Talking Thermodynamics

[Zephyrin]

IMO, measuring the instant gas temp inside a small cylinder is out of reach for the home workshop, unless the cylinder would be made specialy for that, with a large dead volume to insert a fast and sensitive thermocouple (or maybe 2 or 3)...
But it is certainly easier to monitor the internal pressure, I have ordered a MPX2200ap chip for that purpose, but not yet implemented it, a little bit out of touch with the electronics...

as regards the needle valve, I would say that it is a pressure varying device, but some increase in temp is also expected after the small opening.
but I also wait for the response of MJM...

[MJM460]

Hi Paul, regarding your last comment, the good news is that we actually got to the point where these problems appear.  My wife worked helping older frail people with some of the house work.  She always says that based on her observations, growing old is not for sissies.

Drilling a hole in the block is the same basic arrangement as a thermowell, so the extra thermal resistance of a sheath is not necessary.  The cylinder wall temperature varies quite rapidly within each stroke, especially if there is early cut off and some expansion, but even without, the exhaust temperature will rarely be much above 100, yet at the start of the stroke, the incoming steam is nearer your superheater outlet temperature.  Even my little pot boilers have a superheat outlet temperature over 130 C.  It is important to understand the nature of that cylinder temperature.  It varies both in time and location.  It is very difficult to get a location where the temperature is predictable.  As you imply, the slow time constant of most thermocouples will tend to average the reading, but because you can't know exactly the point the thermocouple is contacting, the only really useful information comes from changes in the temperature.  I think this location might be on the side of overthinking the issue, and it would be just as useful if not more so to simply make the best possible measurement of the exhaust temperature, and the inlet temperature.  With these two, you can calculate how much energy is being extracted from the steam, and if you can measure the work done, you can calculate the efficiency.  Then changes in efficiency as the load changes are probably what you are most interested in.  The exhaust temperature will reflect changes in valve cutoff.

Inlet temperature is a bit more problematic, especially when considering changes in inlet throttle valve.  You cannot use the boiler pressure to infer a pressure after the throttle valve so accurate pressure measurement is required.  I have not tested my opinion with a good calibrated gauge, but I am very doubtful that the small gauges we generally see on models are very accurate.  But the simplest thermowell you can design will do the temperature.

For the exhaust the pressure is accurately known if you have a reasonably free path to atmosphere, and again the temperature can be measured with the best thermowell you can devise to most accurately reflect the steam temperature.

For a test rig to apply and measure the load on your engine, I wonder if you can arrange a test rig so the loco driving wheels each rest on two wheels on the test rig, profiled like track cross-section to fit the driving wheels and geared to a flywheel, which can be loaded up with a friction pad on the rim.  The flywheel and gearing can accurately replace the normal linear motion of a locomotive to account for the inertia of the train.  The calculations are relatively simple, not so easy that I can reel them off, but I can help you do them if you go that way.  A digital scale should be accurate enough to measure the friction drag on the pad, and you need to do some electronics for speed measurement.  Again not hard with Picaxe or Arduino.  Should be an interesting exercise to sketch up, then probably not too hard to make up, much of it at a normal desk or tabletop.

Regarding the needle valve, it actually can be said to vary both volume and pressure, but it is not really accurate to say it controls those quantities.

It is better described as a variable restriction orifice.  When the needle is screwed open the area of the flow annulus between the needle and seat increases.  When there is a difference in pressure between the upstream and downstream side, fluid flows through the opening at a rate which is determined by the pressure difference and the open area.  Bernoulli's theorem, which is basically the same as conservation of energy, gives an approximate value for the maximum velocity, but downstream much of this velocity is dissipated in turbulence, and you do not get the pressure recovery Bernoulli would predict on the downstream side.  However, the resultant pressure change means an associated volume change for the mass of fluid.

