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Basic Function of Diesel Engine
The diesel engine (also known as a compression-ignition engine) is an internal
combustion engine that uses the heat of compression to initiate ignition and burn
the fuel that has been injected into the combustion chamber. This contrasts with
spark-ignition engines such as a petrol engine (gasoline engine) or gas
engine (using a gaseous fuel as opposed to gasoline), which use a spark plug to
ignite an air-fuel mixture.
The diesel engine has the highest thermal eciency of any
standard internal or external combustion engine due to its ery high compression
ratio and inherent lean burn which enables heat dissipation by the excess air. !
small eciency loss is also aoided compared to two-stroke non-direct-injection
gasoline engines since unburn fuel is not present at ale oerlap and therefore
no fuel goes directly from the intake"injection to the exhaust. #ow-speed diesel
engines (as used in ships and other applications where oerall engine weight is
relatiely unimportant) can hae a thermal eciency that exceeds $%&.
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INTAKE STROKE
The intake stroke begins at top dead center, and as the piston moes down, the
intake ale opens. The downward moement of the piston creates a acuum in
the cylinder, causing an air to be drawn through the intake port into the
combustion chamber. !s the piston reaches bottom dead center, the
intake ale closes.
COMPRESSION STROKE
The compression stroke begins with the piston at bottom dead center and rising
up to compress the air. 'ince both the intake and exhaust ales are closed,there is no escape for the air, and it is compressed to a fraction of its original
olume. !s the piston reach T, the compression stroke ends.
POWER STROKE
The piston begins the power stroke at T. !t this point, fuel is injected into
combustion chamber and ignited by the heat of the compression. The expanding
forces of burning gases force the piston downward.
EXHAST STROKE
The exhaust stroke begins when the piston nears the end of the power stroke
and the exhaust ale is opened. !s the piston moes upward towards top dead
center, it pushes the burnt gases, resulting from the ignition of the fuel and air
mixture, out of the combustion chamber and through the exhaust port. !s the
piston reaches top dead center, ending the exhaust stroke, the
exhaust ale closes, and the intake ale opens to begin the intake stroke for
the next cycle.
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Diesel Engine Co!"onent
Fun#a!ental O"e$ating P$inci"le of Diesel Engine
*ollowing is a step-by-step iew when you start up a diesel-powered ehicle.
%& 'ou tu$n t(e )e* in t(e ignition&
Then you wait until the engine builds up enough heat in the cylinders for
satisfactory starting. (+ost ehicles hae a little light that says ait, but a
sultry computer oice may do the same job on some ehicles.) Turning the
key begins a process in which fuel is injected into the cylinders under such
high pressure that it heats the air in the cylinders all by itself. The time it
takes to warm things up has been dramatically reduced / probably no more
than 0.$ seconds in moderate weather.
iesel fuel is less olatile than gasoline and is easier to start if the
combustion chamber is preheated, so manufacturers originally installed little
glow plugs that worked o1 the battery to pre-warm the air in the cylinders
when you 2rst started the engine. 3etter fuel management techni4ues and
higher injection pressures now create enough heat to touch o1 the fuel
without glow plugs, but the plugs are still in there for emissions control5 The
extra heat they proide helps burn the fuel more eciently. 'ome ehicles
still hae these chambers, others don6t, but the results are still the same.
+& A ,Sta$t- lig(t goes on&
hen you see it, you step on the accelerator and turn the ignition key to
'tart.
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.& Fuel "u!"s #eli/e$ t(e fuel f$o! t(e fuel tan) to t(e engine&
7n its way, the fuel passes through a couple of fuel 2lters that clean it before
it can get to the fuel injector no88les. 9roper 2lter maintenance is especially
important in diesels because fuel contamination can clog up the tiny holes in
the injector no88les.
A diesel fuel lter.
0& T(e fuel in1ection "u!" "$essu$i2es fuel into a #eli/e$* tu3e&
This deliery tube is called arail
and keeps it there under constant highpressure of :;,$%% pounds per s4uare inch (psi) or een higher while it
deliers the fuel to each cylinder at the proper time. (<asoline fuel injection
pressure may be just 0% to $% psi=) The fuel injectors feed the fuel as a 2ne
spray into the combustion chambers of the cylinders through no88les
controlled by the engine6s engine control unit (>?), which determines the
pressure, when the fuel spray occurs, how long it lasts, and other functions.
