Author Topic: Talking Thermodynamics  (Read 90961 times)

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1020 on: August 10, 2018, 03:04:14 AM »
Hi M.J.M., Here are the charts I mentioned previously, see attachments. I hope they are of some interest.
 Loco specs; 4-4-0 express type; cylinders 19x26 inch, valve travel 3 5/8", 1/8" lead, 1" lap at full gear; Driving wheels 7'1"; boiler 4'4" Dia, 11'4" between tube plates, 240 x 1 3/4" O.D. tubes, copper firebox 5'7" x 3'2" at grate, grate area 18.14 sq. ft., firebox heating surface 112.45 sq. ft., tubes 1246.2 sq. ft., total 1358.65 sq. ft., total area through tubes 2.47 sq.ft.; Boiler pressure 175 p.s.i. Regards, Paul Gough

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1021 on: August 12, 2018, 12:52:52 AM »
Hi MJM, Just a few observations ... today i had a really cold smoothie and i was thinking about warming it up slightly so i was blowing into it. I noticed that when doing this cold air was being returned into my face ?!!! so was this coldness being driven from the smoothie? When you have hot food and drink you blow on it to cool it down .....so does the same thing happen with ice cold drinks in a reciprocative way, to get back to ambient temperature ??  Just wondering.............Paul , those charts will need some figuring out to draw some conclusions......I use to travel on that line years ago when they had steam engines It would have given something to do to while away the time !!!I was stationed at Arborfield cross when i was an electronics apprentice !!



Willy

Offline paul gough

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Re: Talking Thermodynamics
« Reply #1022 on: August 12, 2018, 08:00:47 AM »
Hi Willy, The charts can provide various levels of analysis. You don't necessarily have to jump into applying mathematical methods to get something from them. Firstly, they show the kinds of things that were important to know and what was measured under operating conditions and the results/performance then considered against the existing dimensions of the loco. Some things are obvious once you know them, but often it is not until you see figures for various things and start making comparisons and connections that the relationship between them becomes apparent. Have a look at the boiler and engine efficiencies. Why such a difference?

 Some sense of what is happening and what the driver is doing can be got by comparing the vacuum profiles against the speed and line profiles while noting the indicated horsepower trace. Noting  the normal range for the vacuum in the smokebox, what difference between it and the fire hole door, asking are the changes co-incident at each location, does smokebox temperature and vacuum always parallel each other. Just simple comparisons, maybe obvious, but until the numbers are checked, it is only speculation. The numbers help verify ideas, which might then be improved upon.

The book from which these charts are taken is from; 'A Manual of Locomotive Engineering', by W. F. Pettigrew, Charles Griffin & Co., London, 1899. There are a number of later editions, and in the U.K. it should not be too hard to get a copy. I have always considered this book the best I have come across because it explains things comprehensively but easily. Perhaps there one in a library you can check out. 

 In the end, pondering all these things along with some of awareness of the design reasoning/decisions provides insights into what might or might not be  good in model design, always with the caveat, that not all full size ideas or results can be replicated when scaled down. Hope all this is not boring people. I trust M.J.M. can fathom something from the information that is useful. Regards, Paul Gough

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1023 on: August 13, 2018, 01:24:13 AM »
hi MJM saw this in the newspaper ...i have not seen this before

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1024 on: August 13, 2018, 12:56:47 PM »
Hi everyone, back in civilisation after a few days under the starry skies, with magnificent views of stars, planets and the Milky Way.  Also wonderful surf beach, only one, but 60 km long, magnificent bird life and a four ft. long goanna which was obviously well versed in where there might be food left stored or accidentally dropped.  Got to love the new car (to us), confidently towed the van through deep sand up steep slopes.  Only need a tow out once, but did engage the diff lock a couple of times which got us through.  But my wife says I have to learn to drive more gently, the box of tissues fell off the table, along with two place mats while we came out this morning!

Thanks everyone for keeping up the conversation while I have been thinking of other things.

Hi Derek, not entirely driverless I believe.  I think there are drivers in the control room in Perth, just not on the train.  I wonder if they have changed the electric locomotive that takes over at the end of the line.  It actually had a power cord connected into the mains when I last saw it. 

Hi Paul, those are interesting memories of those times, which do not really seem so long ago to most of us.  We should all  be writing down or recording some of those stories for future generations, we all have them.  And thank you for that interesting information on the locomotive trial runs.

