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The friction and wear of fresh and used engine oils during reciprocating sliding
Wea~ 169 (1993) 167-172
167
The friction and wear of fresh and used engine oils during reciprocating sliding Fawzy M.H. Ezzat Automotive Engineering Department, Faculty of EngineerinN El-Minia University, El-Minia (EgYpO (Received November 23, 1992; accepted April 29, 1993)
Abstract This analysis was undertaken to investigate the friction and wear properties of some engine oils. Fresh and used samples of these oils were examined. An original test apparatus simulating piston-liner movement was used for the purpose of clarifying the effects of various parameters such as load, speed and oil type. Amontons' law was obeyed up to a certain limit for some fresh oils and to lower or greater limits for others. Furthermore, a pronounced drop in friction coefficient was observed with used oils. Wear experiments showed a decrease in wear with the increase of the duration distance (the distance over which the engine oil was used in the vehicle). The electromotive force activity of the oils was shown to affect the wear phenomenon of these oils.
1. Introduction There is general agreement that when operating conditions lead to a sufficiently small separation of the contacting surfaces, high friction and wear will result unless a film of some sort is present at the points of contact to prevent metallic interactions. The type of film and its mode of formation for a given situation are generally in dispute, but most of the factors that influence the situation are well known. They could be classified as those involving operational parameters (load, speed, temperature), liquid-solid interactions and chemical interactions [1]. Another important point is the relationship between friction and wear. It has been pointed out that although friction shows a strong correlation with wear, there is no simple relationship between the two, and low friction does not mean low wear. It has even been suggested that the lowest friction is basically incompatible with the lowest wear [1]. The subject of friction and wear is very important for the piston-liner assembly of automotive engines, and it is clear that this combination dominates the frictional losses of almost all engines [2]. Broadly speaking, it is found that the characteristics of piston-liner lubrication are hydrodynamic except near dead centres, where friction peaks are observed [3], and that wear prevails at the top dead centre, where load and temperature are high and speed is low [4]. The effect of surface roughness has also been investigated [3], and this showed that higher friction peaks
0043-1648/93/$6.00
are observed near dead centres in the case of larger and longitudinal roughness than in the case of smaller and transverse roughness. The purpose of this study was to examine the effects of both oil grade and its duration distance on friction and wear processes during low-speed reciprocating sliding at high contact loads.
2. Experimental The reciprocating test apparatus used in this investigation has been described elsewhere [5] and is shown in Fig. 1. The standard conditions used were 3.70 Hz and 5.95 Hz running frequencies, corresponding to maximum mid-stroke velocities of 0.72 m s -1 and 1.15 m s -1 respectively. The stroke length was fixed at 60 mm. The friction specimens were piston ring samples, the hardness of which was 900 VHN. The moving specimen was a fiat plate with a hardness of 240 VHN. The reeiprocating horizontal motion was resisted by a strain gauge transducer and the friction signal was received through a chart recorder. In friction tests, loads from 46 N to 193 N were used, which correspond to nominal Hertz pressures ranging from 125 MN m -2 to 256 MN m -2, respectively. Wear tests were also carried out using both ring and pin samples. Loads of 105 N and 149 N were used with test times of 1 h and 8 h, respectively. The running frequency was fixed at 3.7 Hz for all tests. The pin diameter was 10 mm and its hardness was measured
© 1993- Elsevier Sequoia. All rights reserved
168
F.M.H. Ezzat I Friction and wear of fresh and used engine oils o°
7200 35o
(a)
]0,1.7 N
134 N
163.6 N
!93 N dc
180 0
54O
360
HS
720 104.7 N
134 N
163.6 N
193 N
dc dc dc
HS
(b) Fig. 2. (a) Typical friction trace for oil A. (b) Friction trace for oil D.
.I(
Fig. 1. Test apparatus. 1" rod; 2: reciprocating block; 3: insulation; 4: fiat specimen; 5 load cell; 6: leverage system; 7: dead weights; 8: guides. to be 200 VHN. The traditional method of measuring wear using a weighing process was used throughout the course of the experiments, with a weighing accuracy of 0.1 mg. Four types of commercial engine oil were chosen for this study and given codes from A to D. Oils B and C were prepared and packed locally. Oil B meets S.A.E. 40 and MIL-L-8104D specifications, and oil C meets S.A.E. 15w/50 specifications. Oils A and D were produced locally. Oil A meets MIL-L-2104B and API CC/ SC specifications, and oil D meets MIL-L-2104B, MILL-46512A specifications.
