Wartsila Technical Journal 02 2010

8/15/2019 Wartsila Technical Journal 02 2010

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WÄRTSILÄ TECHNICAL JOURNAL 02.2010

Cylinder lubrication of two-strokecrosshead marine diesel enginesA U T H O R : O l e C h r i s t e n s e n , G e n e r a l M a n a g e r , C y l i n d e r L u b r i c a t i o n , R & D T w o - s t r o k e P r o g r a m s & T e c h n o l o g i e s ,

W ä r t s i l ä I n d u s t r i a l O p e r a t i o n s

The two-stroke crosshead diesel

engine has, for many years, been

the preferred prime mover for larger

seagoing merchant vessels. This

article discusses means of improving

piston running and reducing cylinder

oil consumption in these engines.

Tere are a number of reasons for thesuccess of the two-stroke crossheaddiesel engine in marine applications:■ It provides a speed and torque

that exactly meets the demandof the propeller, i.e. no

intermediate gear is needed.■ It is easy to maintain.■ It is – still today – by far the most

efficient stand-alone working machine.■ It can operate on heavy fuel oil, which

is basically a residual from the crude oilrefinery process, and hence much lessexpensive than refined oil products.

The challenge posed by fuelCompared with “lighter” diesel fuels,heavy fuel oil (HFO) has a relatively highviscosity. It also contains a number of

substances and components that can beharmful to both the engine and theenvironment if their maximum allowedcontents are exceeded, or if not treatedproperly.

However, residual marine fuels arecategorised in international standards,and all marine engine designers have theirrecommendations as to which types offuels can be applied in a particular engine,and how these fuels shall be treated andconditioned before entering the engine.

Every time a vessel bunkers fuel, the

ship’s chief engineer is given a bunkercertificate with a chemical analysis andinformation about the fuel’s viscosity anddensity. From this he can see whether or notthe fuel is in compliance with the engine

Fig. 1 – Cross section of

Wärtsilä RT-flex82C.

Fig. 2 – Wärtsilä 7RT-flex82C on test bed.

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[ MARINE / IN DETAIL ]

[ M A R I N E / I N D

E T A I L ]

designer’s recommendations, and howthe on board fuel treatment plant needs

to be adjusted.For environmental reasons, marine dieselengines must be capable of operating onthe whole range of diesel fuels. Tis createsnumerous challenges to the machineryon board, and to the engines themselves.

From an engine reliability point of view,and particularly from a piston runningpoint of view, there are two major “enemies”

when the engine is operated on HFO:■ Catalyst particles from

the refining process■ Te natural sulphur content.

In some HFO bunker lots, the content ofcatalytic particles, also known as “catalyticfines” or simply “cat fines”, from therefining process can be extraordinarily high.

Tese catalyst particles consist mainlyof aluminium and silicon oxides typicallyranging in size from 10 to 20 µm. Tey areextremely hard and, therefore, abrasive. Ifthey are allowed to enter the engine, theycan cause severe damage to the fuelinjection system, as well as to the piston

rings and cylinder liners.o safeguard the engine against such cat

fine damage, the allowed maximum content(as Al and Si equivalents) in marine dieselfuels has been set as an internationalstandard. However, it is most importantthat the centrifuges and filters in the vessel’son board fuel treatment plant are correctlydimensioned and properly working atall times.

Crude oil has a natural sulphur content, which will remain in the heavier fractionsof refinery products such as marine dieselfuels. Te average sulphur content in HFO

worldwide is around 2.8% (of mass), but

HFO with a sulphur content of up to 4.5%, which is also the allowed upper limit,is occasionally bunkered.

oday there are no economically viablemethods, either at the oil refineries or onboard ship, to separate the sulphur frommarine diesel fuels. Tis means that thesulphur cannot be prevented from enteringthe engine’s fuel injection system andcombustion space.

