Ammonia is an attractive hydrogen carrier and zero-carbon fuel for decarbonization goals. To address the challenges posed by ammonia's low flame velocity and elevated ignition temperature, a dual-fuel approach combining port-injected ammonia with directly injected diesel has proven effective in enhancing combustion performance. This study focuses on optimizing the thermal efficiency and minimizing emissions in ammonia-diesel direct injection (ADDF) engines by examining the influence of ammonia energy ratio (AER) and fuel injection timing on combustion dynamics and emission patterns. Results reveal that the indicated thermal efficiency peaks at 45.06% when AER approaches 20%, with efficiency declining at both lower and higher ratios. The incomplete combustion of ammonia becomes more pronounced at elevated AER levels, reaching 11.06% unburned ammonia at 40% AER, primarily due to ammonia's sluggish combustion rate and its inhibitory effect on dehydrogenation reactions. Regarding nitrogen oxides, nitric oxide (NO) emissions demonstrate a decreasing trend with higher AER values, attributed to the reduction of NO to molecular nitrogen under specific combustion conditions. The greenhouse gas emissions, predominantly comprising carbon dioxide and nitrous oxide, show an upward trend with increasing AER, as nitrous oxide (N2O) formation is favored in low-temperature ammonia combustion zones. The presence of a well-mixed diesel-air charge during the ignition delay period promotes more complete combustion, thereby reducing residual ammonia emissions. Through meticulous optimization of injection parameters and combustion control strategies, the research achieved a 1.32% enhancement in thermal efficiency, attaining 45.35% at 40% AER, coupled with a significant 29.14% reduction in greenhouse gas emissions.

As a key injection parameter, the liquid-gas momentum flux ratio (J) regulates fuel distribution in the scramjet combustor and thus affects ignition and combustion. In this study, we investigated the effect of J on the process of fuel entering the cavity using an Eulerian-Lagrangian large-eddy simulation method. As a result, a non-monotonic variation in the fuel mass within the cavity is found as J increases. By analyzing the sectionalized flow behaviors induced by the liquid jet, these non-monotonic variations are governed by the penetration depth and the entrainment zone thickness (δs). Increasing J can increase the penetration depth, reducing the fuel mass entering the near-wall region. Meanwhile, increasing J enlarges δs, which in turn allows more fuel above the cavity to be influenced by the downward expansion region in the flow field and subsequently transported into the cavity. The relative variation between these two competing factors ultimately determines the fuel mass within the cavity. A deviation percentage (R) of the fuel distribution region relative to the cavity entrainment zone is defined in this study, and its variation trend aligns well with the non-monotonic trend of the kerosene mass inside the cavity with respect to J. Furthermore, we discuss the implications of this observed non-monotonic phenomenon on the fuel mixing characteristics within the cavity. These results will guide injection design related to ignition and combustion processes in scramjet engines within engineering applications.

This study presents the first systematic optical investigation of spray interaction and combustion characteristics in biodiesel/methanol dual-fuel direct injection system, conducted in a constant-volume combustion chamber using Schlieren imaging. Individual biodiesel and methanol spray behaviors were first compared under nonreactive conditions at injection pressures of 60, 80, and 100 MPa, revealing that biodiesel sprays exhibit longer penetration and narrower cone angles, whereas methanol sprays show finer atomization and wider dispersion. Dual-spray collision and combustion characteristics were then analyzed by varying injection intervals (Delta t) and sequencing. Results indicate that increasing Delta t reduces spray penetration at 60 MPa while enhancing it at 100 MPa. Maximum spray area and lateral dispersion occurs at Delta t = 1.0 ms; beyond this, spatial decoupling limits atomization efficiency. The collision length decreases with increasing Delta t, while collision width peaks at Delta t = 1.0 ms. Methanol-first injection induced localized cooling due to its high latent heat, delaying biodiesel evaporation. In contrast, biodiesel-first injection produced a more cohesive initial spray, followed by rapid methanol dispersion, enhancing overall mixing and spray area. At 100 MPa, longer Delta t reduces spray overlap and interaction, while shorter intervals facilitate greater jet convergence and larger spray areas. Ignition consistently initiates at the spray interaction region, with flame morphology and luminosity strongly influenced by injection strategies. Methanol-first strategies facilitates early ignition but suppresses subsequent biodiesel ignition due to the evaporative cooling, whereas biodiesel-first strategies yield higher overall flame luminosity due to soot formation. This work provides new quantitative insights into how injection parameters affects dual-spray collision and combustion performance, offering practical guidance for optimizing injection strategies in renewable dual-fuel engines.

