Page 20 - Introduction to Marine Engineering

Chapter 2
Diesel engines

The diesel engine is a type of internal combustion engine which ignites
the fuel by injecting it into hot, high-pressure air in a combustion
chamber. In common with all internal combustion engines the diesel
engine operates with a fixed sequence of events, which may be achieved
either in four strokes or two, a stroke being the travel of the piston
between its extreme points. Each stroke is accomplished in half a
revolution of the crankshaft.

Four-stroke cycle

The four-stroke cycle is completed in four strokes of the piston, or two
revolutions of the crankshaft. In order to operate this cycle the engine
requires a mechanism to open and close the inlet and exhaust valves.
Consider the piston at the top of its stroke, a position known as top
dead centre (TDC). The inlet valve opens and fresh air is drawn in as the
piston moves down (Figure 2.1 (a)). At the bottom of the stroke, i.e.
bottom dead centre (BDC), the inlet valve closes and the air in the
cylinder is compressed (and consequently raised in temperature) as the
piston rises (Figure 2.1(b)). Fuel is injected as the piston reaches top
dead centre and combustion takes place, producing very high pressure
in the gases (Figure 2. l(c)). The piston is now forced down by these gases
and at bottom dead centre the exhaust valve opens. The final stroke is
the exhausting of the burnt gases as the piston rises to top dead centre to
complete the cycle (Figure 2.1(d)). The four distinct strokes are known
as 'inlet' (or suction), 'compression', 'power' (or working stroke) and
'exhaust'.
These events are shown diagrammatically on a timing diagram
(Figure 2.2). The angle of the crank at which each operation takes place
is shown as well as the period of the operation in degrees. This diagram
is more correctly representative of the actual cycle than the simplified
explanation given in describing the four-stroke cycle. For different
engine designs the different angles will vary, but the diagram is typical