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Marine Diesel Engines


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Chapter7 MarineDiesel Engines
EditedbyKeeRongWu 7.1 Prospectsin MediumSpeedDi

eselEngines 7.1.1 Meritsof DieselEnginesand Gas Turbinein MarinePropulsion Thissessionwillconcentratesondieselenginesforcruisevesselapplicationswithatotal power requirement ofapprox. 60 MW each installation. A typical dieselelectric drive with fivemediumspeeddieselengineswillbecomparedwiththe58MWCOGES(CombinedGas TurbineandSteam TurbineIntegrated ElectricDriveSystem)[7.1]. Insummer 2000,CelebrityCruises'gasturbinedrivencruising vesselMillenniummade hermaidenvoyage.The91000TonvesselwithaPaxcapacityof 1950(lowerberth)denotesa technologicalshiftincruiseshipdesign,primarilybecausesheisthefirstcruiseshippowered byapuregasturbine plant.Apartfromthis, theship hasthebiggest azimuthpodsever built (twoMermaid pods of19.5MW each). Currently thereare threefurther cruise ships ofthis seriesunderconstruction.Thiscertainlyisamilestoneforgasturbinemovers,themoresoas fourfurthernew Vantageclass cruise ships forRoyal CaribbeanInternational(RCI)arealso specified with turbinebased propulsion plants. Each plant consists of two General Electric LM2500+ aeroderived gas turbines of 25 MW each and an 8 MW backpressure steam turbine. The steam turbine uses steam from the boilers fired by wasteheat from the gas turbines togenerate additional electricalpower.Depending ontheamount ofsteamrequired foronboard services, the complete COGESpower plant is expected to achieve a combined cycle efficiency of between 45 and 50%. This system will provide for all onboard power arrangements,suchaspropulsion,heating,cooling,lighting,ventilation,kitchenandlaundry. However, with about 97% of all existing seagoing ships propelled by two and fourstroke dieselengines(Fig.7.1) duetoitscomparablyhighthermalefficiency(Fig.7.2), itseemstheir manufacturers have so far not seriously been affected by gas turbines in most of their traditionalmarketareas.

Fig. 7.1: A crosssectional view of MAN Fig.7.2: Power efficiencycomparisonatISO 3046[7.1] B&WV40/50 fourstroke Dieselengine In general, diesel engines posses lower initial costs, fuel economy, weight and size as
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comparingwithgasturbine’senvironmentalfriendlinessshowing Fig.7.3and7.4.Thedetail comparisonwillbedescribedinfollowings.

Fig.7.3: Dieselenginesversusgasturbines pro andcons[7.1]

Fig.7.4: NOx emissionof marineprime movers[7.1]

7.1.1.1 Weight and size Gas turbines are known to generate lotsofpowerwhile offering less space and weight than a diesel engine of the same output. The diesel engine's size and heavy mass is an undisputable disadvantage in many applications. However in the new Panamaxsized cruise shipswithanincreasednumber ofdecksbutunchangedwidth,muchweightis needed inthe bottom of ashipforstabilitypurposes,sothevalueof theweightsavingsbygasturbinesmust not be over emphasized. In order to decrease the vertical center of gravity, this weight deficiencycouldbecompensatedbyadditionalfreshwaterorfueltanks.Anotheroptionis to slightly decrease the main deck height or draught ofthe vessel. However, all ofthis would necessitate a new ship design, excluding the use of a common hull form for either diesel engines or gas turbines that would be highly beneficial in order tocut costs.As regards the spacesavingsof gasturbines,thispotentialcannotbefullyutilized:Gasturbineshaveapprox. 15% largerairintakeandexhaustductsascomparabledieselenginesandtheirstartingdevices also occupy much space. Onboard cruising vessels with two gas turbines as prime movers, necessary provisions for a rapid replacement of a gas turbine (or at least its gas generator) within a few hours, with the vessel at sea and underway, occupies extra space. The engine room has to be designed with sufficient free space and all the necessary provisions and equipmentforthisjob,includingstoragespaceforacompletesparegasturbine.Finally,plant availabilityandsafety considerationsmakeatleastoneor twoadditionaldieselenginegensets mandatory to satisfy low power requirements difficult to cover with a gas turbine and as emergencygenerator. Thisdoesnotonlyrestrictthefreedspacefurther, butalsoincreasesfirst costs, operatingcostsandmaintenancecosts. 7.1.1.2 Firstand maintenancecosts Contrary to weight and size, first costs and maintenance costs are lower for the diesel solution, although first costs might be more a political concession. As regards maintenance, RCI has signed a 10 year repair and maintenance contract with General Electric for the vessels' LM2500+ gas turbines at a cost of3$/MW h. The maintenance cost summaryofa multiengineDieselelectricgivesalowerfigure.

