Lean Combustion Research Articles

Resolving NOX formation of ammonia-hydrogen flame utilizing PLIF technique collaborated with flame structures and chemical kinetics analysis

Carbon-free fuels such as ammonia and hydrogen have attracted much attention in response to tackling with climate problem of global warming, but their high NOX emissions limit practical applications unavoidably. Currently, very few studies have addressed the inter-relationship between flame structures and NOX formation. In addition, few previous studies have analyzed ammonia-hydrogen combustion with low hydrogen mixing ratio through in-depth NOX formation mechanisms. In this work, NOX formation of ammonia-hydrogen swirl flame with different equivalence ratios and hydrogen mixing ratios (<30 %) has been comprehensively investigated, and the connection between flame structures and NOX has been reflected based on PLIF technique. The analytical results showed that as equivalence ratio (Φ = 0.6–1.2) increased, NOX concentration increased firstly and then decreased subsequently, and peak NOX value was observed between Φ = 0.7–0.8. Besides, NOX increased as the hydrogen mixing ratio increased from 10 % to 25 %, being capable of reaching up to 2795 ppm. Furthermore with flame structure analysis, the flame structure could be classified into single-front flame, transition flame, and double-front flame, in which transition flame featured with the largest decomposition reaction region contributing to NH3 oxidation to form NOX (intensive OH radicals propagation); while double-front flame characterized by smallest decomposition reaction region inhibiting the NOX formation via OH suppression (weak OH radicals propagation). Based on systematically flame surface density and chemical kinetics analysis, lean combustion benefited the NOX pathway, whilst rich combustion favored the N2 pathway. In addition, as the hydrogen ratio increased, and the reducibility of NH/NH2 to NOX was weakened, which ultimately promoted the production of NOX. The findings achieved suggest that future combustion techniques by the ammonia-hydrogen dual fuel should avoid the occurrence of transition flame, and prone to the generation of double-front flame, which could thus implement effective suppression on NOX formation.

Performance assessment of direct injection stoichiometric and lean burn compressed natural gas engine over port fuel gasoline and compressed natural gas engine

ABSTRACT A direct injection (DI) CNG engine was developed and evaluated under stoichiometric and lean burn conditions, emphasising high compression ratios (CRs). A comparative analysis of PFI gasoline and PFI CNG engines was conducted to provide comprehensive insights. Results showed that the DI CNG engine exhibited a shorter combustion duration, indicating higher combustion speed, and enhanced combustion stability, particularly at low load and speed conditions, with a reduced coefficient of variation of indicated mean effective pressure (COVIMEP) below 1%. The DI CNG engine demonstrated a relative increase in net indicated thermal efficiency by 4–5% compared to the PFI CNG engine. Furthermore, CNG engines operating at a high compression ratio of 16 could run at high loads without encountering knocking issues. Lean burn operation, combined with high compression ratios, improved thermal efficiency while minimising emissions; however, challenges such as combustion instability and increased emissions under lean burn conditions were observed.

An experimental study on the influence of nozzle hole numbers and patterns of a passive pre-chamber spark ignition system

Pre-chamber spark ignition system enhances the combustion stability and efficiency of lean burn engines. Pre-chamber spark ignition system performance directly relies on the characteristics of the turbulent flame jet, which are governed by the pre-chamber geometrical parameters and nozzle hole configurations. Based on the existing literatures, limited research on nozzle hole numbers and patterns is noted with passive pre-chamber, and thus, the impact of these nozzle hole configurations on the combustion and flame development characteristics for hydrogen-methane-air mixtures is investigated in a constant volume combustion chamber. In the current study, single-layer nozzle hole configurations with three, four, six, and eight nozzle holes and double-layer nozzle hole configurations with six and eight nozzle holes are designed and tested in an optically accessible combustion chamber under various equivalence ratio conditions. The results indicated that the combustion characteristics deteriorated with an increase in nozzle holes, except with the single-layer four nozzle hole configuration. Compared to the single-layer eight nozzle hole configuration, the ignition delay and combustion duration for hydrogen-methane-air mixtures with single-layer four nozzle hole configuration was reduced by 31.9% and 30.7% under lean conditions. A reduction in flame jet penetration length, area and velocity and increased adjacent flame jet interaction was noticed for the single-layer configuration with more nozzle holes, and thus, a different nozzle hole pattern was studied in the current study. The double-layer configurations with more nozzle holes showed better combustion and flame development characteristics than single-layer configurations due to their unique flame structure, uniform and faster flame jet distribution, and reduced adjacent flame jet interaction. The ignition delay and combustion duration for hydrogen-methane-air mixtures with the double layer eight nozzle hole configuration were reduced by 30.5% and 40.7% compared to the single-layer eight nozzle hole configuration.

