Multiple Ignition Research Articles

Simulation study on the breakup process of kerosene Gel Jet in transverse airflow

The gel propellant not only inherits the characteristics of liquid propellants, such as high specific impulse, multiple ignition capabilities, and adjustable thrust, but also offers an advantage type of solid propellants, including reduced risk of leakage, long-term storage feasibility, and ease of maintenance. In this study, numerical simulations were conducted to investigate the atomization and mixing process of kerosene gel jets in high-speed crossflows. The study reveals that the jet breakup process primarily involves two forms of breakup: columnar breakup dominated by Rayleigh Taylor’s unstable wave and surface breakup dominated by Kelvin Helmholtz’s unstable wave. These two forms coexist, complement each other, and interact with each other. The numerical simulation of the kerosene gel breakup process provides favorable support for studying its combustion characteristics in the rotating detonation engine.

Горимость липовых лесов в Еврейской автономной области

Based on expeditionary work in the period 2003–2018 we compile a map of linden forests locations in the Jewish Autonomous Region (JAR). An inventory map of forest fires was created using data on forest fire registration of the Forest Management Department of the JAR Government for 2017–2020. The number, area, and configuration of the burned areas were determined by superimposing of these maps. The analysis of fire-damaged forests shows that during the study period fires were observed in all forest formation with linden. Their total number was 174. The most of fires (33%) noted in 2018 and the fewest (15%) – in 2017. The average annual number of fires in 2017–2020 was 44. The largest area affected by the pyrogenic factor was observed in 2018. The smallest area (7.5%) of linden forests were fire-transformed in 2020. The average area of one fire is 653 hectares. This corresponds to 64% of the total fire-affected area in the studied vegetation formations. The black birch and oak park-type forests, sometimes with linden, larch, with silverspike-and-forbs cover and forbs-and-pinegrass meadows were most affected by fire. We analyzed the spatial distribution of the fire-damaged forests and established its main patterns as well as identified the areas of repeated burnout. In the most of the JAR linden forests fires have a one-time character. Local areas are subject to multiple burnout; the causes of their high multiple ignition require further study. In this regard, forest fires is a factor that significantly contributes to the reduction of the honey-bearing lands of the JAR.

Ensemble predictions of laser ignition with a hybrid stochastic physics-embedded deep-learning framework

When investigating stochastic ignition phenomena, high-fidelity simulations and experiments for statistical characterization can be a laborious endeavor. Machine learning (ML), in particular deep-learning, offers an accurate and cost-effective approach for reduced-order modeling of forced ignition. In this work, we investigate the potential and limitations of ML in modeling ignition, especially in the presence of limited data and multiple ignition modes. To this end, we introduce a hybrid stochastic physics-embedded deep-learning framework that combines sparse experimental data from Schlieren measurements with inert large-eddy simulations for predicting the spatio-temporal evolution of ignition kernel location and morphology. This model combines a stochastic differential equation for modeling kernel dynamics and a deep-learning model for representing kernel morphology. We evaluate this modeling approach for laser ignition of a gaseous methane/oxygen (CH4/O2) model rocket combustor, where morphological effects of the ignition kernel significantly influence ignition behavior. Results demonstrate that this model can reasonably capture behavior associated with the three dominant ignition modes, namely direct, indirect, and failed ignition, along with statistics associated with kernel growth and position, at lower computational costs than high-fidelity reacting simulations. In addition, we demonstrate that this modeling framework can be employed for generating spatially resolved ignition probability maps by incorporating physics to represent kernel interaction with the turbulent jet. We note that limitations in accuracy can be observed when predicting with vastly out-of-distribution data. Nevertheless, these results demonstrate that this physics-embedded ML approach can statistically characterize forced ignition in a more cost-effective manner than reacting high-fidelity simulations, as long as sufficiently representative data is available.

