Page 30 - Piston Engine-Based Power Plants

22 Piston Engine-Based Power Plants

There is another branch of piston engines called free piston engines
in which the backwards and forwards motion is used directly rather
than being converted into rotary motion. The power from these
engines is often extracted via the exhaust pressure driving a turbine or
with some form of hydraulic drive. These engines often use two oppos-
ing pistons and cylinders, one driving the piston in one direction, then
the second driving it back. In this case the power strokes in the oppos-
ing cylinders must alternate.
For the more conventional rotating crankshaft engines, there are
different engine cycles. The two most common are the two-stroke cycle
in which each complete cycle of the engine consists of one in-stroke
and one out-stroke, and the four-stroke cycle in which a complete
engine cycle comprises two in-strokes and two out-strokes. There are
also a number of six-stroke engine designs.
Another important variable is the number of cylinders in the engine.
Some simple engines have a single cylinder. However the nature of the
power generation in a crankshaft engine makes the power delivery
from a single cylinder engine very uneven. More complex and sophisti-
cated engines will have multiple cylinders with individual cylinders
delivering power at different points during the rotation of the crank-
shaft in order to smooth the operation.

All reciprocating engines are heat engines that can be analysed in
terms of the thermodynamic cycles which describe how heat can be
used to provide power and work. The amount of power that can be
extracted from a heat engine of this type depends in part on the
temperature and pressure of the working fluid in this case the gases
in the cylinder of the engine so controlling both the temperature and
pressure during the engine cycle plays an important role in determining
efficiency. For modern engines, emissions are also a key consideration.
These will also be affected by the engine operating conditions but often
in a contrary way to efficiency. For example, higher temperatures in
the cylinder can provide higher efficiency but will lead to greater emis-
sions of certain types.

INTERNAL COMBUSTION ENGINE FUNDAMENTALS

The simplest reciprocating engine comprises a single cylindrical cham-
ber housing a piston. The engine cylinder is sealed at one end (but