The aviation sector is essential for global connectivity and economic growth but remains one of the most difficult industries to decarbonize due to its reliance on energy-dense liquid fuels and the anticipated increase in both carbon dioxide (CO2) and non-CO2 climate impacts. As the industry pursues net-zero emissions by 2050, Sustainable Aviation Fuels (SAFs) have emerged as the most practical near- and medium-term decarbonization pathway because they are compatible with existing aircraft and fuel infrastructure. However, uncertainties persist regarding the sustainability, scalability, economic viability, and long-term climate benefits of different SAF production routes. This review provides a comprehensive assessment of SAFs by integrating feedstock availability, conversion technologies, fuel properties, life-cycle greenhouse gas (GHG) emissions, techno-economic performance, and future deployment prospects within a unified sustainability framework. Major SAF pathways, including Oil-to-Jet (OTJ), Gas-to-Jet (GTJ), Alcohol-to-Jet (ATJ), and Sugar-to-Jet (STJ), are critically evaluated, with particular focus on ASTM D7566-certified fuels such as Fischer–Tropsch Synthetic Paraffinic Kerosene (FT-SPK), Hydroprocessed Esters and Fatty Acids (HEFA-SPK), and Hydroprocessed Algae-Based Fuels (HC-HEFA). Comparative analysis reveals significant differences in feedstock requirements, process efficiencies, production costs, fuel composition, and scalability. Life-cycle assessment (LCA) studies indicate GHG emission reductions ranging from 60% to over 100% relative to conventional jet fuel, with FT-SPK and FT-SPK/A offering the highest mitigation potential. Feedstock cultivation, resource extraction, and fuel conversion are identified as the most emission-intensive stages, underscoring the need for upstream process optimization. The review further examines critical barriers to large-scale SAF deployment, including feedstock scarcity, supply variability, high energy and hydrogen requirements, infrastructure constraints, and high production costs. While HEFA dominates current commercialization, scalable aviation decarbonization will require accelerated deployment of FT, ATJ, and Power-to-Liquid (PtL) pathways, supported by technological innovation, sustainable feedstock expansion, and enabling policies.

The use of sustainable aviation fuel (SAF) can reduce emissions, and increasing the flight ceiling is also a key objective of the poppet valves two-stroke (PV2S) aircraft engine. The high-altitude matching characteristics of the combined setup of a mechanical supercharger compressor (MSC) and turbocharger compressor (TC) and the PV2S engine remains unexplored. A 1D thermodynamic model of the PV2S engine with a combined supercharging system (CSS) was established and verified based on the high-altitude simulation tests (HAST). The results indicate that using MSC as the high-pressure stage and TC as the low-pressure stage is a more optimized solution. Adopting a 5:5 pressure ratio distribution at low altitudes and transitioning to 4:6 at high altitudes improves the efficiency of both MSC and TC while improving trapping efficiency and charging efficiency. Moderating valve overlap can enhance the effective work of TC, and pulse turbocharging improves the utilization of exhaust energy. The effective power decay of the engine using RP-3 aviation kerosene and SAF relative to sea-level decreased from 58.7 % and 58.3 % to 45.1 % and 44.6 % at 2400 r/min and 8000 m. This study provides a theoretical foundation for the matching of PV2S engine and CSS and its power recovery.

Water vapor heterogeneous condensation on nanoparticles to form droplets plays a crucial role in many fields. This process involves a complex gas-liquid-solid three-phase transition. There are two main difficulties in measuring the interface phenomenon at the microscopic level: visualization of particles nucleation and controllable condensation of water vapor. So, we proposed an in-situ observation method for controllable condensation of water vapor by constructing a mixed superhydrophobic-fine particle surface, achieving direct observation of particles nucleation by using Environmental Scanning Electron Microscopy (ESEM). The results reveal that nucleation sites are initially located at the junction of any two-close particles. Although we established a nucleation model, determining the energy barrier proved challenging through analytical expressions or definite integrals due to the complexity of finding the original function. So, we developed the code to describe the quadrature region based on Boolean operation, and solved the energy barrier for the first time by using numerical integration methods. Our results show that the critical energy barrier at the junction of two particles is only 1/37 of that on a single particle. Most notably, when water vapor condenses on complex circular and chain-shaped multi-particles, the three-fold symmetry in circled particles will firstly initiate the nucleation.