3. Results and discussion 3.1. Friction results
Four types of fresh engine oil were investigated. The used oil samples were collected from two of these oils. The duration distances were varied from 1000 km to 7200 km. Figure 2(a) shows a typical recording of the friction at a maximum mid-stroke speed of 0.72 m s-~ for oil A. The friction trace appears to be almost constant over the whole track even at dead centres and increases with increasing load. Figure 2(b) shows the friction trends for oil D at maximum mid-stroke speeds of 0.72 m s -~ and 1.15 m s - < Again, at the lower speed the friction was found to be constant over the entire stroke, while at the higher speed frictional variations were observed over the sliding track, especially at dead centres, where frictional peaks were recorded. Figure 3 shows the variation of the friction coefficient with load for the tested fresh samples of engine oils A, B, C and D. A pronounced increase in friction with load was observed up to a certain loading condition,
u
o
.
. 0 ~_
•
Oil A, fresh
o
Oil B, fresh
o
Oil C, fresh
g
Oil D, fresh
m
Vma x
0
40
80
i20
160
.72
200
m/s
240
C o n t a c t load, N
Fig. 3. Effect of contact load on friction coefficient (oils A-D). after which a drop in friction was recorded. Furthermore, this drop occurred at loads that differed from oil to oil. By way of example, for engine oils A and D it occurred at 235 MN m -z, while for oil B it occurred at 213 MN m -z and for oil C at 256 MN m -2. Below these pressures Amontons' law was obeyed. A possible explanation for the rise in friction is the increase in shear strength of the lubricant with pressure, while the drop observed is likely to be due to thermal effects. Tests on used oil samples are shown in Figs. 4 and 5 for oils A and D. In Fig. 4, the used oils showed an increase in average friction coefficient with load to a maximum, after which it decreased. In Fig. 5, a decrease in average friction coefficient was observed until it reached a certain limit, when it rose again. A possible explanation for the general decrease in friction observed in tests with used oils is the tendency of the wear products and contaminants to roll rather than slide [6]. Furthermore, the increase in duration distance of oil appears to affect adversely the drop in friction coefficient. The reason may be attributed to the expected increase of wear grits and debris in used oils as the duration distance tends to increase.
169
F.M.H. Ezzat / Friction and wear of fresh and used engine oils .15 .10
Oil A
o
o .~
8 ~o
.I0
~
.05
.075 • Fresh A
~
. (350
.025
Used, i000 Zm
g
•
Fresh A
R
Used, 1000 Kin
+
Used, 2000 Kin
o
Used, 3420 Km
V
Used, 3420 Km Vma x 1.i5 m/s
,a
o 0
40
80
]20
160
Contact load, N Vma x .72 m/s
Fig. 7. Effect of duration distance on friction (oil A). 0
40
8()
120
160
200
240
Contact load, N Oil n
Fig. 4. Effect of oil duration distance on friction coefficient (oil A).
.3( • Fresh D U Used, 3000 Km X Used, 7200 Km .25
.125 Dead center
Oil D o ~
.20
.10
o -~
.15 .075 Average
o=
.i0 .050 • Fresh D O Used, 3000 Zm
g
.05
÷ Used, 7200 Km
.025
Vmax
0
4(3
80
120
.72 m/s
160
200
Hid stroke
240
Fig. 5. Effect of oil duration distance on friction coefficient (oil D). .15 D V •
10
o=
•
--
~_._--O
8
• Fresh A ~
O Fresh B
x
x Fresh C
.05
g
V
Fresh D Vmax 1.15 m/s
o
0
40
80
!20
(]
40
80
120
i60
200
240
Contact load, N
Contact load, N
160
200
Contact load, N
Fig. 6. Effect of load on friction of engine oils A-D. With the maximum mid-stroke speed increased to nearly o n e and a half times its value, the graphs of the load-friction relationship are shown in Figs. 6 and 7. In Fig. 6, the results were collected for fresh oils
Fig. 8. Effect of contact load on friction (oil D). and showed a decrease in average friction coefficient with load increase. Furthermore, the results collected for flesh oils B and C were nearly the same, except at the contact load of 75 N. In Fig. 7, a comparison is made between flesh oil A and used samples of the same oil at two duration distances, 1000 km and 3420 krn. In this graph the average friction coefficient showed a decrease in value with the increase of the oil duration distance. Table 1 shows a comparison between the friction coefficients at different loads and velocities for flesh engine oils A, B, C and D. As shown in Table 1, oils A and D showed an increase in friction with increasing speed at various loads, while oils B and C showed a decrease in friction with speed, but with deviations at some contact loads. Figure 8 shows the effect of contact load on friction for oil D. The results were collected at mid-stroke and
170
F.M.H. Ezzat / Friction and wear of fresh and used engine oils
