Chapter 7 Marine Diesel Engines

Chapter 7 Marine Diesel Engines

Edited by Kee-Rong Wu

7.1 Prospects in Medium-Speed Diesel Engines

7.1.1 Merits of Diesel Engines and Gas Turbine in Marine Propulsion

This session will concentrates on diesel engines for cruise vessel applications with a total power requirement of approx. 60 MW each installation. A typical diesel-electric drive with five medium-speed diesel engines will be compared with the 58 MW COGES (Combined Gas Turbine and Steam Turbine Integrated Electric Drive System) [7.1].

In summer 2000, Celebrity Cruises' gas turbine-driven cruising vessel Millennium made her maiden voyage. The 91000 Ton vessel with a Pax capacity of 1950 (lower berth) denotes a technological shift in cruise ship design, primarily because she is the first cruise ship powered by a pure gas turbine plant. Apart from this, the ship has the biggest azimuth pods ever built (two Mermaid pods of 19.5 MW each). Currently there are three further cruise ships of this series under construction. This certainly is a milestone for gas turbine movers, the more so as four further new Vantage-class cruise ships for Royal Caribbean International (RCI) are also specified with turbine-based propulsion plants. Each plant consists of two General Electric LM2500+ aero-derived gas turbines of 25 MW each and an 8 MW back-pressure steam turbine. The steam turbine uses steam from the boilers fired by waste-heat from the gas turbines to generate additional electrical power. Depending on the amount of steam required for onboard services, the complete COGES power plant is expected to achieve a combined-cycle efficiency of between 45 and 50%. This system will provide for all onboard power arrangements, such as propulsion, heating, cooling, lighting, ventilation, kitchen and laundry. However, with about 97% of all existing sea-going ships propelled by two and four-stroke diesel engines (Fig. 7.1) due to its comparably high thermal efficiency (Fig. 7.2), it seems their manufacturers have so far not seriously been affected by gas turbines in most of their traditional market areas.

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Fig. 7.1: A cross-sectional view of MAN B&W V40/50 four-stroke Diesel engine

Fig. 7.2: Power efficiency comparison at ISO 3046 [7.1]

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In general, diesel engines posses lower initial costs, fuel economy, weight and size as comparing with gas turbine’s environmental friendliness showing Fig. 7.3 and 7.4. The detail comparison will be described in followings.

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Fig. 7.3: Diesel engines versus gas turbines pro and cons [7.1]

Fig. 7.4: NOx emission of marine prime movers [7.1]

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7.1.1.1 Weight and size

Gas turbines are known to generate lots of power while 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 Panamax-sized cruise ships with an increased number of decks but unchanged width, much weight is needed in the bottom of a ship for stability purposes, so the value of the weight savings by gas turbines must not be over emphasized. In order to decrease the vertical center of gravity, this weight deficiency could be compensated by additional fresh water or fuel tanks. Another option is to slightly decrease the main deck height or draught of the vessel. However, all of this 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 to cut costs. As regards the space savings of gas turbines, this potential cannot be fully utilized: Gas turbines have approx. 15% larger air intake and exhaust ducts as comparable diesel engines and their starting devices 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 equipment for this job, including storage space for a complete spare gas turbine. Finally, plant availability and safety considerations make at least one or two additional diesel engine gensets mandatory to satisfy low power requirements difficult to cover with a gas turbine and as emergency generator. This does not only restrict the freed space further, but also increases first costs, operating costs and maintenance costs.

7.1.1.2 First and maintenance costs

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 of 3 $/MW h. The maintenance cost summary of a multi-engine Diesel-electric gives a lower figure.

7.1.1.3 Fuel and operating costs

As indicated in Fig. 7.2, Diesel engines enjoy further benefits such as lower fuel prices, lower fuel consumption rates at all loads and therefore lower carbon dioxide emission and better load acceptance as well as quicker start-up times after a night stop. For instance, after a night stop, a gas turbine in simple-cycle mode needs 30 minutes until full load is reached, a diesel engine in the same situation less than 5 minutes.

7.1.1.4 Vibration, noise and lube oil consumption

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 direct-resilient mounting of Diesel engines, their structure-borne vibration transmitted into a ship foundation is reduced to a level of approximate below 50 dB at frequencies of above 1000 Hz. Although resiliently seated gas turbines might reach still lower values, design measures aiming at an even further decrease in diesel engines' structure borne noise can be omitted as long as the requirements regarding vibration in the cabins are met.

Unexpectedly the new-building Millennium experienced vibration problems in some areas of the ship under special sea conditions likely to occur in the Caribbean during the windy winter season. The ship had to be dry-docked for technical modifications earlier than planned, following its arrival in New York in November 2000.

