Ignition Phase Research Articles

Ignition and combustion characteristics of n-pentanol–diesel blends in a constant volume chamber

Pentanol is considered as one of the most promising alternative biofuels due to its excellent physicochemical properties. The objective of this work was to compare the ignition and combustion characteristics of different n-pentanol/diesel blends in an optical constant volume combustion chamber. The tested fuels included 20% (D80P20) and 40% (D60P40) of n-pentanol blended with diesel in volume, and pure diesel (D100). Broadband chemiluminescence technique was used to measure the timing and location of spray ignition. A high-speed CCD camera with two ND8 dimmer lenses was used to capture the incandescence radiated from the soot particles during combustion. A wide range of experimental conditions was investigated. The ambient temperature ranged from 800K to 1200K and the oxygen concentration ranging from 10% to 21%, covering both the conventional and low temperature combustion regimes. The results show that pure diesel has shorter ignition delay and distance comparing to pentanol blends. A larger blending proportion of pentanol D60P40 advances the ignition phase more than the D80P20 in low oxygen concentration conditions. Due to the fuel-borne oxygen and the dilution effect, the natural flame luminosity is reduced significantly with the increase of pentanol ratio in most conditions except under the intermediate temperature region of 1000K. In that condition, the shorter ignition delay and flame lift-off length of pentanol blends cause a slightly increase in the natural flame luminosity. The natural flame luminosity images showed that the oxygen-contained structure of pentanol could accelerate soot oxidation under all conditions. This indicates that pentanol blends could decrease final soot emissions in internal combustion engines.

Forced ignition of turbulent spray flames

This paper reviews the current state of knowledge on the initiation of a flame in a spray through the action of a spark or through local deposition of heat, and the subsequent flame development, in uniform and non-uniform dispersions of droplets and in the presence of turbulent flow. These processes are of importance in various applications such as gas turbine ignition (relight) and safety related to flammable liquid mists. The review focuses on the initial kernel development, the evolution of a spherical or edge flame, and the ignition of the spray flame when viewed at the whole combustor scale. The factors that determine success or failure of the ignition process at the various phases of the overall burner ignition are discussed through experiments and Direct Numerical Simulations, while modelling efforts are also assessed. The fuel volatility, droplet size, overall fuel-to-air ratio, and the degree of pre-evaporation are the important factors that distinguish spray ignition from gaseous flame ignition, and the extra fluctuations introduced by the random droplet locations, and how this may affect modelling and flame evolution, are highlighted. The flame propagation mechanism in laminar and turbulent sprays is one of the key aspects determining overall ignition success. Suggestions for future research are discussed.

Operation Mode on Pulse Modulation in Atmospheric Radio Frequency Glow Discharges**supported by National Natural Science Foundation of China (Nos. 11475043 and 11375042)

The discharge operation regime of pulse modulated atmospheric radio frequency (RF) glow discharge in helium is investigated on the duty cycle and frequency of modulation pulses. The characteristics of radio frequency discharge burst in terms of breakdown voltage, alpha(α)-gamma(γ) mode transition voltage and current are demonstrated by the discharge current voltage characteristics. The minimum breakdown voltage of RF discharge burst was obtained at the duty cycle of 20% and frequency of 400 kHz, respectively. The α-γ mode transition of RF discharge burst occurs at higher voltage and current by reducing the duty cycle and elevating the modulation frequency before the RF discharge burst evolving into the ignition phase, in which the RF discharge burst can operate stably in the γ mode. It proposes that the intensity and stability of RF discharge burst can be improved by manipulating the duty cycle and modulation frequency in pulse modulated atmospheric RF glow discharge.

Diesel combustion model for on-board application

Diesel engines exhibit highly efficient, environmentally sound performance under good operational control; however, because of the demand of controlling multiple actuators under various environmental conditions, the conventional experimental method for controlling diesel engines has become increasingly difficult. Therefore, diesel combustion models with less calculation loads were ultimately developed with the aim of implementing such model-based controllers for engine control unit in the future. To achieve the on-board application of the diesel combustion model, the in-cylinder state of a single cycle was discretized into several representative phases such as a valve-opening and valve-closing phase, ignition phase, and maximum pressure phase. Temperature and oxygen quantities in residual gas were considered as the state variables of the system because they have a critical effect on combustion and induce cyclic coupling. The model could take account of the effect of actuators in diesel engines, and the states in each phase were calculated by fundamental thermodynamic equations and some empirical equations. The model was validated against experimental results and had a good agreement with in-cylinder pressures and temperatures at each phase. In addition, the calculation times of the model were confirmed to be capable of on-board application. Furthermore, as a demonstrative example and to show the added value of the model, it was used to synthesize controllers to enable multi-input/multi-output control of a diesel engine in simulation.

