Early Flame Research Articles

An Improved Lightweight YOLOv8 Network for Early Small Flame Target Detection

An Improved Lightweight YOLOv8 Network for Early Small Flame Target Detection

Microwave-induced plasma on spherical flame dynamics of spark-ignited hydrogen-air under water dilution

Microwave-assisted spark ignition (MAI) offers a commercially feasible way to enhance conventional spark ignition performance under extreme conditions. While previous studies suggested microwave pulses had no obvious effect on self-sustained flame from the perspective of energy deposition, the hydrodynamic effect of microwave plasma remained under-explored. This study experimentally investigated the effect of the microwave-induced plasma on lean hydrogen flame dynamics (Lewis number ∼ 0.35), examining the influence of water content (rH2O) in this process under different pulse repetition frequency (PRF) and pressures. Using a linear high-speed shadow imaging system coupled with electrical diagnostics, we synchronously recorded the flame and plasma morphology evolution while monitoring spark and microwave energy. Results revealed that microwave pulses during spark ignition generated wrinkles and perturbances accelerating the cracking and cellularization of the subsequent self-sustained flame. This effect intensified with increasing PRF from 1 to 10 kHz, particularly at high rH2O. The overall microwave impact on early flame development was more pronounced at high rH2O, attributed to stronger interactions between the microwave plasma jet and flame front due to reduced distance. Interestingly, electrical diagnostics showed an inversed relationship between total absorbed microwave energy and rH2O, highlighting the crucial role of the first microwave pulse at 1 kHz PRF. This study demonstrates that early microwave pulses can influence hydrogen flame dynamics even in the self-sustained stage, offering new insights into MAI mechanisms. These findings open avenues for research into active control of self-sustained flame dynamics, potentially leading to improved ignition strategies in challenging combustion environments.

Experimental study on characteristics of methane-coal dust explosions and the spatiotemporal evolution of flow field in early flame

To understand the explosion characteristics of methane-lignite coal dust mixtures and the spatiotemporal evolution of the early flame flow field, explosion pressure, OH* emission spectra, and early flame schlieren images were captured in a 60L constant volume combustion bomb. Schlieren image velocimetry was used to obtain the instantaneous velocity distribution of the methane-coal dust flame front. The results indicate that adding coal dust causes a nonlinear increase in explosion pressure, particularly at low methane concentrations. At a 5 % methane concentration, adding coal dust can increase the maximum explosion pressure by up to 39 % (190 kPa). The trend of the OH* emission spectra corresponds with the maximum explosion pressure. A slight increase in coal dust significantly enhances the OH* emission spectra, especially at lower concentrations, potentially increasing it by nearly five times. However, at higher coal dust concentrations, the OH* emission spectra are significantly reduced. Additionally, within the considered concentration range (≤58.14 g/m³), larger turbulent integral length scales in initial conditions consistently result in the fastest flame acceleration. These findings provide experimental evidence for further exploration of the explosion mechanisms of methane-coal dust mixtures and the development of chemical reaction kinetic models.

Effects of using nanosecond repetitively pulsed discharge and turbulent jet ignition on internal combustion engine performance

