(a) Proven dependability for sustained op-
cost.
eration at rated load.
a. The advantages of diesel engines include:
(b) Can use a variety of liquid and gaseous
(1) Proven dependability for sustained opera-
fuels.
tion at rated load.
(c) Low vibration level.
(2) Efficiency.
3-2
TM 5-685/NAVFAC MO-912
(3) Adaptability for wide range of liquid fuels.
(4) Controlled fuel injection.
b. The disadvantages include:
(1) High initial cost.
(2) High weight per given output.
(3) High noise level.
(4) Specialized maintenance.
(5) Fuel injection system has fine mechanical
tolerances and requires precise adjustment.
(6) Difficult cranking.
(7) Cold starting requiring auxiliary ignition
aids.
(8) Vibration.
3-3. Types of Diesel Engines.
Various configurations of single and multiple diesel
engines, either two-cycle or four-cycle are used to
drive auxiliary generators. Multi-cylinder engines
Figure 3-2 Typical small stutionary diesel
generator unit, air cooled
of either type can be of “V” or in-line configurations.
Figure 3-3. Typical large stationary diesel generator unit.
3-3
TM 5-685/NAVFAC MO-912
Figure 3-4. Typical diesel power plant on transportable frame base.
The “V” configuration is favored when there is a
port (or the exhaust valve closes), then the scaveng-
lack of space because “V” engines are shorter and
ing air port is closed. The piston now compresses
more compact than in-line engines. Most engines in
the air to heat it to a temperature required for
use are liquid-cooled. Air cooling is sometimes used
ignition as the piston nears top dead center (TDC).
with single-cylinder and other small engines (driv-
As the piston nears TDC, a metered amount of fuel
ing generators with up to 10 kW output). Air-cooled
is injected at a certain rate. Injection atomizes the
engines usually reach operating temperature
fuel, which is ignited by the high temperature, and
quickly but are relatively noisy during operation.
combustion starts. Combustion causes the tempera-
a. Two cycle. The series of events that take place
ture and pressure to rise further.
in a two-cycle diesel engine are: compression, com-
(2) Power: As the piston reaches and passes
bustion, expansion, exhaust, scavenging, and air in-
TDC, the pressure of the hot gas forces and acceler-
take. Two strokes of the piston during one revolu-
ates the piston downward. This turns the crank-
tion of the crankshaft complete the cycle.
shaft against the load connected to the shaft. The
(1) Compression. The cycle begins with the pis-
fuel/air mixture continues to burn. As the piston
ton in its bottom dead center (BDC) position. The
passes eighty percent (80%) to eighty-five percent
exhaust valve is open permitting burned gases to
(85%) of the stroke travel towards BDC, it uncovers
escape the cylinder, and the scavenging air port is
the exhaust port (or the exhaust valve is opened).
uncovered, permitting new air to sweep into the
This allows exhaust gas to escape from the cylinder.
cylinder. With new air in the cylinder, the piston
As the piston continues downward, it uncovers the
moves upward. The piston first covers the exhaust
scavenging air port, allowing scavenging air (fresh
3-4
TM 5-685/NAVFAC MO-912
air at 3 pounds per square inch (psi) to 6 psi) to
sweep the cylinder, further purging the exhaust gas
and providing a fresh clean charge for the next
cycle. The piston reaches and passes through BDC.
The compression stroke then begins again.
b. Four-cycle. The series of events taking place in
a four-cycle engine are: inlet stroke, compression
stroke, expansion or power stroke, and exhaust
stroke. Four strokes (two revolutions of the crank-
shaft) are necessary to complete the cycle.
(1) Inlet stroke. As the piston starts downward
from TDC, the inlet (intake) valve opens and allows
the piston to suck a charge of fresh air into the
cylinder. This air may be supplied at a pressure
higher than atmospheric air by a supercharger.
(2) Compression stroke. As the piston nears
BDC, the air inlet valve closes, sealing the cylinder.
