Early Flame Kernel Research Articles

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.

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.

Numerical and experimental investigation of the flame kernel growth in a methane/air mixture near the lean flammability limit

Lean combustion has the potential to improve the thermal efficiency of spark-ignition engines, but it faces the significant challenge of increased cycle-to-cycle variation due to low mixture reactivity and unstable flame dynamics. Computational fluid dynamics (CFD) employing predictive models can guide engine design and optimize operating strategies for lean combustion. However, ignition and combustion models have rarely been validated at fuel-lean conditions, and a fundamental understanding of the early flame kernel growth process is also lacking for a successful sub-model development. The present study develops a numerical simulation framework used to investigate early flame kernel growth in methane/air mixtures. A nanosecond-pulsed discharge (NPD) approach is employed to effectively decouple the flame kernel growth from the electrical discharge due to their difference in timescales, and equivalence ratios near the experimentally measured lean flammability limit (LFL) are selected to focus on challenging mixture conditions. Three numerical investigations, such as the choice of turbulence modeling, grid size, and grid control strategies, are examined to match both LFL and flame kernel structure measured from experiments. It is demonstrated that a quasi-direct numerical simulation (QDNS) with a fixed grid embedding of 10 µm can predict the LFL as ϕCFD=0.61 and match the displacement speed of the kernel's boundary marked in schlieren images. To predict the LFL and flame kernel shape, a fine grid (Δ≤12.5 µm) is needed to capture the consumption of formaldehyde (CH2O) in kernel's reaction branches attached to the anode, and adaptive mesh refinement is replaced with the fixed embedding due to loss of simulation accuracy. Also, it is found that a large-eddy simulation (LES) using the Dynamic Structure model is not suitable for the NPD-induced flame kernel simulation because artificial sub-grid turbulent kinetic energy induced by shock dynamics alters the flow velocity calculation, resulting in divergence of LES from QDNS. Lastly, the simulation well matches the experimental data for the flame kernel evolution in three mixture conditions (ϕ = 0.7, 0.61, 0.55), showing toroidal flame kernel expansion and flame kernel growth/extinction.

Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions

ABSTRACT As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR rates (10−25%), and cross flow speeds (0–30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7–1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

Energy deposition and plasma propulsion during the time window of microwave-assisted CH4-O2 spark ignition with Ar/N2/CO2 dilution

Microwave-assisted spark ignition (MAI) is a promising technology to achieve ultra-lean combustion and thus satisfy the requirements in the low-carbon age. However, the coupling process between microwave and the early flame kernel is still a complex and critical question before its wide application and further optimization. Therefore, this work explores the energy deposition and plasma propulsion effect during the MAI through the ignition experiments fueled by the methane-air mixture under a range of equivalence ratios with argon (Ar), nitrogen (N2), and carbon dioxide (CO2) as the carrier gas. Both the flame and plasma characteristics are recorded and analyzed through the high-speed shadowgraph method. The assessment of the time window and the microwave energy depositing process is based on the voltage of the spark electrode and the detected voltage of reflected microwave power. Results show that in Ar and N2 cases, the microwave enhancing effect on early flame development decreases as the equivalence ratio approaches the stoichiometric ratio, but this tendency is less obvious in the CO2 case. During the MAI, there is a time window for the microwave energy deposition, and once it ends, the following microwave pulses will no longer be absorbed. In successfully ignited cases, the time window in MAI mode is 1.8 ms in the N2 case and 1.5 ms in the CO2 case, while it is extended to 2.4 ms when adopting Ar as the carrier gas causing one more microwave pulse absorbed. Generally, changes in equivalence ratio show no obvious influence on the energy-absorbing process. Microwave pulses during the time window periodically generate a plasma jet that originates from the electrodes gap region towards the flame front. This plasma jet acts as the medium for the microwave influencing the flame development, and thus the relative position between the plasma jet and the flame front determines the final microwave enhancing effect.

Effects of differential diffusion on hydrogen flame kernel development under engine conditions

The early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel.

