Academic literature on the topic 'Early ignition phase'

Natural gas is a promising alternative fuel to petrol for vehicles. However, one of the factors hampering the design of natural gas burning engines for domestic cars is the long delay from the time of ignition to the commencement of significant heat release. This is mainly due to the substantially endothermic phase during the early development of the combustion in natural gas. It is well known that high-energy, extended or multiple ignition sources can reduce this problem. The present article uses a large-scale computer simulation of a natural gas engine to examine the issues affecting the optimization of such ignition sources.

As a candidate for a green hypergolic bipropellant, the combination of highly concentrated hydrogen peroxide and fuel with dissolved sodium borohydride has been widely studied. In this study, a drop test using such a green hypergolic bipropellant was conducted to investigate the stable ignition region in terms of the mixture ratio. As a result, stagnation phenomena of flame growth were observed in high mixture ratio conditions. In addition, impinging-jet tests using a windowed chamber were conducted with the green hypergolic bipropellant to observe the ignition phenomena inside the combustion chamber. As a result, unstable ignition phenomena were observed in oxidizer-lead injection cases. Besides the unstable ignition, hard starts occurred several times during the test series. Data analysis demonstrated that controlling the transient mixture ratio in the early phase of injection is essential for preventing unstable ignition and hard starts. The quantitative threshold of mixture ratio for stable ignition was clarified based on the test results.

The evolution of early stages of homogeneous mixture combustion in spark ignition (SI) engines represents a critical period that greatly affects the whole combustion process. A proper description of this critical phase represents a major issue, which could strongly influence the overall model predictive capability (i.e. model ability to reproduce the real engine behaviour for a large range of operating conditions without any major tuning). Such requirements become even more important for the simulation of last-generation gasoline direct injection or lean stratified engines, where ignition could determine the functionality of the engine itself. In this paper, after a detailed analysis of the ignition physical process and its modelling issues, the predictive capability of the KIVA-3V code has been improved by substituting the original ignition procedure with a more detailed kernel evolution model based on the one presented by Herweg and Maly in 1992. The ignition model introduced in a KIVA-3V version already modified by the authors (re-zoning algorithm, combustion and turbulence models, cylinder wall heat transfer, etc.) has then been tested in order to assess its level of accuracy in describing this complex phenomenon, by varying the most critical engine operating conditions and keeping combustion tuning parameters unchanged. After comparing ignition model results with the corresponding ones presented by Herweg and Maly, a specific application of the overall model (KIVA-3V + ignition model + turbulent combustion model) has been made to perform an analysis of a compressed natural gas (CNG) fuelled engine for heavy-duty applications. To this aim, the in-cylinder combustion history and the related processes as the temperature distribution and NOx formation have been calculated and verified with reference to the experimental data measured in a wide range of operating conditions of an IVECO turbocharged engine.

The combustion process quality is determined by several factors: the composition of the fuel-air mixture in the vicinity of the spark plug and the discharge conditions on the spark plug. This article assesses a high-power ignition system using optical gas flame propaga-tion analyzes. The tests were carried out in a rapid compression machine, using a fast camera for filming. The spark plug discharge quality assessment was determined indirectly by the flame propagation conditions after the ignition of the mixture (during methane combustion). The size of the flame surface and the rate of its change were assumed as a comparative criterion. It has been found that when using an ignition system with high discharge power the rate of flame development is 14% higher with respect to conventional ignition systems. In addition, the shorter development time of the early flame phase after discharge when using the new ignition system was confirmed. Based on the obtained test results and analyzes, modifications of engine operation settings were indicated, resulting from the use of a high discharge power system.

