diesel engine condition monitoring part 1

8/7/2019 Diesel engine condition monitoring part 1

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D IESEL ENG INE D IAGNOSTICSAT WORK

THE COMPLEXITY AND VARIETY OF DIESEL ENGINES PRE-CLUDES THE EASY AND CONSEQUENT INTERPRETATION OFDIESEL ENGINE CONDITIQN MONITORING MEASURE-MENTS.THIS ARTICLE DESCRIBES THE DIAGNOSTIC VALUE OFSEVERAL DIESEL ENGINE PROCESS PARAMETERS. EMPHASISHAS BEEN PLACED ON THE CORRELATION OF DYNAMIC SIG-NALS ('FINGERPRINTS') WITH FAILURE MODES.THE RESEARCH TOOK PLACE IN A CONCEPT .OF OFF-LINE,PERIODICAL DATA-COLLECTION.

1. INTRODUCTION1.1 History.The development of - a conditionmonitoring systemfor dieselengineshasbeen subject to many studies. From 1970on, several systems have been installedon ships, e.g. on the mlv 'Trident Am-sterdam' (PREDIKT 1, NSFI, lit. [2]),the mlv 'Nedlloyd Barcelona' (SEDS,CMO-Sulzer, lit. [3]) and the mlv'Hoegh Multina' (Det Norske Veritas,lit. [4]). Most of these systems werebased on an on-line system philosophywith a central mini-computer, and mostengines involved were 2-stroke low-speeddieselengines.The experiences gained with these sys-tems were not encouraging. Regularcomputer-breakdowns had a negativeand expensive effect on the system-av-ailability. Moreover, sensor-reliabilityproved to bea problem. A choicehad tobemadebetween either a rigid or a sensi-tivesensor, where a combination of bothproperties was required... No adequatesensors were available to determine thecondition of main bearings, large endbearings, exhaust valves and parts of theinjectionsystem.Expertise on the diagnostics of 4-stroke,medium-speed diesel engines is rare,probably causedby the complex natureof its design, and the inherent complex* Summary of anengineering study un-dersupervisionof prof. dr. ir. E. van denPol, Delft University of Technology.The researchtook place inDen Helder atthe floating Diesel Test Platform 'VanSpeyk' of the Royal Netherlands NavalInstitute, with guidancefrom ir. C. A.J.Tromp.

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gas-exchange processes and high cylin-dernumber.1.2 The study.

*PART I

by ir . K. VisserLieutenant-Commander Royal Neth.Navy, MEa NL Submarine Service.

FAn orientation is carried out in modem vert,respromising condition monitoring tech-niques for maritime diesel engines. Re- F g a sfering to historical experience, the re-searchtook place in the concept ofperiod-ical, off-line data collection, within thenext margins:- data-collection to be carried out by aspecialized shore-team, analysis beingexecuted ashore, followed by a mainte-nance advice.- a minimum amount of sensors to beused, sensors to be mounted externallyand after measurements to be discon-nected.- measurements to be carried out in 1or2 days.- asmany defects asjustified and possi-ble are to be covered by the monitoringplan.The study is carried in the context of anengineering study at Delft University ofTechnology. The study took place inDen Helder, at the floating Diesel TestPlaeform=Van Speyk' of the RoyalNetherlands Naval Institute.1.3 Failure analysis.Asurvey ofLloyd's, London, (publishedin lit. [5]) of reported failures of mainpropulsion diesels in the period 1970-1980 showed, that (turbochargers ex-cepted) most failures include main andlarge-end bearings, pistons, cilinder-lin-ers, valvesandvalve-gear. Other surveysof the RNL Navy (lit. [6]),Techno Fysi-ca(lit. [7])and the NSFI (lit. [8])add fail-ures of the injectionsystem.

