Animated Engines, Two Cylinder Stirling


Cylinder Stirling

The Stirling engine is one of my favorites.  It was
invented in 1816 by Rev. Robert Stirling of Scotland. The Stirling is a very simple engine, and was often billed as a safe alternative to steam (since there’s no
boiler to explode).  It enjoyed some success in industrial
applications, and in small appliances like fans and water pumps, but was eclipsed by
the advent of inexpensive electric motors.3  Since it can run
on any source of heat, it now holds promise for alternative fuel
engines, solar power, geothermal power, etc.
Stirling engines feature a completely closed system in which the
working gas (usually air but sometimes helium or hydrogen) is
alternately heated and cooled by shifting the gas to different
temperature locations
within the system.

In the two-cylinder or alpha configured3
Stirling, one cylinder
is kept hot while the other is kept cool.  In the illustration the
lower-left cylinder is  heated by burning fuel.  The other
cylinder is kept cool by an air cooled heat sink (a.k.a. cooling fins).

The Stirling cycle may be thought of as four different phases:
expansion, transfer, contraction, and transfer.

Expansion. At this
point, most of the gas in the system has just been driven into the hot
cylinder.  The gas heats and expands driving both pistons inward.
Transfer. At this point, the gas has expanded (about 3 times in this
example).  Most of the gas (about 2/3rds) is still located in the
hot cylinder.  Flywheel momentum carries the crankshaft the next 90
degrees, transferring the bulk of the gas to the cool cylinder.
Contraction. Now the majority of the expanded gas has been shifted to the cool
cylinder.  It cools and contracts, drawing both pistons outward.
Transfer. The now contracted gas is still located in the cool cylinder.
Flywheel momentum carries the crank another 90 degrees, transferring the
gas to back to the hot cylinder to complete the cycle.
This engine also features a regenerator,
illustrated by the chamber containing the green hatch lines.  The
regenerator is constructed of material that readily conducts heat and
has a high surface area (a mesh of closely spaced thin metal plates for
example).  When hot gas is transferred to the cool cylinder, it is
first driven through the regenerator, where a portion of the heat is
deposited.  When the cool gas is transferred back, this heat is
reclaimed; thus the regenerator „pre heats“ and „pre
cools“ the working gas, dramatically improving efficiency.3

2000, Matt Keveney. All rights reserved.


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