Page 31 - Piston Engine-Based Power Plants

Types of Reciprocating Engine 23

with valves to allow gases in and out) and open at the other end. A
cylindrical-shaped disk of metal, the piston, is designed to fit closely
within the cylinder to seal the open end and this piston is designed to
move backwards and forwards easily within the cylinder. This it does
in response to the pressure changes in the gas contained within the
cylinder at various stages of the engine cycle. The outside of the piston
is connected via levers to a shaft called the crankshaft through which
rotary power is delivered. A cross section of a single piston engine is
shown in Fig. 3.1. This is based on a spark ignition engine.

The operation of an engine of this type depends on repeating a cycle
of events. These are controlled mechanically. For the commonest
engine of all, the four-stroke spark ignition engine (the cycle is slightly
different for a diesel engine) the cycle starts with a mixture of air and
fuel being admitted into the cylinder while the piston moves (out) to
increase the volume of the cavity within the cylinder. Once all the fuel
and air has been drawn in, the piston starts to return, reducing the size
of the cavity and compressing the gas and fuel mixture. When the pis-
ton reaches the top of its stroke and the volume of the cavity is at a
minimum, an electrically generated spark ignites the fuel in the mix-
ture, creating a controlled explosion that forces the piston to retreat.

Figure 3.1 Cross section of a spark ignition piston engine cylinder and crankshaft. Source: US Office of Energy
Efficiency & Renewable Energy.