While methane has traditionally been employed to represent natural gas in combustion studies, actual natural gas contains higher alkanes (e.g., ethane, propane) that may alter fuel reactivity. Therefore, this study aims to investigate the impact of propane component on the combustion of ammonia/natural gas blends. In this work, the ignition delay times (IDTs) of ammonia/propane blends with different ammonia energy share were measured in a shock tube, at 1 atm and at stoichiometric condition, over the temperature range of 1200-1800 K. At the studied condition, the experimental results indicated that the ammonia ratio had small effects on the ignition delay times of ammonia/propane blends, and the small proportion of propane in actual natural gas had small effects on the auto-ignition behaviors of the ammonia/natural gas blends. Furthermore, a kinetic model was developed to simulate the new measured experimental results as well as experimental data in the literature. The model demonstrated good agreement with the measured ignition delay times of ammonia/propane blends, as well as the species profiles of the intermediates during the oxidation of ammonia/propane blends in a jet-stirred reactor at relatively low temperatures. Sensitivity analyses indicated that the interactions between ammonia and hydrocarbons played an important role in predicting the fuel reactivity of ammonia/propane blends at relatively low temperatures, with the effects gradually diminishing as temperature increased.

The ammonia/diesel dual-fuel combustion strategy is gaining increasing attention for its potential to reduce engine CO2 emissions. This study investigates the combustion characteristics of diesel spray in premixed ammonia/air mixtures using constant-volume combustion vessel experiments and numerical simulations. The effects of premixed ammonia/air equivalence ratio (phi(a) = 0 - 0.3), ambient temperature (T-a = 800 - 900 K) and diesel injection pressure (100 - 160 MPa) were examined. Experimental results demonstrate that the premixed ammonia/air mixtures delay auto-ignition of diesel spray, shift the initial auto-ignition locations toward the downstream region of the diesel spray, and increase flame lift-off length. Meanwhile, the recorded flame images indicate that the flame temperature decreases with increasing premixed ammonia/air equivalence ratio. Elevated ambient temperatures reduce ignition delays and increase flame temperatures at different premixed ammonia/air equivalence ratio conditions. Diesel injection pressure shows little effect on ignition delays and flame lift-off lengths at the studied conditions. Numerical simulations employed a reduced chemical kinetic model for ammonia/diesel blends, with diesel represented by a surrogate blend of n-cetane, iso-cetane, and alpha-methylnaphthalene. Simulated ignition delays and flame lift-off lengths show reasonable agreement with experimental data. The simulations indicate that premixed ammonia/air significantly increases NOx (NO, NO2 and N2O) emissions. NO forms primarily in post-flame, high-temperature regions (> 2000K) for all studied cases. Substantial N2O and NO2 are produced in relatively lower-temperature regions (< 1500K) at the flame base in the presence of premixed ammonia/air. Furthermore, under the studied conditions, increasing the premixed ammonia/air equivalence ratio promotes NOx formation, whereas a higher ambient temperature suppresses it.

The influence of ammonia addition on the formation of polycyclic aromatic hydrocarbon (PAH) and soot in 2,5dimethylfuran (DMF) counterflow diffusion flames was investigated by means of laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques. The results indicated that PAH emerged earlier and were more densely distributed in DMF flame than in n-heptane flame. Moreover, the addition of ammonia inhibited the formation of PAH (such as A2, A3, and A4) and soot in DMF flames. In DMF flames, there were two crucial pathways for PAH formation. Path 1 involved small molecules like C2H2, which were derived from the ringopening reaction of DMF, as the key building blocks. Path 2 (via the reaction R2451: 2C5H5 = A2 + 2H) utilizes C5H5, formed through the dehydrogenation and isomerization of DMF, as the key building block. Kinetic analysis revealed that the addition of ammonia initially heightened the competition for H and CH3 radicals in the vicinity of the fuel nozzle. This, in turn, inhibited the dehydrogenation and isomerization of DMF in Path 2 and promoted the ring-opening reaction of DMF in Path 1 (initial benzene ring formation). Further downstream, the increased H radicals inhibited the formation of A2 by suppressing reaction R2451 above. Finally, after the addition of ammonia, both the soot inception rate and the soot surface growth rate were reduced, resulting in a decrease in the soot volume fraction SVF.