Hi Willy, back to smoothie time again?  When you use a straw to blow through the smoothie, the close contact between the drink and the air enables good heat transfer from the warm breath to the cold drink, leaving the air close to drink temperature when it emerges from the surface.  Just simple heat transfer from a hot material (your breath), to a cooler material (the drink).  When you blow on a hot drink, the cooling is mostly by evaporation at the liquid surface, which is accelerated by the breath carrying away the vapour, which is composed of the higher energy molecules in the liquid, so results in cooling of the liquid.  Also the breath is basically cooler than the hot drink so any heat transfer is from hot to cold, but not very good contact, so only a minor contribution to cooling.  If you blow over a cold drink, there is still cooling by evaporation, but there is less evaporation at the lower temperature, so the cooling is not so effective.  Also the breath is actually warmer than the drink, so the small amount of convection heat transfer that occurs is in the direction of heating the drink.  So in each case, heat travels from the hot material to the cooler material.  In your examples your breath temperature is somewhere between the temperatures of the hot and cold drinks, so the heat flow is the opposite direction in the two cases.

Interesting picture of that "firenado", I also have not seen anything like that.  Obviously some sort of effect of an intense heat source in a cold atmosphere, but it must involve something more, that might be explainable to someone who knows more about conventional tornados.  A coincidental occurrence of such a fire and the atmospheric conditions that cause little "Willy Willies" or "dust devils" I suppose.

Hi Paul, I have certainly been interested to see that real locomotive performance data, and to have your drivers view interpretation of them.  They certainly contain a wealth of information. 

I have been fascinated to see the breakdown of where the heat goes on a real locomotive.  There is so much to learn from these figures, and interesting to consider what we can use to help us understand our models. 

As you say, it is always good to go beyond the simple comparison, and try and understand which is cause and which is effect, and the why of those variations in the various parameters.  Understanding some of the issues will require more understanding of some of the systems, but theory can certainly add some understanding of the why.  But your drivers eye looks at what response is required, when the variation is ok and when something has to be done in response.  Even more importantly, what is to be done.  I will definitely leave that part to you.

With regard to your first question, why are the boiler and engine efficiencies so different, perhaps that is a good one for me to start.

We all know that heat is work and work is heat, and that the first law of thermodynamics allows us to calculate the equivalence.  But transferring between them is not so simple or symmetrical.

It is easy to convert 100% of work to heat, but when we try and change heat to work, we find that we always loose some as heat.  This is stated more emphatically in the second law of thermodynamics.

When we look at how the energy in the random motion of molecules is turned into work, we find it is the linear displacement motion component of that random motion that exerts a force on the piston, and produces work.  But the energy of the molecules is not all linear displacement.  There is energy in spin around each axis of the molecule, as Jo mentioned very early in this thread. The spin results in angular momentum, and that angular momentum has minimal effect on the piston.  Not zero, but not very effective at producing work.  That spin energy tends to stay with the molecule through conservation of angular momentum.  We can see the proportions in the steam tables.  The enthalpy column figure is always slightly higher than the internal energy column.  The difference is approximately the amount of energy that can be made to produce work of we expand the fluid to low enough pressure.

When we apply these concepts to the boiler, we burn fuel to release chemical energy.  The energy released increases the temperature of the combustion gases, and the heat involved is all eventually transferred to the surroundings.  With control of excess air to minimise stack losses, generous heat transfer area to help transfer to the water, and insulation to minimise external losses from the boiler, we can get quite good boiler efficiency, measured as the proportion of heat arriving in the boiler water.  I have seen figures in the low 70% for full size high pressure boilers, in your locomotives and even come close in my model boilers. 

When we look at the engine, the most obvious problem is that of the energy delivered to the engine  in the steam, most of it goes out in the exhaust steam, and only a relatively small proportion is turned to work.  There is still energy that can potentially produce work until the steam is expanded to zero absolute pressure.  However, at this pressure, the volume is so large that expansion to this pressure is quite impractical, so not only is it only possible to turn a part of the incoming energy to work, we can't even practically produce work from all of that small proportion.  The difference in enthalpy values of the inlet and exhaust gives us an idea of the maximum possible efficiency of the engine, but in practice there are further losses due to friction of the moving parts, steam leakage, and heat losses from the repeated heating and cooling of part of the cylinder.  The second law of thermodynamics, in saying that when we transfer heat to work we will loose some, really understates the issue.

