23rd International Colloquium Tribology - January 2022 203 Squeeze Film Investigations in a Simulating Piston-Ring Cylinder Liner Experimental Set-up Polychronis Dellis School of Mechanical Engineering Educators, ASPETE, Athens, Greece Corresponding author: pasd@city.ac.uk 1. Introduction The study of overall load carrying capacity of the pistonring and liner lubricated interface leads to the tribological explanation and evaluation of the different parameters that affect the lubricated conjunction and the surface interaction. The main focus is on optimising lubrication and promote effective lubrication of the surfaces in contact. As part of this, friction reduction, cavitation initiation and development, which in turn limits the load carrying capacity and measurement of the oil film thickness, flow rate and oil film pressure has become a priority that eventually leads to emissions reduction. The saviour in lubrication terms is the squeeze film effect at low velocities where a thick enough film to sustain the load capacity is non-existent. According to Stachowiak and Bachelor [1], an extremely useful characteristic of squeeze films is that they provide increased load capacity (although temporarily) when a bearing is suddenly subjected to an abnormally high load. As regards to the motion of either side of the bearing surface, the squeeze film force is always opposite in direction to their motion. 2. Squeeze Film: Minimum oil film thickness (MOFT) - Friction force measurements - Load capacity of piston-ring Evidence of squeeze film effect can be found in the minimum oil film thickness measurements where one can notice that the measurement profile is not symmetric. As Bolander et al [2] have pointed out, the point of absolute minimum of the oil film thickness measurement is shifted a few degrees from the TDC and BDC. Friction peaks correspond to asperity interaction, contact between the piston-ring and liner surfaces where the boundary lubrication prevails. The lubricant begins to squeeze out of the contact area while the pressure generated through this squeezing motion shifts the profile towards the center line [2]. The interpretation of the minimum oil film thickness measurements is important as it identifies the lubrication rheology phenomena that in turn, affect the load capacity of a certain piston-ring configuration. 2.1 Experimental set-up In a simplified single-ring test rig, a steady piston-ring section is placed under a flat surface used as a reciprocating liner. The idealised simulation test rig benefits from simplified lubrication conditions compared to the real engine taking advantage of the simple design layout. As a result, solid and repeatable results are taken allowing the lubricant film characteristics to be examined in isolation. Sensors that measure oil film pressure, thickness (optical - LIF and electrical capacitance), friction and imaging, provide the necessary parametric data to study the effect of speed, load, temperature, piston-ring curvature and variable lubricant properties. When the liner decelerates, the interface reaches a state of mixed lubrication and asperity interaction and frictional losses continue to decrease until the liner reaches boundary lubrication close to the dead centers, as the squeeze film prevails. As the liner accelerates away from the dead centers, the lubricated film begins to develop. While being close to the dead center, asperity interaction between the surfaces remains significant with the squeeze film effect also taking place, resulting in beneficial oil support as it is supported partly by the lubricant present in the contact [3]. Increase in temperature in a lubricant model investigation had the effect of decrease in oil film thickness and advanced the initiation of cavitation and enhanced its intensity [4]. Less viscosity results in less squeezing force from the oil around the dead centers and thus greater asperity contact force is generated to support the radial ring load. Less oil squeezing force is responsible for more asperity contact around the dead centers [5]. The purpose of this study is to show the effect of squeeze film variation and extract useful parametric results that show the effect of different lubricants and setups on friction peaks / losses, correlate and verify them to other measurement techniques for the single ring set-up (such as MOFT measurements). Cavitation initiation and development is another factor that should be taken into account and assess whether cavitation development at the beginning of the stroke together with impeding or aiding factors, play a significant role in friction peaks and MOFT minima. Eventually, a clearer picture will be 204 23rd International Colloquium Tribology - January 2022 Squeeze Film Investigations in a Simulating Piston-Ring Cylinder Liner Experimental Set-up attained to the aspects of load carrying capacity of the ring and the rheological behaviour of chemical additives with a view to establishing the likely performance gains in new lubricant formulations. 