DIESEL ENGINES & FUEL SYSTEMS E-TEXT
D ESEL ENGINE
F U N D A M E N TA L S
4
MECHANICAL FUEL INJECTION SYSTEMS
DIESEL ENGINES & FUEL SYSTEMS E-TEXT
D ESEL ENGINE
F U N D A M E N TA L S
MODULE CONTENTS
SECTION CONTENTS
MECHANICAL FUEL INJECTION SYSTEMS
4
MECHANICAL FUEL INJECTION
SYSTEMS
4.1 FUEL SUPPLY SYSTEMS
Overview
Fuel supply systems
In-line pump
Rotary pump
Common rail
Unit injection
Fuel supply pumps
Plunger pump
Diaphragm pump
Gear pump
Roller cell
Fuel filters
Filtering material
Types of filters
Fuel pipes
Water separators
Filter service
Injector filters
Revision questions
© 2011 DEFS
4.1 FUEL SUPPLY SYSTEMS
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DIESEL ENGINES & FUEL SYSTEMS E-TEXT
D ESEL ENGINE
F U N D A M E N TA L S
MODULE CONTENTS
SECTION CONTENTS
MECHANICAL FUEL INJECTION SYSTEMS
Fuel return line
Injector
Fuel tank
Fuel filter
Distributor
pump
Glow
plug
Fig 2. Schematic diagram of a distributor pump fuel injection system
Courtesy of Bosch
Common Rail Fuel System
A general fuel system layout of a common rail fuel
system is shown in Fig 3. In a common rail system
an electric or gear pump is used to supply fuel to
the high pressure pump. Electric supply pumps are
mainly used on cars and the gear pump is used on
all other commercial applications.
The electric fuel supply pump is located inside
the fuel tank and operates as soon as the ignition
switch is turned on. This ensures fast engine starting
as the low pressure fuel circuit is charged prior to
engine being started.
The gear supply pump on the other hand is flanged
to the back of the high pressure common rail pump
and is driven by it’s input shaft.
Both fuel supply pumps draw fuel via a pre filter in
the fuel tank then pump the fuel through the fuel
filter before delivering it to the high pressure pump.
Fuel bypassed from the fuel rail helps to supplement
the fuel flow in the low pressure circuit and leak off
fuel from within the high pressure pump returns to
the fuel tank.
Fuel filter
Fuel rail
(common rail)
Common rail
fuel pump
Return
line
High pressure
fuel line
Fuel supply
pump
Injector
Fuel tank
Fig 3. Schematic diagram of a common rail fuel injection system.
Courtesy of MAN Nutzfahrzeuge
© 2011 DEFS
4.1 FUEL SUPPLY SYSTEMS
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DIESEL ENGINES & FUEL SYSTEMS E-TEXT
D ESEL ENGINE
F U N D A M E N TA L S
MODULE CONTENTS
SECTION CONTENTS
MECHANICAL FUEL INJECTION SYSTEMS
4
MECHANICAL FUEL
INJECTION SYSTEMS
4.2 INJECTORS
Overview
Injector operation
Injector nozzles
Multi hole nozzles
Pintle nozzles
Mechanical unit injector operation
Electronic unit injector operation
Common rail injector
Injector service (conventional)
Injector service (common rail)
Injector testing
Isolating a faulty injector
Revision questions
Industry updates
Related web sites
Additional diagrams/photos
© 2011 DEFS
4.2 INJECTORS 15
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MECHANICAL FUEL INJECTION SYSTEMS
The injector section consists of a nozzle body and
needle valve assembly. The operation of the nozzle
assembly is identical to that of a conventional
injector in that fuel pressure acts on the needle
valve shoulder lifting the needle valve off its seat
allowing fuel to pass through the multi spray holes
and atomise into the combustion chamber.
Injector operation is described under the following
headings:
• No injection
• Start of injection
• End of injection
Solenoid
valve spring
Ball valve
& seat
Fuel - (from
common rail)
Command
piston
Solenoid
coil
Valve
body
Orifice
plate
Control
chamber
Start of injection
When the solenoid coil is energised by the
electronic control module, the electromagnetic
force draws the valve body upward allowing the
ball seat to open. Fuel in the control chamber now
passes through the orifice plate and flows to the
fuel tank. With the pressure of the fuel in the control
chamber now reduced, the fuel pressure acting
on the underside of the nozzle needle shoulder is
great enough to lift the nozzle needle off it seat.
Fuel under pressure now flows through the nozzle
spray holes and is atomised as it enters into the
combustion chamber as shown in Fig 13.
Note: As fuel flow into and out of the control
chamber is controlled by inlet and outlet restricting
orifii, rail pressure is not noticeably reduced during
injection by the small quantity of fuel returning to
the fuel tank.