Since the 1970s, high energy heavy ion accelerators have been one of the leading options for imploding and igniting targets for inertial fusion energy production. Following the energy crisis of the early 1970s, a number of people in the international accelerator community enthusiastically began working on accelerators for this application. In the last decade, there has also been significant interest in using accelerators to study high energy density physics (HEDP). Nevertheless, research on heavy ion accelerators for fusion has proceeded slowly pending demonstration of target ignition using the National Ignition Facility (NIF), a laser-based facility at Lawrence Livermore National Laboratory. A recent report of the National Research Council recommends expansion of accelerator research in the US if and when the NIF achieves ignition. Fusion target physics and the economics of commercial energy production place constraints on the design of accelerators for fusion applications. From a scientific standpoint, phase space and space charge considerations lead to the most stringent constraints. Meeting these constraints almost certainly requires the use of multiple beams of heavy ions with kinetic energies >1 GeV. These constraints also favor the use of singly charged ions. This article discusses the constraints for both fusion and HEDP, and explains how they lead to the requirements on beam parameters. RF and induction linacs are currently the leading contenders for fusion applications. We discuss the advantages and disadvantages of both options. We also discuss the principal issues that must yet be resolved.

This work studies the characteristics of thunderstorms that cause lightning-caused wildfires in Catalonia, north-east Iberian Peninsula, using lightning and weather radar data. Although thunderstorms produce ~57 000 cloud-to-ground (CG) flashes yearly in Catalonia, only 1 in 1000 end up as a flaming wildfire. Characterisation of thunderstorms that ignite wildland fires can help fire weather forecasters identify regions of increased ignition potential. Lightning data and radar products like maximum reflectivity, echo tops heights and equivalent liquid content were obtained over a 7-year period. Characteristics of thunderstorms that ignite wildfires are examined including storm motion, duration, morphology and intensity. It was found that most probable ignition candidates are lightning associated with cellular thunderstorms and non-linear systems. Radar reflectivity values for lightning that ignites wildfires were found to be below average, these morphological types favouring the occurrence of lightning outside regions of high reflectivity, where precipitation reaching the ground is low or non-existent. Thunderstorms that ignite wildfires are typically of low intensity, with a CG flash rate below average. Most ignitions occur during the maturity phase when the CG flash rate is the highest. A better scientific understanding of the thunderstorms that cause lightning wildfires will help improve early firefighting response.

Hydrogen internal combustion engines (H2-ICEs) have advantages such as clean combustion and zero carbon emissions, and have become one of the important technical routes for decarbonization in the internal combustion engine industry. In this paper, several key factors affecting the performance and emissions of hydrogen internal combustion engines, such as ignition timing, excess air coefficient, and hydrogen injection timing, were systematically studied on a spark ignition multi-point injection (MPI) hydrogen internal combustion engine bench. The experimental results indicate that the ignition timing controls the combustion phase of hydrogen. Moderate early ignition can improve the brake thermal efficiency (BTE) while having little impact on the NOX emissions. Excess air coefficient(λ) can significantly affect the performance and emissions of H2-ICE. Along with the increase of the λ, the NOX emissions first increases and then continues to decline. When the λ reaching 2.1 or above, near zero emissions of NOX can be achieved. The advance of hydrogen injection timing will slightly increase the peak of cylinder pressure and instantaneous heat release rate. However, overall, the impact of hydrogen injection timing on BTE and NOX emissions is not significant on MPI H2-ICE.

The canonical diesel spray A is characterized in an optical Rapid Compression Machine (RCM) at high temperature and density conditions (900 K and 850 K, ρ = 23 kg/m3) using simultaneous high-speed OH* chemiluminescence and two-pulse 355 nm Planar Laser Induced Fluorescence (PLIF). The focus is on the time evolution and the repeatability of the early stages of both cool flame and hot ignition phenomena, and on the time evolution of the fluorescing formaldehyde region in between. In particular, time resolved data related to the cool flame are provided. They show the development of several separated kernels on the spray sides at the onset of formaldehyde appearance. Shortly after this phase, the cool flame region expands at high velocity around the kernels and further downstream towards the richer region at the spray head, reaching finally most of the vapor phase region. The position of the first high temperature kernels and their growth are then characterized, with emphasis on the statistics of their location. These time-resolved data are new and they provide further insights into the dynamics of the spray A ignition. They bring some elements on the underlying mechanisms, which will be useful for the validation and improvement of numerical models devoted to diesel spray ignition.