Non-equilibrium plasma assisted ignition characteristics in premixed ethylene-air flow

Reliable and stable ignition under lean conditions is essential for safe operation of the engine. Nanosecond pulsed discharge non-equilibrium plasma assisted ignition characteristics of premixed ethylene-air flow in an advective combustion chamber were investigated. The effects of the equivalence ratio, discharge gap distance, flow velocity, discharge frequency or inter-pulse time, and pulse number were quantified in terms of ignition probability. Shadow images of ignition kernel development were captured and used to extracted the averaged kernel projected area. The results indicated that increasing the equivalence ratio, a higher flow velocity, a wider discharge gap distance, and a larger number of pulses are all conducive to the increasing of ignition probability via inducing a larger initial kernel. Increasing inter-pulse time has a non-monotonic effect on ignition probability for multiple nanosecond pulsed discharges ignition. As the inter-pulse time decreases, when neighboring kernel boundaries happen to overlap each other, the partially-coupled regime shows a higher ignition probability. Longer or shorter inter-pulse time both cause the decrease in ignition probability. The shortest inter-pulse time shown as the fully-coupled regime is the most favorable for ignition with the highest ignition probability. A method is proposed to estimate the critical frequency at which partially-coupled regime transitions to fully-coupled regime by 95% of the asymptotic time of flame development time.

Preliminary assessment of the potential for rapid combustion of pure ammonia in engine cylinders using the multiple spark ignition strategy

In pursuit of carbon neutrality, the adoption of zero-carbon fuels in internal combustion engines offers a path to zero carbon emissions. Among such fuels, ammonia (NH3) stands out as an effective hydrogen energy carrier with a higher energy density and a more advanced production-storage-transportation lifecycle than hydrogen (H2) fuel, making it a superior alternative. Nevertheless, achieving high combustion efficiencies in engine cylinders requires the use of fuels with fast flame speeds, since the time scales of the combustion event are practically milliseconds. The inherent flame speed of NH3 falls short, typically necessitating augmentation with H2. Yet, the in-situ generation of H2 from NH3 remains constrained, rendering the flame speed of NH3 less competitive against traditional fuels like gasoline and natural gas. To address the dual challenges of slow flame speed of NH3 and the difficulty of generating large amounts of H2 from NH3 onboard, this study evaluates an alternative method of accelerating the ammonia combustion in the engine combustion chamber through the use of multiple spark plugs, which is limited discussed in existing research. Numerical simulations performed in this study show that the application of the multi-spark strategy increases the mass burn rate, advances the combustion phasing, shortens the combustion duration, increases the engine power output, improves the fuel economy, and reduces the unburned ammonia emissions, all of which indicate that rapid combustion of pure ammonia is achieved in the combustion chamber. Moreover, the use of multiple spark points does not lead to a marked increase in nitrogen oxides emissions, thereby ensuring no added stress on exhaust aftertreatment systems. In addition, pure ammonia combustion appears to be very well adapted to this strategy, as evidenced by the avoidance of excessive cylinder pressures and pressure rise rates—a challenge faced by conventional spark ignition engines, given ammonia notably slower flame speed relative to conventional petroleum-based fuels. Overall, all these findings strongly support that the multi-spark ignition system presents a compelling direction for future zero-carbon ammonia engines. However, it is crucial to address design challenges to ensure that the engine head layout provides sufficient cooling, structural robustness, optimal breathing efficiency, and reliable multi-ignition events within the confined space.

Ignition response of NEPE-based propellants under drop weight through experiments and numerical simulation

ABSTRACT Experimental and numerical studies were conducted to investigate the effects of drop height on the ignition mechanism of NEPE nitrate ester plasticized polyether-based propellants. A modified drop-weight system was equipped with a high-speed camera to capture the ignition response images during the whole impact process. A macro-mesoscopic thermomechanical model considering the microcrack and viscous shear flow hotspot formation mechanism was established. The experimental results showed that the ignition responses of the propellants heavily depended on drop height, and the main ignition mechanism was viscous shear flow. The whole loading process could be summarized into three stages: radial extension (viscous shear flow), macroscopic crack initiation and ultimate failure (multiple macroscopic cracks or ignition). The ignition threshold was 35 cm. The posttest scanning electron microscopy (SEM) images further revealed that viscous shear flow played an important role in the ignition of propellants. The numerical simulation results indicated that viscous shear flow was the dominant ignition mechanism compared with the microcrack hotspot mechanism. Point A (along L1) was the most dangerous location under the drop-weight experiment. The predicted features of ignition evolution were consistent with the experimental results and prove that the critical drop height of ignition was 35 cm.

