Flame Kernel Development Research Articles

Stability Limit Extension of a Wet Ethanol-fueled SI Engine using a Microwave-assisted Spark

Advanced engines can achieve higher efficiencies and reduced emissions by operating in regimes with diluted fuel-air mixtures and higher compression ratios, but the range of stable engine operation is constrained by combustion initiation and flame propagation when dilution levels are high. An advanced ignition technology that reliably extends the operating range of internal combustion engines will aid practical implementation of nextgeneration high-efficiency engines. The microwave-assisted spark plug under development by Imagineering, Inc. of Japan has previously been shown to expand the stable operating range of gasoline-fueled engines through plasmaassisted combustion, but the factors limiting its operation were not well characterized. The present experimental study has two main goals: (1) to investigate the capability of the microwave-assisted spark plug towards expanding the stable operating range of wet-ethanol-fueled engines, and (2) to examine the factors affecting the extent to which microwaves enhance ignition processes. The stability range is investigated by examining the coefficient of variation of indicated mean effective pressure as a metric for instability, and indicated specific ethanol consumption as a metric for efficiency. Engine efficiency improved when the engine was run at slightly-lean air-fuel ratios, with the onset of instability eventually eliminating efficiency gains associated with lean-burn when mixtures become too dilute. Microwave-assisted ignition reduced dilution-triggered instability, improving efficiency compared to unstable spark-only operation at ultra-lean conditions. Microwave-assisted spark also promotes faster average early flame kernel development when un-enhanced flame kernel development is sufficiently slow. Correlations between microwave-assisted flame development enhancement and calculated in-cylinder parameters suggest a relation between enhancement and the amount of energy deposited into the flame kernel, but scatter prevented derivation of a unifying empirical correlation governing all tested cases.

Effects of split port/direct injection of methane and hydrogen in a spark ignition engine

This study shows the effects of different injection strategies for methane, hydrogen and hydrogen-enriched methane in a single-cylinder spark ignition engine. In port fuel injected engines, it is well known that hydrogen addition to methane leads to improved combustion stability due to accelerated chemical kinetics, especially at the flame kernel development phase and at higher excess air ratios. In this study premixed (i.e., port fuel injected) methane–hydrogen blends, and split injection of methane (port injected) and comparably small amounts of hydrogen (direct injected) were compared. The split injection of methane alone (i.e., port injected and direct injected) was studied to distinguish between enhanced chemical reactions and flow effects. The direct injection was varied in terms of spatial direction, injection timing and the share of total fuel. The results show that late fuel injection towards the spark plug can produce a significant advantage due to increased turbulence or charge stratification. It is also shown that the extension of the lean limits by the direct injection of comparably small amounts of methane is very similar to the lean limit extension using hydrogen addition.

Parametric study of a laser ignited hydrogen–air mixture in a constant volume combustion chamber

Experiments were carried out in a constant volume combustion chamber (CVCC) to investigate flame kernel development and flame speed of hydrogen–air mixtures having different fuel–air ratios. A Q-switched Nd: YAG laser with 1064 nm wavelength and pulse duration of 6–9 ns was used for ignition by generating laser induced plasma inside the CVCC. In this study, laser induced ignition of hydrogen–air mixtures was investigated using different initial chamber filling pressures (P = 2.5 bar–10 bar) at different initial temperatures (373 K–523 K). A variable optical setup with converging lenses having different focal lengths (f = 100–250 mm) were used to position the plasma at various locations inside the CVCC. A high speed camera recorded the flame kernel development and a piezoelectric pressure transducer recorded the pressure–time history for all the experiments. The main objective of this study was to determine the dependence of combustion properties of laser ignited hydrogen–air mixtures on lasers, optical configurations and initial conditions prevailing in the CVCC.