Les restrictions environnementales abordent la réduction de l’utilisation des sources d’énergie primaires, motivant la recherche pour progresser vers des technologies améliorées. Parallèlement à ces efforts, la fiabilité et les performances doivent être assurées, en particulier dans des conditions délicates de pression et de température, c’est-à-dire en haute altitude. Dans les moteurs à turbine à gaz, ces deux éléments sont cruciaux pour offrir des produits qui répondent aux besoins et aux attentes fixés par le scénario actuel. L’allumage est un processus multiphysique constitué de plusieurs phases et événements qui concourent en couvrant une gamme diversifiée d’échelles de temps caractéristiques. La résolution numérique de la phase d’allumage précoce, pour laquelle des informations fines et détaillées font défaut, est étudiée dans cette étude. L’efficacité de l’allumeur est estimée par calorimétrie dans de l’air sec, ce qui montre que les variations de pression initiale ont une influence sur l’efficacité. La même étude a révélé que la température (20 ° C; - 20 ° C) par ailleurs a un effet négligeable. Les propriétés physiques du noyau en termes de volume, surface, surface de projection, rayon de l’arc électrique établi dans la cavité, sont estimées en adoptant différents diagnostics optiques, notamment de l’imagerie ultra rapide, strioscopie et ombroscopie à 1 MHz. Des calculs sont effectués pour obtenir une évolution temporelle pendant le temps de dépôt d’énergie (130 μs). Un effet de la pression initiale est observé sur les propriétés du noyau de telle sorte que avec la réduction de la pression initiale le volume du noyau augmente. De plus, des visualisations directes filtrées de la cavité de l’allumeur montrent qu’un effet de pression est discerné à partir de 20 μs. La taille du noyau est également mesurée pour des prémélanges de méthane pour différentes richesses. Cela vise à déterminer l’influence de la variation de la composition par rapport à un cas de référence dans du N2 pur qui est comparé aux mesures dans des prémélanges gazeux (à la fois de nature inerte CH4 / N2 et réactive CH4 / O2 / N2). Une comparaison entre les cas inertes et réactifs expose des réactions de combustion actives déjà pendant le dépôt d’énergie. Pour étudier l’exposition aux éléments d’un environnement réel, l’impact d’un écoulement transverse dans des conditions ambiantes est étudié dans une soufflerie. Cela a été adapté pour simuler l’effet combiné de l’écoulement transverse et de l’air de refroidissement auquel l’allumeur est exposé en étant monté dans une douille. L’effet de la douille sur la projection du noyau est étudié, révélant un impact sur la projection et la déformation du noyau en fonction de la vitesse imposée. La génération du noyau est examinée dans un écoulement réactif prémélangé à 0,45 et 1 bar. Le champ de vitesse a été étudié au préalable par PIV pour connaître la vitesse à proximité de l’allumeur et dans le domaine spatial où le noyau est projeté. Trois conditions de vitesse sont retenues pour effectuer la décharge. Il est observé que la pression initiale influence la déformation subie par le noyau en fonction de la vitesse initiale. En effet, à 1 bar, le noyau est préservé plus longtemps. Un effet secondaire de richesse est trouvé. Une étude préliminaire est réalisée pour explorer l’interaction entre le noyau et un spray de gouttes à 0,45 bar et 1 bar. Le fuel utilisé est du décane. L’ombroscopie à fort grossissement est le diagnostique utilisée pour effectuer des statistiques sur une fenêtre spatiale de 2 x 2 cm où des gouttelettes sont observées se déposer sur les électrodes. Des variations de leurs propriétés sont détectées en fonction de la synchronisation avec la décharge. Des visualisations par strioscopie sont ensuite réalisées pour observer qualitativement les phénomènes apparaissant dans une fenêtre temporelle de 1 ms. Le modèle existant de Taylor-Sedov est testé pour déterminer les capacités prédictives
