Combustion Instability Research Articles

Optical engine investigation of ammonia combustion enhancement with polyoxymethylene dimethyl ethers (PODE₃) as pilot fuel

Ammonia, gaining attention as a carbon-free fuel amid declining fossil fuel reserves, faces challenges like combustion instability and misfiring in its application in internal combustion engines. In this study, polyoxymethylene dimethyl ethers (PODE3), a clean, renewable, and electrically synthesized fuel with high cetane number and oxygen content, is evaluated as a pilot fuel for ammonia in dual-fuel engines. The study, performed in a dual-fuel optical engine, investigates the use of PODE₃ compared to diesel as direct injection fuels for ammonia combustion, focusing on the impact of PODE3 injection timing and dwell (the interval between dual-stage injections) on the ignition characteristics of ammonia. The results demonstrate that, compared to diesel, using PODE3 enhances ammonia combustion in dual-fuel engines. This is evident from increased in-cylinder pressure, higher apparent heat release rate (AHRR), advanced combustion phases, and improved indicated mean effective pressure (IMEP), enhancing overall stability. PODE3 also effectively minimizes soot generation, evidenced by a 71.5 % reduction in spatially integrated natural luminosity (SINL), accompanied by a 16.9 % increase in peak flame area. Adjusting PODE3 injection timing is crucial; initial retardation improves combustion performance, marked by higher in-cylinder pressure and AHRR, as well as expanded flame area. However, over-delaying injection timing reduces these gains. The optimal performance is achieved under the SOI-25 condition, balancing combustion and flame development, though some areas remain unburnt. In a dual-stage injection strategy using PODE3, where the second injection timing remains fixed at −25 °CA ATDC, the critical adjustment of advancing the first injection timing to −35 °CA ATDC markedly enhances combustion performance compared to a single injection condition. This setting reduces ignition delays, increases peak in-cylinder pressures, and improves stability, which collectively contribute to a significant 20.4 % increase in the peak flame area, particularly between adjacent fuel jets. Expanding upon this, advancing the first injection timing further to −45 °CA ATDC leads to a decrease in peak in-cylinder pressure and AHRR, accompanied by a notable 40.3 % reduction in the peak flame area. This research affirms the advantages of PODE3 over diesel in ammonia-powered dual-fuel engines, markedly enhancing combustion performance and environmental sustainability, where the injection strategy of PODE3 emerges as a crucial element.

Analysis of Unburned Methane Emission Mechanisms in Large-Bore Natural Gas Engines With Prechamber Ignition

Abstract Although precombustion chambers, or prechambers, have long been employed for improving large-bore two-stroke natural gas engine ignition and combustion stability, their design predates modern analysis techniques. Employing the latest computational fluid dynamics (CFD) modeling techniques, this study investigates the importance of temperature and chemistry for ignition of the main chamber, with an emphasis on eliminating unburned methane. The sensitivity of the ignition and complete combustion to main chamber air/fuel mixture homogeneity was also explored. This study compares the effect of purely thermal ignition, purely chemical ignition, and how their interplay can influence the complete combustion of methane in typical mixtures and in homogeneous distributions of fuel in the combustion chamber. The CFD results demonstrated that temperature and chemistry are equally important in the ignition mechanism, and combining the two phenomena is effective at igniting the main chamber. Reduction of residual methane in the main combustion chamber (MCC) is most effective when chemical intermediates and thermal ignition are combined. A rudimentary analysis of the effect of fuel/air stratification was also conducted, and it demonstrated that a dramatic reduction in methane emissions is observed for homogeneous mixtures. The flow field in the main combustion chamber was shown to create detrimental stratification of the fuel/air mixture, which inhibited complete combustion of the methane in the main chamber. By contrast, in the extreme case of a perfectly homogeneous distribution of both chemical intermediates and fuel in the combustion chamber, it is possible to completely eliminate unburned methane in the main combustion chamber.

