FRICTION AND WEAR CHARACTERISTICS FOR CAM AND FOLLOWER INFLUENCED BY SOOT CONTAMINATION M. SOEJIMA, Y. EJIMA, K. FUKUDA, K. UEMORI, M. KAWASAKI

Department of Mechanical Engineering, Faculty of Engineering, Kyushu Sangyo University, 3-1, Matsukadai 2-Chome, Higashi-ku, Fukuoka, 813-8503 JAPAN; e-mail: soejima@ip.kyusan-u.ac.jp SUMMARY In order to clarify the friction and wear mechanism of the contact between cam and follower in the valve train incorporated with EGR system, an experimental investigation was performed with a cam-follower test rig. A fresh CD-level SAE10W30 multi-grade oil and its deteriorated versions with different contaminants were tested. Changes in friction force and amount of wear were measured during the course of the tests. The influence of the soot in the deteriorated oil was examined by mixing the exhaust gas soot blended with the dispersant and ZnDTP and/or MoDTC additives. Results are summarized as follows. (1) The friction fluctuated and gradually increased with the lapse of time in case of contaminated oils with the soot. (2) Under the coexistence of ZnDTP and MoDTC, however, it became a gradual decrease lower in friction. (3) The soot dispersed in the oil increased wear rate by reducing the anti-wear effect of the ZnDTP. (4) The wear rate became the lowest under the soot contamination as if the anti-wear effect was kept due to the MoDTC.

Keywords: Friction, Wear, Cam Follower, Oil Additives, Soot Contamination

1 INTRODUCTION As referred to some measures to tribology problems for valve trains of internal combustion engines in the preceding papers [1-5], it has been desired to reduce the friction and wear and prevent the scuffing failure by improving the mechanics, the materials, the engine oil properties and others. Recently such technical problems become more serious for the cams and cam-followers of valve trains in advanced engines because of the high contact load of multi-valve, the application of low viscosity multi-grade oils and their deterioration and contamination caused by the carbon soot under the high EGR rate to reduce NOx emission [6-8].

As for the cam and follower contact lubricated with the oil deteriorated and contaminated in the engine, in order to examine the friction and wear characteristics and clarify their mechanisms, the changes of friction force and wear amount with the running time have been measured with the cam follower friction test rig and the test procedure [9]. Particularly, the influences of the soot in the oil on the friction and wear have been examined by mixing with the exhaust gas soot into the oil blended with the additives of the soot dispersant, the wear inhibitor / extreme pressure agent ZnDTP and/or the friction modifier MoDTC.

2 EXPERIMENT METHOD

2.1 Test equipment and method In the friction test rig, as shown in Fig. 1, a follower specimen slips in contact with a cam specimen revolved with an outer electric motor. The contact load between cam and follower was set 930 N in maximum value with an adjusting device of valve spring assembly. The instantaneous change of the friction force with the cam rotation is detected with strain gages adhering to a rod holding the follower and recorded every hour for the test duration of 50 hours long as shown in Fig. 3.

Average values of friction coefficient over the contact duration were estimated from the friction coefficient diagrams and their changes with the running time were compared among the test oils as shown in Fig. 4. The rotating speed of camshaft was constant of 400 rpm as a high wear rate situation [9].

The wear amount in the cam nose direction and the average wear depth over the follower contact face were obtained by measuring the outer diameter with a fine micrometer and the surface profile with a roughness inspector as shown in Fig. 5, respectively, after every test. Such tests were repeated until the stable wear rate was obtained for every case of test oil. The longest test duration was 250 hours as shown in Fig. 6.