I recently saw a report that the newest combined cycle gas turbine power plants from GE had exceeded the previous benchmark a little over fifty percent, but most industrial size plants don't make anywhere near this.  Our little steam plants are nowhere near this level, and the approximately 10% quoted in your tables is representative of real world locomotive performance.   I believe more normal power generation plants achieve around thirty percent.  The efficiency competition results I have seen seem to reach a maximum of around 5% for the best five inch gauge locomotives, and considerably less for smaller gauges.

My own testing so far has all been without load, so quite low pressure.  As there is no work produced above what is consumed by friction, the efficiency is zero.  The adiabatic efficiencies for the steam conditions are also quite low, but should improve when I get to a load test, when higher pressure will be possible with reasonable speed and some work output.

I hope that offers some insight into why the boiler efficiency is so much higher than engine efficiency.  In summary, the boiler is only dealing with heat transfer which can be quite efficient, while the engine is converting heat to work, a process which must always involve losses, which in practice are quite large.

Thanks everyone for following along,

MJM460




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Offline MJM460

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Re: Talking Thermodynamics
« Reply #1025 on: September 12, 2018, 02:59:37 AM »
Hi Willy, good to be thinking about thermodynamics again after a little break.

 I am home from a 6000 km road trip, a great adventure.  Even included some real four wheel drive activity in soft deep sand, and looking forward to next time, but also glad to be home and able to make a bit more progress on my engines.

So, to continue the discussion about heat rising from your engine thread, we can start with the basics.  Heat travels from high temperature regions to low temperature regions.  The effect of gravity, so up or down is not relevant.  I have said it before, but while "heat rising" is still quoted so often, it is worth repeating.

In fluids, whether liquid or gas phase, the material is free to move under what ever forces are in operation, including gravity.  When the fluid is heated, it expands, so has lower density than cooler fluid surrounding it, and so the lighter fluid "floats" to the surface.  A bit like oil on water, but harder to visualise, as the hot and cold fluid do not appear different, and in any case they are continually mixing at the boundaries, which are hence not at all distinct, unlike oil and water.

Solids also expand on heating so their density reduces, but they are not so mobile that they can move under the minute differences in comparison with gravity, friction etc that are also operating.  So in solids, the question of heat rising tends not to arise.  It is easier to accept that heat travels from a hot surface to a cold surface.  In the absence of external heat source (or sink, as in a nearby colder material) the temperature in a solid soon becomes essentially uniform.  Simply because heat travels from a hot area to a cold area until there is no temperature difference.

You mentioned that you were thinking about phase changes.  Phase changes involve energy flow without temperature change, at least no bulk temperature change of the fluid undergoing the phase change.  But evaporation does involve heat transfer, and the heat has to come from somewhere.

Perhaps that is enough of an introduction to help you put some more words around the specific issue you are thinking of.

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1026 on: September 13, 2018, 03:11:37 AM »
Hi MJM, good to be back ...yes i think i know the basics of hot air rising and i guess hot water rises as well. I was thinking about molten steel and other metals though .When you silver solder fabrications the solder will flow from the source of heat ,or is this more to do with capillary action. Do the extra hot molecules of steel rise up in a retort or is that silly question ??. Also there is an ice cream freezer in the local cafe that has a clear sliding top. I was enquiring if they could save electricity by putting an insulating blanket over it. the reply was that it was an ultra modern display case that did not need a blanket and if it did it would over heat ??...Our heatwave is over so it is more comfortable working outside.

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1027 on: September 13, 2018, 12:46:12 PM »
Hi Willy, yes, air, water and even molten metals.  Mind you there are other process going on in the retort that may interfere with the circulation.

Are you sure silver solder runs away from the source of heat?  I m not an expert on silver soldering, so I will leave it to others to answer.  Certainly there is caplilliary action which is why your silver soldered joint must have a little clearance for the solder to run into, for flat surfaces this is done by centre pop marks which create a little dimple to separate the surfaces.

I think the shop owner has not studied much thermodynamics, but the discussion might be frustrating, I would not bother.   Heat travels from hot to cold, so into the cabinet, probably downwards, but by conduction and convection between the glass top and the surrounding air.  It might be that the cover glass is some sort of super insulator, such as double glazed with nitrogen between (I suspect not, but it's possible, used for window glass in Canada, and other cold places) but that reduces the flow of heat inwards but does not totally stop it.  So the blanket would further reduce the heat inflow.  The refrigeration unit for the cabinet will achieve its set temperature with less running so power consumption will be reduced by the blanket, but it will not overheat.  But if the customers cannot see what is in the cabinet, they will either not buy, which does not suit the shop keeper, or they will open the cabinet while they make their selection, which will allow more heat in, so counteracting the effect of the blanket.  So while you are correct in your conclusion based on thermodynamics, it does not suit the shop keeper who wants the product on display.

