Check out the energy density of different technologies and you will understand why chemical energy (petrol, coal etc) is so much better than battery storage. It's true that battery technology has moved forward a lot in recent years but it starts from a disadvantage of an order of magnitude or more.

You could achieve a limited run quite possibly but look at the Isle of Man TT as an example of the relative range possible with motorbikes using the different technologies.

At least with electric heaters, the efficiency of the final stage of conversion is very close to 100%.

Murray

A few calculations shows it's impractical.
Firstly the battery rating is misleading. 20V is the off load fully charged voltage, not the rate voltage which is 18V. (Dewalt tools in the USA are "20V" but the same tool in Europe is 18" due to consumer laws.) So using the correct rating 18V x 4Ah = 72Wh a watt is one joule per second so thats 72 x 3600 = 259.2kJ (about the same as 6g of coal) Specfic heat of water is 4.186j/g or 4.186kJ/l so our battery will raise 1 litre of water 61.9 degrees or from 20 to 81.9 degrees (at ses level ambint pressure)

Even if you start off at boiling, the heat of vaporization of water iss 2261kJ/kg so a fresh battery will boil away 115ml of water.

Robert.

Sounds like introducing another stage in the process with an efficiency of less than 100%.

Would probably work, but not as well as applying heat to the water directly.

Howard

I'm pretty sure a wallpaper stripper would work fine for testing unloaded model steam engines. I think the pressure caps are rated 4psi, enough to turn most small models but not more.

But they use a lot of electricity compared to what a compact battery can deliver.

N.

I think the pressure cap is fitted as a last resort in case the steam outlet got blocked. In normal use the boiler/heater generates very little pressure, probably only slightly above ambient but obviously enough to induce the steam to travel along the large bore outlet hose.

If the boiler output was restricted by being connected to an engine the pressure would build up and possibly move the piston but I suspect it would not be able to generate steam fast enough to keep the crankshaft turning. That's what I think anyway as I have no live steam experience.

Ian P

A typical electric jug element is around 2.4 kW, at least where I am. That is 10 Amps from a 240V outlet. So given that the heater itself is 100% efficient, how big a model engine will that power? Well, IMLEC tells us that model locos achieve an overall efficiency of around 2% at best. Granted, some of the losses are up the chimney and from the outside of the boiler shell, so an electric boiler should do a bit better…but even if we get optimistic and assume that we can get 10% efficiency from our electric version, we are only going to get 240 Watts out of the engine. That's about 1/3 of a horsepower, which suggests to me that such an electric boiler might run something like a Stuart Number 1 OK, but would struggle with anything bigger.

You might think that since (almost) all the heat from the input ends up in the steam that the whole 2.4 kW should be available at the steam engine. Actually this is not the case. Even if the engine was an ideal heat engine using the Carnot cycle, the maximum efficiency would be 11- (Tcold/Thot), multiplied by 100 to express as a percent. With steam at 100 psig, the Thot temperature will be close enough 10 170 degrees, C and exhausting steam to atmosphere means that the Tcold temperature will be 100 degrees C or maybe even a little more. This gives a maximum efficiency of 16 %….for an ideal Carnot cycle heat engine operating between the temperatures that are practical for a model plant. Actually the Rankine cycle that we use is not an ideal Carnot cycle, since for a start all of the latent heat is thrown away, so we will never see a figure as high as 16% at IMLEC. (Absolute temperatures must be used forthe formulae, eg add 273 degrees to the Celsius figure)

We can improve the efficiency in two simple ways. One is to lower the cold temperature and the other is to raise the hot temperature. A condenser would enable us to get the cold end down to about 25 degrees, say 300 K close enough. Raising the pressure would also raise the hot end temperature, but most model plant is not allowed to go over 100 psig, unless you go to a flash steam boiler. The remaining approach is to superheat the steam, the limit here being what the materials and lubricant will stand.

So the tl:dr version…an electric boiler run from the mains will probably serve quite well for trying out typical Stuart size model; engines. Not really practical with a battery for a locomotive.

And then there are those super capacitors

**LINK**

Sam

Interesting point (re 'bomb&#39.

I have a small Microflame jewelers' blowtorch that electrolyses distilled water to fuel it. From what I can see it delivers a mixture of hydrogen and oxygen from the electrolysis tank bubbled through a robust secondary storage / pressure tank (part filled with either Isopropyl Alcohol or Acetone) and has a flash arrestor in the torch handle. I think it relies upon the push fit rubber hose as a safety valve and relatively small volume to prevent major explosion.

The nozzle is a hypodermic needle and yes, when it blows back it can be exciting – the needle gets blown clean off and doesn't need to be pointing towards anybody! On more than one occasion the needle has hit the far wall of the workshop. (great bit of kit for fine work though with very focussed, intense flame)

The start of a flashback can be anticipated as the needle glows red hot and the hot bit creeps back along the needle with a hissing sound until there is a cloud 'crack' and the needle goes flying. At least the needle will be sterile!

More to the point, I have to presume that even a 'perfect mix' of hydrogen and oxygen requires a source of heat to initiate explosion.

Another way of looking at electric vs coal as a way of raising steam is to estimate the number of watts output by the firebox of a real model engine. IMLEC provides measurements.

Coal has a heat value of between 15000 and 30000 kilojoules per kilogram, and I assume IMLEC would use the best coal available – 30000kJ/kg

A Joule is 1 watt per second.

Using Run 13 from IMLEC results in ME 4595, I see Marcus Peel's engine Springbok burnt 1.19lb of coal in 32 minutes. Conveniently, 1.19lb of coal is about 0.5kg, so:

watts = coalConsumed * heatValue * 1000(J) / runtime(S)

watts = 0.5 * 30000000 / 1920 = 7812W

As my learned friend Mr Duncan Webster and several others have pointed out I am not good at maths! However, I reckon the coal burning in Springbok's firebox averaged an output of nearly 8kW. (It could be replicated by three and a half British Standard electric kettles!)

Applying a sanity check, IMLEC calculated Springbok's efficiency at 1.889%, which I call 2%. That makes the power output of the engine about 160W which seems reasonable.

Hope that makes sense. If not where did I go wrong? (Ducks for cover!)

Dave

You could always use Lithium Ion Batteries and set light to them.

(Please don't take me seriously)

Martin

If you look at the link I posted near the top of the thread, you will see that supercapacitors are an order of magnitude even more worser then batteries.

Murray