Robust ignition of hard-to-ignite fuels is essential for future spark ignited internal combustion engines, particularly for introducing efficiency-enhancing diesel-like process parameters like air excess or high amounts of exhaust gas recirculation (EGR). On the one hand, novel plasma-based ignition systems like Nanosecond Repetitively Pulsed Discharge (NRPD) are promising in extending the ignition limits and the early flame development speed. On the other hand, Turbulent Jet Ignition is effective for shortening the combustion duration and decreasing the unburned hydrocarbon emissions.This article investigates experimentally the combination of NRPD ignition and TJI. The aim is to use a technology for robust inflammation (NRPD) in combination with a technology for fast combustion of the bulk charge (TJI). For this purpose, a turbocharged light-duty four-cylinder engine operated with natural gas is used. The engine can be fitted with a classical Open Chamber (OC) spark plug or with Pre-Chambers (PC). The PCs can be filled uniquely with fuel and air coming from the Main Chamber (MC) (“passive PC”), or additional fuel can be added to the PCs (“active PC”). The air-to-fuel ratio and EGR rate can be freely controlled. Five different combustion strategies are investigated with NRPD ignition and compared against an inductive discharge ignition system. The combustion strategies are passive PC with air and EGR dilution, active PC with air dilution, and Open Chamber (OC) with air and EGR dilution. HRR evaluations and a loss analyses are performed to interpret the results.Despite the faster inflammation present with NRPD ignition, similar peak efficiencies and emissions are reached in OC configuration using the inductive discharge and NRPD ignition systems, which are achieved by varying air-to-fuel ratios (AFR) and EGR rates. Above dilution levels for peak efficiency, the efficiency using NRPD ignition decreases at a slower pace and tolerates higher AFR and EGR rates, thanks to a more complete and shorter combustion.For the PC experiments using NRPD ignition, an efficiency increases and a reduction of emissions compared to inductive discharge ignition are present for the investigated AFR and EGR rates for both active and passive PC operations. The efficiency increase is present due to a stronger pre-chamber discharge and thanks to a faster end phase of combustion. Actively fueling the PC results in faster and more complete combustion that is overcompensated by the increased wall heat losses, reducing the overall efficiency. The results show that passive PC with NRPD ignition may be an ideal ignition concept that maximizes engine efficiency and minimizes emissions.

Comparative Analysis Of Cycle-To-Cycle Variabilities And Combustion Development In An Optical Spark-Ignition Engine Fueled By Pure Hydrogen And Propane: Insights From Chemiluminescence and PI

Hydrogen, derived from renewable sources and devoid of carbon emissions, is a pivotal energy carrier for the future. Representing a viable substitute for fossil fuels in internal combustion engines (ICEs), contemporary studies advocate using very-lean and ultra-lean hydrogen-air mixtures, with a fuel-air equivalence ratio below 0.5, as a potent strategy to reduce NOx emissions in hydrogen-fueled ICEs. An experimental setup with an optical spark-ignition engine was devised to investigate the primary factors influencing cycle-to-cycle variations in H2ICEs by optimizing mixture homogeneity and focusing the study on flame interaction with in-cylinder aerodynamics. Simultaneous in-cylinder pressure, chemiluminescence, and PIV analyses were performed to characterize and compare the behaviors of ultra-lean hydrogen-air and propane-air flames, assessing flame development and cyclic variation features. Results indicate that the flame development of hydrogen and propane is significantly different. Propane exhibited increased in-cylinder pressure trace variability at lean and rich flammability limits with a high flame development difference between fast and slow cycles. In contrast, hydrogen-air mixtures under various equivalence ratios (from very-lean to ultra-lean) presented much more stable in-cylinder pressures. Furthermore, fast propane flames were mainly advected to the cylinder's symmetrical axes, while fast hydrogen flames started further away from the spark plug. Horizontal and vertical PIV measurements showed a single structure flow field over the studied conditions with mainly intensity variations over the measured cycles. Fast flames were associated with more intense tumble motion. The differences between fast and slow flames for hydrogen are attributed to the early flame development. However, after initial differences in flame development over several crank angles, there are similarities in all cycles, suggesting that the low variability in in-cylinder pressures is mostly due to the robustness of hydrogen flame ignition and its independence from in-cylinder flow small variations at the early development phase.

Hydrogen explosion characteristics in the obstructed chamber by changing blockage ratio among adjacent obstacles

Using the experimental and large eddy simulation (LES) methods, the impacts of equivalence ratio and adjacent obstacle blockage ratio on flame propagation, flame moving velocity, and explosion overpressure in a closed obstructed duct are investigated. The results indicate that the developed LES method is able to reproduce the tendency of flame propagation, flame moving velocity, and explosion overpressure of BR = 30%, 50%, 70% and BR = 70%, 50%, 30%. The early flame propagation and flame moving velocity are not affected by the first obstacle. As the explosion flame passes through the three obstacles, the flame acceleration and deceleration occur in sequence due to the decreasing cross-section and vortex-flame interaction, eventually forming three ascending peak flame moving velocity. The higher vorticity magnitude of BR = 70%, 50%, 30% and BR = 30%, 50%, 70% is mainly concentrated in the early stage and later stage of hydrogen explosion, which directly results in the fact that the flame acceleration and rise rate of explosion overpressure of BR = 70%, 50%, 30% is higher than that of BR = 30%, 50%, 70%. Additionally, the maximum value of flame moving velocity and explosion overpressure of BR = 30%, 50%, 70% is higher than that of BR = 70%, 50%, 30%.

