Cimac Paper 21

7/26/2019 Cimac Paper 21

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CONSEIL INTERNATIONAL

DES MACHINES A COMBUSTION

INTERNATIONAL COUNCIL

ON COMBUSTION ENGINES

PAPER NO.: 21

Oil stress investigations in Shells medium speedlaboratory engine

Julian Barnes, Shell Global Solutions, [email protected]

Jan Hengeveld, Shell Global Solutions, Germany

Simon Foster, Shell International Petroleum Company, United Kingdom

Thijs Schasfoort, Shell Global Solutions, GermanyRinie Scheele, Shell Global Solutions, Germany

Abstract: Stress on the medium speed engine lu-bricant is increasing due to a combination of factorssuch as engine design, the need for operators to ex-tend time between overhauls and the wide range ofresidual fuel oils available in terms of composition and

quality. Concerning engine design, higher power out-put is being achieved from the same engine size andoil volume in circulation, oil consumption has reducedthrough wide use of the anti-polishing ring. In addi-tion, greater fuel efficiency and lower exhaust emis-sions requires more precise fuel injection with higherfuel injection pressures which require additional mea-sures to reduce fuel contamination of the lubricant.

Shells full scale Wartsila 4L20 engine was up-graded in April 2002 to a higher output (Brake MeanEffective Pressure of 27.3 bar) and provides a uniquelaboratory tool to test lubricant performance in mediumspeed engines. To mimic the effects of fuel contamina-

tion of lubricant in the field, heavy fuel is deliberatelyadded to the lubricant. The engine has been used tostudy the main Oil Stresses of lubricants for mediumspeed engines operating on heavy fuel: acid stress(largely from sulphur acids), thermal/oxidative stressand asphaltene stress (from fuel).

For piston undercrown (PUC) deposits, asphaltenestress of the lubricant and thermal/oxidative stress (inthe thin oil film context) are primary factors. Rapidgrowth of PUC deposits with time is seen in the labora-tory engine at a point that corresponds to a particularused oil condition. For oil technology based on salicy-

late detergent chemistry, BN reflects to an extent lubri-

cant detergency and anti-oxidancy as well as its acidneutralisation capability. Here PUC deposit growthcorrelates with used oil BN and there is a break pointat a BN level of 20 mg KOH/g. Below this level rapidPUC deposit growth occurs. Keeping the oil quality

(and detergency) above this break point will preventexcessive PUC deposit growth. In a related manner, ahigher initial BN for such an oil technology will main-tain PUC deposits at a lower level for a longer time.Benefits for a higher BN oil are also seen for pistonring belt deposits and fuel pump lacquer.

Thermal/oxidative stress of used oils in the labora-tory engine test is higher than that experienced in thefield, based on measurements of residual antioxidancywith differential scanning calorimetry. For oil technol-ogy such as that based on salicylate, a higher BN oilgives a greater degree of anti-oxidant reserve.

PUC deposit levels are strongly influenced by thebatch of heavy fuel. In the field this means that ahigh quality lubricant with a sufficient margin of per-formance should be used to cope with any variationsin fuel composition.

Heavy fuel contamination of the used engine lu-bricant was found to be the main cause of increasedoil viscosity. In view of the strong negative effectsof heavy fuel contamination of engine lubricant, ma-

jor challenges are for a) engine builders to reduce thelevel of contamination through improvements in hard-ware, and b) lubricant formulators to design lubricants

that can better cope with this contamination.

cCIMAC Congress 2004, Kyoto


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1. INTRODUCTION

Global competition and deregulation continues todrive the Marine and Power industry to control very

closely the running costs of ships and power plantsand to optimise utilisation. For technical managersthis translates into a relentless focus on:

Eliminating unplanned downtime/outages,

Reducing maintenance costs,

Extending time between overhauls, and

Fuel flexibility (the ability to use as wide avariety of fuel qualities as possible)

Engine buildersare at the forefront of tackling thechallenge of lower costs for the industry. To keepdown capital cost, higher power output is beingachieved with the same engine size and often withthe same oil volume in circulation. In pursuit oflonger life for cylinder liners, the use of anti-polishing rings (also known as flame or fire orcarbon scraping or cuff rings) is now almostuniversal in trunk-piston engines. These ringsmaintain the oil consumption of the engine at a lowlevel. These engine changes (higher power output,lower oil volume and lower oil consumption)

increase the stress on the oil in terms of the kWhexperienced per gram of oil. In addition, greaterfuel efficiency and lower exhaust emissionsrequires more precise fuel injection which in turnrequires higher fuel injection pressures (as high as2000 bar) which require additional measures toreduce fuel contamination of the lubricant.

