James,

On Tue, 20 Sep 2005 14:15:32 -0700, James Newtons Massmind wrote:

> > 
> > If you have a good espresso machine then you already have a 
> > suitable kettle, with safety valves and all that. One in a 
> > million occasionally blows up. Usually it causes no damage 
> > (there is containment for it).
> 
> Yes, and I thought about building a concrete containment room for a steam
> engine in my back yard... My wife loves me, but...

Tell her it's a hurricane shelter...
 
>...<
> > In fact, why not inject a gas ? Like CO2 ? Then you have 1/2 
> > of a stirling engine (well a hot air/gas engine). The only 
> > thing you want from water is its pressure increase when 
> > heated. A gas can do better here.
> 
> Err... Are you sure? First, water is commonly available and therefore ideal
> for use in a low cost system. Second, I thought the state change was what
> really made the big difference in volume. Water to steam is approx 300 to
> 1... Or rather 1 to 300.

I seem to remember it's actually 1:1500 - one of the reasons that steam is so widely used as a medium to 
convert heat into mechanical energy is this huge expansion (plus the huge latent heat of vapourisation, 640 
times water's specific heat).

> What sort of expansion can you get without phase change?

See Boyles' Law.  Not much, but enough for Stirling engines to work.

> > > What chemical means would trigger this flashing? I've not heard of 
> > > such a thing but that part of it sounds very interesting.
> > 
> > There are several chemicals that will do it, any powder, a 
> > spark (which causes a bubble to form), ultrasound that causes 
> > cavitation etc. I think that it is hard to *prevent* it from 
> > happening while charging the cylinder.
> 
> Sort of like pinging in a gas engine or detonation in a diesel? 

No, they are both detonation, a localised explosion reaction rather than a change of phase, caused by such 
things as too high a compression, the shape of the combustion chamber causing uneven flame-front spread, and 
so on.

Personally I think the whole idea of flashing to steam inside the cylinder is doomed - the temperature cycling 
of the material making up the chamber is a killer for efficiency (see Newcomen's engine, and the vast 
improvements in efficiency that Watt brought about by having the condenser seperate, so keeping the cylinder 
hot).

>I can see
> the point, but again, you have to pre-heat the working fluid and with that
> comes all the issues of pressurized feed, safety, etc... Perhaps a
> combination of heating the water to just below boiling without pressurizing
> it and then injecting it as a fine spray, onto a somewhat convoluted surface
> at top dead center. 

It's all getting terribly complicated!  You now have two locations to heat (the pressurised water storage and 
the cylinder) plus a high pressure injection pump (timed), and valves, and then what do you do with the "wet 
steam" exhaust?  Compare this with a Stirling engine, with one heat location, only a couple of moving parts, 
and no danger of explosions, and I really thing you're on a dead-end path.  Sorry!  :-)

Cheers,


Howard Winter
St.Albans, England


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