400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

SAE TECHNICALPAPER SERIES 981368

Doubling Oil Drain Intervals - The Realityof Centrifugal Bypass Filtration

Paul Coombs, Ian Cox and Andrew SamwaysThe Glacier Metal Company Limited

Reprinted From: Lubricants for Passenger Cars and Diesel Engines(SP-1368)

International Spring Fuels and LubricantsMeeting and Exposition

Dearborn, MichiganMay 4-6, 1998

The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.

SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.

Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.

To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.

ISSN 0148-7191Copyright 1998 Society of Automotive Engineers, Inc.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.

Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.

Printed in USA

All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database

1

981368

Doubling Oil Drain Intervals - The Realityof Centrifugal Bypass Filtration

Paul Coombs, Ian Cox and Andrew SamwaysThe Glacier Metal Company Limited

Copyright © 1998 Society of Automotive Engineers, Inc.

ABSTRACT

The extension of Diesel engine oil drain intervals is nowwidely recognised as an essential part of the global effortto reduce the amount of waste oil generated by society.Engine end users are demanding units which require lessfrequent and less costly servicing, OEM’s are respondingby employing a number of techniques to meet this need.

Bypass centrifugal oil cleaners are known to be effectivein prolonging engine service life. This paper demon-strates, through a series of long term engine tests, thatthe use of a modern bypass centrifugal oil cleaner incombination with a full-flow metal screen can safely dou-ble the oil drain interval of an 8 litre sized diesel engine.The results also show the centrifuge to have a beneficialeffect on the condition of engine components even withthe extended drain period.

INTRODUCTION

Emission limits legislation for diesel engines has nowbeen agreed in many countries. These limits are set toget progressively tighter as we move into the next century[1]. Originally framed for on-highway vehicles, the legis-lation in many countries is now being extended to coveroff-highway, construction and agricultural vehicles. Thisemissions clampdown has prompted engine designers totake a fresh look at the diesel engine and a number ofdesign innovations have reached the marketplace inrecent years. Many of the changes in engine design (forexample the introduction of exhaust gas recircultion(EGR)) have resulted in increased contaminant loading ofthe lube oil.

Whilst manufacturers have been tackling the emissionsissue diesel engine users have been demanding everlonger service intervals to offset the rising costs of ser-vice labour, new oil and filters and used oil disposal. Theresult is that the lube oil of the modern diesel engine isbeing asked to survive longer and tolerate higher levelsof contaminant than ever before. Modern oil additive for-mulations help to contain in suspension the high contam-inant levels while synthetic base stocks are increasinglybeing used to prolong oil life. The control of insolubles

and viscosity levels still remain limiting factors in serviceinterval extension[2].

The application of bypass centrifugal oil cleaners to die-sel engines has long been known to be a powerful tool incontrolling lube oil contamination [3][4][5]. Howeverrecent engine developments have raised new questionsabout the limits of centrifuge performance. The series oftests detailed in this paper were designed in conjunctionwith a major engine manufacturer to define the ability of abypass centrifuge, in conjunction with a long life clean-able full-flow screen, to enable the extension of oil drainintervals on a modern diesel engine. The test resultsshow the effect on a highly rated modern engine using aCG4 oil of a centrifuge and screen filtration system withextended oil drain intervals compared to the standardbarrier media filtration system for the engine.

THE ENGINES

Two engines were taken from the production line andtested by the manufacturer to ensure that they both metthe required performance criteria prior to being shippedto T&N Technology at Rugby, England. The engines weresupplied with a standard sump having a maximum lubeoil capacity of 35 litres.

The lube oil used for the tests was a SAE 15W40 gradeCG4 classification. The total volume of oil used for thesetests came from a single production batch and was sup-plied by the engine manufacturer.

The engines were designated “Engine 1” and “Engine 2”and were tested consecutively in a computer controlledengine test cell using a water brake dynamometer.

ENGINE LUBE OIL FILTRATION

Engine 1 was fitted with the standard OEM filtration of asingle full-flow barrier media filter rated at 40µm absolute.Engine 2 was fitted with a single full-flow mesh screenplus a bypass oil cleaning centrifuge.

The filtration system layout for both engines is illustratedin figure 1.

2

Figure 1. Filtration layout for both engines

THE CENTRIFUGE – The bypass centrifuge used onEngine 2 was a commercially available unit of a clean-able nature having a dirt holding capacity of approxi-mately 600ml. This unit has a quoted performance of7000rpm and 10.5 litres/min throughput at an inlet pres-sure of 7 bar with SAE30 oil @ 75°C [6].

