Tech TipsTesting piezoelectricinjectorsWhen you squeeze quartz
an odd thing happens, it will produce acurrent. When you apply
current to quartz, it will rapidly changeshape and stay in that new
shape. Piezoelectric injectors uses thischaracteristic to provide
fast, precise control of injection timing,while eliminating all
other moving parts. Joe Clarke, from DIT, makes it simple to sort
out problems with these injectors.Figure 1 - A scope trace from a
Euro 5 compliant diesel engine Figure 2 - This injector was
triggered twice in about 1 millisecondFundementalsThe
electromagnetic type injector is controlledwhen its solenoid is
energised, enabling acontrolled leak-off resulting in a
pressuredifference within the injector leading to itsopening. The
closing of this type of injectoroccurs when the electric current is
cut, thesolenoid becomes de-energised, resulting inthe closing of
the control chamber valve via aspring. Because of the design, there
is anunavoidable delay in the time taken for thisspring to close
the control chamber valve. ThePiezo-electric controlled injector
can operateup to four times faster than theelectromagnetic type, as
the control valve isdriven closed instead of relying on a
spring.This extremely rapid open/close time hasenabled up to 5
injections per cylinder percycle (2 pilot injections, split main
injection &post injection) according to runningconditions. As a
result, use of this technologyhas increased since the introduction
of everincreasing emission control standards.OperationThe
mechanical principle of operation issimilar to the electromagnetic
solenoidinjector. The controlled leak off, used to assistin the
opening of this type of injector, utilisesthe inverse
Piezo-electric effect. The injectorcontains a control actuator,
consisting ofseveral hundred thin layers of Piezo crystalmaterial,
(normally quartz). Whena voltage isapplied to the quartz it will
either expand orcontract, according to the polarity of thisvoltage
applied. The Electronic Control Unit(ECU) will continually
alternate the polarity ofthe voltage, enabling
precisionopening/closing of the injector as required. Note: When an
electromagnetic type injectoris disconnected from an electrical
signal, itcloses immediately due to the solenoid valvespring
action. If a Piezo-electric type injector isdisconnectedfrom an
electrical signal whenthe engine is running, the injector may
remainin its open state for a period of time, whichcould lead to
over fueling and possible enginedamage.IdentificationThe majority
of piezoelectric injectors may berecognised by the location of the
leak offpipe, located at the side of the injector andnot on the top
as seen on the electromagnetictype.Safety Precautions & System
Protection The operating fuel pressure withinmodern common rail
systems may exceed1,800 bar, or 26,100 psi. Therefore, beforeany
work is carried out on the high pressurecircuit, it is imperative
that the system isdepressurised after the ignition has beenswitched
off. This depressurisation mayinvolve a procedure using the
diagnostic tool,or waiting a specified time for the pressure todrop
naturally.Note: Recently developed injection systems asused on
hybrid vehicles may retain theirpressure for longer periods of
time, to enableimmediate restart of the engine. If a high pressure
pipe is loosened,disconnected or removed, it should not bereused as
its integrity cannot be guaranteed.These pipes should always be
replaced andtightened to the manufacturersrecommended torque. The
working clearances within theinjector are in the region of 3m, many
timessmaller than that of a human hair. For thisreason cleanliness
must be ensured and allopenings must be sealed followingcomponent
disconnection. If a Piezo-electric injector is dropped orreceives
an impact, its internal crystals may bedamaged resulting in faulty
operation.Joe Clarke, D.I.T.AUTOBIZ TECH TIPSDecember 2012
Compendium >> originally published in Autobiz December
2011TestingUnfortunately there is no single check that canbe
applied to diagnose a faulty injector, asidefrom substitution with
a known goodcomponent. This is not recommended, as afaulty wiring
circuit could lead to damage tothe new injector. The following
tests may be carried out onPiezo-electric injectors. The values
obtainedmay vary according to the injector and theinjection system
variant.Test 1 - Resistance Test: The resistanceof the control
actuator may be measured byconnecting an ohm meter across the
twoinjector electrical pins, when the injector isdisconnected from
the wiring harness. Result: The resistance should be in the
regionof 200K.Test 2 - Capacitance Test: This test iscarried out by
connecting the multi-meter inthe same way as during the resistance
check,except the capacitance test is selected on themulti-meter.
