Page 38 - Piston Engine-Based Power Plants

30 Piston Engine-Based Power Plants

strokes of an Otto cycle engine are followed by a further two during
which a second fluid (such as steam or water) is injected into the cylin-
der, capturing heat that has been wasted during the preceding four
piston movements. Other, compound six-stroke engines have two com-
plementary pistons with one carrying out two strokes while the second
performs the other four. A further variant is a five-stroke engine. This is
a modified Otto engine in which a pair of cylinders share an extra cylin-
der placed between them. The exhaust from each Otto cycle cylinder is
fed into the shared cylinder where it expands, providing additional
work. There are in effect six piston strokes for each cycle, four in an
Otto cylinder and two more in the ancillary cylinder. However the
designers consider it to be a five-stroke cycle because the exhaust stroke
of each Otto cylinder is synchronised with the expansion stroke of the
additional cylinder and this is taken to be a single stroke. These engines
are not widely used.

FREE PISTON ENGINES

A free piston engine is a reciprocating engine that does not use a crank-
shaft to control the motion of the piston(s) or to extract power from the
engine. Instead, when the fuel is burnt in the cylinder of the engine, forc-
ing the gas to expand and the piston to retreat, the force on the piston
must be balanced by some other force acting on it within the engine.
There are various ways this can be achieved. The simplest is to build a
second, sealed, bounce chamber on the opposite side of the piston to the
combustion cylinder and fill this with air. As the piston retreats it will
compress this air and eventually the pressure will be high enough to
force the piston to return into the combustion cylinder.

Another design uses two cylinders and combustion chambers with
their open ends facing one another, each with a piston but with the two
pistons joined together by a single rod. The combustion chambers are
designed to fire alternately. When the cylinder on the right fires it forces
the pair of pistons to the left, compressing the mixture in the left-hand
cylinder. This then fires and returns the piston to the right. There are
various ways that this to-and-fro motion can be used to provide a power
source, with both hydraulic and electric power generation possible.

There is yet another configuration for the free piston engine, the
opposing piston free piston engine in which two pistons share the same