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Hydrogen fuel is gaining particular attention in internal combustion engines. In addition to zero-carbon emissions, major advantages relate to its combustion characteristics, which allow a significant increase in thermal efficiency under ultra-lean operation and with very low NOx levels. The ignition system is one of the main technology enablers, as it determines the capability to control ultra-lean operations, avoid backfire phenomena, and/or reduce the risks of abnormal combustions. The latter results from hydrogen's low ignition energy and it is associated with factors like high-temperature residuals, hot spots, and irregular spark plug discharge. The ACIS gen 2-Barrier Discharge Igniter excels in accelerating the initial flame growth speed by the generation of non-equilibrium low-temperature plasma, a strong ignition promoter for the combined action of kinetic and thermal effects. Moreover, its volumetric discharge facilitates combustion initiation on a wide region, in contrast to the localized ignition of traditional spark systems. In this work we present for the first time, to the best of our knowledge, experimental results showing the performance of a hydrogen engine with a low-temperature plasma discharge. Tests were conducted on a single-cylinder research engine, achieving ultra-lean conditions with cycle-to-cycle variability results below 2.5%. The analysis indicates that the H2-BDI combined solution is capable of accelerating the evolution of the flame front compared to traditional spark plugs, leading to a significant reduction in the cycle-to-cycle variability. A meticulous adjustment of the BDI control parameters further enhances igniter performance and contributes to a deeper understanding of the innovative approach proposed in this study. [ABSTRACT FROM AUTHOR]

As fossil fuels currently account for approximately 85 % of the world's energy demands, the rapid growth of the global population and the economy is projected to increase greenhouse gas emissions. Consequently, there is a growing interest in the concept of ultra-lean combustion. This advanced combustion technology has the potential to effectively reduce both fuel consumption and emissions, offering a more environmentally sustainable approach. This comprehensive review delves into hydrogen ultra-lean combustion, specifically exploring the flame ball phenomenon by specifically investigating computational solutions in uniform and non-uniform mixtures. Noteworthy is the attention given to the emergence of stable flame balls within quiescent premixtures, especially in microgravity conditions. The review elucidates how these flame balls achieve stability through thermal radiation loss. A thorough examination of the role of the Lewis number (Le) in flame instability is conducted, considering its integral connection with diffusive-thermal instability across various experimental setups. Beyond the theoretical aspects, this review encompasses the practical domain of stable spherical flames originating from ultra-lean premixed reactants based on hydrogen. The exploration extends to comprehending the intricate mechanisms governing lean flammability limits, particularly the interplay between low-speed counterflow premixed flames and flame balls. It sheds light on how the concept of flame balls contributes to a more profound understanding of emission characteristics in ultra-lean combustion. Lastly, the review provides valuable insights into potential avenues for further research. It outlines the challenges that lie ahead, which encapsulates experimental observations and in-depth investigations into flame balls and the propagation of hydrogen ultra-lean flames. • Focusing on the phenomenon and characteristics of hydrogen in ultra-lean combustions. • Exploration of the stability mechanisms of ultra-lean combustion flame ball. • Elucidating the role of the Lewis number in flame instability and emission characteristics. • Emphasizing the further investigations into hydrogen based ultra-lean flame propagation. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional numerical model of a hydrogen direct-injection engine was established, and the combustion model was verified by experimental data. The influence of the injection timing and nozzle diameter on ultra-lean combustion was evaluated. The results suggest that, with the delay in the injection timing, the mixture concentration near the spark plug and combustion speed gradually increase. The maximum thermal efficiency increased from 47.44% to 49.87%. The combustion duration and ignition lag are shortened from 19.15°CA to 11.15°CA to 16.13°CA and 5.92°CA, respectively. As the nozzle diameter increased, the injection duration was shortened, and the mixture distribution area became more concentrated. Furthermore, under ultra-lean combustion, the combustion rate is more sensitive to the distribution of the mixture. Appropriately increasing the equivalence ratio near the spark plug can significantly shorten the ignition lag and combustion duration and obtain a higher thermal efficiency. • The laminar flame speed model of hydrogen is calibrated by experimental results. • The G-equation is used to simulation the hydrogen combustion. • The in-cylinder mixture distribution, hydrogen jet and flow field were investigated. • The maximum indicated thermal efficiency reaches 50.05%. [ABSTRACT FROM AUTHOR]

