Low-pressure ammonia injection suffers severe ammonia slip and high NOx emissions. In contrast, dual direct injection of ammonia and dimethyl ether (DME) offersa promising low-carbon alternative. However, the underlyingmechanisms, especially how the ammonia post-injection affects the combustion mode, remain unclear. This study investigates how the injection strategy affects combustion mode transition, performance, and emissions in an ammonia/DME engine. Results reveal two distinct combustion modes governed by ammonia post-injection timing: an early-injection RCCI-like mode and a late-injection DDFS mode, with opposite effects of premixed ammonia ratio on combustion phasing. Crucially, the transition between these modes is not dictated by the injection sequence alone, but arises from a dynamic competition between concentration and reactivity stratifications, controllable through the ammonia pre-injection ratio, DME ratio, and DME injection timing. This stratification competition defines a critical transition point that may shift away from the DME injection timing under weakened reactivity stratification, overturning the conventional understanding. The RCCI-like mode favors higher indicated thermal efficiency, while the DDFS mode shows superior NOx and NH3 emission reduction, providing clear guidance for combustion strategy selection.

This study addresses the critical gap in understanding fire propagation in hybrid electric vehicles (HEVs) by investigating the synergistic effects of battery thermal runaway and gasoline combustion. This research employs both experimental validation and numerical simulation to develop a validated full-vehicle fire simulation framework, incorporating necessary simplifications to analyze fire dynamics, focusing on the influence of battery state-of-charge (SOC) and the effectiveness of aerogel insulation. The model captures the three-phase fire progression: thermal diffusion within the battery, ignition of surrounding materials, and rapid full-vehicle fire spread. Higher SOC levels lead to earlier thermal runaway, faster fire development, and increased overall temperatures. Critically, the presence of gasoline significantly amplifies fire severity. With a 100% SOC, peak combustion occurs at the fuel tank, reaching over 1200 degrees C and a peak heat release rate of 9000 kW. A 100 mm aerogel insulation barrier effectively prevents fuel tank ignition by limiting the fuel tank temperature to approximately 200 degrees C, while a mere 10 mm layer proves inadequate. These findings highlight the critical need to consider SOC-fuel interactions in vehicle safety design and demonstrate the effectiveness of aerogel insulation in mitigating HEV fire risks.

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.

The identification of alternative fuels and the reduction of carbon emissions are key to advancing future engines. Low-carbon-neutral fuels, being more volatile and higher in oxygen than conventional diesel, are gaining widespread use. Moreover, flame-wall interaction is an inevitable consequence of direct fuel injection strategies. To optimize fuel properties and develop efficient, environmentally friendly combustion technologies, it is essential to study the spray and near-wall flame behaviour of various alternative fuels. Polyoxymethylene dimethyl ether (PODE), known for its high cetane number (CN) and oxygen content, can enhance ignition and combustion performance when blended with diesel, particularly in low-oxygen environments. By using a constant volume combustion chamber (CVCC) with integrated optical diagnostics and under different oxygen conditions, the study examined the spray and combustion characteristics of n-dodecane (Nc12) blended with PODE3 at different ratios (pure Nc12, P25 [75 vol% Nc12 + 25 vol% PODE3], and P50 [50 vol% Nc12 + 50 vol% PODE3]). Key findings revealed that wall interaction significantly enhances the influence of PODE3 on ignition behavior and flame lift-off length behavior. The addition of PODE3 results in a noticeable reduction in wall OH* intensity, with this effect being more pronounced at elevated injection pressures and under 21 % oxygen concentration. Notable morphological differences in natural luminosity were observed between the two oxygen levels. The practical implications of this work lie in the potential for PODE3 blends to improve combustion efficiency and reduce soot emissions, which are crucial for advancing environmentally friendly technologies in diesel engines, particularly under oxygen-lean conditions simulating high exhaust gas recirculation (EGR) scenarios.