EXHAUST
Energy supplied by the crankshaft from a flywheel,
or power from other cylinders, forces the piston up-
ward toward TDC, rapidly compressing the air and
increasing the temperature and pressure within the
cylinder.
A .
(3) Power stroke. As the piston approaches
TDC, an amount of fuel (modulated by the governor)
is injected (sprayed and atomized) into the cylinder
which is ignited by the high temperature, and com-
FUEL INJECTOR
V AL VE O P ENS
bustion starts. Combustion, at a controlled rate,
further increases the temperature and pressure to
AIR STARTING
accelerate the piston toward BDC. The expansion of
V A L V E O P E N S 7
the hot gases forces the piston down and turns the
crank against the load. Engine efficiency depends
on the fuel charge being completely burned during
the power stroke.
(4) Exhaust stroke. As the piston passes
through BDC at the end of the power stroke, the
exhaust valve opens. The piston, using stored en-
ergy from the flywheel or from the power stroke of
another cylinder, forces the burned gases from the
cylinder through the exhaust port. As the piston
approaches TDC, the exhaust valve is closed and
the air intake valve opens to begin another cycle.
COMPRESS10
c. Engine timing. Engine timing is critical. Intake
V A L V E O P E N
and exhaust valves have to open and close to allow
O V E R L A P -
S T A R T
the greatest amount of work to be extracted from
V A L V E C L O S E S
combustion. They must also be open long enough to
allow fresh air to flow into and exhaust gas to flow
out of the cylinder. Fuel must be injected at proper
rates during certain periods of time to get smooth
Figure 3-5. Timing diagrams
pressure rise and complete combustion. Timing for
A) FOR A TWO STROKE CYCLE,
two-stroke cycle and four-stroke cycle engines dif-
B) FOR A FOUR STROKE CYCLE.
fers (refer to the timing diagrams in fig 3-5). Dia-
gram A illustrates two forms of the two-stroke cycle
The outer portion covers a port control (uniflow)
engine. The inner portion covers the typical crank-
system. Diagram B illustrates timing for a four-
case scavenging type with uncontrolled fixed ports.
stroke cycle engine.
3-5
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
refined ones (gasoline, benzene, etc.); to meet load
use.
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
out external furnaces, boilers or gas generators.
quicker starting. These fuels are light oils that are
e. Disadvantages. Major disadvantages include: a similar to kerosene. Various additives are fre-need to reduce cranking power by use of compres-
quently used with fuel such as cetane improvers
sion relief during start and a powerful auxiliary
which delay ignition for smoother engine operation,
starting engine or starting motor and battery bank;
corrosion inhibitors, and dispersants. Appendix C
high-pressure, close-tolerance fuel injection systems
contains information related to fuel and fuel stor-
capable of being finely adjusted and modulated for
age.
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
ure 3-6. Information related to cooling, lubrication,
limited number of diesel engines currently in use
and starting systems is also shown. Functional re-
employ a common rail injection system. Electronic
quirements of a diesel engine fuel system include
fuel injection has been developed for use in modern
fuel injection, injection timing, and fuel pressuriza-
diesel engines refer to paragraph 3-4 b(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
and meters fuel supplied to each cylinder of the
through 3-3. Injection of fuel in any system must
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
measured as it leaves the pumping element. Instan-
ending injection is indicated by heavy smoke ex-
taneous rate during injection must deliver fuel to
haust and loud, uneven exhaust sounds. The end of
attain correct propagation of the flame front and
injection (full shutoff) should be total with no
resulting pressure rise.
dribble or secondary injections. Some injection sys-
b. Timing. Fuel injection timing is critical. The
tems include a delivery or retraction valve for fuel
duration of fuel injection and the amount of fuel
shutoff. In other systems, camshafts have cam lobes
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
c. Fuel pressurization. Fuel must be pressurized 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
pressure is required for injection and proper fuel
some of the fuel to flow back (bypass) into the tank.
