Engine oil ageing under laboratory conditions

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    A.B.Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik Scientific and Engineering Centre, LUKOIL Oil Company, Russia

    Abstract

    Keywords

    INTRODUCTION

    Engine Oil Ageing under Laboratory Conditions

    In recent years the role of laboratory methods in investigating the effectiveness of engine oil additives has dramatically in- creased, especially in predicting the performance of engine oils under field conditions. Some results of research under labora- tory conditions on the ageing of engine oils with different performance levels are presented in this paper. The role of the action of antioxidant additives in engine oils for gasoline and diesel engines is described. The paper includes the results of ex- periments over time in laboratory testing of doped motor oils, use of IR spectroscopy, and reference oils. Single- and multi- cylinder engine test results are also discussed. New, effective additive packages for different engine oils are considered.

    engine oil ageing, additives, engine oil testing, antioxidant properties

    In recent years the role of laboratory methods in the investiga- tion of the effectiveness of engine oil additives has increased in importance. This is illustrated, for example, by the inclusion of laboratory methods in the specifications of new groups of high- grade automotive diesel oils: CH-4 AJ?I and E5-99 ACEA.''2 The antioxidant properties of engine oils now have to be eval- uated not only in a Buick engine (64 h Sequence IIIE test), but also by laboratory methods: ASTM 5968-96 (E5-99 and CH-4 oils) and CEC L-85-T-99 (E5-99 oils). This change highlights the importance of good antioxidant properties for producing high-performance, long-life engine oils.

    Usually, the ageing of engine oils is characterised by a complex set of indicators, including the antioxidant, detergent-dispersant, and neutralising properties of the given oil. The antioxidant properties are the most important, as they have a much greater influence on other properties.

    This paper on the investigation of engine oil ageing and prognoses of the performance of oils in an internal combustion

    Lubrication Science 14-3, May 2002. (14) 363 ISSN 0954-0075 $10.00 + $10.00 (2294/0502)

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Figure 1 Correlation between zand Avvloo(15 h tests)

    + SAE 40 CC oil 0 SAE 40 CD oil

    TEST RESULTS LABORATORY METHODS

    + / /

    0 80 160 240 320 t

    engine using laboratory methods, concentrates on the analysis of the oxidation process in the engine and under laboratory conditions.

    It is important that laboratory methods allow one to judge the antioxidant properties of motor oils alongside the results of en- gine tests. A good correlation between the results of laboratory and engine tests was shown by Bouvier and Arn~u ld ,~ where four motor oils (two oils of SF type, two of SG type) were tested both in a Peugeot TU3 engine and under laboratory conditions: 300 cm3 of oil were oxidised at 160-170C with bubbled air (10 lh); the laboratory test durations were 192, 216, and 288 h; the engine test lasted 96 h. The extent of test oil oxidation was evaluated by the viscosity increase at 40C.

    The data showed that differentiation of the antioxidant properties of the oils tested was obtained when the duration of the laboratory tests was 2-3 times longer than that of the engine tests. This means that under laboratory conditions the oil oxidation process was milder than in the Peugeot engine. In

    By

    Lubrication Science 14-3, May 2002. (14) 364 ISSN 0954-0075 $10.00 + $10.00

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Figure 2 Correlation between Tand &,, (SAE 30 oil with different additives, 5 and 15 h tests)

    D,710

    0.08-

    0.06

    0.04

    0.02

    0 45 90 135 180 z

    such a case it is not possible to make a prognosis of engine oil performance characteristics based on a short laboratory test, in particular if the engine oil is a long-life one or of an especially high quality.

    In elaborating a laboratory method for evaluating the antioxidant properties of engine oils, the following criteria were taken into consideration:

    the oil oxidation process in the laboratory must proceed more intensively than in the engine; nevertheless the oxida- tion products in both cases must be similar

    the test duration in the laboratory must not be more than 30-50 h; the test oil quantity should not be more than 50 cm3

    as the oxidation process in internal combustion engines is of a catalytic nature, the influence of this has to be taken into consideration in oxidising engine oils under laboratory conditions.

    For the present work a unit was chosen consisting of a heater and an air feeding ~ y s t e m . ~ . ~ In the heater were placed glass

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Figure 3 Correlation between optical density of IR spectra bands with a maximum at 1710 cm-' 1 oxidation under laboratory conditions 2 single-cylinder engine test

    and at 1770 cm-' (Din,,)

    0.1 0.2 0.3 0.4 0.5 0.6 0.7 D,,,,

    reactors with capillary tubes to spray the air feed. The test oil quantity was 25 g , the oil temperature was 18OoC, and the air supply rate was 12-18 l/h. A copper wire or copper naphthenate (0.05-0.1%) were used as a catalyst. The test duration was 15-30 h.