Numerical modeling of combustion in gas engines with prechamber ignition

A numerical model is developed to predict the combustion processes in large-bore gas engines featuring prechamber ignition systems and being coupled with Computational Fluid Dynamics (CFD). The proposed combustion model is based on the simultaneous modeling of premixed and partially-premixed flames by an extended progress variable approach. By introducing an additional fuel scalar to track the externally injected fuel of the scavenged prechamber and depending on a gradient criteria of the fuel scalar, regions of locally premixed or partially-premixed state can be differentiated. The reaction rate for premixed combustion is described through a turbulent flame speed closure approach, whereas the partially-premixed combustion is described by a pre-tabulated flamelet chemistry approach. For the validation of the combustion model and its performance in the different flame propagation phases, that is, prechamber flame ignition and main-chamber flame propagation, experimental data of two large-bore gas engines with different operating conditions, prechamber configurations and engine geometries are taken into account. A good agreement of the simulations with the experimental results is shown for the variety of operating conditions and engine configurations. The developed combustion model is able to predict the combustion process in the prechamber as well as the ignition of the main chamber charge by means of the protruding flame jets through the prechamber nozzles. The prechamber ignition system accelerates the early flame phase and hence shortens the burning duration due to the deep-penetrating and turbulence-inducing flame jets in comparison to a conventional spark plug engine.

Measurement of laminar burning speed at high pressures using plasma affected flame

Measurements of a propagating flame are crucial to the understanding and verification of important flame phenomena. Specifically, laminar, unstretched flame speed is desired to verify complex flame behavior such as turbulence or chemical kinetic models. This data is also necessary as advanced and more efficient combustion devices operate at these high pressures where this data is less reliable from the onset of flame instabilities. This research provides laminar burning speed (Suo) and Markstein length (Lb) measurements utilizing a novel technique which incorporates flame propagation during and just after the ignition phase. The analysis utilizes a constant pressure technique where the expansion of a spherical flame is visually measured. Flame propagation of stoichiometric flame from 1 to 50 atm are examined up to a radius of 20 mm. The inclusion of early flame propagation improves the measurement of Suo through the addition of a larger data set than usually possible in the conventional method. The measurement method utilizes electrical data of the spark formation with a thermodynamic model to predict the kernel velocity and temperature which result from plasma formation. In addition, morphology of the early kernel at high pressure is reported as well as comparisons to the conventional Suo measurement.

The effect of operating frequency and ignition energy on JP-10/PO/aluminum powder multiphase-pulse-detonation-engine

Pulse-Detonation-Engine is a new engine that uses pulse detonation wave to generate thrust. Previous studies have shown that JP-10/PO/Al dust multi-phase composite fuel has good ignition sensitivity and detonation strength, so the present work studies the application prospect of this optimized composite fuel in the detonation propulsion of Pulse-Detonation-Engine. A numerical model of Multiphase-Pulse-Detonation-Engine was established, and a high-precision Computational-Fluid-Dynamics simulation method was applied to study a series of cyclic processes such as multiphase fuel dispersion, ignition, detonation wave formation, and blowing during Multiphase-Pulse-Detonation-Engine operation. The effects of operating frequency (20–60 Hz) and ignition energy (25–400 J) on the flame acceleration law, thrust, and impulse characteristics in Multiphase-Pulse-Detonation-Engine were quantitatively discussed. The research results indicate that the propulsion effect produced by single-cycle Multiphase-Pulse-Detonation-Engine deteriorates as the operating frequency increases. On the other hand, as the number of operating cycles per unit time increases, the fuel specific impulse gradually increases. But the increase rate gradually decreases, and the operating frequency should not exceed 50 Hz under current inspection conditions. The ignition energy has a promoting effect on the early flame acceleration of multiphase explosions. Due to the cumulative effect of multiple cycles, the impact of ignition energy on the propulsion performance of continuous working Multiphase-Pulse-Detonation-Engine is more significant, with ignition energy increasing from 25 J to 400 J, and specific impulse increasing by approximately 18.3 %.