Sulphur is an excellent lubricant forthe moving parts in the fuel injection

Parameter Unit Bunker limit

ISO 8217: 2005class F, RMK700

Test method

*1)

Required fuel quality

Engine inlet

Density at 15 C [kg/m3] max. 1010 *2) ISO 3675/12185 max. 1010

Kinematic viscosity at50 C

[mm2/s (cSt)]

–700

ISO 310413–17

–

Garbon residue [m/m (%)] max. 22 ISO 10370 max. 22

Sulphur [m/m (%)] max. 4.5 ISO 8754/14596 max. 4.5Ash [m/m (%)] max. 0.15 ISO 6245 max. 0.15

Vanadium [mg/kg (ppm)] max. 600 ISO 14597/IP501/470 max. 600

Sodium [mg/kg (ppm)] – AAS max. 30

Aluminium plus Silicon [mg/kg (ppm)] max. 80 ISO 10478/IP501/470 max. 15

Total sediment, potential [m/m (%)] max. 0.10 ISO 10307-2 max. 0.10

Water [v/v (%)] max. 0.5 ISO 3733 max. 0.2

Flash point [°C] min. 60 ISO 2719 min. 60

Pour point [°C] max. 30 ISO 3016 max. 30

Remark: *1) ISO standards can be obtained from the ISO Central Secretariat, Geneva, Switzerland (www.iso.ch).*2) Limited to max. 991 kg/m3 (ISO-F-RMH700), if the fuel treatment plant (Alcap centrifuge) cannot remove water

from high density fuel oil (excludes RMK grades).– The fuel shall be free from used lube oil, a homogeneous blend with no added substance or chemical waste

(ISO8217:2005–5–1).

Table 1. – Wärtsilä fuel recommendations.

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system and has no negative impact on thecombustion process. However, in

combination with water it creates sulphuricacid, which is highly corrosive. In other words, since it cannot be eliminated beforeentering the engine, it must be dealt withand controlled by other measures, as we

will see later on.

Cylinder lubricationFor marine diesel engines operating onresidual fuels containing sulphur, cylinderlubrication must generally servethe following purposes:■ Create and maintain an oil film to

prevent metal to metal contact betweenthe cylinder liner and piston rings.

■ Neutralise sulphuric acid inorder to control corrosion.

■ Clean the cylinder liner, andparticularly the piston ring pack,to prevent malfunction and damagecaused by combustion andneutralisation residues.

The four-stroke trunk piston engine caseIn four-stroke trunk piston engines, there

are a number of different methods forlubricating the cylinder liners and pistonrings, depending on engine size and make:■ Splash from the revolving crankshaft■ “Inner lubrication”, where the oil

is supplied from the piston side■ “Outer lubrication”, where the

oil is supplied by an external,separate cylinder lubricating devicefrom the cylinder liner side.

In a four-stroke trunk piston engine,the cylinder lubricating oil is identicalto the engine system oil used for bearing

lubrication and cooling purposes. A small amount of the cylinder

lubricating oil by-passes the piston ringsand ends up in the combustion space,

where it is “consumed”. However, thepiston in a four-stroke trunk piston enginehas an oil scraper ring that scrapes mostof the oil supplied to the cylinder linerback to the engine’s oil pan, from whereit is drained, cleaned and recycled.

Normally, a large, modern, wellmaintained four-stroke trunk piston diesel

Fig. 3 – Example of on board fuel treatment plant.

engine will consume some 0.3 to 0.5 g/kWhof lubricating oil.

The two-stroke crosshead engine caseIn the four-stroke trunk piston engine, thecylinder liner is virtually “over-lubricated”

with, as mentioned above, an oil scraperring on the piston scraping the surplus oilback to the oil pan. However, as can beseen in Figure 1, the two-stroke crossheadengine has no connection between thepiston underside space and the bedplate

with the oil pan, and hence cylinderlubrication differs considerably fromthe four-stroke trunk piston engine.

In the two-stroke crosshead engine,the piston has no oil scraper ring and thecylinder oil is not recycled and reused,i.e. once it has left the lubricating deviceit is virtually “lost”, which means thatthe dosage of cylinder oil is crucial.

Te cylinder lubricating oil in a two-stroke crosshead engine is – regardless ofengine size and make – usually suppliedfrom an external, separate cylinderlubricating device via quills in the cylinderliner.

*1)

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E T A I L ]

Fig. 4 – CLU3 system.

Fig. 5 – Pulse lubricating system.

Special two-stroke cylinderlubricating oil

In the two-stroke crosshead engine, theseparation between the piston undersidespace and the bed plate with the oil panallows the use of special cylinder lubricatingoils, tailored for the cylinder conditionand the fuel used. Tis provides a certainflexibility, which the four-stroke trunkpiston engine does not have.

wo-stroke cylinder lubricating oil ismainly characterised by:■ Viscosity grade Normally SAE50.■ Base Number (BN) Te BN

corresponds to the content ofalkaline additives, which are usedfor neutralising the sulphuric acid.

