INSTANTANEOUS PISTON SPEED MEASUREMENT FOR MARINE DIESEL ENGINE COMBUSTION MONITORING

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1 Journal of KONES Powertran and Transport, Vol. 13, No. 2 INSTANTANEOUS PISTON SPEED MEASUREMENT FOR MARINE DIESEL ENGINE COMBUSTION MONITORING Przemysław Kowalak Martme Unversty of Szczecn, Engneerng Department 1-2 Waly Chrobrego, Szczecn, Poland, tel , fax: e-mal:pkowalak@am.szczecn.pl Tadeusz Borkowsk Martme Unversty of Szczecn, Engneerng Department 1-2 Waly Chrobrego, Szczecn, Poland, tel , fax: , e-mal: tborkowsk@am.szczecn.pl Abstract It s mportant that marne desel engnes run n ther normal condton n order to mantan correct effcency, low polluton and hgh relablty. The record of n-cylnder pressure pattern and ts evaluaton s one of the most powerful sources for expermental based analyss of nternal combuston engnes. In addton, collected data and calculated values expand the engne dagnostc system capabltes. From the other hand relable calculaton of mean ndcated pressure and other performance data requres accurate measurement of cylnder pressures and detecton of the crankshaft poston. It s necessary to ensure correct synchronzaton of recorded pressure shape wth crankshaft poston. For ths purpose magnetc pck-up to measure the rotatonal speed from flywheel s usually used as a source of samplng pulses. The nstantaneous pston speed can be nvestgated usng the lnear movement sensor located nsde the crankshaft box. For ths purpose marne propulson testng engne was equpped wth optc fber sensor to perform permanent pston poston measurement. Test bed wth examned engne s equpped addtonally wth: optoelectronc n-cylnder pressure transducer, magnetc pck-up to measure rotatonal speed from flywheel, optc fber sensor to measure the rotatonal speed and angular ncrements from propeller shaft and stran gauge arrangement desgned to provde effectve torque measurement. Keywords: marne desel engne, sea transport, nternal combuston engnes, engne mantenance 1. Introducton Marne engne cylnder dagnostcs s performed through ndcator valves and ppes. Unfortunately, the measurement of the cylnder pressure and TDC poston nsde of a runnng engne on board of a shp wth easy to handle long lastng nstruments s not a smple task. Combuston pressure s analyzed for specfc quanttatve values (ndcated mean effectve pressure, maxmum pressure, frng pressure angle, cycle varaton, etc. durng certan events (valve closures and openng, peak rate of pressure rse, etc., as well as data varaton between cylnders and wthn a specfc cylnder over a certan number of cycles. The fnal am s a calculaton of ndcated power for each cylnder as well as the total engne power derved from the measured cylnder pressures. Engne cylnder pressure analyss s also used to balance and tune the engne: valve and fuel njecton tmng as well as fuel rate and compresson. Improvng the relablty of the engne calls for development of engne dagnostcs. One of the methods used to meet these requrements s a convenent and real-tme cylnder pressure measurng system. Such cylnder pressure sensors could be utlzed n engne control systems. However, dynamc nformaton on cylnder gas pressure n marne engnes s restrcted due to unrelable

2 P. Kowalak, T. Borkowsk synchronzaton of recorded pressure shape wth crankshaft poston [1]. Instantaneous engne crankshaft speed vares durng the whole recprocatng engne operaton cycle. Many possble ways of estmatng the actual pston poston through measured crankshaft sgnals are presented n lterature [2], [3], [4]. Most of the methods do not requre a specfc sensor placed nsde the engne crankcase, but are based on the engne speed measured by the standard pck-up at flywheel. Computatonal modelng of each pston perodc crank shaft poston and speed fluctuatons could be used as a bass for engne dagnoss. Certan parameters were found, such as crank frcton parameter or nstantaneous engne speed that provde a better bass for fault engne dagnoss [5]. The methodology presented n ths paper s based on nstantaneous engne ndvdual pston poston and speed fluctuatons, observed n crankshaft rotaton nsde the engne crankcase. There s an assumpton that a correlaton s present between the measured speed at engne flywheel and selected (one cylnder pston poston (and speed nsde the crankcase. Ths correlaton has been tested usng measured data. 2. Expermental setup The expermental efforts descrbed below were an attempt to observe crank shaft and pston dynamc poston assocated wth measured engne cylnder pressure. Usng test bed engne, havng the specfcaton lsted n Table 1, examnatons were made by means of acquston setup wth transducers lsted as follows: magnetc pck-up on the flywheel - to measure average rotatonal speed, fber optc sensor to provde datum for actual pston poston and optoelectronc pressure sensor. The cylnder pressure sgnal was delvered by means of optoelectronc, dynamc pressure transducer. The sgnal was measured at one of the cylnders of test engne. Durng measurement the engne was loaded by shp fxed ptch propeller submerged n water tank at constant revolutons. Test engne propulson unt and expermental setup s presented n fgure 1. Tab. 1. Test engne specfcaton Maker Buckau-Wolf Type four-stroke 8RDV136 Nomnal rate P e 220 kw Nomnal speed n 380 rpm Bore D 240 mm Pston stroke S 360 mm Connectng rod l 720 mm 1- pston stroke measurement, 2- flywheel, 3 propeller shaft, 4 cylnder pressure measurement Fg. 1. The test engne and expermental assembly 216