The smaller the open area at the needle valve the more pressure is lost.  But the pressure loss also depends on the flow.  So if the upstream or downstream pressure changes, the flow will change, as will the the other pressure and the needle valve would require adjustment to compensate if a fixed value is required.

I hope that I am not being too pedantic here, but I would suggest that the identifying feature of a controller is some sort of variable element that automatically responds to a change to restore the desired set value.  Pressure controllers usually have a diaphragm, needle valve and spring set up so a change in the required pressure is compensated for by adjustment of the needle, which is usually connected to the diaphragm.  They can be set up to control upstream, downstream or differential pressure, which ever is required.  More sophisticated controllers might have separate measurement, then an electronic system, these days involving microprocessors or even computers to compute the required response and drive a diaphragm operated control valve as required.

Hi Zephyrin, great to have you on board again.  I totally agree with you that to measure the time varying temperature in a steam engine cylinder would be beyond what most of us have available.  The normal time constant of a thermocouple means that it would give an average at best. 

I am really interested in that pressure measurement chip.  Will use it with Arduino or Picaxe, or are you into programming other controllers?  I am also interested in how you get the steam pressure to the chip.  Most of the ones I have seen seem only suitable for measuring atmospheric air.  I hope that you will keep us all informed on your progress.

Thanks for your comment about the needle valve.  I think the issue is that sometimes a valve is adjusted to control pressure, while if it is the engine regulator, it is used to control speed, and of course, in a piston engine, speed is about volume.  However, more speed means more work, so generally requires more pressure, again pressure and volume related.  So, at the end of the day, I feel the best description for a simple passive device is adjustable orifice.  The word controller is then reserved for a device that automatically responds to a change to restore the set condition.  The governor Brian is documenting is a very basic self contained device, but definitely complies with the definition of a speed controller.

There is a very small temperature change with throttling that varies with the starting pressure.  At the pressures we are usually talking about, it is very small.  The process is basically adiabatic, so no heat input or loss and no work done, and can be calculated with steam tables.  The pressure-enthalpy diagram best shows what happens.  But when we have throttling, measuring that pressure becomes essential.

Thanks for following along,

MJM460

[Zephyrin]

Thanks for your answer MJM, I like the way you simply describe all the events occuring while opening the steam valve ! it's a great opportunity to revise thermodynamics!

As regards the pressure measurement chip, the signal from the chip must be amplified to give a manageable response, that is all I understand, and I have to wait up to the end of the summer for help from a friend for the electronics...

[MJM460]

Hi Zephyrin, thank you for your kind words.  You have described exactly what I am trying to do, so I take it that at least sometimes I get there.  Enjoying the learning is the main idea.

My apologies also for mis-spelling your name yesterday, I have gone back and corrected it, I hope I have it right this time.

Yesterday's post was getting very long so I did not want to add more, but I thought that in view of some of the comments about needle valves in several other threads, that a little more explanation today might be a good idea.

Typically a needle valve might be used as a control valve for a gas burner, or as Paul suggested, as a regulator for a small engine.  So let's look first at the gas valve used with a burner.

Initially, the needle valve is closed, assuming that any separate isolating valve on the bottle has been fully opened,  it's upstream pressure is the pressure in the gas bottle.  With the needle valve closed, there is no flow, so no pressure loss between the bottle and the upstream side of the needle valve.

The downstream pressure for the needle valve is atmospheric pressure, as while the burner might have small jets, the path to atmosphere is open and again, with no flow, there is no pressure loss between the burner and the downstream side of of the needle valve.

So the pressure difference across the closed needle valve is the difference between the absolute pressure in the bottle and atmospheric pressure, which is of course the gauge pressure in the bottle.