Environmental restrictions tackle the reduction of the use of primary sources of energy motivating research to advance towards upgraded technologies. Alongside with these efforts, reliability and performance need to be ensured, especially for detrimental conditions of pressure and temperature, i.e. high altitude. In gas turbine engines, both these elements are crucial to offer products that fit to both the needs and expectations set by the present scenario. Ignition is a multiphase process constituted by several phases and events that span a diversified range of characteristic time scales. The numerical resolution of the early ignition phase, for which fine and detailed information is lacking, is investigated in this study. The efficiency of the igniter is estimated through calorimetry in pure air, which shows that variations of initial pressure have an influence on efficiency. The same investigation revealed that temperature (20° C; - 20°C) has a negligible effect. Physical properties of the kernel in terms of volume, surface, projection surface, radius of the arc channel in the cavity, are estimated adopting different optical diagnostics, including schlieren and shadowgraphy imaging at 1 MHz. Calculations are done to obtain a temporal evolution during energy depositing time (130 μs). An effect of initial pressure is observed on kernel properties such that reducing the initial pressure, kernel volume increases. Furthermore, filtered direct visualizations of the igniter cavity show that an effect of pressure is discerned from 20 μs. Kernel size is also measured for methane premixed mixtures of different equivalence ratios. This is intended to determine the influence of composition variation with respect to a reference case in pure N2 which is compared to measurements in gaseous premixed mixtures (both of inert CH4 / N2 and reactive CH4 / O2 / N2 nature). A comparison between inert and reactive cases exposes active combustion reactions already during energy deposition. To investigate the exposure to real life environment elements, the impact of a transverse flow at ambient conditions is studied in a wind tunnel. This was adapted to simulate the combined effect of a transverse flow and cooling air spilled from the liner that the igniter is exposed to by being mounted in a sleeve. The effect of the sleeve on kernel projection is investigated, which reveales an impact on projection and kernel deformation depending on the imposed velocity. The generation of the kernel is examined in a reactive premixed swirled mixture at 0.45 and 1 bar. The velocity field have been studied beforehand by PIV to know the velocity in the vicinity of the igniter and in the spatial domain where the kernel is projected. Three velocity conditions are retained to perform the discharge. Initial pressure is observed to influence the deformation the kernel undergoes depending on initial velocity. At 1 bar, the kernel appears to be preserved for longer. A secondary effect of equivalence ratio is found. The existing model of Taylor-Sedov is tested to predict kernel properties and compare them to experimental measurements. A preliminary study is performed to explore the interaction between the kernel and a spray at 0.45 bar and 1 bar. High magnification shadowgraphy is used to run statistics on a spatial window of 2 x 2 cm where droplets are observed impinging on the electrodes. Properties variations are detected depending on the synchronization with the discharge. Schlieren visualizations are further performed to observe phenomena to qualitatively explore the dynamics appearing in a time window of 1 ms