THE FULL-FLOW METAL SCREEN – The full-flow metalscreen fitted to Engine 2 was manufactured from wovenstainless steel wire cloth rated at 45µm nominal, 58-63µm absolute. This material was pleated to form a filtra-tion unit with similar dimensions to the OEM full-flow bar-rier media filter.

SCREEN AND CENTRIFUGE FILTRATION METHODOLOGY

The centrifuge operates in bypass processing approxi-mately 10% of the lube oil before returning it to theengines sump. As the centrifuge is not a barrier type fil-tration device, it does not rely upon a filtration media toremove the contaminant particles from the lube oil. Unlikea barrier media bypass filter which only removes particlesof contaminant larger than the pore size of the media, acentrifuge removes particles based upon their relativedensity.

Oil is pumped into the centrifuge by the engine’s oil pumpat pressure. The oil is directed into a hollow spindlewhere it exits via a cross hole and into the centrifugerotor. The rotor becomes full of pressurised oil which isthen allowed to exit via two tangentially opposed nozzlesin the rotor base. This causes rotation of the free spin-ning rotor assembly thus generating centrifugal forcewithin the rotor. As particles of dirt carried by the lube oilenter the rotor, they are subjected to this centrifugal forcewhich causes them to migrate radially outward to theinner surface of the rotor wall. Over time, these particlesof dirt build up to form a solid annulus of contaminant.

By using centrifugal force, the centrifuge is capable ofremoving a wide range of particles which in theoryextends into the sub-micron range at the extreme and

includes those that are not captured by the full-flow filter.This is confirmed by analysis of the dirt collected by acentrifuge which reveals a capability to remove small par-ticles of less than one micron in size [7]. Figure 2 showsthe principle of a centrifuge in more detail.

Figure 2. Principle of operation for a bypass oil cleaning centrifuge.

By using a centrifuge to remove the bulk of the contami-nants either produced or ingested by the engine, the roleof the full-flow filter device is changed. The purpose ofthe full-flow metal screen in place of the full-flow barriermedia filter is to process the full-flow of the engine’s lubri-cating oil. As the oil is pumped to the engine componentsthe screen prevents large particles of debris from reach-ing the lubricated surfaces and causing catastrophic fail-ure.

ENGINE TEST PROCEDURES

Both engines were tested using a standard engine testprocedure recommended by the engine manufacturer.The test procedure known as a modified life cycle testwas designed to wear the engine over a period of 2100hours. Figure 3 shows the engine test cycle in moredetail.

Figure 3. Engine Test Cycle

E N G I N E

SUMP

PUMP

FULLFLOWFILTER

E N G I N E

SUMP

PUMP

FULLFLOWSCREEN

GLACIER

CENTRIFUGE

ENGINE 1 ENGINE 2

COOLER

COOLER

ENGINE TEST CYCLE

TIME

EN

GIN

E S

PE

ED

PEAK TORQUE

RATED SPEED

10% OVERSPEED

IDLE

FAST IDLE

3

ENGINE BREAK-IN

Prior to shipping the engines, the manufacturer briefly ranboth units to ensure that the power and torque outputswere comparable and within recommended limits.

Prior to the commencement of the 2100 hour test, bothengines were run for a period of 80 hours each, using themodified life cycle test illustrated in figure 3. This break-incycle was conducted using the break-in oil specified bythe engine manufacturer.

Upon completion of the 80 hour break-in cycle, the pro-cedure detailed in table 1 was adopted.

OIL DRAIN PROCEDURES

Engine 1 was run according to the engine manufacturersstandard procedure. The lube oil was drained andreplaced and the full-flow filter was replaced at 350 hourintervals. In order to test the ability of the centrifuge andscreen filtration system to maintain the condition ofEngine 2’s lube oil over extended oil drain intervals, thelube oil was drained and replaced every 700 hours effec-tively doubling the recommended oil drain interval.

The full-flow metal screen remained in service withoutinspection or cleaning for the entire test duration of 2100hours, the pressure drop across the screen being contin-uously monitored. Although the centrifuge used was of acleanable type, the rotor bowl was replaced rather thancleaned every 700 hours at the oil drain. This procedureallowed inspection and analysis of the collected contami-nant at a later date. The service details for both enginesare summarised in table 2.

OIL ADDITIONS PROCEDURE

At the start of each oil drain interval, each engine wasfilled with fresh oil to the maximum mark on the oil levelindicator. Both engines were then run until the oil levelreached the minimum mark on the oil level indicator.Thereafter, the oil level was maintained at the minimumlevel by adding fresh oil to the engine every 24 hours at aquantity equivalent to the engines daily oil consumption.Oil additions to both engines were recorded to enablemeaningful comparisons to be made between the oilanalysis results.