When we open the needle valve enough to cause a small flow for lighting the burner, the pressure drop across the valve causes a flow.  If our gas is butane at moderate temperature, let's assume the pressure is around 200 kPa again absolute.  The gas velocity at the needle valve is roughly proportional to the square root of the differential pressure.  (We more often see this expressed as the pressure drop is proportional to velocity squared).  This flow causes a pressure drop in the burner jet, and assuming the pressure drop in the tube is negligible, the downstream pressure of the needle valve will be the same as the upstream pressure of the burner jet.  If the flow was higher, the burner jet pressure drop would increase, thus reducing the remaining pressure drop available for the needle valve, so the flow will reduce.  You see, the downstream pressure for the needle valve is determined by an equilibrium pressure drop between the burner jet and the needle valve.

With the burner alight, you now open the needle valve some more, thus increasing the flow area.  With more flow area, and the same pressure difference, the flow increases.  This increased flow increases the pressure drop across the burner jet, which reduces the pressure drop available at the needle valve, so the flow quickly stabilises at a new equilibrium, but a higher flow than with the smaller valve opening.

I did assume a very low gas pressure.  The flow of any compressible gas through an orifice is determined by the pressure difference for small pressure differences up to a point where the resultant velocity equals sonic velocity for the particular gas conditions.  The occurs at a point where the absolute pressure downstream of the orifice is approximately fifty percent of the upstream absolute pressure.  Above that point, the velocity through the orifice remains constant, the flow is determined by the upstream pressure only.  If we are using a propane-butane mix with somewhat higher pressure, we will have sonic velocity, so flow is proportional to the upstream pressure only.  The downstream pressure will again be determined by the burner back pressure, and with propane as fuel and a supply pressure greater than 400 kPa absolute you could have a flow sufficient to produce sonic velocity in the burner jet, so the pressure upstream of the burner results from an equilibrium between the flow through the needle valve, and the pressure necessary to get this same flow through the burner jet.  Any flow which gives more that 200 kPa upstream of the burner orifice will give sonic velocity in the burner orifice, so this is quite common with higher fuel pressure.

We can see that the adjustable nature of the needle valve means it is quite useful to adjust flow by changing the area, but the resulting pressure and flow are both determined by the equilibrium determined by all the elements in the system, not just the needle valve.

The other example which gives a little more insight into the humble needle valve is its use as a regulator for a small locomotive.

When the boiler is up to pressure, let's just open the needle valve a little to let's through some steam to warm the cylinder.  Initially the pressure is not enough to move the train, and the build up of pressure is slowed by condensing of the steam, and leakage out of the cylinder drain cocks.  Pressure builds as the cylinder warms to give a higher condensing pressure and eventually we close the drain cocks.  Now the steam flowing through that needle valve is flowing into a closed space.  More mass in the same volume means the pressure builds until it is sufficient to overcome the resistance of the train and start the piston moving.  Initially, the needle valve pressure difference will probably be sufficient that there is sonic velocity at the seat.  But as the pressure rises in the cylinder, the available pressure drop at the needle valve is reduced and flow is determined by the upstream and down stream pressures.  Initially the piston moves slowly, so this determines very small steam flow, so small pressure drop at the regulator and the piston will see close to boiler pressure.  As the train speed increases, the flow to the piston must increase in line with the engine swept volume at increasing rpm.  Eventually, the volumetric flow is sufficient that the pressure drop across the needle valve is just enough to provide only the necessary pressure on the piston to maintain that engine speed. 

If we want to go faster, as the kids always do, we open the needle valve a little further.  At constant speed, the volumetric flow is the same, but with a larger opening of the valve there is less pressure loss, so higher pressure available downstream at the piston.  This provides more force on the piston, more torque, and the train accelerates some more.  And so on until the regulator is full open.  More speed then requires more boiler pressure, or higher fire, and we go back to opening the gas burner needle valve, or shovelling more coal.

In all cases the volume of the fluid at the lower pressure is greater, or the density lower than upstream.  In one case, the downstream volume is determined by pressure drop through the burner jet due to the gas velocity, in the other the volume downstream is determined by the positive displacement engine capacity and rpm.

