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 TM 5-685/NAVFAC MO-912
d. Advantages. Advantages of diesel power for
material can seriously damage moving parts. Con-
generating units include the ability: to utilize spe-
taminated fuel is a major vehicle by which dirt and
cific liquid or gaseous fuel other than highly volatile
water enter the system. Fuel must be filtered before
use.
refined ones (gasoline, benzene, etc.); to meet load
by varying the amount of fuel injected; to utilize a
e. Starting fuels. Diesel engines used for auxil-
relatively slow design speed; and, to operate with-
iary generators usually use distillate fuel for
quicker starting. These fuels are light oils that are
out external furnaces, boilers or gas generators.
similar to kerosene. Various additives are fre-
e. Disadvantages. Major disadvantages include: a
quently used with fuel such as cetane improvers
need to reduce cranking power by use of compres-
sion relief during start and a powerful auxiliary
which delay ignition for smoother engine operation,
corrosion inhibitors, and dispersants. Appendix C
starting engine or starting motor and battery bank;
contains information related to fuel and fuel stor-
high-pressure, close-tolerance fuel injection systems
age.
capable of being finely adjusted and modulated for
speed/load control; weight; and, noise.
f. Injection systems. Diesel engine manufacturers
usually use one of the following types of mechanical
3-4. Diesel fuel system.
fuel injection systems: unit injection, common rail
A typical diesel engine fuel system is shown in fig-
injection, or in-line pump and injection nozzle. A
limited number of diesel engines currently in use
ure 3-6. Information related to cooling, lubrication,
and starting systems is also shown. Functional re-
employ a common rail injection system. Electronic
fuel injection has been developed for use in modern
quirements of a diesel engine fuel system include
fuel injection, injection timing, and fuel pressuriza-
diesel engines refer to paragraph 3-4b(4). Unit in-
tion.
jector, common rail injector, and in-line pump and
a. Fuel injection system. This system measures
injection nozzle systems are described in tables 3-1
through 3-3. Injection of fuel in any system must
and meters fuel supplied to each cylinder of the
engine. Either inlet metering or outlet metering is
start and end quickly. Any delay in beginning injec-
used. In inlet metering, fuel is measured within the
tion changes the injection timing and causes hard
injector pump or injector. In outlet metering, fuel is
starting and rough operation of the engine. Delay in
ending injection is indicated by heavy smoke ex-
measured as it leaves the pumping element. Instan-
haust and loud, uneven exhaust sounds. The end of
taneous rate during injection must deliver fuel to
attain correct propagation of the flame front and
injection (full shutoff) should be total with no
dribble or secondary injections. Some injection sys-
resulting pressure rise.
b. Timing. Fuel injection timing is critical. The
tems include a delivery or retraction valve for fuel
shutoff. In other systems, camshafts have cam lobes
duration of fuel injection and the amount of fuel
injected vary during starting and partial, full, or
designed with a sharp drop to assure rapid fuel
overload conditions, as well as with speed. The best
shutoff.
engine start occurs when fuel is injected at (or just
(1) Common rail injection. The common rail in-
before) TDC of piston travel because air in the com-
jection system is an older system where fuel is sup-
bustion chamber is hottest at that instant. During
piied to a common rail or manifold. A high-pressure
engine operation, the injection timing may need to
pump maintains a constant pressure in the rail
be advanced to compensate for injection lag. Many
from which individual fuel lines connect to the in-
modern injection systems have an automatic injec-
jection or spray nozzle at each cylinder. Fuel is
tion timing device that changes timing to match
drawn from the supply tank by the low-pressure
changes in engine speed.
pump and passed through a filter to the suction side
of the high-pressure pump. The high-pressure pump
to open the injector nozzle because the nozzle (or
raises the fuel to the engine manufacturer's speci-
injector tip) contains a spring-loaded check valve.
fied operating pressure. Constant pressure is main-
The injection pressure must be greater than the
tained in the system by the high pressure pump and
compression pressure within the compression
related relief valve. If pressure is greater than the
chamber or cylinder. Between 1500 psi and 4000 psi
relief valve setting, the valve opens and permits
some of the fuel to flow back (bypass) into the tank.
pressure is required for injection and proper fuel
atomization. Specific information is provided in the
Check valves in the injection nozzle prevent the
engine manufacturer's literature. Fuel system com-
return of fuel oil to the injection system by cylinder
ponents are listed in paragraph 3-4c.
compression pressure.
(2) Unit injection. This system consists of an
integral fuel-injector pump and injector unit. A com-
is manufactured to precision accuracy and must be
very carefully handled. A small amount of abrasive
plete unit is required for each cylinder. Fuel oil is
3-6
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