Characteristics of flame kernal radius in a spark ignition engine according to the electric spark ignition energy

To study flame propagation in a spark ignition engine, it must concentrate on the initial combustion stage, i.e., the formation and development of flame. Therefore, we must study the theoretical calculation of minimum flame kernel radius which affects the formation and development of flame kernel. To calculate the minimum flame kernel radius, we must know the thermal conductivity, flame temperature, laminar burning velocity, etc. To evaluate the accuracy of the minimum flame kernel radius based on the theory, authors compared with those from calculation and those from tests. According to the increase of number of sparks and spark intervals, the flame kernel radii become to increase and the expected lean limits are extended. It is considered that is due to the stability of combustion as increase of flame kernel size according as high ignition energy supplies in initial period and discharge energy period lengthens.

An Experimental Study of Flame Kernel Evolution in Lean and Diluted Methane-Air Mixtures at Engine-Like Conditions

An experimental study was performed to investigate the flame kernel propagation and combustion characteristics of methane (CH4)-air mixture at spark-ignition (SI) engine-like conditions. Tests were performed in a constant-volume combustion vessel (CV) with a product engine spark plug mounted at the top to simulate the engine combustion chamber. In this work, the equivalence ratio of 0.7 for lean methane-air mixture was used as test fuel, and different levels of inert gas were introduced to simulate exhaust gas recirculation (EGR) as in actual engine operations. The visualization of flame kernel formation and flame propagation was performed with a schlieren optical system and flame propagation speed was measured and analyzed accordingly. Global combustion characteristics are acquired and analyzed by utilizing the pressure history of combustion events recorded by high speed and fast response pressure measurements. The flame kernel expansion rates have been reported with quiescent and turbulent conditions. Global combustion characteristics, such as heat release rate, cumulative heat release, and mass fraction burned (MFB) as functions of time, are also reported. The suppression effect of pressure on flame propagation showed the flame slowdown while turbulence played a significant role in changing the combustion mode of the flame kernel and shortened combustion duration. These sets of experimental work and measurements are expected to be useful for development and validation of computational fluid dynamic (CFD) codes that simulate combustion processes for combustion ignition processes inside SI engines.

Investigation of Cycle-to-Cycle Variability of NO in Homogeneous Combustion

Cyclic variability of spark ignition engines is recognized as a scatter in the combustion parameter recordings during actual operation in steady state conditions. Combustion variability may occur due to fluctuations in both early flame kernel development and in turbulent flame propagation with an impact on fuel consumption and emissions. In this study, a detailed chemistry model for the prediction of NO formation in homogeneous engine conditions is presented. The Wiebe parameterization is used for the prediction of heat release; then the calculated thermodynamic data are fed into the chemistry model to predict NO evolution at each degree of crank angle. Experimental data obtained from literature studies were used to validate the mean NO levels calculated. Then the model was applied to predict the impact of cyclic variability on mean NO and the amplitude of its variation. The cyclic variability was simulated by introducing random perturbations, which followed a normal distribution, to the Wiebe function parameters. The results of this approach show that the model proposed better predicts mean NO formation than earlier methods. Also, it shows that to the non linear formation rate of NO with temperature, cycle-to-cycle variation leads to higher mean NO emission levels than what one would predict without taking cyclic variation into account.

Using indium tin oxide material to implement the imaging of microwave plasma ignition process

In this paper, a method is introduced to get global observation of microwave plasma ignition process at high pressure. A microwave resonator was designed with an indium tin oxide coated glass at bottom. Microwave plasma ignition was implemented in methane and air mixture at 10 bars by a 2 ms-3 kW-2.45 GHz microwave pulse, and the high speed images of the ignition process were obtained. The images visually proved that microwave plasma ignition could lead to a multi-point ignition. The system may also be applied to obtain Schlieren images, which is commonly used to observe the development of flame kernel in an ignition process.

Evaluation of the Mixture Formation Process of High Performance Engine with a Combined Experimental and Numerical Methodology

