Measurement techniques of lubricant oil film behavior on

15th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 05-08 July, 2010

- 1 -

Measurement techniques of lubricant oil film behavior on the piston surface

based on LIF and PIV

Masaaki Kato1, Tsuneaki Ishima2, Tomio Obokata3

1: Graduate school of Engineering, Gunma University Kiryu, Tenjin-cho 1-5-1, Japan,

[email protected] 2: Gunma University, Kiryu, Tenjin-cho 1-5-1, Japan

3: Tokyo Denki University, 2-2Kanda-Nishiki-cho,Chiyoda-ku,Tokyo,Japan Abstract Combination method of laser induced fluorescence method (LIF) and particle image velocimetry (PIV) is applied to measure the oil film thickness and its velocity distributions simultaneously with a test engine. The test engine is 4-cycle and 2-cylinder model engine. One cylinder has sapphire cylinder for observing oil film behavior on the piston skirt. The test condition is under motoring condition and engine speed is set to 700, 1200, 1500 and 2000 rpm. Applying the PIV system to the LIF, a dye of Rhodamine B is added to the lubricant oil with 0.5 g/l. The oil with dilute dye presents the fluorescence pattern which is related with oil film thickness. Two images of the oil film pattern with known time interval provide the information of the distance of the change of the oil film in the time. Then, oil film velocity can be calculated by the images and the time interval. The measurement techniques apply to analyze oil film behaviors on two types of pistons. The difference between two pistons is in the clearance between the piston skirt and cylinder. In the results, the difference of the velocity and thickness at the piston skirt can be observed by the present method. The velocity on the piston skirt with large piston skirt clearance is smaller than that of the other one. The thick oil film for the large skirt clearance piston cannot follow quickly to the piston movement. 1. Introduction The friction loss is one of the biggest factors decreasing the engine efficiency. The main friction is occurred between piston and cylinder. Then, many researchers have been studied on the friction between piston and cylinder [1]. The oil film behavior is a key phenomenon to know the mechanisms of the occurrence of friction between piston and cylinder. The oil film also affects oil consumption and durability of the piston and cylinder. Then, it is important to know the oil film behavior between the piston and cylinder. The oil film behavior is affected by the detail piston shape and conditions of piston rings. When oil film behavior is fully understood, optimization of the piston shape and the piston ring specification can be realized by using theoretical background. It is requested to know the oil film thickness and velocity because they are important factors to know the oil film formation and behavior in the engine. In this research field, the laser induced fluorescence (LIF) is applied to measure the oil film thickness [2-7]. Various combinations of dye and light source are tested by these researches. Some researchers are using combinations of Coumarin and He-Cd laser [2-5]. Combinations of the Rhodamine B and Nd:YAG laser are also tested by some researchers [6,7]. Newer works related with the oil film thickness are also using the LIF method [8, 9]. These studies show the LIF method is one of the useful tools for evaluating the oil film thickness. In the studies, the problems of the LIF method are also indicated. The LIF is the relative estimation and then it needs any kind of proofreading to obtaining the oil film thickness. On the other hand, there are a few studies related with the oil film motion. Visualization of the oil film motion is performed by some researchers [10-12], however detailed velocity components are not appeared in the studies. Particle image velocimetry (PIV) has a possibility to measure the oil film behavior as a velocity map. The PIV technique is based on the visualization and then it presents 2 or 3 dimensional velocity vectors. Ochiai et al. applied the PIV to measure the oil film


- 2 -

velocity [13]. However, there are not enough information on relationship between the oil film thickness and oil film velocity. In the present study, a novel experimental technique for simultaneous measurement of the oil film thickness and velocity is developed. The optical access test engine based on the production engine is utilized to apply the developed measurement method. The oil film behavior with different piston skirt shapes is discussed.

2. Experimental techniques of LIF and PIV 2.1 Principle of LIF method The LIF uses photo-luminescence effect. Some dyes have the feature of fluorescence when they are dissolved in liquid. The fluorescence intensity is affected by mainly temperature, dye concentration, liquid thickness, and wave length of illumination light. The intensity of the fluorescence is represented as following equation:

