Oil ageing ‐ drain period in a petrol engine

Introduction

Oil ageing is subdivided into two main cate-gories:(1) Ageing which causes physical and chemi-

cal changes to occur in the oil as a resultof oxidation.

(2) Ageing related to adverse external condi-tions such as sand dust, dirt, fuel, water,blow-by gases or fine metallic particlesbeing introduced into the oil.

When an engine is operated in a long trip,several undesirable things occur in the oil.One of these is oxidation. The mechanism oflubricant oxidation involves a free radicalchain process. The initial products of reactionare peroxides, followed by oxygenated materi-als such as alcohols, ketones, aldehydes andcarboxylic acids. As the oxidation proceeds,these products undergo further reaction toform higher molecular weight species which,at a particular point, become oil insoluble(Coates, 1985). There is an important effectof temperature on the chemical characteristicsof automotive crankcase oils. The lubricant isadversely influenced by hot-spot temperaturesat the surfaces of engine components and thisplays a leading role in the deterioration of theoil. Temperatures above 260°C encouragedeposit formation and rapid deterioration ofthe oil. High temperature becomes a criticalfactor in the thickening of oils and leads to theformation of organic sludges and varnishes(Offune et al., 1991).

Oil oxidation causes depletion of antioxi-dant/antiwear additives. Oil can become con-taminated with acidic compounds that resultfrom the combustion of fuel. If these com-pounds are allowed to accumulate, they canattack the internal metal surfaces and causerapid corrosive wear. Alkaline (“basic”) addi-tives neutralize the acids and minimize thistype of wear. The degree of depletion of over-basing components from the additive gives anindication of its ability to counteract acidiccontaminants which result from the reaction ofthe lubricant with the hot combustion gases(BP Engine Oil-2, 1988). The concentrationof insoluble oxidation products in the oilincreases and the more volatile oil componentsare lost, resulting in an increase in the viscosityof the oil. One of the oil additives that protects

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Industrial Lubrication and TribologyVolume 49 · Number 3 · May/June 1997 · pp. 120–126© MCB University Press · ISSN 0036-8792

FeatureOil ageing – drainperiod in a petrolengine

Hakan Kaleli andIrfan Yavasliol

The authorsHakan Kaleli and Irfan Yavasliol are at Yildiz TeknikÜniversitesi, Istanbul, Turkey.

Abstract

The more frequently an engine oil is changed, the more theoverhaul life of the engine is extended but with anincrease in the cost both of the oil and of the oil drainservices. If engine oil is changed less frequently theassociated costs will decrease. In order to find the opti-mum drain interval, it is necessary to establish the rela-tionship between the cost of the oil and oil drain servicesand the cost of more frequent overhauls. Presents aninvestigation into the degradation of a proprietary lubri-cant marketed in Turkey, and the wear rate of a petrolengine driven in urban traffic. Lubricant samples wereexamined approximately every 2,000km for deteriorationof the lubricant and evidence of wear of the enginecomponents. From the experimental results, determinesthe optimum oil drain period of the engine.

The authors wish to thank Shell Oil Company forproviding test facilities.

the oil from oxidation is zinc dialkyldithio-phosphate (ZDDP). This additive is also anantiwear agent. Thus, loss of ZDDP throughoxidation may have adverse consequences withregard to engine wear. An increase in wear canbe caused by a reduction in viscosity (as aconsequence of fuel contaminating the oil oncold starts) to the point where there is a shiftfrom hydrodynamic to boundary lubrication(Shirley and Donalt, 1988).

Other changes may occur in the oil as partof normal operation, such as abrasive particlecontamination. Such particles entering theengine either through the air filter or the fuelfilter can damage engine parts. This damageoccurs by direct interaction of abrasive parti-cles from the combustion chamber mixingwith the lubricating oil. This will cause wearof lubricated engine parts such as pistonrings, cylinder liners and bearings. The mosteffective method for minimizing friction andwear losses caused by solid contaminants andwear particles is filtration. Filtration of air andfuel serves to reduce the quantity of contami-nants entering the system from the environ-ment. However, some fine dust particles willstill enter the engine, since the filter has thecapacity to filter out only down to certainparticle sizes and, as a result, the filtration willnot provide complete protection under oper-ating conditions (Emad and Aly, 1991).