    Determination of the oxidation product content in the tested oil was carried out by IR spectroscopy, by measuring the value of the photometric coefficient of contamination (acc. GOST 24943), z, and by determination of the oil viscosity increase at 100C, Avloo.

    The IR spectroscopic characteristics were the optical den- sity of bands with a maximum at 1710-1720 cm-I (D,,,,) (carbonyl-containing products of oxidation) and at 1770-1780 cm-l (D1770) (esters of oxyacids, y-lactones), and also the integral absorption intensity in the region 1640-1820 cm-I (El.

    Values of z were calculated using optical density mea- surements of the test oil solutions in a light solvent. From Figures 1 and 2 it can be seen that there is a good correlation between z and Av,,, and between z and D1710; this means that each of these indices could be used to characterise the oil oxidation process, or each could be supplemented by the other.

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Figure 4 Correlation between copper accumulation in SAE 30 CB oil and optical density of the IR spectra band with a maximum at 1770 cm-' ( D,770)

    1 oxidation under laboratory conditions 2 single-cylinder engine test

    COMPARATIVE RESULTS OF LABORATORY AND ENGINE TESTS

    Dl770

    0.3

    0.2

    0.1

    cu (%)

    It is interesting to compare the results of the evaluation of motor oil antioxidant properties by the above-mentioned laboratory method and those of tests in a single-cylinder gaso- line engine (Petter W1; power, 2.2 kW, crankshaft rotation speed, 1500 r ~ r n ) . ~ . ~ To this end an SAE 30 oil corresponding approximately to the classification group API-CB was chosen. Its properties were the following: kinematic viscosity at lOO"C, 9.7 mm2/s; total base number (TBN), 5.5 mg KOWg; sulphated ash, 0.8%.

    During the engine test the oil temperature was 138C and the coolant temperature was 150C. Before the start of the test, 1 kg of oil was added to the crankcase, and every 36 h, fresh oil was added to make up to 1 kg. The total test duration (without oil change) was 144 h. Every 36 h, when the engine was stopped, the piston deposit was evaluated, the lead-bronze bearing weight loss was determined, and a sample of the test oil was taken for analysis.

    The oil oxidation conditions in the laboratory unit were the following: test oil quantity, 25 g; oil temperature, 180C; air supply rate, 12 lfh; catalyst, copper wire; test duration, 15 h. Oil samples for analysis were taken every 5 h.

    It is interesting to consider some similarities between the oxidation process of oil under laboratory conditions and oil age- ing in the engine. As can be seen from Figure 3 the similarity

    ~~ ~

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Table 1 Single-cylinder engine test results

    Engine oil Test Rating* Bearing duration weight

    sticking deposits deposits (inside)

    Piston ring Piston skirt Piston crown loss (mg)

    (h)

    SAE 30 CB oil 36 10 10 8.5 36 72 10 10 6.0 48

    108 10 10 4.0 91 144 10 10 3.5 1958

    SAE 30 CB oil 36 10 10 5.5 23 + 0.1 Yo copper 72 10 10 5.5 1062 naphthenate 108 10 9.0 5.5 2062

    144 10 9.0 4.0 2854 "10 = clean surface.

    between oil oxidation processes in the laboratory unit and in the single-cylinder engine is confirmed by the linear depen- dence. In both cases, almost the same proportions of carbonyl- containing products (D,,,,) and oxyacid esters (D,,,,) were ob- served in the tested From Figure 4, for both tests the same linear dependence is seen for copper and oxidation prod- uct (D,,,,) accumulation in oil (the copper content in oil was determined by atomic absorption, using the analytical line for copper at 324.8 nm). In the laboratory tests, oil-soluble copper compounds were formed as a result of the oxidising oil's contact with the copper wire. In the engine, the oil-soluble copper ac- cumulated as a result of the interaction of acidic oxidation products of the engine oil with bearing surfaces.