Combustion characteristics and misfire mechanism of a passive pre-chamber direct-injection gasoline engine

Engine combustion characteristics and misfire phenomena in a turbocharged direct-injection gasoline engine with a pre-chamber are studied through multi-dimensional modeling of the engine and jet combustion in a pre-chamber vessel. The results show that the high-speed jets generated by the pre-chamber after spark ignition result in multiple ignition spots and enhanced flow turbulence in the main combustion chamber, accelerating the subsequent turbulent flame propagation. It is also found that the momentum of the gas jets directly correlates with the main chamber combustion phasing and duration, which means that varying the design parameters of the pre-chamber to generate higher jet momentum can produce faster flame propagation and better combustion phasing, improving the engine’s thermal efficiency. On the other hand, flame quenching phenomena at the engine low-load conditions were captured in the simulation with the reaction-mechanism combustion model, whose predictivity of jet flame quenching was verified by the pre-chamber vessel experiments. It was found that, in addition to the unfavorable lower in-cylinder pressure and temperature, leaner fuel–air mixture, weaker turbulence, and wall heat losses would lead to slower flame development in the pre-chamber and the jet flame to quench in the main chamber, resulting in engine misfire or ignition instability. The simulation further verified that flame quenching could be mitigated by using a heat-insulated pre-chamber, improving air–fuel mixing in the main chamber and pre-chamber with advanced fuel injection timing, and igniting the mixture at a higher gas temperature and pressure with advanced spark timing.

Remote microwave heating and ignition with an embedded receiving antenna within nanocomposites

This study demonstrates the use of a receiving antenna within a material to transmit microwave energy resulting in dielectric heating and ignition in a direct writing process. The wire antennas themselves do not heat, but rather the induced electric field results in dielectric heating of the sample, and with sufficient electric field strength, triggers ignition. With different length of embedded receiving antenna, local hot spots were generated at the ends of the receiving antenna as imaged with an IR camera. Simulation of a monopole source and receiving antennas, shows a spatial correspondence of the calculated electric field and the measured hot spots. We also demonstrate that receiving antennas can be incorporated to trigger ignition at desired locations, with multiple ignition points from which flame propagations can be induced. This study provides a new way to design and remote control the location of the hot spots and ignition.

A novel multiple spark ignition strategy to achieve pure ammonia combustion in an optical spark-ignition engine

Ammonia is an attractive carbon-free fuel that has the potential to reduce the need for conventional hydrocarbons (HC) and reduce emissions of undesirable pollutants such as CO, CO2, particulates, and unburned hydrocarbons. However, ammonia has different combustion characteristics than conventional HC fuel. Ammonia is difficult to ignite and has a low combustion rate, resulting in large cyclic variations. In addition, nitrogen oxides (NOx) emissions in the exhaust tract are a major challenge when using ammonia as a fuel directly in engines. In this study, the effect of multiple spark ignition sites on the combustion of pure ammonia in an optical spark ignition engine (SI) was investigated. The experiment was conducted with four spark plugs mounted equidistantly on a special metal liner and one spark plug fitted at the top of the cylinder head. The multiple flames emitted from the different spark ignition sites were captured by natural flame luminosity (NFL) imaging. In the results, the conventional single spark ignition is compared with multiple ignitions. It was found that single spark ignition resulted in lower in-cylinder pressure, longer combustion duration, and higher combustion instability due to the poor ammonia fuel combustion rate. However, firing multiple spark plugs significantly improved combustion stability, increased engine power, and shortened the combustion period under the same operating conditions. In addition, the flame kernels produced by multiple ignition sites resulted in higher NOx emissions in the exhaust tract due to the higher temperatures in the cylinder. In addition, this study also investigated the effect of three different air–fuel equivalence ratios (λ) of 1.0, 1.2, and 1.4 on the combustion characteristics of ammonia fuel. The maximum NOx level was obtained for λ: 1.2 because the excess air in the mixture oxidizes the ammonia and provides abundant oxygen to generate more NOx. Further reducing the ammonia fuel and increasing the excess air to λ: 1.4 dramatically reduced NOx emissions due to the lower pressures and temperatures in the cylinder resulting from the longer combustion period.