TABLE 1. Effect of sliding velocity and load on friction coefficient Load (N)
45.9 75.0 104.7 134.2 163.6 193.0
Oil A
Oil B
Oil C
Oil D
~/=0.26 Pa s
",;=0.31 Pa s
"0=0.30 Pa s
"0=0.22 Pa s
0.72 m s -I
1.15 m s -1
0.72 rn s -1
1.15 m s -1
0.72 m s -I
1.15 m s -1
0.72 m s -I
1.15 m s -1
0.0680 0.060 0.0800 0.0920 0.0900 0.0770
0.0850 0.0920 0.0890 0.0950 -
0.0850 0.0740 0.0970 0.0910 0.0870 0.0840
0.0750 0.0830 0.0810 0.0680 -
0.0750 0.0750 0.0750 0.0820 0.0940 0.0850
0.0790 0.0600 0.0790 0.0680 -
0.0840 0.0910 0.1000 0.1000 0.0880 0.0940
0.129 0.110 0.105 0.115
at dead centres. As observed in the figure, high values of friction coefficient were measured over the dead centres. At mid-stroke, friction decreased with the duration distance increase, and a prevailing trend of friction increase with load governed the system behaviour. T h e average values of friction over the whole reciprocating track showed a typical decrease in value with load for used samples of oil D. A coincidence between the values of friction at 3000 km and at 7200 km was observed, with a tendency for the average friction coefficient to be lower than that of fresh oil. 3.2. W e a r results
In this part the experiments were concerned with the wear process of the fresh and used oil samples previewed. Figure 9 shows the variation of wear with oil duration distance for a pin sample and a ring sample of different hardness. As shown in Fig. 9, wear decreased as the duration distance increased. The effect of engine oil type on wear is shown in Fig. 10. T h e results were collected from 1 h tests and showed a decrease in wear with dtlration distance for engine oil D. For oil A, wear decrol!sed with duration 40
distance until it reached a minimum, after which it rose. Table 2 shows a comparison between the wear of the previously mentioned oils at different operating conditions. A sample of used oil at 3420 km, was chosen to show the effect of increasing both sliding distance and contact pressure. Furthermore, Table 2 shows that low wear was obtained with fresh oils B and C, while oils A and D gave higher values, although the contact pressure was nearly twice as high for oils B and C as for oils A and D. As shown in Table 2, oil C seems to be the best at combating wear, and oil D the worst. As already mentioned in the literature [7,8] the antiwear capability of the lubricating oils was directly linked with the thickness of the oil film on the one hand, and roughly with the electromotive force activity (e.m.f.) of the oil additives on the other. A strict relationship relating the e.m.f, to the antiwear performance remains to be demonstrated [7]. Furthermore, it would be dangerous to express the antiwear performance of a certain oil as a high e.m.f. activity of the additives contained, since some of t h e m may lead to wear problems like pitting and corrosion.
.8 0il A
oil A Load 149 N
Load 149 N
Vma x .72 m/s Fin, 200 % ~
Vma x .72 m/s
30 ik IX k k
Ring, 900 VHN ~
Slid. dist• 12.7 Km
d 20 to
C.4
10
.2
0
O
Slid. dist• 12.7 Km
•0
0
1
2
Duration distance,
3
4
Kin x 103
Fig. 9. Effect of d u r a t i o n distance on w e a r (oil A).
i
2
3
Duration distance, Km x 10 3
171
F.M.H. Ezzat / Friction and wear of fresh and used engine oils )
@Oil
A
OOil
D
Loact 105 N
2
Pin, 200 %7-IN
d~3
V
0
1
2
3
4
5
Duration distance,
max
6
.72 m/s
7
8
Km x 103
Fig. 10. Comparison between wear results for oils A-D.
TABLE 2. Effect of sliding distance and contact pressure on wear Sliding distance (km)
Contact pressure (MN m- 2)
Wear (mg) Oil A (duration distance 3420 km)
Oil A (fresh)
Oil D (fresh)
Oil B (fresh)
Oil C (fresh)
12.70 12.70 7.20
1.814 3.318 3.318
31.66 25.86 13.40
31.66 -
31.90 -
17.16
14.60
It was decided to measure the e.m.f, activities of the four engine oils being studied to show to what extent this component might affect the wear phenomenon. Table 3 shows the e.m.f, activities of the four oils, measured in mV. The tests were carried out at room temperature with maximum mid-stroke speed and contact load of 0.72 m s-1 and 105 N respectively. A ring sample was used, the hardness of which was recorded to be 900 VHN, while the hardness of the reciprocating plate was measured to be 240 VHN. The reciprocating plate and the specimen holder were insulated, as shown in Fig. 1, to allow for direct measurements of the e.m.f. The e.m.f, signal was received through a digital voltmeter and a chart recorder of 0.1 mV accuracy. As shown in Table 3, oil B enjoyed the highest activity and oil D the lowest one. Furthermore, the wear of oils B and C proved to be the lower, in spite of the higher contact pressure at which tests were run, but they ranked as follows: W e a r C < Wear B.