3 Instantaneous Pston Speed Measurement for Marne Desel Engne Combuston Montorng To avod any errors when convertng transducer s sgnal nto pressure unt the analyss was conducted n voltage doman. Dgtalzaton of the pressure sgnal was done wth adequate samplng rate that should provde expected accuracy for crankshaft poston approxmaton. The rated speed, for present experment was kept constant by engne governor controller desgn. A basc acquston equpment characterstc s presented n Table 2. The pston stroke measurement was done by means of detecton arrangement, bult n engne crankcase. Tab. 2. Engne tral basc equpment detals Parameter Transducer - part Descrpton Optoelectronc Range: 0 34 MPa, Cylnder pressure Optrand, F532A8-ACu 0, C Fber optc Pston poston Ol and temperature resstance Keyence, typu FU-85Z Flywheel speed Autronca, GF-1 1 pulse/360 PCMCIA - AD126, Data Acquston board 12 bt, samplng rate 175 khz The detecton unt conssts of a fber optc sensor mounted under the cylnder lner and reflectng rod attached to the pston fgure 2. Equdstantly postoned teeth machned n the rm of the reflectng rod were a one way to measure lnear pston speed n the engne crankcase envronment. Developed acquston software was used to measure tme dfferences between the teeth as they pass the sensor. An array of tme dfferences was extracted, where the frst element of the toothed rod corresponds to the pston poston before and after TDC of measured cylnder. From ths tme-stamp array, nstantaneous pston speed as a functon of pston poston was calculated. Movable part of the measurement unt - reflectng rod was not able to cover the whole pston stroke, but gave a measurng range of 47.5 to mm below TDC. That provded to of the crank shaft turn wth 6 mm resoluton and caused 42 pulses for one pston stroke. In the test bed, speed of the flywheel and the pston were measured. One thng that should be mentoned, f a poston dfference between two rotatng engne parts exsts ths fact should be taken account of. In ths work ths occurrence was not defned. Software was created for extractng nstantaneous pston speed data from fber optc sensor assembly. Fg. 2. The pston stroke measurement arrangement 217

4 P. Kowalak, T. Borkowsk Cylnder pressure measurement wthn the combuston chamber was almost exclusvely made by means of pressure transducers and was assocated wth data processng system n tme doman. Sgnals were collected to be able to synchronze pressure and speed sgnals. The pressure sensor (Optrand mounted on the cylnder was precse, and ts sgnal value was approxmately lnear to cylnder pressure. It was not feasble to recalbrate pressure sensor so t s assumed that the lowest pressure n the cylnder s close to the exhaust pressure. All sgnals were measured n the tme doman and further analyss could be appled to transform the measured sgnals onto the angular doman. 3. Development of reference pston poston model For engne cylnder pressure supervson the engne speed measured at flywheel s compared wth the measured pston poston. The estmated engne pston poston s constructed usng approxmatons of the smple formula from whch the speed may be calculated: λ = r ( 1 cosω ( t t0 + ( 1 cos 2ω( t 0 4, (1 S ( t t S ( t pston stroke [m], r crank radal dstance [m], ω rotatonal speed [rad/s], λ rato of crank radal dstance to connectng rod length r/l, t tme [s], t 0 relatve phase lag coeffcent [s]. Consderng an arbtrary tme stamp from the speed sgnal that s wedged n between two flywheel revolutons as angular speed, mean value of sx subsequent engne cycles were calculated, and phase lag covered the tme step of flywheel pulse for one crank shaft turn. The proposed, prelmnary equaton can be expressed: ( = r ( 1 cos( ω( t a1 + ( 1 cos(2 ω( t a1 S t Ŝ ( t approxmaton functon, r, λ - test engne components, t experment perod, ω= rad/s. a 1 = s, λ 4 (2 The man reason for comparng the measured pston poston wth the sgnal from flywheel s to examne f any dynamcal dfferences can be noted. If no major dfferences are seen, data from flywheel may be used as reference data for the model. However, ths may be necessary to examne the flywheel sgnal wth hgher resoluton. To assess the approxmanton qualty the correspondng error can be calculated as follows: = = MSE = n 1 ( y S n ( t 2 = (3 218