Optical study of knocking phenomenon in a spark-ignition engine by using high-speed OH* chemiluminescence imaging: A multiple ignition sites approach

Knock in a spark-ignition (SI) engine is a complicated combustion phenomenon that stimulates high-pressure oscillations inside the combustion chamber and restricts engine performance. This study presents a high-speed OH* chemiluminescence imaging technique to investigate the knock mechanism resulting from firing multiple spark plugs. The experiment was performed using a customized liner having three spark plugs installed at equal spacing, and to compare the results with conventional SI conditions, in which one spark plug was mounted at the center of the cylinder head. In addition, multiple pressure transducers were used at various locations to record the frequencies induced by the pressure oscillations inside the cylinder during knocking events. The results showed that firing a single central spark plug generated mild knock with late combustion phasing and lower power output. However, adding more spark plugs could advance the initiation of autoignition and produce higher knock intensity along with lower combustion duration for the same operating conditions. Additionally, a weak OH* chemiluminescence intensity oscillation was monitored before the autoignition of the unburned charge occurred. The crank angle location of peak OH* intensity and peak HRR showed a good linear curve fit with a positive slope. Furthermore, the larger amount of unburned mass fraction produced stronger pressure waves due to multiple autoignition sites, and the unburned mass fraction exhibited a good linear relationship with unburned temperature and end-gas area at the knock onset point. Moreover, the frequency spectrum recorded by the multiple pressure sensors illustrated that in the case of a single central spark plug only circumferential acoustic waves were formed with low power intensity. However, multiple ignition sites promoted both circumferential and radial pressure waves inside the combustion chamber because of multiple autoignitions occurring both near the center and cylinder wall.

Comparison of combustion duration and end-gas autoignition in inwardly and outwardly propagating flames induced by different ignition configurations

Recently, multiple spark ignition has received great attention since it helps to increase thermal efficiency and to reduce misfire in engines. Multiple spark ignition also affects the combustion duration and thereby it can be used for knock control. However, previous studies reported opposite trends in terms of how multiple spark ignition affects engine knock. This work aims to assess and interpret the influence of flame propagation direction induced by different ignition configurations on combustion duration and end-gas autoignition/engine knock. Two simplified and idealised ignition configurations are studied theoretically and numerically. One is with infinite number of sparks at side circular wall, which induces an inwardly propagating flame (IPF); and the other is with a single central spark, which induces an outwardly propagating flame (OPF). In the asymptotic theoretical analysis, the canonical 1D formulations for IPF and OPF are reduced to 0D model. Based on the 0D model, OPF and IPF at different initial temperatures are studied and compared. Counterintuitively, it is found that the combustion duration of OPF is shorter than that of IPF when there is no end-gas autoignition. On the other hand, the combustion duration of IPF is shorter than that of OPF when end-gas autoignition occurs. Furthermore, end-gas autoignition is found to be more prone to occur in IPF than OPF. These interesting observations are interpreted through assessing the ignition delay time and different components of the absolute flame propagation speed. The theoretical results are validated by transient simulations considering detailed chemistry and transport which are conducted for IPF and OPF in an iso-octane/air mixture at different initial temperatures and pressures. Both theoretical and numerical results suggest that compared to infinite number of ignition sparks at side wall, the single central ignition has the advantages in shortening the combustion duration and reducing the tendency of end-gas autoignition.

