Page 35 - Piston Engine-Based Power Plants

Types of Reciprocating Engine 27

The Otto cycle was modified by Rudolph Diesel in the 1890s. In his
version, air is compressed in a cylinder by a piston to such a high pres-
sure that its temperature rises above the ignition point of the fuel
which is then introduced into the chamber and ignites spontaneously
without the need for a spark. This represents a simplification of the
Otto cycle but is not without its complications, particularly from an
emissions perspective.
At around the same time as the Otto and Diesel engines were being
developed the first two-stroke engine cycle was also proposed. Two
men, German engineer, Karl Benz and an Englishman, Dugald Clark,
independently developed two-stroke engines in 1878. This cycle repre-
sents another important branch of the reciprocating engine family.

Four-Stroke Engines
In a four-stroke engine each piston of the engine, and there can be a
large number depending on the particular engine type and application,
is equipped with at least two valves, one to admit air or an air fuel
mixture and a second to exhaust spent gases after ignition. The open-
ing and closing of these valves is mechanically synchronised with the
movement of the piston backwards and forwards.

The four-stroke cycle derives its name from the four identifiable
movements of the piston in the chamber, two of expansion and two of
compression, for each full power cycle. These have already been out-
lined above but since this is the most important type of reciprocating
engine, they are described again in more detail here, with diagrams to
illustrate the process (Fig. 3.4).

Figure 3.4 The stages of a spark ignition engine cycle. Source: NASA.