In this dissertation, some practical ignition techniques are presented that show how some problems of lean-burn combustion can be overcome. Then, to shed light on the effects of the ignition techniques described, the focus shifts to the more specific problem of the early phase of spark ignition. Thermal models of ignition are reviewed. These models treat the energy provided by the electrical discharge as a point source, delivered infinitely fast and creating a spherically symmetric ignition kernel. The thesis challenges the basis of these thermal models by reviewing the work of many investigators who have clearly shown that the temporal characteristics of the discharge have a profound effect upon ignition. Photographic evidence of the early phase of ignition, as well as other evidence from the literature, is also presented. The evidence clearly demonstrates that the morphology of spark kernels in the early phase of development is toroidal, not spherical as suggested by thermal models. A new perspective for ignition, a fluid dynamic point of view, is described. The common ignition devices are then classified according to fluid dynamics. A model describing the behaviour of spark kernels is presented, which extends a previously established mixing model for plasma jets, to the realm of conventional axial discharges. Comparison of the model behaviour to some limited data is made. The model is modified by including the effect of heat addition from combustion, and ignition criteria are discussed.
Graduate

Forced ignition, the initiation of combustion processes by rapid and localized introduction of energy, is central to the successful operation of many combustion systems. It is therefore of interest to investigate this process, starting from the introduction of energy to the emergence of self-sustained flame or the quenching of an otherwise initialized flame kernel. Since the process is highly non-equilibrium and involves various complex kinetic phenomena, it is important to understand the key aspects that control failed or successful ignition. Detailed studies of the early phases of the ignition process can lead to knowledge of more general characteristics of the problem so that reduced models of the ignition process can be developed. These reduced versions can be used in less costly computational studies to assess various ignition events. This paper reports an experimental and numerical investigations of the early phase of laser ignition. The gas mixtures, air, methane/N2 and methane/air are considered to bring out the effect of heat release on the early flow field. The mixtures are studied at three different energy levels and the Jones blast wave theory is used to deduce the energy responsible for the development of the attendant shock waves. This energy is also used to specify initial conditions for the simulations of air and methane/air processes. Additionally, interferometry is used to resolve the density field within the plasma kernel. For the methane/air simulation two chemical models are used, a global reaction model supplemented by an ignition model and a two-step mechanism. The sensitivity of the simulations to the initial geometry of the laser spark is also investigated. The blast wave and interferometry results show that in the reacting methane/air mixture the resulting shock wave is strengthened by early heat release. It is also shown that the shock wave trajectory is not strongly affected by the initial spark geometry, but it has an impact on the velocity field and on the distribution of thermodynamic properties.

This paper presents a comparative investigation of the early phase of combustion initiated by a focused pulsed laser beam and a conventional spark-discharge. The comparative approach is applied to the characterization of combustible mixtures of natural gas and biogas with varying CO2 content. Interferometry and Schlieren imaging are used to probe flame kernel formation and its subsequent transition to a self-sustained flame. The pressure rise in the chamber is also recorded by means of a fast-response piezo-electric pressure transducer. From these diagnostics, differences between the early phases of laser-induced and spark-ignition are revealed. Unlike in laser-induced ignition, spark-ignition features a delay in the transition from the early flame kernel to a self-sustained flame (attributed to heat loss), which leads to failed ignition for mixtures with high CO2 content. The role of CO2 as an ignition retardant is observed by comparing fuel/air mixtures with and without CO2.

Homogeneous charge compression ignition (HCCI) offers a promising way to improve efficiency and emissions. However, when HCCI is induced by reinducting exhaust gases, less power is produced. A possible solution is to couple HCCI with spark ignition (SI) operation at higher loads. This requires a way to smoothly switch between combustion modes. The authors present a multi-cycle, multi-mode combustion model to aid in understanding and controlling the mode transition. The model captures early ignition and low work after a switch from SI to HCCI. Furthermore, the model reveals a need to coordinate intake and exhaust valve timing to correct the HCCI phase and work. To demonstrate the model, the paper concludes with an example trajectory that maintains constant work and ignition phasing after a switch from SI to HCCI.

The design of a gas turbine combustion chamber integrates multiple contradicting objectives. Among all the parameters available to the engineers, the number of fuel injection systems and their spacing are crucial information which need to be fixed early on in the design phase. Indeed, such choices not only impact the cost and size of the combustor but they also affect the operability of the future engine. One key objective behind these parameters is the ignition time delay needed for the whole combustion chamber to successfully light. To gather knowledge in the ignition process that takes place in real gas turbine engines, current research orient towards the development of experimental facilities that complement high fidelity unsteady numerical simulations. In this context, a multi-injectors experimental set-up located at CORIA (France) is used to validate Large Eddy Simulation (LES) tools developed by CERFACS, IFP-EN and CORIA (France). Preliminary validations against experimental data show that for a given inter-injector distance, LES stationary and ignition transient predictions are very promising and recover the main features found in the experiment. Exit mean and root mean square velocity profiles of the steady flow are in good agreement with measurements obtained for all injectors at multiple axial locations. The simulation of the ignition transient phase well captures global events such as the propagation of the flame front from one injector to its neighbors and the related mechanisms. Improvement is however still needed to recover the proper ignition time of the whole burner.