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 3-4 c.
compression pressure.
d. Fuel contamination. Fuel injection equipment
(2) Unit injection. This system consists of an
is manufactured to precision accuracy and must be
integral fuel-injector pump and injector unit. A com-
very carefully handled. A small amount of abrasive
plete unit is required for each cylinder. Fuel oil is
3-6
TM 5-685/NAVFAC MO-912
Figure 3-6. Diagram of typical fuel, cooling, lubrication, and starting systems.
3-7
TM 5-685/NAVFAC MO-912
Table 3-l. Unit injector system.
of each other. Fuel from the supply tank is passed
through a filter to the injector pump supply pipe.
Component Purpose
The injector pump receives the fuel which is then
Gear pump
Low pressure pump; delivers fuel from tank to
injected into the cylinders in proper quantity and at
injector: fuel also lubricates the pump.
a prearranged time.
Injector
Meters, times, and pressurizes fuel: camshaft-
(3) Electronic Fuel Injection. The electronic
operated by pushrod and rocker arm; one injec-
fuel injection system is an advanced design for mod-
tor for each cylinder.
ern diesel engines, intended to produce improved
Filters
Protect machined components from dirt and
starting and operating characteristics. Several sys-
water in fuel.
tems have been developed, mainly for smaller and
Governor Controls engine speed. Varies position of the
intermediate-sized engines. Similarities to me-
injector plunger to vary amount of fuel in-
chanical injection systems include the following: a
jected.
fuel pump (or pumps), a governor or speed regula-
Table 3-2. Common rail injector system.
tor, filters, and fuel injectors. The major difference
between mechanical and electronic systems is the
Component Purpose
computer which replaces the mechanical compo-
Low and
Low-pressure pump delivers fuel from tank to
nents (cams and pushrods) used to control fuel in-
high-pressure pump
high-pressure pump; high-pressure pump deliv-
jection. The computer processes data inputs (such
ers fuel to injectors at the desired operating
as engine speed and load, desired speed or governor
pressure: fuel lubricates governor and pumps.
setting, engine temperature, and generator load).
Governor
Flyweight-type; controls maximum fuel pres-
Computer output is precisely timed electrical sig-
sure; prevents engine overfueling; controls en-
nals (or pulses) that open or close the fuel injectors
gine idle and prevents overspeeding by control-
ling fuel supply: contained within main pump
for optimum engine performance. Adjustment of in-
housing.
jection timing is seldom required after the initial
setup. Refer to the engine manufacturer’s literature
Throttle
Controlled by the operator; regulates fuel flow
and pressure to injectors.
for maintenance of injectors, pumps, and other fuel
system components.
Injector
Meters, times and pressurizes fuel; camshaft-
operated by pushrod and rocker arm: one injec-
g. The main components of the fuel system. Fuel tor for each cylinder.
supply source, transfer pump, day tank, fuel injec-
tion pump, fuel injection nozzles, and filters and
Filters
Protect machined components from dirt and
water in fuel.
strainers. These components are matched by the
engine manufacturer for optimum performance and
Table 33. In-line pumps and injection nozzle system.
warranty protection.
(1) The fuel supply source is one or more stor-
age tanks. Each tank must have drain valves for
Injection pump Meters, times, pressurizes and controls fuel
removal of bottom water, see paragraph 2-4 for
delivered to the injection nozzles; consists of
genera! requirements. Additionally, the fuel system
single pumping element for each cylinder; tit-
should include a day tank and a transfer pump, see
ted into a common housing; operated by rocker
paragraph 2-4d.
arm or directly from the camshaft.
(2) The following paragraphs cover the fuel in-
Governor Usually the flyweight-type: may be mounted on
jection pump, fuel injection nozzles, and filters and
main injection pump housing; controls fuel de-
strainers.
livery: variable-speed or limiting-speed type is
used.
(3) A fuel injection pump accomplishes the
functions described in paragraph 3-4 b(3). Addi-
Fuel lines High-pressure type; transports fuel from pump
tional details are provided in the following para-
to injection nozzles.
graphs.