4& T(e fuel5 ai$5 an# ,6$e- !eet in t(e c*lin#e$s&
hile the preceding steps get the fuel where it needs to go, another process
runs simultaneously to get the air where it needs to be for the 2nal, 2erypower play.
7n conentional diesels, the air comes in through an air cleaner that6s 4uite
similar to those in gas-powered ehicles. @oweer, modern turbochargers
can ram greater olumes of air into the cylinders and may proide greater
power and fuel economy under optimum conditions. ! turbocharger can
increase the power on a diesel ehicle by $% percent while lowering its fuel
consumption by :% to :$ percent.
7& Co!3ustion s"$ea#s f$o! t(e s!alle$ a!ount of fuel t(at8s "lace#
un#e$ "$essu$e in t(e "$eco!3ustion c(a!3e$ to t(e fuel an# ai$ int(e co!3ustion c(a!3e$ itself&
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Antroduce !dance iesel >ngine ith >lectronic ontrol (ommon Bail
'ystem).
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P$inci"les
'olenoid or pie8oelectric ales make possible 2ne electronic control oer
the fuel injection time and 4uantity, and the higher pressure that the
common rail technology makes aailable proides better fuel atomisation.
To lower engine noise, the engineCs electronic control unit can inject a
small amount of diesel just before the main injection eent (DpilotD
injection), thus reducing its explosieness and ibration, as well as
optimising injection timing and 4uantity for ariations in fuel 4uality, cold
starting and so on. 'ome adanced common rail fuel systems perform as
many as 2e injections per stroke.
ommon rail engines re4uire a ery short (E 0% seconds) to no heating-up
time, depending on ambient temperature, and produce lower engine noise
and emissions than older systems.
iesel engines hae historically used arious forms of fuel injection. Two
common types include the unit injection system and the distributor"inline
pump systems ('ee diesel engine and unit injector for more information).
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hile these older systems proided accurate fuel 4uantity and injection
timing control, they were limited by seeral factors5
• They were cam drien, and injection pressure was proportional to
engine speed. This typically meant that the highest injection
pressure could only be achieed at the highest engine speed and
the maximum achieable injection pressure decreased as engine
speed decreased. This relationship is true with all pumps, een
those used on common rail systems. ith unit or distributor
systems, the injection pressure is tied to the instantaneous pressure
of a single pumping eent with no accumulator, and thus the
relationship is more prominent and troublesome.
• They were limited in the number and timing of injection eents that
could be commanded during a single combustion eent. hile
multiple injection eents are possible with these older systems, it ismuch more dicult and costly to achiee.
• *or the typical distributor"inline system, the start of injection
occurred at a pre-determined pressure (often referred to as5 pop
pressure) and ended at a pre-determined pressure. This
characteristic resulted from DdummyD injectors in the cylinder head
which opened and closed at pressures determined by the spring
preload applied to the plunger in the injector. 7nce the pressure in
the injector reached a pre-determined leel, the plunger would lift
and injection would start.
An common a rail system, a high-pressure pump stores a reseroir of fuel
at high pressure / up to and aboe :,%%% bars (:%% +9aF :G,%%% psi). The
term Dcommon railD refers to the fact that all of the fuel injectors are
supplied by a common fuel rail which is nothing more than a pressure
accumulator where the fuel is stored at high pressure. This accumulator
supplies multiple fuel injectors with high-pressure fuel. This simpli2es the
purpose of the high-pressure pump in that it only needs to maintain a
commanded pressure at a target (either mechanically or electronically
controlled). The fuel injectors are typically >?-controlled. hen the fuel
injectors are electrically actiated, a hydraulic ale (consisting of a
no88le and plunger) is mechanically or hydraulically opened and fuel is
sprayed into the cylinders at the desired pressure. 'ince the fuel pressure
energy is stored remotely and the injectors are electrically actuated, the
injection pressure at the start and end of injection is ery near the
pressure in the accumulator (rail), thus producing a s4uare injection rate.
Af the accumulator, pump and plumbing are si8ed properly, the injection
pressure and rate will be the same for each of the multiple injection
eents.
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Fuel In1ection Pu!"
!n Anjection 9ump is the deice that pumps fuel into the cylinders of a
diesel engine. Traditionally, the injection pump is drien indirectly from thecrankshaft by gears, chains or a toothed belt (often the timing belt) that
also dries the camshaft. At rotates at half crankshaft speed in a
conentional four-stroke engine. Ats timing is such that the fuel is injected
only ery slightly before top dead centre of that cylinderCs compression
stroke. At is also common for the pump belt on gasoline engines to be
drien directly from the camshaft. An some systems injection pressures
can be as high as :%% +9a (;%,%%% 9'A).
