Author Topic: Talking Thermodynamics  (Read 154537 times)

Offline Maryak

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Re: Talking Thermodynamics
« Reply #210 on: August 06, 2017, 11:03:49 PM »
Hi Maryak, thanks for that explanation.  I wonder if optimising the flow for that turbine might be one factor in the choice of crank angles for the Titanic, while for more typical engine arrangements, exhaust pulsations would not be so important so other factors such as balance can be given more importance.

MJM460

I think that may well be the case. I suspect that at cruising speed the LP inlet and exhaust pressures would be higher than without the turbine.  Based on a total HP of 51000 and the difference in propeller sizes I estimate the turbine would be producing some 13000 HP of the total.

Regards Bob
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #211 on: August 07, 2017, 02:01:35 PM »
I have been intending for some time to address Paul's questions from post #208, so this time I will start with those.

So what about the effect of jacketing the cylinder, and even supplying the jacket with much higher temperature steam.  To understand what the jacket heating will do its best to first understand the basic engine cycle and what happens to the cycle if we add or remove heat during the cycle.

Generally, the steam engine is analysed as four processes which are repeated every revolution.  First, adiabatic expansion, then constant volume cooling, third, adiabatic compression and finally constant volume heat input.  This is referred to as an ideal cycle, and the thing most ideal about it is that it can be easily analysed.  Now any real engine is not very much like this, but the ideal cycle does provide a useful basis against which we can compare the performance of our real engines.

Now if we look at the effect of unintended heat input or loss around the cycle, we can see that during the expansion process, we initially approach constant pressure process which requires heat input.  The heat supplied by the incoming steam far outweighs any heat input or loss through the cylinder walls, which can reasonably be ignored.  Then after cutoff, heat input or loss causes the pressure to fall slower or faster than we would expect based on adiabatic expansion.  Heat input means the pressure falls less quickly, so more work output is achieved, so this is beneficial, just difficult to analyse.  Obviously heat loss means faster pressure drop and less work output.

But let's continue around the cycle.  At the end of expansion, we release the remaining steam and associated heat into the exhaust.  It is not entirely constant volume, but continues into the exhaust stroke.  While you are exhausting steam, any heat input is increasing its pressure so is undesirable.   

Next the ideal cycle has adiabatic compression.  In real engines, this occurs only after the exhaust valve closes, but again heat input means that this process occurs at a higher temperature, and this requires more work input, and so again is a reduction of work output.

Finally we have near constant heat input as the steam valve opens and admits more steam.

I don't know if I have painted this clearly enough, but the summary is that during the downstroke, any heat input is an advantage, during the return stroke, heat input reduces the cycle output.  As it is not practical to switch the heating on and off each cycle, any advantage during expansion will be reduced but probably not eliminated on the return stroke.  Similarly, any heat loss is probably more significant on the expansion cycle than the exhaust so there is a net loss.

It is very hard to quantify these effects.  Certainly, the extent is limited by the small surface area available for heat transfer.  The example of full size practice which seems to be to apply lagging suggests there is a slight advantage to insulation, but not enough to justify the complexity of installing and supplying steam to the jacket.  An even higher temperature would give more temperature difference, but again the extent is limited by area, and it would probably be more effective to apply the heat in the boiler or superheater so the supply steam is hotter.  Then the heat is only supplied to the engine when it is needed.  But if a jacket is supplied with steam at about the inlet temperature, this comes close to the same thing as perfect insulation for the enclosed cylinder.  Obviously the jacket itself then needs lagging.

I hope that is enough to satisfy your curiosity for now.

Now on to Willy's question about the stainless steel cladding of the boiler.  First Willy, I have to ask why you would want to?  I know you can silver solder stainless, but I am not sure of the strength of the resulting job.  If you can join it by welding, stainless steel gives a boiler which has higher strength at high temperature than copper, but it is very susceptible to cracking if there are even tiny amounts of chlorides in the water.  These come in even good potable water, and are very hard to eliminate.  Hence the incentive to try and use duplex stainless steels.  And the thermal conductivity is less than copper, so it's main advantage is high temperature strength.  Copper is better for heat transfer.  I am not sure of the effect of plating on silver soldered joints, but I am sure I have seen model boilers that were externally plated for appearance.  I suspect it even has an advantage in lower emissivity that copper that has lost its shine, so less radiant heat loss.  However a simpler approach is insulation.