Fuel quality: At the same time, growing demandfor gasoline and other white products has changedheavy fuel oil production. The refining industry isadopting advanced technologies to improve thehigh-end yield from each barrel of crude, resulting

in changes to the composition of heavy fuel oil.Straight-run product is becoming less common andfuels blended with cracked refinery streams nowaccount for heavy fuels supplied in most majormarkets. This has led to an extension of the fuel oilsupply chain and a growing belief in the market thatheavy fuels are now more variable in compositionand quality than they used to be. In spite of greateruse of standard industry specifications in thepurchase of marine fuels, operators across theindustry and around the world report a risingnumber of incidents involving fuel stability,combustion quality and unexpected effects in use.

Lubricants: The net result of these engine and fueltrends is that lubricants are more highly stressed

since they have to work harder, hotter, in morecompact and lower oil consuming engines andaccommodate a wider spectrum of fuels. Theparticular demands on the lubricant will vary fromengine to engine, but there are three main oil

stresses, discussed in the next section.

2. OIL STRESS

The main stresses experienced by a lubricant for amedium speed engine operating on heavy fuel areacid stress, thermal/oxidative stress andasphaltene stress. These stresses and theirconsequences for the engine and oil condition aresummarised in Table 1. Although there are otherstresses, the three in Table 1 are considered themost important for this type of engine lubricantbased on the laboratory engine studies described inthis paper.

Stress: Caused by: Consequences for engine and

lubricant:

Acid Stress Sulphur acids, oxidation

acids

Corrosive wear, deposits. BN loss and

shorter oil life

Thermal/

Oxidative

Stress

Higher temperatures

giving accelerated

thermal/oxidative

breakdown of lubricant

and fuel

Deposits, sludges, corrosive wear of

bearing material, piston hot corrosion

(from high piston undercrown

deposits). Oil thickening and shorter

oil life

Asphaltene

Stress

Fuel (asphaltene)

contamination of

lubricant

Deposits, lacquers, sludge, fuel

pump sticking, piston hot corrosion

(from high piston undercrown

deposits). Oil thickening and shorter

oil life

Table 1 - Oil stress in medium speed engine lubricants

Acid Stress: Acids entering the lubricant originatefrom combustion of fuel sulphur and, to an extent,from oxidation of fuel and lubricants hydrocarbons.The lubricant has a certain level of basicity forneutralising these acids, expressed as the BaseNumber (BN) which for higher BN oils is measuredby the ASTM D2896 method (unit: mg KOH/g ofoil). BN depletion is the result of acid neutralisationand rapid BN depletion is the most obvious sign ofhigh oil stress. More highly rated engines deplete

BN faster for a given fuel sulphur level due to thehigher sulphur throughput into the engine. If theBN falls too far, corrosion can occur, not only onthe liners but also in the piston ring belt. To avoidthis most engine builders advise a minimum BNlevel of 20 mg KOH/g or around this number forengines running on heavy fuel oil. To maintain thecorrect level of BN of the used lubricant, thereneeds to be a matching of the base level of thefresh oilto the engine type(rated power, fuel andoil consumption) and the fuel sulphur level. Auseful aid to achieve matching of these parametersfor a particular engine application is the Oil Stress

Model [1].

CIMAC Congress 2004, Kyoto Paper No. 21 2


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Thermal/Oxidative Stress: Oxidation of thelubricant, if not controlled, will lead to unacceptableoil thickening and the formation of sludges andlacquers in cooler areas of the oil system. Inaddition, so-called weak acids of oxidation can lead

to corrosion of the bearing material. Control ofoxidation in the thin lubricant film context isimportant, e.g. for piston undercrown cleanliness. Awell formulated engine lubricant must have anadequate level of oxidation inhibition provided byadditives and high quality base oils.

Asphaltene Stress: Asphaltenes will enter theengine lubricant through two routes, blowby and thefuel injector pumps/pump drive. The asphaltenesthrough the blowby route will be burnt or partiallyburnt fuel whilst those by the fuel pump route willbe in raw fuel. Unstable fuel and fuel/lube mixtures

can form lacquer on pump components, leading tosticking or hanging of the fuel pumps when theengine is started. In addition, fuel contamination ofthe oil increases the likelihood of black sludge inengine components, filters and oil ways. Worst ofall, fuel is not designed like lubricant to be thermallystable and will tend to form deposits in the hottestparts of the engine: proper cooling in these areas isvital to avoid excessive build-up of deposits. Themost critical are the piston undercrown (PUC) andpiston groove deposits. At the piston undercrowndeposits form an insulation layer and prevent thelubricant cooling the piston that can lead to hot

corrosion of the piston crown. A modern enginelubricant must be specially formulated to cope withasphaltenes, in particular to disperse them andprevent them forming deposits on engine surfaces.

In addition, ineffective control of any of the abovethree areas of oil stress will lead to shortened oillife, usually through BN loss or viscosity increase,this clearly being undesirable in a climate wherelonger times between overhauls are being sought.