Result: The capacitance should generally bebetween 6F and 8F. Test
3 - Injector Leak-off Test: Allinjector leak-off pipes must be
disconnectedand blocked to prevent leakage. A separatepipe is
connected to each injector, with thefree end in a graduated
cylinder. This testnormally consists of holding the engine athigh
idle for a duration of 2 minutes andobserving the quantity of fluid
contained ineach graduated cylinder afterwards.Result: The quantity
of diesel contained ineach graduated cylinder is normally less
than30ml, with nearly identical leakage in all ofthe cylinders. A
faulty injector controlchamber may be identified by excessive
dieselin the corresponding container, while somefaults can lead to
a reduced leak off quantity. Test 4 - Electrical Control Test:
Anoscilloscope is connected across the two pinsof the injector,
with the engine running orbeing turned over using the starter
motor. Note: Do not display two injectors at thesame time using an
oscilloscope with acommon earth between channels, as thiscould
result in an injector being held open andcausing engine damage.To
display the voltage pulse from the engineECU, the following
oscilloscope settingsshould be selected: Amplitude: 50V/Div Time
scale:1ms/Div Trigger threshold: 40V.Result: Figures 1, 2 & 3
scope traces are takenfrom a Euro 5 compliant diesel engineequipped
with the Continental SID807injection system.Pilot injection is used
to reduce engine noiseand emissions at low to medium enginespeeds.
Above 4,500 rpm, the engine tonewill change as pre-injection is no
longercarried out and the Exhaust Gas Recirculation(EGR) valve will
be closed. Some systems require a learning programmeto be carried
out in order for pilot injection tofunction. This involves driving
the vehiclewithin specified parameters to learn injectordrift. When
injector current is displayed usingthe oscilloscope, the value may
range from 5to 10 amps positive and negative as thecurrent flow
changes direction. Test 5 Injector Flow Correction Test:With the
engine at idle, the ECU will observethe acceleration of the
crankshaft during eachpower stroke and determine an average
value.If one cylinder under-performs, compared tothe average, the
ECU will inject slightly morefuel into that cylinder to compensate.
This canbe viewed using the diagnostic tool in the livedata
section. This correction may be displayedas a percentage, the
injected fuel quantity (inmg/stroke) or the value for each cylinder
maybe given in relation to the average, as shown in Figure 4. If an
excessivedifference is observedbetween cylinders, then aninjector
fault could be thecause. Note: An enginemechanical faultresulting
in reducedcompression could alsolead to excessive fuelbeing
injected tocompensate. This canbe identified byswapping the
injectorsbetween cylinders. Ifthe imbalance stayswith the cylinder,
theinjector is not the fault.If the imbalance moveswith the
injector, thenthe injector is faulty.ConclusionIn order to
efficientlydiagnose a Piezo-electric injector fault, adetailed
knowledge ofthe injection systemsstrategies andcharacteristics
isrequired. A systematicapproach must betaken, involving theskilled
use of variousspecialised tools andequipment, all whileensuring
personaland systemsafety. Figure 3 - This injector was triggered
only once during this event Figure 4 - data from the ECU can be
used to find a faulty injectorA Delphi piezoelectric
injectorCylinder 1 injection time 09.66 msCylinder 2 injection time
09.69 msCylinder 3 injection time 10.08 msCylinder 4 injection time
09.38 msInjector opening correction startegies ActiveCylinder 1
injector flow correction factor 0.860Cylinder 2 injector flow
correction factor 1.098Cylinder 3 injector flow correction factor
1.072Cylinder 4 injector flow correction factor 0.970AUTOBIZ TECH
TIPSDecember 2012 Compendium >> originally published in
Autobiz December 2011