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7.1.1.3 Fueland operatingcosts Asindicatedin Fig.7.2,Dieselengines enjoyfurtherbenefits suchaslowerfuelprices, lower fuel consumption rates at all loads and therefore lower carbon dioxide emission and betterloadacceptanceaswellasquickerstartuptimesafteranightstop.Forinstance,aftera night stop,agas turbine in simplecycle mode needs 30minutesuntil full load is reached, a dieselengineinthesamesituationlessthan5minutes. 7.1.1.4 Vibration,noiseand lubeoilconsumption As regards vibration and noise, multiple cylinder reciprocating engines with their intermittent combustion are at a disadvantage, although sometimes the real differences are exaggerated or erroneously interpreted. By directresilient mounting ofDiesel engines, their structureborne vibration transmitted into a ship foundation is reduced to a level of approximate below 50 dB at frequencies ofabove 1000 Hz. Although resiliently seated gas turbinesmightreachstilllowervalues,designmeasuresaimingatanevenfurtherdecreasein diesel engines' structure borne noise can be omitted as long as the requirements regarding vibrationinthe cabinsaremet. Unexpectedly thenewbuilding Millenniumexperienced vibrationproblems in some areasof theshipunderspecialseaconditionslikelytooccurintheCaribbeanduringthewindywinter season. The ship had to be drydocked for technical modifications earlier than planned, followingitsarrivalinNewYork inNovember2000. Airborneengineroomnoiseof gasturbinesisclaimedtobelessthan85dB(A), whereas the noise emission ofa MAN B&W largebore mediumspeed diesel engine varies between 102and108dB(A)atfullload.Themainreasonfor thisdifferenceisthatmarinegasturbines are installed in acoustically insulated enclosures whereas the noise level for freestanding diesel engines is measured without any soundattenuating encapsulation or lagging. Engine machineroomsarenotamongtheplaceswherepassengersonboardusuallyspendtheirleisure time.Thereforethelowerrunningnoiseofgasturbinesisnotofmajorimportance:outsideof themachineroom, thedieselenginescanbeconsideredto beencapsulated aswell. The specific lube oil consumption of modern gas turbines is typically only 1% of the diesel engines' figure, but high priced synthetic lubes have to be used in comparison to the lowpriced mineral oils forthe Diesel engines. The annual lube oil costsofgas turbines are only about 6% ofthat ofdiesel engines. Ithas to be pointed out that this merit is ofminor importance,sincelubeoilcosts hardlyaffectthetotal operatingcosts. Therealadvantageofthegasturbineis itsecofriendliness asfarasSOxandNOx(not CO ) emissionsare concerned.SOx emissionof gasturbinesisclosetozerobecausethey burn 2 basically sulfurfree fuel (MGO typically contains only about 0.3 % sulfur, HFO for diesel engines up to4.5%).If(higherpriced) lowsulfur orsulfurfree marine diesel oils would be used for diesel engines, there wouldn't be a SOx problem with them either. NOx emission levelsof modernmarinegasturbinesanddieselenginesare listedinFig.7.4. Thereisnobasic technicalrestrictionindecreasingdieselenginesNOxemissiondowntoalevelof 2g/kWhby adopting SCR based exhaustgas cleaning. All today's serial NOx optimized marine diesel engines have to meet IMO NOx restrictions for international shipping valid for new ships (achieved by engine internalmeasures). Bydirect injection ofwaterinto thecylinder orby adopting waterfuel emulsification, a similar NOx emission level as with today's standard marine gas turbines without water injection is achieved. The test results of a MAN B&W 6L48/60 engine in February2000:a NOxcycle value of7.7g/kWh and a fuel consumption

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rate still within tolerance (5%) was measured. This is 40% below the NOx limit set by the IMO.Thisresultwasachievedwithonly15%waterinthewaterfuelemulsionandaslightly retarded injectionbelow80% engineload[7.1]. 7.1.1.5 Efficiency Figure7.2showstheachievableoverallefficiencyleveloftoday's primemovers.Large boremediumspeed engines reachupto47%in simplecycle operationand lowspeed diesel engines even up to 51%. With smaller engines the difference in efficiency and in fuel consumption between diesel engines and gas turbines increases considerably. Figure 7.2 is indicative of the high efficiency level that combinedcycle gas turbines of high unit output (above 50 MW) reach today. Up to now there are only few diesel combinedcycle (DCC) installations in operation.Their number will increase in futurealthough this technology will increasethediesel’sefficiencylevelonlybyafewpercentagepoints.