Ignition and Extinction Characteristics of Electrically Controlled Solid Propellants

The ignition and extinction characteristics of perchlorate-based electrically controlled solid propellants (ECSPs) that can achieve multiple ignition and produce controllable thrusts were investigated. We examined the multiple-ignition and combustion behaviors of ECSPs and identified the basic ignition-model parameters for these materials. Single-frame photography and a schlieren imaging system were used to determine the ignition, combustion, and extinction properties of two different ECSP formulations at various voltages, pressures, and initial temperatures. The results revealed that increasing the voltage, pressure, and initial temperature reduced the ignition delay; whereas increasing the number of ignitions decreased the ignition delay and ignition energy but increased the extinction delay and smoldering time. Furthermore, under the same experimental conditions, the voltage, pressure, and initial temperature produced a larger effect on a nonmetallic ECSP than on an ECSP containing 5% aluminum (Al) powder. The nonmetallic ECSP exhibited a longer ignition delay, higher ignition energy, and better extinguishing performance than the Al-containing ECSP. Moreover, four distinct stages of the ECSP ignition process were identified. The findings of this study demonstrated that the electrical energy consumed during condensed phase heating plays a critical role in propellant ignition.

Prediction of the developing detonation regime in a NTC-fuel/air mixture with temperature inhomogeneities under engine conditions

The developing detonation process resulting in superknock development under realistic engine conditions in the presence of low-temperature chemistry (LTC) is investigated using two-dimensional direct numerical simulations (DNS). A new model is proposed to predict the developing detonation regime under engine conditions. The prediction is validated against the DNS results. Dimethyl ether (DME) exhibiting a typical negative temperature coefficient (NTC) behavior is used as a fuel. In the presence of the NTC regime, the mean distance of dissipation elements of the ignition delay field, l¯DE, is considered as the characteristic length scale of hot spots to improve the predictive accuracy of the model. The model also accounts for the multi-dimensional effect resulting from the interaction and collision of multiple ignition kernels that is found to promote the onset of detonation and reduce the runup distance of detonation initiation as compared to that of 1D laminar detonation cases. The trasient mixture state is also incorporated in the model development. The model is demonstrated to accurately capture the developing detonation boundary that is consistent with the knock intensity level obtained from the statistical analysis of DNS dataset.

Ignition Growth Characteristics of JEOL Explosive during Cook-Off Tests

In order to study the reaction growth process of insensitive JEOL explosive after ignition under cook-off, a series of cook-off tests were carried out on JEOL explosive using a self-designed small cook-off bomb system. A thermocouple was used to measure the internal temperature of the explosive, and a camera recorded macro images of the cook-off process. The temperature change law of JEOL explosive before and after ignition under different heating rates and the smoke ejection caused by the reaction in the slit were studied. The research results showed that the ignition time decreased as the heating rate increased, while the ignition temperature was not sensitive to the heating rate. When the heating rate was faster, the internal temperature gradient of the explosive was larger, and the ignition point appeared at the highest temperature position. As the heating rate decreased, the internal temperature gradient of the explosive decreased, the ignition point appeared random, and multiple ignition points appeared at the same time. The growth process of the ignition point could be divided into severe thermal decomposition, slow combustion, and violent combustion stages. When the heating rate reduced from 7 to 1 °C/min, the burning rate obviously increased.

Laser-induced ignition of methane-air mixtures by a four-beam, pulse-burst mode passively Q-switched Nd:YAG/Cr4+:YAG laser

In this work we report on the characteristics of laser ignition of CH4-air mixtures, using multi-point (up to four points) ignition with single-pulse beams as well as with trains of two pulses. A compact Nd:YAG/Cr4+:YAG laser with four beams, each beam being operated in single-pulse or burst mode (with up to five pulses), each pulse with an energy of 3.8 mJ and 0.9 ns duration was employed in the experiments. A combustion chamber with spherical geometry was used to ignite CH4-air mixtures with various values of the air–fuel equivalence ratio, λ and at different initial pressures of the fuel mixtures. Analysis of pressure curves indicates that using multiple ignition points leads to an acceleration of the flame speed in the early kernel development phase. It has been observed that for an initial pressure of 1 bar in the combustion chamber, ignition at a point with two laser pulses decreases the duration of combustion by about 7 % compared to ignition at a point with a single laser pulse. For ignition in four points with one or two pulses, the combustion is 10 % and 20 % faster, respectively. The combustion efficiency is also analyzed, and we report that a multi-point laser ignition source leads to a slight increase in the efficiency of burning the fuel near the lean limit. The ignition limits, for both lean and rich CH4-air mixtures, have also been determined for initial pressures in the combustion chamber of up to 9 bars and each ignition method, and a discussion of these is presented.