Since analysis of the oil provides an indica-tion of the engine components’ condition, it isnecessary to examine the lubricating oil sothat the changes in its properties can be corre-lated with optimum operation and engine life.

Oil analysis

There are several standard tests that can beused to provide checks on the lubricant dur-ing running in. In this section several types ofoil analysis, especially emission spectroscopyfor detecting iron wear debris, are describedas aids to the interpretation of the results.

ViscosityThis is one of the most important characteris-tics of the oil, and one that must commandimmediate attention. It can show a rise due tooxidation, nitration or contamination; a fallcaused by dilution of the oil by fuel; or break-down of any viscosity index improver present.Certain engine manufacturers impose tightlimits on permissible changes of viscosity, buta useful rule is that a rise or fall of 25 per cent

in viscosity is the maximum acceptable(Coates, 1985). (Refer to the test standard-ASTM D(445/89).

Flash pointFlash point is the temperature at which, underclosely specified laboratory conditions,vapour from heated oil ignites momentarilywhen exposed to a naked flame. In the labora-tory, the test is conducted in either an open ora closed container and the results are knownrespectively as open flash point (COC, orCleveland Open Cup) or closed flash point(PMC, or Pensky Martin Closed Cup). If,after a period in service, the flash point of anoil is significantly lower than the originalvalue, this indicates contamination by fuel.Repeatability of the test is poor with used oil,but a drop of 25 per cent shows that excessivedilution has taken place (ASTM D93/80)).

Alkalinity (total base number, TBN)This measure of an oil’s alkalinity gives auseful indication of the remaining amount ofeffective additive – especially its ability tocounter the corrosive effects from high-sul-phur diesel fuels. TBN, expressed as thealkalinity equivalent in mg potassium hydrox-ide (KOH) per gram, falls initially when an oilis first put into service, and then tends tostabilize at a level determined by the severityof operation conditions and by the rate of top-up. There is a widely held view that in auto-motive applications, oil should be changedwhen its TBN falls to 50 per cent of the origi-nal level (Chawla and Sharma, 1988) (ASTMD445/89).

Insoluble contentsA used oil contains a certain amount of finelydivided solid matter in suspension, and thesesubstances are normally classified under thegeneral heading of “insolubles”. These are theresults of combustion products reaching theoil, together with the resinous material that isgenerated when the oil itself is degraded byoxidation or nitration.

The oils of lower dispersancy may beunsuitable for service at about 2 per centinsolubles, whereas the higher-performancegrades can tolerate 5 per cent or more. Therelative proportions of contaminants can bedetermined through treatment with n-hep-tane or pentane solvents which dissolve themineral oil and leave behind a residue ofinsolubles.

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Oil ageing – drain period in a petrol engine

Hakan Kaleli and Irfan Yavasliol

Industrial Lubrication and Tribology

Volume 49 · Number 3 · May/June 1997 · 120–126

Metal contentsAn ICP (inductively coupled plasma, emis-sion spectroscopy) technique (ASTMD4951/89) is used to measure the parts permillion of metallic wear elements in the oil;this is done by burning a sample of diluted oilin a plasma flame and then measuring thelevel of elements in the oil by spectrographicanalysis of the light emitted. The function ofthe spectrometer part of the system is toanalyse the radiation emitted by the high-temperature vapourized atoms (Richard,1989).

The validity of this analysis depends on thecare with which samples are taken. Samplesshould be taken from a hot engine after run-ning for a given length of time and the con-tainer must be absolutely clean and containno trace of moisture. Samples should becarefully marked, recorded and any risk ofpollution between sampling and arrival in thelaboratory must be prevented (Alphonse,1972).

Experimental work

The petrol engines used for this work weremanufactured by Tofas Ltd (Turkey) underlicence from Fiat Ltd (Italy). Tests werecarried out in urban traffic between themonths of December and February. Twoengines were used: a 1991 Murat 124 Serce1.3l four speed and a 1990 Murat 131 Pahin1.6l five speed. The first engine’s oil was usedto top-up the second engine. Topping up thesecond engine with identical oil from a similarengine helps to minimize misleading results in

the characteristics of tested oil. Table I givesthe properties of the oil used. The test resultsgiven in Table II were taken into account insampling the second engine’s oil (see TablesIII and IV).

