Pumpability of engine oils at low temperatures

Wear, 56 (1979) 19 - 35 0 Elsevier Sequoia S.A., Lausanne

PUMPABILITY

W. J. BARTZ

19 -Printed

in the Netherlands

OF ENGINE OILS AT LOW TEMPERATURES*

and W. WIEMANN

Technische Ahademie Esslingen, Fort- und Weiterbildungszentrum, Anlagen 5, 7302 Ostfildern 2 (Nellingen) (F.R.G.) (Received

February

Postfach

1269, In den

26, 1979)

Summary The pumpability of various engine oils at low temperatures has been investigated using a Haake-Rotovisko RV2 viscometer, a test engine and a laboratory pumping rig. The cold flow and cold pumpability properties of polymer-thickened engine oils can be predicted from the flow curves at low temperatures. Correlation was found between viscosity data and the cold pumpability behaviour of an oil in an engine. The best evaluation of the low temperature pumpability of multigrade oils in the engine used was given by a specially developed laboratory pumpability rig.

1. Introduction With increasing use of lubricating oils containing additives for improving viscosity-temperature behaviour, it becomes increasingly important to measure their viscosity and their flow properties. Owing to the nonNewtonian flow behaviour of such oils, it is necessary both to measure the effect of temperature on viscosity and to determine the flow curve which involves measuring the viscosity dependence on shear rate or on shear stress at given temperatures [l - 51. Because the dependence of the reversible viscosity change on shear rate and shear stress is especially pronounced at low temperatures, these relations in particular should be taken into account for describing the cold startability of engines and the cold pumpability of engine oils. When engines are started at low temperatures different phenomena which are influenced by the lubricating oil, and especially by the low temperature viscosity of the oil, occur simultaneously. Owing to the high viscosity of the oil at low temperatures the friction torque during engine

*Paper 1979.

presented

at the 4th International

Tribology

Conference,

Paisley,

Sept.

10

- 15,

20

starting can increase so much that either the engine starter is not able to reach the required speed or the engine itself is not able to bring up a sufficient torque after ignition and therefore stops. If, despite these problems, the engine does run, the pumpability of the lubricant must be guaranteed. Sufficient oil has to be sucked in by the oil pump to ensure the required oil pressure and all necessary points of lubrication, even rocker-arm bearings, have to be supplied with oil as quickly as possible to avoid damage. Each of these processes has a different shear rate and since multigrade lubricants exhibit non-Newtonian flow behaviour different effective viscosities must be assumed. The SAE engine oil viscosity classification J 300 b [6] which is also used in the DIN standard 51 511 classifies the low temperature startability of engine oils in terms of the cold cranking simulator (CCS) viscosity at one temperature only (-18 “C). The CCS was developed to describe the low temperature startability of engine oil and measures with a shear rate of lo* - lo5 s-i. Therefore this viscometer only shows a good correlation with the starting torque of an engine [ 7 - 111. Difficulties arise in correlating the CCS viscosities of multigrade oils with low temperature pumpability data [ 12, 131 as viscosity measurements in the CCS are carried out at a shear rate much higher than those associated with low temperature pumpability. The different shear rates in the engine and the suspected maximum viscosities during cold starting are plotted in the appendix of the SAE engine oil viscosity classification J 300 b (Fig. 1). Three areas can be distinguished. During starting the shear conditions in the bearings and other points of friction are similar to those of the CCS. With respect to the pumpability of the oils two phenomena are obvious: a shear rate of about 0.1 - 0.2 s-i is assumed for oil flow to the oilscreen and a shear rate of about 10 s-l is assumed in the suction pipe of the oil pump. Viscometers which measure at such shear rates are available but no specific correlation has been attempted between viscosity measurements and low temperature pumpability data for multigrade engine oils. Therefore investigations promoted by the DGMK* were conducted to correlate the

Fig. 1. Shearing *Deutsche

conditions Gesellschaft

in an engine during cold starting. fiir Mineralijlwissenschaft

und Kohlechemie

e.V.