The overall rank might be as follows: Wear C < Wear B < Wear A < Wear D. The e.m.f, activities were ranked as follows: Activity B > Activity C > Activity A > Activity D. Although oil B enjoyed a higher activity than oil C, the wear observed with oil C was less than that with oil B. The reason could be attributed to the high chemical activity of the additives in oil B, which might result in material incompatibility problems or corrosive action. Meanwhile, the results for engine oils A, C and D indicated that the activity rank supposed that t h e antiwear efficiency of oil C might be better than that of A and that of A might be better than that of oil D, which were already recorded (see Fig. 10 and Table 3): W e a r C < W e a r A < W e a r D. Furthermore, the overall rank suggested that the e.m.f, activity, and consequently the reaction film related to it, must be adopted as a contributor to the wear mitigation process.
TABLE 3. Chemical activity and wear of engine oils. Sliding distance= 12.7 km Wear (mg)
Oil type fresh
Activity of oil (e.m.f.) (mV) 10 min
20 min
30 min
40 min
50 min
60 min
Oil Oil Oil Oil
7.70 2.00 29.60 -
13.80 29.70 34.70 0.45
8.40 75.00 48.70 0.45
91.90 76.40 0.45
21.00 108.5 82.20 -
28.20 163.5
A B C D
1.17
1.814 MN m -2
3.318 MN m -2
31.66 -
31.9
17.16 14.60
172
F.M.H. Ezzat / Friction and wear of fresh and used engine oils
4. Conclusions
A study of friction and wear of some commercially produced engine oils were undertaken. Fresh oils showed an increase in friction coefficient with speed increase at various contact loads for some oils (A, D), whilst the rest of the oils (B, C) showed a decrease in friction with speed, but with exceptions at some loads. With increasing load, the friction of fresh oils was increased as a result of increasing viscosity. A further increase of load was found to affect friction adversely, which may be due to thermal effects. Low-speed tests for fresh oils showed a constant friction traction over the whole sliding track (Fig. 2). This is probably due to the formation of a separating layer of constant shear strength [9]. A considerable mitigation of the friction was brought about with used oils at certain combinations of load and sliding speed. This mitigation was observed clearly at the higher velocity (1.15 m s -~) and tended to increase with increasing oil duration distance. The reason may be attributed to the formation of an appreciable hydrodynamic film, which reduces the possibility of metallic contacts even at higher values of the duration distance. Furthermore, the presence of wear debris and grits, which may roll rather than slide, adds a further reason for the observed decrease in friction. At the low velocity (0.72 m s-~), the friction of oil A showed low values adversely affected by increasing duration distance and bounded by high values at low contact loads. On the other hand, oil D showed low values, again adversely affected by increasing duration distance and bounded by high values at both low and high loads (see Figs. 4 and 5). This may be explained bearing in mind that at low velocity the hydrodynamic contribution is small and friction tends to increase with an increasing amount of wear debris and grits in the oil. This was observed clearly at low loads (oils A and D). Further increasing the load increases the lubricant viscosity and consequently reduces the ability of the wear particles and debris to move freely in the lubricant. Furthermore,
the sliding system may behave as a slider moving over very small rolling elements in a thin lubricant film. In this region a dramatic decrease in friction is observed, especially at low values of the duration distance (see Figs. 4 and 5). As the load increases further, a considerable increase in friction is observed. The reason may be attributed to the heat generation during sliding, which reduces the oil viscosity and thus invites more loose wear debris and intermetallic contacts, leading to a considerable increase in friction (oil D). Wear losses were found to decrease with increasing oil duration distance. The antiwear capabilities of the fresh oils were correlated to their e.m.f, activities. The exception was oil B, where its higher activity might result in material and corrosion problems.
References 1 W.E. Campbell, Boundary Lubrication: an Appraisal of World Literature, ASME, New York, 1969, pp. 87-113. 2 M.L. Monaghan, Engine friction -- a change in emphasis, Inst. Mech. Eng., Automotive Division, BP Tribology Lecture, 1987. 3 S. Sanda and T. Someya, The effect of surface roughness on lubrication between a piston ring and a cylinder liner, Proc. Inst. Mech. Eng., C223/87 (1987) 135-143. 4 P.C. Nautiyal, S. Singhal and J.P. Sharma, Friction and wear processes in piston rings, Tribol. Int., 16 (1) (1983) 43-49. 5 F.M.H. Ezzat, Measurement of contact resistance and oil activity of fresh and used engine oils at low speed reciprocating, in press. 6 E. Rabinowicz, Friction and Wear of Materials, Wiley, Chichester, 1965. 7 G. Monteil, J. Lonchampt and C. Roques-Carmes, Study of anti-wear properties of zinc dialkyldithiophosphate through sliding-induced electronic emission, Proc. Inst. Mech. Eng., C224/87 (1987) 531-535. 8 P.E. Fowles, A. Jackson and W.R. Murphy, Lubrication chemistry in rolling contact fatigue. The performance and mechanism of one antifatigue additive, Trans. ASLE, 24 (1981) 107-117. 9 A.F. Alliston-Greiner, J.A. Greenwood and A. Cameron, Thickness measurements and mechanical properties of reaction films formed by zinc dialkyldithiophosphate during running, Proc. Inst. Mech. Eng., C178/87 (1987) 565-572.