Experimental Investigation on Compact Current Non Thermal Plasma Assisted Hydrocarbon Reforming Hydrogen Rich Gas

On-board vehicle hydrogen-rich gas extraction of gasoline reforming utilizing for fuel cells and GDI-engine lean burn combustion enhancement NOx trap applications. A non-thermal plasma discharge electrode that produces a reduce current, increased voltage 15kV source arc plasma fuel reformer is designed to operate at an arc frequency of 50 Hz. The operating factors gasoline equivalence ratio between 4 and 6.and electronically control by the fuel injector and eliminating ground spark plug plasma arc generation. The plasma arc non-thermal reformer gasoline converted into gas that contains partial oxidation contains combustible gases like hydrogen (H2), CO, non-combustible gases like CO2, N2, and a small quantity of H2O. The nonthermal plasma technology alternative for on-board hydrogen vehicle applications.

A Parametric Study on NOx Emissions From Ammonia Containing Product Gas in Rich–Quench–Lean Combustion

Abstract Due to climate change, there has been an increasing demand for fuels that can accelerate the transition away from fossil fuels to clean energy. Humidified product gas obtained from gasifying biomass is emerging as a promising candidate to replace natural gas, as it is composed of a gaseous mixture of hydrogen, steam, carbon monoxide, and methane. However, the gasification process releases ammonia and other nitrogen bearing compounds into the product gas, resulting in substantial increases in nitric oxides, NOx, in the exhaust. As such, there has been a recent push to understand the underlying chemical kinetics that drive NOx formation in order to optimize gas turbines to mitigate emissions at the source. In this study, a simplified chemical reactor network (CRN) model for a gas turbine rich–quench–lean (RQL) combustor was developed in cantera. The following parameters were investigated in this study: equivalence ratio of the primary section, overall equivalence ratio, steam dilution, postflame residence time, and recirculation from the postquench region to the primary section. Additionally, a benchmark CRN representing a lean burner (LB) is also developed. Results of the CRN model suggest that, when comparing to LB, a RQL type combustor delivers up to a 75% reduction in emissions. Additionally, it was found that, for both the LB and RQL combustors, an overall lean to stoichiometric equivalence ratio is well suited to reduce emissions in highly humidified fuels, while for moderately humidified fuels it is preferable to operate in an overall slightly rich equivalence ratio. The difference observed is mainly due to the fact that, at high humidification and lean conditions, the temperature is favorable for the conversion of ammonia to nitrogen, while, at moderate humidification and rich conditions, NO reacts with ammonia in the reburn process. Finally, it is suggested that the incorporation of recirculation from the secondary section to the primary section of the RQL burner results in a broader low emission region, due to more favorable conditions for ammonia conversion to nitrogen in the primary section.

Study on combustion and emission characteristics of hydrogen/air mixtures in a constant volume combustion bomb

