Q: Alternative to model live steam?

Hi Everyone,

I noticed that a lot of models that would normally run on steam are utilising compressed air.

I was wondering if a miniature compressor could be made and incorporated into a model steam locomotive, with the speed regulator and reverse controlled by DCC. The compressor tank could refill by means of a small motor/piston and shut off circuit taking power from the constant track feed.

How small could a compressor be made to be effective i.e. store enough air to power a model loco?

Regards,
Ed.

Intersting concept.

It would need a piston the same volume running at the same speed as the loco, plus a bit for leaks. Or one twice as big at half the speed etc.

Air isn't as smooth as steam for running an engine, a two cylinder engine may run a bit 'lumpy' at low speed. Allow extra pressure/volume to compensate.

You would need to meet the same pressure vessel test for safety, but the boiler could act as an air reservoir.

The pump could be in a carraige behind the loco.

Neil

But surely a major part of the attraction of a steam loco is the sight , sound and smell of the steam ?

'Tis for me anyway

Norman

For what it's worth a steam engine is not a pressure engine but an expansion engine ie relying on the expansion of the steam in the cylinder and not pressure alone.Just my observation.

Mark P.

Storage volume is the big problem. Due to fundamental physics it a lot less efficient to store energy as a compressed gas than it to store liquid and turn it into gas or vapour by injecting energy. The energy to create the vapour can come from from a fire or the surroundings, as with a gas cylinder but either way is effectively stored separately. Obviously liquid volume per unit mass is much smaller than that of the gas. The compressed air system needs to store both material and energy at once so the tank must be much bigger for the same total power output in any given time. Things rapidly become impractical both from tank size and strength viewpoints. Solid and liquid fuels have a very good ratio of energy output to storage volume. Nothing else short of nuclear comes close.

If you are going to have an electric pick up anyway its simpler and more efficient to electric drive with dummy motion.

clive

Some full size examples were made to run on compresses air for the underground if memory serves me - they werent a big hit!

If you are going to go to all that trouble of using a compressor and storing air then you can use a DC electric motor to drive the Loco and bypass all the problems and tow a battery.

No pressure vessel and no test and bags of power. Discuss!

Clive

I think that's missing the point.

Yes, steam engines run better on steam than air, but the original post makes the valid point that very many people run their stationary engines on air, and no-one complains. Clean, simple, quick and you can do it in the living room without setting fire to the carpet or the dog.

So why, in principle, shouldn't the same apply to locos? I think the issue is cheifly going to be supplying enough air at a high enough pressure with a conmpact solution..

Neil

The idea is to drive the pistons of a loco with air as opposed to a noisy motor and gear box which display IMHO unrealistic operation, That's part of the thinking behind not using a motor as the direct drive and it gets away from fire and burning hazards associated with a boiler.

Most electric motor models in the smaller scales are geared way too high to display realistic motion, perhaps a much better gearing ratio would do. Maybe the cons outway the pros for such a concept and it's interesting to read the comments about the physics involved and was one of the reasons to share and ask about the idea.

Regards,
Ed.

Hi Mark P,

I always thought that steam expands whilst it is being heated and the resultant pressurised steam is then channeled to the cylinders (where there is no heat and no further expansion) which forces the piston outwards, yes,no?

Edited By Ed Duffner on 01/04/2013 22:20:15

A lot of models that are running on air are not driving machinery and they only require a small volume of air and will run at a low pressure to turn them over. An engine driving a loco or a traction engine requires a larger volume and pressure to pull a load. This would require a big compressor and receiver to supply the air required.

A 3" scale traction engine for example will run on air from a workshop air compressor (suitable size for spray painting cars) but the 80-100 psi pressure will quickly be exhausted and the pressure may drop to 20-30 psi. This is not enough pressure to pull a load in a loco or traction engine.

Injectors for supplying feed water to the boiler will not run on air.

Also does not smell and sound as good.

Edited By Paul Lousick on 01/04/2013 23:23:49

Consider the ideal gas law:

pressure x volume = number of moles x universal gas constant x temperature

If the steam expands as it is heated then there is no reason for the pressure to rise. If we deal with saturated steam in a fixed volume (a boiler) then the pressure rises because the steam cannot expand; the only thing that can change is the pressure as it is heated.

If steam at boiler pressure goes to the cylinder I agree that it will push against the cylinder and as the cylinder moves, and the volume increases, more steam is admitted to maintain pressure. However, that is incredibly inefficient, as you are exhausting the steam at the same pressure as it entered the cylinder and therefore throwing away pressure and heat energy.

