ELSEVIER JSAE Review 17 (1996) 11-16

Characteristics of friction and lubrication of two-ring piston

Masaaki Takiguchi a, Hajime Ando b, Takahiro Takimoto b, Akiyoshi Uratsuka b a Department of Mechanical Engineering, Musashi Institute of Technology, l-28-1, Tamazutsumi, Setagaya-ku, Tokyo, 158 Japan

b Graduate School, Musashi Institute of Technology, I-28-1, Tamazutsumi, Setagaya-ku, Tokyo, 158 Japan

Received 10 July 1995

Abstract

This study was aimed at clarifying the characteristics of oil film thickness, piston friction and oil consumption in the two-ring piston. Through experimental studies it was found that the oil film thickness of the two-ring piston is thinner than that of the three-ring piston due to the greater amount of blowby through the piston. As a result, increase of oil consumption of the two ring piston was kept to the low level of the conventional three ring piston. A quantitative analysis on the friction force and the relationship between the oil consumption and oil ring movement were carried out using the unique measuring device developed by the authors.

1. Introduction

The development and use of the two-ring piston are being pursued, aiming at reduction of fuel consumption and size of the gasoline engine. However, only a few studies have been reported regarding the effect of the two-ring piston on conditions of lubrication, friction loss and lubricating oil consumption (LOC) [l], though a num- ber of further studies are required to allow discussion on the effectiveness of the two-ring piston.

This paper describes the characteristics of lubrication and the effectiveness of the two-ring piston (2P) on the reduction of friction loss compared with those of the conventional three-ring piston (3P), based on the results of measurements conducted on the oil film thickness at each ring unit and the piston friction force. The analytical results of the relationship between the LOC (there is a concern that the LOC may increase due to the introduction of 2P) and the behavior of the oil ring, and measures to reduce the LOC for 2P considered under this study are also reported.

2. Equipment and method of experiment

2.1. Specifications of test engines and piston rings

Three engines were used for measurements of oil film thickness at each ring unit, piston friction force and LOC. Table 1 shows the main specifications of the test engines.

The same sectional shape and tension were used for the piston rings of the three engines. Seven kinds of piston ring sets - from A through G - shown in Table 2 were prepared and used in the experiments. The A-set with Table 1 three rings (3P-A) is considered here as the standard specification, while D is a two-ring set (2P-D) with the second ring simply removed from the 3P-A. B is a three-ring set (3P-B) with the tension (IV> of oil ring reduced to make the total ring tension the same with that of 2P, while C is another three-ring set (3P-C) with the tensioner seating tab angle (a,> of the oil ring enlarged from 10” to 20”. E, F and G are two-ring sets aimed at the reduction of LOC of 2P. E has an oil ring with the (or value of 20” (2P-El, while F has the top ring with a tapered sliding surface and a particular ring gap form. G is a combination of the oil ring of E and the top ring of F (2P-G). The groove locations of the top ring and the oil ring of 3P remain unchanged for the 2P while only the second ring groove is removed.

2.2. Measuring method and conditions

Test engine-I was used for the measurement of oil film thickness at each piston ring unit by the LIF method [2] at the piston stroke center and at a point on the thrust side, with the coolant temperature CT,) kept constant at 80°C. At the same time, the cylinder temperature (Tc) at the stroke center was measured. Test engine-II was used for the measurement of piston friction force by means of the

0389-4304/96/$15.00 0 1996 Society of Automotive Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved SSDI 0389-4304(95)00050-X JSAE9541362

12 M. Takiguchi et al./JSAE Review 17 (1996) II-16

Table 1 Specifications of test engines

Type

Engine-I

IDI 4 cycle water cooled

Engine-II

SI 4 cycle water cooled

Engine-III

SI 4 cycle water cooled

Bore X stroke 72x12 85x78 77x80 No. of cylinder 1 1 4 Comp. ratio 23:l 7.7 : 1 9.0: 1

floating linear method [3] with T, kept constant at 90°C. Test engine-III was used for the measurement of LOC with the hydrogen engine method [4]. The oil ring upper rail behavior was also measured by means of the electric capacitance method [5], using the same test engine-III used for the LOC measurement, with electrodes embedded at two locations in the radial direction, as shown in Fig. 1, so that motions outside and inside the upper rail could be measured at the same time.

