Testing Piezoelectric Injectors 1

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