The whole purpose of the valve gear in a steam engine is to cut off steam admission to the cylinder early in the cycle, allowing the steam to expand, and drop in pressure, thus extracting some of the energy that has been imparted in the boiler.

Regards,

Andrew

I might have mis read the "rules" for boilers

but it seems to say pressure vessels have one set of critia and WATER BOILERS another ( those with acccess to "regs" please correct me --with quotes--)....now two things follow from this ( maybe mis-) assumption.

Kettles are classed as "boilers"..

gas cylinders are pressure vessels..

ok so far..

What would happen if we filled our "boiler" with oil and run the engin on oil vapour?

1 now its a pressure vessel...thus different rules apply ( and in my reading less harsh rules)

2 lubrication / rust are no longer a problem.

3 thermodynamics might be improved...

..now lets try a simple alcohol say propanol C3H5OH (IPA)...again not water so 1-3 apply

lets try ethanol C2H5OH..(vodka ish)...ditto

lets try methanol C1H3OH..(vodka ish)...ditto

lets try C0H1OH.."Nonaol" ....mmmmm funny stuff this, low tripple point, very high specific heat capacity , high latent heat fusion and vapourisation..looking good..cheap readily available looking fantasic.......
.reactive .......Oh! bu**er.!...

BTW if you need NONAOL it marketed under the name ......Water

I can see problems using air on a steamer, I have a Stuart S9, I run it from my workshop air supply, that consists of an ex freezer compressor, driven by a 1/2hp motor, it can barely keep up. The engine is at the moment coupled to a little alernator that I an developing, when this is hooked up to a 25 watt, 230 v lamp I get a voltage of 80 v, or through a transformer I can light a 12v 5 watt lamp to full power, so you could say I'm using 375W to produce 5W.

Perhaps you could do as a number of people have tried, and use a stirling engine to power the locomotive, I believe LBSC built one. Ian S C

why air as working fluid?..why not hydralic?

Hi Michael,

I can tell you the satll currentb of my motor is 7A at 12V, so the theoretical maximum power as about 80 watts.

With an all-up weight of 23lbs, the tractive effort is probably at least 5lbs.

Design speed is 3.6mph at 200rpM with a 2.7:1 gear ratio.

Coverting to metric that's a pull of 22.3 newtons and a velocity of 1.6 m/s.

That gives a power requirement of 35.2 watts, or 2.9 amps at 12V.

At 60% efficiency to allow for frictional loses, cable resistance, switching losses etc. the amperage requirement will be about 4.9Amps, shy of 60 watts.

Both the power available and tractive effort are comparable to figures quoted for Tich.

I was surprised both by the agreement of the figures, but how well the actual performance (pulling an adult at a walking pace) measured up to the theory.

Neil

Thank you chaps for the inputs. Although some electrical formulae are second nature to me I get a bit lost when the mechanical bits and forces are added ...lots to learn!!

I was hoping to build a small O Gauge or Gauge 1 loco eventualy when I'm more accomplished with machining and as somebody suggested the boiler would have been replaced with an air reservoir.

Back to the drawing board it is then.

Cheers,
Ed.

I'm not convinced entirely by the efficiency arguments. I have a tiny gasparin CO2 engine, and it produces comparable power and duration to a small electric motor for similar weight and size. It is a very simple uniflow design and works expansively (the piston has a point on top that lives a ball valve, so only a short squirt of high pressure CO2 at the top of each stroke, and it gets quite cold in operation). There is no reason why high pressure air wouldn't work expansively to a degree, it just won't be as good.

My airbrush-type compressor is very old, not very powerful but runs a Stuart 10V and comparable engines fast and with a degree of power - enough for gauge 0 or 1. I think you might struggle for passenger hauling, but I think these smaller models could work.

Can I suggest that those of us interested in this who have access to a steam engine and a small compressor do a little experiment? With a plug in power meter, or just reading the data plate, we can estimate the compressor power while driving a weighted wooden lever clamping the flywheel of the steam engine. Add weight to the end of the wooden lever until the engine just slows.

This arrangement is called a De Prony brake and you can then calculate the actual BHP generated by the steam engine using the formula HERE.

I hope to have the time to have a go at this on Thursday or Friday.

Heaven help us, if we got a series of data for various engines under steam and air we could actually have some proper data on the perfomance of small engines!

Neil