Fig. 7.5: Typical part load efficiencies of Fig.7.6:Annualfuelcostsof COGES versus dieselelectricsystem[7.1] primemovers[7.2] The specific fuel consumption rates (SFOC) in g/kWh over the total electrical plant outputforthevarious propulsion conceptsareplottedin Fig. 7.5.From90%powerdownto approx.60%power,thethermalefficiencyis almost constant.Contrarytothis favorably flat fuelconsumptionline,theturbines'consumptionratesarehighlyloaddependent. Atveryhigh rate power, COGES has a thermal efficiency of around 40%, a value that is only slightly higherthanthatofdieselengines'figureindeed.However,thishighelectricalpowerishardly usedincruising: mostofthetime,theturbines havetooperateatpartloadwithmuchhigher specificfuelconsumptionrates. Calculation of the annual fuel costs was based on the following typical weekly load scenario: 60 hoursper weekinports (power requirement10MW): One12V 48/60Dieselengineor onegasturbinewiththesteam turbineinoperation Thissumsupto 3840 operatinghours peryearfor eachof thefivedieselengines, and6 150hours for eachof thetwo gasturbines.For thisloadprofileandfor average August2000 fuelpricesfor North WestEurope, thetotal fuelcostsare showninFig. 7. Thedifferenceinannualfuelcosts betweenCOGES andthedieselelectricoption isUS$7 million.
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Thetotalsumof fuelcosts(Fig.7.6) andlubeoilcostsisUS$13.86millionfor COGES and US$ 7.04mforthedieselelectric system. The difference is US$6.8million peryear. Withthetotalannualnetprofitof onlyUS$2.1 million,itisimpossibleto compensatehigher fuelbillof COGES.WithbunkerpricesinSeptember2000, thereisalossof US$4.7 million everyyearandfor every shipandthisdoesnot includethe higherfirstandmaintenancecosts. In comparison to a COGES system, dieselelectric solutions have clear advantages in many aspects, with the exception of weight and size, and NOx emission and noise. These advantages are of uniform machinery, lower fuel costs and lower fuel consumptions and therefore lower CO emission, lower first costs, operating costs, easier maintenance, lower 2 maintenance costsand wider operational flexibility and redundancy onaccount ofthelarger number ofdieselengines thatareable toburn widely varying fuelqualities. Thegasturbine itself,asanintrinsicallysimplerotatingmachine,ishighlyreliableanddurableasithasfewer movingpartsandlowerfrictionlosses,butthemorecomplicatedCOGESsysteminvolvinga steam turbinegensethadnochanceupto nowto proveitsavailabilityandlongtermreliability incruiseshipping. 7.2 HighSpeed DieselEngines 7.2.1 Introductionto SCANIA12LiterEngine Thefollowing description is referredfromtheuser’smanualofSCANIA12liter diesel engine.

圖7.7:SCANIA 12literengine外觀圖 圖7.8:汽缸體外觀圖

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圖7.9:汽缸套剖面圖

汽缸支架或汽缸本體(cylinderframe) 如圖7.8所示的汽缸本體是用鑄鐵一體成形,每一缸有各別的汽缸蓋,且採用溼式汽 缸套。 汽缸套(cylinderliner) 氣如圖7.9所示的汽缸套,如為水冷式引擎則多用鑄鐵或鋁合金,周圍有散熱片,以 增加散熱面積。水冷式引擎在汽缸蓋周圍有水套環繞,使冷卻水在內循環,以維持引擎 之工作溫度。可更換式汽缸套在汽缸套及汽缸蓋間是用硫化汽缸床墊做為密封,且每一 缸皆有一片硫化汽缸床墊。汽缸套在安裝後稍高於引擎本體表面,再藉由鎖緊汽缸蓋螺 絲加壓於汽缸蓋與硫化汽缸床墊,進而形成理想的氣密效果。汽缸蓋與引擎本體之間的 冷卻水及潤滑油通道,則利用硫化在鋼片上的橡膠墊圈作為封密之用。

圖7.10:汽缸體剖面圖及活塞示意圖

圖7.11:曲柄軸及主軸瓦

汽缸體(cylinderblock) 如圖7.10所示的汽缸體與活塞冠構成燃燒空間,為燃燒室的主要熱傳通道。因在燃 燒室周圍的溫度相高的高,故汽缸套是安裝在比以前較低的位置,可使冷卻水儘可能接
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近汽缸蓋,如此可以降低活塞環的溫度,增加了活塞環及汽缸套的使用壽命。

