2004 AIMETA International Tribology Conference, September 14-17, 2004, Rome, Italy

IMPROVING TRIBOLOGICAL PERFORMANCE OF PISTON RINGS BY PARTIAL SURFACE TEXTURING

Yuri KLIGERMAN(*), Alexey SHINKARENKO(**), Izhak ETSION(***)

Faculty of Mechanical Engineering, Technion Israel Institute of Technology, 32000, Haifa, Israel

Fax +972-4-829-5711 *

Phone +972-4-829-2075, E-mail: mermdyk@tx.technion.ac.il **

Phone +972-4-829-2281, E-mail: alexay@tx.technion.ac.il *** Phone +972-4-829-2096, E-mail: etsion@tx.technion.ac.il

ABSTRACT

An analytical model is developed to study the potential use of partial laser surface texturing (LST) for reducing the friction between a piston ring and cylinder liner. The hydrodynamic pressure distribution and the time dependent clearance between the piston ring and cylinder liner are obtained from a simultaneous solution of the Reynolds equation and the ring equation of motion in the radial direction. The time behavior of the friction force is calculated from the shear stresses in the viscous fluid film and the time dependent clearance. An intensive parametric investigation is performed to identify the main parameters of the problem. The optimum LST parameters such as dimples depth, texture area density and textured portion of the nominal contact surface of the piston ring are evaluated.

NOMENCLATURE

A : reference area, r1W * A : dimensionless area, ( r1W * )/rc2 Bp : axial length of textured zone c(t) : instanteneous nominal clearance between ring and liner C() : instanteneous dimensionless clearance, c/rc Ff*(t) : instantaneous friction force

)(fF : instantaneous dimensionless friction force, Ff*(t)/(paA) Ff : time average dimensionless friction force h : instantaneous local film thickness hp : dimple depth hr : piston ring height H : dimensionless instantaneous local film thickness, h/rc p : instantaneous local hydrodynamic pressure pa : ambient pressure pe : total external pressure on the ring consisting of gas pressure and piston ring elasticity ph : instantaneous average hydrodynamic pressure between piston ring and cylinder liner

pt : pressure above ring pb : pressure below ring P : dimensionless pressure, (p-pa)/pa r1 : half side of imaginary square cell rc : radius of crank rp : base radius of dimple Sp : area density of the dimples t : time W* : piston ring width W : dimensionless piston ring width, W*/rc x, z : Cartesian coordinates in the axial and circumferential directions of the cylinder liner X, Z : dimensionless Cartesian coordinates, x/rc, z/rc : textured portion, Bp/W* : dimple aspect ratio for fully textured piston ring, hp/2rp c : dimensionless dimple depth for partially textured piston ring, hp/rc 1 : bearing number, 3/pa 2 : dimensionless inertia parameter, hrrcpa/2 : dynamic viscosity : piston ring material density : crank angle : dimensionless time, tpa/

INTRODUCTION

Many research works deal with the problem of engine friction force in general and piston rings friction in particular. This is because most of the total engine friction, about (50-60%), comes from the piston-cylinder group.

Ronen et al. [1] examined the piston-cylinder system with laser surface textured (LST) piston rings. The authors studied the potential use of piston ring micro-surface structure in the form of spherical segment micro dimples to reduce the friction between rings and cylinder liner where the entire ring surface in contact with the cylinder liner was textured. It was demonstrated that significant hydrodynamic effects can be generated by surface texturing even with nominally parallel mating surfaces. An optimum value of the micro-dimple depth over diameter ratio was found, which yields a minimum friction force. It was found that a friction reduction of 30% and even more is feasible with textured surfaces. In a subsequent experimental work by Ryk et al. [2] good correlation was found with the theoretical results of Ref. [1].