167
The friction and wear of fresh and used engine oils during reciprocating sliding Fawzy M.H. Ezzat Automotive Engineering Department, Faculty of EngineerinN El-Minia University, El-Minia (EgYpO (Received November 23, 1992; accepted April 29, 1993)
Abstract This analysis was undertaken to investigate the friction and wear properties of some engine oils. Fresh and used samples of these oils were examined. An original test apparatus simulating piston-liner movement was used for the purpose of clarifying the effects of various parameters such as load, speed and oil type. Amontons' law was obeyed up to a certain limit for some fresh oils and to lower or greater limits for others. Furthermore, a pronounced drop in friction coefficient was observed with used oils. Wear experiments showed a decrease in wear with the increase of the duration distance (the distance over which the engine oil was used in the vehicle). The electromotive force activity of the oils was shown to affect the wear phenomenon of these oils.
1. Introduction There is general agreement that when operating conditions lead to a sufficiently small separation of the contacting surfaces, high friction and wear will result unless a film of some sort is present at the points of contact to prevent metallic interactions. The type of film and its mode of formation for a given situation are generally in dispute, but most of the factors that influence the situation are well known. They could be classified as those involving operational parameters (load, speed, temperature), liquid-solid interactions and chemical interactions [1]. Another important point is the relationship between friction and wear. It has been pointed out that although friction shows a strong correlation with wear, there is no simple relationship between the two, and low friction does not mean low wear. It has even been suggested that the lowest friction is basically incompatible with the lowest wear [1]. The subject of friction and wear is very important for the piston-liner assembly of automotive engines, and it is clear that this combination dominates the frictional losses of almost all engines [2]. Broadly speaking, it is found that the characteristics of piston-liner lubrication are hydrodynamic except near dead centres, where friction peaks are observed [3], and that wear prevails at the top dead centre, where load and temperature are high and speed is low [4]. The effect of surface roughness has also been investigated [3], and this showed that higher friction peaks
0043-1648/93/$6.00
are observed near dead centres in the case of larger and longitudinal roughness than in the case of smaller and transverse roughness. The purpose of this study was to examine the effects of both oil grade and its duration distance on friction and wear processes during low-speed reciprocating sliding at high contact loads.
2. Experimental The reciprocating test apparatus used in this investigation has been described elsewhere [5] and is shown in Fig. 1. The standard conditions used were 3.70 Hz and 5.95 Hz running frequencies, corresponding to maximum mid-stroke velocities of 0.72 m s -1 and 1.15 m s -1 respectively. The stroke length was fixed at 60 mm. The friction specimens were piston ring samples, the hardness of which was 900 VHN. The moving specimen was a fiat plate with a hardness of 240 VHN. The reeiprocating horizontal motion was resisted by a strain gauge transducer and the friction signal was received through a chart recorder. In friction tests, loads from 46 N to 193 N were used, which correspond to nominal Hertz pressures ranging from 125 MN m -2 to 256 MN m -2, respectively. Wear tests were also carried out using both ring and pin samples. Loads of 105 N and 149 N were used with test times of 1 h and 8 h, respectively. The running frequency was fixed at 3.7 Hz for all tests. The pin diameter was 10 mm and its hardness was measured
© 1993- Elsevier Sequoia. All rights reserved
168
F.M.H. Ezzat I Friction and wear of fresh and used engine oils o°
7200 35o
(a)
]0,1.7 N
134 N
163.6 N
!93 N dc
180 0
54O
360
HS
720 104.7 N
134 N
163.6 N
193 N
dc dc dc
HS
(b) Fig. 2. (a) Typical friction trace for oil A. (b) Friction trace for oil D.
.I(
Fig. 1. Test apparatus. 1" rod; 2: reciprocating block; 3: insulation; 4: fiat specimen; 5 load cell; 6: leverage system; 7: dead weights; 8: guides. to be 200 VHN. The traditional method of measuring wear using a weighing process was used throughout the course of the experiments, with a weighing accuracy of 0.1 mg. Four types of commercial engine oil were chosen for this study and given codes from A to D. Oils B and C were prepared and packed locally. Oil B meets S.A.E. 40 and MIL-L-8104D specifications, and oil C meets S.A.E. 15w/50 specifications. Oils A and D were produced locally. Oil A meets MIL-L-2104B and API CC/ SC specifications, and oil D meets MIL-L-2104B, MILL-46512A specifications.
3. Results and discussion 3.1. Friction results
Four types of fresh engine oil were investigated. The used oil samples were collected from two of these oils. The duration distances were varied from 1000 km to 7200 km. Figure 2(a) shows a typical recording of the friction at a maximum mid-stroke speed of 0.72 m s-~ for oil A. The friction trace appears to be almost constant over the whole track even at dead centres and increases with increasing load. Figure 2(b) shows the friction trends for oil D at maximum mid-stroke speeds of 0.72 m s -~ and 1.15 m s - < Again, at the lower speed the friction was found to be constant over the entire stroke, while at the higher speed frictional variations were observed over the sliding track, especially at dead centres, where frictional peaks were recorded. Figure 3 shows the variation of the friction coefficient with load for the tested fresh samples of engine oils A, B, C and D. A pronounced increase in friction with load was observed up to a certain loading condition,
u
o
.