ENGINE DATA

Main engine characteristics were monitored during thetesting of both engines including Power, Torque, Blowby,Oil Pressure, Oil Temperature, Oil Additions and Oil Con-sumption. To ensure a truly representative test, the per-formance of both engines was matched as closely aspossible throughout the 2100 hour duration.

OIL AND SLUDGE ANALYSIS

OIL ANALYSIS – In order to asses the performance ofthe two filtration systems, 50ml oil samples were takenfrom each engine every 48 hours and these sampleswere analysed by an independent laboratory. Spectro-graphic analysis was used to determine the levels ofwear elements. Total insolubles measurements weremade using the modified blotter spot technique. Otherphysical properties such as viscosity and TBN were mea-sured using standard oil analysis techniques.

SLUDGE ANALYSIS – The dirt removed by the centri-fuge was collected and subjected to spectrographic anal-ysis and Scanning Electron Microscopy to determine itscomposition.

TEST RESULTS

ENGINE PERFORMANCE – Figure 4 shows the powerand torque produced by both engines over the duration ofthe test. The two engines were selected by the manufac-turer to have very similar performance characteristics.Engine 1 produces slightly more power than Engine 2and this is largely attributed to differences in engine build.The fueling rate and fuel temperature was consistentbetween the two engines.

Table 1. Initial Service Procedure

Engine 1 Engine 2

Lube oil Drain Break-In oil.Fill with test oil.

Drain Break-In oil.Fill with test oil

Filtration Replace full-flow barrier media filter

Replace centrifuge rotor cover

Table 2. Service Timetable.

Engine 1 Engine 2

Every 350 Hours

Drain and replace lube oil.Remove and replace full-flow filter.

Every 700 Hours

Drain and replace lube oil.Remove and replace full-flow filter.

Drain and replace lube oil.Remove and replace cen-trifuge rotor cover

4

Figure 4. Engine Power and Torque Comparison.

Figure 5 shows the cumulative oil additions for bothengines. This graph shows that Engine 2 received feweradditions over the duration of the test which is thought tobe due to reduced cylinder component wear (see sectionon engine wear). The lower oil consumption of Engine 2effectively results in higher lube oil contaminant loadingdue to the lower level of fresh oil additions and hence,less dilution of the contaminated sump oil.

Figure 5. Cumulative Oil Additions for Both Engines.

OIL ANALYSIS RESULTS – Figure 6 shows the level ofiron in the lube oil of both engines. It can be clearly seenthat the level of iron in the oil samples of Engine 1increase consistently over the 350 hours oil drain peri-ods. The increased level in the final oil drain can beexplained as an increase in the amount of natural weartaking place within the engine. This trait was as expectedby the engine manufacturer.

The rate of iron accumulation in the oil of Engine 2 is sub-stantially lower than that of Engine 1. Iron contaminationin the oil of Engine 2 takes approximately twice as long toreach the same level as that of Engine 1.

Figure 6. Spectrographic Oil Analysis of Iron in Lubricating Oil for Both Engines.

This is probably due to a combination of two effects 1) thecentrifuge is removing the dense contaminant wear parti-cles from the lube oil and 2) the engine is actually wear-ing less due to better lube oil contaminant control onEngine 2. The rate of increase of iron in the lube oil ofEngine 2 is consistent over the test and does not exhibitthe rapid increase towards the end of the test that isapparent with Engine 1. This increase in iron levels whenapproaching 2100 hrs is a normal feature of the enginetype when running this test cycle and is an indication ofthe condition of wearing parts, especially cylinder compo-nents.

Figure 7 shows the kinematic viscosity of the oil in bothengines, measured at 40°C. Oil viscosity is a major mea-sure of an oils ability to flow through the engine. Gener-ally, as the lube oil becomes loaded with dirt, theviscosity of the lube oil rises. Figure 7 clearly shows thatthe rate of viscosity increase of the lube oil used inEngine 2 is similar throughout the 2100 hour test and isconsiderably lower than that of the oil used in Engine 1. Itcan also be seen that the rate of viscosity rise for Engine1 is variable and increases over the duration of the test.

Figure 7. Oil Viscosity.