Polyoxymethylene dimethyl ethers (PODEn/OMEx) are increasingly recognized as promising low-emission alternatives or blending components for petroleum diesel, supported by favorable physicochemical characteristics and a strong potential to reduce particulate matter and greenhouse-gas emissions while improving energy security through fuel diversification. This review synthesizes evidence from approximately 300 studies to evaluate PODE across the full value chain, encompassing feedstock options, production routes, techno-economic considerations, catalytic synthesis pathways, and the chemical-kinetic understanding of combustion. Particular emphasis is placed on how PODE's defining properties, high oxygen content, absence of C–C bonds, and elevated cetane number translate into distinct spray and combustion behavior in compression-ignition engines. Compared with conventional diesel, variations in density, viscosity, volatility, and surface tension shape atomization quality, spray penetration, droplet size, and mixing processes, which in turn influence ignition, heat-release characteristics, combustion efficiency, and emissions formation. The review integrates experimental diagnostics (including optical studies), computational fluid dynamics, and mechanistic analyses to consolidate current knowledge and identify critical gaps that limit technological readiness, particularly material compatibility concerns associated with fuel polarity and the need for injection-system optimization to ensure long-term durability and stable spray patterns. By consolidating these opportunities and challenges under realistic operating conditions, this work provides a coherent framework to guide future research on tailored combustion strategies and engine designs, supporting the broader adoption of PODE-based fuels in cleaner, more efficient transportation energy systems.

Rising global energy demand, coupled with concerns over climate change and the depletion of fossil fuels, has intensified interest in carbon-neutral combustion alternatives. E-diesel, produced by synthesizing green hydrogen with carbon dioxide captured from industrial emissions or directly from the atmosphere, represents a promising pathway for reducing greenhouse gas emissions. It shows significant potential as a substitute for conventional diesel in compression ignition (CI) engines. This study experimentally investigates the spray and combustion characteristics of E-diesel under engine-relevant conditions. To support the development of a chemical kinetic mechanism and enable its application in fuel–engine optimization simulations, a surrogate fuel was formulated by blending selected alkanes to replicate the key physicochemical properties of E-diesel. Experiments were conducted in a constant-volume combustion chamber using multiple optical diagnostic techniques under varying ambient temperatures, injection pressures, and oxygen concentrations. These measurements enabled detailed characterization of spray morphology, flame structure, and in-flame soot formation. Conventional diesel fuel was also tested under identical conditions for comparison. The results demonstrate that the surrogate fuel successfully reproduces several physicochemical properties of E-diesel and exhibits similar spray and combustion behavior. Due to the absence of high-boiling aromatic and heavy hydrocarbon components, E-diesel exhibits a shorter liquid penetration length. Its higher reactivity leads to reduced ignition delay and shorter flame lift-off length compared to conventional diesel. Furthermore, in-flame soot formation is significantly lower, despite a relatively higher average equivalence ratio at the lift-off length. This reduction in soot formation is primarily attributed to the near-zero aromatic hydrocarbon content of E-diesel

Wall cooling losses not only constrain engine thermal efficiency but also influence component durability. In this study, numerical simulations were conducted to examine the effects of ambient temperature, oxygen concentration, and ambient density on flame morphology and transient wall heat transfer during the spray-wall impingement process. The results indicate that increasing the ambient temperature shortens the wall heat flux response time, intensifies the thermal motion of gaseous fluids, enhances boundary layer disturbances, and collectively strengthens wall heat transfer. In contrast, reducing the oxygen concentration suppresses the combustion heat release rate, lowers the flame temperature, and decreases wall heat transfer rates. An increase in ambient density promotes gas-liquid two-phase mixing and improves oxygen utilization efficiency, raises the peak wall heat flux. During the spray-wall impingement process, the wall heat flux exhibits a distinct "step-like" pattern. When the ignition delay time (IDT) significantly exceeds the impingement time (Timp), the first increase in heat flux is primarily caused by forced convection, while the second peak arises from combustion heat release. Conversely, when the IDT is close to Timp, only a single heat flux peak is observed.

Ammonia is a promising zero-carbon fuel for heavy-duty transport, but its low reactivity leads to incomplete combustion. The dual direct injection (DI) of ammonia with soot-free, high-cetane dimethyl ether (DME) as a combustion promoter offers an effective solution. However, research on the ammonia/DME dual direct injection remains limited, and the quantitative characterization of ammonia concentration stratification, and its mechanism on combustion phasing and emission pathways, is still unclear. This study employs numerical simulations to investigate the effects of ammonia stratification and DME injection strategies on engine performance and emissions, and develops a method for quantifying the degree of ammonia concentration stratification. The results indicate that a critical injection timing threshold of -240 degrees CA ATDC was identified. Injections before this point yield minimal stratification, but later injections generate substantial stratification, which in turn delays combustion phasing. Moderately delayed ammonia injection reduces unburned ammonia, but overly late injection raises heat transfer loss. Late DME injection shifts the combustion mode to diffusion-controlled combustion, leading to increased flame temperature and significantly higher NOx and greenhouse gas emissions. A synergistic effect was observed between the ammonia and DME injection strategies. The piston motion reduces the variations in ammonia concentration stratification caused by different injection timings. The magnitude of this effect is determined by the DME injection timing: retarded injection reduces the influence of ammonia injection timing, while advanced injection enhances mixture stratification.

