METHODS:  A piston cylinder-liner contact is simulated using a reciprocating test rig, in which a stationary section of piston ring is loaded against a transparent sliding glass liner specimen. This setup, described in detail in previous studies^2,3, enables the contact to be viewed using both an optical interferometer to measure film thickness and a fluorescence microscope to visualize the distribution of lubricant in and around the contact. The Laser Induced Fluorescence (LIF) technique will be the focus of the present study. The LIF set-up is based on the photo-excitation of a fluorescent dye dissolved in the oil and comprises three main components highlighted in Figure 1.

Figure 1 - Schematic representation of the fluorescent cube and the light path inside the microscope.

RESULTS:  A series of pocket shapes (grooves, chevrons, crosshatch) were assessed under both starved and fully-flooded conditions, producing a series of fluorescent images at different crank angles. The results were used to explain the beneficial or detrimental effects of LST under different operating conditions, as well as to assess the cavitation behavior at reversal.

Employing initially a non-textured specimen it was found that immediately after reversal the bearing’s inlet is starved for more than 5% of the stroke length, despite the contact being fully flooded with oil. This behavior, observed for the first time in this research, explains previous studies which have shown localised wear immediately before and after reversal. Figure 2 displays an image captured 6 degrees of crankshaft revolution after the moment of reversal clearly showing strings of cavitation located at the contact’s inlet.  Following the initial study on cavitation behaviour under fully-flooded conditions, various textured patterns were visually assessed under starved lubrication for a thorough understanding of the impact of pocket orientation upon frictional response.

Figure 2 - Oil distribution captured at 6 degree of crankshaft revolution after the moment of reversal for a fully flooded contact.

For textured patterns comprising grooves oriented perpendicular to the direction of sliding, as well as for crosshatch patterns, a highly beneficial surface texture lubricant transport mechanism is put forward after this investigation⁴. It was observed that pockets perform two important functions in addition to friction reduction: i) they prevent oil from being pushed beyond the reversal points away from the wear track and ii) ensure the contact is fully flooded after reversal. Figure 3 shows a fluorescent image captured at the moment of reversal for the textured pattern comprising features transversal to the direction of sliding. In contrast to the non-textured case, it can be observed that a significant amount of lubricant is available at the contact’s inlet immediately after reversal.

Figure 3 - Pockets transversal to the direction of sliding captured at the moment of reversal.

DISCUSSION: The current research investigated – for the first time simultaneously – friction force, oil distribution and cavitation pattern in a reciprocating, laser surface textured, line contact. The beneficial or detrimental effects of laser surface texture were visualised and assessed for different operating conditions. A series of friction and oil consumption reduction mechanisms were identified and described in detail. The issue of the cavitated outlet becoming the starved inlet was shown to be alleviated by the presence of laser-etched pockets, which were clearly observed to deposit oil into cavitated region as they exit the contact. The oil transfer mechanism⁴ results in both lower friction and lower wear, and partly explains the reductions in friction that are known to result from surface texturing. These findings can support engineering decisions regarding surface texturing by providing specific evidence of its benefits for oil consumption, as well as friction and wear reduction.