Ignition Of Fuels Research Articles

Fuel Ignition in HTP Hybrid Rockets at Very Low Mass Fluxes: Challenges and Pulsed Preheating Techniques Using Palladium-Coated Catalysts

In a worldwide scenario which sees an increasing number of small satellite launches, novel mission concepts may be unlocked providing the spacecrafts with the very precise and rapid maneuvering capability that electric thrusters cannot guarantee. In this context, chemical thrusters appear to be a possible solution. This work aimed to experimentally study and solve the problem of ignition for 10 N hybrid rockets based on hydrogen peroxide. Firstly, the study analyzed the performance of a monopropellant engine capable of functioning as a hybrid injection system. In particular, the effects of the liquid mass injected, the initial temperature, and the supply pressure on the pulsed engine performance were experimentally investigated. The injected mass showed a greater impact on the performance with respect to the starting chamber temperature and injection pressure. This thruster also showed a good potential for space applications. In the second part of the work, the objective was to find an ignition procedure that reduced propellant consumption and eliminated the need for a glow plug. This is important because the electrical power consumption in real applications significantly affects other subsystems and is undesirable for chemical engines. Different ignition procedures were tested to emphasize their respective advantages and disadvantages, and the findings indicated that the concept of pulsed preheating is feasible with only a small amount of propellant consumption, while substantially decreasing the ignition duration from approximately 45 min to a maximum of just 3 min. Finally, similar ignition procedures were adopted using different fuels. The results showed that PVC and ABS, under the same operating conditions, ignite more easily than HDPE, which requires an oxidizer consumption approximately double that of the other two fuels. Considerations about the effect of chamber pressure and oxidizer mass flow rate on engine ignition were also included.

Physiochemical View of Fuel Jet Impingement and Ignition Upon Contact with a Cylindrical Hot Surface

This study delves into ignition and flame dynamics involving a cylindrical hot surface impact. Previous studies have focused on the flat-wall hot surface interacting with fuel spray, leaving gaps in understanding the effects of cylindrical hot surfaces on fuel-air mixing and ignition. Using high-fidelity large-eddy simulations (LES), this study investigates how fluid elements, upon contacting an electronically activated glow plug structure, exhibit mixing and thermochemical properties. The analysis examines how this type of structure enhances fuel-air mixing and subsequently influences the thermochemistry behavior in conjunction with the fuel-specific combustion behavior. The study includes scenarios with free spray and non-thermal deposit cases to assess their mixing impact, alongside testing five different electric voltage inputs to study the thermally assisted ignition process. Results demonstrate that the cylindrical structure hinders flow, reducing its inertia and increasing flow residence time. Moreover, a significant Coandă effect due to the circular wall structure is identified, potentially serving as a mechanism for enhancing flame-holding. Furthermore, varying the input voltage notably affects ignition timing, revealing a non-monotonic ignition delay pattern with lower voltages. Detailed analysis highlights the critical role of negative temperature coefficient (NTC)-driven low-temperature chemistry (LTC) in the ignition process.

Detection of Nickel Atoms Released from Electrodes in Spark Discharges Using Laser-Induced Fluorescence.

The reduction of greenhouse gas emissions and the effort of carbon neutrality require the improvement of spark-ignition engines in terms of efficiency and capability to operate on renewable fuels. The electrode wear of spark plugs, used for ignition of novel fuels and lean mixtures, emerges as a significant challenge in this transition. Understanding the physical mechanism and influence of spark operation parameters of the wear process is thus important. Compared to the conventional methodology of performing long-term wear tests, laser-based optical diagnostics methods are capable of assessing electrode wear during one single or a few spark discharges. In this work, the necessary initial steps required for performing optical investigations using laser-induced fluorescence (LIF) are presented. Several excitation pathways of nickel atoms were investigated, and 336.96 nm was identified as the optimal one. This excitation approach yielded emissions between 338.75 and 353.58 nm, effectively avoiding the major interference from N2 plasma emission in spark discharges. Additionally, a linear relationship in fluorescence signal intensity with excitation energy up to 400 µJ was observed. These findings indicate the potential of LIF for in situ diagnostics of electrode wear, contributing to engine development in both efficiency and compatibility with sustainable fuels.

