Physical and Chemical Impact of Sulphuric Acid on Cylinder Lubrication for Large 2-Stroke Marine Diesel Engines

摘要

The influence of sulphuric acid, H_2SO_4, on the physical and chemical behaviour of the engine lubricant in a marine engine cylinder was investigated. To understand the basic interaction of H2SO4 with the lubricant film, the saturated hydrocarbon squalane, C_(30)H_(62), was chosen as a simple model oil in addition to a fully formulated lubricant and its corresponding API Group I base oil HVI160B. To understand droplet formation in the lubricant, the interfacial tension between aqueous H_2SO_4 (0-98%w/w) and C_(30)H_(62) measured in a previous study (Sautermeister, et al. 2011) and is compared with the API Group I base oil HVI160B. Interfacial tension decreases with increasing acid concentration but is much lower for the base oil. Both oils were emulsified with a constant volume fraction of aqueous H_2SO_4. The droplet size was smaller and more distinct for the base oil under the same shearing conditions and the emulsions with base oil were are more stable. The concentration of the dispersed phase had no significant influence on the viscosity of the emulsions compared to the viscosity of the bulk oil alone. In addition to the physical measurements, chemical reaction between H_2SO_4 and the API Group I base oil HVI160B was observed yielding bituminous matter which appears to be a complex emulsion. The critical acid concentration for the formation of bitumen was found to be 40% w/w. Between 50-80%w/w, bitumen was formed spontaneously at 85°C and above 85% w/w, bitumen was formed spontaneously at room temperature. The observed concentrations for bitumen formation correlate well with the first and second dissociation steps of H_2SO_4. (HSO_4)~- ions are likely to be the driving force for bitumen formation. Finally, the viscosity characteristics of two different bitumen were measured. Both were found to exhibit shear thinning and the one formed with highly concentrated H_2SO_4 created a solid deposit on the measurement equipment above 120°C.