Steam bubbles detach very readily, and lead to very high heat transfer coefficients for boiling, so I suspect there is no real advantage in vibrating the boiler.  As I mentioned earlier, the element insulation is probably the main determinant of heat transfer rate, and the consequent element temperature necessary to dissipate the heat.

Having 2 by 500 watt elements means twice the heat input, therefore half the heat up time, and twice the steam production.  It is still necessary to know the mass of copper, and the mass of water to calculate the time more precisely.  It would be interesting to know how long your boiler does need to raise steam.

Thanks Maryak, I think that you are right about those lp cylinder pressures with and without the turbine.  The total pressure range tends to distribute itself over the number of stages, and the turbine is effectively an extra stage when it is on line.  But it depends on valve events and steam chest volumes.  Do you have any more information about that aspect of the triple expansion engine?

Thanks everyone for looking in,

MJM460

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

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #212 on: August 07, 2017, 03:20:11 PM »
Hi thanks for the comments and questions ,I shall endeavour to measure the actually size and water volume and times to steam up soon. Talking about steam jackets the engine i am making ,Woolf Compound by Wentworth and sons is a steam jacketed engine that has a bit later sister engine that is still working. The Ramm Brewery engine is steam jacketed as well but does have wooden cladding although not around the steam chest parts of the engine. In the Beeleigh engine there is no cladding and also no indication of how it may have been attached, ie no threaded bolt holes to secure it. This system of jacketing on this double acting engine means the cylinders have no provision for steam/ condensate removal !! so one assumes that the engine block is first brought up to temperature with the valves in midway closed position, then once the engine block is fully hot the condensate is drained off by a drain cock at the bottom of the cylinder block. the flywheel is then barred into the starting position and the cylinder steam valve opened. This enormous mass and area of cast iron must have been responsible for lots of conduction and radiation of heat continuously during its working cycles !! here are some pics of the two engines, The Beeleigh engine hose burnt down in 1875 so the wooden cladding may have rotted away over the last 140 years

Offline Maryak

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Re: Talking Thermodynamics
« Reply #213 on: August 08, 2017, 01:16:09 AM »
Thanks Maryak, I think that you are right about those lp cylinder pressures with and without the turbine.  The total pressure range tends to distribute itself over the number of stages, and the turbine is effectively an extra stage when it is on line.  But it depends on valve events and steam chest volumes.  Do you have any more information about that aspect of the triple expansion engine?

MJM460

Sorry, I can't pin that down. The information I have been able to find gives conflicting results as to power. i.e. one source suggests 48000 HP with 16000 provided by each of the 3 engines another says total HP 51000. There is information regarding the speeds of the triples at 76 rpm and the turbine at 9psi inlet and 165 rpm. Some of this seems counter intuitive when comparing propeller locations diameters and speeds.

Based on my experience with air pumps in tip top condition a condenser vacuum of 26" Hg is the best I have seen in cold seawater at economical cruising speed.

To give a 9 psi LP exhaust I estimate LP inlet at 24 psi. This suggests that with the turbine out of the loop LP exhaust would now be around 20" Hg and LP inlet around 9 psi, all of this is assuming no linking in or out i.e. line in line valve settings, (which with a standard way shaft is always the case when going astern).