That is rather tedious in detail, but I hope that if you have made it to here, it offers a little more insight into the humble needle valve, and how the pressure or flow changes.  I suppose you could call it a control system if you include the human operator making adjustments the the needle valve in response the a pressure gauge reading of engine speed.  Technically, it is an open loop system.  You can increase the flow by adjusting the valve, but there is no feedback to adjust or control the flow in any automatic manner.  The flow can be changed just as readily by changing the upstream pressure, or the resistance or capacity of the downstream system.

I am still intrigued by your pressure measuring chip, I will have to search for a data sheet.  That should be no issue with the number you gave given, I will have a go in the next few days.  I believe there are available operational amplifiers especially intended as instrument amplifiers.  I will be interested to know which one your friend eventually recommends.  Then I guess you can use your preferred micro controller to produce a readout or data logging facility. 

Sorry to inflict another long post on you.  I do seem to like getting down to the nitty gritty detail.  Probably time now though to move on to a new question, unless I have prompted other issues in the above description,

Thanks for following along,

MJM460

[paul gough]

Hi MJM, Just read your 2nd. needle valve post after another stint in the hospital and escaping at 4pm today. I'm going to avoid ever going there again!!!

I guess I should have clarified what I meant when referring to a loco throttle being a volume control. For the most part steam chest pressure should be boiler pressure or at least very close to it at maximum cylinder demand/power output, all other conditions are largely irrelevant, the boiler, steam circuit, throttle valve should be adequate to ensure this pressure can be maintained in the chest and the ports, valve events should be capable of delivering adequate steam to the cylinders for maximum output. So when under load a locomotive where possible should have the throttle fully open and power out put governed by valve travel (linking up). Fine adjustments to the throttle and valve travel are made by drivers in accordance with the topography and reference to the steam chest pressure gauge and the exhaust back pressure gauge. This also applied to my twelve inch gauge 0-6-0, maintaining boiler pressure in the steam chest and regulating valve travel for more or less power. I cannot speak for smaller gauges and gauge one is something of a challenge operationally and in actually knowing what is going on. Hence my enquiry regarding needle valves. I am aware it may not be possible to recreate all or any of the conditions that might apply in full size or 12 inch gauge but I think it a reasonable course to ponder and investigate until it is shown definitively to be a waste of time or even detrimental. Thank you for the 2nd post on needle valves it further clarified things.

I would very much like to know if ball valves or similar full flow orifice types behave at all differently particularly when partially opened, say 50% or less. I have thought about replacing  the needle valve throttle/regulator on my gauge one loco with a specimen of this type as an experiment. Regards, Paul Gough.

[MJM460]

Hi, Paul, I'm glad you were able to escape, those episodes are not much fun.  Worth working at controlling the causes, but when you need medical help, the hospital is the best place to be.  We are all very glad to have you back.

For maximum power to the wheels, clearly you need a clear open path from the boiler to the valve chest, so there is maximum possible pressure available at the piston face.  The problem is that there are occasions when we don't want the maximum available power.  For example, too much acceleration when starting off might cause wheel slip, too much speed on a down hill stretch with a bend at the bottom will never end happily.  You know the scenarios.  Energy input to the boiler, especially with coal firing, has a very slow time constant, you can't quickly reduce firing to slow down, specially if after that bend is the start of the next uphill grade.  And imagine trying to accelerate a train from stand still by increasing the firing rate when the guard blows the whistle!  So we need a quick and responsive method to match the required quantity of steam to the cylinders and the steam produced by the boiler.