Research of the processes of ignition and combustion of mixed fuels based on coal and wood under different conditions of thermal influence

The ignition processes of mixed fuels formed on the basis of grade 3B coal from the Maikuben deposit and finely dispersed sawmill and woodworking waste have been studied. An analysis of the processes of ignition and combustion of fuel mixtures with different organization of combustion processes was performed. It has been established that the concentration of the wood component has a significant impact on the processes of the onset of oxidation and combustion of mixed fuels. When the proportion of wood component in the mixture increases to 50%, the ignition delay time decreases by an average of 18% in all cases of the studies performed.

Forest structural complexity and ignition pattern influence simulated prescribed fire effects

BackgroundForest structural characteristics, the burning environment, and the choice of ignition pattern each influence prescribed fire behaviors and resulting fire effects; however, few studies examine the influences and interactions of these factors. Understanding how interactions among these drivers can influence prescribed fire behavior and effects is crucial for executing prescribed fires that can safely and effectively meet management objectives. To analyze the interactions between the fuels complex and ignition patterns, we used FIRETEC, a three-dimensional computational fluid dynamics fire behavior model, to simulate fire behavior and effects across a range of horizontal and vertical forest structural complexities. For each forest structure, we then simulated three different prescribed fires each with a unique ignition pattern: strip-head, dot, and alternating dot.ResultsForest structural complexity and ignition pattern affected the proportions of simulated crown scorch, consumption, and damage for prescribed fires in a dry, fire-prone ecosystem. Prescribed fires in forests with complex canopy structures resulted in increased crown consumption, scorch, and damage compared to less spatially complex forests. The choice of using a strip-head ignition pattern over either a dot or alternating-dot pattern increased the degree of crown foliage scorched and damaged, though did not affect the proportion of crown consumed. We found no evidence of an interaction between forest structural complexity and ignition pattern on canopy fuel consumption, scorch, or damage.ConclusionsWe found that forest structure and ignition pattern, two powerful drivers of fire behavior that forest managers can readily account for or even manipulate, can be leveraged to influence fire behavior and the resultant fire effects of prescribed fire. These simulation findings have critical implications for how managers can plan and perform forest thinning and prescribed burn treatments to meet risk management or ecological objectives.

Fuel Ignition Delay Maps for Molecularly Controlled Combustion

Fuel Ignition Delay Maps for Molecularly Controlled Combustion

Characterization of the hypergolic ignition of green fuels with hydrogen peroxide by drop test and jet impingement

This work presents an experimental study on the hypergolic behavior of variants of N,N,N’,N’-tetramethylethylenediamine (TMEDA)/N,N-dimethylethanolamine (DMEA) system, named PAHyp 0, with 93 wt% hydrogen peroxide. Two different copper-based catalyst and one cobalt-based catalytic agent were dissolved in the fuels to trigger the pre-ignition reactions. When employing copper chloride dihydrate, the resultant fuel requires methanol (ternary system) as a stabilizing agent, whereas both carboxylic salts of copper and cobalt dissolved in TMEDA/DMEA deliver a stable formulation (binary system) without the need for any organic solvent. Shadowgraph high-speed imaging was used to capture the ignition process and pressure wave development of over 30 fuel samples in a modified drop test. An ignition/stability envelope was proposed to map the safe and unstable formulations. It was demonstrated that the concentration of TMEDA should be no more than 50% in the binary system, while the amount of methanol should fall within a concentration window of 33 to 50% in the ternary system. Lower loadings of catalyst, ranging from 0.5 to 1.0 wt%, are required to deliver stable fast-igniting fuels. Three promising formulations catalyzed with 1 wt% copper salts were selected to perform impinging jet fire experiments. Impinging jets yielded significantly lower ignition delay times than drop tests due to the better mixing conditions. However, it was verified that ignition may fail for low values of jet velocities, below 0.8 m/s, and for high jet velocities, exceeding 14 m/s. An attempt to explain the ignition and non-ignition events was made by using the fundamentals of explosion limits of the hydrogen/oxygen system and oxidation reactions at low temperature. Remarkably, ultra-fast ignitions of about 10 ms were measured by the impinging jet setup within near-optimal operational conditions. Therefore, TMEDA/DMEA-based fuels with reduced catalyst loadings (0.5–1.0 wt%) offer new opportunities in aerospace propulsion and should be further studied.

