Page 12 - Piston Engine-Based Power Plants

An Introduction to Piston Engine Power Plants 5

pivot to raise and lower a second rod which drove a pump in the mine,
below. The bottom of the cylinder was closed but was connected
through a valve to a boiler that produced very low-pressure steam.
The boiler for the engine was typically a ‘haystack’ boiler, so called
because of its shape, which could produce steam to a maximum of
0.2 0.3 bar above atmospheric pressure. During the first part of the
engine cycle this steam was admitted into the cylinder, forcing the pis-
ton to rise against the pressure of the atmosphere on the top of the pis-
ton. The valve was then closed and a second valve opened briefly,
allowing cold water to spray into the cylinder and cool this steam
which then condensed, allowing atmospheric pressure to force the pis-
ton back down. This cycle was repeated around 12 times each minute.

Operation of the engine required the two valves to be alternately
opened and closed, with this cycle of opening and closing repeated at
regular intervals. The valves were originally operated manually but a
mechanical means of carrying this out was soon devised, an early fore-
runner of the valve mechanisms in modern engines.
The development of steam piston engines continued through the
18th century and into the 19th century with designers such as James
Watt, who developed a more efficient version of the Newcomen
engine. All the early steam engines used low-pressure steam but gradu-
ally, as material technology improved, higher pressure was introduced.

The use of high-pressure steam allowed smaller engines to be built.
Piston steam engine development continued until the late 19th century
when the steam turbine first appeared. This was a much more
advanced, high-speed engine and it quickly superseded steam recipro-
cating engines for most applications. However the use of reciprocating
engines for steam locomotives on railway systems continued, in some
regions, until the end of the 20th century.

While steam, external combustion, reciprocating engines played an
important role in the development of piston engines it is the internal
combustion engine that has proved to be the most enduring engine of
this type. Although the concept was proposed in the 17th century,
internal combustion was virtually abandoned when steam engine devel-
opment began and it was not until the 19th century that practical
engines of this type began to reappear.