Page 33 - Piston Engine-Based Power Plants

Types of Reciprocating Engine 25

Figure 3.3 Schematic of the piston crankshaft linkage. Source: Wikimedia.

mechanical coupling of the piston to the crankshaft then drive the piston
back into the cylinder at the end of the power stroke, so long as the shaft
continues to rotate. The rotation is maintained, and smoothed, by the fly-
wheel attached to one end of the shaft.

The actual energy source for this type of engine is the fuel which burns
in air, releasing large amounts of energy as heat. In the case of the spark
ignition engine the fuel is mixed with air using a special mixing and
metering system and a measured amount is introduced into each cylinder
of the engine at the appropriate point in the engine cycle. A spark is then
used to start a controlled explosion in which the chemical energy in the
fuel is converted first into heat energy and then as the gases expand and
act on the piston, into mechanical energy. This controlled explosion gives
the engines its common name, the internal combustion engine.
As a result of the way in which the mechanical energy is fed to the
crankshaft through levers, the rotational force on the shaft from each
piston is extremely uneven. Most advanced engines have several pis-
tons, synchronised mechanically to provide power sequentially as the
shaft rotates. This helps to smooth the power delivery.

There are two principle types of internal combustion engine, the
spark ignition engine and the diesel or compression ignition engine,
each defined by the way in which fuel is admitted into the engine cylin-
der and how ignition of the air fuel mixture is initiated. The spark
ignition engine, as described above, uses an electrical spark to ignite
the fuel air mixture. The diesel engine takes a different approach.
Instead of a fuel air mixture, only air is admitted into the cylinder
and this air is compressed much more highly than in the spark ignition
engine. The higher compression ratio makes the air much hotter.