3. Measured results

3.1. Piston ring oil film thickness

Figure 2 shows the effect of engine load on the oil film thickness in each ring land in expansion stroke at an engine speed of 1500 rpm, measured for 3P-A and 3P-B ring sets. Matched lines in the figure represent the top ring, second ring and oil ring. It is found from the figure that the oil film thickness of the oil ring and those of the second and top rings of 3P-B are thicker than those of 3P-A, due to the lowered tension of the oil ring. Hence the thickness of oil film remaining on the cylinder wall after the top ring had passed is also thicker for 3P-B, and the LOC is likely to increase as a result. Figure 3 show the results of 2P-D and 2P-F measured under similar conditions. In case of the 3P sets shown in Fig. 2, the oil film thickness of each oil ring is 2 pm or so without significant changes from difference in engine load. In the 2P sets, the oil film

Table 2 Test Ring Sets

Piston / I A-A’

Fig. 1. Measuring device of oil ring motion.

thickness is 2 p,rn or so below the zero load condition, while it becomes thinner as the load becomes higher, with 1 pm thickness under full load. As a result, the oil film thickness remaining on the cylinder wall after the passage of the top ring shows a similar value to that of 3P-A. It is also found by comparing 2P-D with 2P-F that the oil film thickness of 2P-F, having the tapered sliding surface, is thinner than that of 2P-D, though the type of oil ring is the same for both of them. Figure 4 shows the comparison of blowby rate between 3P-A, 2P-D and 2P-F ring sets. The blowby rate of each 2P ring set is greater than that of 3P-A, that is, it is 2.5 times greater for 2P-D and 3 times greater for 2P-F, which increases as the load increases. Thus as the load increases, presumably due to the oil starvation that occurs when the oil around the oil ring is blown down by the blowby. The oil film of the top ring also becomes thinner and the oil film thickness remaining on the cylinder wall after the top ring has passed showed a similar value to that of 3P-A accordingly. Figure 5 shows the oil film thickness of the oil ring and that of the top ring, where the second ring gap clearance (c) is enlarged to increase the blowby rate (Z,) for the determination of the effect of blowby on the oil film thickness of each ring. The figure shows that the oil film becomes much thinner where the clearance is doubled, but the oil film becomes thicker if the clearance is further enlarged. This indicates the existence of a complex relationship between the blowby rate and the ring oil film thickness for 3P, hence the oil blow down function cannot be used in a simple manner.

3.2. Piston friction force

Figure 6 shows the results of the piston friction force measurements with a constant engine speed of 1500 rpm and the constant T, = 90°C for 3P-A, those of 3P-B having the same total ring tension as that of 2P by reducing the oil ring tension, and those of 2P-G having the top ring with tapered sliding surface. It is found from the results that the friction of each piston ring set has significant and the lubrication has characteristics of non-fluid lubrication, and that the friction force in the explosion stroke increases

M. Takiguchi et al./JSAE Review 17 (1996) 11-16

0 Load , 1 I2 Load 414 Load

0

E

E4 E

3P-B E 2

0

Fig. 2. Measured results of three-ring oil film thickness (1500 rpm, expansion stroke, T, = 80°C).