When operating on fuels containing1.5 to 4.5% of sulphur, BN70 oil isnormally used, but when operating forlonger periods on fuels with a lowersulphur content, a lower BN (BN60,BN50 or BN40) is recommended.

■ Detergent additives Te ability toclean the cylinder liner and piston ringpack and minimise deposit formation.

Dosage As mentioned above, the cylinderlubricating oil dosage is vitally importantin a two-stroke crosshead engine. Techallenge is to make the oil properly and

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effi ciently fulfil its tasks before it is “lost”,partly to the combustion space where it is

burned, and partly to the piston undersidespace as sludge.For more than 20 years, CLU3 was the

standard cylinder lubricating oil dosagesystem on Wärtsilä two-stroke crossheadengines. Te CLU3 system was developedin co-operation with the German companyVogel (today SKF). It consists of a multi-element pump unit driven by an electricmotor, and a so-called progressivedistributor for each cylinder unit witha number of quills with a small springloaded membrane accumulator.

Te multi-element pump unit suppliesthe cylinder lubricating oil to theprogressive distributors, ensuring equaldistribution of the oil to each individualquill. Te oil is accumulated in the quills,and when the pressure inside the cylinderat the quill level, which is normally locatedin the upper third of the cylinder liner, issuffi ciently low, the oil is released by thespring force of the membrane accumulator.

Te CLU3 system releases a smallamount of oil to the cylinder liner in eachengine cycle, but the release of the oil is

not timed. Te feed rate is controlled bydisc settings in the multi-element pumpunit, and by varying the rotational speedof the driving electric motor.

Te CLU3 system is simple, robust andvery reliable, but normally requiresa cylinder lubricating oil feed rate inthe range of 1.0 to 1.6 g/kWh.

Due to the increasing price of cylinderlubricating oil, for exhaust gas emissionand environmental impact reasons, andfollowing the introduction of the WärtsiläR-flex engine concept, there has been a

need to find a successor to the CLU3system. Te successor product shouldfit into the concept of the electronicallycontrolled engine, but should also beapplicable to the conventional WärtsiläRA engine and – most importantly –should facilitate lower cylinder lubricatingoil consumption.

Development work for a new cylinderlubricating oil dosage pump was againcarried out in co-operation with Vogel,and in 2006 the CLU4 pump and PulseLubricating System (PLS) were introduced.

Te PLS comprises one CLU4 pumpand 6 to 8 quills per cylinder unit. Itfurther comprises a 50 bar servo oilsystem, which actuates the CLU4 pumps,and a control system.

A

L

T

P

Servo oil in (P)

Servo oil out (T)

Lubricating oil in (L)

Lubricating oil out (A)

Fig. 6 – CLU4 pump.

Te CLU4 pump is a hydraulic, positivedisplacement device, with a number ofindependent cylinder lubricating oil outletscorresponding to the number of quills inthe cylinder liner. Te pump piston isdriven by servo oil, which is pressurised to50 bar, and when the 4/2 solenoid valveis activated, the pump piston delivers afixed, pre-defined amount of cylinderlubricating oil to each quill in the cylinderliner. When the 4/2 solenoid valve isde-activated, the pump piston returns toits starting point, the cylinder lubricating

oil chambers are re-filled from the gravitytank, and the pump is on stand-by forthe next delivery stroke.

As in the CLU3 system, the quills ofthe PLS are mounted in the upper third of

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E T A I L ]

the cylinder liner, but thanks to the CLU4pump, the injection of cylinder lubrication

oil is independent of the pressure in thecylinder. Tis injection time is in the rangeof 8–10 ms, and thus a timed injection ispossible, i.e. it is possible to inject thecylinder lubricating oil into the piston ringpack when the piston rings pass the quilllevel, which is the basic philosophy ofthe PLS.

As mentioned above, the CLU4 pumpdelivers a fixed, pre-defined amount ofcylinder lubricating oil to each quill witheach pump stroke. Te control systemcontinuously measures and calculates the

actual engine load, speed and crankshaftposition, and based on the requested feedrate, calculates the necessary injectionfrequency. Te maximum injectionfrequency is once each engine revolution,and the cylinder lubricating oil is alwaysinjected during the upward stroke ofthe piston.