Air-borne engine room noise of gas turbines is claimed to be less than 85 dB(A), whereas the noise emission of a MAN B&W large-bore medium-speed diesel engine varies between 102 and 108 dB(A) at full load. The main reason for this difference is that marine gas turbines are installed in acoustically insulated enclosures whereas the noise level for free-standing diesel engines is measured without any sound-attenuating encapsulation or lagging. Engine machine rooms are not among the places where passengers onboard usually spend their leisure time. Therefore the lower running noise of gas turbines is not of major importance: outside of the machine room, the diesel engines can be considered to be encapsulated as well.

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 low-priced mineral oils for the Diesel engines. The annual lube oil costs of gas turbines are only about 6% of that of diesel engines. It has to be pointed out that this merit is of minor importance, since lube oil costs hardly affect the total operating costs.

The real advantage of the gas turbine is its eco-friendliness as far as SOx and NOx (not CO2) emissions are concerned. SOx emission of gas turbines is close to zero because they burn basically sulfur-free fuel (MGO typically contains only about 0.3 % sulfur, HFO for diesel engines up to 4.5%). If (higher-priced) low-sulfur or sulfur-free marine diesel oils would be used for diesel engines, there wouldn't be a SOx problem with them either. NOx emission levels of modern marine gas turbines and diesel engines are listed in Fig. 7.4. There is no basic technical restriction in decreasing diesel engines NOx emission down to a level of 2 g/kWh by adopting SCR based exhaust-gas 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 -internal measures). By direct injection of water into the cylinder or by adopting water-fuel 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 February 2000: a NOx cycle value of 7.7 g/kWh and a fuel consumption rate still within tolerance (5%) was measured. This is 40% below the NOx limit set by the IMO. This result was achieved with only 15% water in the water-fuel emulsion and a slightly retarded injection below 80% engine load [7.1].

7.1.1.5 Efficiency

Figure 7.2 shows the achievable overall efficiency level of today's prime movers. Large-bore medium-speed engines reach up to 47% in simple-cycle operation and low-speed 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 combined-cycle gas turbines of high unit output (above 50 MW) reach today. Up to now there are only few diesel combined-cycle (DCC) installations in operation. Their number will increase in future although this technology will increase the diesel’s efficiency level only by a few percentage points.

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Fig. 7.5: Typical part load efficiencies of prime movers [7.2]

Fig. 7.6： Annual fuel costs of COGES versus diesel-electric system [7.1]

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The specific fuel consumption rates (SFOC) in g/kWh over the total electrical plant output for the various propulsion concepts are plotted in Fig. 7.5. From 90% power down to approx. 60% power, the thermal efficiency is almost constant. Contrary to this favorably flat fuel consumption line, the turbines' consumption rates are highly load-dependent. At very high rate power, COGES has a thermal efficiency of around 40%, a value that is only slightly higher than that of diesel engines' figure indeed. However, this high electrical power is hardly used in cruising: most of the time, the turbines have to operate at part load with much higher specific fuel consumption rates.

Calculation of the annual fuel costs was based on the following typical weekly load scenario:

60 hours per week in ports (power requirement 10 MW):

One 12V 48/60 Diesel engine or one gas turbine with the steam turbine in operation This sums up to 3 840 operating hours per year for each of the five diesel engines, and 6 150 hours for each of the two gas turbines. For this load profile and for average August 2000 fuel prices for North West Europe, the total fuel costs are shown in Fig. 7. The difference in annual fuel costs between COGES and the diesel-electric option is US-$ 7 million.

The total sum of fuel costs (Fig. 7.6) and lube oil costs is US-$ 13.86 million for COGES and US-$ 7.04 m for the diesel-electric system. The difference is US-$ 6.8 million per year. With the total annual net profit of only US-$ 2.1 million, it is impossible to compensate higher fuel bill of COGES. With bunker prices in September 2000, there is a loss of US-$ 4.7 million every year and for every ship and this does not include the higher first and maintenance costs.

In comparison to a COGES system, diesel-electric 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 CO2 emission, lower first costs, operating costs, easier maintenance, lower maintenance costs and wider operational flexibility and redundancy on account of the larger number of diesel engines that are able to burn widely varying fuel qualities. The gas turbine itself, as an intrinsically simple rotating machine, is highly reliable and durable as it has fewer moving parts and lower friction losses, but the more complicated COGES system involving a steam turbine genset had no chance up to now to prove its availability and long-term reliability in cruise shipping.

7.2 High-Speed Diesel Engines

7.2.1 Introduction to SCANIA 12-Liter Engine

The following description is referred from the user’s manual of SCANIA 12-liter diesel engine.