Spring
Ball valve
(open)
Fuel - (from
common rail)
Nozzle spring
Nozzle
needle
Command
piston
Solenoid
coil
Valve
body
Reduced
pressure in
control
chamber
Fuel - (to fuel
tank)
Nozzle spring
Nozzle
needle
Fig 12. Schematic diagram of a common rail injector
showing ‘no injection’
Courtesy of Mitsubishi Heavy Industries Ltd
No injection
With the solenoid coil not energised, the solenoid
valve spring presses the valve body and valve ball
onto the ball seat of the orifice plate closing off
fuel flow through the orifice plate. Inside the control
chamber the pressure rises to that of the fuel rail.
The same pressure is also applied to the nozzle
needle shoulder. The fuel pressure now acting on
the nozzle needle shoulder cannot overcome the
combined forces of the fuel pressure acting on
top of the command piston and the nozzle spring.
Therefore, the nozzle needle stays seated on the
nozzle seat as shown in Fig 12.
© 2011 DEFS
Fig 13. Schematic diagram of a common rail injector
showing ‘start of injection’
Courtesy of Mitsubishi Heavy Industries Ltd
The engine control unit determines the length of
the injection period by energising the injector’s
electrical solenoid coil for the pre-injection, main
and possibly post-injection periods. In order to
achieve multiple injections during the power stroke,
some injectors are using a second solenoid coil to
assist in the rapid opening and closing of the nozzle
needle.
4.2 INJECTORS 21
DIESEL ENGINES & FUEL SYSTEMS E-TEXT
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MECHANICAL FUEL INJECTION SYSTEMS
4
MECHANICAL FUEL
INJECTION SYSTEMS
4.3 FLANGE & IN-LINE INJECTION
PUMPS
Overview
Function of injection pump
Flange mounted injection pumps
Construction
Pumping principle
Plunger and helix
Delivery valve function
In-line injection pump
Construction
Camshaft design
Pump lubrication
Automatic advance unit
Pump servicing
Pump to engine timing
Mark method
Spill time method
Advanced and retarted timing
Revision questions
Industry updates
Related web sites
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4.3 FLANGE & IN-LINE INJECTION PUMPS 31
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MECHANICAL FUEL INJECTION SYSTEMS
Inlet
port
Spill
port
Plunger helix
Charging cycle
Port closing
(start of delivery)
Delivery
Spill port opening
(end of delivery)
Fig 5. Charging and delivery cycle (maximum fuel position)
Courtesy of Bosch
Delivery of fuel ceases when the plunger helix
passes the barrel spill port (or control port), and
the delivery valve returns to its seat. During the
remainder of the stroke, the fuel displaced by
the plunger simply returns to the gallery via the
vertical slot, cut away area and spill port. Thus fuel
ceases to be injected when the helix uncovers the
spill port.
Metering the fuel charge
Since the plunger is cam driven, its stroke is
constant and cannot be varied to control the
quantity of fuel injected per stroke. However,
the effective part of the pumping stroke can be
varied to control the quantity of fuel injected per
stroke simply by rotating the plunger in the barrel.
Fuel delivery begins at the instant the top of the
plunger covers the barrel ports and continues until
the helix edge uncovers the spill port, at which
point fuel trapped above the plunger is allowed
to return to the fuel gallery.
Thus the effective pumping stroke ceases
when the spill port is uncovered, and is directly
controlled by the distance through which the
plunger must travel before the edge of the helix
passes the bottom of the spill port.
Spill
port
Inlet
port
Plunger
helix
No delivery
Partial delivery
Maximum delivery
Fig 6. Control of fuel delivery
Courtesy of Bosch
© 2011 DEFS
4.3 FLANGE & IN-LINE INJECTION PUMPS 35
DIESEL ENGINES & FUEL SYSTEMS E-TEXT
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MECHANICAL FUEL INJECTION SYSTEMS
4
MECHANICAL FUEL
INJECTION SYSTEMS
4.4 DISTRIBUTOR TYPE
INJECTION PUMP
Overview
VE Pump
Construction and operation
Fuel supply pump
Pumping and fuel distribution
Fuel metering
Delivery valve
Mechanical governor
Variable speed
Solenoid shut off valve
Automatic advance unit
Air-fuel ratio control
Pump timing
Revision questions
Industry updates
Related web sites
Additional diagrams/photos
© 2011 DEFS
4.4 DISTRIBUTOR TYPE INJECTION PUMP 44
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MECHANICAL FUEL INJECTION SYSTEMS
Overflow restriction
orifice
Maximum fuel
adjust screw
Governor
assembly
Solenoid shut off
valve
Pressure control
valve
Distributor head with
high pressure pump
Fuel supply
pump
Cam plate
Automatic advance
unit
Fig 2. Cut away section of a VE fuel injection pump
Courtesy of Bosch
Fuel Supply Pump
Low pressure charging of the pump housing is
accomplished, by a vane type fuel supply pump
situated at the drive end of the distributor pump.