Biogeographic patterns of daily wildfire spread and extremes across North America

IntroductionClimate change is predicted to increase the frequency of extreme single-day fire spread events, with major ecological and social implications. In contrast with well-documented spatio-temporal patterns of wildfire ignitions and perimeters, daily progression remains poorly understood across continental spatial scales, particularly for extreme single-day events (“blow ups”). Here, we characterize daily wildfire spread across North America, including occurrence of extreme single-day events, duration and seasonality of fire and extremes, and ecoregional climatic niches of fire in terms of Actual Evapotranspiration (AET) and Climatic Water Deficit (CWD) annual climate normals.MethodsRemotely sensed daily progression of 9,636 wildfires ≥400 ha was used to characterize ecoregional patterns of fire growth, extreme single-day events, duration, and seasonality. To explore occurrence, extent, and impacts of single-day extremes among ecoregions, we considered complementary ecoregional and continental extreme thresholds (Ecoregional or Continental Mean Daily Area Burned + 2SD). Ecoregional spread rates were regressed against AET and CWD to explore climatic influence on spread.ResultsWe found three-fold differences in mean Daily Area Burned among 10 North American ecoregions, ranging from 260 ha day−1 in the Marine West Coast Forests to 751 ha day−1 in Mediterranean California. Ecoregional extreme thresholds ranged from 3,829 ha day−1 to 16,626 ha day−1, relative to a continental threshold of 7,173 ha day−1. The ~3% of events classified as extreme cumulatively account for 16–55% of total area burned among ecoregions. We observed four-fold differences in mean fire duration, ranging from 2.7 days in the Great Plains to 10.5 days in Northwestern Forested Mountains. Regions with shorter fire durations also had greater daily area burned, suggesting a paradigm of fast-growing short-duration fires in some regions and slow-growing long-duration fires elsewhere. CWD had a weak positive relationship with spread rate and extreme thresholds, and there was no pattern for AET.DiscussionRegions with shorter fire durations had greater daily area burned, suggesting a paradigm of fast-growing short-duration fires in some regions and slow-growing long-duration fires elsewhere. Although climatic conditions can set the stage for ignition and influence vegetation and fuels, finer-scale mechanisms likely drive variation in daily spread. Daily fire progression offers valuable insights into the regional and seasonal distributions of extreme single-day spread events, and how these events shape net fire effects.

Study on Hydrocarbon Fuel Ignition Characterization Based on Optimized BP Neural Network

The investigation of the ignition delay of hydrocarbon fuel is highly valuable for enhancing combustion efficiency, optimizing fuel thermal efficiency, and mitigating pollutant emissions. This paper has developed a BP-MRPSO neural network model for studying hydrocarbon fuel ignition and clarified the novelty of this model compared to the traditional BP and ANN models from the literature. The model integrates the particle swarm optimization (PSO) algorithm with MapReduce-based parallel processing technology. This integration improves the prediction accuracy and processing efficiency of the model. Compared to the traditional BP model, the BP-MRPSO model can increase the average correlation coefficient, from 0.9745 to 0.9896. The R2 value for predicting fire characteristics using this model can exceed 90%. Meanwhile, when the two hidden layers of both the BP and BP-MRPSO models consist of 9 and 8 neurons, respectively, the accuracy of the BP-MRPSO model is increased by 38.89% compared to the BP model. This proved that the new BP-MRPSO model has the capacity to handle large datasets while achieving great precision and efficiency. The findings could provide a new perspective for examining the properties of fuel ignition, which is expected to contribute to the development and assessment of aviation fuel ignition characteristics in the future.

