Run live steam loco on compressed air

[Porkscratching]

Not sure if this is the right place to ask but....Just a thought, I wonder if it'd be viable to run a mamod type engine on compressed air... I mean along the track, say fed from a cylinder (cylinders?) Located in wagon towed behind....

[peach james]

Yes, you can.  HOWEVER, it is going to take a fair amount of air to do so for any distance.

 

HK Porter in the US was the largest name in supplying mines with compressed air loco's.  There was just a discussion on here somewhere about the French trams that ran on a version.  So, it can be made to work, but it is not a particularly efficient way of transmitting power.

 

One can use CO2, which stores as a liquid at about 600 PSI.  (or dry ice & water, write ups in Model Railroader in the 50's).  

 

All of them are low energy in comparison to burning fuel.

 

James

[Nearholmer]

CA is indeed a poor medium for storing energy, added to which the process of compression is usually wasteful of energy, and even the decompression can be, because it can require extra heat input to prevent is causing freeze-up of mechanisms. However, it has its applications, notably in railway braking.

 

For model use, a really practical problem will be the maximum safe pressure of whatever reservoir might be used. Practical safe pressures at practical volumes are likely to yield impractically low amounts of energy.

 

As a fun experiment, you could try rigging your Mamod up to a fully inflated bicycle tyre (probably 60 to 90psi, depending on type), and seeing what that yields in terms of duration of run 'on the bench'. That will give you some feel for volumes and pressures.

Edited January 24, 2020 by Nearholmer

[Porkscratching]

That's a good plan, try it with a jury rig ' air tank' ie a tyre !.

..I'll have to manufacture a 'tit' of some kind to screw in place of the safety valve or whistle for an air input tube. 

[Sturminster_Newton]

Not being a physicist I am not sure this is all together a practical solution with a conventional Mamod steam plant.

 

Fixed stationary engines will work in compressed air but you have the problem of lubrication to consider.

 

Steam is an expansive product due to its latent heat released as it expands and cools and 1cc of steam will expand 16000 times when released to atmosphere. 1cc of air at 100C will be less expansive due to the much lower density of the air molecules. Water has about 20 times the density of Air.

 

So you'll need a very strong and large reservoir to drive a mamod any more than a few feet on compressed air and 10 minutes of run would need a resevoir that weighed more then the the mamod could haul.

 

You might consider a small coreless motor driving a small compressor powered by a li-po battery and the compressor set to deliver power to the cylinders BUT NOT VIA THE BOILER, unless things have changed mamod boilers are soft soldered brass tube so should not be used as an air pressure vessel due to the variabilty in the alloying metals used. It also wastes engery to charge the boiler rather than drive the cylinders direct.

 

This would drive the mamod but very inefficiently due to all the energy loses in the system.

 

Compressed air storage is being investigated for commercial power generation where the air drives a turbine to give near instant power to the grid to absorb the power dips caused by commercial breaks and power stations by their nature tend not to be the most mobile of items.

 

Small hot air engines can be made to work via the Stirling principle where heat is transferred to and from the driving air but to get real work from a stirling required  a booster to super-heat the charge air. Philips petroleum amongst others were developing high power external combustion engines in the late 1960s and early 70s and did have a 100hp test rig but the practicalities of the engine made it less fuel efficient than existing oil fueled systems.

 

Enjoy your experiments but for most 'on the bench testing' of the principle is about as far as it is practical.

[PatB]

Bottom line is that, for anything but some fairly specialist applications, compressed air is a very poor energy storage medium. Nearholmer and Sturminster Newton have, I think, pretty much covered it. The most important thing to remember is that steam works, not primarily because it's compressed, but because the compression allows it to store more heat energy. When fed to the engine, it is the heat that does the work, not the pressure per se. I'd attempt to explain the thermodynamics, but thermo always made my head hurt at Uni, and I dropped it as soon as I was able.

 

I guess the lubrication problem is less of an issue on a Mamod than it might otherwise be, as Mamod never provided any of their engines with an automatic lubricator, relying instead on frequent manual application of oil to the working surfaces.

 

CA does have a useful role for demonstration running of stationary engines of all sizes, but there it's generally being used as an energy transmission medium, which is a subtle, but significant difference.

 

That is not to say that you shouldn't play with the idea from personal interest and entertainment, but it's important to be realistic about the likely results.