alt.energy.renewable

-> Steam locomotives used what I think was called a "cut-off valve" which
-> allowed full pressure steam to enter the cylinder from TDC for a
-> variable number of crankshaft degrees. From there on the power came
-> from the expansion of the steam within the cylinder. The engine would
-> need multiple stages to get most of the power out of air compressed to
-> 300 atmospheres since the over all CR would need to be close to 300:1
-> or you would get a loud/wasteful pop when the exhaust valve opened.
-> Putting warming fins (as opposed to cooling fins) on the cylinders and
-> heat exchangers between the stages to warm the air that has cooled due
-> to expansion would improve efficiency. But then, how far can we go
-> and still keep it inexpensive and compact?

Ideally, the expansion of the air should be done isothermally, so it
must be supplied with heat *during* its expansion in the cylinder. This
means that there should be plenty of contact between the air inside the
cylinder and the cylinder's wall. Having a long, narrow cylinder would
help. This is the opposite of the situation in internal combustion
engines and steam engines, where contact with the wall is minimized by
using short, wide cylinders.

Having a small radius for the cylinder also means that it can be made of
thinner metal and still be able to withstand the pressure.

dow