2.2 Test materials and lubricants

12

11

12

6

9

78

4

1

2

12

11

12

6

9

78

4

1

2

3

5

10

1 Engine Valve 2 Adjusting Bolt 3 Spring Pre-Load Adjustor4 Adjusting Spacer 5 Valve Springs 6 Bearing Holder-Rod7 Needle Roller Bearing 8 Spherical Joint 9 Follower Specimen10 Follower Holder-Rod 11 Cam SpecimenStrain Gauge

Fig 1: Cam-follower friction test rig

The cam specimen has a base circle radius of 20 mm, an axial width of 13.5 mm and a lift of 7 mm. Materials of cam specimens and follower ones are the hardened ductile cast iron with a Vickers hardness of 6.9 GPa and the chilled cast iron with a Vickers hardness of 7.9 GPa, respectively.

As shown in Table 1, test oils prepared as indices for the present study are a fresh CD-level SAE10W-30 oil coded as A for practical diesel engines and the used one B drained from the engines after the long operation. B oil is much deteriorated and contaminated. It contains heptane insolubles at a weight ratio of 7.7 %. Median value of soot size distribution is 0.054 micron as shown in Fig. 2.

Several test oils from C to I in the table were also prepared to examine the influences of the soot contamination on the friction and the wear. They are SAE20 mono-grade base oil and the SAE20 blend oils with additives of a bis-succinimide dispersant, a secondary ZnDTP and/or an organo-molybdenum compound MoDTC, where each additives is blended at a weight ratio of 3 % and the soot sampled with the exhaust gas filter is dried and mixed at a weight ratio of around 5 % or not. The soot mixed in the test oils was dispersed by a supersonic instrument for more than 24 hours. As shown in Fig. 2 median value of soot size distribution is around 3 micron in D oil without the dispersant but it is 0.3 micron to 0.6 micron in E, F and I oils with the dispersant. The experiment was conducted at a constant oil temperature of 100 degree.

Additionally, as a reason why the SAE20 mono-grade oil instead of the SAE10W-30 multi-grade oil is used for the present experiment, the polymer in the multi-grade oil is easy to be destroyed by the supersonic treatment to disperse the soot in the oil. Under the supersonic dispersion the viscosity of SAE10W-30 oil becomes lower than its origin. Accordingly, it is inevitable to apply the SAE20 mono-grade oil without the polymer having the same high shear rate viscosity as the SAE10W-30 multi-grade oil to the experiment.

3 EXPERIMENT RESULTS

3.1 Friction characteristics In the figures 3 and 4, the changes of the friction coefficient diagram and the average friction coefficient indicate that there are obvious differences for the friction characteristics not only between the fresh oils without soot and the used or mixed oils with soot but also between the oils with and without the additives, dispersant, ZnDTP and/or MoDTC. The following characteristics are found from these test results.

Firstly, during the initial short term of the test, the friction of SAE10W-30 used oil B is lower than that of the fresh one A. This is supposed to be influenced by the soot mediated oil thickening due to the viscosity increase [7] and the promotion of surface running-in due to the soot polishing action.

Secondly, except the case of MoDTC added oils H and I showing the lowest frictions, the friction of the oils

without soot tends to lower gradually with the lapse of time while it is comparatively high just after the start of test. This is due to a general running-in property of rubbing surfaces, that is, the slow change of surface topography, the formation of tribochemical reaction film and others [10, 11]. In cases of the soot mediated oils B, E and F except D, however, such tendency diminishes, and rather the tendency of friction increase with time lapse becomes remarkable as the dispersed soot becomes small in size. But the friction decrease tendency is kept only for the oil I. Further the oil H lowest in friction tends to increase it a little with the lapse of time.

10510.50.10.02

Soot Size , ƒÊ m

q , “

50

40

30

20

10

0

B D E F IOil

Fig 2: Distribution of soot size

Lastly, the timewise fluctuated friction coefficient has an fluctuation amplitude less 0.01 for the oils with no soot A, C, G and H, and the MoDTC added oil with soot I but it has the one more or less 0.02 for the soot mediated oils B, D, E and F. Namely, the friction gradually decreases or increases and fluctuates periodically, and the characteristics of period and fluctuation amplitude depend on the soot mediation and the kind of oil additives. Such variations of friction are supposed to be influenced by the changes of the proportion of true metal contact and boundary lubrication areas to the nominal contact area and the adhesive shear resistance caused by the microscopic change of surface topography, the formation or exfoliation of tribochemical reaction films and others.