MJM460


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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1028 on: September 14, 2018, 02:24:13 AM »
HI MJM , I have spoken to the freezer chap and has said that the suppliers said that it shouldn't have a blanket over it as i would interfere with the  cooling system and potentially mess it up !  . I saw this engine at a car boot sale and the chap wanted 350 for it ...so he went home with it ,anyway i couldn't figure out how the meths burner operated with the pipe coming out of the top. Also the lubricator connections seemed a bit strange !!? The engine is a Maxwell Hemmens from the 80's

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1029 on: September 14, 2018, 12:44:20 PM »
Hi Willy, the refrigeration unit under the cabinet has to reject all the heat it extracts from the cabinet.

This heat is rejected from the condenser on the back of the cabinet to the surrounding air.  If the air flow around the condenser is obstructed, the condenser temperature will rise, with it the condenser pressure, which will overload the compressor motor.  So it is very important that ventilation around the condenser not be obstructed when any blanket is placed. 

Apart from the observation that a blanket carefully placed over the top of the cabinet might reduce power consumption, it is probably not worth following too far, as with it comes the risk that the blanket slips and blocks the ventilation of the condenser and compressor, so the practical compromise is probably to use the cabinet as supplied, without modification, temporary or otherwise.  A night cover would have to be carefully designed to eliminate this risk, which involves more cost.  All these things are a balance of risk and reward.

I had seen an article on a Meths burner with the tube coming from the top, I can't remember the detail inside, but I thought there was an additional closed tube into the flame that generated the vapour.  But the memory of where I saw the article.  Perhaps someone else knows the principal.

MJM460
« Last Edit: September 14, 2018, 12:49:18 PM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1030 on: September 16, 2018, 12:00:07 AM »
Hi MJM, just been looking at the vid of the tiny steam engine ...is there a limit to the speed of engines and does the speed figure in tables sort of backwards ? is there a limit on the speed due to the length weight PSI etc etc  off to Beeleigh Sunday with my engines and will take some photos. the engine will be running with air , but i hope to run it on steam !! Are there limits to the size of engine you can run with a known boiler output ?? ..depending on the type of engine you have...Thanks for the info on the freezer it all makes sense now !!
Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1031 on: September 16, 2018, 11:19:12 AM »
Hi Willy, glad the freezer makes sense now.  It's the other end of the scale from the systems I worked with, but the principals are exactly the same.

I think I would generally agree with your observation that "the speed limit table runs sort of backwards".  Meaning that in general a small engine can run faster than a big one, or in more precise terminology, the maximum speed is inversely proportional to the size.

When I think about why this is so, I think the issues can be divided into two distinct groups.

First is the forces and stresses that increase with the mass of all the moving parts.  For any given size, increasing speed means greater inertia forces, and higher stresses, so that eventually the weakest component breaks.  This is so important that most modern engines and turbines used in industry have not only a governor, but also a totally independent over-speed trip device.  On steam turbines, this device operates a very fast acting trip valve that cuts off the steam supply and shuts down the turbine.  A high speed machine that over-speeds to destruction is very dangerous.

The other group includes all the factors that limit the speed the machine can reach.  Things like the friction loss in the fuel and air or steam passages, steam supply for a steam engine or turbine, friction in bearings, air resistance to the moving parts, viscous forces in bearings and sliding parts and so on.  Unfortunately these do not always become important enough to limit the speed until speeds already exceed that necessary for something to break.

I guess there are also factors in the grey area between the two groups, such as wear rates.  Generally higher speed means more wear and tear, so a decision has to be made as to how much wear and tear is acceptable.  For many of our steam engines, we like this factor to be minimal, so the engine can run as long as possible, before we have to rebuild parts of it.  For our cars, we like them to run reliably until the next service is due, or longer in case we are a bit late.  For a race engine, maximum power and speed is wanted, and the engine will be fully serviced or rebuilt after each race.  So there is a range of what is acceptable.

So if we return to the question of speed vs. size, small engines like the glow plugs Ramon has been showing, might run 20,000 rpm or so, Lohring's amazing engine much faster again.  Our car engines run at a much more moderate speed.  And a large diesel in a container ship will run quite low rpm at maximum speed.  And those are reciprocating engines.  A little turbine might run 50,000 rpm.  A twenty inch diameter rotor on one of my compressor drives runs about 12,000 rpm, while a large lp turbine on a power station might run only 1500 rpm.