Large Eddy Simulation and Acoustic Analysis of Combustion Instability in an Annular Combustor with Low-Swirl Flames

Large Eddy Simulation and Acoustic Analysis of Combustion Instability in an Annular Combustor with Low-Swirl Flames

Effects of in-cylinder water injection on knocking and combustion stability generated by expanding the lean burn limit

The integration of direct water injection (DWI) with the lean burn (LB) process can significantly enhance engine performance. This study reveals that an increase in the water-fuel (W/F) ratio suppressed knocking by extending the ignition delay time. Specifically, the knocking of stoichiometric combustion was suppressed, while that of deeper LB (excess air ratio (λ)˃1.5) was sensitive to a larger W/F ratio. The intensity of knocks during stoichiometric combustion decreased significantly (more than twofold) when the W/F ratio increased from 0.1 to 0.2. Under identical W/F conditions, a shorter ignition delay (CA0–10) and combustion duration (CA10–90) of the main combustion flame were identified as important factors for knocking when λ increased from 1 to 1.5. However, for λ values exceeding 1.5, CA10–90 emerged as the primary knocking determinant. Faster flame propagation and higher oxygen concentration caused by LB and an early spark promoted autoignition of the end-gas and increased the autoignition heat release rate, thus strengthening knock intensity (KI). Furthermore, a minimal amount of water (W/F ≤ 0.3) exerted negligible influence on the stoichiometric combustion stability. As the LB approached its limit, the average KI remained relatively consistent, with combustion stability emerging as the key factor in elevating the LB performance. A 0.3 W/F ratio and precise spark timing ensured that a balance exists between combustion stability and knocking, while simultaneously elevating λ to 2.0.

The Effect of Insulation Thickness on Heat Transfer Characteristics and Flammability in Tube Mesoscale Combustors

The Effect of Insulation Thickness on Heat Transfer Characteristics and Flammability in Tube Mesoscale Combustors

Large-eddy simulation investigation on the effects of inlet swirl/Reynolds number and fuel heating value on a turbulent kerosene spray flame

To progress toward the high efficiency and low emission aviation engine technology, it is mandatory to understand the complex phenomena including kerosene spray atomization, evaporation, turbulent mixing and combustion in the engine combustor. Nowadays with the rapid growth of computational capacity over the world, large-eddy simulation (LES) performs a more and more important role in understanding the complex multi-phase combustion process of kerosene fuels in aviation engine configuration. In the present work, LES is employed to investigate a swirling kerosene spray jet flame in a model combustor, which has been studied experimentally. The turbulent combustion is modeled by the flamelet generated manifold (FGM) approach with a 3-component surrogate kerosene skeletal mechanism. The averaged gas velocity and temperature predicted by LES generally agrees well with the experimental measurements. Local extinction is observed at the inner shear layer of the swirling spray flame, due to droplet-turbulence-flame interactions. A parametric study with 11 cases is then performed to investigate the effects of inlet swirl/Reynolds number and fuel heating value on the kerosene flame. As the swirl number increases, the enhanced turbulent mixing between kerosene fuel and air facilitates the combustion process, but combustion instabilities may occur if the swirl number is too high. A higher Reynolds number can promote the turbulent combustion via a stronger recirculation zone, but it also has strong dilution effects on the combustion field. Variations in the fuel heating value directly impact the temperature field, but the flow field, major species concentrations and droplet phase statistics are only slightly affected. These LES results give clues to achieve optimal combustion performance in realistic aviation engine configurations via better arrangements of fuel and air injections.

Experimental investigation of the effect of variable valve lift on combustion stability and exhaust emissions in a diesel/methane CRDI engine

Diesel/methane combustion still encounters challenges in terms of lower combustion stability and elevated levels of HC and CO emissions. The valve lift (VL) technique is a method that contributes to increased in-cylinder volumetric efficiency, thereby improving engine performance and reducing emissions. Hence, the application of VL in diesel/methane operations would be highly significant. Therefore, an experimental study on the VL for reducing HC and CO emissions and enhancing combustion stability in a diesel/methane CRDI engine was performed at varying methane fractions and torques. In the study, three different cam profiles were used with VL extrapolated using Fourier series and designed: the original VL of 4.46 mm, as well as two different VLs of 4.0 mm and 4.9 mm. The methane in the total fuel energy was initially increased to 20% and later further increased to 40%. The experimental findings demonstrated that the augmented VL mitigated the prolonged ignition delay caused by the increase of methane fraction, thereby ensuring enhanced combustion stability across all torques. On the other hand, in diesel/methane operation, increasing the VL resulted in maximum reductions of approximately 17% in HC, 6% in CO emissions, and 75% in smoke opacity.