21

low temperature pumpability of multigrade engine oils measured in a full scale engine with suitable viscosity measurements and with test results from a specially developed laboratory pumping rig. 2. Test oils Table 1 gives the base oils and Table 2 the viscosity index (VI) improvers used. To investigate the rheological properties of the polymers model multigrade oils were formulated using base oil 2 and the following eight VI improvers: two ethylene-propylene copolymers (EPC); five polyalkylmethacrylates (PAMA); one styrene-butadiene copolymer (SBC). The EPCs, which had mean molecular weights of 30 000 and 50 000 respectively, were dissolved in an oil similar to base oil 2. The PAMAs were dissolved in base oil 2. The exact composition of the VI improvers was unknown. Most of the PAMAs differed only in mean molecular weight: A, 150 000; B, 250 000; N, 350 000; C, 500 000. The chemical structure of these polymers was the same. PAMA M had a different chemical structure but the same mean molecular weight as PAMA B. TABLE 1 Base oils Oil No.

Base oil

Type

Kinematic viscosity (mm2s -1 ) 98.9 “C

2

Solvent refined oil 2.5/50a Solvent refined oil 4/50 Solvent refined oil 11/50

5 6

Pour point (“C)

V.I.

31.8 “C

Paraffinic

4.68

26.82

-15

100

Paraffinic

6.51

47.13

-28

95

Paraffinic

14.86

178.70

-16

89

aThe oil has a viscosity of 2.5 E at 50 “C. TABLE 2 Polymers

ECP I ECP II PAMA PAMA PAMA PAMA PAMA SBC

A B N C M

5Pe

Molecular weight

Polymer concentration (wt.%)

Ethylene-propylene copolymer Ethylene-propylene copolymer Polyalkylmethacrylate Polyalkylmethacrylate Polyalkylmethacrylate Polyalkylmethacrylate Polyalkylmethacrylate Styrene-butadiene copolymer

30 000 50 000 150 000 250 000 350 000 500 000 250 000 Unknown

7 12 50 50 50 50 50 100

22 TABLE

3

SAE 15W-50

____

_

oils blended

Base oil components

Oil no.

with different

VI improvers

~ ~~~~~~~ Polymer

Polymer content (%)

Kinematic viscosity (mm2 s -I) 98.9 “C

37.8”C

CCS viscosity at -17.8 “C (mPa s)

Pour point

VIE

(“0

44

97% G5 +

PAMA-A

8

17.86

116.9

4700

-32

180

45

3% G6 91% G5 +

PAMA-B

6.2

17.98

117.9

4500

-34

180

46

9% G6 85% G5 +

PAMA-C

4

18.01

118.9

4650

-34

179

47

15% G6 97% G5 +

PAMA-M

6.3

18.00

120.0

4700

-27

178

PAMA-N

4.8

17.98

116.3

4500

-33

182

49

3% G6 90% G5 + 10% G6 83% G5 +

OCP II

1.1

18.43

153.7

4300

-24

144

50

17% G6 92% G5 +

OCPI

2

18.28

152.5

4500

-16

143

51

8% G6 70% G5 +

SBC

1.45

17.87

135.3

4400

-27

157

Kinematic (mm2 s-l)

viscosity

31.8 “C

98.9 “C

CCS viscosity at -17.8 “C (mPa s)

OCP PAMA PIBjS SBC PAMA PAMA

144.6 120.7 133.8 138.2 61.9 108.8 139.9

18.00 18.50 17.70 17.50 8.20 19.50 18.30

4000 4100 4000 4600 3700 2400 7600

148 182 156 149 111 213 156

15w-50 15w-50 15w-50 15w-50 35w-20 low-50 2OW-50

-33 -33 -33 -33 -33 -27 -30

PAMA -

64.5 75.4 31.6 62.5

8.70 14.70 4.63 6.61

3910 1300 2420 10800

118 217 46 45

15W-20 SW-40 low 2ow-20

-30 -36 -42 -36

48

30% G6

TABLE

4

Industrial

reference

Oil no.

Polymer

RL RL RL RL RL RL RL

61/l 6211 63/l 65/l 66/l 72/l 73/l

PRO 07 PRO 10 PRO 15a PRO 16a

oils VI

SAE rate

Pour point ?C)

To allow better comparisons of the low temperature behaviour of the different polymer types eight SAE 15W-50 oils (Table 3) were blended with paraffinic base oils 5 and 6. These oils had a similar kinematic viscosity at 98.9 “C in an Ubbelohde viscometer (18.0 mm2 s-l) and the same dynamic

23

viscosity in the CCS at -17.8 “C (4600 mPa s). All the blends were specified in weight % and the molecular weights and pumping rates are mean values. Several industrial reference oils were investigated as well as the model multigrade oils. The oils used are given in Table 4.

