C H A P T E R 1
Introduction.
Aims and Objectives of the W ork
1.1 I n tr o d u c tio n
W idespread autom ation, the use o f highly com plex system s, tight schedules o f perso nal p rivate and p ublic life in the society all these have o f late led to the recogn itio n o f the im portance o f achieving high reliability o f goods and services. T h e internal co m bustion engin e/d iesel engin e has captured a very vast area o f application in the m odern industrial w o rld, m ost significantly in the m arine and rail tran sp o rt and both cap tiv e and standby electricity generatio n plants. A s o f d ate the im portance o f diesel engines in the areas o f co-gen eratio n and transportation cannot be overestim ated. T o cite the Indian scene alone, even as early as 1984 the installed generation capacity w ith I .C . engine plants w as 1632 M W . A lo o k at the ‘U .N . publication on electric p o w er in A sia and the P a c ific ’ w ould show that th e use o f I .C . engines fo r cogeneration has been trem en do usly increasin g o v e r the y ears. A pplication to surface tran sp o rt, particu larly to d iesel electric tractio n is ol p a ra m o u n t im p o rtan ce especially in the case o f the developing co u ntries (as also the developed countries) w h ere norm al life o f p eo p le is so m uch d ep endent on rail traffic.
W hen th e day to day life o f m an has b eco m e too m uch dependen t on diesel en g ines, and w h ile th e p ro p o rtio n o f tim e the engines a re in o p erable conditio n is o f great c o n c ern , any am o u nt o f thought and action tow ards im p ro v in g the reliab ility , n ay a v ailab ility , o f th e in tern al com bustion engines cannot b e u n w arranted. O peration tim e b e fo re first failu re should b e con sid ered a v e ry p rim e fac to r o f service fo r any m ech an ical e q u ip m e n t/
rep a ira b le/ m aintainable system . U sers o f larg e m achinery and e q u ip m e n t a re b eco m in g m ore an d m o re conscious ab o u t the im p licatio n s o f m aintenance c o st. A t least in so m e co u n tries, cu sto m ers have started dem an ding g u a ra n tee /a ssu ra n ce on w ho le life co st.
C o ndition based m ainten ance for optim al reliab ility /a v a ila b ility is g a in in g w id er a c ce p tan c e
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including am ong diesel users.
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1.2 O u ta g e s o f D iesel E n g in e s a n d L osses
T hat the w orld countries are facing en ergy crisis only asserts the need of better efficient operatio n o f m achines and equipm ents. L eavin g alon e the oil producing co u n tries, liquid fuel is liquid gold and w aste o f fuel by w ay o f lesser efficient operation o f internal com bustion engines as w ell as external com bustion engines/m ach in es should b e o f grave concern. E conom y o f fu el oil and lube oil consum ption m ust be receiving m ore attention from rese a rc h e rs, m anufacturers and p ractition ers. A noticeable p a rt o f the G N P o f any country now adays is backed by liquid fuel p o w er pack o f w hich a good share is o f diesel engine plants. O bviously ou tag es du e to unexpected failures can show -up even in the G N P , not to sp eak o f m icro - level production losses, rep a ir/ replacem ent costs, and other losses o f o p p o rtu n ity costs.
1.3 N eed f o r S tu d y o f R .A .M . A sp ects
S ophistication and h ence com plexity o f any ty p e o f m achin ery a re trem endou sly increasing because to h av e the fo rm e r the latter is g en erally in ev itable. A n inversion o f the fo u r b a r ch ain , recip ro catin g m achines a re in h eren tly a bit m o re com p lex in co m p ariso n s w ith any o th e r type o f ro tatin g m achinery. E ffo rts.to p ro d u ce m o re energy w ith b e tte r fuel efficiency and w ith b e tte r p o w er-w eig h t ratio h av e constantly added to the com plexity o f engines because o f th e inev itab ility to use m o re com plex sub-units and feed-b ack o r feed -fo rw ard co n tro ls. E x p erien ce o f the p a st w ould only show that failu re p ro n eness is m o re than p ro p o rtio n a te to co m p lex ity . E ven m o re so is the situation w ith th erm o d y n am ic m achinery and o th e r hot-p rocessin g eq u ip m en ts w hich have to o p e ra te in h ig h -tem p eratu re environ m en ts.
As is also the g en eral case, the financial p e n a ltie s asso ciated w ith en g in e dow n tim e and m aintenance have only sp iralled e v e r u p w a rd . T h e need to
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(spares), and inventories continuously dem and operation and m aintenance to keep pace w ith th e advances in technology that could reduce m ain tenan ce activity o r sim plify the operation o f the diesel plants. A ssessm ent o f reliability, av ailability , and m aintainability have to be an ongoing process in o rd er to op erate fo r optim al availab ility or for optim al m aintenance planning.