Two LST modes are available to reduce the friction loses. The first one is the full width LST mode based on the individual dimple effect (local cavitation in each dimple, see for example Etsion et al., [3]). The second mode is a partial LST, which is based on a so called "collective" effect of the dimples that provide an equivalent converging average clearance between nominally parallel mating surfaces (similar to the "inlet roughness" concept in [4]). This collective effect of the partial LST was demonstrated by Etsion and Halperin [5] for high-pressure hydrostatic mechanical seals and by Brizmer et al. [6] for parallel thrust bearings. It was shown [6] that partial LST increases significantly the load carrying capacity compared to full LST. The focus of the present paper is the potential of partial LST to enhance friction reduction obtained with full LST in the piston-cylinder group (see Fig. 1) by utilizing the collective rather than the individual effect of the micro-dimples.

ANALYTICAL MODEL

Fig. 1(b) shows a piston ring segment with partial LST applied at the middle portion of ring width. The textured portion can be located elsewhere as shown schematically in Fig. 2.

The dimples are uniformly distributed with an area density Sp, within the strip pB , Fig. 2. Each dimple is modeled by an axisymmetric spherical segment having a base radius rp located in the center of an imaginary square cell of sides 11 22 rr where ( ) pp Srr 21 = ,(see [1]).

The main assumptions made in [1] on the piston ring motion with a fluid film between the piston ring and cylinder liner are adopted here. Because of the periodicity of the surface texturing in the circumferential direction z, and the symmetry in each axial dimple column of width 12r about its longitudinal axis, it is sufficient to consider the pressure distribution within just one half of one dimple column [1].

Since the clearance between ring and cylinder liner varies with time during each cycle of the ring reciprocal motion, a simultaneous solution of the Reynolds equation and the equation of piston ring dynamics is required. These equations in nondimensional form are given by [1]

12)(2 133

+

=

+

H

XH

ZPH

ZXPH

X (1)

eh PPC

=

22

2 (2)

where () is a trigonometric function governing the kinematics of crank mechanism [1], and P, Ph and Pe are the corresponding nondimensional pressures (see Nomenclature).

A simultaneous solution of Eqs. (1) and (2) provides the time behavior of both the clearance and pressure between the piston ring and cylinder liner surfaces.

The instantaneous friction force between the piston ring and cylinder liner, due to viscous shear stresses (and negligible pressure gradient effect) in the fluid film, acting on one half the dimples column area *1WrA = is givenby nondimensional form by (see [1]):

)()(

3)()( 1

*

HAptF

Fca

ff

== (3)

where

=A H

AdAH )(11

The time average friction force value over one operating cycle of the engine is defined by:

=0

00

)(1 dFF ff (4)

where is the crank angle and 0 can be 360 or 720 deg. depending on whether the engine is a four or a two-stroke one.

RESULTS AND DISCUSSION

The time average friction force (see Eq. 10) is a good measure of the effectiveness of the surface texture. Hence, a parametric analysis was performed to study the effect of the following parameters on the average friction force:

1. Dimensionless dimple diameter, cp rr /2= 2. Area density of the dimples, Sp; 3. Dimensionless textured portion of the piston ring, =Bp/W* 4. Dimensionless dimple depth, cpc rh /=

5. Inertia parameter, 22 acr prh= 6. Dimensionless Piston ring width,

crWW /*=

7. Bearing number, ap31 = 8. External pressure, ( ) aaee pppP =

The two main goals of the parametric analysis are: To determine the optimum parameters of the partial surface texturing (parameters 1-4

above) for given engine structural and operating conditions (parameters 5-8). To investigate the engine structural and operating condition parameter (parameters 5-8

above) effect on the friction loses between the piston ring and the cylinder liner in the presence of partial surface texturing.

A test ring-cylinder system [2] with the following data was selected as a reference case: crankshaft radius, rc=40 mm piston ring width, W*=2 mm W=W*/rc=0.05; lubricant viscosity, =83 mPa.sec crank angular velocity, =1145 rpm (120 rad/sec) 1=3/pa=3.10-4; external pressure, pe =4 atm Pe=(pe- pa)/ pa=3; piston ring material density, =7800 kg/m3; piston ring height, hr=3 mm 2=hrrcpa/(2)=1.4.107.