Ammonia is a competitive zero-carbon alternative fuel for marine and heavy-duty compression ignition engines, but its inferior combustion properties cause high NOx, NH3 slip and N2O emissions. Ammonia post injection can regulate ammonia diffusion combustion and pollutant formation, yet its coupled control mechanism on engine combustion, performance and multi-pollutant emissions is not fully revealed. This work experimentally explores the combustion and emission characteristics of a diesel/ammonia dual direct injection (DADDI) engine under post-combustion mode, concentrating on the influences of ammonia post injection ratio (APIRpost) and ammonia post injection timing (APITpost) on combustion and emissions. Results indicate that ammonia post injection forms a stable triple-peak heat release rate profile. With the growth of APIRpost, in-cylinder combustion temperature and engine power output drop continuously, and brake thermal efficiency increases first and then peaks at APIRpost of 0.12–0.16. Moderate ammonia post injection effectively reduces NOx emissions through in-cylinder temperature suppression, whereas excessively high APIRpost gives rise to local oxygen shortage and low-temperature combustion zones, leading to deterioration of NH3, N2O, CO and THC emissions. Delaying APITpost from 20° to 30°CA ATDC, NOx emissions showing a fall -then- rise trend and NH3, N2O emissions decreasing continuously with APITpost delay slightly. The optimal operation is achieved at APIRpost = 0.12–0.16 and APITpost = 24° ∼26°CA ATDC, which balances high thermal efficiency, stable combustion and low emissions. This study clarifies the regulation law of ammonia post injection parameters on diffusion combustion and provides technical support for low-emission and high-efficiency operation of DADDI engines.

Investigating the generation mechanism of soot in ammonia/diesel dual fuel engines under road transportation condition is beneficial for developing effective emission control strategies. This study analyzes the physicochemical properties of soot generated by an ammonia/diesel engine under road transportation condition, and further propose the effect mechanism of ammonia. The results show that the presence of ammonia enhances the oxidation activity of soot, with the peak temperature reducing from 418.2 degrees C to 397.6 degrees C. Ammonia significantly influences the content of aliphatic C-H, C=O and COOH functional groups on soot surface, and further promoting formation of nitrogen groups. Pyrrole nitrogen is transformed into pyridine nitrogen and quaternary nitrogen with a more stable structure with ammonia energy ratio increasing. In addition, ammonia weakens the hollow shell structure of soot and inhibits the growth of microcrystal length in the shell, resulting in a decrease of the degree of graphitization. However, an increase in the initial particle size of the soot particle is observed. Ammonia also affects the pore structure of soot, resulting in the decrease of mesoporous volume and specific surface area. This study contributes to control soot emission from ammonia/diesel engines, and further evaluating the environmental effects.

Polyoxymethylene dimethyl ethers (PODEn) have emerged as promising oxygenated synthetic fuels for compression ignition engines applications. While extensive research has focused on the combustion and emissions of PODEn/diesel-fueled engines, fundamental understanding of their ignition behavior remains limited, despite its critical importance for computational fluid dynamics simulations in fuel/engine co-optimization. Therefore, this study measured ignition delay times (IDTs) of stoichiometric PODE2/n-heptane blends in a rapid compression machine at 10 bar and temperatures ranging from 600 to 1000 K. A merged kinetic mechanism for n-heptane/PODE2 blends was developed and validated, showing good agreement with the experimental IDTs. Results demonstrate that blending PODE2 into n-heptane reduces IDTs and enhances mixture reactivity, particularly between 800 and 950 K. When the blending ratio of PODE2 is relatively high, the negative temperature coefficient (NTC) behavior is progressively attenuated, leading to a slight increase in IDTs within the NTC region due to the absence of NTC characteristics in neat PODE2. Kinetic modeling analyses reveal that at 700 K, 40 % PODE2 addition advances the onset of first-stage ignition through enhanced OH radical generation and heat accumulation, though the total IDT is slightly extended. At 900 K, PODE2 promotes earlier ignition, driven by increased OH formation via the CH3 + HO2 -> CH3O + OH pathway despite a reduction in OH production from H2O2 decomposition. Reaction pathway and rate-of-production analyses indicate that H-abstraction and secondary O2-addition reactions of n-heptane dominate ignition chemistry at low PODE2 content, while PODE2 oxidation becomes increasingly influential with rising blend ratio or temperature. These findings provide mechanistic insights into the combustion behavior of PODE2/n-heptane blends and inform strategies for fuel/ engine co-optimization.