Performance and emissions characteristics of a lighting cone for charcoal stoves

A lighting cone is a simple metal cone placed on the charcoal bed during ignition to increase draft. Many traditional charcoal-burning stoves are difficult to light due to poor draft through the fuel bed, so lighting cones are used as an inexpensive accessory to help with charcoal ignition. The goal of this work was to determine the validity of using a lighting cone to decrease the ignition time of traditional Haitian charcoal stoves, and evaluate its impact on stove emissions and fuel consumption during the typically inefficient and slow ignition phase. We found that the lighting cone successfully reduced ignition time by over 50%. Due to a more efficient, shorter ignition stage, charcoal consumption during ignition was reduced by over 40% and carbon monoxide was reduced by over 50%. This suggests that lighting cones are a viable and beneficial accessory for aiding ignition in shallow-bed charcoal stoves.

In-situ optical emission spectroscopy diagnostic of plasma ignition impact on crystalline silicon passivation by a-Si:H films

The influence of the plasma ignition condition during PECVD deposition from a silane/hydrogen mixture on the amorphous silicon passivation of crystalline silicon surface is investigated. The changes in this process step mainly consist in varying the power density for very brief durations in between 1 s and 3 s. We find that the ignition phase contributes significantly in the film growth, especially in the a-Si:H/c-Si interface formation. In particular, the deposition rate increases with ignition power density. TEM cross-section inspection presents a rougher a-Si:H/c-Si interface with higher plasma power and thus, a tendency for nano-clusters formation caused by the crystalline nature of the substrate. In-situ plasma diagnostics reveal the gradual raise up of IHa*/ISiH* with the power density leading to worse SiH* abstraction to the surface. Whereas, time-resolved optical emission spectroscopy explains the possible recombination mechanism in the plasma due to higher-silane related reactive species (HSRS) formation via polymerization reactions. Our results point out that the ignition conditions with a rather low power for longer time give the best passivation, resulting an effective lifetime up to 9 ms.

The role of spray-enhanced swirl flow for combustion stabilization in a stratified-charge DISI engine

Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle.The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum.This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel–air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.

Kinetic studies of NO formation in pulsed air-like low-pressure dc plasmas

The kinetics of the formation of NO in pulsed air-like dc plasmas at a pressure of 1.33 mbar and mean currents between 50 and 150 mA of discharge pulses with 5 ms duration has been investigated both experimentally and by self-consistent numerical modelling. Using time-resolved quantum cascade laser absorption spectroscopy, the densities of NO, NO2 and N2O have been measured in synthetic air as well as in air with 0.8% of NO2 and N2O, respectively. The temporal evolution of the NO density shows four distinct phases during the plasma pulse and the early afterglow in the three gas mixtures that were used. In particular, a steep density increase during the ignition phase and after termination of the discharge current pulse has been detected. The NO concentration has been found to reach a constant value of , , and for mean plasma currents of 50 mA, 100 mA and 150 mA, respectively, in the afterglow. The measured densities of NO2 and N2O in the respective mixture decrease exponentially during the plasma pulse and remain almost constant in the afterglow, especially where the admixture of NO2 has a remarkable impact on the NO production during the ignition. The numerical results of the coupled solution of a set of rate equations for the various heavy particles and the time-dependent Boltzmann equation of the electrons agree quite well with the experimental findings for the different air-like plasmas. The main reaction processes have been analysed on the basis of the model calculations and the remaining differences between the experiment and modelling especially during the afterglow are discussed.

Numerical investigation of frictional behavior and energy loss in mixed-hydrodynamic contact of piston ring pack with deformed cylinder liner during warm-up period of SI-engine

A substantial amount of fuel energy is used to overcome internal friction of an engine. Power loss in piston ring-pack accounts for a major portion of this energy loss. Circumferential clearances caused by distorted bores alter the tribological behavior of the piston ring–liner (PRL) interface. This change in performance of the interface intensifies during warm-up of an engine. Mixed-hydrodynamic lubrication model is considered to evaluate the development of tribo-characteristics in piston ring-pack contact with a non-circular liner during the warm-up phase of a spark ignition (SI) engine. Variations in modes of energy loss caused by bore-distortion during the warm-up period are analyzed. Oil transport to the combustion chamber throughout the warm-up process is studied. Five different viscosity-grades are considered to compare performance of the interface. Results show that viscous shearing of lubricant oil is largely responsible for frictional loss in the warm-up phase. Circumferential variation in film thickness decrease parasitic losses in cold temperatures, but increases oil transport both in cold and in warm conditions.