The combustion process of a Port Fuel Injection (PFI) engine is deeply influenced by the mixture formation process. The needs of reducing engine emission and fuel consumption push the engine manufactures to implement new advanced experimental and numerical techniques to better control the mixture quality. The local mixture air-index at the spark plug is closely related to combustion instabilities and the Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV) well correlates with the variability of the flame kernel development. The control of air to fuel ratio is especially critical for high performance engines: due to the low stroke-to-bore ratio the maximum power is reached at very high regimes, letting little time to the fuel to evaporate and mixing with air. The injector located upstream the throttle causes a lot of fuel to impinge the throttle and intake duct walls, slowing down the dynamics of mixture formation under part load conditions.The aim of the paper is to present a multi-cycle methodology for the simulation of the injection and the mixture formation processes of high performance PFI engine, in order to evaluate both the quality of combustion and the fuel dynamics in the intake duct.The phenomena involved in the process are highly heterogeneous, and particular care must be taken to the choice of CFD models and their validation. In the present work all the main models involved in the simulations are validated against experimental tests available in the literature, selected based on the similarity of physical conditions of those of the engine configuration under analysis. The lagrangian spray is initialized with a semi-empirical methodology, based on available experimental data, and its interaction with the wall is simulated by means of the Kuhnke model, so to take into account the high wall temperature of the intake valves during impingement. The dynamics of the wall film are accurately represented by the activation of momentum equation of wall film and a validation of its dynamics is accomplished against proper test cases.The methodology is applied in two different engine configurations with separate objectives: a part load condition, where the dynamics of fuel in the intake duct is crucial to ensure drivability, a full load configuration, for the evaluation of mixture quality and combustion stability.

Combustion characteristics and flame-kernel development of a laser ignited hydrogen–air mixture in a constant volume combustion chamber

Laser ignition of hydrogen–air mixture was carried out in a constant volume combustion chamber (CVCC) at 10 bar initial chamber filling pressure and 373 K chamber temperature. A Q-switched Nd:YAG laser at 1064 nm with a pulse duration of 6–9 ns was used for plasma generation and ignition of combustible hydrogen–air mixture. Pressure–time history of different hydrogen–air mixtures was measured in the CVCC and flammability limits of hydrogen–air mixture were measured. Flame kernel development was investigated for different air–fuel mixtures using Shawdowgraphy and flame propagation distances were calculated. Minimum ignition energy was measured for hydrogen–air mixtures of different air–fuel ratios and effect laser pulse energy on pressure–time history in the CVCC was experimentally measured. Upon increasing the laser pulse energy, time taken to attain peak cylinder pressure reduced which resulted in faster combustion in hydrogen–air mixtures however the peak cylinder pressure remained similar.

A New Method to Evaluate the Suitability of Ignitor Strength Used in the Tests of Dust Explosions

Based on systematic experiments in the 20-L Siwek sphere, the explosion behaviors of bituminous coal dust were investigated by changing the strength of pyrotechnic ignitor. The effect on the maximum explosion pressure (Pex) and maximum rate of pressure rise ((dp/dt)ex) of the ignitor strength is analyzed. Due to the under-or over-driven phenomena, the overly low or high ignitor strength may result in unrealisticPexand (dp/dt)ex, especially for lower dust concentration. To evaluate the suitability of ignitor strength, the time scales (i.e. the effective action time,teff, and the induction time,ti,) of the ignitor combustion and flame kernel development are introduced by analyzing the ignition process of dust cloud. It can be found that theteffsignificantly heightens and thetigradually decreases with the rise of ignitor strength. Moreover, when theteffis much lower than theti, the under-driven phenomenon will occur, which leads to the underestimatedPexand (dp/dt)ex. Conversely, when theteffis much higher than theti, the explosion tests may be over-driven to cause the overestimatedPexand (dp/dt)ex. For ideal ignitor strength, theteffwould be comparable to theti. Based on these facts, the under-and over-driven phenomena can be effectively verified by comparing theteffand theti. As a result, when the dimensionless time ratioteff/tiis 0.52 or 1.42, the explosion tests in the 20-L sphere would be, respectively, under-driven or over-driven. To obtain the realistic test results in the 20-L sphere, it is suggested that theteff/tivalue should be in the range of 0.8~1.2 as much as possible.

