation of this model is possible if one is geeky enough.
The main reason it was a commercial failure, I think, was that even the model railroading world is basically geek limited.
Though this thread is shaping up to be providing a fair bit of contrary evidence to that idea.
Now look at what happens if you try full-stroke admission with the engine at any particular rotational speed – in a low-drivered engine this might be no more than the 10 to 15 miles an hour where its boiler starts running out of volumetric effectiveness. You have full thrust on the piston all the way to BDC … as the rod starts to move around to the back of the crank and the lubrication starts to suffer … then nearly instantly you relieve the pressure to exhaust and open the other end to steam. Do not expect your rod bearings to survive this treatment very long, or your axle bearings or wedges or pedestals either.
Meanwhile you had to make your rods very stout to absorb the monster thrust. These have appreciable inertial mass, and momentum, and this appears in part as reciprocating augment (the thing overbalance is for). These forces go up as the square of the peak speed, and while you can compensate for this somewhat by making the rods heavier still, you then have the overbalance guiding problems, including severe nosing and hunting, and very few cost-effective ways to deal with them.
This method completely falls apart if you expect to run heavy trains at diameter speed or above, where any real interest in steam power starts [;)] Here the physical time available to get steam mass into and out of the cylinders effectively starts to matter; it would be physically impossible to move valves quickly enough to get long expansion (and in fact when N&W tried this with some of the best-designed Baker gear in the industry, it ‘unraveled’ with somewhat dismal predictability from inertial forces alone when let out to corresponding valve opening) This is where the fast porting/unporting of poppet valves, and separation of admission and exhaust duration, come into their own, and while cutoff much shorter than 24% had little historical value, improved approaches can use shorter overall admission to good result in faster rotation (and hence hig
I did buy one of these, precisely because it was a tour-de-force probably never to be repeated, like those expanding tables on yachts from the same general era, or twelve-cylinder production cars from multiple makers.
The problem I had with it was that, like many live-steam models, what it actually did with the steam is not the same as in a ‘real’ locomotive. The “throttle” is somewhat similar to how you do steam control in a once-through flash boiler: the more you pump in, the more goes out to the cylinders. There is no adjustable valve gear at all, as on a donkey engine, and no care for balance at all. In other words, cool as hell to do, but no more interesting that a typical putt-putt steam launch in complexity. (And you can teach a kid how to set up, run, and put away one…)
The real problem with it was that, at the time, it was too expensive and too operationally limited* for ‘serious modelers’ – it fell into a hole between geek tribes, as it were. That is something I frankly didn’t see coming, and it’s a little sad there will likely never be a ‘re-run’ in properly-improved form.
(* and then, there was the oil issue. If you don’t like smoke on a layout, you won’t like oil…)
On the other hand, I’ve been crying for scale operating rodwork for many years. That is not just scale rods and pin detail instead of soft-metal stampings and slotted screw heads; it’s proportional valve gear actuation from reverse position right through to proper Walschaerts and Baker operation servo-driven proportional either to road speed and ‘load’ (for conventional model throttles controlling DC motors to get speed) or to an actual model power-reverse control
Steam does work in a piston engine by expanding, not by flowing. Expanding results from pressure changes. The boiler is a compressor rather than just a furnace on wheels. The heat is used to manufacture a pressurized gas (the working fluid) , indirectly rather than directly as would be the case for an internal combustion engine (which creates compressed gas directly from air and fuel as its working fluid). The plug in the kettle is the throttle valve allowing boiler pressure and volume to be controlled at a macro level.
(You can make a steam jet engine, apocryphally that was the first and more than 2,000 years old. Hooking it up to do useful work is the challenge there.)
If the pressure expansion comes from the volume of steam all the way back to the throttle valve then the engine will produce the highest torque at the driver crank pin that the engine is capable of. The smallest expansion ratio. That will consume steam at the highest rate for any given wheel rpm, which is of course a consequence of developing maximum torque.
What may be slightly counterintuitive is that most steam locomotives are traction limited, not torque limited. It would be very rare for an engineer to try using maxi
not sure what you mean by efficiency. it typically means extracting the usefullness from something with minimal waste.
i’ve described how allowing steam to expand within a cylinder improves efficiency – more work from the same volume of steam. one way of evaluating this efficiency is the energy in the coal consumed compared to the work produced by the engine
automobile technology has certainly improved since the model-T. internal combustion engine efficiency has certainly improved, but is still only 20%
i don’t think it’s accurate to say “Everything since then is ways to produce higher efficiencies”. there are different goals.
a major drawback of steam engines is the time required to obtain full power. other technologies such as diesel or turbine engines minimize this time. not sure the benefit is efficiency or less complexity and easier maintenance.
not sure how to evaluate the efficiency and benefits of electric vehicles. efficiency may depend on the source of power: coal, nuclear, water, … Benefits may be zero pollution from the vehicle, zero startup time, higher torque, …
while efficiency or fuel cost may be important, performance and other contraints may be even more important. electric locomotives to go underground have a unique purpose
I’m pretty sure he means efficiency in the same sense as you do. You might be leaping to a conclusion or unwarranted inference there.