. 0 ~_
•
Oil A, fresh
o
Oil B, fresh
o
Oil C, fresh
g
Oil D, fresh
m
Vma x
0
40
80
i20
160
.72
200
m/s
240
C o n t a c t load, N
Fig. 3. Effect of contact load on friction coefficient (oils A-D). after which a drop in friction was recorded. Furthermore, this drop occurred at loads that differed from oil to oil. By way of example, for engine oils A and D it occurred at 235 MN m -z, while for oil B it occurred at 213 MN m -z and for oil C at 256 MN m -2. Below these pressures Amontons' law was obeyed. A possible explanation for the rise in friction is the increase in shear strength of the lubricant with pressure, while the drop observed is likely to be due to thermal effects. Tests on used oil samples are shown in Figs. 4 and 5 for oils A and D. In Fig. 4, the used oils showed an increase in average friction coefficient with load to a maximum, after which it decreased. In Fig. 5, a decrease in average friction coefficient was observed until it reached a certain limit, when it rose again. A possible explanation for the general decrease in friction observed in tests with used oils is the tendency of the wear products and contaminants to roll rather than slide [6]. Furthermore, the increase in duration distance of oil appears to affect adversely the drop in friction coefficient. The reason may be attributed to the expected increase of wear grits and debris in used oils as the duration distance tends to increase.
169
F.M.H. Ezzat / Friction and wear of fresh and used engine oils .15 .10
Oil A
o
o .~
8 ~o
.I0
~
.05
.075 • Fresh A
~
. (350
.025
Used, i000 Zm
g
•
Fresh A
R
Used, 1000 Kin
+
Used, 2000 Kin
o
Used, 3420 Km
V
Used, 3420 Km Vma x 1.i5 m/s
,a
o 0
40
80
]20
160
Contact load, N Vma x .72 m/s
Fig. 7. Effect of duration distance on friction (oil A). 0
40
8()
120
160
200
240
Contact load, N Oil n
Fig. 4. Effect of oil duration distance on friction coefficient (oil A).
.3( • Fresh D U Used, 3000 Km X Used, 7200 Km .25
.125 Dead center
Oil D o ~
.20
.10
o -~
.15 .075 Average
o=
.i0 .050 • Fresh D O Used, 3000 Zm
g
.05
÷ Used, 7200 Km
.025
Vmax
0
4(3
80
120
.72 m/s
160
200
Hid stroke
240
Fig. 5. Effect of oil duration distance on friction coefficient (oil D). .15 D V •
10
o=
•
--
~_._--O
8
• Fresh A ~
O Fresh B
x
x Fresh C
.05
g
V
Fresh D Vmax 1.15 m/s
o
0
40
80
!20
(]
40
80
120
i60
200
240
Contact load, N
Contact load, N
160
200
Contact load, N
Fig. 6. Effect of load on friction of engine oils A-D. With the maximum mid-stroke speed increased to nearly o n e and a half times its value, the graphs of the load-friction relationship are shown in Figs. 6 and 7. In Fig. 6, the results were collected for fresh oils
Fig. 8. Effect of contact load on friction (oil D). and showed a decrease in average friction coefficient with load increase. Furthermore, the results collected for flesh oils B and C were nearly the same, except at the contact load of 75 N. In Fig. 7, a comparison is made between flesh oil A and used samples of the same oil at two duration distances, 1000 km and 3420 krn. In this graph the average friction coefficient showed a decrease in value with the increase of the oil duration distance. Table 1 shows a comparison between the friction coefficients at different loads and velocities for flesh engine oils A, B, C and D. As shown in Table 1, oils A and D showed an increase in friction with increasing speed at various loads, while oils B and C showed a decrease in friction with speed, but with deviations at some contact loads. Figure 8 shows the effect of contact load on friction for oil D. The results were collected at mid-stroke and
170
F.M.H. Ezzat / Friction and wear of fresh and used engine oils
TABLE 1. Effect of sliding velocity and load on friction coefficient Load (N)
45.9 75.0 104.7 134.2 163.6 193.0
Oil A
Oil B
Oil C
Oil D
~/=0.26 Pa s
",;=0.31 Pa s
"0=0.30 Pa s
"0=0.22 Pa s
0.72 m s -I
1.15 m s -1
0.72 rn s -1
1.15 m s -1
0.72 m s -I
1.15 m s -1
0.72 m s -I
1.15 m s -1
0.0680 0.060 0.0800 0.0920 0.0900 0.0770
0.0850 0.0920 0.0890 0.0950 -
0.0850 0.0740 0.0970 0.0910 0.0870 0.0840
0.0750 0.0830 0.0810 0.0680 -
0.0750 0.0750 0.0750 0.0820 0.0940 0.0850
0.0790 0.0600 0.0790 0.0680 -
0.0840 0.0910 0.1000 0.1000 0.0880 0.0940
0.129 0.110 0.105 0.115
at dead centres. As observed in the figure, high values of friction coefficient were measured over the dead centres. At mid-stroke, friction decreased with the duration distance increase, and a prevailing trend of friction increase with load governed the system behaviour. T h e average values of friction over the whole reciprocating track showed a typical decrease in value with load for used samples of oil D. A coincidence between the values of friction at 3000 km and at 7200 km was observed, with a tendency for the average friction coefficient to be lower than that of fresh oil. 3.2. W e a r results