Large Eddy Simulation of n-dodecane spray flames using Flamelet Generated Manifolds

In the present study the Engine Combustion Network (ECN) “Spray A” target conditions are investigated using the Large Eddy Simulation (LES) and the Flamelet Generated Manifold (FGM) methods. We investigate n-dodecane spray flames at three different ambient oxygen levels in engine relevant conditions. The flamelet database is generated by simulating the counterflow diffusion flamelets for two recently developed n-dodecane mechanisms with 257-species/1521-reactions (Narayanaswamy et al., 2014) and 130-species/2395-reactions (Ranzi et al., 2014). In addition to validation in non-reacting conditions, we evaluate the performance of the newly implemented FGM model by comparing spray ignition delay times and flame lift-off lengths to the available experimental data within the ECN. The obtained ignition delay times agree well with the experimental data for the mechanism by Ranzi et al., 2014 and are over-predicted for the mechanism by Narayanaswamy et al., 2014. This observation is consistent with a respective trend in the observed flame lift-off lengths. Further, we provide evidence of only minor spray realization to realization variation of the ignition delay time in the present configuration. The spray flame structure is noted to consist of two parts: (1) a diffusion flame enveloping the combusting part of the spray close to the stoichiometric isoline, and (2) a premixed combustion regime in the fuel-rich core of the spray. During spray ignition, the model predicts the spatio-temporal phases of ignition. The model also indicates the presence of a ‘cool flame’ between the flame lift-off length and the nozzle. For the first time, we quantify the size of such a topological structure. In general, the flamelet data showed significant differences in the ignition characteristics between the two chemical mechanisms for all three ambient oxygen cases, but indicated little differences for a steady flame.

Chemical, Thermal and Phase Analysis of Malaysia’s Electric Arc Furnace (EAF) Slag Waste

This study aims to characterize and investigate properties of EAF slag waste obtained from Southern Steel Berhad, Penang (one of Malaysia’s largest steel makers), prior to its recycling into valuable products. Before characterization, lump form of the EAF slag was crushed into micron size powder. The properties investigated were loss of ignition (L.O.I.), chemical composition, leaching behavior, thermal properties and mineral phases present. Through XRF characterization, the chemical composition of the EAF slag was obtained. The slag was found to have extremely low L.O.I. (0.01 wt.%). Leaching test proved the slag is non-hazardous and safe to be handled. Meanwhile, thermal analysis (TGA) revealed the EAF slag is thermally stable and unlikely to decompose upon heating. Phase identification and quantification were performed by applying Rietveld refinement method. The EAF slag consisted of gehlenite (Al2O3.2CaO.SiO2 – 45.3 wt.%), larnite (2CaO.SiO2 – 21.0 wt.%), hematite (Fe2O3 – 13.4 wt.%) and wustite (FeO – 20.3 wt.%) mineral phases. The XRD profile fitted well with agreement indices of the refinement (Rwp: 6.658 and GOF: 2.588). The properties investigated in this study are expected to shed lights in evaluating the potential recycling of the EAF slag into various valuable products such as aggregate, brick, ceramic tile and cementing material.

Comparative study of nonequilibrium plasma generation and plasma-assisted ignition for C2-hydrocarbons

The kinetics of ignition in C2H2:O2:Ar, C2H4:O2:Ar, C2H6:O2:Ar and C2H5OH:O2:Ar mixtures was experimentally analyzed at temperatures above the self-ignition threshold using a shock tube with a discharge cell. For this purpose, ignition delay time was measured in lean and stoichiometric C2H4-containing mixtures and lean C2H6-containig mixtures behind a reflected shock wave after a high-voltage nanosecond discharge and in its absence. Obtained results were compared with previous studies of C2-hydrocarbon plasma ignition conducted with the same setup. Numerical simulations of the discharge and ignition phases were used to show the main mechanisms that control ignition properties of C2-hydrocarbons. The densities of atoms, radicals, excited and charged particles produced in the discharge plasma were calculated and used as input parameters for ignition modeling. Good agreement was obtained between calculated and measured ignition delay times in the mixtures studied with and without the discharge. Sensitivity analysis was used to demonstrate the limiting processes in the discharge and ignition phases. It was shown that electron interaction with hydrocarbon molecules can affect ignition directly, due to additional dissociation, or indirectly, due to additional electron-impact ionization and the increase of energy deposited in the discharge. The first effect is more pronounced in C2H2 and C2H4 ignition in the stoichiometric mixtures, whereas the second effect is more profound for ignition in the C2H5OH-containing mixtures. The comparison of plasma-assisted ignition dynamics for different C2-hydrocarbons leads to the conclusion that the efficiency of nonequilibrium excitation for ignition and combustion control strongly depends on the type of fuel. The fuels can affect the total discharge energy, the efficiency of hydrogen atom and hydrocarbon radical production in the discharge, and the dynamics of oxygen molecule dissociation by quenching of electronically-excited states of buffer gas.