A partly closed regulator, or when necessary totally closed, is an immediate method to reduce steam flow by driver intervention.  As it is an adjustable restriction, it is flexible as to the degree of restriction required.  It is not efficient, but as you say, the aim is to adjust what is available so that eventually the energy produced by the fire is balanced with steam flow to the engine, with minimal inefficient throttling by the regulator.  But this takes time and the train is moving along. Valve gear, by enabling reduction of valve travel, is a more efficient, so more desirable method of adjusting the energy balance, particularly to the extent that it achieves early cut off.  But which ever method is used, it is not without consequences on the boiler and firing.  If the regulator allows less steam flow to the engine, the boiler pressure will tend to rise.  In the absence of other action, the increased pressure means increased temperature so increased heat losses, both through the shell and to the flue gas.  Eventually the safety will lift to achieve the energy balance by "using" the excess steam.  The boiler firing must be reduced to minimise the excess steam production over the requirement.  While temperature and pressure instruments are provided in full size, the practical limitations of the small size of gauge one, (and even considerably larger), mean it is not practical to provide the same level of instrumentation.  Especially when any readings would have to be transmitted to the driver, who is not on board to read simple instruments.

All restrictive regulators act in the same basic manner.  The restricted area means increased velocity and reduced pressure.  Unless the restriction is a carefully shaped Venturi like in an injector, most of this pressure loss is dissipated in turbulence downstream of the restriction, and not recovered, and the overall effect is reduced steam flow, with lower pressure downstream of the restriction.  Not efficient in terms of energy consumption, but quickly achieves the required result, but only as an interim measure to serve until the boiler energy input is adjusted.

The difference between using a needle valve, ball valve, or disk type regulator are essentially in the mechanics.  Operationally, a needle valve offers a predictable progressive opening as the needle is turned, the ball valve initially is progressive, but the opening quickly proceeds to the point where there is no significant throttling, only part way through its quarter turn movement.   A plug valve with oblong openings opens even more quickly, so not very controllable at low flows.  A disk valve can be tailored somewhat by shaping the holes, and a v shaped opening will give better control at small openings than some other shapes.

The mechanical differences between the different types is also significant in longer term use.  Wet steam in particular is quite erosive, and those fine droplets, though hardly visible, are able to cut hardened steel.  A needle valve, just by its shape tends to be least susceptible to this, while a ball valve will soon be damaged to the point where it will no longer shut off.  This means the are normally restricted to applications where they are normally only used full open or shut, with minimal  time throttling.  Disk type regulators seem common in published locomotive designs.  I have no experience with them, but I assume their common use implies that they tend to be satisfactory for the time a model is actually in use.  I would expect you could use which ever type was simplest to fit in your gauge one models, though a ball valve would be a bit twitchy at small throttle openings so a bit harder to control.

I am glad the needle valve explanation was informative.  I think most of the confusion arises from the fact that the restriction actually reduces mass flow by imposing a flow restriction.  The resulting steam conditions downstream of the restriction are really determined by that downstream system, not by the restricting device.  It is reasonable to talk about either the pressure or volumetric flow, depending on the one we actually measure or are most interested in.

Best wishes for a speedy and full recovery so you can get back to the workshop, rail track and the frogs.

Thanks everyone for looking in,

MJM460

[steam guy willy]

Hi MJM, interesting concepts there.... you need the same amount of steam to fill the cylinder but it needs to move slower ?? I have always wondered how fast the pistons were moving when the  126 mph world breaking Mallard was running ? also one needs to take into account the momentum of the engine !! and on a level track how far would the engine travel if the steam was suddenly cut off completely ,a simple maths problem that those schoolboys were given at the time i guess !!    In a lead acid battery ,how faster or slower is the evaporation of the electrolyte ?  just  a practical question this time ....
Willy

[Gas_mantle]

I did a bit of simple schoolboy maths to get a rough idea of how fast the pistons would be moving at 126mph land speed, if I've done it right the figure is a lot lower than I would have expected. I guessed the driving wheels as being about 7ft in dia and the piston stroke as being 3ft - using those figures we get :-)

Wheel circumference 22ft.
1 mile = 5280 ft   5280/22 = 240 wheel revolutions per mile
240 x126 = 30240. Therefore at 126mph the driving wheels make 30240 revolutions per hour.
At 3ft piston stroke we get 3 x 30240 = 90720ft of piston travel in either direction or 181440ft per hour at 126mph.
181440 /5280 = just over 34mph average piston speed.