FtangFC ON NE C TIN G R OD

Fig. 1.1 Dynamics of the running gear.Process-parameters are needed to estab-lish information on defects occurring intheengine. Achoiceofparameters canbederived from the dynamics of the run-ning gear (fig. 1.1). As a result, the gas-forcesand the radialand tangential com-ponent of the axial forcesin the connect-ing rod, all as' a function of time (e.g.crank-angle), may give information con-cerning thenature of the engine-process.An examination of fluid-flows throughtheenginemay complete therequired in-formation.A survey of defects, failure-modes andprocess-parameters, and the possibleconnection between them, isgiven infig.1.2, the defects-matrix. The researchondiagnostic methods has been based onthis matrix. Some important results ofthis researchwill bepresentedin thenextchapters.


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Fig. 1.2 De fe ct ma t ri x1 .4 Mea su r ement s a nd r es ea rch -e ng in e .In order to analyze the influence of en-gine failureson theprocess-parameters, anumber of controlled defects are ex-amined. The failuremodes are:- combustion problems: no injection,injection pump wear, no injectionpressure, decreasedinjection pressure.- sealing problems in the combustionchamber- timing problems: changed valve clear-ance, changed injection timing- changes inmain bearing geometry- changes in power output of respectivecylinders- changes in mechanical properties:crankshaft-system, frame.The failure tests have beenperformed ona medium speed 4 stroke dieselengine,type Werkspoor RUHB 21Sx6, 6 cylin-ders, output460kW, V-angle 4740', seefigure 1.3.The choiceof amedium speed4 stroke engine was influenced by thefact, that relatively little diagnostic ex-pertise is availableon this type of engine,and by the fact that ailemajor diesel-en-gines installed inRNLN schipsare of the4-stroke type.2. CYL IN DER PRESSUREANALYSISThe measurement of the cylinder gaspressure as a function of the crank-angle

has always been an important tool indiesel engine diagnostics. Some impor-tant calculated parameters may be de-rived from this measurement, e.g. theheat-release, the gastemperature and thepressure-change, all as a function of thecrank-angle domain is the period be-tween 'inlet valve closes' and 'exhaustvalve opens', so the scavenging period isnot included. The measurements havebeen executed with a piezo-electricpressure transducer.2 .1 S ea li ng p ro b lem s.When leakages occur in the cylinder, apressure drop of the maximum compres-sion pressure may beexpected, seefigure2.2. However, ambient conditions causethe value of the maximum pressure tohave a range, which makes it hard to de-fme a situation in which a leak occurs. Amore reliablemethod isfound inmeasur-ing the valueofthe shift ofthe angle of themaximum pressure.As stated by Hohenberg (lit. [9]), theangle ofmaximum compression pressureinaclosedcylinder shiftsavalue zx< P withrespect to the TDC, when thermody-namic losses (eitherheat flow or gas flowor a combination ofboth) occur. Inprac-tice, the angle ofmaximum compressionpressure in a dieselengine will thereforeoccur earlier than the TDC. When a gasleakageoccurs over the boundaries of the

Fig. 1.3 Cros s- se ct io nal v i ew o f t h e r es ea rchengine.cylinder volume, the angle shift will beevenmore.In lit. [1], an equation is derived whichquantitative describes the area overwhich the leakage takes place:

A1eak = .1t.n.K1 ! : : : , . < P l30.'IjJ'Y2.B.To

Aleak leak area (rrr')n number of revolutions

perminute! : : : , . < P l extra angleshift with

respect to the situation of ahealthy cylinder (rad)

'IjJ flow constant (c [1]) characteristic gas constant

(J/kgK)To max. temp. in cyl. (K)Kl engine constant

1/2V0(1+ A . )Vo stroke volume (rrr')A . ratio crank length/

connecting rod length.This equation has the advantage, thatmost variables are relatively easy to de-fine, To has to be calculated from thepressure measurement, but a tempera-turefault ofSOKcausesa differenceofnotmore than 1% in the value ofA1eak.The equation is validated by imposing aleak to the combustion chamber throughan opened decompression channeL Thecalculated value of Aleakdiffered notmore than 5% form the smallest area ofthe decompression channel. The angleshift proved to be lessinfluenced by theambient conditons compared with themaximum.compression pressure.