0 Load , 15OOrpm Expansion Stroke

II2 Load 414 Load

4

2P-D ? 2 * UlO

3.5

3

z 25

, c. 2

2 1.5 $ 01 z

0.5

0

Fig. 3. Measured results of two-ring oil film thickness (1500 rpm, expansion stroke, Tw = 80°C)

Fig. 4. Blowby measured results (1500 ‘pm, T, = 80°C).

markedly as the engine load becomes high, without signifi- cant differences in friction force between the three differ- ent ring sets. Figure 7 shows the comparison of piston friction forces shown in Fig. 6, in terms of friction mean effective pressure (FMEP) divided by strokes. The FMEP value of 2P-G and that of 3P-B having the same total ring tension are similar. These values are much lower (by 10% or more under full load and 20% or more under half load) than those of 3P-A. This is due to the lowered piston

2

_ 1.8 E a 1.6 ; 1.4

: 1.2

5 0 1

F 0.8

E 0.6

iT 0.4

E 0.2

0 c(mm)=0.35 0.7 1.0 1.5 L(Umin)=0.87 1.2 1.2 1.96

Fig. 5. Effect of blowby in 3P (1500 rpm, expansion stroke, T, = 80°C).

friction forces in the compression stroke (COMP), intake stroke (INT) and exhaust stroke (EXH) which presumably consist mainly of the piston ring friction forces. It is found by comparing 2P-G and 3P-B that the FMEP value of 2P-G is slightly smaller than that of 3P-B. This is because of the smaller FMEP value in the explosion stroke (EXP) which is affected markedly by the friction force at the

14 M. Takiguchi et al./JSAE Review 17 (1996) 11-16

3P-A

3P-B

~-~___~~-~-_i_-~-_-~___~~-_l t_ 1 j / &+/-i---j4

2P-G

Fig. 6. Measured results of piston friction force (1.500 rpm, T, = 90°C).

piston skirt. In other strokes, the FMEP value of 2P-G tends to become greater than that of 3P-B. This is presum- ably due to the fact that the oil film thickness of oil ring 2P becomes extremely thin under a high load as described earlier, which results in a higher friction force of each ring. The lowered FMEP value of 2P-G in the explosion stroke may be attributed to the difference in the piston itself, the drop of lubricating oil viscosity caused by the temperature rise at the piston rings or reduction of area where the oil

Fig. 7. Comparison of FMEP divided by strokes (1.500 ‘pm, T, = 9O’C).

film exists in the skirt due to the blowby, but the exact cause has not been determined to date.

3.3. Lubricating oil consumption (LOC)

Figure 8 shows the comparison of LOC among individ- ual ring sets of 3P and 2P. It is found that the LOC of 3P-B with the oil ring tension set lower than that of 3P-A is higher by 20% to 30% than 3P-A which shows the lowest LOC. It is also found that the effect of top ring sliding surface shape on the LOC of 2P is small, showing a value between that of 3P-A and 3P-B. Hence the LOC naturally increases by lowering the total ring tension sim- ply by reducing the oil ring tension. The method to reduce the total ring tension by reducing the number of rings, on

0 2000 3000 4000 5000

Engine Speed (rpm)

Fig. 8. Comparison of LOC between 2P and 3P.

M. Takiguchi et al./JSAE Review I7 (1996) II-16 15

= 30 > (? 25 f p 20

8 15

J 10

5

0 2000 3000 4000 5000

Engine Speed (rpm)

Fig. 9. Relationship between LOC and cxE

the other hand, should suppress the increase in LOC owing to the oil blow down function with blowby. Figure 9 shows the comparison of LOC between 2P and 3P ring sets. The LOC values of 3P-C and 2P-E having enlarged (or (from 10” to 20”), and those of 3P-A and 2P-D are compared. It is found that the LOC value becomes much greater for 2P by increasing (or to 20”, while the increased czr does not affect the LOC of 3P. An interesting phe- nomenon found from those results is that the LOC is improved in the high engine speed range of 4000 rpm or higher, if T, is raised from 80°C to 90°C.

