Page 34 - Piston Engine-Based Power Plants

26 Piston Engine-Based Power Plants

When the air is fully compressed, fuel is injected into it and ignites
spontaneously in the hot air. This makes the engine design relatively
simpler and these engines are, because of the higher compression ratio,
potentially more efficient.

Many of the reciprocating engines in use today are based on what
is known as a four-stroke engine cycle. This is the cycle that has been
described above and involves the piston moving in and out twice dur-
ing the full cycle. In consequence, power is delivered to the shaft of the
engine only once during two revolutions. The four-stroke cycle is rela-
tively complex but also allows the most sophisticated engines to be
built. There is a simpler alternative, the two-stroke cycle. In this cycle
a power stroke takes place during each revolution of the shaft. This
type of engine is often used where a small, cheap source of mechanical
power is required. However the engines do have some advantages over
their more complex relatives, particularly a higher power to weight
ratio. The two-stroke cycle is also used in some very large engines used
for power generation since it is capable of high efficiency and tolerat-
ing very poor fuels.

ENGINE CYCLES

The internal combustion engine is a thermodynamic heat engine and
as such belongs to the same category as steam turbines and gas tur-
bines. However the physical nature of the reciprocating engine is very
different to that of the turbines. The reciprocating engine principle can
be traced back to the 17th century but development of the modern
engine belongs to the latter half of the 19th century. Nikolaus Otto is
generally credited with building the first four-stroke internal combus-
tion engine in 1876. In doing so he established the principle still in use
today.
The Otto cycle engine employs a spark to ignite a mixture of air
1
and traditionally gasoline compressed by the piston within the
engine cylinder. This spark ignition causes an explosive release of heat
energy which increases the gas pressure in the cylinder, forcing the pis-
ton outwards as the gas tries to expand. This explosion is the source of
power, its force on the piston turning the crankshaft to generate rotary
motion.

1 Otto’s engine probably burnt powdered coal but gasoline soon became the preferred fuel.