The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type "A" of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH 4 -air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap. [ABSTRACT FROM AUTHOR]

• Liquid EtOH spray with H 2 enrichment combustion were conducted. • Combustion with global equivalence ratio 0.15(67% EtOH/33% H 2) was achieved. • The effects of ignition timing, injection pressure on lean burn were evaluated. • High jet pressure improves ignition stability but causes incomplete combustion. An experimental study was conducted to determine the effects of ignition timings, injection pressures, and hydrogen enrichment on ethanol stratified ultra-lean combustion. The combustion chamber was a constant volume vessel with an injection and ignition system. The ignition and combustion characteristics of ethanol spray with different ignition timings (0–50 ms after start of injection), injection pressures (5, 15, and 25 MPa), and hydrogen premixed equivalence ratios (0.05, 0.1, and 0.2) under ambient temperature 353 K and 1 atm. The direct injection of ethanol and hydrogen enrichment results in an increase in the stable ignition limit to an extremely lean condition: a global equivalence ratio of 0.15 (67% ethanol/33% H 2). Furthermore, the probability of ignition first increased then decreased with the delay in ignition timings; rose with an increase in the injection pressure and hydrogen ratio. However, the delay in ignition timings and high injection pressures resulting in extremely lean zone in the constant volume vessel, which led to incomplete combustion. The addition of hydrogen not only stabilized ignition process but also improved the combustion characteristics. Stable ignition and combustion were achieved at early ignition timing (2–3 ms), moderate injection pressure (15 MPa), and a minimum global equivalence ratio of 0.2 (50% ethanol/ 50% H 2). [ABSTRACT FROM AUTHOR]

Summary Hydrogen is one of the most promising alternative fuels for the transportation industry. The use of hydrogen to enable lean burn in internal combustion engines is an attractive solution for reducing CO2 emissions from two points of views: the substitution of carbon‐based fuels and the increased thermal efficiency due to lean operation. Combining this strategy with the passive pre‐chamber ignition system with gasoline/hydrogen blends is even more interesting. The main limitations of the passive pre‐chamber concept in a high compression ratio spark‐ignition engine were shown through engine experiments. A numerical study was then performed to evaluate the chance of extending the dilution limit by using hydrogen along with this technology. Results show how the use of hydrogen provides considerable benefits in the main chamber combustion process by enhancing the thermo‐chemical properties of the mixture, increasing the flame speed, and improving the flame structure. Using an adequate gasoline‐hydrogen blend proved to enable optimum burning rates at lean conditions, leading to a relevant thermal efficiency gain. [ABSTRACT FROM AUTHOR]

This study investigated the formation of an active prechamber (APC) jet flame based on the synergy of airflow. A single-cylinder gasoline direct injection engine installed with an APC was operated under ultra-lean conditions with a lambda value of 2.5. Different APC structures with nozzle swirl angles ranging from 0° to 30° were designed to produce different flow fields and mixture formations. The results showed that APCs with larger nozzle swirl angles generated smaller wall films and faster fuel–air mixing, forming a more evenly mixed distribution in the APC's top region, which improved the uniformity of the flame jets from different APC nozzles with a smaller variance in jet turbulence kinetic energy and jet velocity. Tumble flow played a dominant role in APCs with a small nozzle swirl angle, and the relatively weak swirl flow field inside the APC resulted in more locally rich mixture regions, reducing the ignition capability of the APC jet flame. APCs with a large nozzle swirl angle can accelerate the evaporation and mixing process of the fuel near the top region of the APC with a stronger swirl, and the wall film diffuses to the central area of the duct through the airflow. • The formation of an active prechamber jet flame based on the synergy of airflow was investigated. • Different prechamber nozzle swirl angles were designed to produce different flow fields. • The fuel-air mixing and spray impingement inside pre-chamber were analyzed. • The synergistic interaction between swirl and tumble flows promotes jet flame ignition capability. • A larger nozzle swirl angle promotes jet flame propagation. [ABSTRACT FROM AUTHOR]