The non-volatile particulate matter (nvPM) emitted by aircraft engines poses significant environmental and health challenges, prompting strict international airworthiness standards. However, previous studies have not sufficiently quantified system-level measurement errors, often directly applying commercial instruments designed for conventional aviation fuel (CAF) to Sustainable Aviation Fuels (SAFs). Given that SAF particles exhibit distinct characteristics, including smaller sizes, more concentrated distributions, and unique morphologies, the applicability of existing nvPM measurement systems requires rigorous evaluation. This study comprehensively evaluates the error sources across the dilution, transport, and measurement stages of the measurement chain and proposes compliance enhancement methods, including background pollution purification, instrument calibration, and a particle size distribution (PSD)-based loss correction model. These strategies reduced the uncertainty of the DF1 dilution factor by 73.8% and the relative error of the Condensation Particle Counter (CPC) to 2.124%. Additionally, a PSD-based fitting correction was developed to address penetration losses, achieving a correlation coefficient (R2) exceeding 0.95. Experimental validation using RP-3 and HEFA-SAF demonstrated that these methods effectively improved measurement repeatability and accuracy, reducing the Coefficient of Variation (CV) of the Emission Index (EI) by over 50% and the measurement error by 24%. Notably, the study confirms that SAF exhibits significantly different microscopic characteristics compared to traditional fuel, showing a reduction in peak particle size by approximately 50%, a decrease in number concentration by one order of magnitude, and a more uniform size distribution indicated by reduced Geometric Mean Diameter (GMD) and Geometric Standard Deviation (GSD). However, under high thrust conditions, the higher proportion of ultrafine particles in SAF emissions increases sensitivity to transport losses, highlighting the necessity of expanding the CPC detection limit in future standards and adopting a unified loss correction model based on full particle size spectra. This work provides a validated framework for improving the accuracy of nvPM measurements and supports the integration of PSD into airworthiness certification.

The gas binary diffusion coefficient (D-ij) between fuels and ambient gases plays a critical role in accurately predicting the characteristics of droplet evaporation and flame dynamics. Given the limited availability for measurement data on D-ij of polyoxymethylene dimethyl ether (PODEn), molecular dynamics (MD) simulations were utilized in conjunction with the Green-Kubo (GK) method to systematically assess D-ij of PODEn in a nitrogen (N-2) environment across temperatures ranging from 500 to 1500 K and pressures from 1 to 100 bar for the first time. The MD simulation results were utilized to optimize the Lennard-Jones (LJ) parameters of the Hirschfelder-Bird-Spotz (HBS) equation, including the characteristic length (sigma(ij)) and the well depth (epsilon(ij)) for PODEn at atmospheric pressure. Moreover, an improved Takahashi correlation was proposed using MD results to accurately predict D-ij at high pressures. This combined approach provides a more accurate and broadly applicable parameter framework for predicting the D-ij of PODEn/N-2 systems than conventional empirical correlations. In addition, the influence of D-ij on single droplet and spray evaporation was examined by single droplet and spray simulations, validated against in-house optical measurements. The results demonstrate that employing D-ij obtained from MD simulations significantly improves the accuracy of evaporation rate, gas-phase penetration, and mixing predictions compared to those based on the empirical correlation. This further validates the applicability of MD-based transport property predictions and highlights the importance of accurate D-ij in single droplet and spray evaporation simulations.

Accurately characterizing organic pollutants, especially gas- and particulate- phase Intermediate-Volatility Organic Compounds (IVOCs) from general Aviation Piston Engine (APE), is crucial for quantifying organic Particulate Matter (PM) contributions, formulating emission control strategies, and integrating regulations into the aviation system. In this study, the emission characteristics of conventional aviation fuel and 100% Sustainable Aviation Fuel (SAF) were evaluated. We simulated APE operation under Landing-Take Off (LTO) cycle conditions, focusing on analyzing IVOCs composition and organic PM modal distribution. Results showed that as engine thrust increased, the mass Emission Factors (EFm) of IVOCs for all three fuels significantly decreased, with SAF showing the most notable emission reduction. Its weighted average EFm value was 59.69% and 51.38% lower than Diesel and RP3, respectively. In-depth research on IVOCs distribution patterns also showed that SAF reduced emissions of C9-C20 but slightly promoted C21-C25 formation. Meanwhile, SAF effectively reduced emissions of 2–3-ring, 4-ring, and 5–6-ring Polycyclic Aromatic Hydrocarbons (PAHs). Notably, organic PM mainly consisted of nucleation mode particles, with IVOCs contributing over 62%. These findings offer core data for incorporating IVOCs and nucleation mode particles into future organic PM emission regulations.