On the ignition and flame development in a spray-guided direct-injection spark-ignition engine

High-speed fuel, flow, and flame imaging are combined with spark discharge measurements to investigate the causes of rare misfires and partial burns in a spray-guided spark-ignited direct-injection (SG-SIDI) engine over a range of nitrogen dilution levels (0–26% by volume). Planar laser induced fluorescence (PLIF) of biacetyl is combined with planar particle image velocimetry (PIV) to provide quantitative measurements of equivalence ratio and flow velocity within the tumble plane of an optical engine. Mie scattering images used for PIV are also used to identify the enflamed region to resolve the flame development. Engine parameters were selected to mimic low-load idle operating conditions with stratified fuel injection, which provided stable engine performance with the occurrence of rare misfire and partial burn cycles. Nitrogen dilution was introduced into the intake air, thereby displacing the oxygen, which destabilized combustion and increased the occurrence of poor burning cycles. Spark measurements revealed that all cycles exhibited sufficient spark energy and duration for successful ignition. High-speed PLIF, PIV, and Mie scattering images were utilized to analyze the spatial and temporal evolution of the fuel distribution and flow velocity on flame kernel development to better understand the nature of poor burning cycles at each dilution level. The images revealed that all cycles exhibited a flammable mixture near the spark plug at spark timing and a flame kernel was present for all cycles, but the flame failed to develop for misfire and partial burn cycles. Improper flame development was caused by slow flame propagation which prevented the flame from consuming the bulk of the fuel mixture within the piston bowl, which was a crucial step to achieve further combustion. The mechanisms identified in this work that caused slower flame development are: (1) lean mixtures, (2) external dilution, and (3) convection velocities that impede transport of the flame into the fuel mixture.

Quantifying the effect of strong ignition sources on particle preconditioning and distribution in the 20-L chamber

Computational fluid dynamics is used to investigate the preconditioning aspect of overdriving in dust explosion testing. The results show that preconditioning alters both the particle temperature and distribution prior to flame propagation in the 20-L chamber. A parametric study gives the fluid pressure and temperature, and particle temperature and concentration at an assumed flame kernel development time (10 ms) for varying ignitor size and particle diameter. For the 10 kJ ignitor with 50% efficiency, polyethylene particles under 50 μm reach 400 K and may melt prior to flame propagation. Gases from the ignitor detonation displace the dust from the center of the chamber and may increase local particle concentration up to two times the nominal value being tested. These effects have important implications for explosive testing of dusts in the 20-L chamber and comparing to larger 1-m3 testing, where these effects may be negligible.

Comparative investigations of flame kernel development in a laser ignited hydrogen–air mixture and methane–air mixture

Concerns about energy availability and pollutant emissions, such as oxides of nitrogen and particulates have driven concerted efforts toward the design of next generation internal combustion engines, capable of using newer fuels, delivering higher efficiencies and lowering emissions. Among various new engine designs and concepts, laser ignition is one of the promising approaches to attain these objectives.In this research, experimental investigations are carried out to study the laser ignition of hydrogen–air mixtures, which are compared with the baseline data of laser ignited methane–air mixtures. Experiments were carried out in a high pressure constant volume combustion chamber (CVCC) to investigate the flame kernel development and visualization, and flame speeds. The initial operating conditions of the experiments were set to 373 K chamber temperature and 10 bar chamber filling pressure. The flame kernel growth was investigated for different hydrogen–air mixture strengths ranging from λ = 2.0–5.0 and compared with the baseline of methane–air mixtures of λ = 1.2. A Q-switched Nd: YAG laser at 1064 nm wavelength with pulse duration of 6–9 ns was used for ignition by generating laser plasma inside the CVCC. The results indicate that the phenomenon of micro-explosions in the wave-front of hydrogen–air mixtures lead to significantly different flame front evolution and higher flame speeds compared to methane–air mixtures.

Simulation Study about Dual Spark Plug Position for the CNG Engine Combustion Process

The paper has modeled and simulated the combustion process of big bore CNG engine by three-dimensional simulation software AVL FRIE. Based on test validation in the model, the effect on position of the dual spark plug for cylinder initial flame kernel development and flame propagation process was researched. The results showed that: When A=0.5, the flame propagation distance shortened by half, a certain intensity of turbulence formed when the compression stroke ended, the combustion speed was the fastest, the combustion duration was the shortest. If dual spark plug spacing is too small, the two flames meet prematurely will cause interfere with each other, and reduce the combustion speed of the overlapping region of the flame surface, increase the heat losses. Otherwise,if the distance is too large , close to the combustion chamber wall, the combustion space is narrow, the flame propagation space is restricted, then flame development is slow.

