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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]

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]

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]

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]

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]