The partial LST parameters for the reference case were: dimple diameter, 2rp=80 m =2rp/rc=2e-3; dimple depth, hp=12 m c=hp/rc=3e-4; width of the textured portion of piston ring, Bp=1.2 mm =Bp/W*=0.6; area density of the dimples, Sp=0.5.

The parameters of the partial surface texturing and the simulation ring-cylinder system were varied over a wide range.

From many numerical simulations it was found that the inertia parameter, 2, has negligible effect on the average friction force; with a large dimples density, Sp>0.5, the average friction force is hardly affected by the dimensionless dimple diameter . Changing from 1.10-3 to 7.10-3 causes an average friction force change of less than 5%, i.e. it is possible to select the dimple diameter from convenience consideration of the laser texturing system. Numerical simulation also showed that the friction force increases with increasing external pressure as would be expected since higher external pressure acts to reduce the clearance between the piston ring and cylinder liner.

Several cases regarding the location of the texturing zone on the piston ring surface (see Fig. 2) were examined. It was found that the position of the textured portion has very little effect on the friction force; hence, the results of the centrally located textured zone case (Fig. 2(a)) are discussed as a typical representation for partial LST.

Numerical simulation showed the dimensionless average friction force decreases monotonically with increasing dimple area density for various textured portion values. Therefore it is recommended to produce the LST with the largest practical density. An area density larger than 0.5 can be technologically difficult, therefore, Sp=0.5 was selected for the subsequent parametric investigation. The textured portion = Bp/W* has the optimum value and it is =0.6 for a wide range of the piston ring-cylinder liner system parameters.

As evident from the results in Refs. [1-3, 6], the dimple depth plays a major role in affecting the tribological performance of LST components. The effect of this parameter, c= hp/rc, in the present work on the average friction is presented for various bearing number, 1, values in Fig. 3.

As follows from Fig. 3 the average friction force monotonically increases with increasing 1 as would be expected. The optimum value of the nondimensional dimple depth for minimum friction decreases with decreasing 1.

Figure 4 presents the dependence of the average friction force, Ff, on the dimensionless piston ring width, W. As can be seen the dimensionless average friction force is smaller for wider piston rings. The optimum value of c is not too sensitive to variation of W in the range from 0.025 to 0.075.

In order to compare the friction reducing efficiency of an optimum partial LST with that of an optimum full LST the results for the latter case are shown in Fig. 5 that is the equivalent of Fig. 4 for partial LST. The dimple area density in Fig. 5 is 0.24 in accordance with the optimum results of Ronen et. al [1]. Note that the dimple depth ph is normalized differently in Figs. 4 and 5, however the normalization of the average friction force is identical.

From Figs. 4 and 5 it is clear that the minimum friction force in the case of the partial LST is significantly smaller than its corresponding minimum in the case of the full LST. The advantage of the partial texturing is greater at larger piston ring widths. For the narrow piston ring (W=0.025) the minimum friction force, Ff, in the partially textured case (Fig. 4), is about Ff =0.34 compared to about Ff =0.52 in the full textured case (Fig. 5) hence, over 30% less friction can be obtained with the partial LST compared to the full LST. For a wider ring (W=0.075) the partial texturing is even more efficient with 55% less friction than the corresponding full texturing case.

From many numerical simulations it was found that the percentage difference between the minimum friction in the two cases of full and partial texturing is independent of the external pressure, Pe, and the bearing number, 1.

CONCLUSION

A theoretical model was developed to analyze the potential of reducing the friction force between a partially surface textured piston ring and a cylinder liner through the collective" hydrodynamic effect of micro-dimples. The advantage of the partial texturing over the full texturing in view of minimizing friction loses was demonstrated.

The effect of texturing parameters on the average friction force between a piston ring and a cylinder liner was analyzed for a wide range of ring width and operating conditions. The following conclusions summarize the results of the present study: 1. An optimum value = 0.6 of the textured portion can be selected for a wide range of ring

width and operating conditions. Similarly, an optimum dimple depth can be found depending on ring width and operating conditions.