Abstract In the present study, mixed-mode combustion of an E30 fuel in a direct-injection spark-ignition engine is numerically investigated at a fuel-lean operating condition using multidimensional computational fluid dynamics (CFD). A fuel surrogate matching Research Octane Number (RON) and Motor Octane Number (MON) of E30 is first developed using neural network based non-linear regression model. To enable efficient 3D engine simulations, a 164-species skeletal reaction mechanism incorporating NOx chemistry is reduced from a detailed chemical kinetic model. A hybrid approach that incorporates the G-equation model for tracking turbulent flame front, and the multi-zone well-stirred reactor model for predicting auto-ignition in the end gas, is employed to account for turbulent combustion interactions in the engine cylinder. Predicted in-cylinder pressure and heat release rate traces agree well with experimental measurements. The proposed modelling approach also captures moderated cyclic variability. Two different types of combustion cycles, corresponding to purely deflagrative and mixed-mode combustion, are observed. In contrast to the purely deflagrative cycles, mixed-mode combustion cycles feature early flame propagation followed by end-gas auto-ignition, leading to two distinctive peaks in heat release rate traces. The positive correlation between mixed-mode combustion cycles and early flame propagation is well captured by simulations. With the validated numerical setup, effects of NOx chemistry on mixed-mode combustion predictions are investigated. NOx chemistry is found to promote auto-ignition through residual gas recirculation, while the deflagrative flame propagation phase remains largely unaffected. Local sensitivity analysis is then performed to understand effects of physical and chemical properties of the fuel, i.e., heat of evaporation (HoV) and laminar flame speed (SL). An increased HoV tends to suppress end-gas auto-ignition due to increased vaporization cooling, while the impact of HoV on flame propagation is insignificant. In contrast, an increased SL is found to significantly promote both flame propagation and auto-ignition. The promoting effect of SL on auto-ignition is not a direct chemical effect; it is rather caused by an advancement of the combustion phasing, which increases compression heating of the end gas.

Abstract This study numerically investigates the effects of radiation from both the soot and gas phase species on the soot kinetics in a quasi-steady state microgravity spherical acetylene-air diffusion flame by coupling the soot and gas phase chemistry with radiative heat transfer processes. The gas phase reaction is modeled by two step chemical kinetics. The soot reaction mechanism includes nucleation, surface growth, oxidation and coagulation steps. The radiation from both soot and the gas phase are calculated by employing a spherical harmonics (P-1 approximation) model. The local Planck mean absorption coefficients of the computed species are specified in the computations. Unlike normal gravity acetylene-air steady state jet diffusion flames in which the radiation from soot dominates gas radiation effects, it is found that for a microgravity diffusion flame, except at very early times, the radiation from the gas products dominates soot radiation. In the microgravity flame configuration, it is noted that the radiation heat loss fraction (defined as the ratio of the radiation heat loss rate to the chemical heat release rate) increases up to 80% shortly after ignition.

<div class="section abstract"><div class="htmlview paragraph">Ammonia (NH₃) is emerging as a potential fuel for longer range decarbonised heavy transport, predominantly due to favourable characteristics as an effective hydrogen carrier. This is despite generally unfavourable combustion and toxicity attributes, restricting end use to applications where robust health and safety protocols can always be upheld. In the currently reported work, a spark ignited thermodynamic single cylinder research engine was upgraded to include gaseous ammonia and hydrogen port injection fueling, with the aim of understanding maximum viable ammonia substitution ratios across the speed-load operating map. The work was conducted under stoichiometric conditions with the spark timing re-optimised for maximum brake torque at all stable logged sites. The experiments included industry standard measurements of combustion, performance and engine-out emissions. It was found possible to run the engine on pure ammonia at low engine speeds at low to moderate engine loads in a fully warmed up state. When progressively dropping down below this threshold load limit, an increasing amount of hydrogen co-fueling was required to avoid unstable combustion. All metrics of combustion, efficiency and emissions tend to improve when moving upwards from the threshold load line. A maximum net indicated efficiency of 40% was achieved at 1800rpm 16bar IMEPn, with efficiency tending to increase with speed and load. Furthermore, comparing spark ignition with active and passive jet ignition (with the former involving direct injection of hydrogen into the pre-chamber only and the main chamber port fueled with ammonia), at different loads it was found that active systems can significantly improve early burn phase and reduce engine-out NOx compared to passive jet ignition and SI. While both Jet ignition systems required supplementary hydrogen, it accounted for ~1% (active) of the total fuel energy at high loads increasing with reduction in engine load.</div></div>