Prechamber-assisted internal combustion engines allow lean limit extension by replacing the spark plug with multiple, high-temperature, radical-rich jets that entrain and ignite the main chamber charge. This work reports the first investigation of active ultra-lean, active rich, and passive prechamber assisted prechamber engine operation (with a fixed prechamber geometry) through 50 kHz cycle-resolved measurements of formaldehyde planar laser-induced fluorescence (PLIF) and OH* imaging in a heavy-duty optical engine operating at 1200 rpm. The third harmonic of a burst mode laser (355 nm) provided the excitation source for formaldehyde PLIF, and a high-speed CMOS camera collected the fluorescence signal from the side window of the engine. Bottom-view imaging of OH* chemiluminescence was achieved using an intensified high-speed camera. The high-speed image sequence of the prechamber jet formaldehyde layer and OH* captures the main chamber heat release stages and the interaction between the moving piston and the prechamber jet. The prechamber-assisted combustion process in the main chamber could be segmented into three phases: initial jet, jet wall interaction, and Post Jet Combustion. The first two phases differ between active and passive operations owing to the global lambda, while the last phases mostly remain flame propagation driven. Jet wall interaction leading to mixing and vortical transport across the main chamber is pivotal in main chamber heat release rate shaping. The study reports the dynamics and evolution of main chamber combustion in detailed time-resolved setups. [ABSTRACT FROM AUTHOR]

Highlights • Ignition characteristics by impinging hot turbulent jet in a dual chamber system are proposed. • The ratio of impinging distance to the nozzle diameter, H / D and the impinging angle, θ are examined. • Using impinging jet ignition, a lower flammability limit of hydrogen/air is achieved. • Numerical simulation of jet impingement process is performed. Abstract The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H 2 /air was studied both experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H 2 /air mixture in a separate small volume called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushed the combustion products into the main chamber through a small nozzle (0.75–4.5 mm in diameter) in the form of a hot turbulent jet, which then impinged on a flat plate. Six different plates with varying impinging heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the ratio of the impinging distance to the nozzle diameter, H / D and the impinging angle, θ were examined. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration/impinging and ignition process inside the main combustion chamber. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the ambient unburned H 2 /air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has a minimum role on main chamber ignition. Employing the stagnation point ignition, a leaner limit of H 2 /air in the main chamber was achieved. Numerical modeling of the turbulent hot jet impingement process was carried out to explain the impinging jet ignition mechanism. It was found that H / D ratio was the controlling parameter between the two ignition mechanisms. The limiting H / D ratio was found to be 21.6, below which ignition occurred via jet impingement. Unlike the H / D ratio, the impinging angle did not affect the ignition mechanism; however, it affected the main chamber burn time. [ABSTRACT FROM AUTHOR]