Although ash deposition is known to promote soot oxidation in gasoline particulate filters (GPFs), the underlying mechanism remains unclear. In this study, two ash surrogates (SiO2 and Al2O3) were employed to investigate the evolution of soot morphology and nanostructure at different oxidation degrees under a 16.6% O2 atmosphere at 650 °C, with a particular emphasis on oxidation mode transition and oxidation-induced fragmentation behavior. Results show that distinct hollow structures are observed in ash-free soot at late oxidation stages. Such hollow structures are largely suppressed in the presence of ash. Ash addition significantly decreases the number and size of primary particles within aggregates, with increased fractal dimension and a more disordered nanostructure. For all samples, the aggregates fragmentation rate decreases with increasing oxidation degree. However, soot oxidized with ash exhibits a higher aggregate fragmentation rate (up to 20% higher) during the initial oxidation stage than ash-free soot. In contrast, the fragmentation rate of primary particles for ash-free soot increases sharply during oxidation, reaching 0.927 in the late stage, whereas this increase is suppressed by ash addition. This indicates that ash impedes oxidant penetration into the interior of primary particles, thereby effectively suppressing internal oxidation while promoting external oxidation. Notably, SiO2 demonstrates superior catalytic performance compared with Al2O3 throughout the oxidation process. This work provides new insights into ash-driven soot oxidation and offers guidance for optimizing GPF regeneration strategies.

The study experimentally investigates the influence of diesel injection timing and methanol energy substitution ratio (MESR) on the performance, regulated, and unregulated emissions of a methanol/diesel dual-fuel marine medium-speed engine. A six-cylinder, turbocharged, 210 mm-bore engine was retrofitted with a port-injected methanol system and tested under four operating loads representative of the ISO 8178-4:1996 E3 cycle. Results show that Medium- to high-load conditions allow for higher methanol substitution, with a peak MESR of approximately 59% achieved at 50% load and an injection timing of 6 degrees CA bTDC. Brake thermal efficiency generally first rises and then declines with advancing injection timing, revealing an optimal timing for each MESR. Advanced diesel injection timing increases NOx emissions owing to prolonged high-temperature residence, while MESR exhibits limited influence except at low load. Particulate mass emissions remain below 1.0 mg under all test conditions, showing no consistent correlation with MESR. Unburnt methanol and formaldehyde emissions increase greatly with MESR and decrease with load, reaching peak values of 12, 523 ppm and 665 ppm, respectively. Diesel injection timing shows little influence on unburnt methanol and formaldehyde emissions, indicating that mixture preparation and temperature distribution dominate its formation. The findings provide new insight into injection strategy optimization for large-bore medium-speed methanol/diesel dual-fuel engines.

Ammonia, as a typical carbon free fuel, is considered the most promising alternative for internal combustion engines. However, the spray and combustion characteristics of ammonia are not clear, especially under diesel/ ammonia dual fuel duel direct injection setting. This study investigates the spray and combustion characteristics of a diesel/ammonia under various injection angle (IA) and injection interval (Delta t) using optical diagnostic techniques. The results show that changing the IA and Delta t can effectively change the diesel/ammonia mixture distribution and achieve concentration stratification and reactivity stratification. At IA = 180 degrees, the liquid- and vapor-phase spray projected areas are 0.76-0.92 and 0.64-0.86 times than those at IA = 90 degrees, respectively. Additionally, increasing Delta t promotes the diffusion and evaporation of spray, thus resulting in a larger vaporphase spray projected area and a smaller liquid-phase spray projected area. Compared with 90 degrees IA, 180 degrees IA increases the relative speed and collision strength of the two jets, inhibits spontaneous diesel combustion, prolongs the ignition delay, and intensifies the combustion process. when IA = 180 degrees and Delta t = 1.0 ms, the collision spray achieves high combustion rate, a short combustion duration, and the least soot generation, which can significantly improve the combustion and emission of engines.