圖7.12:汽缸套及活塞的剖面示意圖

圖7.13:壓縮環及刮油環與活塞的位置圖

活塞(piston) 如圖7.12所示的活塞,其功用是將曲軸回轉運動轉變為上下往復運動,此活塞是採 用兩段式設計,由鋼製的活塞及鋁製的活塞裙兩者組合而成。兩段式活塞的優點之一, 是比一般活塞能承受較高的負載。因此,兩段式活塞設計的引擎可有較高的馬力輸出。 某些引擎也使用鑄鐵製活塞。 活塞頂上的凹處構成所謂的燃燒室,它的形狀類似一個中心凸起的碗。下凹處的設 計可促進空氣與燃油的混合,進而改善燃燒效率(圖7.12)。為了使活塞在汽缸套內順 暢的往返滑動,二者之間留有適當的間隙,因此在活塞上裝了兩條壓縮環(compression ring)來密閉此一間隙,並藉由它將活塞的熱量散出(圖7.13)。 如圖7.13所示的刮油環(oilring),其功用是防止潤滑油由曲軸箱滲入燃燒室而消 耗掉。在刮油環內含有一圈狀螺旋彈簧式擴張環,它可使刮油環壓貼在汽缸壁上。活塞 和活塞環的形狀是很重要的,它可影響引擎的可信度,潤滑油及燃油消耗率。 連桿(connectingrod) 如圖7.13所示的連桿,其上端設計成錐形,目的是提供活塞與連桿兩者有更大的軸 承面積。連桿下端在一斜角處分成兩半,所以活塞及連桿才可以從汽缸套下方抽出(圖 7.14)。為防止連桿及連桿蓋之間產生移動,所以在兩者接觸面上作有環狀凹凸槽。 圖7.13:連桿及活塞組件圖

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圖7.14:連桿大端及軸瓦組立圖

曲柄軸(crankshaft) 如圖7.11所示的曲柄軸,其主要的功用是將活塞之動力輸出由往復式運動,轉變為 回轉式運動而帶動傳動機構的機件,一般四行程的曲柄軸內部鑽有油孔,二行程則無, 曲軸上裝置平衡配重以抵消活塞及連桿的慣性作用一般皆以合金鋼鍛造,再加以車削加 工而成。 對曲柄軸而言,每次壓縮行程就是減速,而動力行程則是加速。曲軸每轉動一圈, 活塞與連桿的運動方向改變兩次。因此,曲柄軸在每次運轉時,承受了許多應力。曲柄 軸所使用的材料。 設計及接面處理將會影響曲軸使用壽命, 尤其是曲軸頸的表面處理, 將 可以防止曲軸因疲勞而破斷。曲軸軸頸與軸肖上的軸承片包含三層不同的材料,最外層 是鋼片、中間層鉛銅為材料,而與軸面接觸的表層則是以鉛銅或鉛錫銅合金組成,它是 最後一層、也是承受磨耗的一層(圖7.11)。為防止曲軸軸向位移,在最後主軸承座的 側邊有止推墊片,為了配合曲軸加工,故此止推墊片有幾種不同的厚度以利調整曲軸軸 向的位移在規範內。為了使軸頸與軸肖在磨耗後,能多次加工延長使用壽命,因此它們 的表面硬化處理,必須有足夠的深度。 曲軸箱通風(crankcaseventilation) 如圖7.15所示的曲軸箱通風系統是透過前搖臂室蓋,將曲軸箱廢氣引入在引擎前方 的通道系統。曲軸箱的廢氣是充滿油氣,當廢氣通過前方通道系統的通道時,因油氣較 重而聚集在通道壁上,並流至通道系統的底部再流回油底殼。確定通道系統沒有阻塞是 非常重要的,若通道系統阻塞,則曲軸箱的所有油 氣將會被渦輪增壓機給吸入。 藉由曲軸箱通風系統 與渦輪增壓機的吸入端相連接, 而保持曲軸箱內輕 微的負壓,以利通風作用順暢。為了調節曲軸箱少 許的真空,當渦輪增壓機的吸入端負壓太大時,則 膜片關閉曲軸箱通至渦輪增壓機的通道, 這情形會 發生在引擎加速油門開度較大時。 1 從前搖臂室蓋之吸入端
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2 油氣收集通道 3 當渦輪增壓機負壓過大時,則膜片將關閉通道 4 膜片 5 渦輪增壓機吸入端 圖7.15:曲軸箱通風示意圖