Abstract The flame dynamics and stabilization mechanism in a novel, ultra-lean, non-premixed, model GT burner is experimentally investigated with optical and laser diagnostics. The burner operating with methane as fuel exhibits a convergent-divergent flow field and is capable of stabilizing a non-sooty flame at ultra-low global equivalence ratio (ϕ glob) down to 0.1. At 1.0 > ϕ glob > 0.6, the flame stabilizes in the diverging section where the flow field is characterized predominantly by the swirl. The flame stabilizes at locations of low velocity and low turbulence where the mean flow strain rates are found to be well below the extinctions strain rates for turbulent non-premixed flames. With decreasing ϕ glob , the flame is located progressively near the burner lip, where a large recirculation zone of the central bluff body in the burner exists. The transition from swirl stabilized to bluffbody stabilized region occurs between 0.6 > ϕ glob > 0.4 and bi-stability of the flame with low-frequency oscillation between the two stabilization locations was observed. The upstream marching of the flame, despite a decreasing ϕ glob , and a corresponding decrease in heat input, is explained in terms of the interaction of the flame front with the evolution of the scalar profile upstream, in presence of the recirculating hot gases. The results support flame stabilization theories based on partial premixing and flow modification through heat release upstream of the flame. For the leaner conditions, 0.4 > ϕ glob > 0.1, the flame is stabilizing in a region characterized by bluffbody induced toroidal flow structures of high strain rates, where the presence of recirculating hot burned gases aid in flame stabilization by the ignition of flammable mixture. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the mechanism of the asymmetric jets of the TJI system. We analyzed the effects of fuel injection and air flows in pre-chamber and main chamber, initial flame kernel, and evolution of turbulent hot jets. The evaluation indexes of asymmetric jet were proposed. The results show when adding fuel injection into the homogeneous PC with off-central spark plug, the average relative velocity range and average degree of velocity non-uniformity increased by 50.8% and 32.8%, which increased by 75.1% and 46.7% for the real engine condition with intake airflow and fuel injection. A similar phenomenon was observed for central spark plug settings but with a much stronger impact on the asymmetry of jets velocity as proved by the increment of 138.5% and 93.2% under real engine condition. The asymmetric jets can be divided into the asymmetry of jets size and the asymmetry moment of flame jets. The former of different jets' velocity is attributed to the tumble flow in main chamber. The latter is attributed to the irregular flame propagation caused by the off-central ignition and the uneven mixture distribution in pre-chamber. The optimization strategies to efficiency utilize the flow exchange between PC and MC were proposed. • The formation of asymmetrical jets from turbulent-jet-ignition system was investigated. • Different PC structures with spark plug centrally and off-centrally installed were designed. • Ideal homogeneous conditions via semi-ideal environment to real engine operations were discussed. • The asymmetric jets phenomenon was a spatial-temporal related to jets size and flame jets moment. • Specific strategies and PC design were carried out to improve the PC jet performance. [ABSTRACT FROM AUTHOR]

A new type of hardware configuration is proposed with direct hydrogen injection combined with gasoline port injection. Different fractions of hydrogen blending were tested under steady-state operating conditions. Independent parameter sweeps were conducted for excess air ratio ( λ ), ignition timing, and hydrogen fraction. The high laminar burning velocity of the hydrogen caused particle emissions to be insensitive to the ignition timing. With air–fuel mixing at λ = 1.1, the number of nucleation particles decreased and the number of accumulation particles increased with increasing hydrogen fraction, indicating that the temperature affected coagulation more greatly than pyrolysis. With mixing under ultra-lean conditions ( λ = 1.2, 1.3, and 1.4), variation of the hydrogen fraction slightly affected the particle number, showing that the critical value for producing particles is the C/O ratio, not the whole mixture equivalence ratio. The decrease of total particle number with increasing λ indicates that this new hardware configuration can take the advantages of lean combustion and avoid the problem of high particle number which is generally associated with this kind of spark-ignition direct-injection engine. [ABSTRACT FROM AUTHOR]