3. Viscosity

measurements

Proposals for measuring the low shear rate viscosity of multigrade engine oils at low temperatures using both rotary and capillary type viscometers have been discussed elsewhere [ 14,151. The Haake-Rotovisko RV 2 viscometer (Fig. 2) was selected for several reasons. Using this rotational viscometer it is possible to control the shear rate over a wide range. Other viscometers, e.g. Brookfield, Hoppler or Ubbelohde viscometers, measure only at specific shear rates which in some cases (Brookfield) cannot be calculated. Unlike the pressurized capillary viscometer, which is also able to measure the viscosities over a wide range of shear rates, the HaakeRotovisko shows no viscosity decrease due to frictional temperature rise.

H = 15.0

(b)

(a) Fig. 2. Haake-Rotovisko

mm

RV 2.

Only the flow curves of some of the model oils investigated are presented. Figures 3 and 4 show the flow curves of two SAE 15W-50 oils formulated with SBC and EPC II. The non-Newtonian flow behaviour of both oils increases with decreasing temperature. The triangles at high shear rates (lo5 s-l) show the CCS viscosities at different temperatures. With respect to this effect attention should be paid to the viscosity properties of multigrade oils containing different types of VI improvers. Figure 5 shows the flow curves of the eight different SAE 15W-50 oils listed in Table 3 plotted at two different temperatures. The non-Newtonian flow behaviour of all oils shown in Fig. 5 increase with decreasing temperatures, but within this group of multigrade oils blended with different VI improvers the oils containing PAMA only show the best flow behaviour. Only PAMA M, which has an unusual chemical structure, shows a viscosity increase like SBC. The highest viscosity increase with decreasing shear rates is obtained in the oils

24

1 , 1.Lm! I , 68la’2 I sWJ12

h 66W

1

shear rate G [s-l]

containing EPC, At lower temperatures be more pronounced.

4, Comparison pumping rig

of viscometric

this non-~ewto~i~

behaviour

will

data with f3ow hehaviow irta laboratory

Zt was necessary to determine whether or not the viscometric data abtained by evaluating the flow ~unres would cur&&e with the flow IxhaviOur in real engines. Tberefure the first step was to ~nv~s~~te the oiXs in a simulated engine oil galIerJr consisting of 2~1oil pump and an oil pipe circuit in which the flow resistance could be varied. Figure 6 shows a schematic drawing of the laboratory test rig which has been desctibed elsewhere [I&].

25

6

A

1 2 3 4 5 6 7 8 9 10 11 12

oil pump driving shaft capillary linner diameter 4 mm) pressure transducer 1 pressure transducer 2 measuring c ylinder temperature control suction pipe with screen oil pan, filled with l&I0 cm3 of oil temperature control obsermtion window insulation of the cooling box

Fig. 5. Comparison of the flow curves of all SAE 15W-50 oils listed in Table 3. Fig. 6. Schematic drawing of the laboratory pumping rig.

In addition to other parameters the time t, defined as the time at which the oil initially reached the end of the capillary was measured. Figure 7 shows to uersus the test temperature for model oils blended with different concentrations of SBC. t,, increases with decreasing temperature and increasing polymer content. The same behaviour was observed with all polymers used. One of the parameters controlling the cold pumpability properties of the oil is the shear rate on the suction side of the oil pump. A mean shear rate can be calculated using the Poiseuille equation for the mean volume of oil pumped through the pipe circuit. For the tests under discussion values between 10 and 50 sK1 have been calculated on the suction side of the pump. Figure 8 shows the time to for the oil to reach the end of the capillary uersus the viscosity of the oils listed in Table 3 at a shear rate of 50 s-l. These viscosities were determined from the flow curves of the oils at several

SBC

Temperature ['Cl

Fig. 7. Pump-up time to of oils blended

-

with SBC us. the test temperature.

PIMIY

D-

.-

.-

-

.-

-

S*E 15W-50 oils

-SBC ECP: L‘P!,

2

-

---

‘

5000

x)m

Fig. 8. Time to us. the effective

viscosity

lsom

ZOOM

of the oils listed in Table 3.

temperatures. Good correlation was found over a wide viscosity range up to 20 000 mPa s. Another parameter describing the flow properties of the oils tested was the mean flow rate of the pumped oil at the beginning of pumping. Figure 9 shows the mean flow rates of pumped oil as a function of temperature. The rate of pumped oil decreases considerably with increasing polymer concentration and decreasing temperature. This is also valid for the polymers not shown. Figure 10 shows the apparent viscosity of the SAE 15W-50 oils blended with different polymers plotted against the mean flow rates measured in the laboratory pumping rig. The good correlation found indicates that the calculated shear rate of 50 s ’ correlates with the real conditions.