Fire zone forefront residual tar combustion characteristics and functional group transformation mechanism: Appling the based on thermogravimetric and in-situ high-temperature FTIR

When preventing spontaneous combustion disasters in extinguished coal fire zones, the influence of the tar re-ignition phenomenon is often overlooked. Therefore, in this study, using thermogravimetric analysis and high-temperature (630 °C) in-situ infrared technology, the combustion reactivity and real-time evolution mechanism of key functional groups for tar were deeply discussed under oxygen-lean conditions. Additionally, a novel two-step baseline drift correction method for spectral graphs at high temperatures is proposed. The results showed that tar combustion could be divided into two stages: pyrolysis-volatilization mass loss and crosslinked structure oxidation combustion. Reduced oxygen concentration limits the violent and stable rapid combustion of tar-crosslinked structures and reduces the re-ignition tendency of tar. Among, with a decrease in oxygen concentration, H (burnout performance), Cb (the reaction ability in the early stage of ignition) and S (overall combustion performance) decreased, while Ti (mass loss endpoint temperature), Th(burnout temperature), and HF (combustion stability) increased. This provides information for the selection of a reasonable process to prevent tar re-ignition. The contents of methylidene and aldehyde carbonyl in tar increased by approximately 2 and 3.7 times, respectively, compared to those of raw coal. In the first thermal decomposition stage of tar, the aromatic hydrocarbons were relatively stable under the action of the structural framework, whereas aliphatic hydrocarbons continued to decrease. Before burnout, a decrease in oxygen concentration increased the crosslinking ability of tar and delayed the tar burnout process. The contents of aromatic hydrocarbons and aldehyde groups are highly sensitive to the evolution of the oxygen concentration. Specifically, the Kubelka–Munk value of the aromatic hydrocarbons increased from 0.015 to 0.10, while that of aldehyde carbonyl groups increased from 0 to 0.23. Additionally, in the early stage of tar combustion, the volatile mass loss was basically not affected by the time-scale effect, and mass loss endpoint residual mass was approximately 48.11 %. In the later stage of tar combustion, owing to the cumulative of time-scale effect, the tar re-ignition tendency increased, H, Cb, and S increased, and HF decreased from 1.388 to 1.098. This research is helpful guiding disaster prevention and control of tar re-ignition in a fire area, which leads to reburning in the surrounding area.

An experimental study on in-cylinder soot formation and flame oscillation of renewable fuel blends in an optical engine

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.

A Bayesian optimization framework for the control of combustion instability of a bluff-body stabilized combustor

Control of combustion instability for a realistic gas-turbine combustor is challenging. This work aims to establish an efficient numerical framework for optimization to improve the combustion stability of a bluff-body combustor. Large eddy simulations of the spray combustion process are conducted, and the experimental measurements are used to evaluate the numerical accuracy of the baseline case. The air preheating temperature, the Sauter mean diameter of fuel droplets, and the location of liquid fuel injection are regarded as input variables. The root mean square of pressure amplitude is regarded as an optimization objective. The Bayesian optimization framework is proposed that includes the sampling process, surrogate model, acquisition function, and genetic algorithm optimizer processes. It is found that PRMS can be reduced by 64% for the optimized case compared to the baseline case using only 17 sample evaluations. This work is promising as it provides an effective optimization framework for the development of next-generation gas-turbine combustors.