Pore condensation enables vapor-liquid transitions below saturation humidity, yet the microscopic dynamics of the embryo formation and cluster growth within the pore remain unresolved. Through molecular simulations on tunable-wettability surfaces (124(degrees)-90(degrees)), we discover that wettability influences condensation kinetics: reducing contact angles 124(degrees)to 90(degrees)from to decreases the molecular displacement by approximate to 60% and the molecular potential energy by approximate to 10% for the condensed water molecules, increasing vapor capture tenfold. Pores enhance nucleation on hydrophobic surfaces (124(degrees)) by restricting molecular mobility, an effect attenuated with increasing wettability. Specifically, we identify four distinct cluster evolution modes: intra-pore (confined growth), on-plane, tumble (pore-to-pore migration), and bounce. Analysis of the above modes reveals that spatial confinement accelerates initial nucleation but suppresses later-stage coalescence. As a result, the cluster growth rate of the intra-pore mode can be initially higher than that of the flat reference but later drops below. This work establishes a microscopic mechanism for pore condensation.

Aviation-emitted soot particles have a significant impact on the global radiation balance and climate change. The emission of these particles around airports poses a serious health risk to surrounding residents. This risk is particularly accentuated when aircraft engines are operating at ground idle and takeoff status during which soot particle emissions are substantially elevated. Consequently, the importance of researching aviation soot particle emissions becomes increasingly evident. The present study investigates the center-staged lean-burn low-emission combustor of a commercial turbofan engine. The impacts of varying different equivalence ratios on the nanostructure, micro-morphology of soot particles during the engine ground idle and takeoff status were analyzed with a combined micro/macro characterization technique. The results indicate that during takeoff conditions, a higher equivalence ratio results in increased graphitization degree of soot particles and fringe length, as well as decreased fringe tortuosity, amorphous carbon content, and oxidation. An increase in the equivalence ratio results in the opposite trends under the ground idle condition. The soot particles during ground idle exhibit lower graphitization compared to takeoff, with the proportion of fringe lengths below 1.5 nm decreasing by approximately 4%. In contrast, the proportion of fringe tortuosity above 1.5 increases by 6%. Additionally, the average particle number concentration increases dramatically, rising from 1.9E4 #/cm3 to 1.1E7 #/cm3. This study provides essential data for optimizing the design of aviation engine combustors and mitigating soot emissions, offering significant implications for improving environmental quality in airport areas.

Ultrafine particles (UFPs) originating from airport operations constitute a significant and growing concern for local air quality and public health. In conventional source apportionment studies, airports have predominantly been treated as a single, aggregate emission source, which overlooks the critical contributions of distinct, ground-based sources within airport boundaries and impedes the development of precise pollution control strategies. To resolve this limitation, a high-resolution spatial monitoring campaign was conducted at representative areas across a major airport, encompassing the terminal area, both ends, and the midpoint of the runway. By applying Positive Matrix Factorization (PMF) to detailed particle number size distribution (PNSD) data, five major emission factors were successfully resolved and quantified: F1—ground service equipment (GSE) (5.8%), F2—aircraft arrivals (20.0%), F3—aircraft departures (53.5%), F4—road traffic (13.9%), and F5—urban background (6.8%). Furthermore, the associations of these source factors with concurrent meteorological parameters, flight activity data and deposited dose of respiratory tract were investigated. This research demonstrated striking spatial heterogeneity in source contributions within the airport, which directly impacts the occupational exposure levels. The 8-h total deposited dose of F3 at site 3# (takeoff runway origin, 6.32 ~ 5.26 × 1011 #) is approximately 55 times that at site 2# (runway midpoint, GSE-dominated). These results underscore the need for spatially refined monitoring and source-specific mitigation policies to reduce population exposure to airport-related UFPs.