ChTFKII ),()(0 λλ= , (1)

where, I is intensity of fluorescence light, I0 is initial intensity of laser source, K(λ) is constant depending on the wave length of source light, F(T, λ) is constant depending on source light and temperature, C is dye concentration and h is oil film thickness. Since I0, K(λ) and C can be treated as constant values in the experiment, the fluorescence intensity is affected by only temperature and film thickness. 2.2 Method of LIF and PIV Normal PIV measurement needs some tracer particles to obtain the particle image. However, the tracer particle sometimes causes the damage of cylinder and piston in the engine measurements. In the section, PIV measurement is tried using LIF images without any additional tracer particles. The oil film pattern doesn’t change during the short time interval of PIV illuminations. Then, velocity vectors are obtained by the movement of oil film within a known time interval. The oil film thickness is represented as gray scales of the LIF images. The brightness distribution in a pair of gray scale images instead of particle images are used for calculation of movement of oil film. 3. Experimental apparatus and conditions 3.1Mesuremanet equipment The first step of the LIF method is a selection of dye and laser. Typical combinations are Coumarin with He-Cd laser and Rhodamine with Nd:YAG laser. Rhodamine B with Nd:YAG combination is selected for proposed measurement technique of LIF and PIV combination because many PIV system are using double-pulsed Nd:YAG laser. Rhodamine B is dissolved in Dichloromethane initially. Then, the solution is added to the oil, because it cannot dissolve to oil directly. The peak excitation wavelength of the Rhodamine B is 552nm and emission wavelength of it is from 560 to 682nm. In the experiments, the excitation light source is Nd:YAG laser with 532nm, then the emission lights are mainly observed in the region less than 600nm. The sapphire cylinder used in the optical access engine has a characteristic of excitation and emission to some wavelength bands but it can be neglected because the wavelength band is higher than that of the Nd:YAG and Rhodamine B fluorescence region. Figure 1 shows an example of LIF image picture. The picture is taken under the following


- 3 -

conditions: Two glasses are put together with a platinum wire with known diameter on only one side. The pair of glasses has a space as shown in Fig. 2. The oil with Rhodamine B is filled between two glasses. Then, the fluorescence light by Nd:YAG laser and Rhodamine B is taken by CCD camera. To determine the concentration of Rhodamine B, fluorescence intensities with various concentrations and oil temperatures are measured. The concentrations from 0.1 to 2.0 g/l are examined. Figure 3 indicates the results of 0.1 and 2.0 g/l of Rohdamine B concentrations as the examples. The oil temperature range is from 25 °C (298 K) to 125 °C (398 K) in the figure. In the engine experiment, the linearity of the fluorescence intensity is needed from 0 to about 150 µm and less sensitivity to temperature is expected. On the other hand, enough intensity of the fluorescence light is needed for PIV measurement. From these reasons, the concentration of 0.5 g/l is selected in the optical access engine test for oil film thickness and correct velocity vectors measurement by the LIF and PIV. Since the calibration slope depends on the conditions of optical setup, camera, and dye concentration, it is reconstructed along the experimental engine. From the calibration slope, the gray scale level of the LIF image is translated to the oil film thickness． 3.2 Optical access engine The novel measurement technique is tested in the optical access engine. The engine specification is shown in Table 1. The engine is a boxer type two-cylinder engine. It is based on the production engine of Fuji Heavy Industries Ltd.

Figure 1 Example of LIF image.

Slide glass Platinum wire(300 µm)

Figure 2 Experimental setup for verification of

oil film thickness.

0

50

100

150

200

250

0 50 100 150 200 250 300Oil film thickness µm

Flou

resc

ence

inte

nsity

25℃ 50℃75℃ 100℃125℃

(a)0.1 g/l

Fluo

resc

ence

inte

nsity

0

50

100

150

200

250

0 50 100 150 200 250 300Oil film thickness µm

Flou

resc

ence

inte

nsity

25℃ 50℃75℃ 100℃125℃

(b)2.0 g/l

Fluo

resc

ence

inte

nsity

Figure 3 Fluorescence intensity with two dye

concentrations and various temperatures.

Table 1 Test engine specification.

NAME SUBARU test engine Cycle 4 cycle Cylinder 2 Bore Stroke [mm] 96.9 × 75.0 Compression Ratio 9.5:1 Operation Motoring Engine Rev. [rpm] 1200, 1500, 2000 Cooling Air


- 4 -

Table 3 Oil specification.

The engine has sapphire cylinder to observe inside the cylinder. Engine speed is 1200, 1500 and 2000 rpm in motoring condition. Table 2 shows the specification of the piston. The piston has two piston rings and one oil ring. There are oil drain holes on the bottom side of oil ring as shown in table 2. The main measuring region is the area from the piston head to bottom of the piston skirt on the upper side of piston which includes the piston rings and oil ring. The difference of the piston is in the skirt shape. The clearance on the piston skirt for the one piston is 100 µm (piston A) and that of the other one is 30 µm (piston B).

