Investigating lubricants base oils reactivity in ammonia/hydrogen engines via accelerated aging

As a carbon-free molecule, ammonia is more and more considered as a relevant fuel for long distance and off-road applications. However, this gas has different combustion characteristics compared to conventional fuels, challenging the suitability of lubricants to such engines. In this work, the evolution of lubricants under conditions mimicking ammonia combustion was assessed. Mineral and polyester lubricant base oils were exposed to oxygen, nitrogen oxides, and ammonia in a pressurized reactor under stirring. Oil aliquots were sampled at regular intervals, and characterized using Fourier Transform Infrared Spectroscopy (FTIR), viscosity and total oxygen and nitrogen contents measurements. Exposure to air containing nitrogen oxides resulted in quicker accumulation of oxidation products compared to neat air, for both the mineral and complex polyester base oil. Besides, exposure to gaseous ammonia in air resulted in a slower oxidation rate for both oils, compared to neat air. A global measurement of the total nitrogen content after 2 h showed a significant increase for both oils. Under similar conditions, the total nitrogen content of the polyester base oil was higher than the mineral oil, indicating a higher affinity of the ammonia with this matrix. Chromatograms obtained with a Gas Chromatography (GC) column associated to a Nitrogen Chemiluminescence Detector (NCD) showed that nitrogen speciation evolved from nitrogen contained in very light molecules, likely dissolved ammonia, to molecular nitrogen distributed across the entire range of hydrocarbon chain lengths. This demonstrates that ammonia reacted with the base oils and their degradation products. Two-dimensional gas chromatography and mass spectrometry will be performed in the upcoming months to identify these new kinds of degradation products. This study paves the way towards a better understanding of oil reactivity when exposed to ammonia combustion byproducts, including outcomes on engine operation and emissions of pollutants.