5. Flow behaviour

in a full scale engine

A test rig with a six-cylinder four-stroke engine based on a test series carried out by ASTM [ 17 ] was developed. Figure 11 shows the test engine

27

-10

-20

-30 Temperature

I0 C]

Fig. 9. Mean flow rates of oils blended with EPC us. the test temperature.

fia[‘rnhj

.

G *so s-1

SAE 15W-50

PAMAB

l

PAMAC

o 1.0 2kL

,

.

PAA'AA

0

PAhUN

0

PAhUM

l

SEC

x

EPC

+

EPCII

+O x '9.3 L..

0.5 -

+p

0 0

SK0

10 am

15 cm

20 0L-m

25cDl 1 [mPa

11

Fig. 10. Mean flow rates of the SAE 15W-50 oils us. the effective viscosity.

which was turned by an electric motor up to 1600 rev min-‘. This engine was selected because of its critical feature of construction at the suction side of the pump. The test oils were run in the engine at various temperatures and the results were classified in terms of the gallery oil pressure-time response as illustrated in Fig. 12. The pumpability conditions, as with ASTM, were classified as follows: (a) normal if the gallery oil pressure was greater than 1.38 bar at all times after 1 min of operation. (b) borderline if the gallery oil pressure was equal to or less than 1.38 bar but greater than 0.41 bar at any time after 1 min of operation.

Fig. 11. The test engine.

3-

2-

1 -

no pumparea 0 0

60

Fig. 12. Different

120

180

pumping

240

conditions.

29

$6 6 E 3 ::

I

2 b -

pressure

in

Mann gallery

0

6

6

0 0

120

360

240

Fig. 13. Pressure-time

curves

6(30

480 testing

measured

time

[I]

in the test engine.

(c) no pump if the gallery oil pressure was 0.41 bar or less at any time after 1 min of operation. The borderline pumping temperature (BPT) is the temperature at which the minimum gallery pressure at any time after 1 min of test is at least 1.38 bar. Some real pressure-time curves for a model oil blended with EPC are shown in Fig. 13. At -10 “C the gallery pressure immediately reaches normal pressure (3 bar). At -25 “C an uneven pressure-time curve at the oil filter shows that air-binding occurred. The gallery pressure also shows a minimum and reaches normal pressure after 4 min testing. At -30 “C violent air-binding can be observed before the oil filter and in the main oil gallery. No pump can be established because after 300 s no pressure is developed in the main gallery. 6. Comparison of engine pumping behaviour in a laboratory pumping

data with viscometric rig.

data and flow

The main objective of this work was to find the correlation between the low temperature engine pumping data for multigrade oils and their low temperature viscosities and flow behaviour in a laboratory pumping rig. Table 5 shows the data obtained at the BPT of the SAE 15W-50 oils. The BPTs of the oils differ by about 4 ‘C. From measurements and calcu-

30 TABLE

5

Data obtained Oil no.

graphically

Polymer

A B C M N

at the BPT BPT

VBPT

(“C)

G = 10 s-l

G = 50 s-l

(mPa s)

(mPa s)

-23.5 -23.5 -23.3 -21.5 -22.5

15700 16500 17000 19900 15800

14500 15500 16000 16300 14800

0.35 0.34 0.33 0.31 0.41

35 37 43 43 41

24600 24600

16300 16300

0.32 0.31

36 35

21200

16000

0.34

38

44 45 46 47 48

PAMA PAMA PAMA PAMA PAMA

49 50

OCP II OCP I

-20.3 -19.6

51

SBC

-21

.o

at

VBPT

at

ti,at BPT .(cm3 s-l)

to at BPT (s)

c .

f f---

0

1

0

1.05

ECPI

D

2.02

ECPI

l

2.5%

ECPI

2

3 ia [cm3/s]

Fig. 14. Comparison of the minimum from laboratory pumping tests.