Interpretation of spectrographic analysisrequires considerable caution. Approximatelyevery 2,000km oil samples were taken careful-ly from the sump via the dipstick hole usingsterile tubes and kept in closed bottles prior toanalysis. The deterioration and wear debrisconcentration of the lubricant for the total15,000km trip were determinated using thefacilities offered by the Shell Quality ControlCentre in Istanbul-Turkey. Leading US man-ufacturers and users have published informa-tion giving the maximum concentration limitsfor wear elements in different engines, andthese can provide a guide when making adiagnosis. For different engines operatingunder normal conditions the limits in Table Vare given as an illustration (Alphonse, 1972and BP Engine Oils-3, 1988).

Determination of the optimum oil drainpoint

According to engine tests by Fiat Ltd (Italy)and Tofas Ltd (Turkey), engine life is 1,000hours or 150,000km between overhauls atmaximum load and revolution conditions.During 1,000 hours of testing, oil drain isevery 150 hours or 15,000kms (Tofas Ltd).From the used oil analysis, iron (Fe) weardebris was 297.4ppm at 15,000km (TableIII). This result extrapolates to 2,974ppm at150,000km.

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Table I Some properties of oil used in the test programme

Performance gradeAPI SG/CDCCMC G4/PD2US Military MIL-L-46152D and MIL-L-2104C ARG BLS 22-OL-06/07/09GM 6048M/6049MPhysical characteristicsSAE viscosity grade 20W50Density (20°C) 0.880 (kg/l)Viscosity min (100°C) (cSt) 17.5Viscosity index (min) 120Flash point (°C) 204Chemical characteristicsSulphated ash (max. weight %) 1.0Zn % (weight) 0.103-0.115Ca % (weight) 0.11-0.14Mg % (weight) 0.129-0.152

Using oil and engine test results we obtain:

Number of oil changes = 2,974/y (1)

where y is the Fe (Iron) contents for each triplength in parts per million (ppm).

Overhaul distance = Number of oilchanges × Oil change distance (km) (2)

Overhaul cost per km = Engine totaloverhaul cost /overhaul distance ($/km) (3)

Total cost of the engine overhaul is the costpaid to overhaul the engine. The total cost is$285.71, which includes the prices of pistons,piston rings, valves, honing the cylinder liner,

cylinder liner, bearings of the crankshaft,camshaft and service. The spare parts areavailable in Turkey.

Oil cost per km = Oil price/oil changedistance ($/km) (4)

Total cost = Oil cost per km + Overhaulcost per km ($/km) (5)

Results

The quantity of iron (Fe as metallic weardebris) was taken as a basis for the determina-tion of oil drain period. This is the most

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Table II Wear element contents in ppm of the first engine for each sampling period

Wear contents(ppm) Pb Ni Fe Mo Cr Sn Si Al Cu Ag

SAE 20W50 3.3 0.9 1.7 0.0 0.0 0.8 3.6 0.9 0.8 0.12,000 km 645.4 1.0 11.0 0.0 0.4 0.9 4.9 1.8 3.2 0.84,000 km 2639 2.5 46.2 0.8 1.7 2.0 11.4 7.0 9.5 0.65,000 km 2946 2.6 51.2 0.5 1.8 1.9 14.1 10.2 27.5 0.56,000 km 3486 3.2 66.5 1.7 3.2 2.2 12.6 11.4 28.0 1.1

Table III Wear element contents in ppm of the second engine for each sampling period

Wear contents(ppm) Pb Ni Fe Mo Cr Sn Si Al Cu Ag

SAE 20W50 3.3 0.9 1.7 0.0 0.0 0.8 3.6 0.9 0.8 0.12,000 km 1357 3.3 16.1 0.3 1.0 2.4 8.9 3.6 110 0.04,000 km 2899 2.2 34.2 0.5 3.5 2.5 9.9 1.3 96.6 1.26,000 km 4241 2.7 57.7 0.3 4.9 3.6 12.7 4.8 93.5 1.18,000 km 6560 3.6 91.3 0.4 6.2 3.9 13.4 7.0 80.9 0.99,000 km 6755 6.8 101 0.7 6.1 3.7 18.0 14.1 72.1 0.710,500 km 4718 5.1 189.6 1.6 4.5 9.3 20.0 10.9 51.5 1.312,000 km 4810 8.0 217.7 1.6 4.9 7.4 50.0 14.8 49.2 0.313,500 km 4685 6.4 251.7 1.6 4.8 1.2 26.6 16.3 52.9 0.415,000 km 5188 8.4 297.4 1.3 10.8 7.7 34.2 12.8 51.6 0.3