To achieve sustainable development in the aviation industry, assessing the environmental benefits of aviation emission-reduction strategies, such as route optimization, requires accurate fuel consumption prediction models. This study proposes a machine learning-based model for predicting aircraft fuel flow rate (FFR) using quick access recorder (QAR) data. We evaluate several machine learning architectures, e.g., the multilayer perceptron (MLP), convolutional neural network (CNN), and gradient-boosted decision tree (GBDT), on two representative aircraft types: the narrow-body Airbus A320 and the wide-body Boeing 787, respectively. Results demonstrate that the GBDT model achieves the highest predictive accuracy, with a coefficient of determination (R2) exceeding 0.97 for the A320, followed by the CNN (R2=0.96) and MLP (R2=0.94). Similar performance trends are observed for the B787. Moreover, the GBDT also demonstrates superior predictive accuracy compared to two open-source models, OpenAP and Acropole. The trained GBDT model is further combined with interpretable analytical techniques to identify the key input features influencing fuel consumption during the cruise phase. As expected, altitude, gross weight, airspeed are the three most significant parameters affecting aircraft FFR. Finally, we present a proof-of-concept demonstration that integrates meteorological data from external sources to predict FFR, laying the groundwork for real-time trajectory optimization in future works.

With the aviation industry committed to achieving net-zero emissions by 2050, sustainable aviation fuels (SAFs)-notably hydroprocessed esters and fatty acids (HEFA)-have emerged as a pivotal decarburization pathway due to their drop-in capability. However, limited understanding of morphological and nanostructure characteristics of soot particles derived from SAF hinders a comprehensive assessment of its environmental and health impacts. This study investigates the microscopic characteristics of soot particles emitted from two general aviation engines operated under low (7%), medium (50% or 60%), and high (100%) loads burning 100% HEFASAF and RP-3 fuels. The results indicate that the Dp of HEFA-SAF soot particles remain smaller than RP-3 across all loads, and HEFA-SAF soot particles feature the morphology of smaller "core" part (amorphous carbon) and thicker "shell" part (concentric graphite layers). Additionally, the findings reveal a close correlation between the elemental ratio of carbon-to-oxygen, internal nanostructures, and the degree of graphitization of soot particles and engine load, while the influence of fuel composition on these parameters demonstrated non-monotonic characteristics, which can be attributed to the kinetic time scales competition between fuel-air mixing and fuel pyrolysis kinetics. Therefore, this study highlights that morphological and nanostructure of soot particles are influenced by coupled effects of fuel composition and operational load. Importantly, while HEFA-SAF effectively reduces soot particle emission concentrations, its derived soot particles microscopic characteristics remain statistically indistinguishable from those of soot particles derived from RP-3. This finding implies that HEFA-SAF implementation can mitigate aviation's particulate climate forcing without enhancing per-particle radiative efficiency, thereby decoupling emission reduction from amplified climate impacts and accelerating aviation industry decarbonization.

Evaporation and condensation have been reported to be able to considerably alter the pinch-off profile of nano liquid threads. However, it is still not well understood how evaporation and condensation will impact the instability of nano liquid threads. In this article, we propose a modified stochastic lubrication equation (MSLE) that incorporates both thermal fluctuations and evaporation-condensation. We conduct stability analysis based on the MSLE and show that the curvature-dependent evaporation tends to enhance the growth of perturbations of small wavenumber but impede those with large wavenumber. For realistic fluids, the effect is actually very small. However, in a supersaturated vapour environment, condensation is dominant. Stability analysis and molecular dynamics simulations both show that the growth of the perturbations is considerably impeded by condensation. The spectrum curves of the perturbations shift to smaller wavenumber and lower magnitude. The effect of condensation becomes very significant on short nano liquid threads whose lengths are around the critical length in Rayleigh's classic theory. Although condensation is usually a slow process, it can completely alter the stability of short liquid threads, rendering an originally unstable thread stable. Our results open up a new avenue to control the instability of nano liquid threads through environmental vapour pressure.