Hydrogen is an environmentally-friendly fuel with the characteristics of low carbon, high calorific value, and extensive combustible range, making it an optimal alternative fuel for internal combustion engines. In this study, an experiment using a spherical constant volume combustion bomb (CVCB) was conducted to investigate the combustion and emission characteristics of hydrogen/air mixtures. The results shows that the flame propagation velocity of hydrogen laminar flow initially increases and then decreases with the excess air coefficient (λ), reaching peak at λ = 0.6, and the flame becomes unstable on the side of lean burning (λ > 1.0). Both the NOx emissions and the average temperature in the CVCB initially rise before eventually declining with the λ, the peak values for both NOx emission and mean temperature are reached at λ = 1.0. The dilution of N2 inhibits the laminar combustion and emission of hydrogen. As the dilution rate of N2 increases, little change occurs in the Markstein length, while flame propagation velocity, NOx emission, and average temperature all decrease. An increase in initial pressure hinders the development of the initial flame, while promoting the formation of cellular structure within the flame. Consequently, unstable combustion leads to a rapid increase in flame propagation speed during later stages. The average temperature inside the CVCB and the NOx emission increase by 6.6 % and 25.9 %, respectively, with the initial pressure increasing from 0.1 MPa to 0.5 MPa. On the other hand, an increase in initial temperature can facilitate flame development and results in an increased flame propagation speed. Especially, Markstein length is less sensitive to changes in initial temperature than to changes in initial pressure. The average temperature and NOx emission present a pattern of increasing first and decreasing then with the increase in initial temperature. These results have provided empirical evidence for optimizing the combustion and emission control of hydrogen engines.

Characteristics of Fuel-Borne Nitrogen Pollutants in Hydrogen-Ammonia Mixtures Using Argon-Oxygen Atmosphere Under Engine Conditions

Abstract Amid climate change, reducing reliance on fossil fuels and transitioning to renewable energy sources is crucial. Ammonia, a carbon-free and renewable fuel, shows significant potential as an alternative energy source. By incorporating hydrogen as an additive, its flammability can be enhanced to suit existing spark ignition engines. However, understanding the characteristics of nitrogen pollutant emissions (i.e., NOx, which includes NO and NO2, and N2O) from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. However, understanding the characteristics of nitrogen pollutant emissions from ammonia-hydrogen combustion is challenging due to contributions from both fuel-borne and air-borne nitrogen. Therefore, a comprehensive understanding of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion is essential. This study focuses on investigating fuel-borne nitrogen pollutants in an argon-oxygen atmosphere, thereby eliminating nitrogen from the oxidizer and its role in thermal NOx formation. The research examines the formation and evolution of fuel-borne nitrogen pollutants during ammonia-hydrogen combustion under engine-like conditions. Results indicate that fuel-borne nitrogen pollutants act as intermediates, potentially originating from chemical equilibrium. While fuel NO predominantly forms in the burning zone, it undergoes reduction in the burned zone. N2O, absent in thermal NOx mechanisms, shows significant concentrations in the burning zone and is mostly converted to N2, leading to limited N2O in the final fuel-borne nitrogen pollutant concentration. Lean burn conditions, hydrogen addition, and oxyfuel combustion promote fuel NOx formation. Additionally, the equivalence ratio affects the ammonia-hydrogen premixed flame structure due to the de-NOx effect of ammonia. Overall, these findings highlight that fuel-borne nitrogen pollutant mechanisms differ from thermal NOx mechanisms, necessitating specially designed reduction technologies for clean spark ignition engines.

Experimental investigation of OH* emission spectrum characteristics and transient ignition dynamics in methane and coal dust mixtures explosions

Combustible gases, dusts and their mixtures are widely present in human production and life，and the fire and explosion disasters caused by them pose a serious threat to the field of energy security applications. Studying the ignition process of mixtures is essential for disaster risk assessment and safety protection. In this work, the explosion characteristics and OH* emission spectra of the mixtures were experimentally tested by varying the fuel equivalence ratio ( ER ≈ 0.79 ~ 1.71), and the evolution of the transient flow field structure during the ignition process was quantitatively analyzed using the schlieren image velocimetry method. The results indicate that the emission spectrum of OH* is closely correlated with the maximum explosion pressure, and the spectral intensity of OH* at 306.4nm is consistent with the maximum rate of explosion pressure rise. The flow field during the ignition process of the mixtures shows that a small amount of coal dust (concentration≤30g/m3) can significantly promote flame acceleration and instability as the methane concentration is in lean combustion or stoichiometric ratio. However, when the concentration of coal dust increases (concentration≥40g/m3), coal dust will suppress flame acceleration and instability. For methane concentration in fuel-rich combustion state, coal dust always suppresses flame acceleration and instability. The experimental results contribute to a further understanding of gas and coal dust mixed explosions and provide a verification database for the construction of chemical kinetic mechanisms.