A Combined Numerical and Experimental Investigation of Cycle-to-Cycle Variations in an Optically Accessible Spark-Ignition Engine

A combined numerical and experimental investigation is carried out to analyze the cycle-to-cycle variations (CCV) in an optically accessible spark-ignition engine with port fuel injection. A stable and an unstable operating condition is considered. Well-established turbulence, combustion, and ignition models are employed in the large-eddy simulations (LES). High-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane are conducted in the experiments. A detailed comparison between LES and experiments is carried out, including the in-cylinder pressure, the flow fields, the spatial flame distribution, and the fields conditioned on fast and slow cycles. Good agreement is achieved for the variables considering all cycles; yet, some discrepancies are observed for the conditionally averaged quantities. A systematic quantitative correlation analysis between the selected influencing variables and the CCV is presented, in which the influencing variables are extracted from different length scales (r = 3 mm, 12 mm, and 43 mm) and the CCV are distinguished between the early flame kernel development and later flame propagation. Even though the most relevant influencing parameters are different for the two operating conditions, the location of the coherent vortex structure is found to be important for the CCV of both cases.

Influence of the spark plug mounting angle on spark ignition process at different flow velocities

Influences of the spark-plug mounting angle on the spark ignition of gasoline-air mixtures in high velocity flows were investigated in a combustion chamber connected at the end of a rapid compression machine. Gasoline-air mixture with an air excess ratio of 1.9 was combusted. A spark plug was mounted on the combustor wall at angles of −30°, −15°, 0°, 15°, and 30°. Flow velocity at the spark plug was varied from 5 to 20 m/s by adjusting the driving pressure of the piston. Time resolved spark-discharge voltage and current were measured with a voltage and a current probe, and the discharge energy was calculated using them. Behaviors of the discharge channel were observed optically with a high-speed video camera in addition to simultaneous measurements of OH* chemiluminescence from the flame kernel with the other high-speed video camera. The results showed that the averaged induction time of flame kernel initiation increased monotonously with an increase in the plug mounting angle from −30° to 30° in flows with velocities of 5 and 10 m/s. However, the increasing trend became less obvious with velocities of 15 and 20 m/s, involving with larger variation in induction time. Discharge duration became shorter with an increase in the plug mounting angle regardless of the same setup. Negative mounting angle resulted in frequent short-circuits/restrikes at the early phase. On the other hand, positive mounting angles resulted in a longer discharge channel length, higher energy deposition before the first restrike and longer induction time of the first restrike. Although the discharge channel at positive mounting angles elongates along with the flow, the early flame kernel was developed toward the center of the swirl. Thereafter, flame propagated outward from the central region, where the flow speed was lower than the downstream area. Negative mounting angle led to a more oval flame kernel shape and the propagation closer to combustion chamber wall, which showed a shorter induction time of flame kernel initiation in the cases of 5 and 10 m/s. However, fragments of the flame kernel tended to be formed due to frequent short-circuits with the velocities of 15 and 20 m/s. This resulted in the large variation of the induction time.

Outwardly growing premixed flames in turbulent media

The propagation of outwardly expanding premixed flames in turbulent media is examined within the context of the hydrodynamic theory wherein the flame, treated as a surface of density discontinuity separating fresh combustible mixture from burned products, is propagating at a speed dependent upon local geometric and mixture/flow characteristics. An embedded manifold approach, one adept at handling multi-valued and disjointed surfaces which are frequently observed in real flames, is used to couple the flow and flame evolution. A sensitivity analysis, based on mixtures with different Markstein numbers, is performed to investigate early flame kernel development in addition to its long-term evolution. The focus is to understand the effect of turbulent flow characteristics, distinguished by the intensity of velocity fluctuations and its integral length scale, in addition to intrinsic flame instabilities (predominantly the Darrieus-Landau instability) on flame propagation. The overarching objective is to quantify their influence on the flame morphology and burning rate and to construct scaling laws for the turbulent flame speed through appropriate modifications of Damköhler’s first hypothesis. Flame-turbulence interactions are inferred from statistical quantities based on its developing flame topology, including local flame curvature and hydrodynamic strain, and their combined effects integrated into the flame stretch rate experienced by the flame and the local flame speed deviation from the laminar flame speed.

Numerical investigation of a fueled pre-chamber spark-ignition natural gas engine

