Page 56 - Piston Engine-Based Power Plants

48 Piston Engine-Based Power Plants

Diesel engines have been used for heavy transportation applications
such as locomotives and marine engines and many of these heavy
engines have been adapted for power generation use too. Most are
four-stroke engines but some of the very heavy engines are two-stroke
engines.

DIESEL ENGINE FUNDAMENTALS

The diesel engine is a heat engine that utilises the properties of a gas to
covert heat energy into mechanical energy. When a mass of air is con-
tained in a restricted volume such as the cylinder of an engine and
then heat is added to it, the pressure of the gas increases. This increase
in pressure can be exploited to generate a mechanical force, power.
The cross section of a diesel engine cylinder is shown in Fig. 5.1.
Most diesel engines have four strokes, exactly like the spark ignition
engine. For an idealised engine these four strokes are an intake stroke
when air is drawn in to the cylinder through a valve as the piston
moves away from the top dead centre position (TDC see Chapter 4)
towards the bottom dead centre position (BDC). When it reaches
1
BDC, the valve closes and the piston returns towards TDC, compres-
sing the air inside the cylinder as it does so. When it reaches TDC
again, diesel fuel is injected into the compressed gas, which is now very
hot as a result of being compressed and the fuel burns, increasing the
temperature and hence the pressure inside the cylinder further. This
additional pressure on the piston head forces the piston back towards
the BDC position, providing the power stroke of the engine that can
be harnessed to provide mechanical drive. Finally, at BDC the piston
returns again, this time with a second, exhaust valve open when the air
and combustion products are expelled from the cylinder.

The stages of the cycle can be represented by a pressure volume
diagram that represents the gases inside the engine cylinder. This
is shown in idealised form in Fig. 5.2. This diagram ignores the first
stroke of the cycle which draws air into the cylinder, and the last
stroke which expels the combustion gases because these two strokes,
ideally, involve no exchange of energy. (In practice, they do require
energy to complete but the amount is small compared to the energy

1 In real engines the opening and closing of the valves is actually offset from BDC and TDC.