Experimental and simulation study on lean combustion characteristics of passive pre-combustion chamber ignition boosted GDI engine

In order to verify the potential application of passive pre-combustion chamber technology in commercial GDI engines, the efficiency characteristics, combustion characteristics (CA50, Knock Index and COV) and emission characteristics of a pre-combustion chamber ignition engine in a boosted environment were studied by three-stage injection strategy to form a lean burn environment (λ = 1.3). A 3D simulation model of pre-combustion chamber ignition engine is established. Combined with the engine bench test results, the process of intake, mixture formation, combustion and emission generation under different working conditions is simulated by simulation method. The development process of TKE (turbulent kinetic energy), air-fuel ratio, temperature and combustion emissions in the main/pre-combustor is studied. It is found that under high load conditions in boosted pre-combustion chamber engine, the average knock index can be controlled between 2 and 3.4 bar, and the COV can be controlled between 3% and 3.5%. The indicated thermal efficiency reaches over 40% within the load range of IMEP from 10 to 14 bar. When the load increases, NOx and soot emissions show a downward trend, the HC and CO emissions slight increase. The pre-combustion chamber engine exhibits a tumble intake pattern around the cylinder axis perpendicular to the cylinder axis. The TKE reaches its maximum value 20 CAD before the top dead center and then rapidly decreases. The use of a three-injection strategy can form a uniform stratified mixture at the ignition moment. The fuel air equivalence ratio in the ignition area of the pre-combustion chamber is 0.8–0.9, slightly higher than the average fuel air equivalence ratio of 0.75.

Effects of industrial hydrogen-rich gases on the performance of the spark ignition engine under various combustion modes

Hydrogen is emerging as a promising alternative to traditional fossil fuels for the transportation. Additionally, the hydrogen-rich gases (HRG) can produce on a large scale during the process of chemical production. In this study, the influences of HRG on the performance behaviors of the spark ignition (SI) engine are comprehensively analyzed under various combustion modes and operating conditions. The experimental results indicate that with increasing the lambda, the torque of the test engine can maintain the target value under equivalent ratio and lean combustion modes, while it sharply decreases under ultra-lean combustion. When the value of the lambda is 1.5, the brake thermal efficiency (BTE) and brake specific hydrogen consumption (BSHC) of the HRG SI engine respectively reach up to 33.6% and 89.1 g/kW.h, yielding superior fuel economy. The NOx emissions of the HRG SI engine firstly increase and then decrease with increasing lambda, reaching the peak value at the lambda of 1.1. In addition, the NOx, CO, and HC emissions of the HRG SI engine greatly reduce when the lambda is approximately 1.5. Consequently, using HRG partially substitute the traditional fossil gasoline in the SI engine can benefit to achieve energy conservation and emissions reduction in transportation section.

Experimental and numerical study of lean combustion of propane in divergent porous media burners

Porous inert media burners for industrial applications demand the utilization of flame front stabilization methods, among which the convective or cross-section variation technique stands out. In this work, combustion of lean premixed propane-air mixtures in cylindrical and divergent alumina packed-bed porous inert media burners was numerically and experimentally conducted. Three burners of 0°, 15° and 30° of divergence were used to study the variation of equivalence ratio and inlet velocity. A bidimensional transient mathematical model was developed and solved numerically, whose results show good agreement with experimental measurements. The maximum temperature along the central axis is location dependent, with upstream augmentation of up to 86 %, meaning an increase of ∼ 815 K in the case of the 30° burner. Combustion front propagation velocity of upstream regime flames show acceleration in both divergent burners due to heat transfer and increment of thermal power in relation to cross-sectional areas when reaching the inlet. In comparison to cylindrical burner, a concave shape of the combustion front and a stretch of the maximum temperature zone was obtained, due to the diffusive nature of the velocity field. Stabilized flames on both divergent burners were achieved in the equivalence ratio range from 0.4 to 0.6. Residence time of product gases is greatly prolonged with burner wall angle and the decrease in overall temperature towards the outlet.