Experimental and numerical investigation of the auto-ignition characteristics of cylinder oil droplets under low-speed two-stroke natural gas engines in-cylinder conditions

Auto-ignition of cylinder oil droplets in the cylinder of low-speed two-stroke gas engines may cause the pre-ignition. To verify and understand the mechanism of the auto-ignition of cylinder oil droplets, experimental research and numerical analysis were carried out to investigate the auto-ignition behaviors of cylinder oil droplets under in-cylinder conditions of low-speed two-stroke natural gas engines. The experimental equipment includes an optical rapid compression machine to simulate the engines’ in-cylinder conditions and a dual-camera system to capture the cylinder oil droplets' ignition processes. The numerical simulation model contains a transient multi-component evaporation model and a multi-step chemical reaction kinetic mechanism. The evaporation of cylinder oil droplets and the ignition of oil vapor in the cylinder of the rapid compression machine were simulated. The optical experiment verified that breaking a single large-size droplet into the droplet group consisting of the small basic droplets could greatly shorten ignition delay and form multiple ignition hot points intensifying the pre-ignition. The overall ignition delay decreases with the increase of the temperature for large oil droplets, while the ignition delay is not monotonous with the temperature change for small oil droplets. The ignition radius and the vapor/air ratio of ignition moment were obtained by combining experiment and simulation. The ignition radius is reduced as the temperature and pressure increase. And the vapor/air ratio of ignition moment could be reduced by decreasing ambient temperature and increasing ambient pressure. The effect of methane on the overall ignition delay of oil droplets depends on the initial droplet diameter. The auto-ignition of small-size single oil droplets could induce natural gas pre-ignition was valid. And these findings are beneficial to understanding the mechanisms of pre-ignition induced by cylinder oil droplets auto-ignition, and the design of low-speed two-stroke gas engines.

CFD Study of Dual Fuel Combustion in a Research Diesel Engine Fueled by Hydrogen

Superior fuel economy, higher torque and durability have led to the diesel engine being widely used in a variety of fields of application, such as road transport, agricultural vehicles, earth moving machines and marine propulsion, as well as fixed installations for electrical power generation. However, diesel engines are plagued by high emissions of nitrogen oxides (NOx), particulate matter (PM) and carbon dioxide when conventional fuel is used. One possible solution is to use low-carbon gaseous fuel alongside diesel fuel by operating in a dual-fuel (DF) configuration, as this system provides a low implementation cost alternative for the improvement of combustion efficiency in the conventional diesel engine. An initial step in this direction involved the replacement of diesel fuel with natural gas. However, the consequent high levels of unburned hydrocarbons produced due to non-optimized engines led to a shift to carbon-free fuels, such as hydrogen. Hydrogen can be injected into the intake manifold, where it premixes with air, then the addition of a small amount of diesel fuel, auto-igniting easily, provides multiple ignition sources for the gas. To evaluate the efficiency and pollutant emissions in dual-fuel diesel-hydrogen combustion, a numerical CFD analysis was conducted and validated with the aid of experimental measurements on a research engine acquired at the test bench. The process of ignition of diesel fuel and flame propagation through a premixed air-hydrogen charge was represented the Autoignition-Induced Flame Propagation model included ANSYS-Forte software. Because of the inefficient operating conditions associated with the combustion, the methodology was significantly improved by evaluating the laminar flame speed as a function of pressure, temperature and equivalence ratio using Chemkin-Pro software. A numerical comparison was carried out among full hydrogen, full methane and different hydrogen-methane mixtures with the same energy input in each case. The use of full hydrogen was characterized by enhanced combustion, higher thermal efficiency and lower carbon emissions. However, the higher temperatures that occurred for hydrogen combustion led to higher NOx emissions.