Ammonia combustion in heavy-duty diesel engines commonly suffers from low reactivity, the difficult joint control of uNH3, NO+NO2 and N2O, and constrained thermal efficiency; optimizing the diesel injection strategy is therefore a key lever for simultaneous improvement. This study investigates the coupled effects of injector nozzle angle (INA145 degrees, INA90 degrees, INA60 degrees), pre-injection timing (PIT), and diesel pre-injection strategies (Single/ Dual-PI) on spray-wall interaction, mixture stratification, combustion dynamics and emissions in an ammoniadiesel dual-fuel heavy-duty engine operating at 50 % ammonia energy fraction. Results show that, compared with Single-PI, Dual-PI generally strengthens low-temperature heat release, advances the combustion center, and increases indicated thermal efficiency, while reducing smoke, CO, and THC. The wide-angle INA145 degrees combined with Late-PIT promotes premixed compression ignition in a hotter environment, which markedly lowers uNH3 and N2O but keeps NO+NO2 relatively high. In contrast, narrow-angle INA60 degrees with Early-PIT confines the spray to the bowl center, minimizes wall-wetting and yields a compact main heat-release phase with peak indicated thermal efficiency of about 49.9 % and low NO+NO2, at the expense of higher N2O. Medium-angle INA90 degrees provides an intermediate balance, with moderate ITE and a narrower NO+NO2/N2O band. By mapping these behaviors for three injector angles and both Single- and Dual-PI strategies under a fixed, practically relevant lean condition, the study identifies two actionable operating windows and clarifies the NO+NO2-N2O trade-off, providing engine-relevant guidance for calibrating low-carbon heavy-duty ammonia-diesel dual-fuel engines.

Low-pressure direct injection hydrogen engines show promise for achieving zero-carbon emissions in heavy-duty commercial vehicles, but the underlying mechanisms and key factors governing mixture formation and flame propagation remain poorly understood, especially for the hydrogen engines with side-mounted injectors and flatroof-and-shallow-bowl combustion chambers. This paper aims to address this knowledge gap by conducting three-dimensional computational fluid dynamics simulations. The results reveal that the bulk flow and wallguided hydrogen motion determine the mixture distribution. At spark timing, a band-shaped region with high excess air ratios (2) is observed, with two vortices formed near the spark plug. The combined effects of the high-2 region and turbulence influence the flame propagation, leading to a three-stage heat release process: the initial development stage, the primary heat release stage, and the after-burning stage. Owing to the presence of the high-2 region with lower turbulent intensity, flame propagation toward the end gas is decelerated. During the after-burning stage, the reduction of temperature, local equivalence ratio, and turbulent kinetic energy downstream of the flame front suppresses flame propagation in the end gas region, ultimately leading to local flame extinction and the formation of residual hydrogen.

It is important to investigate the first-stage ignition of alkane fuels as it is responsible for the cool flame heat release in combustors, particularly engines. In the present study, a new set of ignition delay time (IDT) data of npentane is measured in a rapid compression machine (RCM) at phi = 1.0, p = 30 atm, and T = 685-994 K. Moreover, the species concentration profiles of major intermediate species, including alkenes, cyclic ethers, and aldehydes are measured in an RCM at a two-stage ignition condition (T = 730 K) using an updated 2 x fast-acting-valves sampling system. A new kinetic model has been developed to simulate this data. Both the core chemistry and thermochemistry of the low-temperature species associated with n-pentane have been systematically updated. It is found that updating the HO2 + HO2 reaction, which leadstwo OH radicals and O2, has no obvious influence on the 1st-stage ignition but significantly affects the prediction of the total IDT. This is because OH radicals are mainly produced from the formation and consumption of carbonyl-hydroperoxide species before the 1st-stage ignition; HO2 radical recombination and the reaction H2O2 (+M) <-> OH + OH (+M) become the main source of OH radical production only at/after the 1st-stage ignition. The updated thermochemistry data inhibit both the 1st-stage and total IDTs due to the shift towards reactant in the equilibrium of the RO2 <-> QOOH reaction. The key reactions involved in the low-temperature chemistry are optimized using the Optima++ code within the uncertainty limits of reviewed rate constants in the literature. The present model can predict the experimentally measured data well and shows an improvement compared to previous models.