In this part the experiments were concerned with the wear process of the fresh and used oil samples previewed. Figure 9 shows the variation of wear with oil duration distance for a pin sample and a ring sample of different hardness. As shown in Fig. 9, wear decreased as the duration distance increased. The effect of engine oil type on wear is shown in Fig. 10. T h e results were collected from 1 h tests and showed a decrease in wear with dtlration distance for engine oil D. For oil A, wear decrol!sed with duration 40
distance until it reached a minimum, after which it rose. Table 2 shows a comparison between the wear of the previously mentioned oils at different operating conditions. A sample of used oil at 3420 km, was chosen to show the effect of increasing both sliding distance and contact pressure. Furthermore, Table 2 shows that low wear was obtained with fresh oils B and C, while oils A and D gave higher values, although the contact pressure was nearly twice as high for oils B and C as for oils A and D. As shown in Table 2, oil C seems to be the best at combating wear, and oil D the worst. As already mentioned in the literature [7,8] the antiwear capability of the lubricating oils was directly linked with the thickness of the oil film on the one hand, and roughly with the electromotive force activity (e.m.f.) of the oil additives on the other. A strict relationship relating the e.m.f, to the antiwear performance remains to be demonstrated [7]. Furthermore, it would be dangerous to express the antiwear performance of a certain oil as a high e.m.f. activity of the additives contained, since some of t h e m may lead to wear problems like pitting and corrosion.
.8 0il A
oil A Load 149 N
Load 149 N
Vma x .72 m/s Fin, 200 % ~
Vma x .72 m/s
30 ik IX k k
Ring, 900 VHN ~
Slid. dist• 12.7 Km
d 20 to
C.4
10
.2
0
O
Slid. dist• 12.7 Km
•0
0
1
2
Duration distance,
3
4
Kin x 103
Fig. 9. Effect of d u r a t i o n distance on w e a r (oil A).
i
2
3
Duration distance, Km x 10 3
171
F.M.H. Ezzat / Friction and wear of fresh and used engine oils )
@Oil
A
OOil
D
Loact 105 N
2
Pin, 200 %7-IN
d~3
V
0
1
2
3
4
5
Duration distance,
max
6
.72 m/s
7
8
Km x 103
Fig. 10. Comparison between wear results for oils A-D.
TABLE 2. Effect of sliding distance and contact pressure on wear Sliding distance (km)
Contact pressure (MN m- 2)
Wear (mg) Oil A (duration distance 3420 km)
Oil A (fresh)
Oil D (fresh)
Oil B (fresh)
Oil C (fresh)
12.70 12.70 7.20
1.814 3.318 3.318
31.66 25.86 13.40
31.66 -
31.90 -
17.16
14.60
It was decided to measure the e.m.f, activities of the four engine oils being studied to show to what extent this component might affect the wear phenomenon. Table 3 shows the e.m.f, activities of the four oils, measured in mV. The tests were carried out at room temperature with maximum mid-stroke speed and contact load of 0.72 m s-1 and 105 N respectively. A ring sample was used, the hardness of which was recorded to be 900 VHN, while the hardness of the reciprocating plate was measured to be 240 VHN. The reciprocating plate and the specimen holder were insulated, as shown in Fig. 1, to allow for direct measurements of the e.m.f. The e.m.f, signal was received through a digital voltmeter and a chart recorder of 0.1 mV accuracy. As shown in Table 3, oil B enjoyed the highest activity and oil D the lowest one. Furthermore, the wear of oils B and C proved to be the lower, in spite of the higher contact pressure at which tests were run, but they ranked as follows: W e a r C < Wear B.
The overall rank might be as follows: Wear C < Wear B < Wear A < Wear D. The e.m.f, activities were ranked as follows: Activity B > Activity C > Activity A > Activity D. Although oil B enjoyed a higher activity than oil C, the wear observed with oil C was less than that with oil B. The reason could be attributed to the high chemical activity of the additives in oil B, which might result in material incompatibility problems or corrosive action. Meanwhile, the results for engine oils A, C and D indicated that the activity rank supposed that t h e antiwear efficiency of oil C might be better than that of A and that of A might be better than that of oil D, which were already recorded (see Fig. 10 and Table 3): W e a r C < W e a r A < W e a r D. Furthermore, the overall rank suggested that the e.m.f, activity, and consequently the reaction film related to it, must be adopted as a contributor to the wear mitigation process.