Experimental Investigation of the Effect of Air/Fuel Ratio Change on JP8 Swirl Flame Characteristics with Image Processing Methods

In this study, the stable combustion ranges and emission values of JP8 fuel, which is used in military aviation, were tested and optimum conditions were determined. The AFR (Air/Fuel Ratio) of the JP8 flame was increased experimentally at constant thermal power in a gas turbine combustion chamber. In the study, AFR were tested as 30/1, 33/1, 36/1, 39/1 and 42/1. A generator with 0.4 swirl number is placed at the burner outlet. Under these conditions, the effect of external acoustic enforcement frequencies on dynamic instabilities at different AFR was investigated. Flame length and thickness data were extracted from the images obtained from the image processing technique. The effect of AFR change on the combustion chamber temperature was examined and pollutant emissions were measured. The results showed that the most stable flame appeared in AFR 33/1 case at 155 and 300 Hz resonance frequencies under acoustic enforcements. Additionally, when the AFR was increased to 42/1, flame instability increased at all frequency values. Moreover, it has been determined that the most optimum mixture in terms of combustion chamber exit temperature and CO emissions is AFR 33/1. There was an increase in NOX emissions by up to 39/1 with the increase in AFR.

CO2, Ar, and He dilution effects on combustion dynamics and characteristics in a laboratory-scale combustor

This study aimed to investigate the impact of CO2, Ar, and He addition on combustion instability under varying acoustic frequencies, dynamic pressure amplitudes, and light emission intensities. Experiments involved using pure methane as fuel at a 1.0 equivalence ratio, with three distinct acoustic frequencies. The burner operated at 5 kW, and the combustion chamber was excited at frequencies of 90 Hz, 160 Hz, and 320 Hz. It was observed that with dilution, the temperatures decreased more from the burner exit temperature to the exhaust section. As the oxygen concentration in the oxidizer decreases in Ar, He, and CO2 dilution, CO emissions increase, but CO2 emissions decrease. The addition of Ar, He, and CO2 causes an effect that reduces the flame temperature and brightness while causing the flame length to extend due to the diluting effect. While a decrease in instability was observed in He-Ar-CO2 dilution in 20 % oxygen, instability increased again in the three 19 % cases. It is observed that mixtures under the diluent effect approach the blowoff limit due to the increase in ignition delay. It has been determined that the addition of diluent increases the tendency for the flame to go out, especially due to the thermal effect. The methane fuel with an 18 % oxygen concentration in the oxidizer of Ar remained below lean extinction limits, undergoing blowoff. The temperature was measured as 1156 K at 21 % oxygen concentration, 584 K at 19 % oxygen concentration for carbon dioxide, 877 K for Helium, and finally 777 K for argon. In the case of pure air for the combustion of methane, CO emissions were measured at 140 ppm. Average CO levels at 20 % oxygen concentration in the oxidizer are 1254 ppm, 356 ppm, and 308 ppm with carbon dioxide, argon, and helium, respectively. The study also shows that an increase in O2 in oxidizing air leads to a decrease in CO emissions, while an increase in He/Ar/CO2 gases leads to an increase in CO emissions. Regarding NOX emissions, undiluted conditions measured a value of 14 ppm. NOX emission levels approached 0 ppm in all dilution cases and achieved its goal.

A comparative study on NH3/H2 and NH3/CH3OH combustion and emission in an optical SI engine