POW ER & TORQUE CURVES

0 350 700 1050 1400 1750 21000

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

0

500

1000

1500

2000

2500

3000

ENGINE HOURS

EN

GIN

E P

OW

ER

(K

W)

@ 2

200

rpm

EN

GIN

E T

OR

QU

E (

Nm

) @

140

0 rp

m

ENGINE 1

POW ER

ENGINE 2

POW ER

ENGINE 1

TORQUE

ENGINE 2

TORQUE

O I L A D D I T I O N S

0 350 700 1050 1400 1750 21000

10

20

30

40

50

60

70

ENGINE HOURS

gT

ho

usa

nd

s

ENGINE 1

ENGINE 2

I R O N

0 350 700 1050 1400 1750 21000

50

100

150

200

ENGINE HOURS

SP

EC

TR

OG

RA

PH

IC A

NA

LY

SIS

(p

pm

)

ENGINE 1

ENGINE 2

KINEMATIC VISCOSITY @ 40 °C

0 350 700 1050 1400 1750 210090

100

110

120

130

140

150

160

ENGINE HOURS

KV

, cS

t

ENGINE 1

ENGINE 2

5

Figure 8 shows the level of total insolubles within the lubeoil of both engines. The level of total insolubles indicatesthe level of dirt within the lube oil. As the oil becomesloaded with dirt, the total insolubles level rises. The lubeoil becomes unservicable at a predetermined insolubleslevel and hence must be changed. The engine manufac-turer has determined that a total insolubles level of 3% byweight should not be exceeded for this engine. Figure 8clearly shows that the total insolubles level of Engine 2increases at a slower rate than that of Engine 1. It canalso be seen that the rate of increase in Engine 2 is rea-sonably repetitive and controlled. Towards 2100hrs theoil in Engine 1 exceeds the 3% condemnation limit. Thisis due to wear in the power cylinder components which issupported by the iron levels shown in figure 6.

Figure 8. Total Insolubles.

Figure 9 shows the remaining active dispersancy withinthe lube oil of both engines. This is a measure of the oilsability to disperse particles of soot thus preventingagglomeration. As the oil becomes loaded with dirt, thelevel of remaining dispersancy reduces. Despite theextended drain intervals of Engine 2 the level of activedispersancy in this engine falls at a similar rate to that ofEngine 1. It is thought that the mechanism responsible forthis is the removal from the lube oil of contaminant parti-cles by the centrifuge before they become fully saturatedwith dispersant molecules [8].

An oil’s Total Base Number (TBN) is a measure of theoil’s reserve of alkalinity which is used to neutralise acidsproduced by combustion and the reaction of water withother contaminants. As the lubricating oil is used, its abil-ity to neutralise acids is reduced as the alkalinity addi-tives are consumed.

Figure 9. Remaining Dispersancy.

Figure 10 compares the TBN of the lube oil in bothengines for the duration of the test. The TBN of the oilused in Engine 1 depletes rapidly over the 350 hour draininterval. The TBN of the oil used in Engine 2 reduces ini-tially at a similar rate to that of Engine 1, but after approx-imately 250 hours the rate of depletion reduces, showinga tendency to “level off”. The TBN level of the oil inEngine 2 does not fall below the 2 mg KOH/g warninglevel recommended by the engine manufacturer duringany of the service intervals.

Figure 10. TBN.

The continued effectiveness of the oil’s basisity up to 700hours is demonstrated by the levels of lead found by thespectrographic analysis of the samples. The lead levelsat oil drain for Engine 2 were consistently low, reachingbetween 6 and 12 ppm at the end of each 700hr period.

TOTAL INSOLUBLES

0 350 700 1050 1400 1750 21000

1

2

3

4

ENGINE HOURS

% W

EIG

HT

ENGINE 1

ENGINE 2

REMAINING DISPERSANCY

0 350 700 1050 1400 1750 210050

60

70

80

90

100

ENGINE HOURS

%

ENGINE 1

ENGINE 2

T B N

0 350 700 1050 1400 1750 21000

2

4

6

8

10

12

ENGINE HOURS

mg

KO

H/g

ENGINE 1

ENGINE 2

6

CENTRIFUGE SLUDGE – Over the duration of the test,the centrifuge fitted to Engine 2 removed a total of 1,389gof dirt from the lube oil. Figure 11 shows the breakdownof this total over the three oil drain periods. From thisanalysis, it was determined that the centrifuge removedmore dirt from the lube oil as the engine hours increasedand hence component wear increased. Through spectro-graphic analysis, we are able to understand in moredetail the composition of the centrifuge dirt.

Figure 11. Debris Collected by Centrifuge.

Figure 12 illustrates the averaged analysis of the dirt col-lected by the three rotor covers. This indicates that themajority of the dirt is soot.

Figure 12. Analysis of Dirt Collected by Centrifuge.

Figure 13 shows the three centrifuge rotor covers used inthe tests. The first cover is on the left of the picture andthe last is on the right.

Figure 13. Centrifuge Rotor Covers from Engine 2.