When a linear dimension is doubled, the mass increases eight times and the forces increase with the mass.  If the speed is halved, the forces reduce to quarter, not quite enough, so the bigger engine has to go even slower for similar forces.  Similarly, for the same pressure, if a piston diameter is doubled, the force is four times, again adding to the increase of stress on a larger engine.  But of course we have the famous equation, F= m.a.  Or a= F/m.  From this, we can see that as the mass increases, the acceleration due to a given force is smaller.  Or put the other way, for a given force, a smaller piston accelerates to much higher velocity than a bigger one, so easily reaches a higher speed, even with lower pressures.

The fluid dynamics and other factors which limit the speed attainable are perhaps a bit more complex to analyse, but there is no incentive to design a system to supply fuel to accelerate an engine to the point of destruction, so there are probably some deliberate built in limitations in that area, at least when the engine is fully loaded.  One easy factor to consider is piston speed, usually calculated as the total distance the piston moves in a given time.  This is actually an average speed which is about seventy percent of the maximum speed, but a good indicator of wear rate on rings etc.  Obviously a small piston on a short stroke can go many more rpm to have the same piston speed of a longer stroke engine.  So double the stroke, you have to halve the rpm for the same piston speed and wear rate.

With regard to boiler size, it does not matter if the boiler is too large, apart from weight if it has to be portable, or cost if the boiler is built for the purpose.  The boiler controls, including the safety valve, will look after the boiler even if the engine is drawing no steam.  So if you take your engine along to the Beeleigh museum, and hook it up to a connection on a suitable steam line on the full size boiler, it will be fine.  (Particularly as I am sure it is not highly superheated, or really high pressure, which would each require extra consideration of the connection details.)

There is no one size of boiler that is correct.  Over a reasonably wide range, a larger boiler will allow the engine to do more work, while a smaller one may limit the engine output.

As the boiler gets smaller, it will be limited in the volume of steam it can produce at any given pressure.  The engine will take a certain volume of steam for each stroke, and if this is more than the boiler can provide, the boiler pressure will fall.  The engine will run slower at the lower pressure, so requires less steam, and so on until the boiler is no longer able to supply enough pressure to overcome engine friction at the lowest speed the engine will run.  The limit is when the pressure on the piston does not produce enough force on the piston, hence torque, to accelerate the flywheel to the minimum speed at which the stored energy will carry it past the top and bottom dead centres.  Obviously it has ceased to do any useful work before this.  So a boiler can be too small.  As a guide, it should be able to produce a volume of steam at the required working pressure for the piston displacement at the required engine speed.  It can be a bit smaller if the valve gear provides for earlier cutoff.  The potential steam volume can be calculated once the burner fuel consumption is known, as we have discussed before.

Glad you will run your engine on steam, surely that is required for a "real" steam engine.  However, I can see the convenience of using air for a first test run, quick demonstrations or display days, when insurance and public safety come into play.  But at the end of the day, I think an engine should at least have a test run on steam to be complete.  It is not too hard to build a small boiler you can use in your workshop or garden, so long as you have an adequate size torch.  Or better still, borrow one ( preferably with operator attached).  It is always helpful to have extra hands and even a second torch maintaining the general heat level on a larger job.

I hope that covers all the questions,

Thanks for looking in.

MJM460
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1032 on: September 18, 2018, 12:19:41 AM »
Thanks for the very detailed answers MJM,  I have been looking at Flash steam boilers for early hydroplane engines and notice that the copper tube is wound in coils. Would there be any advantage in a back and forth winding rather than a circular arrangement ??. I will need a bit more time through your last explanations. The HP of an engine depends on the speed the engine is running , so with an ic engine that is continually changing speed does the HP change as well ?  Was there a measurement of power before Watt's 33000 Horses were all coupled together ?? :mischief: !!!
Willy

Offline Zephyrin

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Re: Talking Thermodynamics
« Reply #1033 on: September 18, 2018, 08:22:37 AM »
Nice engine you have Willy, looking as could be overhauled easily...
It is a fast running engine, for driving a boat at a good speed, 8 km/h I would say.
For the oiler, I suppose that the "tube" in the bottom is just holding the oiler on the tube, and not connected with it, otherwise it cannot displace oil, unless this tube is prolonged high in the oiler, up to the oil level.
For the other mystery with the alcohol reservoir, as there is a tap in the alcohol feed tube, this could work by siphoning maybe ?
the alcohol tank may also be filled with cotton wool or another absorbing matter, to damper liquid movements in the tank (it is steam plan for a boat model!), as it was common in old tin plate toys.