3.3 Oil conditions The specification of oil is indicated in table 3. Comparison of the features of dissolvent of Rhodamine B shown that the CD class #30 has better quality for using the LIF measurement than the other one. Then, CD class #30 is used for the experiment. The oil temperature is not controlled during the experiment. The oil temperature affects the measurement accuracy in the oil film thickness. However selected dye concentration is insensitive to the oil temperature in the present study. The oil temperature is not controlled but cylinder temperature is monitored during the acquisition. When the cylinder temperature exceeds 350 K, the data acquisition is stopped immediately. 3.4 Measurement setup The basic experimental apparatus is shown in Fig.4. The measurement equipment consists of Nd:YAG laser (New Wave Research, SOLO III-15, maximum power 50mJ, wave length 532nm), CCD camera (Kodak ES-1.0 1008 pix. x 1018 pix.) and PC. In the experiment, it is difficult to illuminate the oil film from the perpendicular direction to the CCD camera. Then, the laser beam is arranged to the same direction of CCD camera. The laser beam is expanded by a convex lens (focal length is 8 mm and diameter is 5 mm) because of getting larger illumination area. Two same notch filters (Edmund, Rugate notch filter, center wave length is 532nm, half width is 26.6nm) are applied to cut the noise light. Timings of laser light illumination and shuttering of CCD camera are controlled by PIV processor (DANTEC, FLOWMAP 2000) and an encoder is used to pick up the crank angle information. The oil film

Total ring tension N 51.0

Tp µm 294

Bd µm 100

TpL mm 19.3

BdL mm 9.1

Skirt

profile

Ovality µm 560

Skirt Length L mm 42.1

Type of oil drain hole Hole type

SAE

#30

CD class #30

Additive [wt%] ― 8.5

Base Oil 500SN [wt%] 100 73.0

Density(15 ºC) [g/cm3] 0.884 0.887

Kinetic Viscosity [mm2/s]

at 100 ºC 10.94 10.87

Kinetic Viscosity [mm2/s]

at 40 ºC 96.97 90.28

Viscosity Index [-] 97 105

Sulfated Ash [wt%] ― 0.77

TBN(HClO4) [mg KOH/g] ― 6.04

Table 2 Piston specification.

Tp

TpLBdL L

Bd

Oil drain holes


- 5 -

thickness is measured by intensity of gray scale of LIF picture. 3.5 Coordinate system and calibration Figure 5 shows an example of LIF image in optical access engine. In the actual measurements, a pair of LIF images is taken. From a pair of images, movement of oil film pattern is calculated. Main measuring regions are on the piston, the piston skirt and the piston rings. The piston moves in horizontal direction in the engine. The measurement is carried out at the upper side of the piston as shown in Fig. 5. To take the LIF image of upper side piston, a mirror is attached on the top of the cylinder. Conversion from the LIF image to the oil film thickness needs calibration table. The known oil film thickness in the cylinder is set by using a spacer called as “feeler gauges” which have constant thicknesses from 5.0 µm to 200 µm. The fluorescence intensity from the given oil film thickness is measured. The reference points are top, center and bottom regions of the cylinder with each 10 degree in the cylinder rotating angle. The preliminary test provides the calibration table. The table uses for translation of the oil film thickness from the LIF image. From the pair of LIF images, the velocity vectors are calculated. The size of captured LIF images is 136 mm x 135 mm. The interrogation area is 64 x 64 pixels.

4. Experimental result 4.1Oil film motion around 4-stroke Oil film motion in each 60deg. from 0deg. to 660deg. with 2000 rpm of piston A is shown in Fig.6. When the piston moves to the cylinder bottom side, thick oil film is introduced into clearance between piston and cylinder. Because of the thrust force, the area in contact piston with cylinder for 360 and 420 degrees is clearly seen more than that of 0 and 60 degrees.

Figure 5 Example of LIF image in the transparent cylinder.

Figure 4 Experimental apparatus.


- 6 -

4.2 Effects of engine speed to oil film thickness Distributions of oil film thickness with different engine speeds at 390deg. are shown in Fig.7. At this crank angle, piston is thrust to the cylinder liner. The tendencies of the distributions of the oil film are almost same in each engine speed. In the upper part of the piston skirt, thicker oil film than 50µm adheres near the oil ring. In the bottom part of the piston skirt, thin oil film is observed where the piston contacts with cylinder liner. For the case of high engine speed, oil film around center of the piston skirt is thinner than that of the low engine speed cases.