3. STUDYING OIL STRESS WITH THESHELL W4L20 LABORATORY ENGINE

In 1994 a full size Wrtsil 4L20 research enginewas installed in the Shell laboratory at Amsterdamto develop the concept of Oil Stress for mediumspeed engines running on heavy fuel. The use ofthe Oil Stress Model to study engine, fuel sulphurand lubricant factors was reported as a CIMACpaper [1]. At that time, the predominant concernwas Acid Stress, namely depletion of thelubricants Base Number.

In April 2002, Shells Wrtsil 4L20 was uprated toa Brake Mean Effective Pressure (BMEP) of 27.3

bar, the Shell laboratory believed to be the firstcustomer for Wrtsil with this configuration. Acomparison of the engine parameters of this Doutput mode with the former B output engine

(BMEP of 22.5 bar) is shown in Table 2.

B output

D output#

Bore mm 200 200Stroke mm 280 280Speed rpm 1000 1000Output (constant) kW/cyl 165 200BMEP bar 22.5 27.3Fuel injection pressure bar 1300 1500Peak combustion pressure bar 175 200Injection timing BTDC 12 12Exhaust temp. degC 460 470Oil inlet temp. degC 65

65Test duration h 500

320 - 500Sump size l 250

250Oil consumption g/kWh 0.2 - 0.4

0.1 - 0.2Fuel consumption g/kWh 229

226

Fuel sulphur %m 3.2 - 3.4

3.2 - 3.5y-factor (see text) ## % 0.068

0.072

Test measurements: PUC deposit thicknessPiston groove and land deposits

Piston ring wearLiner wear and lacquer

Fuel pump lacquer and stickCrankcase and rocker deposits

Detailed used oil properties

# Final test method with 2.5% heavy fuel added to the sump and top-up## Based on tests carried out with the same lubricant

Table 2 Shell W4L20 laboratory engine methods

The exhaust temperature for the D output modewas higher by 10 C and templug measurementsshowed that temperatures on the pistonundercrown and piston (crown, lands and grooves)were on average higher by 15 and 8 Crespectively. In addition, the oil consumption of theD output test was reduced to 0.1-0.2 g/kWhcompared to the earlier 0.2-0.4 /kWh and to mimicthe effects of fuel contamination, heavy fuel oil wasdeliberately added to the lubricant. Theseimprovements in the test method allow a study ofthe effects of thermal/oxidative stress andasphaltene stress, in addition to acid stress.

3.1 Engine operation and lube oil stress

The Oil Stress Factor, OSF, is defined as below [1],[2], [3].

OSF = 1/R * (1 e[-Rt/V]

) (kWh/g) (Eq. 1)

Where R is the specific oil consumption (g/kWh), tis the oil hours (h) and V is the specific oil volume(g/kW).

In terms of physical and chemical processes, theOSF (in kWh/g) reflects the energy each gram of oilhas been exposed to over time t and therefore OSFis directly related to the amount of combustion



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Where BNt and BN0 are the BN at time t and theinitial (fresh oil) BN respectively, S is the sulphurcontent of the fuel (%m/m), F is the fuelconsumption (g/kWh). The factor 0.35 convertschemical equivalents of S into chemical equivalents

of BN. The factor y is an empirical number that ischaracteristic of a particular engine and engineoperation and a particular lube oil technology. It isthe rate of lubricant BN depletion normalised fordifferences in engine power, oil consumption, fuelconsumption and fuel sulphur content. It is usefulfor determining whether acid stress on the lubricanthas changed.

products, soot, acids and insolubles that haveentered the crankcase.

This parameter allows different engines andconditions to be compared and an assessment

made of their relative severity in terms of oil stress.For example, the Oil Stress Factor and BNdepletion of a 50 BN lubricant tested in the newShell W4L20 D output test are compared in Figure1a and 1bto a typical Marine and Power engine inthe field.

Figure 1a - Oil Stress Factor versus t ime, lab engine versus the field

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500 3000 3500 4000

OIL HOURS (h)

OSF(kWh/g)

Typical Power Gen: fuel S: 2.2, sump charge: 801 g/kW, SLOC: 0.4 g/kWh

Typical Marine: fuel S: 3.3, sump charge: 1000 g/kW, SLOC: 0.4 g/kWh

W4L20 Shell Lab: fuel S: 3.3, sump charge: 275 g/kW, SLOC: 0.1 g/kWh

3.2 Development of a new Shell W4L20 engine testmethod

To measure the effect of changing from B to D

output tests and of adding fuel as a contaminant,comparative engine tests were carried out with a 40BN reference oil (labelled R40) with these three testvariants. For the third test variant, a level of fuelcontamination of 2.5% heavy fuel was used both forthe initial oil charge and the top-up oil, this levelbeing determined through discussions with EngineManufacturers on the typical level of heavy fuelthought to contaminate used oils in service.