In particular, as the soot size becomes small the influence becomes intensive, because the smaller particles of soot are easier to be induced into the interface. Since the change and fluctuation of friction are mainly caused by the formation and the exfoliation of the soft surface layer formed under the tribochemical reaction of oil and additives, it is obvious that the mediation of soot promotes such instabilities. In case of the oils B, E and F, the oil starvation caused by the soot aggregation on the oil entrainment side is supposed to make the oil film thin. The thinner oil film increases the friction and the wear under the higher adhesive shear resistance due to much metal contact as shown in Fig. 7 [12]. Thus the soot contamination increases the friction between cam and follower.

Soot Size, µm

q,

%

Kinematic Viscosity

Metal Content ( ppm )

FeZnPCa

( HCI )( HCIO 4 )

57.58.31.00.5

0.8

4.4

620140160190

+ Soot(5%Wt)

+ Dispersant(3%Wt)+ZnDTP(3%Wt)

56.68.14.20.5

4.0

3.9

15018002000290

51.17.23.70.7

220002500

< 1

Test Oil

Ultra Centrifugal Coagulated

Coagulated Pentan Insoluble( mass% )

SAE20 Base OilCD ClassSAE10W-30Engine Oil

Fresh Used

66.411.40.70.0

3.3

1.5

< 100< 100< 100< 100

46.76.80.00.0

127.016.75.65.4

20.8

5.6

7.7

810015001800300

72.411.02.7

14.515.5

590011001300

00.4332.9350.054 0.323

0.0

0.0

+MoDTC(3%Wt)

+Soot

51.7 61.77.2 8.74.5 4.90.8 0.9

0.0 4.0

0.0 3.9

< 100 1902300 21002900 2500< 1 130

0.556

Code B CA E F G H ID40 °C

Total Acid Number ( mg KOH/g )100 °C( mm /s )2

Soot Size ( Median Value µm )

( mg KOH/g )Total Base Number

Blendof

Table 1: Properties of test oils

In the case of the oil I containing both ZnDTP and MoDTC, however, the friction is the lowest and also the instability of friction caused by the soot mediation is slight. This is supposed to be due to the increase of ability to form the lower shear resistance surface film like MoS2 rich in interface lubricity tribochemically formed under the coexistence of these two additives [13, 14].

3.2 Wear characteristics From the comparison of the wear rates among the test oils shown in Fig. 7 it is made obvious that the influences of the soot contamination on the wear are able to be characterised by several factors of wear such as follows.

Firstly, in all cases of test oils, the wear rate of cam nose is higher than the one of follower. This is supposed to be caused by the reasons that the material of cam is a little harder and the substantial rubbing area of cam nose slipping on the follower is smaller than those of follower, respectively.

Secondly, as a typical influence of test oil on wear, the oils A, G and H formulated or added with additives have low wear rates of 0.03-0.05 micron/hr for the cam nose and 0.01 micron/hr for the follower. But the oils E and F except I with the dispersed soot have wear rates over 0.1 micron/hr. Also the oil B with much insolubles of soot smallest in size indicates the highest one around 1 micron/hr. Thus the wear rate is recognised to vary exponentially among these oil groups.

Accordingly, it is found that the effect of secondary ZnDTP additives on the wear prevention is as significantly large as the one of it on the scuffing resistance [5]. And the influence of soot mediation on the wear is remarkably large so that the effect of ZnDTP tends to be considerably degraded. Further it is resultant that the soot in oil dispersed to the smaller particle size with the dispersant tends to increase the wear rate.