Natural gas is a highly promising low-carbon alternative fuel for internal combustion engines, but its high-octane number and elevated auto-ignition temperature limit direct application in conventional compression-ignition (CI) engines. Glow plug (GP) assisted ignition, relying on high-temperature surfaces, provides an effective thermal strategy to overcome these challenges. In this study, numerical simulations using KIVA-3V systematically investigate GP-assisted natural gas ignition in a high-pressure direct-injection (HPDI) engine. The effects of GP surface temperature, fuel injection angle, and shield design (including the number and spatial arrangement of openings) were examined for three key ignition characteristic parameters: ignition delay by pressure (IDP), ignition delay by temperature (IDT), and ignition delay difference (IDD). The study presents a comprehensive multi-parameter analysis of GP ignition, revealing how shield-opening geometry influences the coordination between ignition onset and flame propagation. For single-and square-opening shields, IDP exhibits angular symmetry with respect to injection angle, primarily determined by initial flame propagation characterized by IDD, with the square-opening design showing the largest IDP fluctuations. In contrast, the diamond-opening shield achieves the closest correlation between IDT-IDP and IDD-IDP, reflecting an effective balance between fuel inflow and flame propagation and yielding the most stable and optimal ignition. Increasing the number of openings without proper geometric arrangement can exacerbate asynchrony between IDT and IDD effects on IDP, thereby reducing ignition reliability. Overall, GP ignition performance is controlled by the complex interaction of thermal, flow, and geometric factors. These findings provide insights into the thermal and geometric design of GP-assisted ignition systems, and offer practical guidance for improving ignition stability and performance in low-carbon engine applications.

Flash-boiling occurs during fuel injection when the ambient pressure falls below the fuel's saturation pressure. This study employs numerical simulations to investigate flash-boiling spray behavior across a wide range of superheated conditions. To accurately reproduce the radial expansion characteristics of spray plumes in the near-nozzle region, a physically adaptive spray cone angle correlation was developed using computational fluid dynamics (CFD) calibrated data. This correlation incorporates the degree of superheat (Rp), fuel surface tension, and the ambient-to-liquid density ratio. Validation across multiple fuels, including iso-octane, n-hexane, ethanol, and ammonia, demonstrates that the proposed correlation accurately predicts spray morphology over a broad range of Rp, effectively capturing key phenomena such as plume collapse and coalescence. The correlation is subsequently applied to high-pressure ammonia-diesel dual direct-injection engine simulations to evaluate its impact on mixture formation. Results reveal that the cone angle expands with increasing Rp and is further modulated by fuel properties and ambient conditions, thereby fundamentally influencing the mixture formation efficiency. Specifically, for transitional ammonia flash-boiling sprays, a wider cone angle increases the spray surface area and mitigates plume collapse. This enhances evaporation and spatial uniformity, which consequently amplifies vaporization cooling, retards ignition, and reduces nitrogen-related emissions. Furthermore, retarding the injection timing weakens the flash-boiling intensity but exacerbates localized cooling effects, leading to significant in-cylinder temperature stratification.

Understanding the physicochemical properties of aviation soot emissions is crucial for elucidating its environmental and public health implications. This study examines the nanostructure, the degree of graphitization, and the surface functional groups of soot collected from the exhaust of CD-135 aviation piston engines (APE) burning RP-3 fuel at 7%, 50% and 100% load, and contrasts theses with soot-generated by the latest MiniCAST 6204C (CAST). The flame characteristics of 6204C are quantitatively described using the global equivalence ratio & empty;, with operating conditions involving fuel-rich, stoichiometric and fuel-lean scenarios. The objective is to evaluate the feasibility of using MiniCAST as a cost-effective soot generator to represent aviation soot emissions. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analysis demonstrate that the nanostructure and the surface oxygen-containing functional groups of CAST soot are similar to those of APE soot. CAST soot exhibits smaller primary particle diameters (D-p) than APE soot. The D-p of CAST soot increases as & empty; rises, while the D-p of APE soot grows with increasing engine load. The geometric mean diameter (GMD) of the electrical mobility diameter of the soot aggregate generated under fuel-lean condition is the closest to that of APE soot aggregates. The low matching of CAST soot with APE soot in the D-p and GMD levels is noteworthy. Raman spectroscopy indicates that the average degree of graphitization of CAST soot under fuel-lean operating condition (& empty;=0.97) is comparable to that of APE soot at all loads compared to fuel-rich operating conditions (& empty;>1). In addition, our results indicate that the & empty; of MiniCAST has a monotonic effect on soot characteristics, while engine load/speed has a non-monotonic effect on the distribution or abundance of graphite defect types and surface functional groups in soot, which is crucial for predicting aviation soot emissions characteristics under different operating conditions. If this discrepancy is not critical for the properties under investigation, we recommend prioritizing the use of soot generated by MiniCAST under fuel-lean conditions as a surrogate for aeroengine soot and further simulating the trend in nanostructure and graphitization changes with increasing engine load by raising & empty;.