Te Pulse Lubricating System with theCLU4 pump has proven to be as robustand reliable as the CLU3 system, but issuperior in terms of flexibility, and itfacilitates a cylinder lubrication oil feed

rate in the range of 0.7 to 0.9 g/kWh, whichis substantially below the CLU3 level.

DistributionOnce the dosage pump has delivered thecylinder lubricating oil via the quills in thecylinder line to the piston ring pack, thekey to success is to facilitate and ensurea proper distribution of the oil onthe cylinder liner running surface.

Te challenge is not only to ensurethat the oil film on the cylinder liner

wall is well maintained, but also

continuously refreshed in order toprovide sufficient additives for the acidneutralization and cleaning processes.

For example, the Wärtsilä 84 enginehas a bore of 84 cm and a stroke of 315 cm.Each cylinder liner, therefore, hasa running surface of 8.3 m² to belubricated. Each cylinder liner has 8quills equally distributed at the samelevel 900 mm from the top, the cylinderoil dosage pump delivers 310 mm

3 to

each quill per stroke, and dependingon the engine load and the applied

cylinder oil feed rate, the dosage pumpinjects every 2 to 5 engine revolutions.Consequently, a very large cylinder linerrunning surface has to be maintained bymeans of a very small amount of oil.

How is this possible when the cylinderoil must be distributed both vertically and

horizontally?Te vertical distribution of the cylinderoil is mainly performed by the piston ringsduring their reciprocating movement. Tecylinder oil is injected into the piston ringpack during the piston’s upward stroke,and factors such as oil viscosity and feedrate, as well as the amount of oil perinjection are important here. A correctviscosity is important in order to ensurethe spreadability of the cylinder oil, andthe applied feed rate and injected amountof oil per stroke are key factors in the

delicate balance between under- and over-lubrication:■ Under-lubrication If too little

cylinder oil is supplied, starvation will occur which might result incorrosion, accumulated contaminationfrom unburned fuel and combustionresidues, and in the worst case, metalto metal contact, known as “scuffing”.

■ Over-lubrication If too muchcylinder oil is supplied, the loss offresh, unused oil in the scavenge ports

will be high, and the piston rings

might be prevented from moving(rotating) in their grooves by the so-called “hydraulic lock”. Furthermore,the cylinder liner running surfacestructure might over time becomeclosed and smooth like a mirror, and

will no longer be able to retain thelubricating oil. Tis is sometimescalled “chemical bore polish”, and

when alkaline deposit build-up on thepiston top land from excessive cylinderoil is in contact with the cylinderliner running surface, it can cause

what is sometimes called “mechanicalbore polish”. All of these phenomenamight eventually result in scuffing.

In attempting to achieve the best possiblehorizontal or circumferential distributionof the cylinder oil, the first executionof the quill for the Pulse LubricatingSystem was the so-called Pulse Jet.

Te Pulse Jet quill could deliver thecylinder oil either into the piston ring packor directly to the cylinder liner runningsurface, and was expected to provide safeand reliable operation.

Service experience, however, revealed anumber of piston running problems andscuffing incidents, particularly on the 96Cengines. Te Pulse Jet principle was thenreplaced by the so-called Pulse Feed

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Fig. 7 – Pulse Jet principle.

Fig. 8 – Pulse Feed principle with and without “zig-zag” distribution groove.

principle, which had shown goodperformance on engines where the old

CLU3 systems had been retro-fitted withthe PLS.

Nevertheless, here too there wereproblems. Initially the cylinder liner justhad outlets from the quills, but soon itbecame obvious that some additional aid

was needed to achieve a proper horizontaldistribution of the cylinder oil, andconsequently the so-called “zig-zag” groovebetween the quills was introduced.

As the piston passes the “zig-zag” groove,the groove makes a short circuit and thepressure difference transports the oil

concentrated at the quill outlet towardsthe groove.

Te “zig-zag” groove principle has provento work very well after it was retro-fitted toa high number of large bore cylinder liners

Piston Pressure high

Pressure low

Top ring

Second ring

Fig. 9 – Function of the “zig-zag” distribution groove.