Maritime transport is a major component of global trade and an important source of atmospheric pollutants and greenhouse gases. In the decarbonization era, ship emission research is moving beyond conventional emission estimation toward integrated assessment frameworks that connect high-resolution inventories, atmospheric transformation, health and ecosystem impacts, climate forcing, and mitigation pathways. This review focuses primarily on characterizing and quantifying shipping-related primary emissions and emission inventories, which constitute the source term for atmospheric impact assessment. Ambient concentrations and environmental impacts are further governed by plume dispersion, transport, chemical transformation, and secondary pollutant formation. Using a PRISMA-style screening process, this review synthesizes recent advances in ship emission sources, inventory methodologies, spatiotemporal patterns, atmospheric impacts, and air pollution–climate co-mitigation strategies. The evolution of ship emission inventories is first reviewed, from fuel-based top-down approaches to activity-based bottom-up methods and AIS-driven high-resolution models, with emphasis on emission factors, load factors, operating-mode identification, auxiliary-engine and boiler emissions, uncertainty quantification, and multi-source validation. The review then summarizes the spatial and temporal characteristics of ship emissions, highlighting global shipping lanes, regional chokepoints, port-city interfaces, coastal corridors, straits, and island regions. Atmospheric processes and environmental impacts are further examined, including primary pollutants, secondary PM2.5 formation, nonlinear O3 chemistry, population exposure, health risks, atmospheric deposition, and climate forcing. The evidence indicates that ship-related impacts are shaped not only by emission magnitude, but also by chemical regimes, meteorology, receptor proximity, fuel quality, and policy context. Mitigation policies and energy-transition pathways are critically assessed, including international regulations, emission control areas, shore power, vessel speed reduction, alternative marine fuels, market-based measures, and Well-to-Wake life-cycle assessment. Overall, ship emission control is shifting from single-pollutant regulation toward integrated air pollution–climate co-mitigation. Future research should prioritize uncertainty-aware high-resolution inventories, observation-constrained emission verification, coupled air-quality–health–climate–economic assessment, AI-enhanced emission modeling, and region-specific strategies for coastal, port, strait, and island environments. An integrated data-to-impact-to-policy framework is essential for supporting cleaner, healthier, and lower-carbon maritime transport.

Exploring the changes of soot emissions from ammonia/diesel engines in the exhaust process is essential for developing specialized emission control strategies. This study investigates the evolution pattern in physiochemical properties of soot from ammonia/diesel engines along the after-treatment devices. The results show that the C-H and C-N functional groups are formed after soot passing through diesel oxidation catalyst (DOC), while the catalytic diesel particulate filter (CDPF) facilitates the decomposition of C-H groups and the conversion of C-N to C=N functional groups. The degree of soot graphitization gradually increases in the exhaust process, with DOC exhibiting the significant effects on the D1 carbon component, while the CDPF mainly acts on the D3 and D4 carbon components. DOC reduces the degree of soot agglomeration, causing a slight shift in the particle size distribution towards small diameter. In contrast, diesel particulate filter (DPF) or CDPF enhances the soot agglomeration. The orderliness of soot nanostructure increases in the exhaust process, and the length of microcrystals shifts towards large microcrystalline size, accompanied by soot edge oxidation affected by CDPF. The DOC and DPF reduce the content of C=O functional groups, while the CDPF promotes the generation of oxygen-containing groups. In addition, the nitrogen-containing groups such as pyridine structure compounds (N-6), pyrrole structure compounds (N-5) and nitrogen oxides (N-Ox) gradually decrease during the exhaust.

Ash deposition in gasoline particulate filters (GPFs) can promote soot oxidation, yet its underlying catalytic role remain unclear due to variations in soot properties under practical engine conditions. In this study, the catalytic effects of two ash surrogates (i.e., SiO2 and Al2O3) on gasoline direct injection soot oxidation were investigated, with a particular emphasis on the evolution of pore structure and surface functional groups during oxidation. Results show that under the test conditions of 650 degrees C and 16.6% O-2 atmosphere, ash significantly accelerates soot oxidation, reducing total oxidation time and increasing the apparent rate constant by up to 2.2 times compared with ash-free soot. Ash-free soot expands during oxidation, while ash addition alters the soot oxidation mode by initiating contact oxidation which promotes the formation of mesopores (2-10 nm) and suppresses macropores (>50 nm) through pore filling and structural collapse. This shifts the pore size distribution toward smaller diameters, decreases the total pore volume, and enhances intimate soot/ash contact. Moreover, Al2O3 preferentially oxidizes aliphatic C-H groups at early stages due to its active hydroxyl sites, while SiO2 sustains oxidation in later stages, enabling deeper oxidation of soot. Therefore, SiO2 exhibits superior catalytic performance compared to Al2O3. This work provides mechanistic insights into ash-catalyzed soot oxidation and offer guidance for optimizing GPF regeneration strategies.