Soot formation in diesel-like spray combustion remains a critical issue in the design of low-emission engines. In particular, wall-impinging sprays, which are common in modern high-pressure fuel injection systems, introduce complex interactions between flame, wall, and flow field that significantly affect soot evolution. This study employs numerical simulations to investigate soot formation and oxidation in wall-impinging diesel sprays, with variations in wall distance, wall temperature, injection pressure, ambient temperature, and ambient oxygen concentration. Virtual Particle Tracking (VPT) method is used to analyze the thermochemical history of soot parcels. Results reveal that the increase in total soot mass in wall-impinging cases is primarily caused by soot accumulation near the wall while the influence of low wall temperature on the formation of soot is limited. Shorter wall distance impingement case exhibits suppressed soot formation and oxidation throughout the entire process due to strong thermal and spatial constraints imposed by the wall. Lower injection pressure shortens liftoff length, resulting in a locally higher equivalence ratio that favors soot formation. Additionally, the reduced spray momentum prolongs the residence time in regions favorable for soot formation. Elevated ambient temperature and oxygen concentration both promote soot formation and oxidation. The influence of elevated ambient temperature on the formation of soot is more significant than oxidation of soot, which widens the gap between formation and oxidation rates and leads to greater soot accumulation. In contrast, under high ambient oxygen concentration, the effect on soot formation is relatively weak, resulting in lower overall soot mass. These findings highlight the key role of wall effects in shaping soot behavior, providing useful insights for optimizing injection and wall design to control soot emissions.

Polyoxymethylene dimethyl ethers (PODE), as a potential e-fuel, can realize the carbon neutrality for internal combustion engines. Existing studies on PODE are primarily engine-based, leading to contradictory conclusions about fuel consumption and pollutant emissions due to the different engine specifications and operating conditions. This work applies the second law of thermodynamics under constant-volume conditions to analyze fuel economy and emissions characteristics without the influence of particular test conditions or engine types. The fundamental fuel economic and emission-related behaviors of pure n-dodecane and PODE3/n-dodecane blended fuels were numerically studied. For the spray and combustion processes, compared with n-dodecane, the blended fuel exhibits the low-temperature heat release (LTHR) in the region with leaner fuel/air mixture and higher temperature. However, the high-temperature heat release (HTHR) of the blended fuel is closer to the stoichiometric combustion but with lower temperatures. Blending PODE3 into n-dodecane increases the exergy destruction induced by chemical reactions but decreases the exergy destruction related to heat conduction and mass transfer, resulting in a basically unchanged overall potential maximum fuel economy. Heightened sensitivity of the exergy destruction from chemical reactions to temperature and equivalence ratio is found under lowtemperature and high-equivalence ratio conditions. This sensitivity trend is nearly consistent for different PODE3/n-dodecane blends. The exergy destruction arising from chemical reactions for the LTHR of PODE3 is higher than that of n-dodecane. Less exergy destruction induced from chemical reactions can be achieved by controlling the combustion temperature higher than 1760 K and 1900 K respectively for n-dodecane and the blended fuel. Moreover, both nitrogen oxide (NOx) and soot are reduced for the blended fuel compared with ndodecane. Notably, the trade-off relationships of NOx-chemical exergy destruction as well as NOx-soot, can be improved by blending PODE3 into n-dodecane.