Figure 1b - BN depletion versus time, lab engine versus the field

0

5

10

15

20

25

30

35

40

45

50

55

0 500 1000 1500 2000 2500 3000 3500 4000

OIL HOURS (h)

B

N(mgKOH/g)

Typical Power Gen: fuel S: 2.2, sump charge: 801 g/kW, SLOC: 0.4 g/kWh

Typical Marine: fuel S: 3.3, sump charge: 1000 g/kW, SLOC: 0.4 g/kWh

W4L20 Shell Lab: fuel S: 3.3, sump charge: 275 g/kW, SLOC: 0.1

Changes in oil properties for the 40 BN referenceoil are shown in Figures 2a to 2e and arediscussed below.

3.2.1 BN depletion

Figure 2asuggests that the rate of BN depletionincreases as one moves from B to D and D + fueltests. Increased BN depletion from B to D testswould be expected due to the higher fuelthroughput (and sulphur acids arriving in the luboil)combined with the lower oil consumption of the Doutput test (around 0.2 compared to 0.3 g/kWh forthe B output test).

Figure 2a - Influence of test method, BN with time

0

10

20

30

40

50

0 100 200 300 400 500

Test time, h

BN,mgKOH/g

D + fuel

B

D

This shows that at around 500 hours the enginetest, due to its low specific oil volume and oilconsumption and the relatively high fuel sulphurlevel used, gives around the same level of OilStress and BN depletion as 3,000 - 4,000 hoursexperienced in the field. This increased severityaffects engine performance (e.g. piston undercrowndeposits) making the engine a powerful tool fordiscriminating between lubricants, illustrated later.

The OSF concept and the above equation (Eq. 1)can be extended with the BN level of the used oiland it has been shown that BN decreases in directrelation with the OSF at time t [1], [2], [3]:

BNt = BN0 0.35 * S * F * y * OSFt (Eq. 2)

The above equation (Eq. 2) can be used tocalculate the y factor for the three engine methods.



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Plots to determine the y factors, as the gradients ofstraight lines, are shown in Figure 2b.This showsthat the y factor for the B output test is slightly lower

Figure 2c - Influence of test method, viscosity with time

14

15

16

17

18

19

20

0 100 200 300 400 500

Test time, h

Viscosityat100C,cSt

B

D

D + fuel

OEM Vk limit

Figure 2b - Plot to determine the y factor (see text)

y = -0.0718x + 39.389

y = -0.0679x + 38.824

y = -0.0711x + 40.895

0

10

20

30

40

50

0 100 200 300 400

1/R * (1 - e(-Rt/v)) * 0.35 * S * F

BN,mgKOH/g

D + fuel

B

D

Plots of viscosity versus the Oil Stress Factor forthe three test modes are made in Figure 2d. Thisshows very clearly that the B and D output tests

give the same viscosity change with Oil StressFactor whereas the D + fuel test data show that fora given Oil Stress (say of 1.0) the viscosity issignificantly higher.

than those for the D output and D + fuel tests(0.068 compared to 0.072) indicating that the Doutput and D + fuel tests are slightly more severe.The y factor calculated means that for the ShellW4L20 engine around 0.07% of the fuel sulphurends up in the lubricant as sulphuric acid where it isneutralized by the alkalinity of the lubricant. Thevalues for B versus D + fuel engine modes wereconfirmed by further engine tests with the same oiltechnology. Thus although the engine factors thatdeliver acids to the lubricant, such as combustioncharacteristics, blowby and luboil wetting of theliner, are similar for the two Wrtsil 4L20 enginemodes they are slightly more severe for the D +fuel mode.

Figure 2d - Viscosity versus Oil Stress Factor

y = 3.1646x + 14.627

y = 2.3846x + 14.895

y = 2.5305x + 14.771

14

15

16

17

18

19

20

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

1/R * (1 - e(-Rt/v))

Visc

osityat100C,cSt

D + fuel

D

B

3.2.2 Viscosity and insolubles

Figure 2cshows that viscosity increase with time isslightly greater with the D compared to B outputtest but that addition of fuel substantially increasesthe rate of viscosity increase so that the EngineManufacturers limit (25% increase on initialviscosity) is met after only 400 hours, at which pointthis test was stopped. The viscosity data can benormalised for changes in oil consumption (g/kWh)and specific oil volume (g/kW) across tests usingthe Oils Stress Factor determined by the earlierequation (Eq. 1).

Figure 2e shows the Index of Contamination (IC), ameasure of total insolubles. IC is measured bypaper chromatography followed image analysis ofthe used oil spot. The IC value increases to around1.5% at the end of test but the evolution of IC with

time is similar between the engine test modes. TheMerit of Dispersancy (a measure of dispersancyusing the same measurement technique) of usedoils from engine tests without fuel contaminationstarted at around 90% then dropped fairly quickly toaround 60% where it stayed relatively stable formost of the test. In contrast, with fuel contaminationdispersancy dropped to a stable level of around45%. This difference likely reflects the negativeinfluence of heavy fuel contamination on the abilityof the lubricant to disperse insoluble material.