L ife tim e m aintenance costs are determ ined by the d iffe re n t w ays in w hich a p ro d u ct can fail to p erform p ro p erly , the frequencies w ith w hich these d iffe re n t failures can o ccu r, the nature o f th e repairs req u ired fo r co rrecting each ty pe o f failure, and the ex tent o f the ro u tin e servicing p rescribed by the m anufacturers. R .A .M . studies can only o ffer clues fo r im p ro v ing o r altering designs for better reliab ility . R A M studies w ill also help evolving designs fo r m inim al m aintenance expense.
1 .4 E a r ly D ia g n o sis a n d A d v a n ta g e s
M aintenan ce costs totalled o v er th e useful life tim e o f a p ro d u ct w ould in m any cases exceed the o rig in al purch ase cost. O ver and abov e th e m aintenance costs, there a re the oppo rtun ity costs because o f no ‘p ro d u ctio n ’ d u rin g d o w n-tim e. T h e m aintenance cost is only a m ajor constitu en t o f the dow n-tim e c o st. T h e d o w n tim e can be split into the follow ing fractions:
(i) A d m in istrativ e tim e; tim e fo r m anagerial decisions an d san ctio n s, w aiting tim e fo r the re p a ir facility; tim e fo r m ovem en t to and fro m the re p a ir w orks; d elay due to non-av ailability o f spares, etc p u t to g eth er,
(ii) D iag n o sis tim e,
(iii) A ctu al re p a ir tim e, and
(iv) T estin g and reco m m issionin g tim e.
L ite ratu re on industrial m ainten an ce w ould show th at a b ig c h u n k , a ro u n d
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60% o f the dow n-tim e is diagnosis tim e. T he p a y o ff corresp o n d in g to a given p ercen t reduction will be the biggest if it is in the m ore w eighty constituen t o f the dow n -tim e.
C learly it is the diagnosis tim e one has to h it at fo r cutting dow n d o w n -tim e costs.
A nd this is w here lies the im portance o f the quantitative techniques o f condition m onitoring.
E arly d iagnosis o f im m inent failures - prognosis - w ould cut d o w n-tim e very significantly. It w ould also help av ert further pro p ag atio n o f a fault and hence m ore severe and costly dam ages. C ondition m onitoring can d ire c t posting o f m ajor m aintenance to tim es w hen m ore econom ical handling w ould b e possible. It is vastly rep o rted that the British Industry has cu t dow n-tim e costs d rastically by the app licatio n of conditio n m onitoring. C .M . enables to avoid unnecessary p rev e n tiv e m aintenance schedules, w hich in turn subscribes to b e tte r reliability. Raj B. K. N . R ao, in his introduction note to C O M A D E M ’8 8, m entions ‘unless top m anagem ent adopt a holistic p ractice to m onitor, detect, diagnose, and pro gn ose the u nd esirable sym ptom s in ev ery p art ol th eir system , it w ill be d ifficu lt fo r them to keep up w ith the rapid p ace o f d ev elop m ent o f techn olog y and to reap the m axim um benefits from i t ’.
1.5 P re m is e to th is W o rk a n d O b je c tiv e s
W h ile en g in e failures - d eg rad atio n and catastrophic - a re m ostly the cause o f outages o f diesel p lan ts, th e attention and activity o f the m ain tenan ce seem to b e aro u n d the classical ap p ro ach . In terd iscip lin ary sp ecializatio ns lik e R eliab ility E n g in eerin g and C onditio n M o n ito rin g techniques founded on D ig ita l S ignal P ro cessin g do n ot seem to have p e rc o late d significantly to p ra c tic e level to im p ro v e the m ain ten ance scene at large. C on sisten t an d persisten t co n scien tio u s e ffo rts a re req u ired fo r derivin g extend ed periods o f failu re-free op eratio n an d reliab le se rv ic e o u t o f d iesel en gine application s.
F a ilu re -fre e operatio n an d reliab le serv ice a re n o t on ly co n sid e ratio n s
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need to reem phasize engineering m ore reliability into in tern al com bustion eng in e vX-
designs. A vailability and m aintainability are im ages in differen t p ersp ects. S o m e o f the aspects o f m echanical reliability in the context o f I.C . engines a re highlighted in this w ork.
A clear vision o f the failure m echanism s o f a system is required for a com preh en sive qualitative vision o f its reliability scene. A v ery convenient and easy way to have this is already available, the fault-tree. But how fa r this is consciously used in practice is not m uch know n. A t least in the context o f this w ork it has been observ ed this w as not tru ly taken advantage o f in the diesel p ractice. F au lt-tree analysis w ould be very m uch useful for assessing reliability at the tim e o f desig n and d ev elo pm ent p ro vid ed failu re data fo r all the co m p o nents/parts a re at h and . Standard failu re d ata-b ase fo r r a substantial num ber o f m echanical p arts h aving n o t been g e n erated , fau lt-tree cannot be o f m uch use this w ay. H o w ev er atlases o f fau lt-trees can b e a very p o w erfu l aid fo r trouble-shoo ting and fo r training and im parting skills to m ain tenance p erso n n el especially novices. Sam ple fau lt-tree fo r a m odel loco d iesel eng in e is w orked herein hopefully to d raw th e attention o f diesel p ractition ers to the v a lu e o f such a sim ple and conven ien t tool and im press upon them the need to m ake such stan d ard fau lt-trees and p ro p ag ate through th eir m en fo r education on b etter m aintenance action s. C lo sely allied w ith fau lt-tree is a n o th e r technique, F M E A (failure m odes an d effects an aly sis).