Admittedly 34mph is an average speed over an estimated 3ft stroke and the speed will be considerably higher at midstroke than at the endstroke but it still is lower than I would have guessed.

I think I've calculated correctly but it is possible I've overlooked something 

[MJM460]

Hi Willy, basically to answer your questions we have to consider the locomotive as a system.  If we look at the whole system, which we may represent as a square box (not a black one please, - we do know what goes on inside!), we have a system with fuel input and work out.  Perhaps we should include waste heat out in the exhaust steam, flue gas and other heat losses.  It basically operates in equilibrium, or steady state, when the work out plus the losses exactly consumes the input energy.  If we demand more work output by entering an uphill section of track, or adding more load, without changing the energy input, the train will slow down to use the same amount of power by doing the required work at less speed.  Remember power is work per unit time, measured in watts.  And the fuel input can also be expressed as energy per unit of time, or watts.  And of course we all know the song, heat is work and work is heat.  (We had better be careful here, or Zee might start singing again!)

On a flat track with constant load, if we want to slow down in preparation for arriving at a station, we basically have to reduce the fuel input. 

Of course, as I mentioned yesterday, the time constant associated with reducing fuel input is too long for practical purposes, so we have to do something else for a quick response, and then deal with the consequences of the excess heat input.

If we look inside the box representing the whole system, we see two subsystems with some control gear between, a boiler with its firebox and fuel supply, and an engine which takes in steam, and outputs work at the shaft/wheels, and exhaust steam.

If we look at the boiler, it takes in fuel, outputs steam, and there are losses which take care of the balance.  If we put in more fuel, there will be more steam out.  On the other hand, if we restrict the output steam, with the same fuel input, the boiler pressure will rise, losses in flue gas and convection losses all increase, and if this is not enough to find a new equilibrium, the safety valve will lift.  If we try and take more steam on the outlet side, with the same fuel input, boiler pressure and so temperature will fall and the steam output will settle at the point where the steam plus losses carry away all the input energy.

At the other end of the overall system is the engine.  Steam at the inlet is admitted to the piston where the pressure exerts a force on the piston.  This force acts through the piston rod, conrod, and crank shaft and exerts a torque on the driving wheels which reacts with the track to make a force on the locomotive frame in the appropriate direction.  If this force is sufficient, the locomotive will move, resulting in the piston moving, and as we have previously learned, when a force, like that on the piston, moves through a distance, work is done.  If the force on the piston is more than sufficient to just start the locomotive moving, the excess force where the wheels meet the track will accelerate the locomotive, and once the drawbar slack is taken up, eventually the whole train.

As the train accelerates the piston requires more steam, otherwise the pressure will fall.  Thus the engine accelerates until equilibrium is achieved where the steam required to maintain the pressure at the piston face is just equal to the steam produced by the boiler.  If the boiler is producing more steam than the engine requires at the current speed, the pressure at the piston face will increase, and the engine will accelerate. 

Ultimately the two systems operate at steady conditions when each has found the point where there is equilibrium between input energy and energy consumption.

As we saw yesterday, that control gear between the two systems, let's just assume a throttle valve regulator for the moment restricts the flow of steam, thus the transfer of energy from the boiler to the engine.  When the regulator is adjusted, the flow of steam happens almost immediately, the increase in boiler pressure is a bit slower, so the fireman has some time to react.  The lower steam flow means the steam flowing to the cylinder reduces in pressure, hence lower force on the piston, lower torque at the wheels, and less force to maintain the train velocity so the train slows down.

This brings us to your momentum question.  I have said it before, but can't say it too often, momentum is a very fundamental quantity in physics.  We all have an intuitive idea of what it is, but it can be very precisely measured, and it's effect in a moving system is precisely predictable.