Vehicle emission inventories are highly sensitive to vehicle activity data, yet annual vehicle kilometers traveled (VKT) is still commonly represented using generalized default values whose representativeness at the city scale remains uncertain. In this study, large-scale vehicle inspection data from Haikou, China, were used to derive inspection-based VKT estimates and to quantify how activity assumptions affect urban vehicle emission inventories and policy evaluation. By holding vehicle population and emission factors constant across scenarios, we explicitly isolated the effect of activity representation on emission estimates. An inspection-based, age-sensitive VKT framework was further developed to capture within-fleet heterogeneity. The results showed that inspection-derived VKT accounted for only 36-75% of guideline-recommended values across major vehicle categories, with the largest discrepancies observed for diesel freight vehicles. As a result, the use of guideline-based VKT produced higher emission estimates by 34-39% for carbon monoxide (CO) and volatile organic compounds (VOCs) and by approximately 66-67% for nitrogen oxides (NOx) and particulate matter (PM). The influence of activity representation was also evident in policy assessment. In a case study of old diesel vehicle retirement, guideline-based VKT produced estimated emission reduction benefits that were more than 120% higher for most pollutants and nearly 200% higher for NOx than those derived from inspection-based VKT. These findings demonstrate that generalized activity assumptions can substantially affect both emission inventory estimates and policy-oriented assessments. Rather than merely refining a local mileage parameter, this study highlights a potential representativeness limitation of generalized activity assumptions when they are applied to city-specific emission inventories, particularly in medium-sized or geographically constrained urban systems. The inspection-based, age-sensitive approach proposed here provides a practical pathway for improving activity representation in data-rich urban environments, while its transferability should be evaluated according to local fleet structure and transport conditions.

Ammonia fuel is expected to emerge as an effective alternative to fossil fuels due to its zero-carbon nature, high-efficiency storage and transportation advantages, and extensive industrial manufacturing infrastructure. This study discussed the impacts of compression ratio and injection timing on combustion and emission characteristics of an ammonia/diesel dual-fuel (ADDF) engine using numerical simulation. Results indicated that the corresponding optimal indicated thermal efficiency (ITE) continuously increases with an increasing compression ratio. When the compression ratio is 15:1, the injection timing corresponding to the maximum indicated thermal efficiency is −18 °CA after top dead center (ATDC). When the compression ratio ranged from 16:1 to 19:1, the corresponding optimal ITE was achieved at a retarded injection timing of −12 °CA ATDC. At a compression ratio of 19:1, the optimal ITE reached 47.9%. The in-cylinder formation regions of nitrous oxide (N2O) are closely correlated with NH3, NO, and temperature distributions, being primarily located at the interface between high-concentration regions of unburned NH3 and NO. Under the comprehensive impact of increased compression ratio and advanced injection timing, both N2O and unburned NH3 emissions show a tendency of increasing first and then decreasing, while NOx emissions demonstrated a monotonically increasing behavior.

The future prominence of extensive hydrogen utilization in heavy-duty gas turbines is noteworthy. In the Chinese market, numerous natural gas turbines are in operation, where the matching of components during hydrogen blending plays a crucial role in performance and safety. Considering the significant impact of component performance in analysis, this study focuses on a certain F-class heavy-duty gas turbine, employing a high-precision mechanism model. By extracting component characteristics from actual operational data and analyzing operational boundaries, the research explores the mechanisms and methods for optimizing hydrogen blending matching. The study highlights that the core issue of matching lies in the increase of corrected mass flow rate in the turbine under hydrogen conditions. Subsequently, based on the operational safety boundaries of the unit and without altering the unit components, three basic matching methods are compared and analyzed. Among them, the two methods that maintain the combustion chamber outlet temperature at the original design value are impractical due to the risk of surge. Therefore, maintaining the compressor inlet guide vane at its design angle while adjusting fuel consumption to reduce the combustion chamber outlet temperature emerges as the only viable option-resulting in a temperature reduction of approximately 32 degrees C under full-load pure hydrogen conditions. It is also regarded as the optimal method as it strives to maintain efficiency while meeting surge margin requirements. Furthermore, the study demonstrates its feasibility at part loads and different hydrogen blending ratios, indicating the relationship between turbine inlet temperature and hydrogen blending ratios. The matching methods and rules outlined in this study can be widely applied to similar units. The research findings offer valuable insights for control and hold practical application value.