Ammonia is an attractive carbon-free fuel but suffers from poor combustion characteristics and heavy emissions. Green hydrogen and methanol are recognized as the most promising combustion promoters for enhancing ammonia reactivity in pursuit of a carbon–neutral solution. Nonetheless, a comparative study on ammonia combustion and emissions, incorporating hydrogen and methanol enrichment, is currently absent under actual engine conditions. In this work, the combustion performance, visualized flame characteristics, and pollution emissions of ammonia were investigated in an optical engine, allowing for low-level hydrogen and methanol enrichment, specifically at energy ratios of 5%, 10%, and 15%. We have identified the crucial role of hydrogen and methanol enrichment in enhancing various aspects of ammonia engine performance, including thermal efficiency, combustion stability, flame initiation and propagation, and nitrogen-based emissions. Specifically, hydrogen enrichment demonstrates a more pronounced effect on reducing cyclic variations, whereas methanol enrichment exhibits a greater influence on increasing thermal efficiency, manifesting improved controllability in combustion phasing. Combustion visualizations show the contribution of hydrogen enrichment to the formation process of the initial flame kernel, which is reflected as a lower cyclic variation from a macroscopic perspective, and meanwhile, methanol enrichment features a stronger acceleration effect during the turbulent flame propagation process. Furthermore, the quantification of flame response to turbulence is solved and observed much higher effects for NH3/CH3OH blends than for NH3/H2 blends, indicating a distinctly different mechanism of facilitation by methanol enrichment. Regarding nitrogen-based emissions, both hydrogen and methanol enrichment lead to a substantial reduction in NH3 slip and N2O emissions, consistent with observed variations in combustion stability. However, methanol enrichment induces noticeable decreases in NOx emissions, while hydrogen enrichment exhibits an opposing trend. The present study demonstrates the beneficial impacts of hydrogen and methanol enrichment on improving ammonia combustion and emissions, with the underlying reason correlated to flame dynamics and thermochemical interactions.

Study on the Flame Transition Characteristics of a Gas Turbine Combustor

Gas turbines are widely used as important equipment for electricity generation on islands and offshore platforms. During the operation of a gas turbine, the flame shape in the combustion chamber undergoes variations in response to changes in parameters such as gas turbine load, fuel distribution, and burner structure. These alterations in flame shape exert influence on combustion instability, emissions, and load characteristics. This study explores the variations in flame transition, emissions, and operating parameters among three distinct center stage structures: namely, the non-premix center stage (NPCS), premix center stage (PCS), and enhanced premix center stage (PCSE). The investigation is conducted using a heavy-duty gas turbine hybrid burner on a full temperature, full pressure, and full-size single burner experimental bench. Simultaneously, a multi-parameter numerical simulation regarding the influence of the central fuel split on flame shape analysis was conducted using the PCS burner under the design point for a more in-depth understanding of the mechanisms and for influencing factors associated with flame transition. The findings indicate that variations in flame transition loads among different central stage structures: for the NPCS burner, the transition occurs between 45% and 50% load; for the PCS burners, it takes place between 60% and 65% load; for the PCSE burners, it shifts between 55% and 60% load. Additionally, a reduction in NOx emissions is observed during the flame transition process. Furthermore, it was found that decreasing the central stage fuel results in a decline in flame angle for the same burner structure. As the central stage fuel diminishes to a specific value, the flame shape undergoes a sudden change. Further reduction in central stage fuel does not significantly affect the flame shape and temperature distribution.

Characterization and kinetic analysis of lignocellulosic and algal biochar combustion

The combustion characteristics of biochar obtained from barley straw (BS) and brown algae (BA) are explored. Four different heating rates are utilized to determine the respective activation energies. A master plot analysis is used to identify the appropriate reaction model. The results show that the activation energies vary in the ranges of 6.86–48.36 and 46.34–77.51 kJ mol−1 for BS and BA biochar combustion, respectively. As the heating rate increases, most of the combustion characteristic parameters increase, while the combustion stability index decreases. These observations help provide a deeper understanding of the combustion of lignocellulosic and algal biomass-derived biochar.

Technical evaluation of low-carbon fuels as a decarbonization pathway of the light-duty transport sector