We all know about the law of conservation of energy.  It is a fundamental law of physics, and leads to the first law of thermodynamics.  Not so well known is that conservation of momentum is an equally fundamental law which also always applies.  And it is even easier to use than conservation of energy for many of the problems where conservation of energy is often quoted.  I am thinking of wings, sails and propellors for a start.  So let's look at what momentum means on our moving train as per your question.

Momentum is a property of any moving mass.  It is quantified mathematically as mass times velocity.  SI units come into their own here, there are no arbitrary constants such as "g" required.  Mass in kg times velocity in m/s equals momentum in kg.m/s, ( read this as kilogram meters per second).

The law of conservation of momentum means that a body continues at rest or uniform motion in a straight line unless it is acted on by an external force.  Now, where have we heard that before?  It is Newton's first law of motion. (Or is it the second?  Not really important which order he discovered them.)

Now when a force acts on that moving body for a time, we get a change in momentum.   Change of momentum per unit time equals force.  So if we know the velocity of the train in kg, and measure how many seconds it takes to come to a stop when the steam is suddenly cut off, we can calculate the resistance or total drag on the train.  Of course, this simple calculation requires one extra assumption, that the force is constant.  We know that the air resistance portion of the force is proportional to the velocity squared, but it is also probably quite low compared with the total rolling resistance, unless of course you are thinking of that Mallard at record speed.  It also requires sniffting valves to be fitted to admit air freely, so the engine does not do work creating vacuum or pressure somewhere.  But over a small speed change, where the change in variable resistances can be ignored, the calculation is quite exact.

To calculate the piston speed we need to know the driving wheel diameter and the piston stroke, then it is relatively simple maths to work out how far the piston travels each revolution of the drive wheels and how many revs per unit time the driving wheels rotate at any given speed.

I notice that Gas-Mantle has done the piston speed calculation while I have been writing.  Give or take any minor arithmetic error or inaccuracies in the measurement estimates that should give the correct answer.  The piston speed is a maximum in mid stroke and zero at each dead centre, but the conventional method of specifying piston speed for engines and compressors for industrial machines uses the same simple method, so it is really an average piston speed.  Ring wear is generally roughly determined by piston speed so engineers typically specify a piston speed limit for an acceptable design.  I can't remember the typical figures, but it is a relatively modest figure that can be achieved with long stroke and low rotational speed or short stroke and higher rotational speed.  My compressors were generally classed as low speed, typically about 400 rpm, and typically had about 12 inch stroke.  Oil field compressors were generally described as high speed and were typically 1000-1200 rpm and about six inch stroke.  But the same piston speed limit applied to both.  Of course these criteria apply to industrial machines expected to work 24/7 for long periods without interruption.  I am sure a different criterion would be used for a race engine, or a record attempt, but don't try and run those special engines 24/7 for very long.

Hi Gas-Mantle, thanks for looking in and doing that calculation.  Good to hear from you again here in addition to your other posts.

I suspect any experiment involving momentum and the Mallard would be limited by the available length of straight, level track.  Predicting the total distance requires knowing the total mass, and the total rolling resistance.  A more practical experiment would be to measure time for a relatively small velocity change, and repeat the measurement over a similar speed change but from different starting speeds.  Each measurement enables a drag calculation for that speed range.  Assembling the results on a graph enables construction of a curve showing resistance with speed.  From this curve, the total distance on that ideal never ending straight, level track can be calculated.

On the lead acid battery question, in the old days, batteries had a screw cap with a small vent hole in each cell.  It was important to check battery liquid levels frequently, as it did evaporate.  It could even boil quite vigorously if charged at too high a charging current.  I am sure that you can remember those days.  Then we had sealed maintenance free batteries.  I assume the evaporation resulted in pressure change within the casing.  Severe overcharging could increase the pressure sufficiently to damage the casing.  I don't know if they had any specific over pressure vent.  Perhaps another forum member can comment on that.  Now we have gelled electrolyte batteries and AGM, and as far as I know there is no evaporation problem.  If you are really up to date, lithium Iron is the go!