The condensers would now be "pulling" steam through the engines and the 4th stage expansion would be lost. I suspect CW pumps would also need to be operated at higher rpm to cope with the additional heat arriving in the condensers.
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Offline paul gough

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Re: Talking Thermodynamics
« Reply #214 on: August 08, 2017, 01:46:33 AM »
Thanks MJM for considering and running through what is going on with a 'heated cylinder'. I am pleased you did not think it an altogether flippant enquiry. I have been interested in whether attempts to keep cylinders 'warm' in a steam engine is/was of any significant value in practice, especially for models. Thus trying to understand what might go on in an extreme situation can deepen understanding. As I am primarily interested in locomotives it is, (usually), to these I address my investigations. All this line of inquiry was prompted by noting that the full size Lion locomotive, (Titfield Thunderbolt), had the top of the inside cylinders forming the base of the smokebox so the steam chest at least received some heat from the flue gases, but maybe the cylinders also got a bit warmer on a longer run?? As the Lion would, I presume, have been a coke burner like most locos, (railway not colliery lines), before 1850, there might not have been too much soot/ash build up to create an insulating layer on top to impede heat transfer. So I wondered if any noticeable benefit might have occurred beyond the obvious hopeful reduction in condensation formation due to the locos maximum boiler operating pressure of 50 p.s.i. I also notice on my drawing of Stephensons Patentee loco it had the cylinders and steam chest, except for cyl. covers, totally enclosed in the smokebox, presumably it would have achieved a greater heating effect on the cylinders than Lion. Regards Paul Gough.

 
« Last Edit: August 08, 2017, 06:29:41 AM by paul gough »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #215 on: August 08, 2017, 01:33:46 PM »
Hi Willy, interesting that those engines had steam jacketing.  I suspect the boiler pressure was not high, and probably minimal if any superheat, so minimising losses was probably even more important than for a modern engine.  I suspect that the tapped opening you have arrowed is the condensate drain for the jacket.  Today it would have a steam trap on it, but I don't know if they had been invented then.  The engineer probably had to regularly drain the jacket by opening a valve briefly.  The engine could be cleared of condensate by barring it over when it had heated well, before opening the steam valve.  It is great that you have two contemporary engines to help you work out the authentic arrangement after so much time has passed since they were new.

Hi Maryak, I wonder if the 165 rpm is actually the turbine speed or is it the propeller speed?  Modern turbines would certainly be much faster for good efficiency, and use a gear to give the slower required propellor speed.  Certainly the engines would have been designed to be optimum when going forward with the turbine in operation.  Then operation in reverse without the turbine would be an off design condition, where lower efficiency would be acceptable.  I also assume you would not often if ever need full power in reverse.  When the lp cylinder is connected directly to the condenser, it would of course have higher than normal power due to the higher differential pressure,  but I suspect there would be some ripple effect through the ip and hp cylinders which might each operate under slightly modified pressures.  Usually a multi stage engine design aims to have roughly equal pressure ratio in each stage, and equal pressure ratio gives roughly equal power, so your figure of 13,000 HP for the turbine out of a total of 51,000 looks about right.  It would mean about 19,000 HP for each of the triple expansion engines.  I guess further explanation will have to wait until we have more information.

A vacuum of 26 inches of mercury, means an absolute pressure of only 13 kPa.  I assume that 9 psi is a gauge pressure, so absolute pressure of 163 kPa.  This means a pressure ratio of 12.3:1 for the turbine, enough for two extra reciprocating stages, but they would be impractically large.  However, a turbine can handle the volume of steam easily in a moderate size machine, even if with lower efficiency.  A really ingenious arrangement for good fuel efficiency.

No problems answering questions, Paul.  I think this thread probably more useful when it addresses the questions people are wondering about, rather than some subject no one is interested in.  It is interesting that heating or cooling of the cylinder is an asymmetric problem.  We know that heat loss is detrimental, but we can reduce heat loss until it is hardly significant by insulation, which has low conductivity.  We can even reduce heat loss to zero by steam jacketing, so the cylinder has no temperature difference to drive heat transfer.  I presume we would also lag the jacket to minimise the steam requirements. Interesting that insulation works by addressing the heat transfer coefficient, while jacketing addresses the temperature difference.