The need to rapidly reduce greenhouse gas emissions from the transportation sector has renewed interest in low-carbon fuels, both from the production and utilization standpoint. The maturing of alternative processes to synthesize fuels from atmospheric CO2 and renewable non-edible feedstocks constitutes an interesting proposition, which needs to be matched by favorable drop-in properties and environmental performance once applied to internal combustion engines. To this end, the authors investigate the use of diesel-like high-reactivity and low-reactivity LCFs in a light-duty compression-ignition engine by experimentally assessing their implementation with existing hardware (drop-in) and discussing possible modifications to achieve improved results through calibration. The results revealed that high-reactivity fuels could operate under similar conditions to the standard diesel fuel, exhibiting similar efficiency and soot emissions combined with important reductions in NOx emissions. On the other hand, low-reactivity LCFs show advantages in promoting alternative combustion concepts because of the longer ignition delays, which make it possible to use lower injection pressures and have more stratified air–fuel mixtures, but at the same time, the results revealed that the implementation of these fuels might suffer from hardware limitations which worsens the combustion stability and the operation at low-load. Although the fuels tested exhibited variations in Tank-to-Wheel due to their carbon content and fuel economy, it was found that Well-to-Wheel CO2 intensity is eventually dominated by the renewable content of the fuel and the CO2 is greatly impacted by its production pathway, rather than the fuel consumption optimization. Finally, the study confirmed that, for the successful adoption of oxygenated low-carbon fuels (including alcohols), appropriate material selection in the engine fuel system and fuel additives are necessary to prevent components and seal performance degradation.

Experimental Combustion and Flame Characterization of a Chemical Looping Based Oxidative Dehydrogenation Byproduct Fuel Mixture Containing High CO2 Dilution

Abstract This study investigates the combustion performance of CO2 rich fuel mixtures containing ethane and methane as active species using a constant volume combustion chamber. This fuel is obtained as byproducts of a chemical looping based oxidative dehydrogenation (Cl-ODH) process ethylene production. The byproduct gas mixture has 40.79% CO2, 39.49% ethane, and 4.88% methane by weight with other minor compounds. After initial combustion modelling, the gas fuel mixture was reduced to just the major species: CO2, ethane, and methane. The mixture was then tested for flammability limits and combustion performance under spark-ignition conditions. Effects of ambient conditions and stoichiometry like temperatures between 300 to 400 K with initial pressures from 1 to 10 bar were tested. The fuel mixture showed an overall reduced flame velocity compared to gasoline. Instability in combustion was believed to be caused by the dissociation of ethane under elevated conditions. At higher pressures, the flame produces lower cumulative heat release. Simulations were also performed using a model tuned to replicate the operations of the combustion chamber used in the experiments. Heat release and unburnt fuel mass data were calculated to identify the discrepancies in the combustion completeness at elevated pressures. The effects of CO2 quenching the flame coupled with the increased dissociation of the fuel species can lead to up to more than 75% of the fuel mixture being unburnt. Data from this study was used to modify a small-scale spark-ignition engine to use this fuel and produce usable energy.

Role of proton exchange membrane fuel cell in efficiency improvement of ammonia–hydrogen fusion zero-carbon powertrains for long-haul, heavy-duty transportation

Ammonia fuel, a transport zero-carbon energy carrier with a much lower life-cycle greenhouse gas footprint than existing fuels and the ability to overcome the storage and transportation challenges of hydrogen fuel, is an effective means of realizing the energy transition for long-haul, heavy-duty transportation such as heavy trucks. Although the efficient use of liquid ammonia fuel cannot be realized directly at this stage, it can be realized indirectly by converting ammonia fuel into hydrogen and combining the hydrogen generated with ammonia in an ammonia–hydrogen fusion fuel supply method. In this study, we assess four ammonia–hydrogen fusion-fueled zero-carbon powertrains for heavy-duty commercial vehicles, each of which is designed to determine its most efficient green zero-carbon fuel utilization scenario at the point of steady state operation as well as over the entire power-demand interval. It was found that the required engine capacity decreases with the increase of the hydrogen energy ratio in the ammonia–hydrogen composite fuel. Considering the indicated thermal efficiency of the engine and the combustion stability in the cylinder, the ammonia–hydrogen composite fuel with 30% hydrogen energy share was selected for the matching design of the powertrains. The study results show that powertrain equipped with engine and fuel cell are more energy efficient than others. The powertrain with range extender consisting of the engine and fuel cell takes ammonia dissociation separation unit and fuel cell as an important part of engine exhaust gas heat recovery, which reduces the dependence of ammonia cracking hydrogen production on electric heating power. By regulating the energy distribution ratio between the engine and the fuel cell, the system can be maintained at a high level of energy efficiency. Under different power requirements of highway application scenarios, the system efficiency is not less than 36%, and the system energy utilization efficiency is higher than 44%. Compared to engine direct drive powertrain, engine range-extended powertrain, and fuel cell range-extended powertrain, the average system efficiency increases by 30.8%, 8.68%, and 14.24%, while the average system energy utilization increased by 41.12%, 25.26%, and 39.43%, respectively.