High pressure ignition kernel development and minimum ignition energy measurements in different regimes of premixed turbulent combustion

A high-pressure, double-chamber explosion facility possessing controllable ignition and turbulence properties is used to measure flame kernel formation and its minimum ignition energy (MIE) of methane–air mixtures at the equivalence ratio ϕ=0.7 with various turbulent intensities (u′/SL) up to 80 covering from flamelet to distributed regimes, where SL is the laminar burning velocity. The facility applies a high-power pulse generator to control ignition energies of a spark-electrode at the centre of a large inner cruciform burner that is lodged in a huge high-pressure outer chamber. The cruciform burner is equipped with a pair of counter-rotating fans and perforated plates capable of generating controllable near-isotropic turbulence. Two statistical methods for MIE determination are discussed. A high speed Schlieren imaging system is applied to see different modes of flame kernel development with island formation and quench. Results show that MIE decreases considerably with increasing pressure (p) at any given u′, while MIE increases with increasing u′ at fixed p. It is found that the increasing slopes of MIET/MIEL=Γ curves with increasing u′/SL change drastically from linear to exponential at different critical values of u′/SL≈22 (1atm) and 60 (3atm) showing MIE transition, where the subscripts T and L represent turbulent and laminar values with MIEL=0.73mJ (1atm) and 0.23mJ (3atm). We introduce a modified reaction zone Péclet number, Pe*=PeRZ(p/p0)−1/4 estimated just at the instant of the formation of the flame kernel, indicating the surface diffusivity ratio between turbulence and chemical reaction around the kernel with pressure correction, where p0=0.1MPa. The aforesaid two different sets of Γ vs. u′/SL curves can be thus merged into a single Γ vs. Pe* curve having one critical value of Pe* across which MIE transition occurs. A model is proposed in attempt to explain these results.

Laser-Induced Spark Ignition of Premixed Confined Swirled Flames

Optimization of the ignition location is of crucial importance for many combustion systems and requires advanced knowledge of the ignition process for both fundamental and applied configurations. In this article, a premixed swirl burner was designed to experimentally study the impact of the spark location on successful ignition and to detail the scenario from ignition to flame stabilization. Two swirl numbers were investigated to evaluate their impact on the ignition process. Particular attention was paid to providing accurate data on cold flow velocity field statistics (obtained by stereoscopic particle image velocimetry) as well as on ignition conditions. Ignition probability maps were obtained for a constant level of deposited energy. Contrary to previous studies, no correlation between local turbulent kinetic energy and ignition probability was observed, and a deeper analysis of the temporal evolution of the flame kernel within the combustion chamber is required. Coupling fast flame visualization with the corresponding pressure signal demonstrated that the efficiency of the ignition location was not only controlled by the local flow properties, but also by the early flame kernel development, linked by its typical trajectories within the combustion chamber.

Experiments and zero D modeling studies using specific Wiebe coefficients for producer gas as fuel in spark-ignited engines

The paper addresses experiments and modeling studies on the use of producer gas, a bio-derived low energy content fuel in a spark-ignited engine. Producer gas, generated in situ, has thermo-physical properties different from those of fossil fuel(s). Experiments on naturally aspirated and turbo-charged engine operation and subsequent analysis of the cylinder pressure traces reveal significant differences in the heat release pattern within the cylinder compared with a typical fossil fuel. The heat release patterns for gasoline and producer gas compare well in the initial 50% but beyond this, producer gas combustion tends to be sluggish leading to an overall increase in the combustion duration. This is rather unexpected considering that producer gas with nearly 20% hydrogen has higher flame speeds than gasoline. The influence of hydrogen on the initial flame kernel development period and the combustion duration and hence on the overall heat release pattern is addressed. The significant deviations in the heat release profiles between conventional fuels and producer gas necessitates the estimation of producer gas-specific Wiebe coefficients. The experimental heat release profiles are used for estimating the Wiebe coefficients. Experimental evidence of lower fuel conversion efficiency based on the chemical and thermal analysis of the engine exhaust gas is used to arrive at the Wiebe coefficients. The efficiency factor a is found to be 2.4 while the shape factor m is estimated at 0.7 for 2% to 90% burn duration. The standard Wiebe coefficients for conventional fuels and fuel-specific coefficients for producer gas are used in a zero D model to predict the performance of a 6-cylinder gas engine under naturally aspirated and turbo-charged conditions. While simulation results with standard Wiebe coefficients result in excessive deviations from the experimental results, excellent match is observed when producer gas-specific coefficients are used. Predictions using the same coefficients on a 3-cylinder gas engine having different geometry and compression ratio(s) indicate close match with the experimental traces highlighting the versatility of the coefficients.