5 Instantaneous Pston Speed Measurement for Marne Desel Engne Combuston Montorng MSE mean square error of approxmaton, S approxmaton functon for experment perod, ( t y nstantaneous poston dstance to TDC, n number of pston stroke measured samples (n=545, t nstantaneous tme. Another approach to equaton 2, where ω and a 1 value was establshed by means of least square technque, gves: G t ( = r ( 1 cos( ω( t a1 + ( 1 cos(2 ω( t a1 G ( t approxmaton functon, ω= rad/s, a 1 = s. λ 4, (4 Agan, the approxmanton qualty wth the correspondng error can be tested as follows: = = MSE = n 1 ( y G n 2 ( t 2 = , (5 Results from both comparson calculatons show that the flywheel sgnals are not much dfferent n ther propertes and sgnfcant error found. Therefore, t s proposed to exchange constant angular crank shaft speed wth tme - dependant expresson: ω (6 ( t = a2 + a3 sn( a4 t + a5 ω ( t - approxmaton functon of angular speed, a 2..5, - determned coeffcents (least square soluton. Ths approach should elmnate sgnfcant error observed n recorded data caused by small speed fluctuatons. Updated approxmaton equaton for pston poston n tme doman may be calculated as shown: H λ = r ( 1 cos( ω ( t a1 + ( 1 cos( ω( t ( t a1 4, (7 ( t ( t a determned coeffcents (least square soluton, 219

6 P. Kowalak, T. Borkowsk a 1 = , a 2 = , a 3 = , a 4 = , a 5 = Wth use of a 1..5 n equaton 6 and 7 mean square error test apples the comaprson wth measured pston poston data as follows: = = MSE = n 1 ( y H n 5 ( t 2 = , (8 Equaton 8 hghlghts the mprovement of the reconstructed pston poston f fluctuatons are expressed as varable tme - dependent. Pston poston and speed fluctuatons are present even n motorng condton. As t can be observed n equatons 3, 5 and 8, where the comparson s carred out, the qualty of approxmaton functon s dfferent. The best result acheved s stll unsatsfactory. The consderaton need to be modfed when the motored and compressonexpanson strokes occurred. Nevertheless, t has to be consdered that snce there are eght engne cylnders and eght combustons accordngly, all strokes are supermposed. Ths means there are 90 degrees shft between workng strokes of the followng cylnders (frng order and all o them nfluence the speed varaton at flywheel. It would be approprate then to compute approxmaton functon takng nto account the measured pston poston data over the whole cycle, leadng to: I λ = r ( 1 cos( ω ( t a1 + ( 1 cos( ω( t ( t a1 4 (9 ( t ( t a 1 = , a 2 = , a 3 = , a 4 = , a 5 = Wth use of a 1..5 n equaton 9, mean square error test apples the comaprson wth measured pston poston data as follows: = = MSE = n 1 ( y I n 5 ( t 2 = (10 4. Expermental results As t has been found, pston nstantaneous poston and speed s depended over an engne cycle on specfc stroke and n-cylnder events. Its fluctuatons are manly due to n-cylnder pressure varatons. The relable calculaton of mean ndcated pressure and other performance data requres accurate measurement of cylnder pressures and detecton of the crankshaft poston. To meet ths need, detaled pston poston and speed on ndvdual engne cylnder unt s expected. The essental dea for the pston poston and speed reconstructon s that t can help establsh ndvdual cylnder angular coordnate. It could be utlzed wth flywheel speed measurement. The expermental valdaton of the pston poston and speed reconstructon has been carred out usng engne runnng test. The test under steady state condtons covers one arbtrarly chosen engne load. The effectve propeller load power to speed relaton for expermental test-bed can be wrtten: P e 2,7759 = 0,88993 n (11 220