oil pressure

after 60 s and the mean flow rates

lations using the Poiseuille equation the shear rate in the suction side of the pump at borderline conditions was found to be 50 s-l. Therefore it is not surprising that the viscosities at BPT at this shear rate differ less than at the lower shear rate of 10 s-r . The mean value at 50 s-l is about 15 700 mPa s. At ashearrate of 10 s-l viscosities range from 15 700 mPa s to 24 600 mPa s. The mean flow rates and the time to when the oil first reaches the end of the capillary show good correlation at the BPT. The mean values are

31

”

J

n” . 7

SAE 15 W - 50 oils & PAMA A 0 PAMA B D PAM4 C x PAW N + PAM M _ . ECP I . ECP II . SBC

3, [cm’ls]

Fig. 15. Comparison of the minimum oil pressure after 60 s and the mean flow rates from laboratory pumping tests.

o CI l

,

1.5% PA&IA 7.SSPAhWB 7.5% PAMAC 7.5% PACE

-

\ \ \ 50

im

t, Is1 Fig. 16. Comparison of the minimum oil pressure and the time to from laboratory pumping tests.

0.338 cm3 s-l and 38.5 s respectively. Figures 14 - 1’7 show the correlation. In Figs. 14 and 15 the mean flow rate is plotted against the minimum oil pressure in the gallery. The same good correlation can be seen in oils differing in oil content as well as polymer type. In Figs. 16 and 17 the mini-

! j

SAE

15w -50

0

Lt

PAh'A A

0

PAMA

B

0

PAMA

C

x

PAMA

N

+

PAMA

M

.

ECPI

.

ECPII

.

SBC

t

I

Fig. 17. Comparison pumping tests. TABLE

of the minimum

graphically

oii

BPT

RL 61 ,‘l RL 62/l RL 63/l RL 65/l RL 72/l RL 73/l PRO 07 PRO 10 PRO 15a PRO 16a

oil pressure

and the time to from laboratory

6

Rata obtained

no.

_

at the BPT

(“C)

?jupT at 50 (mPa s)

-21.6 -25.5 -20.8 -22.0 -27.5 -21.5 -24.0 -31.1 -29.1 -19.2

13800 23000 10500 13900 23900 18500 10900 14600 12000 12500

S-l

7jBp~

at 10 s-l (mPa s)

6, at BPT (cm3 s-l)

to at BPT (s)

15500 27000 12000 16000 29000 19300 11000 16500 12090 12500

0.46 0.37 0.45 0.51 0.45 0.47 0.66 0.41 0.48 0.50

36 43 35 32 26 31 20 37 39 38

mum oil pressure after 60 s is compared with to; again there is good correlation. Table 6 gives data obtained at the BPT for the industrial reference oils listed in Table 4. The mean value of the viscosities at the BPT and a shear rate of 50 s ‘, which is about 15 400 mPa s, corresponds to the mean value of the viscosities calculated for the model oils (Table 5). However, the zone of dispersion is wider than that calculated for the SAE 15W-50 model oils. While the mean values of the times to for the reference oils (34 s) and the model oils (38 s) correspond quite well the mean values of the mean flow rates differ. Mean values of 0.45 cm3 s-r and 0.34 cm3 s‘ ’ were calculated for the reference oils and the model oils respectively.

33

o .D . .

7.5 I PAMAA 7.5ZPAMAB 7.5 % PAMAC 1.5% PAMAN

. /

60

40

M

0

RAOTIv, [ * 1

Fig.

8. Comparison of the RAOT and the time to from laboratory pumping tests.

I

I

I

;.

SAE . d o q

oxg

0

x

+

Y +

a’ . I 50

A 100

15W - 50 oils SEC PAMA A PAM4 B PAMAC PAMA N PAMA m ECPI

150

zal RAO T1.v.

[sl

Fig. 19. Comparison of the RAOT and the time to from laboratory pumping tests.

Other authors [17] have attempted to describe the low temperature pumpability of engine oils in terms of the rocker arm oiling time (RAOT), i.e. the time when the oil first reaches the rocker arms. Figures 18 and 19 show attempts to correlate the RAOT with values of t,, obtained from laboratory tests. Where data for model oils with different PAMAs were plotted (Fig. 18) no correlation was found. Some correlation between RAOT and to, especially for RAOT values greater than 50 s, is indicated in Fig. 19 where data for SAE 15W-50 oil are plotted. Using laboratory pumping rig data a mean value of 38.5 s was calculated for t,. Figure 19 shows that this value corresponds with a value of RAOTr v of about 80 s. Nevertheless the measurement of the RAOT is very subjective and therefore a very large error is to be expected.