Influence of the flame jet on heat loss and thermal efficiency of lean burn pre-chamber natural gas engine with various geometrical combustion chambers

Due to relatively high thermal efficiency and low NOx emission, the pre-chamber type lean burn natural gas engine for cogeneration is expected to be spread rapidly in the near future. To offer a guidance for designing the pre-chamber natural gas engines, in the present study, the influence of the flame jets on combustion process and thermal efficiency of pre-chamber type lean burn natural gas engine was investigated with various pre-chamber orifice diameters and angles and piston bowl shapes by numerical simulation. The presented results showed that when the orifice diameter is smaller, it results in higher heat loss with the stronger gas flame jets impinging on piston top and cylinder liner walls, but the higher degree of constant volume heat release can be also obtained. Consequently, there is no significant difference in the thermal efficiency for these four orifice diameters. Increasing the angle of the orifice and the depth of the piston bowl can avoid the heat loss caused by the early impact of flame jets on the piston top wall. In addition, compared to the diameter of piston bowl, the depth of piston bowl, which is related to the distance from the orifice to the piston bowl, should have more effect on the combustion process and thermal efficiency. For wider orifice angle, the heat loss was also increased, particularly to cylinder head. Finally, the optimal thermal efficiency obtained by balancing heat loss and the degree of constant volume heat release is 44.73%.

Experimental analysis and optimization of the variable valve timing on attaining high efficiency with low NOx emission of a direct-injected hydrogen engine

The direct-injection hydrogen engine (DI-H2ICE) has demonstrated zero carbon emissions with high brake thermal efficiencies (BTE). Due to the high stoichiometric air–fuel ratio of hydrogen, with a lean burn strategy the gas exchange process of a hydrogen engine is different to a gasoline engine, and the valve timing controlling strategy requires reinvestigation. In this research, the optimal variable valve timing (VVT) is investigated in a 2.0 L DI H2ICE. The intake and exhaust valve timing controlling strategies are analyzed experimentally over the whole operating map. A multi-indicator decision-making method is applied to determine the entropy weight of BTE and NOx emissions. Results indicate that BTE greatly benefits from advanced intake valve timing, and exhaust valve timing affects BTE through pumping losses. Advanced intake and exhaust valve timings are employed in the hydrogen engine to enhance the BTE until the NOx emissions rise rapidly at high loads. The retarded exhaust valve timing helps reduce NOx emissions and avoid the risk of abnormal combustion at high loads. Therefore, earlier IVO and later EVC are applied simultaneously for DI-H2ICE compared to the gasoline engine. A maximum power of 124.8 kW at an engine speed of 4500 rpm and a BTE of 42.57 % is achieved with optimized VVT. Furthermore, NOx emissions are controlled to under 20 ppm over nearly half of the engine operating map. The conclusions are valuable in the development of high-performance H2ICEs, the numerical simulation studies, and hydrogen fuel applications.

Effect of mainstream-forced-entrainment control strategy on the combustion performance of a cavity-based combustor

The trapped vortex combustor (TVC) displays potential for use in advanced aircraft engines because of its excellent combustion stability with a compact geometry. However, the existence of forced-entrainment phenomenon in the mainstream poses obstacles to the practical application of TVC. This study introduces passive control strategies using modified radial struts to address the issue of mainstream-forced-entrainment in a TVC. Additionally, experimental and numerical studies were conducted to investigate how the mainstream-forced-entrainment control strategy on the combustion performance of a TVC. The findings suggest that the inclined strut (case-2) and longer-vertical strut (case-3) are more effective than the short-vertical strut (case-1) in reducing mainstream entrainment into the cavity. This results in a larger vortex structure and a more concentrated fuel distribution within the cavity of case-2 and case-3, as compared to case-1. Consequently, case-2 and case-3 achieved much better ignition and lean blowout limit and combustion efficiency performances compared to case 1.