Compression

Intake

Expansion

Exhaust

0deg. 60deg. 120deg.

180deg. 240deg. 300deg.

360deg. 420deg. 480deg.

540deg. 600deg. 660deg.

Figure 6 Oil film motion every 60deg. from 0deg. to 660deg. with 2000rpm of piston A

0 100 200 300 µm


- 7 -

4.3 Effects of piston skirt clearance to oil film thickness Figure 8 shows the oil film thickness at 390 deg. of crank angle with both pistons. Both pistons, there are two oil trails on the piston skirt from oil drain holes. Thin oil film area located bottom of the piston skirt of piston B is distributed wider than that of piston A. In addition, the shape of the area is different. This is caused by an assembled clearance of piston B which is smaller than that of piston A.

Figure 9 shows the oil film thickness distributions on center line of the piston skirt with 2000 rpm of engine speed and 390 deg. of crank angle. The oil film thicknesses of the Fig. 9 are picked up from Figs. 8 (a) and 8 (b). The measurement lines are the piston center. The positive position of y is directed to right hand side in the Fig. 8. The origin of the measurement position is set on the piston head. Both pistons have no difference in piston shapes on the centerline, but The oil film thicknesses of piston A at bottom of the piston skirt, y ≤ -55mm, and under the oil ring, -20 mm ≤ y ≤ -

0 100 200 µm

(a) 700rpm (b) 1200rpm (b) 1500rpm (b) 2000rpm

Figure 7 Oil film thickness distributions by LIF in each engine speed with piston A

Oil

film

thic

knes

s µm

y mm

0 100 200 µm

(a) Piston A (b) Piston B

Figure 8 Oil film thickness distributions by LIF with 2000 rpm of engine speed

0

20

40

60

80

100

- 60 - 50 - 40 - 30 - 20 - 10 0

PistonA(CL=100µm)

PistonB(CL=30µm)

Figure 9 Oil film thickness distributions by LIF with 2000

rpm of engine speed

1st ring

2nd ring

oil ring


- 8 -

17mm, are about twice larger than that of piston B. The thinnest oil film thickness is observed in the region from -30mm to -50mm for both pistons. The oil film thickness in the region is about 8µm and it is as large as maximum surface roughness (Rmax) of the piston skirt.

Figure 10 shows the variation of oil film thickness taken over one cycle from 0deg. to 690deg. of clank angle at the center of the piston skirt with 2000 rpm of engine speed. Thickness data are taken every 30deg. of crank angle. Center of the piston is located 21mm from bottom of the piston skirt on the centerline. It is cleared that thicknesses of the oil film near TDC become thin as large as Rmax in both pistons. In contrast, during the intake and exhaust strokes where the piston is moving to down side, oil films become thick. When the piston passes over the center of the cylinder, increase rate of the oil film thickness of piston B is larger than that of piston A. As the result, oil film thickness of piston B becomes thicker than that of piston A at BTC. This result seems to have negative relation with the piston shape, however this has to be considered with overall oil film behavior.

4.4Oil film velocity Figure 11 shows velocity maps on the piston with piston illustration. The engine speed is 2000 rpm and the crank angles of the both results are 300 degree. In the figure, right hand side is piston top side. The velocity vectors can be observed around piston rings and oil rings. The vectors have almost same magnitude and directions. In the area the vector maps are independent from the piston conditions. There are also many vectors on the piston skirt. The distributions of the velocities are different with piston conditions. Since the piston A has broad space on the piston skirt, the oil film velocity becomes smaller than the other piston.

Piston moving direction (Cylinder axis direction) 5m/s

(a) Piston A (b) Piston B

Fig. 11 Oil film velocity maps at 2000 rpm at 300deg.

0

5

10

15

20

25

30

35

40

45

0 90 180 270 360 450 540 630

A-CL100µm

B-CL30µm

Figure 10 Variation of oil film thickness taken over one cycle at

the center of the piston skirt with 200rpm

Oil

film

thic

knes

s µm

Crank angle deg.


- 9 -

Figure 12 shows the velocity distributions of the oil film and the theoretical piston speed with 2000 rpm of engine speed at 300 degree in crank angle. The oil film velocity is calculated on the middle line of the piston skirt, which is 21mm down from the oil ling. More than half of the part on the line, velocity on piston B with 30µm of clearance is larger than that of piston A with 100µm of clearance. Maximum velocity of piston B reaches 8.5m/s, which is almost the same with the piston speed at 300deg. of crank angle. Spatial mean velocity of the oil film for the piston B on the line is almost half value compared with the piston speed. The mean velocity of the oil film of the piston A is about half of that with piston B. This means it is about one quarter of the piston speed. At this line, the oil film velocity of piston A becomes small due to the large clearance of the piston skirt.