2 .2 Combu st io n p ro b lem s.The trend of the appa re nt h e at r el ea se dur-ing the closed cycle of the dieselprocessappears to reveal information concerningcombustion problems. Theheat releaseis


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1 c y li nd er p re ss u re ( bar )2 tem p (K)from PV= MRT3 heat release D QID A (K]/(KM OL *RAD ))4 p ress ur e c ha ng e DP IDA (B AR SlR AD )

crankangle (degrees)Fig. 2.1 Pressure-measuremen t in the combus tion chamber.a function of the crankangle, and itscaluclation concerns an iteration processbased on the measured pressure diagramand the fuel consumption.In fig. 2.3 a model of a heat-release dia-gram is shown. In the first period, the re-lease has a negative value caused by thefact, that energy is needed for the vapori-zation of the injected fuel droplets. In thepremixed phase, ignition occurs of theair/fuel mixture which has been formed.

1 = w it ho ut l ea k2 = l eak in troduced

Cran kang le i n d eg re esFig. 2.2 T he in flu en ce o f a g a s l ea ka ge i n t he c yli nd er .

In the last period, a more diffusive com-bustion form takes place when the fuelinjectd after the ignition is being burned.An experimental heat release diagram hasbeen shown in fig. 1.2.The high peak in the heat-release, the re-sult of the rapid burning of the premixedfuel/air, gives diagnostic informationconcerning the quality of the combus-tion. When the combustion conditionsdecrease due to a defect, ignition will

start later, allowing more fuel and air tobe mixed. In that case the heat-releasepeak will be subsequently higher after ig-nition takes place.This is shown in fig. 2.4. An injection de-fect (lower injection pressure) results in ahigher heat-release peak. The informa-tion is even more clear during partialload: the lower gastemperature enhancesthe impact of the defect.A high quality gasoil (NATO F-76) has

I" H--------~L->: I,/ I,// I./ I/ I/ I/ I

/ I/ IIIIIIII

~..J~ I!)Vl z ~ ' "..J Z~a: a:< ~~:I: ~Vl~ > ~i= ~:3 a:~ " " ",< , " r-,


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Fig. 2,5 T ransduc er l o ca t ion s i n t h e c y li nde r head .been used during' these experiments.Fuels with a lower quality (and hence alonger, ignition delay) are expected togive an even more profiled performanceconcerning this diagnostic method.

1 c yl in d er p re ss ur e ( ba r)2 tem p. (K )from P V=M RT3 h ea t r el ea seDQ?DA (K] /( KMOL*RAD ))4 p re ss ur e c ha ng eD P ID A (BAR SlR AD )

crank.nele 1n decrc rankangle in degrees

Fig. 2.6 Pressure measurement in t he decompression channel .able location for the mounting of thepressure transducer is the connectionwith the decompression channel. Thischannel can cause a serious decrease ofthe quality of the pressure-measurement.In fig. 2.5 two transducer locations areshown in the cylinder head of the RUHB215 engine. Location A, in which orig-nally the startvalve was placed, has been

2.3 The influence of the decom presssionchannelOn most diesel engines, the only avail-

I : : l Hea lt h y c y li nde r - -, .- -- -o L ow er in jectio n p ressu re -- ----o Valv e clearance offset - . - .-X Gas l eakage c y li nde r v o lume* no inject ion----- - --

-- ~iT' > !' UI',; < a:::JC f II>, '. II>UI, If50 '. 0 '5 0 . . . . . . 300 . .0 4 .0E FF EC TI EF V EA M OG EN ( KN )

Fig. 2.7 The in fl uence o f de fe ct s on t he max imumgaspres su r e.

used for the preceding measurements.Location B is the end of the decompres-sion channel. The result of a measure-ment on this location is shown in figure2.6. Itis clear, that the pressure trend andthe pressure values differ with e.~. thetrend of fig. 2.1. The heat release in fig.2.6 shows negative values after the igni-tion, which is physically impossible.

injection 5' b e fo r e TDC------ in jection 5' a ft er TDC

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I \I \J \_ . . ./ '---o 1 8 0 360 S40

--cEGREES CRANKANGLE

Fig. 2.8 The inf lu enc e o f c ombus ti on in je ct ion t im ing ont he max imum gaspres su r e.

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A Pumpspillclol 8deg approll.k./em~ STDCFuel ....lv.opef1s 4degapP fO .