圖7.7：SCANIA 12-liter engine外觀圖

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圖7.8：汽缸體外觀圖

圖7.9：汽缸套剖面圖

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汽缸支架或汽缸本體（cylinder frame）

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（cylinder liner）

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。

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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

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汽缸體（cylinder block）

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（oil ring），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（connecting rod）

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。

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圖7.13：連桿及活塞組件圖

圖7.14：連桿大端及軸瓦組立圖

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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（crankcase ventilation）

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

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

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圖7.16：閥機構組立圖

圖7.17：閥門座及進排氣閥的佈置圖

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汽門機構（valve mechanism）

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(Valve arm)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（timing gear）

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）。

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圖7.18：引擎裝配噴射泵的正時齒輪

圖7.19：引擎裝配噴射泵的正時齒輪

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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（driving belts）

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（lube oil system）

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。

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圖7.21：潤滑系統

圖7.22：機油冷卻器

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機油濾清器（oil filter）

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。

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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

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潤滑油道（lube oil passage）

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。

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圖7.25：潤滑油道示意圖

圖7.26：活塞的潤滑油道示意圖

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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。

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圖7.27：渦輪增壓機剖面圖

圖7.28：渦輪增壓機作動示意圖

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7.3 Medium-Speed Diesel Engines

7.3.1 Introduction to Wärtsilä L46 Diesel Engine

The Wärtsilä 46 (Fig. 7.3.1) is a medium-speed engine for which reliability and total Economy have been the guiding principles. Extensive testing in Wärtsilä modern diesel laboratory by several thousand running hours has made the Wärtsilä 46 a really reliable diesel engine. Laboratory testing is full-scale 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 of such factors as heat balance, fuel and lube oil consumption, exhaust emission, noise and vibration level.

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Fig. 7.29 General view of Wärtsilä 46 engine

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The following is a summary of Wärtsilä NSD’s approach to design and technology in the Wärtsilä 46 engine [7.3]. The main technical data and rated power are listed in Table 7.3.1 and 7.3.2, respectively.

Table 7.3.1: Main technical data of Wärtsilä 46 marine engine

Table 7.3.2: Rated power data of Wärtsilä 46 marine engine

7.3.1.1 Low NOx Combustion and Direct Water Injection

Any hydrocarbon can be burnt provided the temperature is right and there is sufficient oxygen. However, the way it is burnt has a great effect on thermal efficiency and exhaust emissions, particularly NOxformation. Wärtsilä NSD has developed a low NOxcombustion process that reduces the NOx level up to 50% without compromising on thermal efficiency. The low NOxcombustion technology is based on the following:

(1)A higher combustion air temperature at injection start drastically reduces the ignition delay

(2)A retarded injection start and shorter injection period means that combustion takes place at the optimal point with respect to efficiency

(3)Improved fuel atomization

(4)Modified combustion space for improved mixing

Fig. 7.30: Conventional combustion versus Wärtsilä 46 low NOx combustion

The engine with direct water injection is equipped with a combined injection valve and nozzle that allows injection of water and fuel oil into the cylinder. This means that neither of the modes (water on/off) will affect the operation of the engine. Water is fed to the cylinder head at high pressure, 210 bar. High water pressure is generated in a high-pressure water pump module. The pumps and filters are built into modules to enable easy, and require a minimum of space. A flow fuse is installed on the cylinder head side. The flow fuse acts as a safety device, shutting off the water flow into the cylinder if the water needle gets stuck. Water injection timing and duration is electronically controlled by the control unit, which gets its input from the engine output. NOxreduction of 50-60% can be reached without adversely affecting power output.

7.3.1.2 Major Improvement in Engine Components

(1) Injection pump

As shown in Fig. 7.31, the mono-element design is a rigid and distortion-free solution even at high injection pressures. A constant pressure relief valve eliminates the risk of cavitation erosion by maintaining a residual pressure, which is on a safe level over the whole operating field. A drained and sealed-off compartment between the pump and the tappet prevents leakage fuel from mixing with lubricating oil. Precalibrated pumps are interchangeable.

(2) Injection valve

The valve (Fig. 7.32) is designed to have a small heat absorbing surface facing the combustion space together with efficient heat transfer to the cooling water. This eliminates the need for a separate nozzle temperature control system. The fuel is transported the shortest way from the pump to the valve, i.e. via a high-pressure pipe in the cylinder head.

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Fig.7.3.3: Fuel pump

Fig. 7.32 Injector valve

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(3) Turbocharging system

Wärtsilä 46 is provided with Spex (Single pipe exhaust) system and with high efficiency 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 efficiency at partial loads. The Spex system is practically free from interference. This means very small deviations in the scavenging between the cylinders and consequently an even exhaust gas temperature. The modular-built exhaust gas systems are durable enough to handle high-pressure ratios and pulse levels, but at the same time elastic enough to cope with thermal expansion in the system. The turbocharger has the highest available efficiency (Fig. 7.34). The turbocharger is equipped with plain bearings and there is no cooling water. The turbocharger is fitted with cleaning devices for both the compressor and the turbine side. Exhaust waste-gate and air by-pass are used to obtain specific requirements on the operating range, load response or partial load.