Differences in the operation of steam engines with real steam and compressed air

[Werner SchleidtParticipant @wernerschleidt45161]

Hi all,

in the german steam forum we had a discussion between the differences of operating small table or gauge 1 and 5 inch steam engines or road steamers.

Here are some of the findings we thought they are correct;

For small engines with nearly no expansion setting there is no big difference in the behaviour, but subjectivly the engines run better with steam, because of a better lubrication with steam oil and wet steam.

Enines who have to create working power for road and rail steaming there is the difference by oiling and which gear type is used. The best is the walschearts gear, because of the constant pre opening of the steam ports. With this gear my loco runs with expansion very good under low load with less steam use and powerful with ful open gear. Marshal or german patent stearing they are similar in the behaviour and they are good for narrow gauge locos, who often stop. The expansion is only in a smal range useful, because of the fixed and reduced pre opening. On my roadsteamers, a merrywheather fireking and a jaxon steam car, they behave similar with the same gear. Best running fully open gear or a small reduction.

This is the summary of the findings of three people, but the dicussion went somehow out of control because the people with table engines could not understand why we discus this, they are proud that the engines are turning with no load.

Any commands are welcome when they are serious to find a better understanding.

 

Best regards

Werner

 

[SillyOldDufferModerator @sillyoldduffer]

Steam engines are heat engines, and their energy is comes from heat stored in the working fluid.  Compressed air contains relatively little heat compared with steam, so steam outperforms air.

The difference isn’t obvious when engines are run without a load because the engine does very little work.  If there’s enough heat in the air to overcome friction, the engine runs and looks good.   But the engine is easily stalled – not enough energy in air for it to do work : it’s weak.

The same engine powered by steam has much more energy available and it can do much more useful work.

You can’t meaningfully compare unloaded engines.  A brake dynamometer is needed.  By putting a known load on the engine with a brake, the engine’s power output can be measured.    A dynamometer reveals torque (turning power in Newton/metres), work/energy (Joules), and rate of work (Watts).  For most practical purposes, compare each engine’s power output in watts.  Numbers and graphs, not guesswork.

In the UK a competition called IMLEC collects performance data: fuel burned, load pulled, and time taken.   Copying what IMLEC do will highlight the difference between air and steam.

Steam doesn’t perform as well as thermodynamic theory predicts because so much heat is lost between boiler and cylinder.  Full size steam engines are thermally inefficient due to heat losses and models are much worse.  Size matters – small engines are very difficult to insulate, so most of the heat is lost.  Results further confused because wet steam lubricates, covering up low efficiency.

Measure with a dynamometer.  Pull a trailer full of weighed concrete blocks and measure how quickly it accelerates, how far it pulls the load before running out of steam (or compressed air), and for how long.  Measure drawbar pull with a strain gauge – a suitcase scale should do.

Data reveals all.  Opinion is unreliable, especially if the engines are only spinning on a bench.

Dave

 

 

 

[old martParticipant @oldmart]

A lot of small engine makers will only run them on compressed air because of the regulations regarding certification of live steam boilers here in the UK. I am helping make a twin cylinder double acting self starting model engine from an incomplete set of castings which will be lubricated with motor oil and only used on compressed air for a short time.

[BazyleParticipant @bazyle]

An added factor is the use of superheated steam which in some aspects should behave like a pressurised gas (which it is) but unlike air it has an energy reserve liberated when it loses the superheat during expansion.

[JasonBModerator @jasonb]

I find an engine run on air does not need as much lead of the eccentric as one run on steam. As there is no expansion of the air retiming helps a lot as does reduced lap.

Although I tend to run my steam engines on air they will still do work and you can hear the change in sound when they are loaded up. You wil also find that those running “tabke top” engines on air tend to use very low pressures less than 0.5 bar is typical and regulate dright down a swe like a nice slow running engine but if you up the pressure to what you might expect from a boiler 4-6bar and open up th eregulator then they will do a lot more work as what energy goes in must come out

[Werner SchleidtParticipant @wernerschleidt45161]

First of all thank you for helpful replies, I did not expect so much replies in this short time !

IMLEC is a very interesting Challenge and I am always surprised if I study the results. For my own models I try to optimize them as far as possible, but I often see that my steam colleagues have not so much understanding for that.

But in the other hand I build a steam car with propane firing and at the beginning I was able to drive only 2 to 3 Km with 1 Kg propane gas. With some optimizations of the burner and gas control I came to 4 to 5 kilometers with a max speed of 4 to 5 Km/h.

Then I changed the engine with some other steam routing, this was very difficult the space was limited, two valves and a receiver in the top of the boiler housing, changed this to a switchable normal or compound engine.The cylinder ratio is one to one, but my highest boiler pressure is only 6 bar.