What do you mean, ‘megafire’?

AbstractBackground‘Megafire’ is an emerging concept commonly used to describe fires that are extreme in terms of size, behaviour, and/or impacts, but the term’s meaning remains ambiguous.ApproachWe sought to resolve ambiguity surrounding the meaning of ‘megafire’ by conducting a structured review of the use and definition of the term in several languages in the peer‐reviewed scientific literature. We collated definitions and descriptions of megafire and identified criteria frequently invoked to define megafire. We recorded the size and location of megafires and mapped them to reveal global variation in the size of fires described as megafires.ResultsWe identified 109 studies that define the term ‘megafire’ or identify a megafire, with the term first appearing in the peer‐reviewed literature in 2005. Seventy‐one (~65%) of these studies attempted to describe or define the term. There was considerable variability in the criteria used to define megafire, although definitions of megafire based on fire size were most common. Megafire size thresholds varied geographically from > 100–100,000 ha, with fires > 10,000 ha the most common size threshold (41%, 18/44 studies). Definitions of megafire were most common from studies led by authors from North America (52%, 37/71). We recorded 137 instances from 84 studies where fires were reported as megafires, the vast majority (94%, 129/137) of which exceed 10,000 ha in size. Megafires occurred in a range of biomes, but were most frequently described in forested biomes (112/137, 82%), and usually described single ignition fires (59% 81/137).ConclusionAs Earth’s climate and ecosystems change, it is important that scientists can communicate trends in the occurrence of larger and more extreme fires with clarity. To overcome ambiguity, we suggest a definition of megafire as fires > 10,000 ha arising from single or multiple related ignition events. We introduce two additional terms – gigafire (> 100,000 ha) and terafire (> 1,000,000 ha) – for fires of an even larger scale than megafires.

Effect of spark ignition location on the turbulent jet ignition characteristics in a lean burning natural gas engine

The Turbulent Jet Ignition is an effective concept to achieve stable lean burning for natural gas engines due to the multiple ignition sources, high ignition energy, and fast combustion rate. A variation of the ignition location has a non-negligible effect on the ignition performance of the TJI system. The present work aims to provide more details on this effect by numerical simulations. Both factors of the additional fuel supply to the pre-chamber and the in-cylinder flow field are taken into consideration in this study. A numerical model is built based on a lean burning natural gas engine and validated by experimental results. Five different spark ignition sources are equally arranged on the vertical axis of the pre-chamber, with different distances from the connecting orifices. Simulations are carried out under the same initial and boundary conditions except for the location of the ignition source. Combustion pressure, in-cylinder flow field, fuel mass fraction distribution, and heat release rate are analyzed to study the in-cylinder ignition and combustion process. The results show that a rotational flow and a non-uniform fuel distribution are formed in the pre-chamber during the compression stroke. The turbulent jet characteristics are significantly influenced by the coupling of two factors: the combustion rate inside the pre-chamber as well as the flame propagation distance from the ignition source to the connecting orifices. Rapid combustion rate and shorter flame propagation distance both lead to the earlier ejection of cold jets and hot jets. Among five ignition sources, the one located closest to the connecting orifices generates earlier hot jets with the highest mean velocity. The jets are more effective to ignite the lean mixture and could decrease the combustion duration of the main chamber.

Effect of Ignition Position on Pre-chamber Jet Combustion

This study focused on the effect of ignition position in the pre-chamber on front chamber jet combustion. To investigate the effect of ignition position, we designed a pre-chamber visualized cylinder head with multiple ignition positions that can capture the pre-chamber combustion. From the visualized images, it was found that two different jet morphologies, a pre-existing jet and a pre-existing unburned mixed gas jet, are produced depending on the ignition position. In the case of the pre-existing jet, it was confirmed that the combustion pattern of the sub-chamber affects the main chamber flame spread velocity.