The effect of the fuel injection ratio on the combustion and emission characteristics of stratified lean mixture combustion was investigated for a spray-guided liquefied petroleum gas (LPG) direct injection engine. Inter-injection spark-ignition combustion-a specially designed combustion strategy for LPG fuel derived from a two-staged injection-was employed to maximize the improvement in thermal efficiency when combustion stability is secured. When changing the fuel injection ratio, the optimum spark advance and fuel injection timings were experimentally determined to maximize the thermal efficiency based on sweeping timings. The optimum fuel injection ratio with the highest thermal efficiency (42.76%) and stable operation was 60%/40%, with the optimization of the spark advance and fuel injection timing, because of the locally rich mixture region in the recirculation zone. NOx emissions were at their highest level with a fuel injection ratio of 60%/40% because of the high combustion temperature, and the levels of total hydrocarbon and CO emissions with 50%/50% and 60%/40% fuel injection ratios were similar, whereas emissions at 70%/30% were significantly higher because of fuel wetting and the formation of over-lean mixture. [ABSTRACT FROM AUTHOR]

Super lean (usually Φ < 0.5) and ultra-lean (usually Φ < 0.3) combustion are regarded as promising combustion techniques owing to high fuel economy and low emission. Ethane is one of the key components of natural gas. In the present study, the ignition delay times (IDTs) of ethane/oxygen/argon mixture were measured behind reflected shock waves at three equivalence ratios of 0.5, 0.1, 0.05, two pressures of 1.68, 9.0 atm, and in the temperature range of 1120 – 1351 K. The OXYMECH2.0, proposed in our previous works, is modified in the present study by updating several elementary reactions. The modified OXYMECH is validated by ethane IDTs of super lean combustion measured in the present study, in high CO 2 atmospheres and O 2 /Ar atmospheres from the literature as well as laminar flame speeds (LFSs) under O 2 /N 2 and O 2 /CO 2 atmospheres. Aramco3.0, San Diego, NUIGMech0.9, and CRECK-GB models are also evaluated by ethane IDTs measured in the present study, the results show that modified OXYMECH, NUIGMech0.9, and CRECK-GB models provide equally accurate predictions for experiments. The NUIG Mech0.9 and CRECK-GB are further evaluated using the ethane IDTs in high CO 2 atmospheres and experimental LFSs of ethane under O 2 /N 2 and O 2 /CO 2 atmospheres, the results show that modified OXYMECH has good performance in predicting IDTs and LFSs of ethane. The detailed comparisons of IDTs and LFSs prediction between modified OXYMECH and CRECK-GB models are conducted. The effects of superabundant air, which is the air excess with equivalence ratio decreases from Φ 1 to Φ 2 , on the ignition in super lean combustion consist of the dilution and oxidation effects, which are studied in the present study in detail. It is very interesting that the dilution effects promote the ignition while the oxidation effects inhibit the ignition at Φ < 0.3 and P = 9.0 atm. [ABSTRACT FROM AUTHOR]

The demand for gas turbines that accept a variety of fuels has continuously increased over the last decade. Understanding the effects of varying fuel compositions on combustion characteristics and emissions is critical to designing fuel-flexible combustors. In this study, the combustion characteristics and emissions of methane and hydrogen-enriched methane were both experimentally and numerically investigated under ultra-lean conditions (Ø ≤ 0.5). This study was performed using global mechanisms with a one-step mechanism by Westbrook and Dryer and a two-step mechanism with an irreversible and reversible CO/CO2 step (2sCM1 and 2sCM2). Results show that the 2sCM2 mechanism under-predicted the temperature, major species, and NO x by more than 100% under ultra-lean conditions; thus, we proposed a modified-2sCM2 mechanism to better simulate the combustion characteristics. The mechanisms of Westbrook, 2sCM1, and modified 2sCM2 predicted the temperature and the CO2 emission with an average deviation of about 5% from the experimental values. Westbrook and 2sCM1, however, over-predicted the NO x emission by approximately 81% and 152%, respectively, as compared with an average under-prediction of 11% by the modified-2sCM2 mechanism. The numerical results using the proposed modified-2sCM2 mechanism shows that the presence of hydrogen in the fuel mixture inhibits the oxidation of methane that led to the formation of unburned hydrocarbons in the flame. We also showed that for any given fuel compositions of H2/CH4, there is an optimum equivalence ratio at which the pollutant emissions (CO and NO x) from the combustor are minimal. Zero CO and 5 ppm NO x emissions were observed at the optimal equivalence ratio of 0.45 for a fuel mixture containing 30% H2. The present study provides a basis for ultra-lean combustion toward achieving zero emissions from a fuel-flexible combustor. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13–0.21) on flame configuration, combustion stability, combustion efficiency, and NO x emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NO x generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NO x emission of below 10 ppm (@ 15 vol% O 2) due to low oxygen concentration and flame temperature. Furthermore, part of NO x entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H 2) to further lowering NO x emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields. • Low NO x emission with high stability can be achieved via a simple structure. • Steadily combustion results from the optimization structure of this combustor. • Low combustion temperature is the principal reason for the low NO x emission. • Generated NO x can be reduced by the stratified species distribution. • The flow field can promote the transport of the NO x to the reduction region. [ABSTRACT FROM AUTHOR]