Optical study on a heavy-duty natural gas dual-fuel engine applying POMDME as pilot fuel

In this study, a fully optically accessible single-cylinder research engine is the basis for the visualization and generation of extensive knowledge about the in-cylinder processes of mixture formation, ignition, and combustion of alternative fuels for the dual-fuel combustion process. POMDME substitutes the fossil pilot fuel as a drop-in, non-sooting alternative to widely eliminate the NOx-PM tradeoff. Furthermore, an optimized ignition behavior, increased degrees of freedom in combustion phasing, and the pilot’s energy content are expected. The flame luminosity transmitted via an optical piston was split in the optical path to record the natural flame luminosity simultaneously with an RGB high-speed camera. The second channel consisted of OH chemiluminescence recording, isolated by a bandpass filter via an intensified monochrome high-speed camera. To investigate the combustion process spectrally, spatially, and temporally resolved in more detail, selected operating points were re-recorded via a high-speed imaging spectrograph. POMDME is benchmarked against regular diesel oil, according to EN590. Synthetic natural gas is applied as the primary gaseous fuel. Experimental sweeps along the overall pilot’s energy content (2%, 5%, 10%), injection pressure (500–1600 bar), and start of energizing (5–55 CAD bFTDC) are carried out. The given conditions result in decreased liquid-penetration length between 25% and 30% for the oxygenate, larger for earlier SOE and higher dilution. The lift-off length is nearer the liquid penetration length, increasing for higher rail pressures. The light-based ignition delay for EN590 is enlarged by 0.8 CAD after adding methane, while the oxygenate is not visibly retarded. Without methane, the oxygenate preceded EN590 by 0.6 CAD. The temporal and spatial position and extent of premixed, diffusive, and OH*, change significantly. RCCI operation at practically relevant 18.4 bar IMEP is demonstrated, highlighting the influence of the start of energizing variation with 51% decreased burn duration in the first half of combustion.

Effects of nano-metal oxide additives on ignition and combustion properties of MICs-boron rich fuels

Boron has been considered a promising powdered metal fuel for enhancing composite propellants' energy output due to its high energy density. However, the high ignition temperature and low combustion efficiency limit the application of boron powder due to the high boiling point of the boron oxide layer. Much research is ongoing to overcome these shortcomings, and one potential approach is to introduce a small quantity of metal oxide additives to promote the reaction of boron. This study prepared boron-rich fuels with 10 wt% of eight nano-metal oxide additives by mechanical ball milling. The effect of metal oxides on the thermo-oxidation, ignition, and combustion properties of boron powder was comprehensively studied by the thermogravimetric analysis (TG), the electrically heated filament setup (T-jump), and the laser-induced combustion experiments. TG experiments at 5 K/min found that Bi2O3, MoO3, TiO2, Fe2O3, and CuO can promote thermo-oxidation of boron. Compared to pure boron, Tonset can be reduced from 569 °C to a minimum of 449 °C (B/Bi2O3). Infrared temperature measurement in T-jump tests showed that when heated by an electric heating wire at rates from 1000 K/s to 25000 K/s, the ignition temperatures of B/Bi2O3 are the lowest, even lower than the melting point of boron oxide. Ignition images and SEM for the products further showed that the high heating rate is beneficial to the rapid reaction of boron powder in the single-particle combustion state. Fuels (B/Bi2O3, B/MoO3, and B/CuO) were mixed with the oxidant AP and ignited by laser to study the combustion performance. The results showed that B/CuO/AP has the largest flame area, the highest BO2 characteristic spectral intensity, and the largest burn rate for powder lines. To combine the advantages of CuO and Bi2O3, binary metal oxide (CBO, mass ratio of 3:1) was prepared and the test results showed that CBO can very well improve both ignition and combustion properties of boron. Especially B/CBO/AP has the highest burn rate compared with all fuels containing other additives. It was found that multi-component metal-oxide additive can more synergistically improve the reaction characteristics of boron powder than unary additive. These findings contribute to the development of boron-rich fuels and their application in propellants.