However, it is more difficult to drive this further by adding heat to the cylinder to increase the work out.  We can use high temperature in the jacket, though we probably don't have a steam source at higher temperature than the engine supply.  The size of the cylinder severely limits the area available for heat transfer, so it is difficult to achieve much heat input.  Compare the geometry and temperature difference with that of the boiler to get an idea of the relative magnitudes.  If we had a higher temperature steam available, we would probably be better to supply it to the engine directly, rather than try and drive it through the cylinder wall.  Even a high temperature source such as the smoke box is limited by the film coefficient of the gas to metal, so a high temperature heat source such as the smoke box probably does not achieve much more than a standard jacket.  However it has a great advantage in simplicity, no moving parts or extra connections, and in fact difficult to see how it could be avoided on a typical locomotive.

Now I have introduced power and efficiency this time, so next time. I had better define those terms and their units of measurement.

Thanks for looking in

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

Offline MJM460

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Re: Talking Thermodynamics
« Reply #216 on: August 09, 2017, 01:24:47 PM »
For the dinosaurs amongst us

Came across some dinosaur foot prints near the road today.  Well not really near, more like 110 km off the road really, a little detour from the long paddock, but interesting to see a moment in time captured by nature, the time of a stampede.  And not found unaided of course.  The palaeontologists told us where they were, and how they solved the puzzle, and how this moment is still preserved for the future.

Very early in this thread, I looked at the SI metric system and the definitions of force and work.  I have avoided introducing other terms as long as I could, because in the explanation of how engines work, I wanted to concentrate on what work is, and it's definition and how any engine turns heat into work.  No fundamental difference between big and small.  However it was necessary to talk about power in the discussions on the engines of the Titanic, so I had better define it.

If you want to lift a mass of 1 kg to a height of 10 metres, you need a force of 1 kg x 9.8 m/s^2  = 9.8 N.  The distance is 10 metres, so the work required is 98 N.m.  Now any size engine can, in principal, do this.  My tiny single acting Mamod engine an with sufficient gearing and given enough time, can do the job, though I admit that it would have to be very good quality gearing with near perfect bearings so that friction did not require more work than the lifting task.  On the other hand a full size ships engine would not even notice if this task was added to its normal load, and would do it in the blink of an eye.  While the amount of work done is the same in each case, there is clearly a difference in the engines.  This difference is power.  So what is power?

Power is defined as the rate of doing work.  Alternatively, it is the amount of work done per unit of time.  The unit for measurement of power is the Watt, which is defined as one Newton.metre per second.  You can see a Watt is the power required to do one Newton.metre of work in one second.

The power developed by engines we are familiar with varies over quite a wide range.  The little Mamod engine I have already mentioned would be measured in thousandths of a watt, or milliwatts.  Similarly, many low temperature difference Stirling engines would be measured in milliwatts.  For most of my life in making model boats, 10 W was considered almost unattainable, certainly for mere mortals.  How things have changed with the advent of better batteries and brushless motors.  A typical sewing machine motor, or a mini-lathe might be 150 W.  My lathe motor is about 1 thousand watts, or one kilowatt, or kW.  Our cars might be a bit less than 200 kW, while formula one, I should know, but it escapes me for a moment, come in Jo, can you help us here?

The compressors used in automotive service centres for tyre inflation are around 5 kW, while the ones I am familiar with from my working career range from about 600 kW to 20 MW, 20 million watts, 20 megawatts.  I am sure that you can all think of others, larger and smaller.

Now we sometimes see an engine described as "powerful".  We need some understanding of what this might mean.  We have on this forum, an amazing thread by strictlybusiness on a 0.9 cu. in. engine developing 7.2 HP.  Amazing for a small engine, for a model aeroplane or boat, but not very impressive for a Formula one race car.  Unless you mean 7.2 extra HP, of course, when in that context it would probably be considered a major breakthrough.  None of which takes anything away from the incredible feat of getting so much power out of such a tiny engine.  Similarly, a formula one engine would be no use in a model in the usual size range.  So, is there a measurement which allows us to compare our model engines despite the differences size?