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Figure 2e - Influence of test method, insolubles with time

0

0.5

1

1.5

2

0 100 200 300 400 500

Test time, h

IndexofContam

ination,%

D + fuel

D

B

3.2.3 Piston undercrown deposits

Measurements of thickness of deposits on the

piston undercrown (PUC) showed that B and Doutput tests gave similar deposit levels (around the0 to 10 micron range following 500 hours) but thatwith the addition of fuel contamination depositsrose to >100 micron after only 400 hours of test.This shows the profound influence of heavy fuelcontamination on the formation of PUC depositsthat can increase deposits by a factor of 10.

It was decided to fix the test method for the newtest with 2.5% fuel contamination since the test wasmore severe in terms of Oil Stress and had thepotential for discriminating between lubricants in

terms of viscosity control and PUC deposit control.

3.3 Lubricant effects

Tests with the final method (D + fuel) on 30, 40 and50 BN reference oils (R30, R40 and R50) with thesame additive technology are shown in Figure 3a-

3d. Results may be compared directly betweenthese tests without need for normalisation of datasince each test experienced low and similar oilconsumption. As well as monitoring oil conditionduring the test and measuring engine condition atthe end of test, intermediate deposit measurements

were made by pulling one piston at intervals duringthe test and measuring PUC deposits and pistonland deposits with rings in place. The piston inquestion was then re-installed and the enginecontinued on, with the turn-around time from enginestop to engine restart of only 3-4 hours.

Figure 3a shows BN depletion for the three oils,the 30 BN oil reaching a BN of 10 mg KOH/g at theend of the test (320 hours for the 30 BN oil).Although this level is well below that recommendedby the Engine Manufacturers, checks on used oilcondition revealed that wear metals levels did not

increase which shows that, for a very limited time atleast, this level of BN can be run in an engine

without causing acid corrosion problems but with aninfluence on PUC deposits, see below.

Fig 3a - BN with time for three lubricants with different initial BN

0

10

20

30

40

50

0 100 200 300 400 500

Test time, h

BN,mgKOH/g

30 BN

40 BN

50 BN

Figure 3bplots the viscosity profiles, showing thatthe 30 BN oil viscosity climbed the soonest andreached the Engine Manufacturers limit in onlyaround 320 hours. This can be compared to the 50BN oil which reached this limit at around 500 hours.

Fig 3b - Viscosity with time for three lubricants with different

initial BN

14

15

16

17

18

19

20

0 100 200 300 400 500

Test time, h

Viscosityat100C,cSt

30 BN 40 BN

50 BN

OEM Vk limit

Piston undercrown deposit thicknessmeasurements (Figure 3c) show a rapid,accelerating increase in PUC deposits and (again)this occurs earliest for the 30 BN oil and latest forthe 50 BN oil. The PUC deposit behaviour of the 30BN oil reflects a more rapid deterioration of oilproperties, particularly oil detergency (discussedlater).



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Fig 3c - PUC deposits with time for three lubricants with

different initial BN

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400 500

Test time, h

PUCdeposit,micron

40 BN

50 BN

30 BN

A different 40 BN oil (coded S40) was tested induplicate to evaluate engine test repeatability anddiscrimination. The PUC deposit results are

compared to the reference oil R40 and shown inFigure 3d.

Fig 3d - PUC deposits with time for two 40 BN lubricants (full

lines)

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400 500

Test time, h

PUCdeposits,micron

R40 (40 BN)

R50 (50 BN)

R30 (30 BN)

S40 repeat

tests

They show that S40 gave improved control of PUCdeposits with time, this difference judged to bestatistically significant based on PUC depositrepeat measurements. The data show that theengine test can discriminate between different 40BN oils. The duplicate tests on S40 (two solid bluelines) have PUC deposits at 400 hours of 70 micronin the first test and 72 micron in the second test.This is surprisingly good test repeatability and otherrepeat tests (including with earlier B output tests)indicate a more realistic PUC deposit repeatabilityat 95% confidence of 15%, these error bandsbeing shown in Figures 3c and 3d.

Piston belt cleanliness (lands and groves) and fuelpump lacquer are also evaluated at the end of theengine test, the latter giving pump stick if severeenough. The piston cleanliness results showedthat piston lands were similar for the 30, 40 and 50BN oils tested but top grove cleanliness for the 30

BN oil was worse than that for the 40 and 50 BNoils. Fuel pump lacquer results on the plunger andbarrel are plotted in Figure 4and show that the 50

BN oil (R50) gives directionally cleaner resultscompared to the 30 and 40 BN oils.