The cold start issue of methanol engines limits their wide application in cold seasons and regions. To explore the underlying mechanisms and propose effective improvement measures, the evaporation characteristics of methanol at 243-303 K were first investigated by the single droplet method in this study. The effects of ambient temperature, initial diameter, fuel temperature and intake air flow velocity on evaporation of methanol droplets were quantitatively analyzed and their guidance for improving the cold start performance of methanol engines were discussed. The results revealed that the evaporation process of methanol droplets in low temperature and humid environment showed two-stage feature due to its hygroscopicity, including pure methanol evaporation and water-dominated evaporation of methanol-water mixture, but it could be regarded as the evaporation of a pseudo single component and the same was true for other binary mixtures. The existence of water in methanol droplets not only led to their incomplete evaporation at temperatures below 263 K, but also caused the linear change of their evaporation rates with temperature, which was different from the exponential change of pure methanol evaporation rate with temperature. Increasing the ambient temperature from 243-283 K to 293 K was optimal for promoting methanol evaporation. Reducing droplet diameter inhibited the water absorption of methanol droplets, thus enhancing their evaporation rates. Increasing fuel temperature could not promote droplet evaporation, but mainly influenced water absorption of methanol droplets. The promotion effect of air flow on methanol evaporation became weaker with the increase of intake air flow velocity, especially when it exceeded 3 m/s. The findings of this study suggest that multiple methods should be combined to improve the cold start performance of methanol engines.

It is necessary for gasoline surrogate models to simulate the effect of NOx addition on fuel auto-ignition behavior, as NOx can affect engine combustion via exhaust gas recirculation (EGR). Toluene is often used as a representative aromatic component in gasoline surrogate models, and hence it is important to investigate the effect of NOx addition on its auto-ignition behavior and to fully understand the interaction chemistry between toluene and NOx. In this paper, high-pressure shock tubes and a rapid compression machine are used to measure the ignition delay times (IDTs) of toluene in 'air' mixtures with and without the addition of nitrogen dioxide (NO2), at a pressure of 20 atm and at temperatures in the range 600-1400 K. The IDTs of n-heptane, iso-octane and a mixture of toluene/n-heptane/iso-octane are measured at the same conditions for comparison. The experimental results show that the auto-ignition behavior of toluene exhibits significantly different sensitivity to NO2 addition compared to n-heptane and iso-octane. NO2 significantly promotes the reactivity of toluene at low temperatures (600-1000 K), in which the IDTs decreased by two orders of magnitude when 1000 ppm of NO2 is added, whereas there is an order of magnitude decrease with the addition of 200 ppm NO2. The promoting effect of NO2 on toluene oxidation reduces significantly at temperatures above 1000 K. The experimental results also show that NO2 addition exhibits a slight promoting effect on the reactivity of n-heptane and iso-octane at temperatures above 750 K at the conditions studied. A kinetic model is proposed based on C3MechV3.3 in which the interaction chemistry between these gasoline surrogates and NOx is updated. The proposed kinetic model can simulate well the effect of NO2 addition on the auto-ignition behavior of these surrogates. Flux and sensitivity analyses are performed to highlight the important interaction reaction pathways.