%& Inline In1ection Pu!"
An-line injection pumps to meter,
and raise the pressure of the fuel.
The basic principle is for a plunger
to act on a column of fuel, to lift an
injector needle o1 its seat.
Anside the pump is a pumping
element, and a deliery ale for
each cylinder of the engine. The
element has a barrel, and a plunger
that 2ts inside it. Their accurate 2t
and highly-polished 2nish ensures
only minimal fuel leakage past them, without needing positie seals. The
barrel usually has two holes, or ports, called the inlet port, and the spill
port. They connect the inside of the barrel with the gallery. The gallery
contains 2ltered fuel from the low-pressure system. !t the top of the
barrel is a deliery ale, deliery ale holder, and the pipe to carry fuel
to each cylinder.
The upper end of the plunger has a ertical grooe, extending from its top
to an annular grooe. The top edge of this annular grooe is cut in a helix,
also called the control edge. 'ome pumps hae a helix cut on top of the
plunger.
! camshaft, cam follower and spring, moe the plunger in a reciprocating
motion.
hen the plunger is below the ports, fuel from the gallery enters the
barrel aboe the plunger. This ensures the barrel is full of fuel. !s thecamshaft rotates, the plunger is pushed past the ports. The highly
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polished surfaces cause a sealing e1ect, trapping the fuel aboe the
plunger. +oing the plunger further raises the pressure of the fuel. This
forces the fuel out past the deliery ale, along the fuel line to the
injector.
*uel Hows to the injector until the control edge uncoers the spill port. The
pressuri8ed fuel aboe the plunger then moes down the ertical grooe,
to the annular grooe, and into the spill port. The deliery ale stops fuel
leaking from the pipe back into the element. At reduces pressure in the
fuel line to ensure there is no dribbling by the injector.
The deliery ale has a relief plunger, and a conical face which is held
against its matching seat by the deliery ale spring. The relief plunger
on the ale is a close 2t inside the bore of the deliery ale seat.
hen the fuel pressure rises, the deliery ale is lifted o1 its seat. hen
the plunger is clear of its bore, fuel Hows to the injector. hen injection
ceases, the pressure below the deliery ale drops to gallery pressure.
*uel pressure aboe the deliery ale forces the ale towards its seat.
The relief ale enters the seat bore, sealing the olumes aboe and
below the deliery ale. *urther moement of the deliery ale towards
its seat, increases the olume in the injector pipe, and reduces the
pressure in there. This drop in pressure causes the injector needle to snap
shut, helping to preent fuel dribble from the injector. The conical face ofthe deliery ale then contacts the seat, further sealing the plunger from
the injector pipe.
Botating the plunger controls the length of the stroke for which the spill
port is coered. This is called the e1ectie stroke. At inHuences how much
fuel is deliered to the injector. ! short e1ectie stroke means a small
amount of fuel is injected. ! longer e1ectie stroke lets more fuel be
deliered. To stop the engine, the ertical grooe on the plunger is aligned
with the spill port, which stops pressure in the barrel rising.
The plunger is rotated by a control sleee, a rack, and a pinion. +oing
the rack rotates the pinion, the control sleee, and then the plunger. The
rack6s moement is controlled by the goernor.
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+& Dist$i3uto$ T*"e Fuel In1ection Pu!"
The distributor-type pump
uses a ane-typetransfer pump to 2ll the
single pumping element.
This then raises fuel
pressure to injection
pressure.
! distribution system then distributes fuel to each
cylinder, in the 2ring order of the engine.
The most common type in light automotie use is the 3osch I> pump.
! drie shaft drien from the engine, rotates a plunger, and a cam disc.
ams on the face of the disc hae as many lobes as cylinders in the
engine. ! plunger spring holds the cam disc against rollers that rotate on
their shafts.
The lobes moe the plunger to-and-fro in its barrel, making it rotate, and
reciprocate, at the same time. Ats rotation operates the fuel inlet port to
the pumping chamber, and at the same time distributes pressuri8ed fuel
to the correct injector. The reciprocating motion pressuri8es the fuel in thepumping chamber.
The plunger6s pumping action forces fuel through a deliery ale, to the
injector. This pump is for a ;-cylinder engine, so it has ; deliery ales.
