Ammonia is a prospective zero-carbon-emission fuel for use in large marine diesel engines. Current research focuses on several technical aspects, such as injection strategies or exhaust gas aftertreatment options, but investigations regarding the impact of ammonia on engine oil degradation are largely absent from the literature. This study provides a methodology with which to evaluate this phenomenon via artificial oil alteration. By using an admixture of various contaminations to air, such as ammonia and its partial combustion product nitrogen dioxide, their respective impacts on chemical oil degradation were assessed. Subsequently, the lubricating performance of altered oils was investigated, with a focus on corrosion properties, deposit formation, and load-bearing capability. Although the application of a stoichiometric ammonia–air mixture resulted in less pronounced thermo-oxidative degradation compared to alteration with neat air, static and dynamic deposit formation as well as corrosion properties and load-bearing capability were severely impacted by the presence of ammonia. On the contrary, nitrogen dioxide contamination resulted in higher oxidation and acidification of the oil, but altered samples performed considerably better than ammonia-altered aliquots in terms of coking tendencies, corrosivity, and load bearing.

Low vapor pressure and several other outstanding properties make room-temperature ionic liquids attractive candidates as lubricants for machine elements in space applications. Ensuring sufficient liquid lubricant supply under space conditions is challenging, and consequently, such tribological systems may operate in boundary lubrication conditions. Under such circumstances, effective lubrication requires the formation of adsorbed or chemically reacted boundary films to prevent excessive friction and wear. In this work, we evaluated hydrocarbon-mimicking ionic liquids, designated P-SiSO, as performance ingredients in multiply alkylated cyclopentane (MAC). The tribological properties under vacuum or various atmospheres (air, nitrogen, carbon dioxide) were measured and analyzed. Thermal vacuum outgassing and electric conductivity were meas- ured to evaluate ‘MAC & P-SiSO’ compatibility to the space environment, including the secondary effects of radiation. Heritage space lubricants—MAC and perfluoroalkyl polyethers (PFPE)—were employed as references. The results corroborate the beneficial lubricating performance of incorporating P-SiSO in MAC, under vacuum as well as under various atmospheres, and demonstrates the feasibility for use as a multifunctional additive in hydrocarbon base oils, for use in space exploration applications.

Introduction: Grease oxidation in rolling bearings causes a loss of grease in the track and hence a loss of lubricity in the bearing. This work addresses loss of grease via evaporation and oxidation due to thermal-oxidative stress applied to two lubricating greases.Methods: For this purpose, the oxidation stability of two greases was determined using a modified micro-coking test over a wide temperature range from 100°C to 190°C. Grease analyses by means of antioxidant content, oxidation and weight loss were used to develop a lifetime model to describe the change in mass as a function of temperature, time, and grease type due to oxidation based on the Arrhenius equation.Results and discussion: The developed model describes the correlation between the findings from grease analyses and shows increased oxidation and evaporation after the induction period when antioxidants are almost consumed.

Due to the demand for higher efficiency, gas engine designs are changing in that gas engine oils are exposed to higher thermal stress and face challenges regarding the prevention of deposits. In this work, the “oil chute” laboratory test was used to study the high-temperature deposit-formation tendency of various paraffinic and naphthenic base oil blends. The oil chute is a setup that circulates oil through two different temperature zones, the hot zone being a heated metal chute that triggers deposit formation. In addition, the thermo-oxidative stability of the oil blends was investigated using an artificial alteration method. The results showed that naphthenic base oils have the capability of substantially reducing high-temperature deposit formation. However, they tend to degrade faster under thermo-oxidative stress. Therefore, finding the right balance between paraffinic and naphthenic base oil components will allow the formulation of gas engine oils that provide the right properties to cope with the higher stress levels they are subjected to in modern gas engines.