BTOCInjection SpiUopens , 2 d89 aopro.pressure beioreTOC0 Fuel ",''''. closes 16deg approll.e ATDCReflected pressurew . " . due lo f ue lvalve closing, Plrtialequilibrium 20degaporox.

G Iniection periodP, Residue' fuelpressure~Pf~ RaleotflJel

p ressur e r is abetor.fuel. . , . I . . . . op.ns 2.5aooro ll .

'Po FueJvllva openingpressure J50kglcm''Po Ang 'e I twhi ch f ue l....tvaopefflrelatlvlI toTOC 4degapprOIl.

BTDCFPyuMaximum fuel

pumpdischarge ...,kglem'""nlur.

Fig . 3.1 Fu el i nj ec t io n p re ss ur e mod e l.

I N JEC T ION L INE F IG . 32 I N JE C T IO N T R A N SD U C E R

Fig . 3.2 In jec ti on t r an sdu rerIt is apparent, that great cautiousness isneeded while using decompression chan-nel mounted pressure transducers fordiagnostic pressure measurements. A lo-cation placed flush with the internal com-bustion wall is recommended.

2 .4 Peak -p re ss ur e mea su r emen ts .Peak-pressure measurements devices arerelatively easy to use. However, asshowed in fig. 2.7, the peak pressure inseveral simulated defect situations didnot differ significantly from the healthysituation. If a pressure change occurs, themost likely reason may be a change in in-jection moment, see fig. 2.8.

Finally, cautiousness is needed in the useof mechanical pressure-meters. Themechanical pressure-reading at the end ofa decompression channel may well beequal to the pressure reading of a very

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sensinve flush mounted piezo-electrictransducer. In that case, the mechanicalmeasurement is influenced by two oppo-site phenomena: a pressure-increase be-cause of pressure waves in the channel,and a pressure-decrease because of theadded volume of the mechanical sensorto the combustion space.

2 .5 Con cl us io n s c o nc er n in g c y li nd e r p r es su reanalysis.Cylinder pressure analysis can be a valu-able tool in dieseldiagnostics. Compres-sion pressure angle shift measurementsgive information concerning gas leakageareas. Heat release analysis, preferablyduring part load conditons, can indicatecombustion problems. The quality of thepressure measurement is a very impor-tant condition for a good diagnostic con-clusion. Pressure measurements are in-fluenced by decompression channels.

3. FLUID FLOW DYNAMICPRESSUREANALYSIS3 .1 In je c ti on p r es su re mea su rement s.Injection system defects are responsiblefor combustion problems. An analysis ofthe injection pressure as a function oftime (e.g. crankangle) may give informa-tion concerning injection defects. In fig.3.1 a fingerprint of the injection signal isgiven. Injector and fuel pump defectscause changes in the form of the finger-print.

3.1 .1 Th e senso rThe sensor choice is very important. Apiezoelectric transducer is expensive andhas to be mounted in the injection pipe.There the fuel flow can be influenced,which decreases the quality of the mea-surement. A strain gauge element ismounted externally, but the strain levelon the outer surface of the injection pipeis relatively low, and a grease-free con-nection between element and pipe re-quires a conditoned pipe surface.These sensor problems resulted in an ex-perimental sensor design first publishedby Heggie in 1977. The design was basedon the p la st ic co ld }l ow principle (free flowof solid resins at low strain levels). Incooperation with the Internal Combus-tion Engines Section of Delft Universityof Technology a pressure transducer hasbeen produced in order to examine thecapabilities of this concept. This sensor isshown in fig. 3.2.A relatively rigid steel enclosure (1) isclamped around the injection line (2).The space between enclosure and injec-tion line (4) is filled with epoxy resin. Inthe steel enclosure, an air cavity (5) ismanufactured over which a resistancestrain element (3) is attached. When apressurre wave inside the line passes in-side the fuel line, the pipe wall will bestrained, resulting in a decrease of theavailable volume within the enclosure.This causes the resin to flow into the cav-ity,' followed by an increase of the ele-ment strain. The small dimensions of thecavity result in an element strain which isgreater than the strain at the outer surfaceof the injection line: a strain amplificationhas been created.