The example given is Model T to modern car. Not steam locomotive to modern car (or truck).
Steam locomotives are not less efficient than diesel electric but they do cost way more to run.
Sailing ships were way more efficient than steam ships (and, bonus, zero emissions rated) but far more expensive to run.
In human economies, efficiency is measured by output per man hour (or per man horsepower if you prefer) which is quite different to real engineering efficiency. The leaf blower springs to mind as illustrative, for some reason I can’t quite put my finger on.
But don’t make the mistake that this ‘fine-tunes torque’ exactly; it does adjust it, but makes it peakier at the same time, which contributes to both a propensity to stall and to spin at times … not a fun place to be!
Better to think of the controls of a steam locomotive as double-salient in a sense: the throttle controls the admission effective pressure, and the cutoff controls the mass flow available for expansive working starting at that pressure. There will be times – plenty of times, on excursions! – where you don’t need more than 150psi or so throttle pressure, but you need to start a train without slipping on uncertain track and then operate it efficiently. You do this by coordinating the throttle and reverse appropriately. For fast or ‘most efficient’ running, you want minimum actual impediment in the steam flow … and the poppets in a multiple front-end throttle are an impediment. Then you crack it open and, once you’re at a speed where surge and low-speed augment are no longer problematic, drive on the reverse … until you get into the range of ‘high-speed slipping’. At that point things start to get interesting…
Be careful with analogies to internal-combustion engines. The point of turbocharger boost is to increase the amount of fuel that can be burned during a stroke, and there will be diminishing returns at some point where the complications of higher peak firing pressure, peakier torque, and exhaust back-pressure outweigh the added boost.
Except that ain’t what it does. You get equivalent work from a lower MASS of steam which is allowed to expand into a (varying, but bounded by cylinder dimensions) volume. The valve gear meters the amount of mass that is admitted, and the control is to adjust the timing and duration of that admission to produce the ‘desired’ power at speed with the minimum of confusion and delay.
Note that one problem is that conventional steam locomotives are designed around what the English called ‘automatic action’ – as you opened the throttle and tinkered with reverse, the exhaust energy in the front end more or less controlled the fire efficiently to match the added or decreased power. In some locomotives starting in the late '30s this got out of whack with improved low-back-pressure design: it got to the point that engines would make desired horsepower at high speed but not produce enough physical draft to keep steam generation going (some of the early UP double-stack 800s had this problem). Sometimes ‘good’ can be too good…
Remember that an important way steam engines are better than IC engines is that they utilize the Rankine cycle, not something more simplistic like the Carnot or Brayton cycles that are ‘more efficient’ as heat engines but can’t use recuperated waste heat effectively (no, I’m not going into entropy here!) Even on locomotives it was possible to use exhaust steam for a wide variety of useful purposes, from very good feedwater heat to gas-producing firing or primary-air preheat. This is brought to a fine edge in powerplant design (as anyone who has seen and comprehended a heat-balance diagram will know) and it accounts for much of the phenomenal overall efficiency of steam-turbine baseline power
They are far, far less efficient than diesel-electric, and in some uses (flat switching being one) compellingly so even if designed to be as ‘diesel-like’ as possible (see the N&W M-2 ‘Automatic’ for a poignant example). It is almost astounding how much more effective a pair of 4400hp ‘power-by-the-hour’ modern diesels is than any 8800-hp external-combustion equivalent … it takes a great difference in fuel, and care with other support provision, to make them economically competitive with diesels, which is a very, very different thing from efficiency. They can be made roughly ‘as cheap’ to run mechanically, but it takes lots of work and detail design to make it so.
Sure, if you don’t mind ridiculous lack of actual cargo space if you want any kind of speed, and don’t care when the cargo gets to port reliably, and can keep well out of the track of storms … and there are interesting automation schemes for much of the expense of running traditional multimast sailing ships, but they ain’t cheap and not always good in ‘anomalous conditions’. The British took over the trade of the world with dirty iron tubs … and remade the Scots into respectable engineers in the process, by making the actual things constituting ‘efficiency’ in shipping cost-effective (or more reliable).
Note that subsequent attempts to utilize ‘the power of the wind’ for commercial shipping, some of them very sophisticated, have not panned out, in any service requiring profitability. I have always thought it strange that Flettner rotorships were never tried; they do involve some motorization but far less than what would be needed for equi
Possibly not the best idea to take up the efficiency issue about sailing ships. They were of course much more efficent cargo carriers. No steam engine gubbins, no fuel bunkers or tanks and so on. Size limits had nothing to do with the sailing technology. It took a long time for steamers to pack more cargo and deliver it faster. Just BTW the hard driving skippers made a point of not avoiding storm tracks. Ever heard of the roaring forties?