Towards hydrogen gas turbine engines aviation: A review of production, infrastructure, storage, aircraft design and combustion technologies

This study explores the potential of hydrogen gas turbine engines for sustainable aviation, with a particular focus on their development for low subsonic to transonic flight regimes. Hydrogen offers notable advantages, including zero CO2 emissions and high energy density. However, its widespread adoption is prevented by significant challenges in production, infrastructure, storage, aircraft design, and combustion technology. The limited availability of green hydrogen – the global production totaled only 127 kt in 2023, with 76% produced in China - raises concerns about the feasibility of a rapid shift to hydrogen-powered general aviation. Despite numerous pledges for hydrogen adoption in the Western world, major aircraft manufacturers have primarily engaged in marketing activities rather than substantial product innovation. To date, the only hydrogen aircraft demonstrator from this century is the 2012 Boeing Phantom Eye, which utilized modified hydrogen internal combustion engines rather than a gas turbine engine. The only hydrogen aircraft demonstrator with a gas turbine engine was the 1988 hybrid Tupolev Tu-155 which coupled one hydrogen gas turbine engine to other two jet fuel engines. This paper evaluates the current state of hydrogen gas turbine technology, comparing its combustion characteristics with traditional jet fuels and examining various NOx emission control techniques. It also investigates the potential of micro-mix lean combustion and staged combustion to achieve efficient and low-emission hydrogen combustion in gas turbines. Given the more extensive experience with terrestrial gas turbines compared to their aeronautical counterparts, the review includes insights from terrestrial power generation applications. Additionally, it considers hythane - a blend of hydrogen and natural gas - as a transitional solution for significantly reducing CO2 emissions while progressing towards net-zero targets. The review underscores the urgent need for advancements in green hydrogen production, technology, and infrastructure to overcome existing barriers and enable a more sustainable aviation future powered by hydrogen. Importantly, the review emphasizes the need to move beyond mere virtue signaling and make tangible progress toward a hydrogen economy.

Promotion of La2O3 on Pd/Al2O3 three-way catalyst for passive selective catalytic reduction operation

For three-way catalysts (TWC) applied in lean burn gasoline engines for passive selective catalytic reduction (pSCR) operation, besides the conversion efficiency of HC/CO/NO, the production of NH3 is also an important concern. In this work, La2O3 was introduced into the Pd/Al2O3 catalyst system, the influence on its TWC performance for pSCR operation was investigated, and the physicochemical properties were characterized by a range of techniques. The results reveal that the introduced La2O3 mainly interacts with Al2O3 by preferentially anchoring to the tetra-coordinated Al sites on the surface. When the catalyst is modified by appropriate amount of La2O3 (4 wt%), higher dispersion of Pd species, larger amount of Pd2+ and active oxygen species are achieved, which well contributes to improved low-temperature reduction ability and NH3 desorption property of the catalyst. As a consequence, excellent catalytic conversions of HC/CO/NO are obtained, and meanwhile, higher concentration of NH3 is generated, making the modified catalyst more promising for pSCR operation.

Numerical modeling and suppression of combustion instabilities in a partially premixed combustor

To suppress the combustion instabilities faced in the lean premixed combustion, the impacts of swirler hub configurations on combustion instabilities under elevated pressure are investigated using large eddy simulation combined with a flamelet generation manifold model. Good agreement between the numerical predictions and experimental data is achieved. The flow fields of the combustors with three distinct swirler configurations are simulated: prototype, swirler with lobes on the hub of pilot stage, and with lobes on the hub of the first main stage. Furthermore, dynamic mode decomposition (DMD) is used to extract the dynamic characteristics, and a flame transfer function (FTF) is employed to characterize the fluctuation characteristics. The results show that the prototype combustor demonstrates a coupled fluctuation between flow and heat release. Influenced by the precessing vortex core (PVC), the flame angle varies between 70° and 90° and the first DMD modes of axial velocity, temperature, and heat release rate are all at a frequency of 470 Hz. The lobes on the hub of the pilot stage suppress the formation of PVC, making the combustion very stable. The flame angle remains constant at 80°, and the gain of FTF is lower than 1. However, adding lobes to the first main stage makes the combustion extremely unstable. The flow field structure undergoes drastic changes, mimicking a “breathe” process. The flame surface is highly distorted, and flashback phenomena occur.