4. Oil ring motion and LOC

Figures 10(b) and (c> show measured results of motions of upper rails of 2P-E and 2P-D where T, was kept constant at 80°C. The solid lines in the figures represent the outer (OUT) motions of upper rails while the dotted lines represent the inner (IN) motions. Differences between the two kinds of motions are seen most clearly at BTDC 180” CA. Both the outer and inner motions of the rail are

completely apart from the groove upper surface where czr is lo”, while the motions at that point are extremely small due to the increased pushing force of the rail against the groove upper surface where LYE is 20”. The rail inner side in particular hardly moves from the groove upper surface during the compression stroke if the engine speed is high. The motion is contrary to the check valve motion of oil ring proposed by Furuhama et al. [6]. Furthermore, the effect of oil blown down by the blowby in the compression stroke may not be exerted properly. Hardly any difference in motion is found between them at other crank angles, and the difference in upper rail behavior found in the region of BTDC 180” CA to the compression stroke is considered as the cause of the large increase in LOC of 2P-E shown in Fig. 9. Figure 10(a) shows the measured results of motion where T, was increased up to 90°C for 2P-E ((w, = 20”). It is found that the outer side of the rail is separated from the groove upper surface at BTDC 180” CA as the oil ring friction force increases at the top and bottom dead centers. As the engine speed becomes higher than 4000 ‘pm, the inner side of the rail is also separated from the groove upper surface positively, though the amount of motion is small, and the rail behavior becomes closer to that where czr is 10”. It is deduced from the above results that it is necessary to reduce the force that restrains the upper rail to the ring groove upper surface, in order to allow the motion to follow the check valve motion, and to use the oil blown down function caused by the blowby effectively in the region of BTDC 180” CA to the compression stroke.

5. Conclusions

The ring oil film thickness, piston friction force, lubri- cating oil consumption, etc. were measured for two-ring

(a)2P- E (Tw=SO”C)

TDC TDC TDC TDC TDC TDC TDC

3000rpm 4000rpm

Fig. 10. Measured results of oil ring motion (full load),

TDC TDC

5000rpm

16 M. Takiguchi et al./JSAE Review 17 (1996) 11-16

and three-ring pistons, and the following conclusions were reached:

(1) As the oil around the oil ring of a two-ring piston is easily blown down by blowby, oil starvation tends to occur in the oil ring, which reduces the oil film thickness. As a result, the oil film thickness of the top ring and that on the cylinder wall also become thinner, and the amount of increase in lubricating oil consumption (LOC) over that of the standard three-ring piston is relatively small.

(2) The piston friction force can be reduced regardless of the number of piston rings, by reducing the total tension of piston rings, but the lowered tension of oil ring results directly in increase of LOC.

(3) For the oil ring of a two-ring piston, it is necessary to reduce the force that restrains the upper rail to the ring groove upper surface, and to use the oil blown-down function caused by the blowby effectively in order to reduce the LOC.

(4) In case of the two-ring piston, the oil film thickness of the oil ring is affected markedly by blowby. With the

effective use of blowby, the oil ring tension can be reduced without increasing the LOC. Therefore, there is a high possibility of reducing the friction loss.

References

[ll

121

[31

[41

El

[61

Inoue, T. et al., A Study of Friction Reduction on Piston Rings (in Japanese with English summary), Proceedings of JSAE, No. 9436215 (1994). Wong, V. and Hoult, D., Experimental Survey of Lubricant-Film Characteristics and Oil Consumption in a Small Diesel Engine, SAE Paper 910741 (1991). Takiguchi, M. and Furuhama, S., Measurement of Piston Friction Force in Actual Operating Diesel Engine, SAE Paper 790855 (1979). Furuhama, S. and Hiruma, M., Some Characteristics of Oil Consump- tion Measured by Hydrogen Fueled Engine, Journal of the ASLE, Vol. 34, No. 12, pp. 665-675 (1978). Furuhama, S. and Hiruma, M., Axial Movement of Piston Rings in the Groove, ASLE Transactions, Vol. 15, No. 4, pp. 278-287 (1972). Furuhama, S. and Hiruma, M., Same Factors on Engine Oil Con- sumption through a Piston, Proceedings of the JSLE International Tribology Conference, pp. 301-306 (1985).