SPEED IRREGULARITY CHARACTERISTIC OF LOW SPEED, TWO - STROKE MARINE DIESEL ENGINE APPLIED AS VESSEL S MAIN PROPULSION

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1 Joural of KONES Powertrai ad Trasport, Vol. 15, No SPEED IRREGULARITY CHARACTERISTIC OF LOW SPEED, TWO - STROKE MARINE DIESEL ENGINE APPLIED AS VESSEL S MAIN PROPULSION Przemysaw Kowala Maritime Uiversity of Szczeci - Departmet of Egieerig Waly Chrobrego 1-, Szczeci, Polad tel.: , fax: powala@am.szczeci.pl Abstract The pisto egie irregularity is a cosequece of the ueve power supply from the egie uits. As such it cotais valuable iformatio about egies uits worig process. Ufortuately it is strogly iflueced by torsioal vibratio, resoace, ad propeller behavior. I order to better uderstad the speed irregularity pheomea the istataeous speed was recorded ad its characteristic agaist egie mea speed was developed. Some statistical parameters: stadard error ad deviatio as well as variace were determied. The results were compared with propulsio plat torsioal vibratio. The preseted results show that the irregularity of the log stroe egie s speed is strogly iflueced by torsioal vibratio pheomea. The aalyzed egie has a bared speed rage rpm. Exactly aroud that rage the speed irregularity reaches its maximum values of 18% of mea speed. The aalysis ca be cosidered as qualitative loo ito the pheomea. However eve based o the preseted aalysis oe ca observe some practical ifereces. The commo type of electroic idicators is used to simplify the measuremet by assumig that the shaft speed is equal i betwee the picup pulses. I order to icrease the accuracy of the idicated power determiatio the use of such istrumet should be avoided i the speed rage which is a multiple umber of the barred speed. Due to the fact that the shaft speed is irregular especially i the rages where torsioal resoace may occur the pisto positioig (for example for TDC determiatio) purpose procedure should tae it ito accout. Keywords: trasport, marie diesel egies, combustio processes, speed measuremet, speed irregularity 1. Itroductio The torsioal vibratio of the vessel s propulsio shaft is particularly supervised by the Ship s Classificatio Societies [3, 5]. As the eergy i the pisto egie is provided periodically ad the shaft speed of the ship s mai propulsio egies is low, the ris of resoace with the shaft s free vibratio emerges. I [6] the MAN B&W compay estimates that due to resoace the shaft s tagetial stress may reach eve from 5 up to 50 times higher value tha the stress at omial load. To protect the shaftig agaist damage the Classificatio Societies set two levels of allowed stress [6] which are usually called 1 ad (Fig. 1). The propulsio set is allowed to operate below the level 1 without ay limitatios; however, the geerated vibratios may be upleasat for the crew or eve harmful for some of the ship s equipmet. I case the level 1 is exceeded the barred speed rage has to be applied as a coutermeasure. The egie ca be operated i this rage oly for a very short time ecessary to pass up quicly to above or dow to below the barred speed. If the higher level of stress is exceeded the egie will be totally baed from operatio i the rage of speed whe excessive stress is observed. Whe the ew propulsio uit is beig desiged the costructor has to pay particular attetio to avoid the exceedig of i the egie s operatioal speed rage. I some cases the torsioal vibratio damper may be helpful to reduce the stress dow to below the. The variable tagetial stress i the shaft lie is the cosequece of the variable torque betwee the egie ad propeller. The torque applied to the shaft is twistig it accordig to the