When viewed closely, the amount of dirt collected and itsconsistency can be seen more clearly. All three rotorsdisplayed compacted dirt of a dry nature with a minimalamount of lubricating oil present. Figures 15, 16, 17 showthe three rotor covers individually in more detail.

Figure 14. Centrifuge Rotor Cover from Engine 2 (First Oil Drain - 0-700 Hours)

DEBRIS COLLECTED BY CENTRIFUGE

0 - 700 HRS 700 - 1400 HRS 1400 - 2100 HRS

0

100

200

300

400

500

600

ENGINE HOURS

g

AVERAGED ANALYSIS OF DIRT COLLECTED BY THE CENTRIFUGECOMPOSITION ANALYSIS (%)

SOOT

W EAR METALS

SPENT ADDITIVE

OIL & SOOT PARTICLES

69.7

0.8

8.4

21.1

Figure 15 - Centrifuge Rotor Cover fromEngine 2 (Second Oil Drain - 700-1400Hours)

Figure 16 - Centrifuge Rotor Cover fromEngine 2 (Third Oil Drain - 1400-2100Hours)

ANALYSIS OF THE FULL-FLOW SCREEN- Over the duration of the test on Engine 2,the full-flow screen collected a total of294mg of contaminant. This debris wasanalysed to reveal its composition and theresults are illustrated in figure 17.

Figure 17 - Analysis of Debris Collected byFull-flow Screen.

The minimal amount of debris collected bythe full-flow screen can be attributed to theability of the centrifuge to remove the bulkof the contaminants generated or ingestedby the engine.

ENGINE WEAR

On completion of the engine tests, bothengines were returned to the manufacturerfor analysis. The engines weredisassembled and inspected to comparethe effects of the two filtration systems onthe wearing components. All the majorengine components from Engine 2 werefound to be visually cleaner that thoseremoved from Engine 1. Components fromEngine 1 exhibited signs of normal wear.Components from Engine 2 exhibitedconsistently less wear than those fromEngine 1. Of particular note were theconnecting rod bearings which showed aworn and wiped appearance in Engine 1with debris scratches. The bearings fromEngine 2 however exhibited a goodcondition with only one shell from thetwelve exhibiting wear to the overlay.

CONCLUSION

As engine designs continue to evolve tomeet future emissions legislation, thecontaminant loading placed on the engineslubricating oil will increase. Maintaininglubricant performance under theseconditions will require a filtration systemcapable of consistently removing largevolumes of contaminant from the lube oil.

8

The results of these engine tests demonstrate that thecombination of a bypass centrifuge and a large pore size,mesh, full-flow screen can effectively control both thefine, carbonaceous contaminants that cause viscosityincrease and engine wear, and the large solid particleswhich cause short term damage to engine components.Furthermore, it has been shown that the benefits of thecentrifuge and screen system with extended oil drainintervals are sustainable over the working life of theengine.

It is concluded therefore that centrifuge and screen filtra-tion technology provides a real alternative to conventionalbarrier media filtration in extended drain applications.

ACKNOWLEDGMENTS

The authors would like to take this opportunity to extendtheir thanks to the staff of the Advanced Instrumentationand Testing Department of T&N Technology Ltd., espe-cially Mr. B. Fitzsimons and Mr. R. Williams.

REFERENCES

1. Bunting, A., “Squeezing Diesel Emissions”, Truck Maga-zine, pp26-27, January 1997.

2. Miyahara, M., Watanabe, Y., Naitoh, M., Hosonuma, K.,“Investigation into Extending Diesel Engine Oil Drain Inter-val (Part 1) _ Oil Drain Interval Extension by IncreasingEfficiency of Filtering Soot in Lubricating Oil”, SAE paperNo.912339

3. Bowen, AD., “Centrifugal Filtration of Lubricating Oil - Labo-ratory Test Results and Fleet Experience”, T&N TechnicalSymposium 1990, paper No. 31.

4. McNair, J., “Comparison Between Different Bypass Lubri-cating Oil Cleaning Systems”, SAE Paper No. 930996.

5. Backhouse, M.E., Purcell, D.C., “Cleaning of Lubricating Oil- The Needs of the Future”, T&N Technical Symposium,Würzburg & Indianapolis, Paper No. 5, 1995.

6. Glacier Metal Company Limited, “GF060 Centrifuge DataSheet” 1993.

7. Rodibaugh, S.A., “Diesel Engine Lube Oil ContaminantSize and Comparison by Analysis of Solids Collected by OilCleaning Centrifuge”, SAE Paper No. 920928.

8. Glacier Technical Bulletin No. 7/86 “The Effect of Centrifu-gal Separation on a Modern Lubricating Oils Additive Pack-age”.