Early flame development characterization of ultra-lean hydrogen–air flames in an optical spark-ignition engine

Hydrogen-fueled internal combustion engines (H2ICEs) represent a promising technology for decarbonizing the transport sector. In pursuit of this goal, research is needed on lean hydrogen–air combustion under engine conditions aiming to mitigate NOx emissions. However, there is a limited amount of experimental data focusing on flame development at high pressure and temperature, especially in ultra-lean mixtures, which are recently considered to play an important role for future H2ICEs. This study investigates the behavior of very-lean to ultra-lean hydrogen–air flames (0.20 <ϕ< 0.55) in an optical spark-ignition engine, focusing on the early flame development phase. Fuel–air equivalence ratio and engine speed (900 <N< 1500 rpm) effects on hydrogen combustion are addressed with pressure-based heat release analysis and chemiluminescence flame imaging techniques. The findings revealed an intrinsic association between these two approaches. Hydrogen flames development measured before 2% of Mass Fraction Burned (MFB) strongly correlates with pressure-based quantities up to 50% MFB, underscoring the critical role of understanding the early flame phase for predicting total combustion duration and in-cylinder pressure traces. A model for hydrogen turbulent flames in the initial combustion stages is formulated based on the Zimont equation and the experimental data. An equation for 0D/1D combustion duration prediction up to 10% MFB is proposed, demonstrating its validity from the wrinkled flame to the well-stirred reactor premixed combustion regimes. Additionally, flame chemiluminescence images for each zone within the Peters–Borghi diagram are presented, offering valuable insights into the diverse characteristics of these flames.

Study on the effects of elastic modulus of constructions on heat and mass transfer of gas explosion

The factor of combustion and explosion remains one of the main constraints on coal mining and management. To clarify the impact of structural properties on the consequences of gas explosion disasters during coal mining, this article conducts a study on the impact of the elastic modulus of structures on gas explosion disasters. The research results indicate that in the case where structures with high elastic modulus must exist, the elastic modulus of the structure has minimal impact on the structure during the early stage of flame development. The area of flame front and the degree of deflagration also decrease with the increase of elastic modulus, but the disturbance degree of air-flow and flame in the pipe increases with the increase of elastic modulus. The peak flame velocity at elastic modulus of 0.7 GPa and 2.8 GPa increased by 3.56% and 7.47% compared to elastic modulus of 0.18 GPa, respectively. The upstream overpressure peak increased by 24.63% and 42.52%. The downstream overpressure peak increased by 11.19% and 20.62%. The peak values of flame velocity and overpressure increase with the increase of elastic modulus, while the explosion intensity and pressure rise rate increase with the increase of elastic modulus. The explosion intensity index at elastic modulus 2.8 GPa is approximately 1.45 times that at elastic modulus 0.18 GPa. Therefore, it is necessary to choose structures with smaller elastic modulus as much as possible to achieve the best fire and explosion suppression effect.

Calosidade do solo de mudas de uva enxertadas como uma característica positiva para as mudanças climáticas

Abstract Nowadays, some relative warming temperatures related to climate change may be provided at the grafting time. Therefore, this study was conducted during two seasons (2018-2019) to study the effect of three callusing method (callusing room, callusing soil, callusing tunnel) and four grafting date (15 Jan., 1Feb., 15 Feb., 1 Mar.) for early (Flame seedless), medium (Thompson seedless) and late (Crimson seedless) grape varieties on grafted grape cuttings as short methods for transplant production. The results indicated that, the early grapes variety achieved higher grafting success on 1st Feb. grafting date as well as the late grape variety in callusing room and callusing soil methods. Also, Callusing soil achieved grafted success by 72.9%, 68.55% and 77.94% compared to callusing tunnel 37.3%, 45.9% and 55% for Flame seedless, Thompson seedless and Crimson seedless, respectively as mean of both seasons. High grafting success resulted from the high content of indole and sugars, along with low phenol content before callusing stage, as well as high indole and low sugars of grafts partner after callusing stage. while, higher phenols was accumulated in rootstock after callusing stage. There is no antagonistic effect between grafts partners. Callusing soil may be considered as an eco-friendly, sustainable and cheaper alternative tool for callusing of grafts cuttings.