圖7.16:閥機構組立圖

圖7.17:閥門座及進排氣閥的佈置圖

汽門機構(valvemechanism) 閥(Valve)是負責進排氣的開與閉,進汽門較大而薄,是以鎳鉻合金鋼製成耐熱約 500℃。排汽門較小而厚,一般是以釸鉻合金鋼製成耐熱約800℃;閥搖臂(Valvearm)是 傳遞由凸輪軸來的動力給汽門,做開閉的動作,一般是以鉻鉬合金鋼製成。如圖7.16所 示的閥機構, 其主要功用就是配合曲軸及活塞的相關位置, 在正確時間開啟或關閉汽門。 凸輪軸是由正時齒輪驅動,其轉速為曲軸的一半,凸輪軸有兩種型式,一種是每缸有二 個凸輪,另一種則每缸有三個凸輪(PDE)推桿一端是放在舉桿上,另一端則頂在汽門搖 臂上,在搖臂與推桿接觸端有一個下端為球狀的調整螺絲,並緊抵著推桿,使舉桿能隨 著凸輪軸作動。如圖7.17所示的閥門座,是採用一種特殊耐用的金屬材料,故可以耐久 使用、且是緊壓在汽缸蓋上,在磨損後可單獨進行更換、降低維修成本。為使空氣容易 進入汽缸中,故每缸採用四汽門以增加進氣面積,同時也可減少排氣所消耗的能源。由 於四汽門的設計而降低氣流阻力,使引擎效率提升,進而使耗油率降低(圖7.17)。 正時齒輪(timinggear) 如圖7.18所示的正時齒輪是裝在引擎後方,其理由為:因安裝位置靠近飛輪,所以 曲軸所產生的扭震較少,則噴射泵及汽門機構得以獲得精確控制及減少噪音。正時齒輪 的安排方式則依據,裝噴射泵及單體式噴油器(簡稱PDE)有所不同。裝配噴射泵的引擎 曲軸齒輪驅動兩個惰輪及機油泵齒輪,其中一惰輪本身有外內兩齒輪,其內齒輪是匼動 凸輪軸及噴射泵,而外齒輪驅動方向機泵,而另一個惰輪則驅動空壓機(圖7.19)。 圖7.18:引擎裝配噴射泵的正時齒輪 引擎裝配噴射泵 1 機油泵驅動齒輪 2 空壓機驅動齒輪 3 曲軸齒輪 4 惰輪 5 噴射泵驅動齒輪 “中国海员之家”网站(http://www.seamancn.com )搜集整理,四分之一船长@中国海员之家网站制作。 6 凸輪軸驅動齒輪 7 方向機泵驅動齒輪

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圖7.19:引擎裝配噴射泵的正時齒輪

圖7.20:無冷氣增壓機(左)

導輪裝有冷氣壓縮機一個導輪(右)

驅動皮帶(drivingbelts) 水泵、 冷氣壓縮機及發電機的皮帶驅動則如圖7.20所示, 驅動皮帶是採用寬板V型多 槽式皮帶。此型式皮帶可允許在皮帶外側安裝導輪以便增加皮帶盤與皮帶的接觸面積。 為了維持正確的皮帶張力,故裝有自動皮帶張力調整器。 潤滑系統(lubeoilsystem) 如圖7.21所示的潤滑系統是透過由曲軸齒輪所驅動的機油泵,使潤滑油流經機油冷 卻器及濾清器後到達各個潤滑點。滑油自油底殼被機油泵(圖7.23)吸入之前,先經過 一個初級濾清器(圖7.24),過高的機油壓力將使機油泵及其他潤滑機件受到過高的應 力而發生損壞,故潤滑油出機油泵後會流經一安全閥。為了維持潤滑油循環不息且有效 地潤滑及冷卻每一個潤滑點,因此機油泵必須能隨時提供足夠的油壓。自機油泵後所有 的潤滑油,經汽缸體的孔道流經裝在引擎前側蓋內側的機油冷卻器(圖7.22),並藉由 引擎的冷卻水來冷卻潤滑油機,來防止因潤滑油溫度過高,油膜保持困難而造成各摩擦 機件的損壞。在機油冷卻器外殼上有一控制閥,它是控制活塞冷卻之用,在引擎怠速時 活塞不需冷卻。 圖7.21:潤滑系統

