[Gasification] Underwater gasification?

[Daniel Chisholm]

A Humphrey is a low-compression ratio, low pressure piston engine.  As such
it will have a lower thermal efficiency than a higher compression ratio
engine, though that does not necessarily mean it is a bad solution.  The
elegance of a Humphrey comes from the fact that its piston and its workload
happen to be one and the same thing (a column of water), which allows it to
avoid high speed machinery, reduction gearboxes, rotary pumps etc, all of
which have a dollar and efficiency cost.  (and a side comment for what it's
worth - the combustion process in a Humphrey engine would be a lot closer to
being a constant volume rather than a constant pressure one).

With a reaction engine, the lower the speed of the vehicle, the more
strongly its propulsive efficiency depends on accelerating a large mass a
small amount.  Marine applications are very slow speed, which is why
efficient marine propulsion systems use very large propellers, which
accelerate enormous columns of water a very small amount.

This is why a container ship uses large propellers rather than a jet boat
type of propulsion.

Propelling a ship by means of a steam jet would be an even bigger mismatch
(an exhaust velocity of hundreds of meters per second, versus a vessel speed
of only ten or so m/s), the efficiency would be miniscule.

Perhaps this might help to understand why a reaction engine's exhaust speed
affects its propulsive efficiency:

   - The energy that it takes to accelerate a mass jet is proportional to
   the mass flow rate, times the square of the jet velocity (which is to say -
   to the kinetic energy that you are delivering to the jet).
   - The thrust that that mass jet produces is proportional to the mass flow
   rate times the jet velocity (which is to say - to the momentum that you are
   delivering to the jet)

If you look to optimize the problem (reduce the amount of energy it takes to
produce a given level of thrust), you will see that propulsive efficiency is
improved by accelerating a larger amount of mass by a smaller delta-V.


-- 
- Daniel
Fredericton, NB  Canada
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