Figure 13 shows ensemble average velocity of oil film at the center of the piston skirt with each engine speed. The oil film velocity is obtained by averaging the velocity for 4 cycles. The mean velocities of the oil film with piston B are larger than that of piston A with 1200, 1500 and 2000rpm. The oil film on the piston B follows to the piston quickly compared with the piston A. Maximum velocities increase in proportion to the engine speed in the both piston conditions. At the piston located on TDC or BDC which is represented as every 180 deg. in crank angle, the velocities denote almost 0 m/s. It seems that the oil film retained on the piston skirt can be unaffected by the inertia force. The velocity variation around the TDC has similar tendency to that of the piston. On the other hand, oil film velocities around the BDC of both pistons seem not to follow the piton motion. The thickness of oil film around BDC is thicker than that around TDC as indicated in Fig. 10. Thin oil film formed at TDC can follow the piston motion.

Piston speed at 300deg.

Oil film mean velocity PistonB

Oil film velocity distribution PistonB

Oil film mean velocity PistonA

Oil film velocity distribution PistonA

Middle of the piston skirt

Velocitym/s

-2

0

2

4

6

8

10

Fig. 12 Oil film velocity distributions on the middle of the piston skirt with 2000 rpm of engine speed at 300deg.

-4

-2

0

2

4

0 90 180 270 360 450 540 630 720

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 90 180 270 360 450 540 630 720

-8

-6

-4

-2

0

2

4

6

8

0 90 180 270 360 450 540 630 720

-6

-4

-2

0

2

4

6

0 90 180 270 360 450 540 630 720

(a) 700rpm

(b) 1200rpm

(c) 1500rpm

(d) 2000rpm

Crank angle

Crank angle

Crank angle

Crank angle

Velo

city

m/s

Ve

loci

ty m

/s

Velo

city

m/s

Ve

loci

ty m

/s

piston A piston B piston speed

Figure 13 Variation of oil film velocities at the center of the piston skirt


- 10 -

5. Conclusion

Oil film thickness and velocity are measured simultaneously by using LIF and PIV. With techniques of combination of the LIF and PIV is tested with the pistons with different skirt clearance. The conclusions are followings:

1. Oil film thickness and velocity with each crank angle can be measured by LIF and PIV. 2. Using LIF images, PIV can provide reasonable velocity vectors. The proposed method

promises the simultaneous measurement of oil film velocity and thickness from the LIF images. 3. The velocities of oil film become larger with small clearance than that with large clearance. 4. Oil film thicknesses at the center of the piston skirt with 2000rpm of engine speed are

independent from the skirt clearance.

Acknowledgments

The authors would like to thank to Hidekazu SUZUKI and Yasukazu BABA for their help at making fundamentals for this experiments.

References

1. Shuzou SANDA and Tsuneo SOMEYA, Trans. of JSME Ser. C 55-516. 2. Hoult ,D.P. and Takiguchi, M., STLE Tribology Trans., 34 (1991), 440. 3. Takiguchi, M. et al., Trans. of SAEJ, 29-2 (1998), 71. 4. Sanda, S. et al., Trans. of JSME, Ser. C, 64‐623, (1998), 2653. 5. Murakami, M. et al., Trans. of SAEJ, 25-4 (1994), 74. 6. Thirouard, B. and Hart, A.P., SAE Paper, NO.997203 (1999). 7. Saeki, S. et al., J. of the Visualization Society of Japan, 21 (2001), 19. 8. Sato, Y., et al., SAE Paper, NO. 1999-01-0878 (1999). 9. Nakayama, K., et al., SAE Paper, NO. 2003-01-0243 (2003). 10. Suzuki, H. et al., Trans. of SAEJ, 36-5 (2005), 163. 11. Ting L.L, Trans. of ASME. Ser. F, 102-2, (1980), 165. 12. Suzuki, T. et al., Trans. of JSME, Ser. B, 57-542, (1991), 3603. 13. Ochiai,Y. et al., Proc. of 8th Int. Symp. on Fluid Control, Measurement and Visualization

2005, (2005), Paper No.344. 14. Azetsu., A.. and Ikeda, K., Trans. of JSME, Ser. C, 64-621, (1998).

Documents

Measurement techniques of lubricant oil film behavior on