I suggest that when we say an engine is powerful, we actually imply "powerful for its size", so we need a meaningful measure of size to convey the idea with any precision.  We could use some representative length, but the commonly used measure is the mass of the engine, which in the SI metric system would be in kilograms.  If we describe an engine as producing so many watts per kilogram, we would have a measure, generally referred to as power to weight ratio, that is meaningful for a wide range of engine sizes.   Power to weight ratio has dimensions (W/kg in the metric system), so the numbers will depend on the units you are using, you need a conversion factor if you want to compare W/kg with HP/lbm for example which would be the equivalent in imperial units.  Power to weight ratio is an important consideration in comparing engines for a specific application.  For an aeroplane, for example you need a high power to weight ratio, but can afford to check and maintain it often.  A steam engine with its boiler, and condenser is at a severe disadvantage for this application.   For a national grid electric power generation, weight is just not an issue, you put much more priority on reliability and low fuel and maintenance costs.  The turbines used in a typical national grid power station were never meant to fly or move in any manner at all, but they reliably operate 24/7/365.

So to get back to Willy's question on the horses power of his engine, first we had better look a bit closer at the commonly used units for power.  I have already mentioned the metric unit of power is the Watt.  Interesting that this watt is exactly comparable with the electrical unit of Watts, calculated by multiplying volts by amps (by power factor if the voltage is AC).  I never have quite understood  how the definitions of volts and amps could come up with such an elegant result.  Or is the definition of the unit of resistance (ohm) the secret?  In imperial units, power is measured in Ft.lbf/second.  However we normally use the unit horsepower, first defined by James Watt, I believe.  The horsepower is defined as 550 ft.lbf/sec or 33,000 ft.lbf/min. *  I have heard that James Watt must have had a very large horse, as the figure is apparently a bit high for normal horses, however, it gave a basis for comparing the power of his steam engines with the contemporary technology, which at that time was horses.  Fascinating that Willy's engine name plate specifies the power in horses power, but the concept of the engine being able to match the power of a number of horses makes sense for his advertising purposes.  It obviously later became modified to horsepower. 

We can now see that to calculate the number of horses power of the engine, we need to know the pressure in the cylinder and the piston diameter so we can calculate the force in lbf exerted by the piston rod.  We then need to know the stroke length in feet, so we know how much work is done by each stroke.  Finally we need to know the number of strokes per minute, or hour that the engine completes.  So we multiply force by stroke length by strokes per minute and divide by 33000 * to get horses power.  And because electrical watts are watts in both imperial and metric systems, we all know that one horse power is equal to seven hundred and forty six watts.  (1 HP= 746 W, or 0.746 kW)

Next time I will,talk about efficiency, which surprisingly is a little more complex, as there are many definitions in common use.

I know the thread needs pictures, but unfortunately dinosaur footprints do not photograph very well. However, Mr Google is quite good at finding dinosaur stampede photographs that are way better than I was able to take.

Thanks everyone for reading

MJM460

* corrections 10/8/17 -  1 HP = 550 ft.lbm/sec
« Last Edit: August 10, 2017, 11:57:15 AM by MJM460 »
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Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #217 on: August 09, 2017, 02:24:31 PM »
Hi MJM, I think they increased the Horses power rating by adding more weight to the flywheel and increasing the steam pressure ? presumably this would work. ?? On my BMW motor bike the R69 engine of 600 cc is rated at 35 horsepower., but the R60 engine of 600cc is only rated at 25 horse power !! I think the only difference is the compression ratio is higher for the R69 engine ??!! so does compression ratio come into the equation ?? also the R69 engine can increase the rpm by 1000 to actually go up to 100MPH. With marine and areo engines one can feather the propellers to give more speed/power so is this part of the equation as well ??  Sorry i am using the question key such a lot perhaps i could remove it !!!
« Last Edit: August 09, 2017, 02:28:19 PM by steam guy willy »

Offline paul gough

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Re: Talking Thermodynamics
« Reply #218 on: August 09, 2017, 10:09:25 PM »
Horsepower??? For the novice, a confounding species with too many breeds to choose from! Eg., Brake H.P, Indicated H.P., Shaft H.P., Boiler H.P., Drawbar H.P., etc, etc. Maybe a quick explanation why the need for various sorts is in order. Regards, Paul Gough.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #219 on: August 10, 2017, 01:45:44 PM »
A little more horsepower?