2. The average friction force is not affected by the dimple diameter and it decreases monotonically with increasing dimple area density. Hence, the largest possible density that is dictated by the LST system should be selected.

3. The influence of the ring width and operating conditions on the average friction force in the case of the partial texturing is similar to that in the full texturing case. The dimensionless friction force, Ff, is smaller for smaller bearing number, 1, smaller external pressure, Pe, and larger ring width, W. The average friction force is not affected by the inertia parameter, 2.

4. The minimum average friction force for the optimum partially textured piston ring is significantly lower than that for the corresponding optimum fully textured ring. The difference varies from about 30% reduction for narrow rings to about 55% reduction in wide rings. These differences are independent of the external pressure, Pe, and the bearing number, 1.

REFERENCES

[1] Ronen, A., Etsion, I., and Kligerman, Y., Friction Reducing Surface Texturing in Reciprocating Automotive Components, STLE Tribology Transactions, Vol. 44, 2001, pp. 359-366.

[2] Ryk, G., Kligerman, Y., and Etsion, I., Experimental Investigation of Laser Surface Texturing for Reciprocating Automotive Components, STLE Tribology Transactions, Vol. 45, No. 4, 2002, pp. 444-449.

[3] Etsion, I., Kligerman, Y., and Halperin. G., Analytical and Experimental Investigation of Laser-Textured Mechanical Seal Faces, STLE Tribology Transactions, Vol. 42, 1999, pp. 511-516.

[4] Tonder, K., Inlet Roughness Tribodevices: Dynamic Coefficients and Leakage, Tribology International, Vol. 34, 2001, pp. 847-852.

[5] Etsion, I. and Halperin, G., A Laser Surface Textured Hydrostatic Mechanical Seal, STLE Tribology Transactions, Vol. 45, 2002, pp. 430-434.

[6] Brizmer, V., Kligerman, Y., and Etsion, I., A Laser Surface Textured Parallel Thrust Bearing, STLE Tribology Transactions, Vol. 46, 2003, pp. 397-403.

Figure 1. Segments of piston rings: (a) fully textured; (b) partially textured.

Full LST piston ring segment

Partial LST piston ring segment

Textured Friction Surface

(a) (b)

Figure 2. Different locations of the textured zone: (a) symmetrically in the center; (b) symmetrically at both ends; (c) arbitrarily at a distance d from the ring center

Figure 3. Dimensionless average friction force, Ff, vs. dimensionless dimple depth, c, for various bearing numbers, 1.

Bp

W *

(a)

z

x

d

x

W * Bp

(c)

z

x

Bp/2

(b)

W *

z

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1 2 3 4 5 6 7Dimensionless dimple depth, cx104

Dim

ensio

nle

ss fr

ictio

n fo

rce,

F f

=0.6, Sp=0.5, W=0.05, =0.002, P =3 =0.002

1111=5x10-4

1111=3x10-4

1111=1x10-4

1111=0.5x10-4

Figure 4. Dimensionless average friction force, Ff, vs. dimensionless dimple depth, c, for

various ring widths, W.

Figure 5. Dimensionless average friction force, Ff, vs. dimple aspect ratio, ( )cp rh 2= , for fully textured piston ring with various width, W.

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

1 2 3 4 5 6 7

Dimensionless dimple depth, cx104

Dim

ensio

nle

ss fr

ictio

n fo

rce ,

F f

W=0.025

W=0.05

W=0.075

=0.6, 1=3x10-4, Sp=0.5, =0.002, Pe=3

0.45

0.50

0.55

0.60

0.65

0.70

0.05 0.1 0.15 0.2 0.25

Dimple aspect ratio for fully textured ring,

Dim

ensio

nle

ss fr

ictio

n fo

rce ,

F f

W=0.025

W=0.05

W=0.075

=1, 1=3x10-4, Sp=0.5, =0.002, Pe=3