Table IV Viscosity, total base number, flash point and insolubles data of the second engine

Total baseSecond test Viscosity Viscosity number Flash point Insolubles inengine (40°C, cSt) (100°C, cSt) (mgKOH/g) (°C) the oil (%)

SAE 20W50 160.16 18.09 7.5-8.5 204 0.02,000 km 127.38 15.91 7.78 162 0.264,000 km 123.31 15.48 4.65 142 0.226,000 km 128.10 15.74 4.38 148 0.268,000 km 120.71 16.02 4.16 149 0.239,000 km 130.62 15.70 4.18 150 0.2710,500 km 108.51 14.09 3.71 128 0.3712,000 km 98.60 12.92 3.73 108 0.3213,500 km 102.52 13.45 3.92 110 0.3615,000 km 103.02 13.24 3.48 112 0.30

important criterion for calculating the num-ber of oil changes, the engine overhaulmileage and their relative costs in the life ofthe engine.

As seen in Table V, there is a wide iron (Fe)concentration range between 40 and 200ppm(parts per million). Figure 1 shows the ironconcentration and trip length profile for datafrom Table III; this points to severe wear after11,000km.

Figures 2-5 show the viscosities, flashpoint, total base number and trip lengthprofile where data are taken from Table IV.There are apparent decreases in viscosity (at40°C and 100°C), flash point and total basenumber starting at around 9,500km.

Figure 6 plots oil insolubles against journeylength using data from Table IV. It is consid-ered that the results are satisfactory since theinsolubles are less than 2 per cent.

Applying the experimental results to equa-tions (1), (2), (3), (4) and (5) for distancesbetween 2,000 and 15,000km we obtain theoverhaul and oil costs data in Figure 7 andTable VI.

According to Figure 8, the lowest total costis 9,000km at which point the oil should bechanged. This point represents the optimum

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Table V Maximum concentration limits for different wearelements in engines

ContentMetal (ppm)

Lead 5 to 40Silicon 25Iron 40 to 200Chromium 30Aluminium 15 to 40Copper 5 to 40Tin 5 to 15Silver 5 to 10

32028024020016012080400

Iron Fe (ppm)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Maximum

Minimum

Figure 1 Iron wear debris and trip length profile

180

150

120

90

60

30

0

Viscosity at 40˚C (Cst)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Figure 2 Viscosity at 40°C and trip length profile

225

187.5

150

112.5

75

37.5

Flash point (˚C)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Figure 3 Flash point and trip length profile

21

17.5

14

10.5

7

3.5

Viscosity at 100˚C (Cst)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Figure 4 Viscosity at 100°C and trip length profile

10.625

8.5

6.375

4.25

2.125

0

Total base number (mgKOH/g)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Figure 5 Total base number and trip length profile

oil drain period. It can be seen that the lowestcost interval lies between trip lengths of about7,000km and 10,000km. Data for total costare shown in Table VII.

Discussions

The economic aspects of oil drain period arethe overhaul cost and the cost of oil deter-mined by the number of oil changes up to theoverhaul time.

In Germany in the 1950s, the crankcase oilchange for passenger automobiles was recom-mended every 2,500-4,000km. Oil changeperiods of 7,500-10,000km were usual in1979, although power density and oil temper-ature have increased (Hans and Helmut,

1979). In the early 1990s the average intervalswere 12,000km with a range of 10,000 to20,000km (Caines and Haycock, 1996). InTurkey, investigation has shown that oil drainperiods are between 3,000-5,000km, which isshort when compared to other Europeancountries.

Based on the equation (1) the iron contentof an optimum oil drain distance of 9,000kmis:

Number of oil changes = 2,974ppm/100ppm = 29.

According to equation (2):

Overhaul distance = 29 x 9,000 =261,000km.

Taking into account 4,000km as an average oildrain period in Turkey:

Number of oil changes = 2,974ppm/34.2ppm = 87.

According to equation (2):

Overhaul distance = 87 x 4,000 =348,000km.

Excess number of lubricant changes are:

87 – 29 = 58.