Pre-chamber spark-ignition (PCSI) is a leading advanced ignition concept for internal combustion engines with the potential to enable diesel-like efficiency in medium-duty/heavy-duty (MD/HD) natural gas (NG) engines. By leveraging distributed ignition sources from multiple turbulent jets, the PCSI technology can deliver extremely short combustion duration in ultra-lean mixtures and significantly improve the engine thermal efficiency. However, in the automotive industry there is a lack of adequate science base and predictive simulation tools required for commercial development of PCSI engines. In this study, Reynolds-Average Navier-Stokes simulations are carried out to describe the combustion process in lean-burn NG engines, focusing on the combustion modeling approach. Two combustion models, multi-zone well-stirred reactor (MZ-WSR) and G-equation, are used to simulate the combustion process in an MD NG engine equipped with a fueled-PCSI system for four operating conditions close to the lean operating limit. A skeletal chemical mechanism and a laminar flame speed tabulation are used to compute the combustion accurately. Simulation results are compared with experimental data regarding measured cylinder pressure, heat release rate, and combustion duration. By dividing the PCSI combustion process into four distinct phases, the difference between the two models’ results for each phase is analyzed in detail. The MZ-WSR model overestimates the combustion duration for early flame kernel growth in the pre-chamber due to the lack of a specific formulation to take turbulence-chemistry interaction into account. Despite the prolonged combustion duration and low pressure built-up inside the pre-chamber, the model matches the combustion rate in the main-chamber. In contrast, the G-equation model delivers good agreements for the pre-chamber combustion and turbulent jet-driven combustion processes. However, the model starts to underestimate the combustion rate in the main-chamber, especially under ultra-lean mixture conditions. Finally, improvements are needed for both models to simulate the later combustion stage that occurred in the near-wall regions.

G-equation based ignition model for direct injection spark ignition engines

To accurately model the Direct Injection Spark Ignition (DISI) combustion process, it is important to account for the effects of the spark energy discharge process. The proximity of the injected fuel spray and spark electrodes leads to steep gradients in local velocities and equivalence ratios, particularly under cold-start conditions when multiple injection strategies are employed. The variations in the local properties at the spark plug location play a significant role in the growth of the initial flame kernel established by the spark and its subsequent evolution into a turbulent flame. In the present work, an ignition model is presented that is compatible with the G-Equation combustion model, which responds to the effects of spark energy discharge and the associated plasma expansion effects. The model is referred to as the Plasma Velocity on G-surface (PVG) model, and it uses the G-surface to capture the early kernel growth. The model derives its theory from the Discrete Particle Ignition (DPIK) model, which accounts for the effects of electrode heat transfer, spark energy, and chemical heat release from the fuel on the early flame kernel growth. The local turbulent flame speed has been calculated based on the instantaneous location of the flame kernel on the Borghi-Peters regime diagram. The model has been validated against the experimental measurements given by Maly and Vogel,1 and the constant volume flame growth measurements provided by Nwagwe et al.2 Multi-cycle simulations were performed in CONVERGE3 using the PVG ignition model in combination with the G-Equation-based GLR4 model in a RANS framework to capture the combustion characteristics of a DISI engine. Good agreements with the experimental pressure trace and apparent heat-release rates were obtained. Additionally, the PVG ignition model was observed to substantially reduce the sensitivity of the default G-sourcing ignition method employed by CONVERGE.

LES investigation of cycle-to-cycle variation in a SI optical access engine using TFM-AMR combustion model

Multi-cycle large-eddy simulations (LES) are performed to investigate combustion cycle-to-cycle variability (CCV) in a gasoline spark ignited optical access engine operating under homogeneous stoichiometric conditions. Combustion is addressed with the Thickened Flame Model (TFM) and finite rate chemistry is accounted for through a reduced oxidation reaction mechanism. In view of the fact that computational costs of LES engine simulations are still very high today, this work investigates the use of adaptive mesh refinement (AMR) in the flame zone in conjunction with the artificial flame thickening applied by the TFM model. The paper discusses how the resulting coupled TFM-AMR combustion model allows good resolution of the flame, maintaining accuracy at acceptable costs. First, the details of the coupled model are presented and the effects of the parameters are explored, highlighting their impact on the combustion prediction. Then, computational fluid dynamics (CFD) simulation results are validated against experimental data collected in a low-speed low-load engine point, by comparing 20 LES cycles and 100 measured cycles, for mass fraction burned, combustion phasing, flame images and CCV indices. Lastly, a detailed investigation on the fastest and slowest numerical cycles is presented, analyzing instantaneous flame structures, ignition behaviors, propagation speeds, and probability density function (PDF) of the instantaneous velocity fluctuation around the spark region. The results show that combustion variability is highly correlated to the resolved velocity field and the resolved turbulence intensity, which is found to be the main cause of CCV and affects the early flame kernel growth. This work is an early attempt to use TFM-AMR combustion model for LES simulations of internal combustion engines.