F M E C A (failu re m odes effects and critically analysis) is also th ere. T h ese h av e also sim ilar uses as fault-tree. A D elphi version o f th e F M E A w hich c a n b e used fo r ran k in g failure m odes in the o rd e r o f severity
has been attem pted.
T h oug h trials w ere m ade in the p a st fo r th e d e v e lo p m en t o f v ib ro m etry -b ased d iag n o stic testing m eth o d s fo r in te rn al c o m b u stio n en g in e s, till now no em phasis has been placed on such in v estig atio n s. B ecause a v iew p re v a ile d th at
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the condition o f internal com bustion engines could be relativ ely sim ply, and m ore reliably , diagnosed with o th er m ethods w hich 'w e re consid ered classics. T h e m ethod o f testing cylind ers, valves, the ignition system , and the fuel supply system w ere deem ed to be fully developed. T echnologists seem ed to have refrain e d from m aking further vibro m etry investigations because o f the m any difficu lt p ro b le m s that had to be confronted in the case o f I.C . engines. T he engine is a dynam ic m echanism in w hich, besides the cran k m echanism , there a re other m oving p arts (e.g . tim in g ), that m ay excite intensive vibratio n s, even if the engine is in good condition. T h e vibration o f a host o f au xiliary system s (w ater pum p, dyn am o , oil pum p, fan , g e n e ra to r, etc all o f w hich m ust be operating harm oniously fo r the p ro p e r functioning o f th e engine) ad d to the already com plex vibration s excited by th e eng in e. In m obile en gin es the v ib ratio n s arising in the chassis (tru ck ), shock absorp tio n , and carriag e a re fu rth er additives. A ll these m ake th e identification o f the characteristic vibrations o f the in d iv idu al p a rts very difficu lt. B ut, o f late, it could be noted that, intense-efforts to find o u t w ays o f ex p loitin g v ib ratio n ob servations fo r diesel fault diagnostics w ere going o n . T h e shaftin g o f an internal com bustion engine w ith all its crank s, pistons, flyw heel, a n d d riven m ach inery is too com plicated a stru ctu re that determ in ation o f exact to rsio n al n atu ral freq u en cy is n ear im possible. Y et it is felt that an g u lar velocity fluctuations o f th e c ra n k shaft can serve a m ost uncontam inated and p o w erfu l signal that can b e used fo r the early d etectio n o f im pending failures especially those pertainin g to the c y lin d e rs. R ecen tly som e w o rk related to an g u lar velocity m o n ito rin g has been rep o rted . A ttem pt to actually m easure an g u lar velocity is a difficult p ro p o sitio n b efo re c o m m e rc ial v ersio ns o f an g u la r v elo city sensors a re av ailab le on the m ark et. O f course fo r e x p e rim e n tal w o rk , a n g u la r v elo city m easurem ents a re being d one using in d irect w ays. A gain th is w o rk o n ly a ttem p ts to reem p h asize the pro sp ects fo r c ondition m onitoring techniques, p a rtic u la rly , v ib ro m e try tech n iques in health assessm en t/ fault d iagnosis o f I .C . en g in es. In clu d ed a re so m e dem o n stratio n o f fault diag n osis by sim ulation using ap p ro x im ate p .v - d iag ra m s.
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T h e objects o f this study had b een set as
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(i) T o collect, classify, and study and critically review the av ailable literature on RA M studies and fault diagnosis o f I.C . engines.
(ii) T o highlight som e o f the aspects o f R A M techniques in the co n tex t o f diesel p ractice.
(iii) T o exp lo re the possibility o f diagnosing cy linder related faults w hich a re fundam ental through m onitoring an g u lar velocity signature.
(iv) T o conduct v ib ro m etry experim ents to be able to draw in feren ces/clu es w hich w ould help assess the health condition o f the engine and decide the tim ing o f next m ajor ov erh au l/ m aintenance so that unnecessary prev entiv e m aintenance action s can be forsaken.
T h e w o rk could be carried o u t to fulfill m ore o r less th e set objectiv es. T h e rep ort on the w o rk is presented in the subsequent ch ap ters. An am ou nt o f theo retical b ackground text-book m aterial is also quoted in som e o f the ch apters fo r the sake o f co ntinu ity and e x tra clarity. H e re follow s the second c h a p te r o f a b rie f sum m ary o f the relev an t available literatu re studied in this context.
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