Check out this piston fluid accumulator.
 It looks very similar to David's  drawing.  I have  not yet found a
source.
Jerry

http://www.engineersedge.com/hydraulic/accumulator_piston.htm

Yes they have been around for 100 years. They are usually designed for
1000 psi and above. As such they are rather heavy. Every airliner has one.
Paul Lamar


---------------------------
Try
http://www.pegasusautoracing.com/group.asp?GroupID=OILACCUS

made for oil accumulators
Bill Schertz
KIS Cruiser # 4045


There is a failure mode I don't like about the idea.
If steam is generated for what ever reason the piston or spring bottoms
out and your radiator splits.

"One gallon of water makes [1325] gallons of steam."

Paul Lamar ...No rotor no motor.

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