I can now drive much faster, 6 to 7 Km/h ( legal in Germany ) and have now a range 6.5 to 7.5 Km. Then I changed the gear ratio for a better hill climbing, the engine has now an increased rpm, but the mileage was nearly the same. In september I noticed subjectivly that in the receiver is an amount of condensed water collected and this increased over longer runs the engine in compound runs then not so like in the start range. I added then 4 heat pipe at the receiver and this looks then similar to a smithies boiler. I added in totall 1.2 meter 6mm copper pipe. It was the much I was able to install in the boiler housing. I was very astonished how the engine changed in compound performance, it was hereable by the sound much smoother and feelable by driving. I had severall starts with a warm engine, were I had forgot to start in normal mode valve setting, in compound mode with no problem I noticed it only as I want to switch in compound mode and the lever was still there.

Next year I will make indication measurements by driving up the hill track. I made this as I changed the system to compound mode. There I have two different types of inclinations. One part I can drive in compound and the more inclinated part is measured in two cylinder normal mode. The inclinations are in the range of 2 to 4% and 8 % and this is all driven in one gear. There is no traffic and so I can drive there with my laptop on my knees and looking to the track and the installed sensors. I measure piston displacement. receiver pressure, cylinder pressure of each cylinder and driving speed. This is the same setup as in mid 2024 as I start changing to compound.

Werner

[JAParticipant @ja]

Interesting. I would like to have a good think about this but life does not allow it.

Air is a perfect gas while steam which is, or will end up, wet is not. To state the obvious steam engines are designed to use steam. If air is used the expansion will be PV^n = constant. The power n could be anything between 1.4 and 1.0. 1.4 is adiabatic (where not heat is lost during expansion) to 1.0 (where expansion is cold, difficult to visualize). High speed engines will be close to adiabatic, most slow running engines won’t. While a steam engine’s valve will allow steam to be admitted for a short time I think the exhaust will open far too early to make full use of a gas’s expansion.

This is complex and DO NOT compare it with the Otto or Brayton cycle.

JA

Werner, please keep us informed of your results.

[BazyleParticipant @bazyle]

In a compound engine the setting of the valves to control the amount of expansion in each cylinder will depend on the amount of superheat. It is quite complex to control when condensation occurs in the still expanding and cooling steam. There will be details in university text books from 100 years ago when engineers still had to be trained how to design this type of engine.

When it comes to running on air have a look at this at around 15 minutes in. “Restoring my steam loco #3 machining the tyres”  https://www.youtube.com/watch?v=v3vkxgYzphw&t=330s

[SillyOldDufferModerator @sillyoldduffer]

[Werner SchleidtParticipant @wernerschleidt45161]

Thank you for your replies!

The video is very interesting and nice to see what people do with their possibillities.

Dave , this is a lot of information and I have to study this, many thanks for your work.

From the thread theme this is different this represents only the steam part.

Unfortunatly the winter is coming and so I have to make a pause till next year with my measurements to compare my continous optimizing of my steam car.

After making the measurements next year I will give an update of it. To have an idea a few informations from last year the measurement setup under the seat of the steam car

 

On the left side receiver pressure sensor, cylinder pressure sensor and the attachment to hold the piston travel sensor. On the right a cylinder pressure sensor and an attachment for the piston travel sensor.

This was the outcome of the measurement.

In green the high pressure cylinder and in violet the low pressure cylinder with high pressure over the valve who combines both valve boxes for two cylinder driving. The green curve had some oszillations, later found by a loose stuffbox and a not good guiding of the valve rod. Caused by the valve there are some throtteling losses so the green and violet curves are not identical. The next exhaust pressure is not identical, because of different routing of the steam exhaust, one is direct the violet and the green have to go through a valve  for changing to two cylinder or to compound action. Combining the valve boxes in that way is a compromise for easy use and my space here is very limited, but in the beginning I never planned to do such a conversion to compound action.

That is a picture of me with my car

Werner

 

[JAParticipant @ja]

Werner

Many thanks. My problem is that I only try to think about these things when it is cold. When it is warm I am, or should be, in the workshop.

Dave

Just a comment: Strange as it may seem the Diesel and Brayton cycles are the same (ideally adiabatic compression, constant pressure combustion followed by adiabatic expansion). Some gas turbines, before 1935, have used the Otto cycle where constant volume combustion is used. This has, unsuccessfully, been revisited in the last twenty years.

I have a few thermodynamics and fluid dynamics text books from my youth and they are very interesting but do not make light reading.

JA

[Julie AnnParticipant @julieann]

An adiabatic process is one in which no heat is lost, or gained, from an external source and that is also reversible, in a thermodynamic sense. No real steam engine can be fully adiabatic as any real material will conduct heat to/from the outside world.