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Fig. 10 – Example of good condition.

system, which can be installed on all newengines, but also retro-fitted to engines

already in service. Tis system measuresthe temperature in two diametricallyopposite positions near the running surfacein the upper part of each cylinder liner. Itthen filters and interprets the developmentof the temperatures, and in case thetemperature level escalates, a “high frictionalarm” is generated. Te alarm alerts theengine crew only, and is not applied inthe cylinder lubrication control system.

Economical and environmental aspectsMore than ever before, ship owners and

operators are focusing on their operationalcosts, and cylinder lubricating oil costshave become extremely relevant.

Example:

A 6500 EU container vessel has a12-cylinder Wärtsilä RA96C main engine

with a MCR power of 63,000 kW. Temain engine operates 6000 hours per yearat an average load of 65% of MCR. It usesa CLU3 cylinder lubricating system, andthe average cylinder lubricating oilfeed rate is 1.2 g/kWh.

■

CLO consumption = 63,000 x 0.65 x6000 x 1.2 / 1,000,000 = 295 tons/year

■ One metric ton of cylinder lubricatingoils cost approximately 1750 USD,meaning: CLO operational cost =295 x 1750 = 516,000 USD/year

In 2006, when the CLU4 cylinder oildosage pump and the Pulse LubricatingSystem were introduced, it becamepossible to substantially reduce the cylinderoil feed rate, down to approximately0.8 g/kWh.

PLS became the standard cylinder

lubricating system, and was installed onnew buildings from May 2006 on. Inparallel, however, Wärtsilä developed andintroduced the Retrofit Pulse LubricatingSystem (RPLS). Te idea of the RPLS

was to offer an upgrade of the old CLU3based system to a modern CLU4 basedsystem on engines already in service.Tis offered the potential for significantcylinder lubricating oil cost reductions.

With the specific cylinder lubricating oilconsumption reduced from 1.2 to 0.8 g/kWh,the figures from the above example now

become:■ CLO consumption = 295 / 1.2 x 0.8

= 197 tons/year■ CLO operational cost = 197 x 1750

= 345,000 USD/year

Fig. 11 – Example of poor condition.

in service. With its introduction, piston

running problems and scuffing incidents were substantially reduced, and today itis standard in all Wärtsilä two-strokecylinder liners.

Surveillanceoday’s two-stroke cylinder lubricatingsystems are essentially pure feed-forwardsystems, i.e. there are no sensors in thecylinder liners to inform the control systemof the actual piston running condition,and to enable the control system to reactintelligently in case something is wrong.

Consequently, the cylinder lubricationof a two-stroke crosshead marine dieselengine is highly empirical. Based onthousands of running hours and numerousinspections of different engines, both on

test beds and in service, comprehensive

instructions with recommendations forhandling various situations are available,but at the end of the day it is up to thechief engineer and his crew to monitorthe situation and react accordingly.

oday some vessels have the means tomeasure the BN and iron content of theoil in the piston underside space, andthereby get an indication of the pistonrunning condition. However, the best –and most recommended – way to keepthe piston running condition undersurveillance is to carry out regular piston

underside inspections, where cylinderliners, pistons and piston rings are visuallyinspected through the scavenge ports.

Wärtsilä offers a piston runningsurveillance system, known as the MAPEX

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Tus the annual savings would be:■ CLO consumption = 295 – 197

= 98 tons■

CLO operational cost =516,000 – 345,000 = 171,000 USD

Te first RPLS installation wascommissioned and entered into service ona 12-cylinder Wärtsilä RA96C engine inSeptember 2006. Since then it has beenfitted to some 150 engines, to the benefitof both RPLS customers and theenvironment.

As can be seen, the feed rate reductionresults in almost 100 tons less of cylinderlubricating oil being consumed per year.

Tis means - for this particular engine 100 tons less harmful emissions per year,partly as gases and particulates to theatmosphere from the funnel, and partlyas sludge from the piston underside space,

which must be removed and incinerated.

OutlookBecause of constantly changing boundaryconditions, such as engine specificationstowards higher ratings, specifications offuels and lubricants, legislation, and marketprices in general, there will be a continuous

need for understanding and improvingpiston running as well as for reducingcylinder oil consumption.