    In recent investigations'-'' it was shown that in labora- tory tests with a catalyst - copper wire or oil-soluble copper compound -the accumulation of oxidation products is similar. Therefore, it was interesting to ascertain if such an observa- tion is true in the case of engine testing. Additional tests of the SAE 30 CB oil were carried out: in both cases (laboratory unit and single-cylinder engine) 0.1% copper naphthenate was in- troduced into the oil. In these tests copper naphthenate was used as a model for oil-soluble copper salts found in the oil dur- ing its oxidation in the presence of ~opper.~~'l As can be seen from Table 1 and Figure 5, addition of copper naphthenate to the oil speeds up the oxidation process of the latter; at the

    Lubrication Science 14-3, May 2002. (14) 368 ISSN 0954-0075 $10.00 + $10.00

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Figure 5 Correlation between optical density of IR spectra bands with a maximum at 1710 cm-' (Ol7,J and at 1770 cm-' (Din,,) 1 oxidation under laboratory conditions 2 single-cylinder engine test 0 o SAE 30 CB oil A x SAE 30 CB oil + 0.1% copper naphthenate

    0.1 0.2 0.3 0.4 0.5 0.6 0.7 D~~~~

    same time the proportion of oxidation products, characterised by the indices D,,,, and D1770, remains unchanged. This con- firms the similarity of the oxidation processes in both cases.

    A detailed analysis of the results of these tests is given el~ewhere.~ Here it is of interest to note some additional simi- larities. As seen from Figures 3 and 5, oxidation products characterised by D,,,, are of a secondary nature. Their accumu- lation in oil is preceded by the formation of carbonyl- containing oxidation products (D1710); the former accumulate in the oil only at the later stage of further oxidation. It is also im- portant to note that 10 h of oxidation under the laboratory conditions chosen here corresponds to -100 h of testing in the Petter W1

    The test results discussed above indicate that the conditions in the laboratory test are rigorous. Therefore, this method can be used not only for the investigation of the antioxidant action of different additives: but also for the evaluation of the perform- ance level of engine oils belonging to different categories.

    A number of commercial engine oils of SF/CC, CD/SF, and CF-4/SG type were chosen; they were compared with

    EVALUAT'oN OF DIFFERENT ENGINE OILS

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Table 2 IR spectroscopic analysis of oils oxidised under laboratory conditions (15 h)

    Sample Engine oil no.

    1 Commercial oil SAE IOW-30

    2 Experimental oil

    3 Commercial oil

    SAE 15W-30

    SAE 15W-40

    4 Experimental oil

    5 Commercial oil

    SAE 15W-40

    SAE 20W-40

    6 Experimental oil SAE 20W-40

    API group

    SFICC

    SFICC

    C DIS F

    CDISF

    CF-4lSG

    CF41SG

    Optical density, D

    1720 cm-' 1780 cm-'

    0.25 0.07

    0.13 0.02

    0.20 0.06

    0.20 0.03

    0.21 0.06

    0.14 0.02

    Integral absorption intensity, E

    cm-' cm-'

    12.3 2.6

    1640-1740 1740-1820

    5.5 0.7

    11.7 2.0

    8.2 1.5

    7.1 2.4

    5.4 1 .I

    experimental oils produced using new additive packages for the authors' company. In these tests the air supply rate was 18 l h ; other test conditions were similar to those described in the previous section.

    The results of IR spectroscopic analysis of the tested oils after 15 h oxidation (Table 2) indicate that is possible to eval- uate the antioxidant properties of engine oils in the laboratory. Moreover, it can be seen that there is a certain difference be- tween the tested oils, in particular if the integral absorption intensities in the region 1640-1740 cm-' are compared. The SAE 15W-30 experimental SF/CC oil containing the additive package had a certain antioxidant capacity in reserve. This ob- servation has been confirmed in additional experiments using scanning calorimetry and engine tests.I2

    Concerning the test results of the SAE 15W-40 experi- mental CD/SF oil (sample 4 in Table 2), it is interesting to note that this oil was also tested in a tractor diesel engine; results of the IR spectroscopic analysis for this test are shown in Table 3. If these results are compared with the data in Table 2, it can

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    A.B. Vipper, 1.1. Zadko, M.V. Ermolaev, and J.Y. Oleinik: Engine oil ageing under laboratory conditions

    Table 3 IR spectroscopic analysis of an SAE 15W-40 CDfSF oil" tested in a D-245 tractor diesel engine (120 h)

    PREDICTION OF DIFFERENTIATION OF ENGINE OILS

    Test Optical density, D Integral absorption intensity, E

    1780 1640- 1740 1 740- 1820 (h) cm-' cm-' cm-' em-' 40 0.12 0.03 3.8 0.6 80 0.21 0.03 4.6 1.1 120 0.31 0.04 6.5 1.3

    duration 720

    'Sample no. 4 in Table 2.

    be seen that 15 h of oxidation in the laboratory unit leads to practically the same accumulation of oxidation products as in the oil tested in the tractor engine for 120 h.

    For the final stage of the experiments, a number of railway engine oils were chosen: M-14G2 (SAE 40 CC oil; TBN, 5.5 mg KOH/g), M-16DR (SAE 40 CD oil; TBN, 9.7 mg KOH/g), M-448 (SAE...

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