Methanol's low reactivity impedes its application as a renewable fuel in compression-ignition (CI) engines. This study investigates the viability of methanol as a low-reactivity fuel (LRF) in hydrogenated catalytic biodiesel (HCB)-assisted dual-fuel (DF) combustion under reactivitycontrolled compression ignition (RCCI) conditions. Using Reynolds-Averaged Navier-Stokes (RANS) simulations, the effects of varying methanol concentrations on ignition characteristics and flame structure are quantified. The results reveal that both single-fuel (SF) and DF spray combustion exhibit a two-stage ignition process, though this phenomenon is weaker in SF combustion. Moreover, ambient methanol and temperature variations significantly influence both low- and high-temperature combustion. The sensitivity of low-temperature combustion decreases once the premixed methanol equivalence ratio (phi m) reaches 0.5, whereas the induction time continues to increase with higher methanol content. Additionally, the flame kernel location and peak heat-release rate are closely linked to regions of maximal formaldehyde (CH2O) consumption. Following HCB injection cessation, high-temperature combustion, characterized by rapid CH2O depletion, dominates the spray interior. In DF operation, substantial premixed methanol combustion occurs, with OH and C2H2 diffusing into fuel-lean zones. These insights into HCBmethanol RCCI interactions inform optimization of operational parameters for controlled ignition and efficient energy release, advancing methanol's viability as a clean CI engine fuel.

Application of renewable fuels in compression ignition engines provides one solution to reduce greenhouse gas emission in heavy-duty transportation sector. However, it is a big challenge to find a single alternative fuel to replace the traditional diesel under full load conditions. In this study, the combustion characteristics of two kinds of renewable fuels with different reactivity, namely methanol and hydrogenated catalytic biodiesel (HCB), were investigated in an optical engine under partially premixed combustion (PPC) and reactivity-controlled compression ignition (RCCI) modes, respectively. A two-color (2C) method was employed to quantify the inflame soot formation. Meanwhile, the flame oscillation phenomenon was captured and evaluated. The results indicate that the RCCI mode exhibits reduced peak cylinder pressure, resulting in smoother and more stable combustion compared to that of the PPC mode. Moreover, the more uniform fuel distribution in the RCCI mode further decreases soot formation from the blended fuel. Additionally, with increasing methanol proportion, the rise in oxygen content within the blended fuel significantly reduces soot formation. The flame oscillations are primarily related to the rate of cylinder pressure variation. Under PPC mode, multi-point autoignition causes a rapid increase in cylinder pressure, leading to larger oscillation amplitudes (M15: 0.268, M25: 0.772). Conversely, under the RCCI mode, the heat release process is more stable, resulting in weaken oscillations (M15: 0.161, M25: 0.064) compared to that of PPC.

The escalating global energy demand and urgent need to reduce greenhouse gas emissions have intensified interest in carbon-free fuels for internal combustion engines (ICEs). Ammonia (NH3) is a promising candidate for decarbonizing transport sectors and this review investigates its potential for ICEs owing to its high hydrogen density and compatibility with existing infrastructure. However, challenges such as low flammability, high autoignition temperature, and elevated NOx emissions limit its direct applications. This study highlights the various production technologies of NH3, fuel properties, combustion mechanisms, and its performance and emission characteristics with conventional and alternative fuels. Blending NH3 with high-reactivity fuels (diesel/hydrogen) enhances combustion stability and efficiency, enabling up to 95 % NH3 utilization. Despite its lower thermal efficiency at higher blends, improved performance occurs under high compression ratios and loads. The review synthesizes technological barriers, emission mitigation strategies, and the need for sustainable NH3 production, underscoring its potential in future low-carbon energy systems.

Accurately controlling combustion and achieving cleaner emissions in compression ignition engines need a better understanding of soot evolution. In the present study, the transient characteristics of in-flame soot evolution of spray A within the engine combustion network are numerically investigated under high-temperature and high-pressure conditions using a parcel tracing methodology. A virtual parcel driven by the current flow field, recording the local information to understand the evolution of soot. A two-equation soot model has been implemented based on the OpenFOAM platform, coupling with a ∑−Y Eulerian spray model, and unsteady flamelet progress variable combustion model. The computed results indicate good agreement with the experimental ones under studied conditions. Results show that oxygen concentration significantly affects the onset soot location, which occurs at some radial distance away from the spray axis under low oxygen conditions, while it aligns along the spray axis for high oxygenconditions. The peak soot location of quasi-steady flames is concentrated around an equivalence ratio region (2<Φ<2.2) regardless of operating conditions. The chemical source of soot production is dominated by surface growth rate after the traced parcels pass through flame liftoff length where the local temperature exceeds 1500 K.