Shipboard cabins may experience persistent emissions of oil aerosols and vapors during mechanically intensive operation, while limited ventilation promotes long-term accumulation on surfaces and in materials. To characterize such contamination, we developed a compound-resolved multiphase mass-transfer model coupling gas–particle-surface exchange, dynamic organic-film growth on impermeable surfaces, and depth-resolved transport in permeable materials. Based on qualitative source characterization, 32 oil-borne organic pollutants were simulated under an equal-mass assumption to examine compound-dependent distribution, accumulation, and exposure. Validation of the air–particle–dust transport framework against published indoor measurements showed satisfactory agreement for airborne and settled-dust concentrations. Under zero external ventilation, total suspended particles, particle-phase oil, and gas-phase oil stabilized at 3.2 mg/m3, 1.75 × 103 µg/m3, and 1.5 × 102 µg/m3, respectively, compared with 0.05 mg/m3, 2.15 µg/m3, and 1.75 µg/m3 under ventilation. In permeable materials, the near-surface dust concentration reached about 2.0 kg/m3 under zero external ventilation, approximately 40 times the ventilated value, while the total internal oil concentration was nearly 3.5 × 109 µg/m3 under zero external ventilation and on the order of 107 µg/m3 under ventilation. Upward-facing oil films reached about 700 nm by day 365 under zero external ventilation. Under the conservative continuous-exposure scenario, total daily intake decreased from approximately 5 × 102 µg/(kg·day) under zero external ventilation to 4 µg/(kg·day) under ventilation, while the corresponding screening-level hazard index decreased from 49–62 to 0.25–0.46. These results provide a mechanistic basis for evaluating multiphase pollutant accumulation, crew exposure, and contamination-control strategies in confined shipboard environments.

Insufficient condensation and inefficient droplet removal on functional surfaces limit the performance of water harvesting and thermal management systems. To address these challenges, molecular dynamics (MD) simulations were performed to investigate the condensation and self-driven transport of water molecules (WMs) on wedgeshaped composite copper surfaces with continuous wettability gradients (WGs). The results reveal that condensation efficiency and droplet mobility are jointly governed by the WGs, the degree of surface hydrophilicity, and the wedge vertex angle. Expanding the hydrophilic region and increasing the vertex angle significantly enhance condensation efficiency, whereas droplet transport is regulated by the balance among the WG-induced driving force, Laplace pressure, and adhesive resistance. Notably, a direct comparison between continuous WGs and fixed CA designs inside the wedge demonstrates that continuous WGs provide superior condensation and drainage performance. In particular, the 150/120-0 degrees gradient at alpha = 14 degrees increases the number of condensed WMs by 26.3% and the drainage number by 64.5% compared with the corresponding fixed CA counterpart. Moreover, the droplet migration velocity exhibits a non-monotonic dependence on the WG, and the condensation enhancement by the gradient diminishes rapidly with increasing vertex angle. The continuous gradient improves drainage far more than condensation, indicating that droplet transport is more sensitive to continuous surface energy variation than condensation. These enhancements are attributed to reduced energy barriers and the coordinated driving effect of Laplace pressure and the surface energy gradient. Overall, this study provides valuable molecular-level insights into droplet dynamics on structured WG surfaces, offering theoretical guidance for designing surfaces with optimized condensation and drainage properties.