Page 70 - Piston Engine-Based Power Plants

62 Piston Engine-Based Power Plants

An Alpha Stirling engine has two synchronised pistons that oscil-
late, but with a time lag so that they do not move in or out either
exactly together or exactly in opposition. This is important for the
operation of the engine. Fig. 6.1A represents an Alpha engine at the
point in its cycle when most of the working fluid is inside the hot cylin-
der and is being heated by the hot energy source. At the same time
the piston in the cold cylinder is close to its minimum position and
little working fluid is in contact with the cold sink. As heat passes into
the gas from the hot source, the working fluid pressure increases and it
expands, placing an additional force on the cold piston which starts to
move so that the volume in that cylinder increases. Meanwhile the lag
between the pistons means that the hot cylinder piston does not move
significantly at this point in the cycle.

As the cycle continues the cold cylinder piston moves out, and the
volume of the cold cylinder expands until the combined volume in
the hot and cold cylinders reaches it maximum (Fig. 6.1B). At this
stage the maximum amount of heat is being extracted from the cold
cylinder and the mechanical linkage ensures that the hot piston now
starts to move in, reducing the volume of the hot cylinder and forcing
more of the gas into the cold region.
Now the amount of gas in the hot region approaches the minimum
(Fig. 6.1C) and the extraction of heat from the cold cylinder starts to
cause the gas volume to contract so that the cold cylinder piston starts to
move down, reducing the volume of the cold cylinder too. As this con-
tinues the system reaches its minimum volume, as shown in Fig. 6.1D.
At this stage the mechanical linkage sets the hot piston in motion,
expanding the hot cylinder volume and the cycle begins once more.
One key element of the Stirling engine is missing from these simpli-
fied figures, a regenerator. This is a heat absorbing device that sits
inside the tube connecting the hot and cold cylinders of the engine.
The regenerator captures some of the heat from the gas that would
otherwise pass out through the cold sink as the gas flows in that direc-
tion and then returns it to the gas as it passes back into the hot cylin-
der. This helps improve the efficiency of the engine. A schematic of an
Alpha Stirling engine with a regenerator is shown in Fig. 6.2.

The other main category of Stirling engine is the Beta Stirling
engine. This has two pistons contained within a single cylinder, with a