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圖7.22:機油冷卻器

機油濾清器(oilfilter) 潤滑油從機油冷卻器後,所有的潤滑油流過一個全流式濾清器(圖7.24),以過濾 鐵屑、碳粒等雜質,濾清器是採濾紙式。在濾清器座上有一旁通閥,它的功用是當濾清 器阻塞時,旁通閥將會打開,潤滑油將透過此閥流至引擎潤滑,但此時潤滑油並未經過 濾清。機油濾清器必須依據保養規範定期更換。如圖7.24所式的離心式濾清器係流經全 流式機油濾清器後,潤滑油流入主油道,在油道上有些潤滑油將被導入離心式濾清器過 濾後流回油底殼。為避免潤滑系統油壓過高,過多的潤滑油經由流量閥流回油底殼。在 離心式濾清器裏有一轉子,在轉子的底部都有兩個噴咀, 當轉子內部的油壓,將潤滑油 自噴咀噴出時,反作用力便會使轉子轉動。由於離心力的作用,雜質便被甩往轉子的壁 上集中,進而凝聚成固態廢棄物。

圖7.23:機油泵及入口管示意圖

圖7.24:全流式濾清器與離心式濾清器

潤滑油道(lubeoilpassage) 如圖7.25所示的潤滑油道係經由引擎本體上的機油道,潤滑油可至凸輪軸軸承及曲 軸主軸頸潤滑,曲軸內的油道再導引潤滑油至連桿大端的軸承處進行潤滑。從主油道直 接透過潤滑油道,使得潤滑油至搖臂潤滑。經由凸輪軸軸承上的油道,潤滑油到達舉桿 軸、舉桿軸上有油道可使潤滑油至舉桿潤滑。 引擎潤滑油同時也用來冷卻引擎活塞(圖7.26),在每一個汽缸有一機油噴咀,將 機油從活塞下面往上噴。 潤滑油透過活塞散熱片上的孔進入, 再由散熱片上另一孔流回。 圖7.25:潤滑油道示意圖

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圖7.26:活塞的潤滑油道示意圖 渦輪增壓機(turbocharger) 圖7.27所示的渦輪增壓機,其主要功用是增加進入汽缸中的空氣量,增加進氣量即 意味著可燃燒更多燃油,因此渦輪增壓引擎比自然進氣引擎可提供更多馬力。渦輪增壓 機是由一渦輪與增壓機組合而成,渦輪機是由引擎排出的廢汽所推動,而增壓機是把更 多的新鮮空氣壓入引擎的汽缸內。壓縮葉輪與渦輪葉輪兩者共同裝配在一支轉軸上(圖 7.28),而介於兩葉輪之間的是一軸承座殼。當引擎的輸出馬力升高時,也意味著就廢 氣排出量增加,因此會使得渦輪和增壓機葉輪的轉速同時提昇,在這種方式下,根本不 需要其他特殊控制的輔助,進氣量便能隨著引擎的需要自動作最恰當的控制了。渦輪增 壓機的轉速相當高,當引擎全負荷時,轉速約達每分鐘十萬轉,同時渦輪機的溫度則超 過600℃。因此對於渦輪增壓機旋轉部份的零件來說,平衡,冷卻與潤滑都是非常重要。 在軸承座殼中有兩個浮動軸承。另外,利用類似活塞環, 將渦輪機與增壓機兩端作密封 及隔離。

圖7.27:渦輪增壓機剖面圖

圖7.28:渦輪增壓機作動示意圖

7.3 MediumSpeedDieselEngines

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7.3.1 Introductionto Wrtsil L46DieselEngine The Wrtsil 46 (Fig. 7.3.1) is a mediumspeed engine for which reliability and total Economy have been the guiding principles. Extensive testing in Wrtsil modern diesel laboratorybyseveralthousandrunninghourshasmadetheWrtsil46areallyreliablediesel engine. Laboratory testing is fullscale engine testing: it covers various types of endurance testing, and also combustion measurements and system optimizations. All these confirm theoretical calculations, simulations as well as performance mapping ofsuch factors as heat balance,fuelandlubeoilconsumption,exhaustemission,noiseandvibrationlevel.

Fig. 7.29 General view of Wrtsil46 engine

The following is a summary ofWrtsil NSD’s approach to design and technology in the Wrtsil 46 engine [7.3]. The main technical data and rated power are listed in Table 7.3.1 and7.3.2, respectively. Table7.3.1:Maintechnical dataof Wrtsil 46marineengine

Table7.3.2:Rated power dataof Wrtsil46marineengine

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7.3.1.1 LowNOx Combustionand DirectWaterInjection Any hydrocarbon can be burnt provided the temperature is right and there is sufficient oxygen. However, the wayit is burnt has agreat effect onthermal efficiency and exhaust emissions, particularly NOxformation. Wrtsil NSDhas developed alow NOxcombustion process that reduces the NOxlevel up to50%without compromising onthermal efficiency. ThelowNOx combustiontechnologyisbasedon the following: (1) Ahighercombustionairtemperatureatinjectionstartdrasticallyreducestheignition delay (2) A retarded injection start and shorter injection period means that combustion takes placeat theoptimalpointwithrespectto efficiency (3) Improvedfuelatomization (4) Modifiedcombustionspacefor improvedmixing