An activity-oriented direct reaction screening (AODRS) method is proposed to simplify the reduction of detailed chemical kinetic mechanism. This method requires only four parameters (cGISS, cLO, cT, and cRIT) to generate a skeletal mechanism. Extensive experience or expertise is not necessary for identifying important species, as these species are determined locally and dynamically based on their absolute molar converting fluxes. cGISS is used to dynamically identify and construct the global important species scope, while cLO is used to dynamically identify the local target species. Reactions with higher contribution coefficients than cT and having species all belonging to the global important species scope are classified as locally important. Then locally important reactions with higher importance tendency than cRIT are identified as global important reactions. To validate the proposed method, detailed mechanisms for NH3-CH4 and n-heptane are reduced stepwise. As cGISS and cLO increase while cT and cRIT decrease, the relative errors in ignition delay time, laminar flame speed, and species concentrations generally decrease. With the criterion of 5 % maximum relative error in ignition delay time, a skeletal mechanism achieving reductions of 62 % in reactions and 59 % in species is obtained for the NH3-CH4. Meanwhile, a skeletal mechanism for n-heptane within 10 % relative error in ignition delay time achieves reductions of 67 % in reactions and 58 % in species. Additionally, ignition delay time, laminar flame speed, and species concentrations are extensively evaluated. Results indicate that, with a comparable number of reactions, skeletal mechanisms generated by AODRS exhibit better agreement with detailed mechanisms compared to skeletal mechanisms generated by DRGEPSA and DRGEP methods. Finally, the recommended value ranges for cGISS, cLO, cT, and cRIT have been further constrained.

In pursuit of alternative fuels to reduce fossil fuel dependence and achieve carbon neutrality, this study explores biodiesel from mixed Castor-Jatropha seeds. Combining Jatropha's lower viscosity and higher heating values with Castor's higher yield, various biodiesel blends were obtained and tested in a single-cylinder, four-stroke diesel engine: pure Castor, 75% Castor-25% Jatropha, 50% Castor-50% Jatropha, 25% Castor-75% Jatropha, and pure Jatropha. The study examines the engine performance, emissions, and sustainability of these blends. The results demonstrated that the mixed biodiesels have similar physical and chemical properties to diesel fuel, allowing their application in conventional engines without modifications. The high flash point and cetane number as well as the oxygen content of these biodiesel blends, enhanced combustion stability. Mixed biodiesels exhibited lower HC, CO2, and smoke emissions, but higher BSFC, EGT, and NOx than diesel. The 75% Jatropha and 25% Castor blend exhibited superior performance and lower emissions through Self-Organizing Map analysis (SOM). This blend demonstrates a higher eta th and equivalent fuel consumption to diesel, particularly at high engine loads. Meanwhile, the 75% Jatropha and 25% Castor blend exhibited a reduction in NOx emissions due to its balanced oxygen content. At higher engine loads, the 75% Jatropha-25% Castor biodiesel blend (B60) resulted in a reduction in CO2 emissions by 75% compared to that of pure diesel. Overall, biodiesel from mixed Castor and Jatropha seeds emerges as a viable substitute for conventional fuel in diesel engines, indicating potential advancements in fuel technology in the foreseeable future.

Renewable methanol and hydrogen have emerged as great potential alternative fuels for internal combustion engines in recent research. Hydrogen exhibits the ability to mitigate cycle-to-cycle variations in methanol engines thanks to its rapid combustion rate. Furthermore, the integration of machine learning with genetic algorithms provides a solution to achieve efficient and clean combustion in hydrogen-enriched methanol engines. Nevertheless, this approach is constrained due to the limited predictive accuracy in small-sample learning. This study aims at introducing an enhanced model based on the automated machine learning framework, and provide a comparative analysis of the enhanced model and computational fluid dynamics simulations in terms of multi-objective optimization performance and computational time. Additionally, this study further considered the effect of engine cycle-to-cycle variations on performance, ultimately attaining optimal operating parameters for stable combustion. The results indicate that datasets actively created by genetic algorithms are more effective than those generated through Latin hypercube sampling. The introduction of a classification model for the identification of misfires has enhanced the efficiency of dataset utilization. Through feature construction, the automated machine learning model demonstrates improvements in R-square and mean-squared error for all parameters on the test dataset. Furthermore, by employing a larger overall population size, the automated machine learning model has not only achieved a higher-quality Pareto fronts compared to the computational fluid dynamics method but has also significantly reduced computation time, particularly upon introducing additional engine cycle-to-cycle variations constraints.