Fig. 3: Changes in friction diagram with running time

Fig 4: Changes in average friction coefficient during the test

(a) Direction of cam sliding

(b) Transversal direction of cam sliding

Fig 5: Wear profiles of follower at 150 hr

Running Time, h

Wea

r Am

ount

, µ

m

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10

100

1000

0 100 200 300

Oil :Cam Nose :

Follower :

A B

Running Time, h

Wea

r Am

ount

, µ

m

1

10

100

1000

0 100 200 300

Oil :Cam Nose :

Follower :

A B

Fig 6: Changes in wear amount during the test

From the fact that the wear rate changes with the soot contamination and the soot size the following physical and chemical wear factors can be considered.

As supposed in the foregoing paragraph, the smaller particles of soot are easier to be induced into the interface between cam and follower. The EHL oil film thickness at the cam nose contact timing is theoretically estimated to be around 0.034 micron under the present test conditions [3]. All of the median values of soot size for the test oils are larger than this oil film thickness but

the one of oil B showing the highest wear rate is comparable with it.

Then it is supposed that the wear depends on the quantity of the soot abrasion comparable with the oil film thickness in size and the intensity of participation with the wear. Namely, the soot acts onto the rubbing surface as a hard abrasion like a wear-debris or a soft

abrasion like a carbon-graphite. Here, as shown in Fig. 7, the wear rate for the cam comparatively low in material hardness increases more relatively than the one for the follower as the soot becomes more dispersed. Then it is found that the abrasive wear is predominant in the present tests because the wear rate is in inverse proportion to the material hardness.

Wea

rRat

e , µ

m/h

0.01

0.1

1

10Cam : Hardened Ductile Cast IronSlip Follower : Chilled Cast IronPre-Load of Valve Spring : 730 NMax. Load of Valve Spring : 930 NRotational Speed : 400 rpmTemperature : 100 °C

A B C D E F G H I

Cam Nose Slip Follower

Wea

rRat

e , µ

m/h

0.01

0.1

1

10Cam : Hardened Ductile Cast IronSlip Follower : Chilled Cast IronPre-Load of Valve Spring : 730 NMax. Load of Valve Spring : 930 NRotational Speed : 400 rpmTemperature : 100 °C

Wea

rRat

e , µ

m/h

0.01

0.1

1

10Cam : Hardened Ductile Cast IronSlip Follower : Chilled Cast IronPre-Load of Valve Spring : 730 NMax. Load of Valve Spring : 930 NRotational Speed : 400 rpmTemperature : 100 °C

A B C D E F G H I

Cam Nose Slip Follower

Fig 7: Comparison of wear rates among test oils

Further, as also discussed in the foregoing paragraph, it is another wear factor whether the oil starvation is easy to be occurred by the soot aggregation on the oil entrainment side or not. The thinner oil film causes the adhesive wear besides the abrasive wear [9, 12].

On the other hand, concerning the change of anti-wear effect for ZnDTP with the soot contamination, the equilibrium between the formation and the exfoliation of tribochemical reaction film rich in anti-wear property, the degradation of additives with the soot and the absorption of additives onto the soot surface enlarged by the dispersion [15] can be considered as a few chemical wear factors.

Lastly, the oil D with only the soot larger in size and the oil I blended with ZnDTP and MoDTC additives have the lowest wear rates of all the present test oils mixed with the soot. Also the wear rate for the oil C with no soot is comparatively high. For the oils H and I the lower and more stable friction characteristics were recognised in the foregoing paragraph. It is noteworthy that both the friction and the wear rate are the lowest for the oils blended with both ZnDTP and MoDTC when the oil is contaminated with the soot. Namely, the MoS2 film formed under the coexistence of ZnDTP and MoDTC additives is also able to maintain the anti-wear property under the soot mediation. It is supposed to be desirous to select and combine the additives so highly potential in effect of additives as to realise both low friction and high anti-wear property under the soot mediation in oil.

