On Thu, Sep 22, 2005 at 10:46:24PM +0100, Howard Winter wrote:
> 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...

Funny. I did have a thought. I realized that instead of putting the engine in
the focal point of the collector, that instead use another mirror to beam the
concentrated sunlight down onto the engine. So the focal point will have a lot less
sun blockage. The next thing is that the engine itself is going to have to track with
the collector. Imagine the whole setup on a beam that swings from east to west each
day.

>  
> >...<
> > > 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.

But medium to high efficiency Stirling engines have their problems too. Air isn't
a real good gas to use, and good Stirling engines need a good working gas and
high pressure to be effective.

I think that steam can be usable in this setup. I'll take a crack at some of the
issue you raise below.

> > > > 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.

Since we're talking about external heat engines, I'll let this one go by the wayside.

> 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).

What would be the temp cycling for superheated water injected into the cylinder?
What pressure would superheated water need to be in order to get in the ballpark
temp of a 600-700C steam chamber?

> >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)

In James' suggestion the water isn't pressurized. However it would be better if
the water slug to be injected could be superheated under pressure. More on this
in a minute.

> plus a high pressure injection pump (timed),
> and valves,

We're talking about an engine that already has valves in it right? We would set the
timing so that the exhaust valve would be open for the return stroke moving the
wet steam out of the cylinder. I think that's what James' original proposal states.

I'll come back to my idea in a later post...

BAJ
> 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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