Fig.7.30: ConventionalcombustionversusWrtsil 46lowNOxcombustion The engine with direct waterinjection is equipped with acombined injection valve and nozzle thatallows injectionofwaterandfueloilintothecylinder.This meansthatneitherof themodes (wateron/off) will affect theoperationoftheengine. Wateris fed tothecylinder head at high pressure, 210 bar. High water pressure is generated in a highpressure water pump module. The pumps and filters are built into modules to enable easy, and require a minimumofspace.Aflowfuseisinstalled onthecylinderheadside.Theflowfuseactsasa safety device, shutting off the water flow into the cylinder if the water needle gets stuck. Waterinjectiontiminganddurationiselectronicallycontrolledbythecontrolunit,whichgets itsinputfromtheengineoutput.NOx reductionof5060%canbereachedwithoutadversely affectingpoweroutput.

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7.3.1.2 MajorImprovementin Engine Components (1) Injectionpump As shown in Fig. 7.31, the monoelement design is a rigid and distortionfree solution even at high injection pressures. A constant pressure relief valve eliminates the risk of cavitationerosionbymaintainingaresidualpressure,whichisonasafeleveloverthewhole operating field. A drained and sealedoff compartment between the pump and the tappet prevents leakage fuel from mixing with lubricating oil. Precalibrated pumps are interchangeable. (2) Injectionvalve The valve (Fig. 7.32) is designed to have a small heat absorbing surface facing the combustionspacetogetherwithefficientheattransfertothecoolingwater. Thiseliminatesthe needforaseparatenozzletemperaturecontrol system.Thefuelistransportedtheshortestway from the pumpto thevalve,i.e.viaahighpressurepipeinthe cylinderhead.

Fig.7.3.3:Fuelpump

Fig.7.32 Injector valve

(3) Turbochargingsystem Wrtsil46isprovidedwithSpex(Singlepipeexhaust)systemandwithhighefficiency turbocharger (Fig. 7.33). The Spex turbocharging system is an exhaust gas system that combines the advantages of both pulse and constant pressure charging. Compared with a constant pressure system, the ejector effect of the gas pulses will provide better turbine efficiencyatpartialloads.The Spexsystem is practically freefrominterference.This means very small deviations in the scavenging between the cylinders and consequently an even exhaustgastemperature.Themodularbuiltexhaustgassystemsaredurableenoughtohandle highpressureratiosandpulselevels,butatthesametimeelasticenoughtocopewiththermal expansioninthesystem.Theturbochargerhasthehighestavailableefficiency(Fig.7.34). The turbochargeris equipped withplain bearings andthereis nocooling water.Theturbocharger is fitted with cleaning devices forboththe compressor and the turbine side. Exhaust waste gate and air bypass are used to obtain specific requirements on the operating range, load responseorpartialload.

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Fig.7.33: Wrtsil 46Spex (Singlepipeexhaust)system

Fig.7.33: Wrtsil46 turbocharger (4) Piston andpistonringset Acompositelowfrictionpistonwithanodularcastironskirtandasteeltopasshownin Fig.7.34.Thespecialcoolinggallerydesignassuresefficientcoolingandhighrigidityforthe piston top. The design can handle combustion pressures beyond 200 bar. Hardened top ring groovesassurealonglifetime.Lowfrictionisensuredbytheskirtlubricationsystem.Awell distributedcleanoilfilm thateliminatestherisk ofpistonring scuffing andreducesthewear rate. Cleaner rings and grooves free from corrosive combustion products. Hydraulically dampedtiltingmovementsprovidedbyanoilpadbetweenthelinerandthepiston,resultingin lessnoiseandwear.

Fig.7.34: Generalviewof Wrtsil46piston

Fig.7.35:GeneralviewofWrtsil46piston ringsets

Pistonringset(Fig.7.35)isprovenoflowfrictionthathasspecialwearresistantcoating for the compression rings and dimensioned and profiled for maximum sealing and pressure balance. (5) ConnectingRod Theconnecting rod(Fig. 7.36)is athreepiece marine design, wherecombustion forces

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are distributed over a maximum bearing area and where the relative movements between mating surfaces are minimized. Piston overhauling is possible without touching the big end bearingandthebearingcanbeinspectedwithoutremovingthepiston.Thethreepiecedesign alsoreducesthepistonoverhaulingheight. Allnutsaretightenedwithhydraulictool.