Oops, one horsepower = 550 ft.lbm/sec or 33000 ft.lb/min, and no one picked me up!  I have corrected yesterday's post, I hope no one has wasted too much time worrying about that one.  It would have been a quite puny horse.  My apologies if I have led you astray by this one.

Thanks Willy and Paul for your questions, please don't remove the question key, either of you.  Your questions prompt me on areas where people need a bit more clarification, and I am sure that you are not alone with these questions.

On the power of those two ancient engines, the increase in steam pressure does increase the force on the piston rod, hence the work done on each stroke.  Increased work per stroke at the same number of strokes per minute means more power.  So increasing the steam pressure would have increased the power output.  Depending on the load, it may even increase the number of strokes per minute.

The flywheel however does not produce any power.  It simply stores energy when the engine tends to speed up, and returns power when the engine tends to slow down.  Remember when I looked at the torque produced by the force in the piston rod?  It rises to a peak, and falls to zero twice every revolution.  The engine accelerates under the higher than average torque, and slows when the torque is below average.  Extra weight in the flywheel is a disadvantage because it increases the bearing loads and hence losses, but it is a necessary side effect of adding extra moment of inertia.  I would assume they added the weight to the rim where it has the most beneficial effect.  However, increasing steam pressure increases that peak of torque twice each revolution, while zero is still zero, so increasing the power by increasing steam pressure increases the speed fluctuation of the engine, not usually desirable.  It is also possible that the speed fluctuation before the modifications was already higher than desirable, thus increasing the necessity for increasing the flywheel inertia when the steam pressure was increased.

On your modern BMW engines, I assume twin cylinder, those are beautiful machines.  I don't know the specific details of the engine, but in principle you are quite right, increasing the compression ratio does contribute to increased power output.  It effectively increases the mean effective pressure, so increases the torque produced by the engine.  However there are many other aspects to tuning up, or hotting up an engine.  Some of the things that can be done include grinding the top of pistons so each has the same compression, and grinding the bottom of the piston skirts to make the two the same weight.  I have it on good authority that checking the pistons at top dead centre and making them within 0.001" of each other and grinding the bottom so the weights are within 0.1 gm makes a significant difference for a street machine, but not even close to good enough for the race track.  Paying this level of attention to bearing clearances, and every other aspect of the engine construction will significantly increase its power with otherwise standard components, and will increase the maximum rpm.  Modification to cam profiles, valve spring rates, port polishing and even different air cleaners can all make a difference, especially if you have access to a dynamometer to help you see the differences.  Not really necessary to do anything heroic to make quite a difference.  However things which increase torque are the most desirable, as torque gives acceleration, while higher top speed only loses your licence, unless you confine yourself to the track of course. 

Feathering an aeroplane propellor does not change the power output of the engine, rather it changes the load.  This allows the engine to run at the optimum rpm for the immediate task.  When climbing, maximum power is required and a certain pitch will allow the engine to run at the right rpm to provide maximum power.  Slowing down and descending requires less power, but purely reducing rpm is not necessarily good for fuel efficiency.  A lesser pitch will allow the engine to run at the optimum rpm with much lower fuel consumption.  Perhaps flyboy Jim (or we may have other pilots on the forum), can elaborate on this one. I hope that makes it a bit clearer.

Paul, thanks for pointing out the different ways in which power is measured.  I have simply provided the definition, 1 Newton.metre per second = 1 watt, however everyone wants to claim that their engine is more powerful than the competition.  There is kudos in having the highest power engine, size does matter, it seems.  The engine maker does not want to have his figures compromised by what is added after the engine is sold, while the end user only wants to know if there is enough power available to drive his generator, propellor or compressor, or just make his car or bike go faster.  So everyone measures the power in the way that best suits the vested interest.  Let's have a look at some of the common terms, and how they reflect different measurement locations.  Remember above all they nearly all use the same definition of one horse power = 550 ft.lb/sec, or 1 watt = 1 Newton.metre/sec.  Of course horse power is a specific  imperial unit, in SI terms, is is usual to just refer to power.