2 P. Kowala formula: M l, (1) G I o Whe the ati-torque geerated by the propeller is smaller tha the torque produced by the egie the shaft lie is forced to rotate. I the theoretical situatio of balace betwee the torque ad ati-torque the rotatioal shaft speed would be costat. I fact the propeller s ati-torque fluctuates ad the egie produces highly pulsatig torque. As a cosequece the shaft rotates with variable agular speed which ca be described as: d ( t ) ( t), () dt By meas of the Fourier trasform the costat compoet of the agular speed ca be distiguished ad a ifiite amout of the periodic compoets as well. Those periodic compoets are resposible for the pheomea called irregularity of the shaft rotatioal speed. As it was preseted above the shaft s rotatioal speed irregularity ca be related to its tagetial stress. a shaft stress due to vibratio b 1 barred speed shaft speed [% ] Fig. 1. Shaft stress ad barred speed rage over a marie egie speed The istataeous values of the shaft speed ad the torque orieted to istataeous pisto positio determiatio lays i the field of the author s iterest. So the possibility of the low speed, two - stroe marie diesel egie s speed irregularity characteristic aalysis appeared as very iterestig.. The propulsio plat ad the measuremet coditios The measuremet was carried out o the 1719 TEU cotaier ship. As the prime move the log stroe, low speed MAN 6L70ME - C egie was istalled. The egie drives the ship s costat pitch propeller directly. The istalled propulsio plat allows the ship to reach cotract speed of 1 ots. The diagram of the propulsio plat is show o Fig. a ad its basic techical data i Tab

3 Speed Irregularity Characteristic of Low Speed, Two - Stroe Marie Diesel Egie Applied as Vessel s Tab. 1. Basic techical data of the propulsio plat Deomiatio Value Egie s omial power W Egie s omial speed 98.3 mi -1 Propeller s shaft diameter 579 mm Propeller diameter 6700 mm Propeller s blades umber 5 Barred speed rage rpm To elimiate sea coditio ifluece o the measuremet results the experimet was coducted i the equatorial area oly, with the sea state 3 or lower. The ifluece of the ship s load coditio was ot tested. a) b) Fig.. The propulsio plat: a) schematic diagram; 1.Mai egie;.propulsio shaft; 3. Rotary icremetal ecoder; 4. Ster tube with ster bearigs; 5. Propeller; b) Ecoder attached to the crashaft The mai egie is of the electroic cotrol type. The fuel ijectio ad exhaust valve are hydraulically actuated ad cotrolled by desigated digital multipurpose cotrollers (MPC). The MPC utilizes the sigal from a icremetal ecoder for the crashaft s positioig. The ecoder is attached to the crashaft free ed by meas of a bellows clutch (Fig. b). As the ecoder geerates 360 pulses of TTL stadard per revolutio it was possible to utilize the sigal for precise rotatio speed measuremet. The used measuremet istrumetatio is preseted i the Tab.. Tab.. Measuremet istrumetatio data Istrumet Measuremet board Ecoder Type NI USB-61, Natioal Istrumets, with couter iput gate Icremetal ecoder ITD. 44 A 4 Y134, MAN B&W The methodology described i [7] by Pita was utilized for istataeous speed determiatio. The ecoder s TTL sigal was coected directly to the digital couter iput gate of the measuremet board, the the sigal was compared with a fast, 80 MHz, o-board frequecy geerator. I such a way the difficult aalog sigal of fast samplig was avoided. As a result the time of oe period of TTL sigal was geerated. A schematic diagram of the measuremet is preseted i Fig 3. 65

4 P. Kowala Fig. 3. Measuremet basics; 1. Iput sigal from ecoder;. O board, 80MHz pulse geerator, 3. Trigger ad reset modulus; 4. Pulse couter; 5. Pulse umber to period coverter, 6. Sigal recorder 3. Statistical speed aalysis The measured sigals were recorded at differet speed settigs of the mai egie. The sigals were divided ito segmets icludig oe complete egie worig cycle. The example of the speed course for 3 cosecutive egie cycles is preseted o Fig 4. shaft speed [rpm] CA [deg] Fig. 4. Shaft speed course over its agular positio For every egie cycle the maximum, miimum ad a mea value of the speed was determied. Ad the speed irregularity was calculated as followig: max mi, (3) I order to get a more precise statistical image of the rotatioal speed alteratio a stadard deviatio, variace ad stadard error of the mea value was calculated additioally [1]: s 1 i 1 ( i ), (4) s 1 i1 ( i ), (5) s SE. (6) 66

