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?
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:
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.
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.
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.
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.
“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
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?
"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.
yes, the pressure at the cylinder valve on the boiler side
any contained fluid will only flow if there’s a diffrence in pressure. so obviously, the steam chest pressure has to be lower than the boiler pressure for steam to flow into the steam chest.
and when the cylinder valves open, the steam chest pressure drops a bit as steam flows into the cylinders. but when the valves closes, pressure in the steam chest builds trying to equalize to the boiler pressure
at high cutoff, the valves are closed less of the time and the steam chest pressure has less time to recover (increase).
at low cutoff, there is more time for steam chest pressure to recover.
so at low cutoff, the pressure of the steam entering the cylinder will be higher.
Pressure develops torque not power. Power results from the rpm of the drive wheels.
Can one infer the pressure effects from the chuff?
Low cut off should produce shorter and sharper chuffs?
High throttle settings (more open) should produce much more energetic chuffs regardless of cut off setting.
The steam chest seems to be a pressure wave absorption device as well as providing a volume of steam at throttle pressure sufficient to fill the cylinders often enough to sustain maximum torque.
Pressure and flow are not at all the same thing.
Very little steam flow needs to occur in order to develop maximum torque or a locomotive would fail to move from a standstill.
Think of a closed hydraulic brake system (or airbrakes if you prefer). Lots of force transmitted but very little fluid flow required.
The word throttle is descriptive, for steam engines as for ICE where incoming air is throttled. The cutoff works a bit like valve timing on ICE especially variable lift variable timing valvetrains such as BMW and FIAT Group have devised.
The main difference between throttling ICE and steam engines is where the heat is added to the working fluid. ICE create the working fluid by adding the heat after the air is metered (throttled). Steam engines start with a useable energized working fluid. The principles of developing torque by pressure at the piston face and power by expansion of the working fluid are exactly the same.
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 Duk
“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’.
There is a big difference between pressure and mass.
If you can follow the lengthy description above this difference is explained. At least I think it is explained. Hard to say. I did find the idea that an ICE sucks in air and so is fundamentally different to a steam engine which blows in steam fascinating. So a blower on an ICE fundamentally changes the physics of induction?
Steam has no magical properties. Water is a relatively heavy molecule and when excited it can do a fair amount of work before it gets cold.
Steam power is proportional to the mass and temperature of the heated gas, not the “pressure” per se.
Pressure is a version of heat content. Conceptually using the Greek image of atoms, more atoms (mass) in a smaller volume (pressure) can do more work more quickly (power). Discussions of just pressure rises and falls isn’t enough to understand how the throttle and cut off interact. You can have more atoms (molecules) or have the molecules try to move faster (higher temperature) and preferably both. The throttle meters the available mass indirect by reducing volume flow. There will always be a pressure differential between the steam dome and the steam chest. The cut off meters used mass by controlling the number of molecules admitted to the cylinder. The pressure in the steam chest should not fluctuate, ideally. In fact I suspect the main function of the steam chest is to damp out valve pressure fluctuations. Unlike an ICE which gains efficiency by exploiting pressure waves caused by valve operation a steam engine has no such potential.
Steam power depends on converting that heat into kinetic energy at the drivers. The limit on heat extraction is provided by the condensation temperature and pressure.
Generally speaking, the MEF at the piston face depends on the mass of the steam, not the volume. Describing quantity of steam by reference to volume flow is not useful because
Why would there be a pressure difference on each side of the throttle?
The pressures should equalize or the throttle wouldn’t throttle. The constriction of the throttle reduces the flow rate, less mass of steam gets through, but the pressures have to equalize or the flow rate through the throttle would just keep rising.
The pressure of the steam in the steam chest as it enters the cylinder should be the same as at the input side of the throttle. The cut off position should have no effect on the steam pressure as it passes into the cylinder. Closing the valve earlier only affects the total mass of high pressure steam entering the cylinder. Pressure drops only after the valve closes.
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,