As found through the present study, the soot contamination of engine oil is one of the friction and wear factors so that the size of soot and the interaction of soot with oil additives are complicatedly related with the influence of it on the practical friction and wear. Accordingly, some important subjects to be furthermore clarified are left in the future studies on the difference of

influencing intensity of abrasion on wear between the soot and the wear debris, the mechanism how the soot abrasion is interacted with the ZnDTP and MoDTC additives to reduce the friction and wear and others.

4 CONCLUSIONS From these experiment results the friction and wear characteristics of cam and follower lubricated with the soot contaminated oil are summarized as follows.

(1) The friction fluctuates more intensively than the one without the soot and increases gradually with the lapse of time in contrast with it.

(2) However, under the coexistence of the two additives, ZnDTP and MoDTC, in the oil, it gradually decreases with the lapse of time and becomes the lowest.

(3) The soot dispersed in the oil increases the wear rate by reducing the anti-wear effect of the ZnDTP.

(4) The wear rate becomes the lowest under the soot contamination as if the anti-wear effect of the ZnDTP was kept due to the MoDTC. 5 ACKNOWLEDGMENT It is mentioned that financial supports for the present study have been given by the Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science (C : 12650151). 6 REFERENCES [1] Wakuri, Y., et al.: Studies on Friction Characteristics of Reciprocating Engines. SAE Paper 952471 [2] Soejima, M., et al.: Experimental Evaluation of Scuffing Resistance of Cam and Follower. Proc. Int. Tribology Conf., Yokohama, Japan (1996) 3, 1483-1488

[3] Soejima, M., et al.: Studies on Tribology of Cam and Tappet in a Diesel Engine. Proc. 22nd CIMAC Int. Congress on Combustion Engines (1998) 3, 593-608 [4] Soejima, M., et al.: Studies on Friction Characteristics of Cam and Roller Tappet for Engine Valve Train. Proc. 15th ICE Symposium, Seoul, Korea (1999), 185-190 [5] Soejima, M., et al.: Improvement of Lubrication for Cam and Follower. STLE Tribology Transactions, 42 (1999) 4, 755-762 [6] Sasaki, M., et al.: The Effect of EGR on Diesel Engine Oil, and Its Countermeasures. SAE Paper 971695 [7] Kornbrekke, R. E., et al.: Understanding Soot Mediated Oil Thickening. SAE Paper 982665 [8] Gautam, M., et al.: Effect of Diesel Soot Contaminated Oil on Engine Wear - Investigation of Novel Oil Formulation. Tribology International, 32 (1999), 687-699 [9] Soejima, M., et al.: Friction and Wear Characteristics of Cam and Tappet. Proc. 6th Int. Congress on Tribology, Budapest, Hungary (1993) 4, 329-334

[10] McGeeham, J. A., et al.: Some Effects of Zinc Dithiophosphates and Detergents on Controlling Engine Wear. SAE Paper 852133 [11] Willermet, P. A., et al.: The Composition of Lubricant-Derived Surface Layers Formed in a Lubricated Cam/Tappet Contact. Tribology International, 28 (1995) 3, 163-175 [12] Colacicco, P. and Mazuyer, D.: The Role of Soot Aggregation on the Lubrication of Diesel Engine. Tribology Transactions, 38 (1995) 4, 959-965 [13] Kubo. K., et al.: The Effect of Aging during Engine Running on the Friction Reduction Performance of Oil Soluble Molybdenum Compounds. Proc. Int. Tribology Conf., Yokohama, Japan (1996), 745-750 [14] Muraki, M. and Wada, H.: Friction Properties of Organomolybdenum Compounds in the Presence of ZDTPs under Sliding Conditions. In: Lubricants and Lubrication, Elsevier 1995, 409-422 [15] Kagaya, M., et al.: Study on Diesel Soot - A New Proposal on the Mechanism of Wear Promoted by Soot. Journal of Sekiyu Gakkai in Japanese, 40 (1997) 6, 494-499