A CFD ignition model to predict average-cycle combustion in SI engines with extreme EGR levels

Control of the combustion process in Spark-Ignition (SI) engines operated with extreme dilution from exhaust gas re-circulation (EGR) represents one of the major limitations in the industrial design of such technology. Numerical approaches able to describe in detail the formation of the early flame kernel become essential to face such an ambitious task. This work presents a RANS-based multi-dimensional model of the combustion process, including an advanced description of the ignition stage to consider its stochastic re-ignitions within the average cycle prediction. The spark-channel is described as a column of Lagrangian parcels that represent early flame kernels, whose growth is controlled by the laminar flame speed and energy input from the electrical circuit. The spatial evolution of each parcel is computed according to a scaled value of the average-flow speed, to mimic the smooth but short elongation of the mean-cycle channel produced by stochastic restrikes affecting the single-cycle arcs. To clarify this phenomenon and assess the proposed CFD method, a series of experiments are performed in a single cylinder SI engine with optical access, running at a low-load cruise-speed operating condition. Increasing EGR levels are tested up to the onset of misfire, with measurements of the secondary-circuit features and of the flame evolution through high-speed imaging. Satisfactory results are achieved in terms of numerical-experimental comparison of the cycle-averaged in-cylinder pressure, discharge parameters, and spatial flame distribution, demonstrating the reliability of the proposed numerical approach.

Comparative study on combustion and flame characteristics of NH3 dual-fuel engine under CI vs SI combustion modes

Ammonia can be used for engines to reduce carbon emissions, while its poor combustion performance limits commercial applications. Thus, combustion promotors are needed to improve the combustion properties of NH3. In this study, the combustion and flame characteristics of NH3-heptane dual-fuel engines under CI vs SI combustion modes are comparably studied. The initial flame formation and its development are addressed by high-speed direct photography, whereas simultaneous pressure acquisition is applied to quantify the engine performance. The results show N-heptane can significantly improve the combustion of NH3 under both SI and CI modes, while the effective energy fraction of N-heptane is beyond 30%. Besides, a critical direct injection timing exists for the effect of N-heptane on NH3 dual-fuel CI combustion, before/after which the combustion phase will delay. Flame images show that the color of ammonia-based fuel flame tends to feature an orange-yellow color. When comparing the SI and CI mode, the thermal efficiency of CI combustion is much higher because of the multipoint auto-ignition combustion in CI mode. Statistical analysis of flame shows that the early flame duration of CI mode is short and the corresponding cycle-to-cycle variation is low. More specifically, the flame of CI is nearly symmetrical distribution and the high probability is at the center, while that of SI is at the side.

Comparative analysis of thermal and non-thermal discharge modes on ultra-lean mixtures in an optically accessible engine equipped with a corona ignition system

Radio-Frequency corona-based ignition systems are currently being investigated by the engine research community for their capability to ensure robust combustion at challenging operating conditions such as lean mixture and/or high exhaust gas recirculation (EGR) dilution. The low-temperature plasma (LTP) produced by the corona discharge accelerates the early flame development thanks to the production of combustion precursors like radical and excited species. Plasma benefits can be maintained with extensive management of the control parameters, preventing the transition from the non-thermal discharge mode to a thermal one.However, under ultra-lean conditions, where the ignition probability significantly decreases, the LTP discharge may not be suitable for promoting the combustion process because of insufficient thermal energy transfer and scarcity of available fuel, which is essential for radical production. Therefore, it may be necessary to increase the volume in which the plasma interacts to ensure robust ignitions and stable flame propagations. For this reason, the present work aims to assess and quantify the differences, in terms of combustion performance, between a corona igniter operating at LTP mode and with electric arcs occurring, at ultra-lean operating conditions. In arc-mode, the igniter produces volumetric discharges, facilitating extensive mixing of the mixture and resulting in advantageous effects on kernel formation and propagation. Tests were carried out in a single-cylinder optical access engine under low load and low speed, using gasoline as fuel. Close to the leanest stable limit found, the LTP mode shows a lower cycle-to-cycle variability. Above such a threshold, the thermal discharge can guarantee stable combustion events while the LTP discharge leads to unstable working conditions. Indicating, imaging, and pollutant emission analysis are carried out to evaluate the effects of thermal and non-thermal plasma modes on combustion characteristics and their environmental impact.

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Effects of turbulence on variations in early development of hydrogen and iso-octane flame kernels under engine conditions

The understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.