Fig.7.36: Crosssectionalviewof Wrtsil46 Fig.7.37: Generalviewof Wrtsil 46 cylinderliner connectingrod (6) CylinderLinerandAntipolishingRing Cylinder liner (Fig. 7.37)deformations arenormally causedby cylinder head clamping, thermal and mechanical load. A special design with a high collartostroke ratio, the deformations in this liner are very small. A round liner bore in combination with excellent lubricationimprovesconditionsforthepistonringsandreduceswear.Toeliminatetheriskof borepolishing,thelinerisprovidedwithanantipolishingringintheupperpart.Thepurpose ofthis ring is to “calibrate” the carbon deposits on the piston top land to a thickness small enoughtopreventcontactbetweenthelinerinnerwallandthedepositsonthepistontopland. “Borepolishing” can lead to local liner wear and increased lube oil consumption. The temperature distribution in the cylinder liner is important not only in terms of stress and deformation but also decisive for the cylinder liner wear rate.The temperature must remain abovethesulfuricacid dewpointtoavoidcorrosion,butatthesametimeremainsufficiently lowtoavoidlubricatingoilbreakdown.Thematerialcompositionisbasedonlongexperience withthespecialgraycastironalloydevelopedfor excellentwearresistanceandhighstrength. (7) Crankshaftandbearings The crankshaft design allows foruse ofhigh combustion pressure and still maintains a conservative bearingload.Asshownin Fig.7.38,thecrankshaftis forgedinonepiece,fully machinedandrigidduetomoderatebore/strokeratioandlargepinandjournaldiameters.The crankshaft also fitted with counterweights onevery crank web. Itis designed forfull power takeoff,alsofrom thefreeend. Fig. 7.38: General view of Wrtsil46 crankshaft

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Fig. 7.39: General view of Wrtsil46 bearingpad

Thethickpad bearing (Fig. 7.39)design emphasizesonekeyconceptofreliability. The bearingloadshavebeenreducedbyincreasingcrankshaftjournalandpindiametersaswellas length. Lowbearing loads allow forsofterbearing materials with greaterconformability and adaptability.Thismakesthe bearingvirtuallyseizurefree. (8)Coolingandlubricationoilsystems The fresh water cooling system (Fig. 7.40) is divided into high temperature and low temperaturecoolingsystem.Thehightemperaturecoolingwatersystemoperatesconstantlyat ahigh temperaturelevel tomake thetemperaturefluctuations in thecylinder componentsas smallaspossibleandpreventingfromcorrosionduetoundercooling.Forobtainingmaximum heatrecovery thechargeaircoolerissplitintoahighandlowtemperaturesection. Enginedrivenpumpscanbeprovidedasanoptionfor marineapplication.In powerplant application,thesearestandard.

Fig. 7.40: Schematic of Wrtsil 46 cooling Fig. 7.41: Schematic of Wrtsil 46 system lubricationoilsystem Marineengineshavedrysumpandpowerplantengineswetsump.Thelubeoilistreated outsidetheenginebycontinuousseparating.Onthewaytotheengine,theoilpassesthrough a lubeoilcooler,afullflowautomaticfilterunit andasafetyfilterforfinalprotection.Forthe purposeofrunningin,provisionhasbeenmadeformountingspecialrunninginfiltersin the crankcase in frontofeachmain bearing. Engine drivenlube oilpump canbe provided asan option for marineapplication.In power plantapplication thisisstandard.

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Chapter7MarineDieselEngines
7.1 Prospects inMediumSpeedDieselEngines 7.1.1 Meritsof Diesel EnginesandGasTurbineinMarinePropulsion 7.1.1.1 Weightandsize 7.1.1.2 Firstandmaintenancecosts 7.1.1.3 Fuelandoperatingcosts 7.1.1.4 Vibration,noiseandlubeoilconsumption 7.1.1.5 Efficiency 7.2 HighSpeedDieselEngines 7.2.1 IntroductiontoSCANIA 12LiterEngine 7.3 MediumSpeedDieselEngines 7.3.1 IntroductiontoWrtsil46 DieselEngine

Reference:
[7.1]Koehler, H.W.,2000,“DieselEngines andGasTurbines inCruiseVesselPropulsion”, TheInstitutionof DieselandGasTurbineEngineers,London. [7.2]MAN B&W,2001,“Very Large Diesel Engines forIndependent PowerProducers and CaptivePower Plants”, Denmark. [7.3] Wrtsil NSD, 2001, “Wrtsil 46TechnologyReview”, Finland.

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