7 Instantaneous Pston Speed Measurement for Marne Desel Engne Combuston Montorng P e engne effectve power [kw], n - engne runnng speed [rpm], The pston poston evaluaton wth correspondng motored cylnder pressure can be seen n fgure 3. Usually, n normal engne servce operaton there s a necessty to relay on statc TDC orentaton. However, t does not correctly estmate cylnder dynamc TDC what can be seen as a dfference wth reconstructed pston poston trace and statc TDC pont n fgure Calculated Measured Stroke [mm] Cylnder pressure [bar] ,0 0,2 0,4 0,6 0,8 1,0 Tme [s] pston poston trace; motored pressure; thermodynamc TDC; flyng wheel mark - statc TDC; Fg. 3. Pston poston reconstructon (cylnder No: 3, n = 282 rpm The effect of ths error can be mnmzed f proposed approxmaton of pston poston and speed s mplemented. Fgure 4 shows dscussed pston poston approxmatons error comparson aganst reference pont. A more sophstcated algorthm s requred for precse pston poston acquston, so far speed sgnal detecton on flywheel shows mprovement from 18 mm (maxmum value S(t approxmaton to 11 mm (maxmum value H(t approxmaton. A notcable dfferences are seen between reference lne and I(t approxmate value even though maesurment data was used. That dfference would be dentfed wth compresson and expanson strokes where the errors for all approxmatons are serous and confrmed [6]. The man reason of uncertantes n TDC and BDC poston where all expected error curves were not made s lack of expermental data. Ths could be establshed when the addtonal measurement for pston poston wll be prepared n ths range. Calculatons and comparsons of MIP to other cylnder unts are pontless wthout correctons of ths naccuracy. 221

8 P. Kowalak, T. Borkowsk H(t I(t G(t S(t Poston error [mm] Reference poston Stroke [mm] ,3 0,4 0,5 0,6 0,7-50 Tme [s] Fg. 4. The approxmatons error comparson over the full workng cycle 5. Conclusons The n-crankcase pston poston and speed measurement system has been shown to be an effectve technque for detecton of specfc speed fluctuatons. The tral results from the dagnostc system showed that each pston stroke related to workng cycle s dfferent and can be dstnctvely recognzed. As seen n fgures n prevous sectons, each engne operatonal stroke creates dfferent pston speed resonance. Obvously, ths causes torsonal vbratons n the crankshaft. The speeds from the pston measurement are sgnfcantly phase shfted comparng to the flywheel speed fluctuatons. Ths may be due to engne - propeller confguratons, but t s more lkely due to dynamcs between the flywheel and the specfc pston unt n the test-bed envronment. Furthermore, the cylnder pressure estmaton based on reconstructed pston poston could be more relable f the perodcty n the speed fluctuatons wth respect to crank angle wll be modeled. Varatons n ths perodcty are strong ndcators for ndvdual cylnder and may be detected by comparson wth the reference level. References [1] Borkowsk T., Lstewnk J., [CD-ROM], Cylnder pressure data analyss to estmate NO x emsson on large bore slow speed marne engnes. Internatonal Councl on Combuston Engnes CIMAC Congress 2004, Paper No. 231, p. 15, Kyoto (Japan [2] Wang Y., and Lm T., C., An analyss of modal dampng sources n recprocatng engne, Journal of Sound and Vbraton, 242(1, pp , [3] Nlsson M., Modelng flywheel-speed varatons based on cylnder pressure, Master s thess, Lnköpng Unversty, nr: LTH-ISY-EX ,

9 Instantaneous Pston Speed Measurement for Marne Desel Engne Combuston Montorng [4] Traver M. L., Atknson R. J., Atknson C. M., Neural network-based desel engne emssons predcton usng n-cylnder combuston pressure, SAE, 1532, 01, [5] Twddle J., A., Jones. N. B., A hgh-level technque for desel engne combuston system condton montorng and fault dagnoss, Proc. Instn Mech. Engrs., Vol 216 Part I: J Systems and Control Engneerng, pp , IMechE [6] Haller C., O Connell M., Advances n desel engne sgnature analyss recent experence wth technques for medum and slow-speed desel engne, Congress CIMAC, Hamburg 2001, [7] Tsuchya K., Nagashma k., A., A calculaton method for ndcated mean effectve pressure based on harmonc analyss of pressure waveform, Int. J. Engne Res. Vol. 4 No 2, IMechE

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