2.2 Results In a set of experiments focused on high temperature testing, high friction peaks were noticed when oil viscosity changed to lower values as lubricant temperature increases and MOFT decreases significantly [6]. At higher temperatures the asperity interaction at the boundarymixed lubrication region is intense giving considerably higher friction results than the ones taken at lower temperatures. As MOFT decreases with high temperature, friction force peaks move closer to the dead center of the stroke with absolute friction values that are significantly higher. This gives evidence that the squeeze film effect does not have a strong impact at high lubricant temperatures. Figure 1: Temperature effect on friction force peaks at 300 rpm, 3371 N/ m load, top dead center [6]. In previous publications it was shown that for the MOFT measurements, high load testing is combined with squeeze film movement towards the dead centers of the stroke. High temperature testing showed that the MOFT decreases significantly from ambient temperature (33 °C) to 50 °C and that the squeezing action is getting marginal. The same action is shown in Figure 2 for the friction peaks [7]. For a set of different lubricants, the properties of which can be found in Table 1, friction force peaks have different behaviour close to the dead centers. Figure 2 shows that for similar speed and load and temperature testing conditions friction force peaks move closer to the dead centers for the lubricant that has the lowest VI, V40 and HTHS (High Temperature, High Shear) viscosity. Figure 2: Lubricant properties effect on friction force peaks at 300 rpm, 971 N/ m load. It has been verified that different forms of cavitation appear after the dead centers of the stroke that accompany the squeeze film which is measured in the capacitance and friction signals [4, 6, 7, 8]. The geometry of the piston-ring affects the friction force as well. The flatter the piston-ring, the lower the friction force peak and they also appear earlier in the stroke [6]. Table 1: Oils tested for temperature-friction investigations Blend Code 003B 006E/ 02 005A/ 02 002A/ 02 Grade 0W-30 0W-40 0W-20 10W-40 HTHS(mPas) 3.30 3.4 2.14 4.05 V 100 (cSt) 12.16 12.8 6.04 14.97 V 40 (cSt) 68.93 66.8 31 97.8 VI 182 196 146 160 3. Conclusions • The squeeze film effect between the liner surface and the piston ring, shift the friction force peaks, as it forces the lubricant flow to delay compared to the liner movement. • With an increase in load in every lubricant the flow reversal due to the squeeze film effect appears closer to the dead centers. That could be also attributed to the fact that the viscous film impedes the liner’s reciprocation. • With an increase in reciprocation speed for constant load, friction force maxima have lower absolute measurements and appear at a greater distance from the dead centers. • Large radius of curvature for the ring profile promotes effective squeeze action at the ends of the stroke, as the flatter ring enhances a stronger squeeze effect than the curved ring at the dead centers. • Different oil blends produce different appearance for the friction peaks in terms of their distance from the dead centers. • Load capacity is affected by the cavitating region. 23rd International Colloquium Tribology - January 2022 205 Squeeze Film Investigations in a Simulating Piston-Ring Cylinder Liner Experimental Set-up References [1] Stachowiak G.W. and Bachelor A.D., “Engineering Tribology”, ELSEVIER, 1993. [2] Bolander, N. W., Steenwyk, B. D., Sadeghi, F. and Gerber, G. R., “Lubrication Regime Transitions at the Piston Ring - Cylinder Liner Interface”, Proceedings of the Institution of Mechanical Engineers Part J: Journal of Engineering Tribology, Vol 219, 2005. [3] Dellis P.S., “Piston-ring performance: limitations from cavitation and friction, International Journal of Structural Integrityˮ, Vol. 10 No. 3, pp. 304-324, 2019. [4] Nouri J. M., Vasilakos I., Yan Y., “Cavitation between cylinder-liner and piston-ring in a new designed optical IC engine”, Int. J. of Engine Research, (on line first) 9 Apr 2021. [5] Tian, T., Wong, V. W., and Heywood, J. B. “A Piston-Ring Pack Film Thickness and Friction Model for Multigrade Oil and rough surfaces”, SAE paper 962032, 1996. [6] Dellis P., “Effect of Friction Force between Piston Rings and Liner: a Parametric Study of Speed, Load, Temperature, Piston-Ring Curvature and High-Temperature, High-Shear Viscosity”, Proc IMechE, Part J: J Engineering Tribology, 224(5): pp. 411-426, 2010. [7] Dellis P., “Oil Film Thickness Measurements Combined with High Temperature Friction Investigations in a Simplified Piston-Ring Lubrication Test Rig”, Tribology in Industry, 41 No. 4, 471-483, 2019. [8] Dellis P., “Cavitation initiation and patterns in engine lubricants as a result of different operating conditions and lubricant properties”, STLE Virtual Annual Meeting and Exhibition, May 17-20, 2021, New Orleans, USA.