Fig 4: Fuel pump rating (blue bar is plunger, brown bar is barrel)

5

6

7

8

9

10

R30 R40 S40 (test 1) S40 (test 2) R50

Lubricant

Meritrating(10=clean)

Thus both the piston cleanliness and fuel pumplacquer results show that the 50 BN oil has theoverall highest performance. Since these testswere carried out, a fuel pump without a sealing ringon the plunger has been installed in the engine tosimulate older fuel pump designs in the field, wheremixing of fuel and lubricant will occur and give morelacquer and potential for stick. The pump without aseal shows lower (= worse) lacquer ratings allowingbetter discrimination between different lubricantsand fuels.

4. DISCUSSION

4.1 Thermal/Oxidative Stress

A measure of thermal/oxidative stress on thelubricant in the thin film context can be madethrough high pressure Differential ScanningCalorimetry, DSC (method: isothermal mode at 210C with high pressure oxygen atmosphere) thatgives the induction period (IP) in minutes. Thelonger the IP of a used oil, the time beforeaccelerating oxidation, the greater its anti-oxidant

reserve. Plots of IP with engine test time for the 30,40 and 50 BN reference oils tests with the D + fuelmode (including a repeat test with the 30 BN oil)are shown in Figure 5.



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Figure 5: Residual antioxidancy (by DSC) of used oils

0

10

20

30

40

50

60

0 100 200 300 400 500

Engine test time, h

InductionPeriod,min

30 BN test 1

40 BN50 BN

30 BN test 2

DSC induction

period of samples

from the field

40 BN, no fuel

This shows that anti-oxidant reserve of the threelubricants decreases with time. The 30 BN oilreaches the lowest point first because its starting

anti-oxidancy was lower. For comparison sake,Figure 5 also includes measurements of DSCinduction period of several samples from the fieldfrom three engines (two Marine and one Power).This shows that the Shell W4L20 laboratory enginesamples, with induction periods of 60 to 11minutes, are more oxidatively stressed in the thinfilm context towards the end of test than the fieldsamples with values of 55 to 25 minutes.

The additional plot (with black dashed line) of the40 BN oil test without fuel contamination gives ashallower reduction in anti-oxidant reserve with

time (compared to the yellow line) showing thatwhere contamination is present there is moreoxidative stress to the lubricant through theoxidation of the heavy fuel.

Separate measurements of the actual level ofoxidation of used oil samples of the 30 BN oil,measured by differential infrared (DIR)spectroscopy and using the characteristic carboxylpeak at 1720 cm

-1showed a maximum level of 2.

Thus although anti-oxidant reserve had beendramatically reduced in the test, this level of oiloxidation was still very low. This indicates that

althoughbulk oil oxidation

is not playing asignificant role in the phenomena seen in theengine, oxidation in the thin filmsituation and in thepresence of fuel will likely be important. Overall, theresults show that a high performance lubricant isneeded to cope with thermal/oxidation stress insevere engine applications.

4.2 Piston Undercrown deposits and Oil Stress

The results in Figure 3c and 3d show theimportance of controlling deposits in the hightemperature region of the PUC. This is especially

true if fuel is present in the lube oil (asdemonstrated in section 3.2.3). Deposits on thepiston undercrown may rise excessively and as

little as a few hundred microns of deposit may raisethe temperature of the piston into the danger zonefor hot corrosion. In the worst case the piston willburn through and/or crack and fail.

Why do the PUC deposits build-up almostexponentially with test time rather than plateau?The data indicate that once deposit build-up starts,deposition accelerates with time and this is mostlikely due to the non-equilibrium conditions in thelaboratory engine where used oil properties rapidlydeteriorate as the test progresses. This contrasts toa substantial proportion of field experience whereused oil condition is near to or at equilibrium anddeposit build-up would be expected to equilibrate ata finite value. However, this finite value of PUCthickness will depend on used oil condition, withpoorer used oil condition expected to give a higher

equilibrated PUC level. Nevertheless, as illustratedearlier, the laboratory test enables deposits to bebuilt up rapidly to levels of the same order assometimes seen in the field, but within 500 hoursrather than several thousand hours.

It is instructive to plot the PUC deposit data versusused oil BN. This is shown in Figure 6a for severaltests on lubricants of differing initial BN but havingthe same additive chemistry. The results show thatonce BN drops to below ca. 20 mg KOH/g, depositsstart to increase rapidly. It is well known that someBN providing additives also give detergency power

to the oil, to keep the hot surfaces of the engineclean. And certain alkaline additives also provideanti-oxidant capacity.

Fig 6a: PUC deposits versus used oil BN

0

20

40

60

80

100

120

140

160

180

01020304050

BN of used oil, mg KOH/g

PUC,micron

For the lubricant technology shown in Figure 6a thealkaline additives are based on salicylate detergentchemistry which imparts both detergency and anti-oxidancy to the oil and explains why BN providesan overall measure of oil performance in this case.A correlation of PUC deposits with used oil BN maybe less apparent with other lubricant technologiessince not all sources of BN have such powerful

detergency and anti-oxidancy.