A Pressure-Oscillation-Based RON Estimation Method for Spark Ignition Fuels beyond RON 100

Knock in spark ignition (SI) engines occurs when the air–fuel mixture in the combustion chamber ignites spontaneously ahead of the flame front, reducing combustion efficiency and possibly leading to engine damage if left unattended. The use of knock sensors to prevent it is common practice in modern engines. Another measure to mitigate knock is the use of higher-octane fuels. The American Society for Testing and Materials’ (ASTM) determination of the Research Octane Number (RON) and Motor Octane Number (MON) of spark ignition fuels has been based on measuring cylinder pressure rise at the onset of knock since its inception in the 1930s. This is achieved through a low-pass filtered pressure signal. Knock detection in contemporary engines, however, relies on measuring engine vibrations caused by high-frequency pressure oscillations during knock. The difference between conditions in which fuels are evaluated for their octane rating and the conditions that generate a knock intensity signal from the knock sensor suggests a potential difference between octane rating and the knock limit typically identified by a contemporary knock sensor. To address this disparity, a modified RON measurement method has been developed, incorporating pressure oscillation measurements. This test method addresses the historical lack of correlation between RON and high-frequency pressure oscillation intensity during knock. Using toluene standardization fuels (TSFs) as a reference, the obtained results demonstrate excellent high-frequency knock intensity-based RON estimations for gasoline. The method is able to differentiate between two fuels that share the same ASTM RON, associating them with a RON-like metric that is more aligned with their performance in a modern SI engine. This alternative method could potentially serve as a template for an upgrade to the existing ASTM RON method without significantly disrupting the current approach. Additionally, its capability to evaluate fuels beyond RON 100 opens the door to assessing a wider range of fuels for antiknock properties and the intensity of fuel oscillations during knocking combustion.

Combustion Behavior of Fuel Droplets with Metallic, Non-Metallic and Organo-Metallic Boron Additives

ABSTRACT There is considerable interest in the utilization of fuels derived from boron materials, with their high calorific value, in various applications ranging from propellants to pyrotechnics. Conversely, their impact on the combustion behavior of conventional hydrocarbon fuels remains largely unclear. In this study, ignition, combustion, micro-explosion and simultaneous atomization behaviors of gasoline-based alternative fuels containing metallic (28–35 µm MgB2), nonmetallic (1 µm amorphous boron, 10 µm AlB12 with 86–88% and 95–97% purity) and organo-metallic boron derivatives (triethyl borate (TEB) containing C2H5 groups and trimethyl borate (TMB) containing CH3 groups) were investigated. The experiments were carried out at droplet scale and recorded using a high-speed camera and a thermal camera. The findings revealed a systematic reduction in ignition delay for each gasoline fuel enriched with boron derivatives. 2.5%AlB12/G and pure TMB droplets were the fastest extincting fuel droplets (0.9678 s and 1.245 s, respectively). The highest maximum flame temperature was recorded as 626 K and 610 K for pure TMB and 80%TEB/G, respectively. Droplet diameter regression analyses showed that the diameters of fuel droplets containing predominantly metallic and nonmetallic boron derivatives decreased in accordance with the d 2 -law. This study demonstrated that cost-effective and easily producible amorphous boron and organometallic boron derivatives are promising energy carriers for hydrocarbon fuels.

