EUREKA AUG 2000 COVER FEATURE STORY

Steam gives power on a miniature scale

Innovative models show that micro-scale steam has considerable potential application. Tom Shelley reports

A retired designer is building 'OO' gauge model locomotives that run on real live steam.

Serious toys for boys, the miniature locomotives show the potential of steam power on a very small scale.

Energy efficiency may be low, but specific power per unit weight is high, and the techniques developed to make the model engines function shows great potential for other devices, especially those that are only required to function occasionally.

Nuclear and fossil fuel power electric generating stations and the space shuttle show that the power and efficiency of steam remains undimmed if the scale is big enough. Richard Hallam, on the other hand, has now shown that steam power can also be appropriate on the small scale.

His model City of Nottingham can happily pull 15 coaches up a 1-in-95 gradient at speeds that a conventional model equipped with a conventional electric motor would find impossible.

Steam is raised in a 25ml boiler in the tender equipped with an electric immersion heater. It then passes to a superheater in the engine, which uses almost as much power as the heater in the boiler. Since the steam pipes are only 1.5mm in diameter, and passages between valve and piston heads, 0.6mm in diameter, it is essential to superheat the steam delivered to the two 1/4 in (6.3 mm) pistons so it does not condense before it gets there.

The engine makes efficient use of the water: 25ml lasts for about an hour of continuous running. But it is less efficient with the electricity. The two heaters, which are connected in parallel, consume about 3.5A at 12V, compared with about 0.75A at 12V for a conventional electric motor powered model.

Every effort has been made to prevent heat loss. The cylinder block is connected to the rest of the construction by the minimal amount of metal, and parts carrying steam are thermally insulated as much as possible. Energy efficiency in steam railway locomotives was always a problem. The last full-sized, British steam railway locomotives managed just 8 per cent efficiency, while the French managed 12 per cent and the vast American ‘Big Boy’ locos are said to have achieved 14 per cent. By comparison, according to the Energy Efficiency Best Practice Programme, 90 per cent of the energy used to compress air in a typical factory pneumatic system is lost as heat. And according to other sources, only a quarter of the remaining 10 per cent is thought to transfer to the task.

Hallam believes the efficiencies of steam locomotives could have been much improved, but the designers became set in their ways and never looked seriously at ideas like waste heat recovery, using flue gas to pre-heat boiler water. Traditional engines also had very inefficient grates. Because lumps of coal get smaller as they burn, and are liable to be carried over in the gas stream, more than 50% of the coal that was fed into the firebox could under some circumstances be ejected from the smokestack as cinders.

On the other hand, although Hallam’s model locomotives are hand crafted, steam technology is fundamentally simple and low cost. Steam locomotives always cost much less to build than diesels, although in service they required more maintenance.

All the steam tubes, parts and valves in the model are made of ordinary brass. The valves require no seals, just well-finished surfaces and a film of oil. The piston rods are packed with nothing more elaborate than mineral fibre saturated with graphite and oil. Oil is delivered from a container by the differential pressure across the output side of the system. When the regulator steam valve is closed, there is no differential pressure and so no oil is delivered.

Hallam’s control system is ingenious in that it requires no electronics. When he wants to change locomotive speed or direction, he drops track voltage below 9V, and a relay in the locomotive then changes over to connect power to a geared electric motor with an approximately 1,000 to 1 reduction ratio. The motor drives a rotary regulator valve. Reversing the direction of current reverses the direction of motion of the valve. Pressing a button on a control box drops the supplied voltage for short periods so that the valve can be inched. A mechanical linkage also acts on the main rotating slide valve, which directs the steam to the two sides of the cylinders and moves this between forward and reverse settings. In between forward and reverse position, the regulator valve supplies steam to the whistle. As well as adding realism to the operation of the model, this tells the operator where the valve must go. The engine can be gently eased back onto a line of waiting carriages to pick them up, just like a real one.

Fundamentally, as a system of fluid power, a steam system does not need a pump, only a source of heat. It has therefore been considered as a source of prime movement for occasional usage. A concept we have seen is to generate steam for oil and gas industry valve operation on the sea bed, where there is no possible shortage of feed water. In one scheme, electrical heat is generated by discharging a large capacitor.

Steam actuation is also a serious possibility for micro-engineered devices, although surface tension effects caused by liquid water must be taken into account. Heaters can easily be incorporated into etched silicon devices at much less cost than a mechanical pump. Electrical consumptions are so small that thermal efficiency is not normally an issue.

Chemical sources of occasional steam are also a possibility and still provide the motive power for some spacecraft. They are the prime power source of big commercial space launch vehicles such as the space shuttle, being the propellant gas produced by burning hydrogen with oxygen.

Hallam is currently working on a new Stanier Black 5 locomotive, in which he has simplified and compacted the internals. Commercial possibilities for model steam engines are being handled by Inventorlink, which says that several commercial makers of model railway equipment are showing a keen interest.

Information on projects to build new, full sized, modern philosophy, steam engines can be found on the internet (at www.trainweb.org/tusp/index.html ). Two Cuban railways are said to have ordered four new steam locomotives burning crushed sugar cane stalks.

Design Pointers

Small scale steam power has the advantages of low capital cost, simplicity, reliability and high power to weight ratio

The steam power needs to be used fairly close to the point of generation

Energy efficiency tends to be low, but can be much better than many pneumatic systems and may not matter if usage is only occasional

Postscript

Since writing the original story, it came to my our attention that there are even smaller live steam model locomotives.

Chris Dowlen, a lecturer at South Bank University e-mailed us to tell me about model locomotives made by Brian Caton of Thorpe Willoughby in Yorkshire, which run on 9mm gauge track. These are on a scale of 4mm/ft, which represents 2ft 3in gauge full size, as used by the Talyllyn railway in Wales.

Steam cashes in its chips


The above photograph is of a steam powered micro actuator developed at the Sandia National Laboratory in the US.

The water inside the cylinder chamber is heated by an electric element which causes it to vapourise and push the piston out. The unit is so small that capillary action is all that is required to draw the piston back to its original position. Researchers at Sandia have developed multiple iterations of the design, including a three cylinder version.

The problem with using steam on this scale is its speed of reaction - this is why the majority of micro devices under development at the moment are utilising electrostatic forces.

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