If you have a battery that is loosing electrolyte, it is probably worth checking the regulator charging rate.  Internal shorts and other cell faults might also be the cause.  These days we do not have those external bars connecting the individual cells so we can't check the voltage of each cell separately.  I assume we can deduce a short circuit by testing the voltage after completion of a suitable charging cycle, and measuring the voltage after a rest time after disconnecting the charger with no load.  It should still be about 12.5 volts minimum to 12.8 or so depending on temperature and whether the battery is fully charged.  A shorted cell would immediately drop this by about 2 volts.  I hope that helps.

Thanks everyone for looking in,

MJM460

[paul gough]

Hi All, Mallards driving wheel dia. is 80 inch and cylinder stroke is 26 inch. At 126mph = 530 rpm or 8.8 revs a second. Piston speed 1060 strokes/min X 2.167ft = 2297 ft/min. Mallards highest horsepower output that I know of was just under 2500 H.P. Now it was going down a slight grade when doing the speed record so would not have been near this figure but consider 8.8 revs per second, the reciprocating masses and whatever power was applied from the cylinders and you can understand why it was damaged doing this run. However, a little more modest speed of 100 mph for the very best modern express locos, on suitable sections of track, was actually timetabled on some of the named passenger expresses in the U.S. and I presume elsewhere. Considering the forces involved at over 8 revs a second with the acceleration and deceleration of the piston and rods all coming to a stop and then reversing direction one can only have the greatest respect for these machines. Consider what it would be like to be the crankpin! Paul Gough.

[MJM460]

Hi Paul, thank you for the information on the Mallard.  As you can tell, I have not looked too much into its history, or history in general, for that matter.  My grandson brought home an HO gauge electric locomotive, a Triang/Hornby model of it, as his souvenir from the trip his family took to Europe and UK last year, so at least I had heard of it and its famed speed.  When you are thinking of the forces, it is not only the crank pin, but also the cross head pin that has to take all that force.  Of course the whole engine and its frame has to be designed for the forces, and the maximum piston rod force turns out to be the controlling load for the whole frame and moving parts.

Thanks for following along,

MJM460

[steam guy willy]

Hi MJM,  'the fireman needs to act' would this entail putting more coal on the fire to take out some of the heat and then delay a good head of steam to egress the platform again ? lots of interesting info again, cheers.

Willy

[Gas_mantle]

Willy, this video gives a good introduction in how to fire a steam loco. It's actually a far more skilled job than a lot of people think 

<a href="https://www.youtube.com/watch?v=NHo860Q66Gw" target="_blank">http://www.youtube.com/watch?v=NHo860Q66Gw</a>

[steam guy willy]

hi thanks for that..interesting and informative...I noticed on the film of the Tornado trip ,the fireman wasn't shovelling coal all the time !!....Hi MJM....Also on the news it said that after the heatwave of 52 days when the rain came the water temp dropped 10 degrees and killed all the fish because the oxygen content dramatically decreased !!They then put in fountain pumps to try and get some oxygen back into the water the duck weed also increased dramatically ? I have also heard that here in norfolk they used flint to help break up the clinker in the fire box...it would heat up and explode/shatter !!?

[steam guy willy]

Hi MJM,...a friend has a flat in the attic of a victorian house and she is suffering from the high temps about 32 degrees. She is wondering how to keep it cool and says a fan doesn't make any difference. I have given her the info about closed windows   blankets etc. She says the window  perhaps is not glass but some sort of acrylic. it is a rented flat and there is possibly no insulation in the roof. I was wondering if muslin cloths in buckets of water might cause cooling by evaporation ?  Have you any ideas please....thanks

Willy