Critical condition for the ignition of reactant mixture by radical deposition

Motivated by recent interests in plasma assisted combustion (PAC), theoretical investigation on the ignition of pre-mixture by radical deposition is performed. Thermally sensitive intermediate kinetics are considered and the governing equations for temperature and mass fractions of fuel and radical are solved analytically. The correlation depicting the evolution of flame kernel resulted from radical and/or heat deposition is derived. Based on this correlation, we study the flame bifurcation and critical condition for the ignition caused by radical as well as heat deposition. The emphasis is placed on investigating the effects of fuel and radical Lewis numbers. For ignition by radical deposition only, it is demonstrated that the fuel Lewis number has a pronounced influence on the flame propagation and flame bifurcation. With the increase of the fuel Lewis number, new flame branches and bifurcations are observed. It is found that there are two regimes in the change of the minimum chemical ignition power with the fuel Lewis number. Unlike the fuel Lewis number, the radical Lewis number only has a quantitative influence on the flame bifurcation and critical ignition condition. For ignition by radical and thermal deposition, the flame trajectory and flame bifurcation are found to be strongly affected by additional heat deposition. The minimum chemical and thermal ignition powers are compared, and the ignition efficiency of pure chemical ignition and pure thermal ignition is shown to strongly depend on the fuel and radical Lewis numbers. Moreover, the validity of theoretical results describing the effects of fuel and radical Lewis numbers on the minimum ignition energy is confirmed qualitatively by transient numerical simulations including thermal expansion and detailed chemistry.

Droplet/turbulence interaction and early flame kernel development in an autoigniting realistic dense spray

A very detailed evaporating/reacting spray simulation has been conducted extending our previous liquid atomization data in order to understand the physical processes leading to group combustion. The head region of a realistic dense Diesel jet spray is considered, and the droplets and diffusive layers are resolved. At this stage, the droplets have the relative velocity to the high-temperature air flow and this generates comparable-size turbulent eddies around the droplets. By these eddies, mixing is enhanced more strongly than molecular diffusion and merged vapor clouds are formed especially in the area of large droplet number density. This suggests that external group combustion is likely to occur and this is also confirmed by the estimation of group combustion number. This finding requires a modification to the point-source model so that the droplet-generated mixing enhancement effect should be more accurately included. A simple time scale analysis shows that final ignition occurs after the liquid jet head disappears and the resident time is sufficiently large enough.

Optical investigations of the early combustion phase in spark ignition boosted engines

An improved understanding of the thermo-fluid dynamic phenomena that occur during the combustion process in boosted spark ignition engines is necessary for future developments in these engines. In particular, increased understanding of flame kernel evolution is fundamental since the initial growth of the flame contributes significantly to cycle-to-cycle variation in engine performance and emissions. In this work, flame inception and early stages of the combustion process were investigated in an optically accessible single-cylinder ported fuel injection engine. This engine was equipped with the cylinder head of new generation spark ignition turbocharged engine with the same characteristics as the research engine. Cycle-resolved visualization was applied to follow the flame propagation from the spark ignition to the flame kernel growth. A retrieval procedure for the optical data was realized to obtain information about the flame radius evolution. Natural emission spectroscopy in the ultraviolet–visible wavelength range allowed the detection of the chemical markers of the combustion process such as CH, OH, and HCO radicals, and formaldehyde molecules. In-cylinder optical investigations were correlated with engine parameter measurements obtained by conventional methods.