First the one exception, the boiler horsepower.  It is actually defined in terms of raising a defined quantity of steam at 0 psig and 212 deg F, from water at zero psig and 212 deg F.  It is approximately 33475 Btu/hr or 9811 watts.  This rate of energy transfer is about(?) 13.1547 times the equivalent of the mechanical horse power definition.  It is not a measure of mechanical power, but intended to be an indication of the size engine the boiler would be suitable for.  A nice lead in to our discussion on efficiency, as it implies a steam plant efficiency of about 7.6%.  So hold that idea of efficiency, but otherwise it is not really a useful concept.  Better to define a boiler capacity in terms of rate of steam production at a specified pressure and temperature, and match it to the engine specifications.

For a given engine, the indicated horsepower can be expected to be the highest.  It is calculated from the area of the indicator diagram which we looked at earlier in this thread.  It gives a measure of the power provided by the action of steam on the piston.  It does not allow for friction in the rings or bearings, or the power required by the feed water or lubrication pumps, or condensate pump,or air pump.  So a nice big figure, also useful when looking at and optimising valve events, but does not help much in assessing the size of generator the engine could drive, or the power available to drive the ship or aeroplane propellor.  Again, some of the marine engineers might like to come in and explain a little more about how they use these diagrams.

Brake horse power and shaft horse power are very similar.  I am never sure whether there is actually any difference in the numbers.  Brake horse power is the term used in the engine is tested on a brake, an artificial load used for the specific purpose of measuring the engine output.  Shaft horse power is I believe the same number, but the term is used to refer to the power available at the output shaft to drive the end users load.  In each case there is room for some ambiguity, and in specifying an engine it is preferable to separately list the power consumed by engine accessories, such as oil pumps, cooling water pumps, and radiator fans and specifically identify the available power after allowances for all necessary accessories.  There is some justification for the engine makers desire to not include all these loads.  In many industrial applications there are good reasons for driving these accessories with electric motors, thus leaving the entire engine output available to drive the intended load.  And of course, they are all measured with the engine new and clean.  Usually, power output gradually declines for ever afterwards, depending on the quality of maintenance.

Drawbar power is useful for prime mover applications and might apply to a traction engine as well as a locomotive.  A typical traction engine requires a considerable amount of power just to move itself.  A prospective purchaser who wanted it to pull logs out of the forest, or a plough or a trailer wants to know how much power is available for this purpose.  It would indicate the drawbar pull and towing speed.  The engine efficiency competitions run by some clubs for locomotives actually measure draw bar power and compare this with fuel consumption to calculate efficiency, another neat introduction to that topic.  If the engine is used as a stationary power supply, say for a saw mill, the power available would be much higher than the drawbar power.

That probably enough for now, does anyone have any other definitions of power output they have come across?

Thanks for following along,

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

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #220 on: August 10, 2017, 02:05:44 PM »
Hi MJM, thanks for the reply there is lots more to increasing Horse power than i thought......also i can give you another definition of power output from this little booklet i have in my possession   4 Man Power !!!!! thats a new one on me !! however it is in print so must be true !!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #221 on: August 10, 2017, 02:22:03 PM »
Hi MJM ,just looked up Bisschof gas engines  So can be run by any boy or girl !! the beginning of girl power then !! Also engines available in  ! man and a half power !! Brilliant stuff.........