TABLE 3. Chemical activity and wear of engine oils. Sliding distance= 12.7 km Wear (mg)
Oil type fresh
Activity of oil (e.m.f.) (mV) 10 min
20 min
30 min
40 min
50 min
60 min
Oil Oil Oil Oil
7.70 2.00 29.60 -
13.80 29.70 34.70 0.45
8.40 75.00 48.70 0.45
91.90 76.40 0.45
21.00 108.5 82.20 -
28.20 163.5
A B C D
1.17
1.814 MN m -2
3.318 MN m -2
31.66 -
31.9
17.16 14.60
172
F.M.H. Ezzat / Friction and wear of fresh and used engine oils
4. Conclusions
A study of friction and wear of some commercially produced engine oils were undertaken. Fresh oils showed an increase in friction coefficient with speed increase at various contact loads for some oils (A, D), whilst the rest of the oils (B, C) showed a decrease in friction with speed, but with exceptions at some loads. With increasing load, the friction of fresh oils was increased as a result of increasing viscosity. A further increase of load was found to affect friction adversely, which may be due to thermal effects. Low-speed tests for fresh oils showed a constant friction traction over the whole sliding track (Fig. 2). This is probably due to the formation of a separating layer of constant shear strength [9]. A considerable mitigation of the friction was brought about with used oils at certain combinations of load and sliding speed. This mitigation was observed clearly at the higher velocity (1.15 m s -~) and tended to increase with increasing oil duration distance. The reason may be attributed to the formation of an appreciable hydrodynamic film, which reduces the possibility of metallic contacts even at higher values of the duration distance. Furthermore, the presence of wear debris and grits, which may roll rather than slide, adds a further reason for the observed decrease in friction. At the low velocity (0.72 m s-~), the friction of oil A showed low values adversely affected by increasing duration distance and bounded by high values at low contact loads. On the other hand, oil D showed low values, again adversely affected by increasing duration distance and bounded by high values at both low and high loads (see Figs. 4 and 5). This may be explained bearing in mind that at low velocity the hydrodynamic contribution is small and friction tends to increase with an increasing amount of wear debris and grits in the oil. This was observed clearly at low loads (oils A and D). Further increasing the load increases the lubricant viscosity and consequently reduces the ability of the wear particles and debris to move freely in the lubricant. Furthermore,
the sliding system may behave as a slider moving over very small rolling elements in a thin lubricant film. In this region a dramatic decrease in friction is observed, especially at low values of the duration distance (see Figs. 4 and 5). As the load increases further, a considerable increase in friction is observed. The reason may be attributed to the heat generation during sliding, which reduces the oil viscosity and thus invites more loose wear debris and intermetallic contacts, leading to a considerable increase in friction (oil D). Wear losses were found to decrease with increasing oil duration distance. The antiwear capabilities of the fresh oils were correlated to their e.m.f, activities. The exception was oil B, where its higher activity might result in material and corrosion problems.
References 1 W.E. Campbell, Boundary Lubrication: an Appraisal of World Literature, ASME, New York, 1969, pp. 87-113. 2 M.L. Monaghan, Engine friction -- a change in emphasis, Inst. Mech. Eng., Automotive Division, BP Tribology Lecture, 1987. 3 S. Sanda and T. Someya, The effect of surface roughness on lubrication between a piston ring and a cylinder liner, Proc. Inst. Mech. Eng., C223/87 (1987) 135-143. 4 P.C. Nautiyal, S. Singhal and J.P. Sharma, Friction and wear processes in piston rings, Tribol. Int., 16 (1) (1983) 43-49. 5 F.M.H. Ezzat, Measurement of contact resistance and oil activity of fresh and used engine oils at low speed reciprocating, in press. 6 E. Rabinowicz, Friction and Wear of Materials, Wiley, Chichester, 1965. 7 G. Monteil, J. Lonchampt and C. Roques-Carmes, Study of anti-wear properties of zinc dialkyldithiophosphate through sliding-induced electronic emission, Proc. Inst. Mech. Eng., C224/87 (1987) 531-535. 8 P.E. Fowles, A. Jackson and W.R. Murphy, Lubrication chemistry in rolling contact fatigue. The performance and mechanism of one antifatigue additive, Trans. ASLE, 24 (1981) 107-117. 9 A.F. Alliston-Greiner, J.A. Greenwood and A. Cameron, Thickness measurements and mechanical properties of reaction films formed by zinc dialkyldithiophosphate during running, Proc. Inst. Mech. Eng., C178/87 (1987) 565-572.