Abstract: It has been reasoned that the structures of strongly cellular flames in very lean mixtures approach an array of flame balls, each burning as if it were isolated, thereby indicating a connection between the critical conditions required for existence of steady flame balls and those necessary for occurrence of self-sustained premixed combustion. This is the starting assumption of the present study, in which structures of near-limit steady sphericosymmetrical flame balls are investigated with the objective of providing analytic expressions for critical combustion conditions in ultra-lean hydrogen-oxygen mixtures diluted with N2 and water vapor. If attention were restricted to planar premixed flames, then the lean-limit mole fraction of H2 would be found to be roughly ten percent, more than twice the observed flammability limits, thereby emphasizing the relevance of the flame-ball phenomena. Numerical integrations using detailed models for chemistry and radiation show that a one-step chemical-kinetic reduced mechanism based on steady-state assumptions for all chemical intermediates, together with a simple, optically thin approximation for water-vapor radiation, can be used to compute near-limit fuel-lean flame balls with excellent accuracy. The previously developed one-step reaction rate includes a crossover temperature that determines in the first approximation a chemical-kinetic lean limit below which combustion cannot occur, with critical conditions achieved when the diffusion-controlled radiation-free peak temperature, computed with account taken of hydrogen Soret diffusion, is equal to the crossover temperature. First-order corrections are found by activation-energy asymptotics in a solution that involves a near-field radiation-free zone surrounding a spherical flame sheet, together with a far-field radiation-conduction balance for the temperature profile. Different scalings are found depending on whether or not the surrounding atmosphere contains water vapor, leading to different analytic expressions for the critical conditions for flame-ball existence, which give results in very good agreement with those obtained by detailed numerical computations. The one-step chemistry employed in the present work, which involves a non-Arrhenius rate having a cutoff at the crossover temperature, applies with excellent accuracy to the description of lean premixed hydrogen-air combustion, i.e, for at atmospheric pressure, and could be used for instance in the numerical simulation of the propagation of curved or cellular flames in ultra-lean reactive atmospheres, of interest for safety analyses related to the storage, transport, and handling of hydrogen. [Copyright &y& Elsevier]

Abstract: Rotating counterflow twin premixed flames of methane–air were numerically simulated with detailed chemistry based on a similarity solution to explore the leanest extinction limit without preheating and to elucidate the mechanism of “ultra-lean” combustion. We focused on high rotation rate cases in which unrealistic backflow from infinity is allowed to occur since ultra-lean combustion was found to be realized in such a situation. It was found that the reaction zone is in the backflow zone, where the flame’s apparent burning velocity is negative, and that the flame zone width is much smaller than that of a 1-D planar premixed flame due to an inversion of the convexity directions of the profiles of temperature and main species concentrations. The decrease of the flame width seems to make the flame less extinguishable. The equivalence ratio of the leanest flame obtained neglecting radiative heat loss is 0.32, while that obtained with an optically thin radiation model is 0.42, which is still much leaner than the ratio of 0.49 for a 1-D planar premixed flame generated using the same radiation model. [Copyright &y& Elsevier]