The cost of oil (3l, SAE 20W50) is $7.14. Thenumber of automobiles in Turkey is3,058,511 (General and Statistical InformationBulletin of Turkish Automotive Manufacturers,

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Table VI Overhaul and oil costs data according to the trip length

km × 1000 2 4 6 8 9 10,5 12 13,5 15

Fe (ppm) 16,1 34,2 57,7 91,3 101 189 218 252 297Overhaul cost ($/km) 10–4 7,73 8,21 9,24 11 10,8 17,3 17,4 17,9 19Oil cost ($/km)10–4 35,7 17,9 11,9 8,93 7,94 6,8 5,95 5,29 4,76

Table VII Total cost data according to the trip length

km × 1000 2 4 6 8 9 10,5 12 13,5 15

Total cost ($/km) 10–4 43,4 26,1 21,1 19,9 18,7 24,1 23,4 23,2 23,8

0.4

0.3

0.2

0.1

0

Oil insoluble (per cent)

Length trip (km × 1,000)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Figure 6 Insoluble in the oil and trip length profile

20

16

12

8

4

Overhaul cost (S/km) 10–4

Length trip (km × 1,000)2 4 6 8 9 10.5 12 13.5 15

Overhaul cost

Oil cost

4036322824201612840

Oil cost (S/km) 10–4

Figure 7 Overhaul and oil costs and trip length profile

5045403530251510

Length trip (km × 1,000)2

Total cost (S/km) 10–4

4 6 8 9 10.5 12 13.5 15

Figure 8 Total cost and trip length profile for an optimum oil change point

1996). Therefore the excess cost of engine oilnationwide could be:

3,058,511 × $7.14 × 58 = $1,266,590,575.

From this result the degree of economic lossattributed to the lubricant is very high; thequantity of engine oil purchased is far greaterthan necessary in Turkey. The experimentalwork used only a single engine. The researchis the basis of further investigations aimed atinforming Turkish drivers of the cost of earlydrain periods.

Conclusions

(1) The optimum oil drain period was foundto be 9,000km for the petrol engine testedin this work.

(2) When the oil drain period is reduced, thetime between overhauls is increased, thusreducing the overhaul cost per kilometre;when the oil drain period is extended, oilcost decreases but the overhaul cost perkm increases.

(3) If the oil drain period for all Turkishengines was 4,000km instead of 9,000km,the extra total costs for the lubricantwould be $1,266,590,575.

References

Alphonse, S. (1972), Automobile Engine Lubrication,Scientific Publication (GB) Ltd, Broseley, Shropshire,pp. 10.35-10.45.

BP Engine Oils-2 (1988), Assessment and Engine Monitor-ing, BP Oil Ltd, BP House, Victoria Street, LondonSW1E 5NJ, catalogue No. 16540.

BP Engine Oils-3 (1988), Assessment and Engine Monitor-ing, BP Oil Ltd, BP House, Victoria Street, LondonSW1E 5NJ, catalogue No.16539.

Caines, A. and Haycock, R. (1996), Automotive LubricantsReference Book, Mechanical Engineering Publica-tions, Bury St Edmonds, pp. 179-180.

Chawla, O.P. and Sharma, G.K. (1988), “Modelling oflubricant oil alkalinity in diesel engines”, TribologyInternational, Vol. 21 No. 5.

Coates, J.P. (1985), “Infrared spectroscopic methods forthe study of lubricant oxidation products”, ASLETransactions, Vol. 29 No. 3, pp. 394-401.

Emad, A.K. and Aly, M.N. (1991), “A review of the effect ofsand dust and filtration on automobile enginewear”, Wear, Vol. 141, pp. 349-71.

General and Statistical Information Bulletin of TurkishAutomotive Manufacturers (1996), Vol. 29 Part 1, p. 41.

Hans, K. and Helmut, T. (1979), “The importance of oilfiltering to the useful life and practical application ofdiesel engines”, Wear, Vol. 53, pp. 53-60.

Offune, G.C., Maduako, A.U. and Ojinnaka, C.M. (1991),“Studies on the effects of temperature on thechemical characteristics of automotive crankcaseoils and their base oils”, Tribology International,Vol. 24 No. 3, p. 178.

Richard, M. (1989), Condition Monitoring as a Basis forPlant Maintenance, Finning Ltd, 7 Watling Street,Cannock, Staffs WS11 3LL, UK.

Shirley, E.S and Donalt, J.S. (1988), “Development of anautomatic engine oil-change indicator system”,Society Automotive Engineers, No. 870403, p. 7.62.

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Oil ageing ‐ drain period in a petrol engine