Piston ring lubrication-influence of sulphuric acid formation from high sulphur content fuel

Large 2-stroke marine diesel engines burn high sulphur content residual fuel. The sulphur converts to aqueous sulphuric acid, H2SO4, and condenses on the cylinder liner surface, where it causes corrosive attack when not neutralised by the high alkaline reserve in the lubricant. Cylinder liner wear and deposit formation are observed to reflect poor lubricant distribution, surface temperature and sulphur fuel content, which all influence acid concentration. Currently neglected by the industry, is the catalysing action of H2SO4 on lubricant degradation and the rheology of entrained droplets in the oil film, which was the motivation for this study.

To understand the basic interaction of H2SO4 with the lubricant film, the Saturated hydrocarbon squalane, C30H62, was chosen as a simple model oil in addition to fully formulated lubricants and their corresponding API Group I
base oils. Interfacial tension between oil and aqueous solutions of H2SO4 was measured. Contact angles of aqueous solutions of H2SO4 immersed in C30H62 on piston ring chrome coating compared to grey cast iron cylinder liner material were measured. Fully formulated lubricant and API Group I base oil was used pure and emulsified with H2SO4 to lubricate a piston ring/ cylinder contact on a Plint TE77 Tribometer. The main analytical techniques were the light microscope, SEM/EDX, XPS, FTIR, laser diffraction particle size characterisation, white-light-interferometer and viscometer.

Inter facial tension measurements of aqueous H2SO4 against C30H62 reveal a close relationship with the dissociation of H2SO4, which is also reflected by the formation of bituminous emulsions with API Group I base oil when above
40% w/wH2SO4 and formation of solids when above 80% w/wH2SO4 and high temperatures of around 120–165°C or long storage times. For pure water, the contact angles were found to be large while they were small for concentrated sulphuric acid. Contact angles on chrome coating were bigger
than on grey cast iron. Advancing contact angles were larger than static and receding contact angles, which was expected.During the TE77tests,the degradation temperature of the base oil was lowered when in contact with
H2SO4 from 250 to 80°C for oxidation, from 300 to 170°C for nitration and from 300 to 120°C for deposit formation. Over all the appearance of the surfaces improved when sulphuric acid was added to both, the API Group I base oil and the fully formulated lubricant.