Abstract: The stability of ultra-lean mixtures in a porous media burner is numerically investigated. The one-dimensional model considers separated energy equations for the solid and gas phases and accounts for radiative heat transfer in the solid porous foam. A simplified reaction kinetics (two-step mechanism) was employed in order to perform a linear stability analysis. This results are used to investigate the stability of the steady solutions as a function of burning velocity and flame location. The present analysis includes the effects of volumetric ratio, inlet mixture temperature and equivalence ratio on the flame stability. From the linear stability results it can be concluded that the insertion of inert porous media for premixed combustion promotes the ultra-lean burning of mixtures. [Copyright &y& Elsevier]

In order to overcome the instability of ultra-lean combustion, a novel vortex-tube combustor with axial fuel injection was proposed. The stability limit, flame configuration, NO x emissions, and burn-off rate of the combustor were investigated experimentally under various global equivalence ratios (ϕ g) and fuel flow rates (q f). Results show that the combustor exhibits a large stability limit with ϕ g and q f as low as 0.01 and 6 × 10−5 m3/s respectively. Complete combustion and ultra-low NO x emissions of less than 3 ppm can be achieved at ϕ g of 0.3 and q f of more than 30 × 10−5 m3/s, indicating that the combustor has a good potential for ultra-lean combustion and low NO x emission. The pressure fluctuation amplitude is always less than 1300 Pa during the entire experiments. The vortex-induced flame has a diffusion-like flame structure, which provides a suitable equivalence ratio zone under ultra-lean conditions, whilst the high peak combustion temperature indicates an intensified combustion, which is responsible for the large stability limit and low-pressure fluctuation amplitude. Subsequently, the enhanced stabilization can enable the ultra-lean combustion and the ensued low temperature to conduct, whilst the vortex-flow can decrease the local flow velocity and enable the turbulent diffusion velocity of NO to be dominant, which can make the NO emission reduced further. The decreased area of high-temperature region and the appearance of the negative reaction rate region of NO is the essential reason for the decrease of the NO emission at lean cases. • One novel vortex-tube combustor is researched with diffusion peculiarity. • This combustor can achieve a large stability limit and low NO emission. • The diffusion flame structure provides a NO reduction zone and promote stability. • The decreased high temperature area can decrease the thermal NO. • The rich-fuel region and the flow field structure can promote the NO reduction. [ABSTRACT FROM AUTHOR]

The complexity of modern hybrid powertrains poses new challenges for the optimal control concerning, on one hand, the thermal engine to maximize its efficiency, and, on the other hand, the vehicle to minimize the noxious emissions and CO2. In this context, the engine calibration has to be conducted by considering simultaneously the powertrain management, the vehicle characteristics, and the driving mission. In this work, a calibration methodology for a two-stage boosted ultra-lean pre-chamber spark ignition (SI) engine is proposed, aiming at minimizing its CO2 and pollutant emissions. The engine features a flexible variable valve timing (VVT) control of the valves and an E-compressor, coupled in series to a turbocharger, to guarantee an adequate boost level needed for ultra-lean operation. The engine is simulated in a refined 1D model. A simplified methodology, based on a network of proportional integral derivative (PID) controllers, is presented for the calibration over the whole operating domain. Two calibration variants are proposed and compared, characterized by different fuel and electric consumptions: the first one aims to exclusively maximize the brake thermal efficiency, and the second one additionally considers the electric energy absorbed by the E-compressor and drained from the battery. After a verification against the outcomes of an automatic optimizer, the calibration strategies are assessed based on pollutant and CO2 emissions along representative driving cycles by vehicle simulations. The results highlight slightly lower CO2 emissions with the calibration approach that minimizes the E-compressor consumption, thus revealing the importance of considering the engine calibration phase, the powertrain management, the vehicle characteristics, and its mission. [ABSTRACT FROM AUTHOR]