Experiments on symmetry breaking of azimuthal combustion instabilities and their analysis combining acoustic energy balance and flame describing functions

Combustion instabilities in annular systems raise fundamental issues that are also of practical importance to aircraft engines and ground-based gas turbine combustors. Recent studies indicate that the injector plays a significant role in the stability of combustors by defining the flame dynamical response and setting the inlet impedance of the system. The present investigation examines the effects of combinations of injectors of two different types ( $U$ and $S$ ) on thermoacoustic instabilities in a laboratory-scale annular combustor and compares different circumferential staging strategies. The combustor operates in a stable fashion when all injection units belong to the $S$ -family, but exhibits large amplitude pressure oscillations when all these units are of the $U$ -type. When the system comprises a mix of $U$ - and $S$ -injectors, it is possible to determine the number of $S$ -injectors leading to stable operation. For a fixed proportion of $U$ - and $S$ -injectors, some arrangements give rise to stable operation while others do not. Results also show that introducing symmetry-breaking elements affects the system's modal dynamics. These experimental observations are interpreted in an acoustic energy balance framework used to derive an expression for the growth rate as a function of the describing functions of the flames formed by the different injectors and their respective azimuthal locations. Growth rates are determined for the different configurations and used to explain the various observations, estimate the system damping rate and predict the location of the nodal line when the standing mode prevails.

Comparison and optimization of strategies for ammonia-diesel dual-fuel engine based on reactivity assisted jet ignition and reactivity turbulent jet disturbance under high load conditions

Ammonia (NH3), as a zero carbon, easy to synthesize, and easily obtainable fuel, is one of the choices for future internal combustion engine fuels. However, the slow laminar flames speed and high ignition temperature of NH3 seriously hinder the development of NH3 engines. Efficiently igniting NH3 and ensuring its stable combustion is currently a challenging research topic. Utilizing diesel to optimize the NH3 combustion process is one effective solution. However, the increase of unburned ammonia (UNH3) and unstable combustion, under high load conditions have always been technical challenges that constrain the development of ammonia-diesel dual-fuel (ADDF) engines. Previous studies have found that pre-chambers (PC) have a significant effect on improving ammonia combustion characteristics. Therefore, based on the PC system, this study proposes two combustion modes: Reactivity Assisted Jet Ignition (RAJI) and Reactivity Turbulent Jet Disturbance (RTJD), which greatly optimize the combustion characteristics of ADDF engines under high load conditions. The research results indicate that the RAJI mode effectively improves the thermal environment inside the cylinder. Setting the start of diesel injection of pre-chamber (SODI-PC) at −14 °CA after top dead center (ATDC) can increase the cylinder temperature by 25.05% before the start of diesel injection of main chamber (SODI-MC). The improvement of thermal environment reduces the average generation of N2O to 13.56% of the traditional mode. However, if SODI-PC is postponed to −8 °CA ATDC, the effect of creating a thermal environment is no longer significant. Research on the RTJD mode has found that adopting the RTJD mode can significantly optimize emissions. Compared with traditional modes, N2O, UNH3, and Soot decreased by 28.81%, 27.79%, and 35.08%, respectively. Additionally, SODI-PC is delayed until after 4 °CA ATDC, CO and UHC will be further reduced to 50.00% and 12.24% of the traditional mode, respectively. In addition, the RTJD mode overcomes the deficiency of the slow laminar flame speed of NH3, shortening the combustion duration by 23.46%, and effectively improving the gross indicated thermal efficiency (ITEg) by an average increase of 1.08%, with the highest increase reaching 47.57%. This paper provides ideas for optimizing the structure and strategy of ADDF engines, as well as theoretical support and empirical reference for achieving clean and efficient combustion of engines.