Effect of fuel composition on early flame kernel development process

点火による初期火炎核形成から定常火炎伝播に至る過程は火花点火機関の燃焼変動に影響する．近年では従来燃料にフラン類を混合すると希薄限界が改善されることが報告されているが，その過程については十分明らかでない．本研究では定容容器内に充填した予混合気をレーザ点火で着火させ，火炎核の成長過程を撮影した．それらの結果から燃料種が火炎核成長過程に及ぼす影響を考察した．

Large-eddy simulations of a stratified-charge direct-injection spark-ignition engine: Comparison with experiment and analysis of cycle-to-cycle variations

Multiple-cycle large-eddy simulations (LES) have been performed for an optically accessible, single-cylinder, four-stroke-cycle, spray-guided direct-injection spark-ignition (SG-DISI) engine operating in a stratified globally fuel-lean mode. The simulations combine a standard Smagorinsky turbulence model, a stochastic Lagrangian parcel method for liquid fuel injection and fuel spray modeling, a simple energy-deposition spark-ignition model, and a modified thickened flame model for turbulent flame propagation through highly stratified reactant mixtures. Comparisons between simulations and experiments include individual-cycle and ensemble-average pressure and apparent-heat-release-rate traces, individual-cycle and ensemble-average indicated mean effective pressures (IMEP), and instantaneous two-dimensional vapor-equivalence-ratio contours. Although the number of LES cycles is small (35), the results show that the simulations are able to capture the global combustion behavior that is observed in the experiments, including cycle-to-cycle variations. The simulation results are then analyzed further to provide insight into the conditions that lead to misfire versus robust combustion. As has been reported in earlier experimental and LES studies for homogeneous-charge SI engines, local conditions in the vicinity of the spark gap at the time of ignition largely determine the subsequent flame development. However, in contrast to homogeneous-charge engines, no single local or global quantity correlates as strongly with the eventual peak pressure or IMEP for each cycle. Rather, it is the interplay among the early flame kernel, the velocity field that it experiences, and the fuel distribution that it encounters that ultimately determines the fate of each combustion event. Deeper analysis and quantitative statistical comparisons between experiments and simulations will require the simulation of larger numbers of engine cycles.

Strengthening effect of microwave on spark ignited spherical expanding flames of methane-air mixture

The Microwave Assisted Spark Ignition system is a novel and potential ignition method to improve efficiency of spark ignition engines. To better understand the coupling mechanisms/characteristics between microwaves and spark plasma, experiments were conducted to visualize spark discharge and flame development with microwave emitted in a Constant Volume Combustion Chamber. The operation parameters varied included equivalence ratio, ambient pressure, microwave pulse repetition frequency, and peak power. To clearly illuminate the coupling mechanism between microwave and spark plasma, a small-diameter antenna device was specifically designed to minimize heat loss. The microwave assisted spark ignition and flame propagation characteristics of methane-air premixed spherical flames were studied first. Subsequently, non-reactive discharge was experimented to exclude the effects from combustion. Results showed that microwave increased the development speed of an early flame kernel by 60% with 1 kHz microwave pulse repetition frequency and 1000 W peak power under the condition of equivalence ratio 0.6 and ambient pressure 0.1 MPa. The enhancement was believed to be the result of enhanced chemical reaction and the induced flame deformation by collisions between high energy electrons and other particles. As the ambient pressure increased, the enhancement effect of microwaves on flame kernels was found to diminish. In this situation, non-reactive discharge and related MAI experiments showed that a higher pulse repetition frequency or peak power was beneficial to strengthen the effect of microwave as much higher energy absorption efficiency was achieved.

Experimental study of microwave assisted spark ignition on expanding C2H2-Air spherical flames

Microwave Assisted Spark Ignition (MAI) is a potential technology to improve lean combustion characteristics in spark ignition engines. This paper investigates the MAI performance with C2H2-Air spherical expanding flames in a Constant Volume Combustion Chamber (CVCC) system under ambient pressures from 0.1 to 0.6 MPa and equivalence ratios (φ) from 0.5 to 1.9. Results show that MAI greatly increases early flame kernel radius compared with SI mode particularly in the cases close to lean or rich limit under 0.1 MPa. The enhancement decreases obviously as equivalence ratio approaches stoichiometric ratio. With ambient pressure increasing, the MAI enhancement gets weakened. Normalized Reduced Electric Field (REF) is found to correlate well with the decreasing trend of normalized flame radius enhancement with increasing ambient pressure, indicating that REF is probably one of the key factors affecting MAI performance. The influence of microwave Pulse Repetition Frequency (PRF) on MAI under PRF from 1 to 80 kHz are studied and discussed for both non-combustion discharge and ignition tests. Increasing PRF, the enhancement of MAI is strengthened as more microwave energy is injected in high-absorption-efficiency period which is dominated by electron density. At last, the formation mechanism for “fluorescence spot” and its dependence on pressure are offered to explain the observed increasing light intensity in the flame kernel with ambient pressure.