The barrel has 0 intake port and ; distribution ports. The plunger has a
central passage, a connecting passage to the distributing slit, and a cross-
drilling to a control sleee. !s the plunger rotates, each intake slit aligns
with the intake port, and the distributing slit with the distributing port.
!s the plunger rotates, the intake slit moes away from the intake port. !t
the same time, the plunger is acted on by the cams, causing it to moe
axially along the barrel, pressuri8ing the fuel in the pumping chamber.
The distributing slit now uncoers the distribution port, and the
pressuri8ed fuel passes through deliery ale to the injector. *urther
rotation of the plunger closes o1 the distribution port, and opens the
intake port. !t the same time, the plunger spring moes the plunger back
along the barrel for the next pumping stroke.
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*or intake, fuel from the feed pump reaches the open intake port in the
barrel. The intake slit aligns with the intake port, and fuel 2lls the pumping
chamber and passages in the plunger.
*or injection, the plunger rotates to close o1 the intake port, and moes
along the barrel, to pressuri8e fuel in the pumping chamber. The
distributing slit aligns with the distribution port and the pressuri8ed fuel
forces the deliery ale o1 its seat, and reaches the injector. An this
phase, a cut-o1 port in the plunger is coered by the control sleee.
To end fuel deliery, the plunger6s cut-o1 port moes out of the control
sleee, and lets pressuri8ed fuel spill back into the pump housing. This
reliees pressure in the pumping chamber, the deliery ale closes, and
injection ceases.
+etering the fuel is controlled by e1ectie stroke of the control sleee,
and that6s determined by the action of the goernor sliding the control
sleee along the plunger. 'liding it one way opens the cut-o1 port earlier,
and reduces e1ectie stroke. 'liding it this way delays its opening, and
increases e1ectie stroke.
The goernor changes the position of the control sleee to ary the
4uantity of fuel deliered, according to throttle position and load.
hen the ignition is switched o1, an electrical solenoid closes o1 the
intake port, and stops fuel deliery.
Co!!on Rail
The ommon Bail iesel Anjection 'ystem deliers a more controlled
4uantity of atomi8ed fuel, which leads to better fuel economyF a reduction
in exhaust emissionsF and a signi2cant decrease in engine noise during
operation.
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An the ommon Bail system, an accumulator, or rail, is used to create a
common reseroir of fuel under a consistent controlled pressure that is
separate from the fuel injection points.
! high-pressure pump increases the fuel pressure in the accumulator up to
0,J%% bar or :;,:%% 9'A. The pressure is set by the engine control unit and
is independent of the engine speed and 4uantity of fuel being injected into
any of the cylinders. The fuel is then transferred through rigid pipes to thefuel injectors, which inject the correct amount of fuel into the combustion
chambers.
The injectors used in ommon Bail
systems are triggered externally by
an >lectronic iesel ontrol, or >
unit, which controls all the engine
injection parameters including thepressure in the fuel rail and the
timing and duration of injection.
iesel fuel injectors used in
ommon Bail injection systems
operate di1erently to conentional
fuel injectors used in the jerk pump
system, where the plungers are
controlled by the camshaft position
and speed. 'ome common rail injectors are controlled by a magneticsolenoid on the injector. @ydraulic force from the pressure in the system is
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used to open and close the injector, but the aailable pressure is
controlled by the solenoid triggered by the >lectronic iesel ontrol unit.
'ome injectors use 9ie8o crystal wafers to actuate the injectors. These
crystals expand rapidly when connected to an electric 2eld. An a 9ie8o
inline injector, the actuator is built into the injector body ery close to the
jet needle and uses no mechanical
parts to switch injector needles.
The electronic diesel control unit precisely meters the amount of fuel
injected, and improes atomi8ation of the fuel by controlling the injector
pulsations. This results in 4uieter, more fuel ecient enginesF cleaner
operationF and more power.
Fuel In1ecto$
The fuel injectors are controlled by either a solenoid or pie8o actuator.
They are energi8e se4uentially by the engine >+. The >+
simultaneously switches a lie oltage supply and an earth for each
injector. +ultiple injection processes per cylinder combustion are possible.
Co!!on Rail Diesel Fuel
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A#/antages of co!!on $ail
• *uel pressure aailable on demand
• @igher injection pressure and 2ner atomi8ation of fuel
• Anjection pressure created independent of engine speed
• +ultiple injection per cylinder combustion are possible
Bene6t of Co!!on Rail
• Beduction of oerall exhaust emission
• Beduction of particulate emission
• Beduction of noise emission
• Amproe fuel eciency
• @igher performance