The combustion and emission performance of ammonia/diesel dual-fuel (ADDF) engines is critically influenced by the diesel injection strategies and combustion regimes. However, there has been limited research performed on this topic. This paper has systematically compared the diesel single/double/triple injection strategies and the early/late compression ignition regimes, in terms of the heat release pattern, combustion performance, and pollutant emissions. The experiments are carried out on a 1.85 L PFI-DI single-cylinder ADDF engine. It's found that diesel pre-injection strategies can effectively improve the combustion and emission performance of an ADDF engine, whether in the ECI (early compression ignition) or the LCI (late compression ignition) combustion regime. From single to double and triple injection, ITE (indicated thermal efficiency) increases while PPRR (peak pressure rise rate) decreases, indicating smoother and more efficient combustion, and smoke opacity, THC, CO, uNH3 (unburned ammonia), NOx, and N2O emissions are all reduced. Compared to the LCI combustion regime, the ECI combustion regime produces a higher ITE, comparable PPRR, lower smoke opacity, THC, CO, and N2O emissions, and higher NOx emissions with the diesel double/triple injection strategies, but the result is reversed with the diesel single injection strategy. The experimental results indicate directions for the optimization of ADDF combustion.

The effective ammonia decomposition on the anode surface directly impacts direct ammonia solid oxide fuel cell (DA-SOFC) performance. This paper uses electrochemical impedance spectroscopy and Tafel curve to study the impact of directly coating 30 mu m Fe catalyst layer (Fe//SOFC) on performance and electrode reaction of DA-SOFC. The electrochemical performances of H2, NH3, and NH3 d.e.g (75% H2 + 25% N2) were compared within the temperature range of 550-700 degrees C. The results indicate that, when NH3 is used as fuel in SOFC, NH3 decomposition causes significant performance loss of the cell, especially during low-temperature operation. In Fe//SOFC, the decomposition rate of NH3 is notably enhanced. The high porosity of the catalytic layer has an aggregation effect on the reaction gas, increasing the reactant concentration near the active sites. This reduces the mass transport loss of the cell by 50%, doubles the exchange current density, and significantly improves the cell performance (close to 20%). Nevertheless, the directly coated Fe catalytic layer covers some active sites and pores on the electrode surface, increasing the polarization resistance and the ion transport resistance.

The pyrolysis of C2H4/NH3 mixtures was conducted in a plug flow reactor (PFR) in the temperature range of 973 K-1373 K. The pyrolysis products, including C2H4, NH3, C2H2, C6H6 and HCN, were quantified using gas chromatography (GC) and Fourier transform infrared (FTIR) spectroscopy to elucidate the thermal decomposition behavior of C2H4 and NH3, as well as the effects of NH3 on the formation of gaseous soot precursors. The results indicate that both C2H4 and NH3 conversion increase during co-pyrolysis compared to their individual pyrolysis. Moreover, C2H4 shows a more pronounced promoting effect on NH3 decomposition. Kinetic analysis reveals that the reactions C2H4 + NH2 and NH3 + CH3 are primarily responsible for the increased conversion of C2H4 and NH3, respectively. The effects of NH3 on soot precursors formation (e.g., C2H2 and C6H6) exhibit a non-monotonic trend with reaction temperature. Specifically, NH3 addition promotes soot precursors formation below 1273 K but inhibits it above 1273 K. This trend is determined by the competition between NH3-induced enhancement of C2H4 decomposition and the effects of C-N interactions. The former consistently promotes the formation of soot precursors, while the latter becomes significantly effective in inhibiting their formation only above 1273 K by removing C atoms from participating in soot precursors formation. This finding is supported by FTIR measurements with a significant increase of HCN being formed at temperature at 1273 K. It should be noted that as the temperature further increases, the concentration of HCN decreases due to its involvement in the formation of N-containing polycyclic aromatic hydrocarbons (NPAHs). Meaningfully, the molecular structure of NPAHs were identified using gas chromatography-mass spectrometry (GC-MS). Notably, existing kinetic mechanisms are unable to satisfactorily predict the quantitative trends of the experimental results, highlighting the need for further mechanism improvement and refinement.