Co-optimization and prediction of high-efficiency combustion and zero-carbon emission at part load in the hydrogen direct injection engine based on VVT, split injection and ANN

Hydrogen fuel holds great promise for accelerating the energy sector's decarbonization efforts. However, hydrogen's unique properties pose challenges for hydrogen direct injection (HDI) engines, such as differences in airflow exchange capabilities for very low density and reduced combustion rates due to lean burn conditions. This study investigates the impact of combined variable valve timing (VVT) and split injection strategies on HDI engine combustion, efficiency, and emissions. Using Artificial Neural Network (ANN) models and experimental data, predictive models for Brake Thermal Efficiency (BTE) and Nitrogen Oxide (NOx) emissions were developed. Adjusting intake and exhaust valve timings improved BTE by up to 2.5 % and 1.8 %, respectively, while reducing NOx emissions by 48.9 % and 31.1 % under specific conditions. Optimizing split injection alongside VVT further increased BTE by 1.2 % and 0.9 %, but increased NOx emissions by 29.9 % and 26.5 %. The flexible application of VVT and split injection strategies aims to balance efficiency gains with emissions control. The ANN-based models demonstrated high accuracy (R2 > 0.974), proving reliable substitutes for real-engine testing in certain conditions. In conclusion, this research highlights the potential of VVT and split injection strategies to enhance HDI engine performance while mitigating environmental impact, supporting the transition to sustainable energy solutions.

Study of Efficient and Clean Combustion of Diesel–Natural Gas Engine at Low Loads with Concentration and Temperature Stratified Combustion

The approach for achieving efficient and clean combustion in a diesel–natural gas (NG) heavy-duty engine at low loads was studied by computational fluid dynamics simulation. This study proposed the concentration and temperature-stratified combustion technology and clarified its mechanism. The results revealed that different stratified combustions can be organized by controlling the pressures, timings, and durations of diesel and NG injections, and stratified combustion can be classified into moderate, lean, and rich stratified combustion modes. Efficient and clean combustion can be realized simultaneously at low engine loads: the gross indicated thermal efficiency (ITEg) of engine breakthrough was improved to 47.9%, and the indicated-specific emissions of unburned hydrocarbon (ISUHC) were greatly reduced to 1.6 g/kWh, while the indicated-specific emissions of nitrogen oxide (ISNOx) remained at 0.6 g/kWh. Moreover, the detailed analysis of three typical stratified combustion modes demonstrates that coupling control of the concentration and temperature of the charge is the key to obtaining excellent engine performance. Most of the NG-stratified mixture should burn in the react ratio range of 0.4 to 0.8 for low unburned hydrocarbon emissions, low nitrogen oxides emissions, and rapid combustion. The proper temperature stratification should ensure that a high-temperature charge is around the over-lean NG mixture. This study can provide the fundamentals of stratified combustion control and feasible solutions for commercial applications of NG engines.

Nonpremixed Approaches for Fuel Flexible, Low NOx Combustors in High-Efficiency Gas Turbines

Abstract Lean, premixed combustor designs have almost completely replaced non-premixed combustors for industrial gas turbine applications where NOx emissions are regulated. Nonetheless, these designs have also introduced turndown and fuel flexibility constraints and made combustion instabilities and flashback more problematic. Future gas turbine applications will require combustors to accommodate a range of alternative fuels, provide operational flexibility, and compete for services with a host of new technologies, including energy storage and fuel cells. The purpose of this paper is to propose nonpremixed, multi-stage designs for the next generation of high turndown, high fuel flexibility, low NOx combustion designs – referred to here as a Nonpremixed, Rich, Relaxation, Lean (NRRL) combustor. The key concept we explore is non-premixed combustion, followed by additional fuel mixing to locally fuel-rich conditions, a relaxation stage, and then a lean stage. This non-premixed approach can handle essentially any fuel composition, including pure hydrogen, liquid fuels, pure methane, pure ammonia, and any combination in between while breaking the NOx-CO tradeoff and reducing combustion instability risk. This paper provides chemical reactor network modeling calculations to identify key kinetic processes and time scales required for such a concept. This concept has completely inverted sensitivities from lean, premixed systems which prefer short residence times, low pressures, and low temperatures to minimize NO formation. This concept prefers long residence times, high pressures, and high temperatures, indicating a very different set of design trades for part load and off-design performance.