During recent years, Wärtsilä hasdeveloped and tested some very promisingtools and methods for assessing theperformance of cylinder lubricating systems.From this we have gained a lot of newknowledge and experience, not only aboutthe cylinder lubricating systems themselves,

but also about the cylinder lubricating oil.Tese tools and methods are today

intensively applied for improving andoptimising our current systems, and they will be huge assets in the developmentof future systems. Furthermore, they willhelp us to support the oil companies intheir development of new and improvedformulations of cylinder lubricating oils.Te focus areas are:■ Distribution and refreshment of the oil

on the cylinder liner running surface■ Influence of the fuel’s sulphur content

on the cylinder lubricating oil’sperformance

■

Influence of engine operation at verylow load over longer periods of time

■ Influence of engine operation inareas with high ambient humidity

■ Investigation of the stress level of the oilon the cylinder liner running surface.

Te outcome of the above mentionedfocus areas is expected in the short tomedium term range. On the slightlylonger term perspective , Wärtsilä isinvolved in a number of activities underthe umbrella of HERCULES BEA,

a joint development project funded byEU, where the main focus areas are:■ Development of a test rig for intensive

studies of the lubricating oil filmbetween a piston ring segment andcylinder liner segment under variousconditions, such as contact pressure,piston ring speed (up to 15 m/s) andliner surface temperature (up to 350 ºC).

■ Development of sensors and methods

for dynamic cylinder lubricatingoil film thickness measurements on

the Wärtsilä RX4 test engine.■ Development of a mathematical model

for advanced simulation of lubricatingoil film behaviour.

Also, a number of activities are ongoing onthe applied hardware and component side.

For the upcoming 35 and 40 bore Wärtsilä R-flex engines, a new cylinderoil dosage pump is currently beingdeveloped in co-operation with SKF. Tisnew dosage pump, which has been namedCLU5, will be double acting in order to

fulfil the demands for dynamics andextended time between overhauls, and

will be capable of delivering the cylinderlubricating oil into the piston ring pack

within 3–4 ms in order to ensure correctinjection timing and thus a low feed rate.

Fig. 12 – Test rig for lubricating

oil film studies.

Fig. 13 – Instrumented piston for

Wärtsilä RTX4 .

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E T A I L ]

Some years ago, an Inner LubricationSystem concept was studied at Wärtsilä,

and a prototype was developed and tested with positive results. Based on this, andon some good results from applying an oilscraper ring in the piston ring pack on a

Wärtsilä RA96C engine, where an oilsupply from the piston side might beadvantageous, it has been decided to takethis concept up again and develop it toa commercially applicable level.

A slightly more ambitious idea is toapply the four-stroke trunk piston enginecylinder lubrication concept to the two-stroke crosshead engine, i.e. to “over

lubricate” the cylinder liner, apply an oilscraper ring, and then collect the surplusoil, clean it, and recycle it. Tis will ofcourse be a radical change of concept, and

whether or not it is viable remains to bedemonstrated, but an outline exists and apatent is pending. Te aim is to increasescuffing resistance and to achieve the samelow specific oil consumption level as onthe four-stroke trunk piston engines.

Closing remarksIn many ways, the two-stroke world is

quite different from the four-stroke world,and this is particularly the case, when itcomes to the testing and validation of newcomponents and systems. A four-strokeengine can be operated on a test bed fora considerable number of hours within areasonable budget, but this would not bethe case for a two-stroke engine, at leastnot for a large bore, multi cylinder one.

Terefore, most testing and validation ofnew two-stroke components and systemsmust be carried out in service, and aprerequisite for our development work is

to have access to a number of vessels, which requires good relations with ownersand operators.

esting and validation in service is veryoften both a time consuming and slowprocess, because it requires thousands ofrunning hours. In many cases it would bedesirable to accelerate this process, andmodern advanced computer simulationtools are becoming more and more precise

and useful in complement to field tests, butin the meantime we still need to rely on

full-scale measurements on board ship.

Fig. 15 – Inner Lubrication System.

Fig. 14 – CLU5.

6 feeding bores 1.5 mm

Distributor grooves

Control unit

Electro magnetic valveand mechanical bypass valve.

Electric motorwith variable speed.

Volumetric feed pumpwith defined flow foreach cylinder

Feed pipes i = 16 mm,length = 6–7 m

1 pressure retaining valve

Cyl. 3

Cyl. 4

Feed systemfor inner lubricationthrough piston skirt

Test engine 4RTX-36 feeding bores inpiston skirt

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Documents

Wartsila Technical Journal 02 2010