5 Speed Irregularity Characteristic of Low Speed, Two - Stroe Marie Diesel Egie Applied as Vessel s Heywood [4] proposed to apply a coefficiet of variatio i idicated mea effective pressure as a evaluatio of idicated wor variability. The coefficiet is equal to the stadard deviatio error to the mea value ratio. As the speed irregularity ca be determied for every egie cycle separately as well, the coefficiet of variatio i istat egie speed was calculated adequately: s X 100%. (7) Based o the preseted formulas the characteristics were draw versus the egie s crashaft rotatio speed (Fig. 5). stadard dev. s,4 1,8 1, 0,6 0, variace s shaft speed [1/mi] shaft speed [1/mi] stadard error 0,15 0,10 0,05 0, speed irreg. [%] shaft speed [1/mi] shaft speed [1/mi] variatio coeff. X [%] shaft speed [1/mi] Fig. 5. Calculated statistical parameters versus shaft speed 4. Coclusio The preseted results show that the irregularity of the log stroe egie s speed is strogly iflueced by torsioal vibratio pheomea. The aalyzed egie has a bared speed rage rpm. Exactly aroud that rage the speed irregularity reaches its maximum values of 18% of mea speed. Other, additioally calculated, statistical parameters show similar behavior. It is worth to otice that a small pea ca be observed i the rage of 78-8 rpm rage which is twice the barred speed rage. I that rage of speed a upleasat extra vibratio o board the ship was 67

6 P. Kowala observed. This rage was usually omitted by avigatio officers as disturbig oise ad vibratio of equipmet occurred the. It ca be observed that the irregularity icreases whe the egie speed approaches the speed level of 0 rpm, too. As the egie s operatio rage of speed is 4-98 rpm the test for a very low speed close to 0 rpm was impossible. The aalysis ca be cosidered as qualitative loo ito the pheomea. However eve based o the preseted aalysis oe ca observe some practical ifereces. The commo type of electroic idicators is used to simplify the measuremet by assumig that the shaft speed is equal i betwee the picup pulses [, 8]. I order to icrease the accuracy of the idicated power determiatio the use of such istrumet should be avoided i the speed rage which is a multiple umber of the barred speed. Due to the fact that the shaft speed is irregular especially i the rages where torsioal resoace may occur the pisto positioig (for example for TDC determiatio) purpose procedure should tae it ito accout. Nomeclature agle of shaft lie twistig or rotatio, T torque, l shaft measure distace, G shear modulus of shaft material, I o shaft lie mass momet of iertia, (t) agular speed of the shaft, t time, speed irregularity, mi miimum shaft speed i worig cycle, max maximum shaft speed i worig cycle, umber of ecoder s pulses per crashaft revolutio, s stadard deviatio of the speed i oe worig cycle, s variace of the speed i oe worig cycle, SE stadard error of the speed i oe worig cycle, X coefficiet of variatio of the speed i oe worig cycle. Refereces [1] Dobosz, M., Wspomagaa omputerowo statystycza aaliza wyiów bada, Warszawa: Aademica Oficya Wydawicza 004. [] Gaeci, W., Tomcza, L., Przeoe systemy diagostycze dla procesu spalaia w ortowych siliach wysooprych, XVIII Midzyarodowe Sympozjum Siowi Ortowych, s , Gdyia [3] Germaischer, L., Rules for Classificatio ad Costructio,I - Ship Techology, Part 1 - Seagoig Ships,Chapter - Machiery Istallatios, Editio 000. [4] Heywood, Joh. B., Iteral Combustio Egies Fudametals, McGraw-Hill Iteratioal Editios, Sigapore [5] Larse, O. C., Søtvedt, T., Prevetio of Harmful Egie - ad Propeller - Iduced Vibratios i the Afterbody of Ships., Det Norse Veritas Iformatio No. 9, Oslo, August 197. [6] MAN B&W Diesel A/S, Vibratio Characteristics of Two - stroe Low Speed Diesel Egies d editio revised from P , Copehage [7] Pita, A., Olchowi, W., Trawisi, G., Aaliza moliwoci wyorzystaia pomiarów chwilowej prdoci obrotowej wau orbowego do ocey stau techiczego silia, XXX Jubileuszowe Ogólopolsie Sympozjum Diagostya Maszy, Wgiersa Góra 003. [8] Wimmer, A., Glaser, J., Idyowaie silia, Warszawa, Istytut Zastosowa Techii rozdzia 9, s