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Re: Talking Thermodynamics
« Reply #222 on: August 10, 2017, 02:42:31 PM »
Many many years ago, aspiring race mechanics like me were not only matching pistons in our Austin Mini's, we were balancing up the combustion chambers by grinding them out and measuring each volume by introducing water with a pipette through a tiny v cut in the edge of a piece of perspex placed over each chamber in turn. I'm not sure it went any faster but it felt good! Before that I skimmed the heads of my Ariel Arrow 2 cycle bike and fitted padding in the crankcase: now that DID go better!
John

Offline Dan Rowe

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Re: Talking Thermodynamics
« Reply #223 on: August 10, 2017, 03:02:44 PM »
MJM,
Some nameplates have a continuous and max or peak HP rating. If you google horsepower litigation you will find the current legal wrangling. There are cases that go back to the time of James Watt. The invention of the steam engine indicator was kept secret for many years and is first mentioned in a court case about engine horsepower. You can not always take nameplate or catalog data as true information.

Dan
ShaylocoDan

Offline MJM460

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Re: Talking Thermodynamics
« Reply #224 on: August 11, 2017, 01:00:25 PM »
Horse power or man power

Great examples of different ways of rating engines.  It shows ingenuity to the fore in the days before standard measurements, and manufacturers tried various ways to convince potential customers that theirs was the one to buy.  But of course, there was no standard for power measurement, so all those terms could not be precisely compared, until James Watt defined the horsepower.

Interesting that boys and girls were treated as equal in that add, Willy, pity it all seems to have gone down hill from there.

Hi simply loco, great to have you on board.  I think smoother running resulting from better balancing of weight, compression ratio, and fuel mixture always results in an engine being able to run faster, as there are losses associated with rough running.  And vibration is a reasonable criteria for calling it the maximum power.  But the tuning tricks you learned with those activities have a long and honourable tradition.  And very rewarding to my way of thinking, what ever directions your career took later.  Unfortunately my time was spent in other areas, so I never got to learn some of those things. 

Hi Dan, modern engines still have a published maximum or peak rating, as well as a continuos rating.   Suppose there is always something that limits the maximum power output from an engine in an absolute way, but most engine ratings are determined by the criteria you impose.  A formula 1 race engine is expected to run at the maximum power possible for a bit over two hours then the mechanics get to restore it for next time.  Even then, I understand they will get up to tricks like disconnecting oil or water pumps for a little extra power on the road for those critical final timing laps.  They know just how far they can push it without damage.  We expect our personal cars to last much longer between servicing, and expect the servicing costs to be more moderate.  So continuous operation might be limited by some critical temperatures, perhaps exhaust, water or oil temperature.  If you exceed this limit, you will shorten the life of the engine, but you can exceed it for a short time, occasionally, with minimal effect on reliability.  And it's not totally unreasonable, even in your car you want to be able to put your foot down to get to the top of a hill, then are happy to coast down the other side while the engine cools a little.  Car engines are rated nearer peak power, which cannot be used continuously, while my industrial machines had to run at 100% load continuously.

I think all those legal problems can be traced back to someone seeking advantage.  The manufacturer exaggerates his claims, while the purchaser wants to pay the least even when it means buying a smaller engine, so in a way they collude with each other, and it often ends with tears.

I guess catalogue and nameplate rating are at best a reasonable average for similar engines, but even when you specify a test run and certified test results, the results really only apply to the specific conditions of the test.  And of course the test is at best only a few hours, and no real indication of how long it could run at those conditions.  Most of my clients in industry expected the engine on the test stand to demonstrate at least 10% more power than that required by the driven machine, which was also demonstrated on the test stand.  But they then expected the equipment to run 24/7 for three years or more.  Not so hard for a centrifugal machine where there is really only one moving rotor, and hydrodynamic bearings mean nor rubbing or wearing parts, even seals are separated by a gas film.  But quite a feat for a large reciprocating compressor.   They weren't portable by any reasonable assessment.

No new topic today, I will be entering tin can and string territory for communication for the next week or ten days on the long paddock, so I don't expect to be able to make regular posts.  I will check in when a freak of the atmosphere, together with adequate solar power enable it.  I believe it usually involves standing on the left foot with your right elbow in your left ear while you hold the phone high over your head pointing in the precise direction of the nearest tower which is beyond the horizon.  Or something like that.

So thanks for following along, keep thinking about thermodynamics of your engines and the regular discussion will return in about two or three weeks, rather than try to continue through odd unpredictable times in that period.

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