Investigation of cycle-to-cycle variations in a spark-ignition engine based on a machine learning analysis of the early flame kernel

High-speed Mie scatter imaging and in-cylinder pressure measurements are performed in an optically accessible four-stroke spark-ignition engine to investigate cycle-to-cycle variations (CCVs). Droplet Mie scattering is used to measure the cross-sectional flame contour. Machine learning (ML) methods are applied to predict combustion cycles of high maximum in-cylinder pressure based on flame cross-sections at -15 °CA (crank angle degrees before top dead center). All tested ML methods (decision tree-based, multi-layer perceptron, and logistic regression) are able to predict high energy engine cycles given only partial flame topology information at a temporal snapshot of the flame propagation. Feature importance analyses, employed using decision tree methods, reveal that a combination of flame position and shape features of the flames' cross-section is necessary in achieving a high prediction accuracy. These sensitivity analyses can be used to gain insight into the combustion processes and to strategically reduce the number of features. This enables the addition of new physical quantities in future investigations.

The role of differential diffusion during early flame kernel development under engine conditions - part I: Analysis of the heat-release-rate response

Although experimental evidence for the correlation between early flame kernel development and cycle-to-cycle variations (CCV) in spark ignition (SI) engines was provided long ago, there is still a lack of fundamental understanding of early flame/turbulence interactions, and accurate models for full engine simulations do not exist. Since the flame kernel is initiated with small size, i.e. with large positive curvature, differential diffusion is expected to severely alter early flame growth in non-unity-Lewis-number (Le ≠ 1) mixtures as typically used in engines. In this work, a DNS database of developing iso-octane/air flame kernels and planar flames has been established with flame conditions representative for stoichiometric engine part-load operation. Differential diffusion effects on the global heat release rate are analyzed by relating the present findings to equivalent flames computed in the Le=1 limit. It is shown that in the early kernel development phase, the normal propagation velocity is significantly reduced with detrimental consequences on the global burning rate of the flame kernel. Besides this impact on the overall mass burning rate, the initial production of flame surface area by the normal propagation term in the flame area balance equation is noticeably reduced. By using the optimal estimator concept, it is shown that strong fluctuations in local heat release rate inherent to Le ≠ 1 flames in the thin reaction zones regime are mainly contained in the parameters local equivalence ratio, enthalpy, and H-radical mass fraction. Differential diffusion couples the evolution of these parameters to the unsteady flame geometry and structure, which is analyzed in Part II of the present study (Falkenstein et al., Combust. Flame, 2020).

The role of differential diffusion during early flame kernel development under engine conditions – part II: Effect of flame structure and geometry

From experimental spark ignition (SI) engine studies, it is known that the slow-down of early flame kernel development caused by the (Le > 1)-property of common transportation-fuel/air mixtures tends to increase cycle-to-cycle variations (CCV). To improve the fundamental understanding of the complex phenomena inside the flame structure of developing flame kernels, an engine-relevant DNS database is investigated in this work. Conclusive analyses are enabled by considering equivalent flame kernels and turbulent planar flames computed with Le > 1 and Le=1. In Part I of the present study (Falkenstein et al., Combust. Flame, 2020), a reduced representation of the local mixture state was proposed for the purpose of this analysis. Fluctuations in heat release rate attributed to differential diffusion were found to be governed by the parameters local enthalpy, local equivalence ratio, and H-radical mass fraction. Here, a coupling relation for the diffusion-controlled mixture parameter local enthalpy with local flame geometry and structure is derived, characterized by the key parameters κ and |∇c|/|∇c|lam. The analysis shows that the large positive global mean curvature intrinsic to the flame kernel configuration may detrimentally affect the local mixture state inside the reaction zone, particularly during the initial flame kernel development phase. External energy supply by spark ignition may effectively bridge over this critical stage, which causes the impact of global mean flame kernel curvature to be small under the present conditions compared to the overall effect of Le ≠ 1 observed in a statistically planar flame. Once ignition effects have decayed, the mixture state inside the reaction zone locally exhibits an identical dependence on |∇c| as in a strained laminar flame. This implies that differential diffusion effects at Karlovitz numbers representative for part-load conditions are not weakened by small-scale turbulent mixing, which is undesirable for the engine application, but can be favorable in terms of modeling.