Page 35 - Introduction to Marine Engineering

22 Diesel engines

After passing through centrifuges the cleaned, heated oil is pumped to a
daily service tank. From the daily service tank the oil flows through a
three-way valve to a mixing tank. A flow meter is fitted into the system to
indicate fuel consumption. Booster pumps are used to pump the oil
through heaters and a viscosity regulator to the engine-driven fuel
pumps. The fuel pumps will discharge high-pressure fuel to their
respective injectors.
The viscosity regulator controls the fuel oil temperature in order to
provide the correct viscosity for combustion. A pressure regulating valve
ensures a constant-pressure supply to the engine-driven pumps, and a
pre-warming bypass is used to heat up the fuel before starting the
engine. A diesel oil daily service tank may be installed and is connected
to the system via a three-way valve. The engine can be started up and
manoeuvred on diesel oil or even a blend of diesel and heavy fuel oil.
The mixing tank is used to collect recirculated oil and also acts as a
buffer or reserve tank as it will supply fuel when the daily service tank is
empty.
The system includes various safety devices such as low-level alarms
and remotely operated tank outlet valves which can be closed in the
event of a fire.

Fuel injection
The function of the fuel injection system is to provide the right amount
of fuel at the right moment and in a suitable condition for the
combustion process. There must therefore be some form of measured
fuel supply, a means of timing the delivery and the atomisation of the
fuel. The injection of the fuel is achieved by the location of cams on a
camshaft. This camshaft rotates at engine speed for a two-stroke engine
and at half engine speed for a four-stroke. There are two basic systems
in use, each of which employs a combination of mechanical and
hydraulic operations. The most common system is the jerk pump; the
other is the common rail.

Jerk pump system
In the jerk pump system of fuel injection a separate injector pump exists
for each cylinder. The injector pump is usually operated once every
cycle by a cam on the camshaft. The barrel and plunger of the injector
pump are dimensioned to suit the engine fuel requirements. Ports in the
barrel and slots in the plunger or adjustable spill valves serve to regulate
the fuel delivery (a more detailed explanation follows). Each injector
pump supplies the injector or injectors for one cylinder. The needle