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Another interpretation of Figure 6a could be thatacid stress (giving BN loss) results in high PUCdeposits. However the relatively low levels ofoxidation acids seen in used oils from the 4L20 testcombined with the absence of piston ring and liner

corrosion show that acid stress is under control.Thus the primary influences on PUC depositswould appear to be asphaltene stress and thin filmthermal/oxidative stress which become more criticalat lower lubricant detergency (lower BN) levels.

4.3 Effect of fuel batch

Additional 4L20 engine tests with another batch oftest fuel (batch 15) have shown that the growth ofPUC deposits with used oil BN is less pronouncedthan with the earlier fuel (batch 14) discussed so far

in this paper (see Figure 6b).

Fig 6b: PUC deposits versus used oil BN, two series of tests with

different batches of heavy fuel

0

20

40

60

80

100

120

140

160

180

01020304050

BN of used oil, mg KOH/g

PUC,micronFuel batch 14

Fuel batch 15

Thus fuel composition will strongly influence PUCdeposit growth even if the standard analyticalproperties of the fuels are quite similar, as shown inTable 3. This means that any engine testcomparisons across different lubricants need to bemade with the same fuel batch.

Property Unit Batch 14 Batch 15

Kinematic viscosity, 50C mm2/s 426 378

Density, 15C kg/m3

0.9905 0.9882

CCAI 850.3 849.2Asphaltenes %m/m 7.0 6.9

Micro carbon residue %m/m 16.1 14.7

Sulphur %m/m 3.35 3.54

Vanadium mg/kg 73 106

Sodium mg/kg 11 19

Aluminium mg/kg 1 4

Nickel mg/kg 24 32

Silicon mg/kg 2 7

Initial boiling point C 208 212

Table 3 - Properties of the heavy fuels used for engine tests

From a practical viewpoint the results show that ahigh quality lubricant with a sufficient margin ofquality should be used in order to cope with thevariations in fuel composition that occur in the field.

From a research perspective, a betterunderstanding of heavy fuel composition onmedium speed engine performance, particularly on

PUC deposits and fuel pump lacquer, is needed inthe future.

4.4 Viscosity increase and Asphaltene Stress

What are the main causes of viscosity increase fora medium speed engine giving high oil stress (asshown in Figure 3b) among, for example, soot,oxidation, fuel contamination (asphaltenes) andother insolubles? Oxidation would appear to be anunlikely cause on the basis of the low level ofoxidation measured in a used oil from thelaboratory engine, discussed earlier. One possiblecause is asphaltenes. To investigate this used oilsex the 4L20D were analysed for asphaltene contenttogether with used oils (based on same oiltechnology) from the field covering fiveMarine/Power engines of different makes.

Asphaltene content was measured by sizeexclusion chromatography which is underinvestigation within the CEC group [4]. A plot ofviscosity at 40 C versus asphaltenes is shown inFigure 7 (viscosity at 100C versus asphaltenesgives a similar plot).

Figure 7: Viscosity versus asphaltene content of used oils

120

140

160

180

200

220

240

260

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Asphaltenes, %m

Viscosityat40C,cSt

This shows a distinct trend of increased viscositycorrelating with increased asphaltenes with theconclusion that fuel contamination of the lubricant isa primary cause of viscosity increase. The ShellW4L20 engine test samples (blue labels) lie close

to the correlation line at its lower part showing thatthe engine test is reproducing field experience inmost cases with the exception of one particular setof field samples (with brown circular labels) thatdiverge from the correlation. This suggests thatalthough asphaltenes from fuel are an importantcause of viscosity increase, there can be othercauses, for example soot or another form ofinsolubles. Further investigation is in progressfocusing on the nature and type of the particles inthese used oil samples. Standard analyses suchas n-heptane insolubles and toluene insolublesmeasurements did not reveal particularly high

contaminant levels that could be correlated with thehigh values of viscosity at 40 C (in the range 200-



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230 cSt). In principle, n-heptane insolubles givestotal insolubles and the difference between n-heptane and toluene insolubles gives a measure oflubricant-derived degradation products.

In view of the negative effects of heavy fuelcontamination of the lubricant on PUC deposits,viscosity increase and fuel pump lacquer, a majorchallenge for the future is for a) engine builders isto reduce the level of heavy fuel contamination ofthe lubricant, and b) for lubricant formulators todesign lubricants that can better cope with thiscontamination.