« Last Edit: September 18, 2018, 08:41:57 AM by Zephyrin »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1034 on: September 18, 2018, 10:47:58 AM »
Hi Zephryn, good to hear from you again.  Is it possible you are looking at the photos of the engine Willy saw at a car boot sale, but decided the price was too high?  I think you are probably on the right track thinking the burner might work by syphon with the tube extended down to near the bottom.  It would not be able to empty the tank that way, but the tank floor may be above the base of the cylindrical walls.  Interesting to look at the lubricator arrangement also. 

Hi Willy, with power, it helps if you remember some basics.  Power is rate of doing work, or work per unit time.

Work is force times distance, or, for rotating systems, torque times angle turned through.  In fundamental units, angles are measured in radians.  So power is force times distance per second, or force times velocity.  Again, for rotational systems, torque times angle rotated through per second.  We don't normally measure angles using radians, usually revolutions unless we are talking about less than one or two revolutions.    We tend to calibrate tachos in rpm. There are 2 times Pi radians each revolution, so one revolution per minute equals 2 times Pi divided by 60 radians per second.

So, power for a rotational system, such as an engine, is calculated using:-

P = 2 x Pi x T x N divided by 60. Or P=2.pi.N.T/60 where N is rotational speed in rpm.

In SI units, Torque is measured in Newton meters, and power in Newton meters per second.  One Newton meter per second is given the special name Watt.  It is not a coincidence that this is the same unit as given to electrical power, and they are indeed equivalent, though I don't know how the early pioneers in this field managed to align the two.  As I understand it, there was no agreed unit of power before Watt decided his number of ft. lb. / min as the power developed by a good workhorse.  I was always taught that he must have had a very good horse as his "standard" horsepower, but I don't know much about horses.

So determining the power output of a motor requires measurement of both torque and rpm.  The characteristic performance curve for a motor is a graph of torque vs. speed (rpm).  The power at any speed comes from the calculation.  The torque and power both vary with the rpm.  And the maximum power does not correspond with the maximum torque.  There is a separate curve for each throttle setting of an internal combustion engine, while for an electric motor, there is a separate curve for each applied voltage.

When an engine is running, the power developed does indeed vary as the speed varies, and also with throttle setting.

Flash boilers, those hydroplanes were very interesting beasts.  I have had Benson and Rayman's book on experimental flash steam on my shelf for many years, and it appeals as an interesting way of generating steam without the heavy boiler making, though I would be aiming for something a bit more tame than the ones described in the book.  But I often go back and read it again.

The principal is simple enough, the engine driven pump puts water into one end of a coiled tube heated by a blow lamp so that at some point in the coil, or over a short distance, the water flashes into steam, at which point the volume of the water increases close to 1000 fold.  In a confined tube, this increase in volume results in a very large pressure increase.  The operating steam pressure and outlet temperature is determined by the balance between the heat input from the burner, water supplied and the steam consumed by the engine.  I believe they tried measuring the outlet pressure on one of their models and concluded the pressure was frightening!  And there would be a big pressure loss between the pump discharge and the steam outlet due to the small tube diameter.

To start up, I assume you pump in a small amount of water, then light the burner. When some steam is generated, the volume of steam in the confined space of the tube means the pressure would rise pretty quickly, and the engine would be running and continue pumping water.  I can see potential for some wild action until the procedure is all sorted out, but nothing that can't be tamed by a more moderate burner output.

There were some articles in Model Engineer magazine some years back by someone (maybe in France?) who was experimenting with flash steam for a locomotive.  He had some very interesting ideas that would be quite applicable to a model boat or even a stationary engine.  I seem to remember the system had an air tank as an accumulator to smooth the pressure and make the system a bit easier to control.  I wonder if I can still find the articles.

The issue I see with coils is that each turn has a low point for water, not usually a good idea in systems involving a phase change, though the steam generation would move it through pretty quickly, so even a cold spot would not result in water slugs for more than a few coils, so I don't suppose it matters.  I assume you would get a real two phase flow over part of the coil before the water is all turned to steam.  However, flat coils which are basically flat or a continuous slope in one direction might be slightly better, especially in a unit operating at more moderate conditions.  I think the main thing is for the water to enter in the coolest section then move progressively to the hottest section at the outlet.  The simple coil is probably the easiest to form.

Are you thinking of a flash boiler for your new engine?

Thanks everyone for looking in,

MJM460
The more I learn, the more I find that I still have to learn!