Investigation of Effects of Air Jet Pressure and Temperature on High-Pressure Air Jet Controlled Compression Ignition Combustion Based on a Novel Thermodynamic Cycle

To control premixed charge compression ignition (PCCI) phasing in internal combustion engines, a novel method called high-pressure air jet controlled compression ignition (JCCI) based on compound thermodynamic cycle was proposed to achieve diesel premixed combustion in all load operations. The method is implemented in a hybrid pneumatic engine (HPE). The application of HPE is also beneficial to the fuel consumption and emissions because of its several flexible operation modes. The purpose of this paper is to investigate the in-cylinder high-pressure air JCCI combustion processes in the power cylinder of HPE. A three-dimensional (3D) computational fluid dynamics (CFD) model coupled with reduced n-heptane chemical kinetics has been developed to analyze the effects of high-pressure air jet pressure and temperature on the in-cylinder ignition and combustion characteristics. The results demonstrated that the diesel premixture combustion phasing can be controlled near the top dead center (TDC) with the variatio...

Formaldehyde preparation methods for pressure and temperature dependent laser-induced fluorescence measurements.

Formaldehyde is an excellent tracer for the early phase of ignition of hydrocarbon fuels and can be used, e.g., for characterization of single droplet ignition. However, due to its fast thermal decomposition at elevated temperatures and pressures, the determination of concentration fields from laser-induced fluorescence (LIF) measurements is difficult. In this paper, we address LIF measurements of this important combustion intermediate using a calibration cell. Here, formaldehyde is created from evaporation of paraformaldehyde. We discuss three setups for preparation of formaldehyde/air mixtures with respect to their usability for well-defined heating of formaldehyde/air mixtures. The "basic setup" uses a resist heater around the measurement cell for investigation of formaldehyde near vacuum conditions or formaldehyde/air samples after sequential admixing of air. The second setup, described for the first time in detail here, takes advantage of a constant flow formaldehyde/air regime which uses preheated air to reduce the necessary time for gas heating. We used the constant flow system to measure new pressure dependent LIF excitation spectra in the 343 nm spectral region (41 (4) absorption band of formaldehyde). The third setup, based on a novel concept for fast gas heating via excitation of SF6 (chemically inert gas) using a TEA (transverse excitation at atmospheric pressure) CO2 laser, allows to further minimize both gas heating time and thermal decomposition. Here, an admixture of CO2 is served for real time temperature measurement based on Raman scattering. The applicability of the fast laser heating system has been demonstrated with gas mixtures of SF6 + air, SF6 + N2, as well as SF6 + N2 + CO2 at 1 bar total pressure.

A numerical investigation of the influence of radiation and moisture content on pyrolysis and ignition of a leaf-like fuel element

The effects of thermal radiation and moisture content on the pyrolysis and gas-phase ignition of a solid fuel element containing high moisture content were investigated using the coupled Gpyro3D/FDS models. The solid fuel has dimensions of a typical Arctostaphylos glandulosa leaf which is modeled as thin cellulose subjected to radiative heating on one side. We incorporated a five-step extended Broido-Shafizadeh reaction model for thermal degradation, moisture evaporation, and pyrolysis of cellulose in Gpyro3D. The solid-phase model was successfully validated against published data. Ignition of the modeled leaf-like fuel element at three initial moisture contents (5%, 40%, or 80%) exposed to a 1500 K radiant source was simulated. Lower moisture content resulted in earlier onset of pyrolysis and ignition resulting in higher solid and gas phase temperatures. Local moisture evaporation and temperature rise were observed in all three cases and a significant amount of moisture remained in the sample during ignition. The numerical results suggested that moisture content not only affected the process of pyrolysis, but also influenced the ignition and gas phase combustion of the solid fuel.