As an efficient hydrogen carrier, ammonia can be applied in internal combustion engines to reduce greenhouse gas emissions in the transportation sector. At present, small-displacement light-duty trucks account for a large proportion of urban transportation. However, limited research has been conducted on small-displacement ammonia-diesel dual-fuel (ADDF) engines. To promote the practical application of such engines, this study focuses on investigating the effects of ammonia on the performance of small-displacement diesel engines. This study employs a numerical simulation model to investigate the in-cylinder combustion and emission characteristics of a small displacement ammonia diesel dual-fuel under different ammonia energy ratios and start of diesel injection (SODI) conditions. Gaseous ammonia is supplied via the intake port, while diesel is directly injected into the combustion chamber. The simulation results indicate that under low ammonia energy ratio conditions, the increase in ammonia energy ratio raises the in-cylinder pressure and peak heat release rate, thereby improving the thermal efficiency of the engine. However, due to the decrease in the in-cylinder temperature during the late combustion process, the soot post-oxidation weakens, leading to an increase in soot emissions. Additionally, owing to the influence of the ammonia thermal nitrogen oxides reduction (de-NOx) process, the increase in ammonia energy ratio from 0 % to 30 % reduces nitrogen oxides (NOx) emissions by 31.7 %. Meanwhile, the nitrous oxide (N2O) emissions increase. The advance of SODI results in excessively high incylinder combustion temperatures, significantly increasing NOx emissions. Meanwhile, excessively advanced SODI increases engine negative work, consequently reducing thermal efficiency. At a SODI of -7.2 degrees CA, the engine achieves an indicated thermal efficiency of 45.47 %. Meanwhile, greenhouse gas emissions are measured at 483.3 g/kW & sdot;h, representing a 15.4 % reduction compared to pure diesel mode. The above results demonstrate the feasibility of using a low ammonia energy ratio in small-displacement engines. However, the excessively high NOx emissions still require optimization of the injection strategy for further reduction.

The opposed rotary piston (ORP) engine has the advantages of compact size and high power-density without connecting rod mechanism, which makes it an ideal power source for long-endurance and heavy-duty unmanned aerial vehicles (UAVs). Ammonia-hydrogen blends serve as carbon-neutral energy carriers with significant potential for decarbonizing ORP engines. This study systematically examines the effects of altitude on combustion and emissions of an ammonia-hydrogen ORP engine. The results demonstrate that elevated altitude conditions lead to a systematic reduction in both peak in-cylinder pressure and heat release. As a result, the indicated thermal efficiency decreases from 36.63 % to 34.45 % when the altitude increases from 0 m to 5000 m, and the emission of NO decreased from 0.763 mg to 0.374 mg. However, N2O emission increases with the elevation. This study further investigates the optimization of combustion characteristics in high-altitude (5000 m) through systematic adjustment of the ignition timing parameters, the results demonstrated that advancing the ignition angle can reduce residual hydrogen and ammonia concentrations in the cylinder, consequently elevating both peak cylinder pressure and heat release rate during the combustion process. A shorter combustion duration is observed after enlarging ignition advancing angle, indicating enhanced combustion velocity under modified ignition timing conditions. Consequently, the indicated thermal efficiency increased from 30.97 % to 37.23 % with the ignition advance angle advancing from 12 degrees to 18 degrees. Furthermore, the optimized ignition timing strategy exhibits an additional benefit of reducing NOx emissions.