Early smoke and flame detection based on transformer

Fire-detection technology plays a critical role in ensuring public safety and facilitating the development of smart cities. Early fire detection is imperative to mitigate potential hazards and minimize associated losses. However, existing vision-based fire-detection methods exhibit limited generalizability and fail to adequately consider the effect of fire object size on detection accuracy. To address this issue, in this study a decoder-free fully transformer-based (DFFT) detector is used to achieve early smoke and flame detection, improving the detection performance for fires of different sizes. This method effectively captures multi-level and multi-scale fire features with rich semantic information while using two powerful encoders to maintain the accuracy of the single-feature map prediction. First, data augmentation is performed to enhance the generalizability of the model. Second, the detection-oriented transformer (DOT) backbone network is treated as a single-layer fire-feature extractor to obtain fire-related features on four scales, which are then fed into an encoder-only single-layer dense prediction module. Finally, the prediction module aggregates the multi-scale fire features into a single feature map using a scale-aggregated encoder (SAE). The prediction module then aligns the classification and regression features using a task-aligned encoder (TAE) to ensure the semantic interaction of the classification and regression predictions. Experimental results on one private dataset and one public dataset demonstrate that the adopted DFFT possesses high detection accuracy and a strong generalizability for fires of different sizes, particularly early small fires. The DFFT achieved mean average precision (mAP) values of 87.40% and 81.12% for the two datasets, outperforming other baseline models. It exhibits a better detection performance on flame objects than on smoke objects because of the prominence of flame features.

Optical Study on Spark Plug Gap in Extending Methane Lean Combustion Limits under High Ignition Energy Conditions

&lt;div&gt;Lean combustion has the potential to achieve high thermal efficiency for internal combustion engines. However, natural gas (NG) engines often suffer from slow burning rates and large cyclic variations when adopting lean combustion. In this study, the effects of spark plug gaps (SPGs) on methane lean combustion are optically investigated under high ignition energy conditions. Synchronization measurements of in-cylinder pressure and high-speed photography are performed for combustion analysis. The results show that large SPGs with high ignition energy exhibit great improvement in engine combustion stability and power capability. Under ultra-lean conditions, a large SPG with a high ignition energy of 150–200 mJ can extend the lean limit to 1.55. Combustion images indicate that this is contributed by the enlarged initial flame kernel, which promotes early flame propagation. Besides, an empirical criterion is adopted to quantify the underlying mechanism, and the results confirm that a more stable early flame development with a faster burning rate can be obtained by a larger SPG and higher ignition energy under lean conditions. Therefore, a large SPG is an effective way to improve combustion stability and thermal efficiency for NG engines.&lt;/div&gt;

A universally applicable 0D combustion model with a novel analysis of the critical factors affecting combustion in spark-ignition engines

Notwithstanding the continuous technological development of spark-ignition engines, the physics of in-cylinder phenomena has not been explicitly clarified to date. Furthermore, the advent of diverse engine technologies and in-cylinder compositions has increased the difficulty of understanding these phenomena using existing analysis methods. Under those circumstances, the modeling based on the fundamental combustion behaviors is becoming critical for analyzing combustion. Built upon the zero-dimensional flame surface density model, therefore, the models proposed herein attempt to reproduce a vast majority of combustion behaviors revealed to date, such as differential diffusion, thermo-diffusive instability, cutoff scales, distributed reaction, and flame quenching. As a result, we successfully predicted the combustion of numerous operating points of four different SI engines, including the extremely lean operating points of gasoline/air mixture up to [Formula: see text]. Furthermore, the extensive thermodynamic ranges are explored for the validation of the models with the aid of continuously variable valve timing modules; the pressure and the calculated temperature of the unburned mixture at ignition timing are varied from 4.5 to 28.6 bar and 559–794 K, respectively. In this study, we suggest an expression for a fully-developed turbulent flame speed well-fitted with the explored ranges of chemical and thermodynamic states by inspecting the influencing factors for flame wrinkling. Additionally, we analyze combustion using the developed models and identify several distinctive features of both stoichiometric and lean-burn points. Lastly, this study reveals the importance of flame thickness (or inner cutoff), which has a comparable or more severe effect on early flame propagation compared to planar laminar flame speed.