A numerical study of the effects of primary reference fuel chemical kinetics on ignition and heat release under homogeneous reciprocating engine conditions

In the current work, numerical simulations are used to evaluate the effects of detailed reaction chemistry of different primary reference fuel (PRF) blends of iso-octane and n-heptane on heat release in one-dimensional engine simulations. A simplified slider-crank model was used to represent the engine cycle. The contributions of specific reaction classes to ignition and heat release were quantified. Maps of ignition phasing and heat release were created as a function of pressure and temperature to indicate the change in reactivity (defined by the first and second stages of ignition) as a function of state conditions as well as the fraction of heat release associated with the two stages of ignition. For the conditions studied, the reactivity of the second stage of ignition always increased with increasing temperature, i.e. the phasing of autoignition advanced with increasing temperature, whereas the reactivity of the first stage of ignition exhibited negative temperature dependence where increasing temperature delayed the first stage of ignition and decreased the heat release at the first stage of ignition for some conditions. The results show low-temperature chemistry radicals like C7RO2 species are not uniquely indicative of low-temperature heat release, but they are formed at earlier times, higher rates and higher concentrations with PRF blends with higher fractions of n-heptane. A modified approach to the Livengood–Wu integral is presented to capture the integrated effects of the compression stroke on the potential for using the first stage of ignition to distribute heat release. The results of the modified ignition integral analysis are presented as a function of engine speed and fuel/air preheat temperature and demonstrate the utility of the approach to design and interpret fueling strategies of fuel blends.

Evaluation of non-steady state condition contribution to the total emissions of residential wood pellet stove

In order to evaluate non-steady phase contribution to the total emissions of a pellet stove in real domestic operations, particulate matter and gaseous emissions were determined separately for different operating conditions, i.e. ignition, partial load, increase in power and nominal load. TSP (Total suspended particulate) was sampled with a dilution system and characterized for TC (total carbon), PAHs (polycyclic aromatic hydrocarbons), the main soluble ions, Ni, As, Cd and Pb. Gas monitoring shows that CO and NO emission factors in ignition phase markedly differ from other operating conditions: NO emission factor is lower, while CO one is much higher, since it is a product of incomplete combustion. Start-up phase emission factors are also higher for TSP, Cd and other products of incomplete combustion, i.e. TC and PAHs. Despite being a non-steady phase, the increase in power phase emission factors appreciably differ from steady state ones only for PAHs. Moreover, the PAHs emitted in non-steady state phases have a higher toxicological burden. In conclusion, in order to evaluate the real impact of pellet stove on the environment, transient conditions should be taken into account. The ignition phase, even though it lasts only 20 min, can significantly contribute to pollutant emission.

Modelling pyrolysis of charring materials: determining flame heat flux using bench-scale experiments of medium density fibreboard (MDF)

Medium density fibreboard is a homogenous wood product which is a suitable candidate for validating the pyrolysis model of charring materials. For comparison between model and experiment, this article presents the burning behaviour of MDF under cone calorimeter with different experimental conditions. The total incident heat flux, as a significant boundary condition for pyrolysis modelling, is specifically studied both experimentally and theoretically. The experimental conditions were found to have no significant impact on the ignition phase however the sample thickness would lead to different burning behaviours. The total incident heat flux led by cone and flame at sample surface was experimentally measured and the experimental results were evaluated using non-flaming experiments with inert gas. An analytical model based on classical ignition theory is developed to address the mechanism of mass loss caused by incident heat flux, involving the effects of char layer and back boundary. The model shows that the mass loss rate correlates linearly with the total incident heat flux for a specific char layer thickness and back boundary condition. The model was validated for the non-flaming and flaming experiments to further justify the reliability of heat flux measurements. An empirical equation describing the flame heat flux is proposed.

Model of Pulsed Electrical Discharge Machining (EDM) using RL Circuit

This article presents a model of pulsed Electrical Discharge Machining (EDM) using RL circuit. There are several mathematical models have been successfully developed based on the initial, ignition and discharge phase of current and voltage gap. According to these models, the circuit schematic of transistor pulse power generator has been designed using electrical model in Matlab Simulink software to identify the profile of voltage and current during machining process. Then, the simulation results are compared with the experimental results.

Pulses Model of Electrical Discharge Machining (EDM)

This article presents a model of pulses inElectrical Discharge Machining (EDM) system. There areseveral mathematical models have been successfully developedbased on the initial, ignition and discharge phase of currentand voltage gap. According to these models, the circuitschematic of transistor pulse power generator has beendesigned using electrical model in Matlab Simulink software toidentify the profile of voltage and current during machiningprocess. Then, the simulation results are compared withexperimental results.