Evolution of lubricant degradation and lubricant behaviour in a piston assembly of a reciprocating gasoline engine

The most difficult and challenging area for the tribology of reciprocating internal combustion engines to improve efficiency and life is the lubricant health within the piston ring pack. It is here where extreme temperatures, pressures and noxious gases interact with the small volume of lubricant designed to protect the components. There is a lack of knowledge of the correlation between the lubricant condition and its performance. This research is an original contribution to this field and addressed the influence of lubricant degradation on lubricant film thicknesses and residence times in the piston ring pack.

A laser induced fluorescence (LIF) system was first implemented on an operating motored gasoline engine to examine the piston ring to cylinder wall lubricant film thickness. A bespoke optical setup was designed and developed with excellent spatial resolution incorporating an argon ion laser, operating at 488 nm, combined with photomultiplier tubes to measure reference laser and incoming fluorescent light via a sapphire window in the
cylinder liner. Lubricants were doped with Pyrromethene 567A fluorescent dye and the fluorescence signals were calibrated through a strict method, which allowed the fluorescence of degraded samples, and hence the lubricant film thickness, to be quantified. A range of degraded engine lubricant samples were acquired from Mercedes Benz, Leeds, UK and Southwest Research Institute, Texas, USA. The LIF system was then adapted and transferred to a high speed, fired Ricardo Hydra single cylinder gasoline engine. The capability of the LIF system was finally extended to examine lubricant flow in the piston ring pack through a
novel tracer technique, which enabled direct measurement of piston ring pack lubricant residence time.

A LIF system was developed that could clearly distinguish between lubricants of different viscosities and degradation state. It was found that degraded lubricants, with increased
viscosity compared to fresh lubricant, produce thicker lubricant films in the piston ring to cylinder wall interface, which would directly impact on engine efficiency. Additionally, it was found that engine speed, load and lubricant viscosity influence the piston ring pack
lubricant residence time and the replenishment of the lubricant within the piston ring pack.