[Karen W. (OP)]

 Something cheap or free?  Perhaps solar heated air or something REALLY different?

 When you click on the site please be sure to scroll down to see the operating engine.

Thanks Bob..

http://www.greensteamengine.com/index.html

[lyner]

If you're after  a 'moderately hot air'  engine, then the Stirling Engine is what you want.
It won't produce much power (TNSTAAFL) but you could operate it from a solar panel -
http://www.google.co.uk/imgres?imgurl=http://blog.steamshift.com/wp-content/uploads/moved/stirling_engine.gif&imgrefurl=http://blog.steamshift.com/2005/05&h=115&w=115&sz=4&tbnid=O9Nph48QjuAJ:&tbnh=115&tbnw=115&prev=/images%3Fq%3Dstirling%2Bengine&sa=X&oi=image_result&resnum=1&ct=image&cd=1
(Right at the bottom of the web page)

[McQueen]

It certainly does seem to be a very original design, its a pity it has pistons in it though!
A pure rotary motion would have increased the engines efficiency several fold. Interesting concept though. Unfortunately an engine that works on 20 psi is not going to be very efficient.

[lyner]

It would be, basically, a 'niche' application - perfect in the right place,

[another_someone]

It certainly does seem to be a very original design,

The concept was original in 1816 - although enhancements have been made since then.

its a pity it has pistons in it though!
A pure rotary motion would have increased the engines efficiency several fold.

Depends on what you mean by efficiency.

Rotary engines tend to have a good power to weight ratio - but that matters for a motor vehicle, but not for a stationary engine.

In practice, fuel conversion efficiency is no better in rotary engines than in piston driven engines.  The theoretical cycle efficiency is the same in both, and frictional losses, although different, tend to be just as bad in rotary engines as in piston engines.

Another thing to bear in mind - more important than the pistons, is the fact that it is an external heat machine, rather than an internal combustion engine.  This always introduced losses, but increases flexibility (you can use almost any heat source - try doing that with an internal combustion engine).  The fact that, unlike a steam engine, it does not require a phase change in the working fluid, further adds to its flexibility (it can use any range of temperature differences).

Interesting concept though. Unfortunately an engine that works on 20 psi is not going to be very efficient.

The working fluid is at 20psi, but the working fluid operated on a closed cycle, so there is no efficiency loss in that (once the fluid is pressurised, it need not lose that pressure).  Do not confuse that pressure with the open cycle high pressure required in turbo-charged vehicles or diesel cars.

In practice, sterling engines have always been regarded as having high theoretical promise in terms of conversion efficiency, but often failed to deliver on that promise (as is true of so many other technologies, including rotary technologies).  One area where they do deliver is in high reliability, and low maintenance.

[lyner]

I've always fancied owning a stirling engine with some proper application for it.

[qazibasit]

yes ethane, butane, propane and others.

[Bored chemist]

What, no verb?

[lyner]

Verbs as fuels?
I've heard of 'hot air' in speeches but selecting the verbs is definitely a new direction for green technology.