The perfect expansion in a turbine/steam engine is isentropic, ie, entropy is constant but enthalpy changes. The expansion is represented by a vertical line on the Mollier diagram. Conversely perfect throttling through a nozzle is isenthalpic, ie, enthalpy is constant and entropy changes. The expansion is represented by a  horizontal line on the Mollier diagram.

In Victorian times it was noted that energy was lost in the expansion process beyond what calculated. This lost energy was call the ‘missing quantity’. Experimental work showed that it was partly due to condensation, and partial re-evaporation, on the cylinder walls and partly due to leakage past the then common slide valves. Superheating is a way of minimising the losses due to condensation and re-evaporation. Of course superheating steam adds energy but more importantly it raises the temperature of the steam above the temperature of saturated steam at the same pressure. Consequently the steam can be expanded further before reaching a point on the saturated steam curve beyond which the steam will condense.

Julie

[JAParticipant @ja]

Julie

To be pedantic, no system can be adiabatic. To do so it would contravene the Second Law of Thermodynamics. (As an aerothermal engineer “enough said, laws are laws”).

JA

[duncan webster 1Participant @duncanwebster1]

All getting a bit heavy, if running at long cut off air or steam no difference. If short cut off air loses pressure more quickly than saturated steam because it has a higher expansion index (1.4 vs 1).

Julie’s comments on missing quantity are bang on the nail. More important in little engines as the surface are/volume is higher. Prof Hall did a lot of experimental work to actually measure missing quantity and cylinder wall temperature fluctuation. Reported in SMEE journal. Main reason for superheat is to reduce condensation.

As Prof Hall is deceased it is probably difficult to get copyright permission or I’d send a copy to MEW.

[simondavies3Participant @simondavies3]

Sheer curiosity – what would an engine designed from the start to run on compressed air look like?

 

[old martParticipant @oldmart]

Could a triple expansion steam engine  run on compressed air?

[duncan webster 1Participant @duncanwebster1]

Not very well, with a compound you are probably aiming for a larger expansion ratio, so as explained above, air would lose even more pressure. Of course you could alter the cut off in each cylinder to compensate, but as the main reason for compounding is to avoid condensation, it seems a big pointless. All the extra gubbins whirling round absorbing power.

[JasonBModerator @jasonb]

[Charles LamontParticipant @charleslamont71117]

Looking at the interesting indicator diagrams, the first things I would observe are the pointy bits at the right, and the sloping first half of the exhaust line. These suggest that the exhaust is opening too late on both cylinders, and that the exhaust port or passage area is too small.

[duncan webster 1Participant @duncanwebster1]

If you go tohttp://www.douglas-self.com/MUSEUM/LOCOLOCO/airloco/airloco.htm#p there is a lot about compressed air locos. They had reservoirs working a very high pressure. Used in mines, explosive factories etc.

[Werner SchleidtParticipant @wernerschleidt45161]

Hi ,

Charles, thank you for your hint I keep in mind your finding and I make a check to the raw data. The shown data  have some smoothing that looks good, but is not always the truth. Next years measurement session I will see how the measurement looks like then. In between there is some run in now, I drove in two years 470 Km with steam. I made some refinements more subjective and I have no indicator measurements for that work. Therefore the measurement session next year. I have only for comparison data of many runs with an overall charateristic. I can see there speed, water level and steam pressure with a low data rate of 10 sample per channel.

Duncan, thanks for the interesting link about all the airlocos I need some time to study them. At a first look very interesting.

 

[SillyOldDufferModerator @sillyoldduffer]

[old martParticipant @oldmart]

We had an air engraver for part marking certain aircraft parts which was a straight version of a dentists drill. It took 1/2mm tip diameter carbide spherical tips with five flutes cut in them and ran at 250000 rpm. The power came from air striking a wheel of about 5mm diameter tangentially which had about 20 tiny scallops cut out of it, in the same manner as a Pelton wheel. When it was in use, some people dissapeared out of earshot, it sounded like a mad dentist was torturing somebody. It used the smallest ball races that I have ever seen.

[JAParticipant @ja]

I am sure Jason is right but my memory from 60 years ago, so not very good, suggests that compressed air tools used a number of pistons in a rotating block attached by gears to the output shaft. The pistons acted against a stationary swash plate. However I could be very wrong since us spotty apprentices were not not allowed to go anywhere near compressed air (very wise).

JA

[JasonBModerator @jasonb]

My comment about not many pistons referred to the Ingersoll Rand link, only the very large there seem to be piston based.

It is possible air tools had/have a nutating disc but there are quite a few with various blades or vanes where the air enters at a tangent to drive them. Such as this drill

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