Injection nozzle Spring-loaded; hydraulically operated valve that (a) The fuel injection pump must perform
is inserted in the combustion chamber: one
two functions: first, deliver a charge of fuel to the
nozzle for each cylinder.
engine cylinder at the proper time in the engine
Filters Protect machined components from dirt and
operating cycle, usually when the piston has almost
water in fuel.
reached the end of the compression stroke; and sec-
ond, measure the oil charge delivered to the injector
supplied to the cylinders by individual pumps oper-
so the amount of fuel is sufficient to develop the
ated from cams located on a camshaft or on an
power needed to overcome the resistance at the
auxiliary drive. The pumps operate independently
crankshaft.
3-8
TM 5-685/NAVFAC MO-912
(b) The fuel injection pump consists of a bar-
quency of cleaning and replacing filter elements.
rel and a reciprocating plunger. The reciprocating
Adjust the frequency to meet unusual local operat-
plunger takes a charge of fuel into the barrel and
ing conditions. Generally, all metal-edge and wire-
delivers it to the fuel-injecting device at the engine
mesh devices are called strainers, and all replace-
cylinder.
able absorbent cartridge devices are called filters.
(4) Fuel injection nozzles for mechanical injec-
Fuel filters approved for military use consist of re-
tion systems are usually of the spring-loaded,
placeable elements mounted in a suitable housing.
needle-valve type. These nozzles can be adjusted to
Simplex and duplex type fuel filters are available.
open at the predetermined pressure. Consult the
Fuel strainers and filters must not contain pressure
manufacturer’s specifications before adjusting fuel
relief or bypass valves. Such valves provide a means
injection valves. The nozzle components are as-
for the fuel to bypass the strainer or filter, thereby
sembled carefully at the factory and must never be
permitting the fuel-injection equipment to be dam-
intermixed. Most manufacturers use an individual
aged by contaminated fuel. Filter capacity is gener-
pump for each cylinder (pump injection system) and
ally described in terms of pressure drop between the
provide each cylinder with a spring-loaded spray
input and output sides of the filter. However, fuel oil
valve. The spring keeps the needle from lifting until
filters must be large enough to take the full flow of
the pump has delivered oil at a pressure greater
the fuel oil pumps with a pressure drop across the
than the spring loading. As soon as the pressure
filter not to exceed the engine manufacturer’s speci-
lifts the needle, oil starts to spray into the engine
fications. Fuel filter elements should be changed
cylinder through an opening in the valve body.
whenever the pressure drop across the filter nears
(5) Diesel fu 1
e suppliers try to provide clean
or reaches a specified value. Refer to manufactur-
fuel. However, contaminants (water, sand, lint, dirt,
er’s instructions for information on the replacement
etc.) are frequently found even in the best grades. If
of filter elements. Filter capacity at a given pres-
foreign material enters the fuel system, it will clog
sure drop is influenced by the viscosity of the fuel.
the nozzles and cause excessive wear of fuel pumps
The filter should have ample capacity to handle fuel
and injection valves.
demand of the engine at full load. The larger the
(6) Sulphur, frequently found in fuel oil, is very
filter, the less frequently it will have to be cleaned
undesirable. When sulfur is burned (during combus-
and the better the filtering performance will be.
tion), sulfur dioxide and sulfur trioxide form. Both
substances will combine with water condensates to
3-5. Diesel cooling system.
form sulfuric acid. The maximum amount of sulfur
Diesel engines are designed to be either air cooled
acceptable in fuel oil must not exceed one percent.
or liquid cooled. Cooling is used to prevent the cyl-
The engine manufacturer’s recommendation should
inder walls, the head, the exhaust manifold, and the
be used if acceptable sulfur in fuel oil requirements
lube oil from overheating.
are more restrictive. Strainers and filters capable of
a. An air-cooled system depends on an engine
removing fine particles are placed in the fuel line
driven fan to blow ambient air over the fluted or
between supply tank and engine, or between engine
finned surfaces of the cylinder head and through a
transfer pump and injection pump, or sometimes at
radiator type oil cooler, and over the exhaust mani-
both places. The basic rule for placement of strain-
fold. The exterior surfaces must be kept free of