The increasing global energy demand has led to the dominance of fossil fuels, but they also contribute to harmful emissions and global warming, and biofuels offer a promising alternative. This study aims to extract bio-oil from mixed Jatropha-Castor seeds as an alternative fuel, overcoming atomizing challenges with current biofuels through precise extraction methods. A comprehensive exploration of various extraction methods, including solvent, hydraulic, and screw pressing techniques, is undertaken. Special attention is given to optimizing the screw pressing method in terms of yield, energy and time consumption. In the realm of screw pressing, the oil yield is enhanced by elevating the extraction temperature and screw speed across all proportions. The yield showed a significant correlation with temperature, while speed significantly impacted energy and time consumption. The screw press method demonstrates considerable suitability for processing mixed Jatropha-Castor seeds in comparison to solvent and hydraulic methods. Quantitatively, the screw-pressing enhances the yield by 34.27-131.96% compared to hydraulic method, albeit with a minor decrease of 2.41-32.21% compared to solvent method. Nevertheless, the screw press achieves notable savings in extraction energy (35.65%) and time (92.91%). The results from mixed seeds demonstrate good consistency with diesel properties and could contribute to advanced fuel technology.

Methanol has been widely recognized as a promising alternative fuel for mitigating greenhouse gas emissions. However, its application in internal combustion engines often necessitates blending with a high-reactivity fuel to address challenges related to ignitability and combustion stability. In this study, a two-color method was employed in a single-cylinder optical engine to investigate the characteristics of soot formation and flame oscillation of methanol blended with a high-reactivity fuel, hydrogenated catalytic biodiesel (HCB), under partially premixed combustion (PPC) modes. The blended fuels include M15 (15% methanol, 60% HCB, and 25% n-octanol) and M25 (25% methanol, 60% HCB, and 25% n-octanol), with pure hydrogenated catalytic biodiesel M0 (100% HCB) serving as the reference fuel. N-octanol was utilized as the co-solvent to enhance the stability of the mixture. The results indicate that, employing double-injection strategies, as the methanol content increases in the blended composition, leading to a postponed peak cylinder pressure and heat release rate during combustion. Thanks to the high-level premixed combustion and higher oxygen content, the increases of alcohol blending significantly reduced in-cylinder soot formation. Double-injection strategies reduced soot production effectively and resulted in a more uniform soot distribution compared to single-injection strategies. In terms of flame oscillation, its amplitude primarily correlates with the rate of cylinder pressure rise, while the methanol content within the blends exerts a minor impact on flame oscillation. Double-injection strategies contributes to a more uniform fuel distribution, ensuring a smoother heat release process and significantly reducing flame oscillation amplitudes compared to single-injection strategies.

As one of the most popular synthetic fuels (E-fuels), Polyoxymethylene dimethyl ethers(PODEX) have garnered significant attention among researchers in the compression ignition engine field. This work aims to provide an exhaustive analysis of the combustion characteristics of PODE3, which typically constitutes the primary component of PODEX. This research seeks to deepen our understanding of the flame structure of PODE3, particularly in relation to ambient temperature variation, and unravel the underlying mechanisms driving the observed trends. To achieve this aim, a range of optical techniques was employed within a high-temperature high-pressure combustion vessel to quantify various general combustion parameters and assess OH* chemiluminescence, formaldehyde, and soot distribution. Two additional reference fuels, pure n-dodecane and a blend of 50 % vol n-dodecane and 50 % vol PODE3 were also tested. Additionally, some analysis from chemical kinetics and 1D spray model calculations were used to substantiate the experimental observations. The findings reveal that the flame structure of PODE3 differs significantly from traditional diffusion flames associated with diesel-like sprays. The flame lift-off length (LOL) of PODE3 aligns closely with that of n-dodecane, and maintains the twolobe structure under high-temperature conditions. In contrast, under low ambient temperatures, the LOL of PODE3 significantly extends, accompanied by a concentration of OH* radicals at the spray center. The notable sensitivity of PODE3 flames to temperature variation is related to the chemical kinetics of formaldehyde and CO. As a result of the analysis, conceptual models of PODE3 flames under varying temperatures were proposed in the end.