For the purpose of extracting the frictional vibration characteristics of the friction pair during friction and wear in different friction states, the friction and wear tests of friction pair in different friction states were conducted on a testing machine. Higher-dimensional fractal and multifractal characteristics hidden in time series can be examined by multifractal detrended fluctuation analysis (MFDFA) method. The frictional vibration time-domain signals, the friction coefficient signals and the frictional vibration frequency-domain signals were analyzed and multifractal spectra were acquired by using the MFDFA algorithm. According to the spectra, the multifractal spectrum parameters of these signals were calculated to realize the quantitative characterization of frictional vibration characteristics in different friction states. The analysis shows that it is symmetric in the variation trends of the multifractal spectrum parameters of the frictional vibration signals and the friction coefficient data. Based on the multifractal spectrum parameters of frictional vibration, the principal component analysis (PCA) algorithm was applied to establish the friction state recognition method. The results show that the multifractal spectra and their parameters can characterize the frictional vibrations, and the friction state recognition can be realized based on the multifractal spectrum parameters of frictional vibrations.

Since the implementation of the sulfur cap legislation in 2020, marine very-low-sulfur fuel oil, often known as VLSFO, has become a crucial source of fuel for the contemporary shipping industry. However, both the production and utilization processes of VLSFO are plagued by the poor miscibility of the cutter fraction and the residual fraction, which can result in the precipitation of asphaltene. In this study, biodiesel was chosen as a cutter fraction to improve the stability and compatibility of asphaltene in VLSFO because of its environmental benefit and strong solubility. The average chemical structure of asphaltene derived from the marine low-sulfur fuel oil sample was analyzed using element analysis, FTIR, ¹HNMR, and time-flight spectroscopy. The composition of biodiesel was analyzed using GC-MS. It was found that the asphaltene had a feature of a short side chain, low H/C ratio, high aromaticity, and a high proportion of heteroatoms. Both laboratory experiments and molecular dynamic simulations were applied to investigate the dispersion effect and mechanism compared with other dispersants. The dispersion effect of biodiesel was studied using measurements of the initial precipitation point (IPP), dispersion improvement rate, and morphology of asphaltene in the model oil. Experimental results revealed that biodiesel was fully compatible with heavy fuel oil and that it can postpone the IPP from 46% to 54% and increase the dispersion improvement rate to 35%. Molecular dynamics (MDs) simulation results show that biodiesel can form strong interactions with the fused aromatics structures and heteroatoms in the asphaltene; such interactions can increase the solubility of asphaltene and acts as a “connection bridge” to promote the dispersion effect of asphaltene molecules.

In this study, the effect of mixing a small amount of ammonia on the ignition delay time of methyl decanoate under different conditions was studied from the perspective of the combustion mechanism. The effect of adding ammonia on the ignition delay time of methyl decanoate at different pressures and temperatures was studied by means of simulation calculations and numerical comparison. Integrating the detailed mechanism and reaction path of methyl decanoate, the sensitivity of the ignition delay time was investigated. Analyses of the ignition delay time and rate of production were conducted to explore the transformation and influence of ammonia on the oxidation/decomposition process of the main elementary reaction during the ignition of methyl decanoate. The research illustrated that the ignition delay time of methyl decanoate increased with the number of moles of mixed ammonia at a certain temperature range, and in the negative temperature coefficient region, the effect of ammonia on the ignition delay time was the greatest. In addition, the susceptibility and yield analysis of methyl decanoate showed that the addition of ammonia had a weakening effect on the elementary reactions that originally promoted and inhibited methyl decanoate, and its consumption and production rates were reduced.

The instability of marine low-sulfur fuel oil caused by asphaltene precipitation poses significant operational challenges in the shipping industry. This study systematically investigates the dispersing effects of biodiesel derived from three different feedstocks—palm oil, waste cooking oil (WCO), and microalgae oil—and their individual fatty acid methyl ester components on asphaltenes extracted from VLSFO. Biodiesel was selected as a dispersant due to its renewable nature, polar ester functional groups, and variable unsaturation levels, which enable favorable interactions with asphaltene molecules through hydrogen bonding and π-π stacking. Using UV–Visible spectrophotometry, the dispersion performance was quantitatively evaluated under various conditions including dispersant concentration, temperature, storage time, and molecular structural characteristics. The results demonstrate that microalgae oil biodiesel exhibits the most superior asphaltene dispersion capability among the three biodiesels, with a dispersion improvement index of 35% at 12 g/L, compared to 28% and 22% for waste cooking and palm oil biodiesels. Optimal performance is achieved at 80 °C, where the asphaltene concentration increases by 68% relative to the control, and remains stable within the first 10 days of storage but deteriorates significantly after 30 days due to oxidative degradation. Among individual FAME components, the dispersion effectiveness increases with alkyl chain length from C10 to C20, with the latter reaching a 30% improvement index. Functional group polarity plays a critical role, with carboxylic acid exhibiting a 45% improvement at 14 g/L, substantially outperforming alcohol at 32% and ester at 28%. The degree of unsaturation further enhances dispersion, as the improvement index rises progressively from 20% for saturated methyl stearate to 42% for tri-unsaturated methyl linolenate, representing a 2.1-fold increase. Dynamic light scattering (DLS) measurements confirm that biodiesel addition reduces asphaltene particle size from the micrometer range of 2 to 5 μm down to submicron levels of 200 to 500 nm, while microscopic observations reveal inhibited aggregation. These findings provide theoretical foundations for biodiesel application in marine fuel systems.