>DB> Could you make a piston big enough cheap enough to do anything 
 >DB> useful with 15 PSI?
 > 
 > I have never worked with these [air] engines but I once understood 
 > steam engine cycles and practical working examples.
The very first steam engines worked on atmospheric pressure. Steam was 
injected into the cylinder to displace the air, then water was injected to 
condense the steam. The resulting vacuum caused the atmospheric pressure on 
the other side to push the piston through its stroke. These were walking beam 
engines, used to pump water from coal mines in England. No crankshaft or 
other rotating nonsense. :-)
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