In the development process of all gas turbines, the thermoacoustic behavior of the combustion system is of crucial importance for the overall performance in terms of lifetime, emissions, and operational flexibility. An efficient design process requires a methodology which is able to analyze this behavior in an early phase. This method is at the same time expected to give insight into the underlying thermoacoustic mechanisms of relevant acoustic modes. Based on an example case for an Alstom reheat combustor relying on the auto-ignition principle, the paper describes such a method for low frequency modes, which is based on unsteady reactive CFD, flame transfer function analysis and acoustic network modeling. Results are validated against full-scale engine measurements. Moreover, physical insight of the coupling mechanisms responsible for the dynamic response of the flame is given.

Bio-coke (BIC, highly densified biomass briquette), a newly developed biomass fuel as an alternative to coal coke which utilized in blast furnace, is employed in this study. This fuel is manufactured in highly compressed and moderate temperature conditions and has advantages in its versatility of biomass resources, high volumetric calorific value and high mechanical strength. Japanese knotweed is chosen as a biomass resource and is shaped into cylinder (48 mm in diameter and 85 mm in length). One of the most important characteristics of BIC is its high apparent density (1300 kg/m3; twice or more than that of an ordinary wood pellet). In the present study, combustion characteristics of a single BIC fuel in high temperature air flow (473–873 K, 550–750 NL/min.) are investigated. Air is preheated and blown to the bottom surface of the BIC. Ignition and subsequent combustion behavior are observed with monitoring gas temperature near the BIC, surface and inside the BIC temperature, and time dependent mass loss of the BIC is measured. In the case with low air temperature, low heat flux from the fuel surface leads to the broad temperature distribution inside the BIC accompanied by the increase in ignition delay time and, then, once ignition takes place degradation rate becomes larger than the case with high temperature air. On the other hand, mass loss rate for the case of solid surface combustion in the high temperature air does not depend on the air temperature but does depend on the air flow rate, which is a result of reduced degradation rate relating to narrow temperature distribution in depth caused by short ignition delay time. Consequently, it is suggested that the history of preheating, i.e. the preheated condition which is determined by air temperature and air flow rate, is an essential factor to determine the ignition mode in the early stage of BIC combustion and the mass burning velocity in the period of main combustion with flame. It is found that the mass loss rate of BIC in the gas-phase combustion period increases with decrease in supplied air temperature in this study.

<div class="section abstract"><div class="htmlview paragraph">It is well known that ethanol can be used in spark-ignition (SI) engines as a pure fuel or blended with gasoline. High enthalpy of vaporization of alcohols can affect air-fuel mixture formation prior to ignition and may form thicker liquid films around the intake valves, on the cylinder wall and piston crown. These liquid films can result in mixture non-homogeneities inside the combustion chamber and hence strongly influence the cyclic variability of early combustion stages. Starting from these considerations, the paper reports an experimental study of the initial phases of the combustion process in a single cylinder SI engine fueled with commercial gasoline and anhydrous ethanol, as well as their blend (50%_vol alcohol). The engine was optically accessible and equipped with the cylinder head of a commercial power unit for two-wheel applications, with the same geometrical specifications (bore, stroke, compression ratio). Ultra-violet (UV) natural emission spectroscopy measurements ranging from 250nm to 470nm wavelength and simultaneous thermodynamic analysis were used to better understand the effect of ethanol content on flame kernel inception and development. All experiments were conducted at wide open throttle (WOT), with stoichiometric air-fuel mixtures, fixing the engine speed at 2000rpm. Optical investigations allowed to follow the evolution of chemical species that marked the spark discharge (cyano CN and hydroxyl OH radicals) as well as flame front initial growth (OH and carbyne CH radicals). Vibrational and rotational temperatures were calculated during the arc and glow phase by the ratio between the emission intensity of CN and OH radicals. Results were compared with adiabatic flame temperature traces obtained by applying a two zone model.</div></div>