34

7. Conclusions The following conclusions can be drawn from the data presented. (1) With the rotational viscometer used viscosities can be measured over a wide range of shear rates which are adjusted, controlled and maintained constant during measurement. (2) The cold flow and cold pumpability properties of polymer-thickened engine oils can be predicted from the flow curves at low temperatures. (3) At a shear rate of 50 s-l a correlation was found between the viscosity data and the cold pumpability behaviour of an oil in a real engine oil circuit. (4) The best evaluation of the low temperature pumpability of multigrade oils in the engine used was given by a specially developed laboratory pumpability rig. (5) Mean flow rates or the time to, defined as the time when the oil first reaches the end of the capillary in a laboratory pumping rig, correlate well with engine pumping data. (6) Critical data to describe the BPT of an engine oil can be obtained. It must be emphasized that these conclusions are valid for predicting the pumpability of multigrade oils in engines of similar construction (oil pump, suction side of the oil pump etc.) to the engine used in this work. Investigations with several different engines which are at present being conducted will show how far these conclusions can be used for a general prediction of the pumpability of multigrade engine oils. References 1 W. J. Bartz, Fliessverhalten von Mehrbereichsmotorenijlen bei tiefen Temperaturen, Miner&Z-tech., 19(7) (1974) 1 - 29. 2 W. J. Bartz, N. Nemes, J. Herritage and A. L. Neville, Untersuchungen zum Fliessverhalten polymerhaltiger Schmierole, DGMK-Compendium, 1974/1975, pp. 496 506. 3 W. J. Bartz, About the reversible and irreversible flow properties of polymer thickened engine oils, Proc. Colloque Polyme’res et Lubrification, Coil. Int. CNRS, 233 (1974) 123 - 136. 4 W. J. Bartz, About the influence of viscosity index improvers on the cold flow properties of engine oils, Tribology, 9 (1976) 13 - 19. 5 W. J. Bartz and N. Nemes, Flow properties and thickened effect of different viscosity index improvers used in multigrade engine oils, Lubr. Eng., 33 (1) (1977) 20 - 32. 6 Crankcase oil viscosity classification, SAE J 300 b, SAE Handbook, Sot. Automot. Eng., Warrendale, Pa., 1979. 7 W. Lemke, Influence of viscosity on oil supply at low temperatures, VDI Ber. (Ver. Dtsch. Zng.), 177 (1972) 59 - 62. 8 B. Partington and W. C. Pike, The effect of lubricants on the starting ability of automotive engines, Wear, 17 (1971) 351 - 365. 9 R. Hollinghurst, C. E. S. Hackett, K. Marsen and R. A. Wright, The cold cranking simulator and British engine cranking studies, SAE (Sot. Automot. Eng.) Tech. Pap. 720526, 1972. 10 Evaluation of laboratory viscometers for predicting cranking characteristics of engine oils at 0 “F and -20 OF, CRC Rep. 409, April 1968.

35 11 W. A. P. Meyer, T. W. Selby and H. R. Stringer, Evaluation of laboratory viscometers for predicting the cranking characteristics of engine oils at 0 “F and -20 OF, SAE (Sot. Automot. Eng.) Tech. Pap. 680065,1968. 12 R. M. Smith and J. P. Graham, Pumpability of multigrade engine oils at low temperatures, SAE (Sot. Automot. Eng.) Tech. Pap. 710139, 1971. 13 R. M. Stewart and C. R. Spohn, Some factors affecting the cold pumpability of crankcase oils, SAE (Sot. Automot. Eng.) Tech. Pap. 720150, 1972. 14 W. J. Bartz, Flow properties of multigrade engine oils at low and high temperatures, Miner&.%tech., 19 (7) (1974) 1 - 29. 15 W. J. Bartz and W. Wiemann, Determination of the cold flow behaviour of multigrade engine oils, SAE (Sot. Automot. Eng.) Tech. Pap. 770630, 1977. 16 W. J. Bartz and W. Wiemann, Flow properties of polymer blended engine oils at subzero temperatures, ASTM Trans., 21(2) (1978) 152 - 160. 17 Low temperature pumpability characteristics of engine oils in full-scale engines, ASTM Data Ser., 57, 1975.