The advantages of this design are:- good protection of the resistance strainelement- the resistance element can be mountedin controlled conditions outside the en-gineroom- strain amplification is created- no influence on fuel flow in injectionline, easy mounting

457


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METIriG 2strainelement < Dptezoelement(2)

IQ2 160289n = 700 rpmpe = 11.8' bari

450-65cr+

Fig. 3.3 Compa ri so n p la st ic c o ld jl ow s en s or andp i e zo -e le c tr ic s en s or

1V = 1bar Enh Time eh.B Real Lin Avg: 50RUSREf 22. .. . . . J ~ - - + - ~..-..- - - ..~ -_ __ - _ _ x[V).07 , - - . . . . . . . . . . _ _ _ _ .

1 3

-.07

Ill. ~L-.14($] .0156

NORMA L b ea rin g c lea ra nce.

Fig. 3.4 Dynam ic l ub r ic a ti on o il p r e ss ur e- low material costs- potentially very rugged

tric signal. The strain signal before andafter injecsion may have to be subject tomore conditioning.The reproducability of the signal provedto be excellent, although an offset of thezero-pressure line was noticed, propablycaused by thermal expansion of the resin.An optimized redesign, with a minimumresin volume, may avoid this.The limited project period did not allowfor a comparison of measured strain le-vels with quantitative model predictions.However, the simple sensor geometrymakes the development of a numencmodel of the sensor worthwile.

3 .1 .2 P ro to ty p e p er fo rma nc e.A prototype of the senor has been placedon the fuel line of the experimental 1Q2diesel engine of the University Laborato-ry, close to a sensor block in which a pie-zo-electric transducer was placed. Acomparison of the results is given in figu-re3.3.The figure shows an excellent dynamicperformance of the strain sensor. Impor-tant moments in the injection signal ap-pear simultaneously with the piezoelec-

3 .2 Lub ri ca t io n o il p r es su re mea su remen t s.The radial component of the axial con-necting rod force isresponsible for the lo-ad of the main- and big end bearings (fig.1.1). An analysis of the lubrication oilpressure as a function of the crank anglemight consequently give diagnostic in-formation concerning the involved gas-en inertia processes. But, with the radialcomponent being considered as an excita-t ion , and the oil pressure as a responsion,the mechanical properties of the systemare involved too, in this case the geometryo f t he b ea rin gs a nd t he lu br ic at io n o il p ip e-lines.Schiffbaenker and Thien were the firstwho published information concerningthe condition monitoring of bearing geo-metry using dynamic lubrication oilpressure signals [lit. 13]. The methodwas based on the measurement of the dy-namic oil pressure of an e xt e rna ll y d r iv e ndiesel engine. The pressure sensor wasplaced outside the diesel engine, and themethod was meant to be a factory test ofnew engines.In the course of this research project, theeffect of bearing geometry defects on thedynamic lubrication oil pressure has beenexamined when combus t io n t a ke s p la c e inthe cylinders. The results were remar-kable.

3 .2 .1 M e as ur em e nt r es ult s.A pressure transducer was placed outsidethe diesel engine, in the pressurized partof the lubrication system, between the oilfilter and the internal distribution line.The pressure signals are presented as afunction of crank-angle.In figure 3.4, the 'fingerprint' of a 'healty'system is shown. 2 cycles of 720 crank-angle-degrees are presented, includingthe firing order of the engine. Six irregu-lar pressurepeaks including high-fre-quency components appear, which canprobably be related to the engine cilin-dercount of six.The influence of the radial forces wasexamined by blocking the fuel pump ofcylinder Ll , thus eliminating the com-bustion pressures in the relevant cylin-der. No changes in the lubrication pres-sure signal were observed.However, the characteristics of the lubri-cation signal changed significantly whenbearing geometry changes were introdu-ced. Fig. 3.5 shows the signal with a de-creased bearing clearance, a wear patternprior to the jamming of the crankshaftsystem ifthis is caused by bearing materi-al being stuck in the clearance. The figureshows, that the high-frequency compo-nents in the pressure signal have disappe-

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diesel engine condition monitoring part 1

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