5. CONCLUSIONS

The Shell W4L20 medium speed engine runningon heavy fuel has been modified to run at aBMEP of 27.3 bar, from an original value of 22.5bar, resulting in high Oil Stress (in kWh/g). Alaboratory method with this engine has beendeveloped for lubricant evaluation that includesdeliberate contamination of the lubricant withheavy fuel to mimic this occurrence in the field.With this severe engine test procedure aduration of 320 500 hours is equivalent, interms of Oil Stress and BN depletion, to severalthousands of hours in the field.

Oil and engine performance of differentlubricants have been interpreted in terms of OilStress, its three main components in mediumspeed engines operating on heavy fuel beingacid stress, thermal/oxidative stress andasphaltene stress from fuel contamination.

Lubricant BN depletion is higher for the higherengine output mode, as would be expected fromhigher acid stress resulting from higher fuelsulphur throughput into the engine combinedwith lower oil consumption. In addition, brake

specific BN depletion of the higher output modeis slightly more than that of the lower outputshowing that the engine factors that deliveracids to the lubricant are slightly more severefor the higher output mode.

For piston undercrown (PUC) deposits,asphaltene stress of the lubricant andthermal/oxidative stress (including oxidativestress of the fuel) in the thin lubricant filmcontext are primary factors. Thus heavy fuelcontamination of the lubricant is a primary causeof PUC deposits.

For oil technology such as that based onsalicylate detergent chemistry, BN reflects to an

extent lubricant detergency and anti-oxidancy inaddition to its acid neutralisation capability. Withsuch an oil technology, PUC deposit growthcorrelates with used oil BN, there being a breakpoint at a BN level of 20 below which rapid PUC

deposit growth occurs. Keeping the oil quality(and detergency) above this break point willprevent excessive PUC deposit growth and theresulting piston over-heating and hot corrosion.In a related manner, a higher initial BN for suchan oil technology will control PUC deposits at alow level for longer. Benefits for a higher BN oilare also seen for piston ring belt deposits andfuel pump lacquer.

Thermal/oxidative stress of used oils in the latterpart of the Shell W4L20 laboratory test is higherthan that experienced in the field, based on

measurements of residual antioxidancy withscanning calorimetry. For oil technology suchas that based on salicylate, a higher BN oil givesa greater degree of anti-oxidant reserve.

PUC deposit levels in the laboratory engine arestrongly influenced by the batch of heavy fuel. Inthe field this means that a high quality lubricantwith a sufficient margin of performance shouldbe used to cope with any variations in fuelcomposition. In the research context, sincestandard analytical properties of the fuels testeddid not give insights to explain the engine

differences it is recommended that further workshould be carried out on the relationshipbetween engine deposits and heavy fuelcomposition.

Asphaltene contamination of the used enginelubricant increases oil viscosity. A generalcorrelation of viscosity increase with increasedasphaltene level (from fuel contamination) wasfound though one set of field samples was offthis correlation suggesting another cause ofviscosity increase in this case.

In view of the strong negative effects of heavyfuel contamination of engine lubricant, majorchallenges are for a) engine builders to reducethe level of contamination throughimprovements in hardware, and b) lubricantformulators to design lubricants that can bettercope with this contamination.



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ACKNOWLEDGEMENTS

The authors wish to express their thanks to FransWortel, Tom Borst, Richard Hoff and ArjenNieuwhof for the engine tests, oil analyses and

DSC measurements. The authors would also like tothank Dr Holger Gehring of MANB&W Augsburg forthe asphaltene measurements of used oils. And toShell International Petroleum Company and ShellMarine Products for their support.

REFERENCES

[1] HENGEVELD, J; CANNON, M.J andSCHEELE, M.J, A model for lubricant stress inmodern medium speed diesel engines and its

verification is a Wrtsil 4L20 laboratory engine,XXII CIMAC Congress, 1998, paper D-78.

[2] HENGEVELD, J; CANNON, M. J; SCHEELE,M.J and LOGTENBERG, J; Lubricant stresspredictions and some operating counter-measuresto prevent frequent oil drain, Presented at Institutfur Schiffsbetriebsforschung, Flensburg, 1999.

[3] CANNON, M.J, HENGEVELD, J; SCHEELE,M.J and FOSTER, L.J.S, Interactions betweenengine design, oil consumption and lubricantperformance, Fifth CEC International Symposium,

1997, paper CEC97-EL09.

[4] VROLIJK, D; BARNES J; DOYEN, V; DUNN,A; FABRIEK, W; GEHRING, H; LIM, KC;NAUDTS, B. Report of Sub-Group on Raw FuelContamination to the CEC Special Project Group:Marine and Large Engines, 1

st October 2003, St

Nazaire, France.

ADDRESS OF AUTHOR

Correspondence to:J.R. Barnes, OGML/7Shell Global Solutions Deutschland,

PAE Labor, Hohe-Schaar Strasse 36,D-21107 Hamburg

Tel (switchboard): +49 40 7565 0Fax: +49 40 7565 4504Email: [email protected]

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Cimac Paper 21