The opposed rotary piston (ORP) engine offers advantages such as compact size, high power-density, and structural simplicity, making it a promising candidate for long-endurance, heavy-load unmanned aerial vehicle (UAV) applications. Unlike conventional engines, the ORP engine eliminates intake and exhaust valves, instead controlling the gas exchange process through phase adjustments of the exhaust ports on the cylinder block. Investigating the impact of exhaust port structural parameters on exhaust behavior and the realization of internal exhaust gas recirculation (iEGR) is essential for reducing exhaust losses, mitigating knock tendencies, and improving emission performance. In this study, the structural parameters of the ORP engine's exhaust port were defined, and the exhaust characteristics under various port configurations were analyzed. A strategy for achieving iEGR through adjustments of the exhaust early closing angle (EECA) was proposed, and its effects on intake characteristics, in-cylinder combustion behavior, and overall engine performance were evaluated. Results indicate that an exhaust advance angle (EAA) of 18 degrees yields the highest indicated work, representing a 13.75 % improvement over the baseline configuration. Increasing the exhaust tilt angle (ETA) reduces expansion losses but simultaneously increases exhaust losses, resulting in a net decrease in indicated work, which declines by 2.71 % when the ETA reaches 30 degrees. The EECA influences the residual gas content by altering the exhaust stroke termination timing and shows a positive correlation with the iEGR rate and a negative correlation with charging efficiency (CGE). At an EECA of 50 degrees, the iEGR rate reaches 15.91 %. An EECA range of 10-20 degrees is recommended for optimal fuel economy and combustion stability. Conversely, if emission performance is the primary concern, a range of 20-30 degrees is more appropriate.

Methanol high-pressure direct injection (HPDI) engines are a promising low-carbon solution for heavy-duty applications. Multi-dimensional computational fluid dynamics (CFD) simulations play a crucial role in the design and optimization of these engines. However, the distinct physical properties of methanol, compared to diesel, lead to significant differences in spray behavior, including atomization and development, raising doubts about the applicability of diesel spray models to methanol. While most existing studies focus on low-pressure low-reactivity methanol spray or diesel-like high-reactivity fuel spray behavior, there is a lack of spray data for methanol HPDI, leaving the question of whether diesel-based models are suitable for methanol unresolved. To address this gap, this study investigates the spray characteristics of methanol in a constant volume combustion chamber using Schlieren imaging to measure key spray parameters. Spray penetration length, spray cone angle, and other characteristics were systematically examined with the varying injection pressures of 30-90 MPa over the ambient temperature range of 300-500 K at ambient pressures of 0.5 and 3.5 MPa. The results showed that injection pressure, ambient temperature, and pressure significantly affected spray penetration length and spray cone angle, consistent with known spray behavior trends, thus validating the experimental data. Most importantly, based on these experimental results, a non-dimensional analysis of spray penetration length was conducted, demonstrating a linear relationship between dimensionless spray penetration length and time, with a coefficient of determination close to unity. This strong correlation confirms that methanol spray behavior can be effectively modeled using gas-phase jet flow models, similar to diesel spray in engines. This study provides valuable experimental data for calibrating methanol HPDI spray models and lays the theoretical foundation for simplifying the spray modeling approach in engine simulations.

Polyoxymethylene dimethyl ethers (PODEn) are promising synthetic fuels (e-fuels) with the potential to decarbonize the heavy-duty and marine sectors. However, it is unclear how blending monomolecular PODEn with different CH2O chain lengths affects the diesel spray. This study therefore compares the spray and combustion characteristics of neat diesel and diesel/PODEn blends (blending with three monomolecular PODEn for n = 2, 3, 4 at a constant percentage of 20 % by volume) under non-evaporating, evaporating, and burning conditions. The finding suggests that diesel/PODEn blends are more advantageous to spray quality than diesel fuel under the nonevaporating condition because they demonstrate an increasing trend in spray tip penetration, cone angle, and area. Blends with longer CH2O chain lengths in PODEn present a longer spray tip penetration but a smaller spray cone angle and spray area. Under the evaporating condition, substituting diesel with PODEn containing a longer CH2O chain length generally increases the maximum liquid-phase penetration length of blended fuels, together with a slightly accelerated spray cone angle and spray area, indicating a lower evaporation rate. Shorter ignition delay and flame lift-off length while increased flame area are observed with the increasing CH2O chain length in PODEn. A trade-off relationship occurs between the influences of the CH2O chain lengths in PODEn on soot generation of diesel/PODEn blends, which leads to slight discrepancies in soot concentration (KL factor).