This study investigates the potential of using three high specific heat capacity gases-21 % O2 + 79 % H2O, 60 % O2 + 40 % H2O and 100 % O2-as alternatives to 21 % O2 + 79 % N2 in the cathode of DA-SOFCs, aiming to reduce system BoP parasitic power consumption while analyzing their impact on the internal species, thermal, and electrical performance of the fuel cell. A single-channel multi-physics coupled model was developed for four DA-SOFCs, incorporating 21 % O2 + 79 % N2, 21 % O2 + 79 % H2O, 60 % O2 + 40 % H2O and 100 % O2 as cathode gases. Experimental validation showed good agreement with the simulations, with a maximum voltage error of <= 4.9 %. Results show that the power density of DA-SOFCs with cathode gases of 21 % O2 + 79 % H2O, 60 % O2 + 40 % H2O, and pure O2 increases by of 0.5 %, 38.0 %, and 48.7 %, respectively. Changes in the cathode gas composition significantly affect electrochemical reactions and the NH3 decomposition rate at the anode. Due to the higher diffusion coefficient of O2 in H2O compared to N2, the O2 molar fraction under the ribs of the 21 % O2 + 79 % H2O DA-SOFC is higher. This results in a smaller concentration polarization, which explains why the power density of the DA-SOFC with 21 % O2 + 79 % H2O is higher. For the DA-SOFC with 21 % O2 + 79 % H2O, the maximum temperature gradient showed no significant change compared to the DA-SOFC with 21 % O2 + 79 % N2 with a power density increase.

A combined experimental and Reactive Force Field molecular dynamics (ReaxFF MD) study is conducted to investigate the chemical interactions between NO and large hydrocarbon species (e.g., soot and its gaseous precursors). Co-pyrolysis experiments of C2H4/NO mixtures are carried out based on a plug flow reactor (PFR) and the outlet concentrations of C2H4 and C2H2 as well as C6H6 are measured using gas chromatograph (GC). Then ReaxFF MD simulations are performed to depict the time-resolved process, including fuel thermal decomposition, formation and growth of PAHs, as well as soot formation in the co-pyrolysis of C2H4/NO mixtures. It is experimentally observed that NO addition reduces the rate of thermal decomposition and conversion of C2H4. In addition, the addition of NO inhibits the formation of C2H2, C6H6 and soot. The ReaxFF MD results reveal that most NO molecules (92 %) initially undergo the separation of N and O atoms by reacting with hydrocarbons or H/H2 species, thus converting NO into nitrogen-containing species (mainly NH and NH2 radicals) and oxygen-containing species (mainly O2 and OH radicals). Especially, the chemical interaction between NO and large hydrocarbon species (such as C3H5, C6H3 and C12H5) is observed first, which suggests the existence of richer reaction pathways than those in the current reaction mechanism and provides new insights for improving it. At the end of the simulations, the majority of N atoms (87 %) exist in the form of HCN or C2N, while O atoms are present in CO and H2O. Importantly, direct oxidation of NO on soot particles is not observed, revealing that the formation of HCN and CO reduces the migration of C atoms into larger hydrocarbons, thereby decreasing soot formation.

Microwave-assisted spark ignition (MAI) offers a commercially feasible way to enhance conventional spark ignition performance under extreme conditions. While previous studies suggested microwave pulses had no obvious effect on self-sustained flame from the perspective of energy deposition, the hydrodynamic effect of microwave plasma remained under-explored. This study experimentally investigated the effect of the microwave-induced plasma on lean hydrogen flame dynamics (Lewis number similar to 0.35), examining the influence of water content (r(H2O)) in this process under different pulse repetition frequency (PRF) and pressures. Using a linear high-speed shadow imaging system coupled with electrical diagnostics, we synchronously recorded the flame and plasma morphology evolution while monitoring spark and microwave energy. Results revealed that microwave pulses during spark ignition generated wrinkles and perturbances accelerating the cracking and cellularization of the subsequent self-sustained flame. This effect intensified with increasing PRF from 1 to 10 kHz, particularly at high r(H2O). The overall microwave impact on early flame development was more pronounced at high r(H2O), attributed to stronger interactions between the microwave plasma jet and flame front due to reduced distance. Interestingly, electrical diagnostics showed an inversed relationship between total absorbed microwave energy and r(H2O), highlighting the crucial role of the first microwave pulse at 1 kHz PRF. This study demonstrates that early microwave pulses can influence hydrogen flame dynamics even in the self-sustained stage, offering new insights into MAI mechanisms. These findings open avenues for research into active control of self-sustained flame dynamics, potentially leading to improved ignition strategies in challenging combustion environments.

