[Stoves] Green Steam Engine

[Ken Boak]

List,

The "Green Steam Engine" has being doing the rounds of the internet for
about the last 5 years, with totally unproven claims regarding it's
efficiency.

There is nothing about its design to suggest that it should be any more or
any less efficient than a conventional steam engine of similar cylinder
dimensions.

Small engines tend to have poor efficiency because the ratio of surface
area of the cylinder to the swept volume is greater than that of a larger
engine, and so small engines lose considerably more heat energy in cylinder
losses.  This is why any practical small engine will have insulation
wrapped around the cylinder to conserve heat.

The efficiency of the engine is determined by the temperature and pressure
of the steam admitted and the steam exhausted,  dictated by the laws of
thermodynamics which have over 200 years of history.   The mechanical
linkage coupling the cylinders to extract the mechanical power, has very
little influence on the overall efficiency - so an engine with poor
thermodynamic design will not become super-efficient by fitting a miracle
new linkage.

Small scale steam has always been notoriously inefficient, and there is
nothing about this engine design to suggest otherwise.  Thermo-mechanical
efficiencies rarely exceed single digits.  Our forefathers measured
efficiency in the pounds of steam needed to be raised to produce a
horsepower for an hour.  Small engines often took 40 or 50 pounds of steam
to produce 1hp hr and rejected 95% of this thermal energy to the atmosphere.

By comparison, wood gasification has been shown to convert woodchips to
electricity using a conventional spark ignition engine, or modified diesel,
with an overall efficiency of over 20%.

This means that every pound of firewood or biomass you have access to, will
go 5 times further by following the gasification route rather than the
steam route.
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