Fuel flow is from the fuel tank through the fuel filter
and into the vane pump, from here it enters the
pump housing at pressures that vary between 360
kPa and 810 kPa. Generally there is no fuel feed
pump fitted to this fuel system, as the vane pump
serves this purpose.
Being a constant displacement pump, the fuel
supply pump can deliver several times the amount
of fuel required for injection.
Fuel from tank
Pressure control
valve piston
To pump
housing
Rotor
Vane
Therefore, when the pump housing pressure
reaches a predetermined level, excess fuel delivery
is relieved via a pressure control valve and returned
back to the inlet side of the fuel supply pump, as
shown in Fig 3.
The pressure control valve is located beside the
fuel supply pump and is of the spring loaded piston
type. This valve is pre-set on manufacture and
requires no further adjustment.
For the purpose of self bleeding and cooling of the
entire pump, fuel circulates through an overflow
restricting orifice back to the fuel tank. The overflow
restricting orifice is 0.6 mm in diameter and is
situated in the banjo bolt in the fuel return line on
top of the pump housing as shown in Fig 2. While
this orifice allows fuel to return to the fuel tank, it
offers sufficient restriction to fuel flow from the fuel
supply pump to cause the injection pump housing
to be pressurised. Further, in conjunction with the
fuel supply pump pressure control valve, the orifice
is responsible for the pump housing fuel pressure
necessary for the charging of the high pressure
chamber and the operation of the automatic
injection advance unit.
Drive shaft
Fig 3. Schematic diagram of the operation of a
vane type fuel supply pump
© 2011 DEFS
4.4 DISTRIBUTOR TYPE INJECTION PUMP 46
DIESEL ENGINES & FUEL SYSTEMS E-TEXT
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MECHANICAL FUEL INJECTION SYSTEMS
4
MECHANICAL FUEL
INJECTION SYSTEMS
4.5 GOVERNORS
Overview
Function of a governor
Classification of governors
Types of governors
Governor terminology
Graphs of governor control and engine fuelling
Mechanical governors
Constant speed
Variable speed
Idle maximum speed
Pneumatic governor
Electronic governor
Revision questions
© 2011 DEFS
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MECHANICAL FUEL INJECTION SYSTEMS
Control
rack
Control
lever
Governor
spring
As the shaft rotates, centrifugal force causes the
flyweights to move outwards from the shaft, the
lever arm thrusting against the sleeve. Thus the
sleeve is balanced between spring force on the
one end and the force exerted by the flyweights on
the other.
The governor mechanism connects to the pump
rack via the pivoted fork, one end of which
engages in a groove in the sleeve with the other
end connecting to the rack via a link.
Should the engine speed drop due to an increase
in engine load, the centrifugal force acting on the
weights will decrease, allowing the spring to push
the sleeve along the shaft. This movement will move
the rack, via the pivoted fork, to increase the fuel
supply to the engine.
Centrifugal
weights
Fig 3. Mechanical governor with centrifugal weights
Courtesy of Bosch
Simple Constant Speed Governor
Constant speed governors are fitted to engines that
are required to run at a set or constant speed, and
are governed to this set speed.
Applications include engines that power alternator
sets, water pumps, conveyors, etc.
The simple constant speed governor as shown
in Figs 4 consists of two pivoted flyweights, fixed
to a pivot plate, which rotates with the pump
camshaft, a sliding control sleeve, a pivoted fork
and a governor spring. Spring force acts against
the sleeve, forcing it against the lever arm of the
flyweights, which are forced in towards the shaft.
On the other hand, should the engine speed
increase due to a lightening of the load, the
subsequent increase in centrifugal force will fling
the flyweights outwards and the lever arms will
force the sleeve along the shaft against the spring.
Movement in this direction will move the rack to
reduce the fuel delivery from the pump.
Thus any change in the engine speed will cause an
immediate change in the quantity of fuel injected,
which will compensate for the speed change.
Simple Variable Speed Governor
In applications where engines may be required to
operate at any selected speed, variable speed
governors are used. These governors govern
the engine at any set engine speed, from idle
to maximum. Governors of this type are used
extensively in engines for earthmoving equipment
and farm tractors.
Fuel control rack
Fixed
throttle
position
Centrifugal
weights
Maximum fuel
rack stop screw
Fulcrum lever
Governor spring
Pumping
element
Sliding sleeve
Fig 4. A simple constant speed governor
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4.5 GOVERNORS 61