Partially replacing diesel with carbon-free ammonia fuel is gaining more interest in tackling the issue of greenhouse gas emissions in freight transportation applications. But in ammonia-diesel dual-fuel engines, the combustion and emissions are of particular complexity, with complicated trade-offs among the rough combustion, the thermal efficiency, and the CO2/N2O/NO+NO2/NH3 emissions. To this end, this paper conducts systematical experimental tests on a modified PFI-DI single-cylinder heavy-duty diesel engine with ammonia injected in the intake manifold and diesel directly injected into the cylinder, containing ammonia energy fraction ranging from 0% to 50%, engine loads from IMEP 0.5 to 1.3 MPa, and sweeping diesel injection timing. Results show that the heat release of ammonia-diesel dual-fuel combustion (ADDC) possesses two exothermic stages, and as the ammonia energy fraction increases, the heat release process of ADDC in the first stage is intensified and that in the second stage is just the opposite, which causes rougher combustion but lower indicated thermal efficiency (ITE). Increasing the ammonia energy fraction also makes NO+NO2 and CO2 emissions reduced but NH3 and N2O emissions increased. N2O emissions with stronger greenhouse gas impact usually offset the reduction of CO2. Advancing diesel injection timing can improve ITE, but the cost is the rapidly increasing PPRR and NO+NO2 emissions. Besides, as the engine load increases, the ignition delay gets shortened and the combustion process becomes concentrated and efficient, the resistance to ammonia in ADDC gets better, and NH3 emissions are reduced at the cost of more NO+NO2 emissions.

Direct ammonia SOFCs (DA-SOFCs) are numerical studied to investigate the effects of bipolar plate flow channel configuration on thermal-electric performance. This study establishes a planar DA-SOFC model and investigates triangular, quadrangular, and oval flow channel configurations' impact using numerical simulations. Results reveal oval cross-section channels outperform trapezoidal and triangular ones, achieving power density increases of 15.6%, 16.1%, and 18.6%, respectively, compared to rectangular channels at 0.6V. The oval channel displays superior velocity and vorticity, enhancing mass transfer between the main flow and reaction zone. Increasing channel width reduces rib width, promoting O2 distribution, temperature improvement via exothermic reactions, NH3 decomposition into H2, and improved cell performance. Rib width increase shortens diffusion and reduces concentration polarization, but increases ion transport and ohmic polarization. A theoretical formula for optimal rib width (Wrib0) to minimize polarization loss is provided. These findings offer a comprehensive reference for optimizing DA-SOFC design.

This study firstly experimentally determined the performance of direct ammonia solid oxide fuel cell (DA-SOFC) under various O2-enriched cathode gas conditions at 750 degrees C and 700 degrees C. Results indicate a significant 40.4% decrease in DA-SOFC power density when temperature dropped from 750 degrees C to 700 degrees C, more pronounced than in H2-SOFCs. O2-enriched operation improved DA-SOFC power density, with increases ranging from 7.9% to 22.9% as O2 molar fraction at the cathode inlet rose from 21% to 100% at 750 degrees C. And at 700 degrees C, such promoting effect on DA-SOFC was more significant than on H2-SOFC. Interestingly, DA-SOFC open circuit voltage (OCV) showed a nonmonotonic trend with increasing O2 molar fraction, peaking between 30% and 60% O2. Subsequently, multi-physics modeling of DA-SOFC were performed. The numerical results revealed that the observed increase in power density of DA-SOFC in O2-enriched operation mode was primarily resulted from the reduction in concentration polarization, rather than the changed OCVs/Nernst voltages.