Numerical investigation on pyrolysis and ignition of ammonia/coal blends during co-firing

Co-firing ammonia with coal is a promising and feasible technology for reducing coal-related carbon emissions. Pyrolysis and ignition of ammonia-coal blended fuels are the key steps for flame stability and boiler operation safety throughout the conversion process but remain unclear. In this work, an extended Euler–Lagrange framework coupled to detailed solid-phase pyrolysis kinetics and gas-phase reactions mechanism is introduced and validated against experimental results for ammonia-coal co-firing in a two-stage flat flame burner. First, the CRECK-S coal pyrolysis model was validated with TGA experiments and the CPD model at different heating rates to determine parameters for a competing two-step model for CFD simulations. Second, a detailed gas-phase mechanism with 116 species and 1513 elementary reactions was derived from the volatile and ammonia reaction mechanisms. Thirdly, the co-firing simulations were validated for the ignition delay time for various ammonia co-firing ratios. The results show that increasing the co-firing ratios from 0.0 to 1.0 results in gradually increasing ignition delay times in a low-oxygen atmosphere. Further analysis demonstrates that adding ammonia accelerates the coal particle heating rate due to the reduced coal particle number density and ammonia reaction induced gas-phase temperature increase, and thus the coal devolatilization rate is increased. The latter plays a dominant role. Hydrogen produced from ammonia pyrolysis is negligible, so ammonia predominantly participates in the ignition. Time scale analysis shows that homogeneous ignition is dominant during the ignition process. The presence of ammonia inhibits the inward oxygen diffusion and causes the reaction zone to move away from the pulverized coal particle flow. The oxygen diffusion inhibition and low reactivity of ammonia compared to volatiles lead to an increase in ignition delay time for ammonia co-firing, even though pulverized coal devolatilization is accelerated.

A Raman spectroscopy based chemometric approach to predict the derived cetane number of hydrocarbon jet fuels and their mixtures

Fuel ignition quality, measured in the form of Derived Cetane Number (DCN), is an important part of integrating fuels, including sustainable aviation fuels, in compression ignition engines. DCN has been correlated with simulated and/or real spectroscopic measurements as well as other physical and chemical properties, but rarely have these correlations developed into a pathway to application. One application of the correlations is the use of miniaturized onboard fuel sensors that could assist, by using predicted DCN, in real-time feedforward engine control. To aid in the application of developing such DCN fuel sensors, Raman spectra coupled with chemometrics and a selection of influential spectral features were investigated. In this study, the Raman spectra were obtained from a database that included jet fuels, jet fuel mixtures, pure hydrocarbon components, and their weighted mixtures. The resulting Raman spectral database from the experimental measurements included spectra of components that span a wide range of DCNs and covered all the expected chemical functional groups present in a standard jet fuel. Chemometric models were developed to associate Raman spectra with DCN in subsets of the spectral range to aid in sensor miniaturization. The models were tested on jet fuels such as National Jet Fuel Combustion Program fuels designated A-1, A-2, and A-3 along with mixtures of jet fuels that spanned a wide range of DCN, simulating fuels that could represent real-world scenarios. An Artificial Neural Network (ANN) model trained on the fingerprint region (500 cm−1 – 1800 cm−1) of the Raman spectra was able to capture the non-linearity of the association between the Raman spectra and DCN with a test R2 score of 0.926, a test MSE of 3.61, and a test MPE of 3.41. Around 97 % of the unseen test samples were predicted within 10 % of the DCN measured with an Ignition Quality Tester. One hundred features of the fingerprint region influencing DCN predictions in the optimal ANN model were extracted using a Global Surrogate (GS) model. A reduced ANN model trained on only these one hundred features performed slightly better with a test R2 score of 0.935, test MSE of 3.19, test MPE of 3.20 and with the entire set of unseen test samples predicted within 10 % of the measured DCN. For assessing applicability of real-time and online DCN sensing, the Raman spectrometer was integrated with a flow cell capable of allowing measurements of DCN in flowing fuel samples and included the optimal ANN model of the fingerprint region and the 100-feature GS-ANN model on a Raspberry Pi computer. A number of unseen F-24/alcohol-to-jet fuel mixtures composed of unknown volumes were tested using the flow cell for DCN, and all of these samples were predicted within 10 % of the measured DCN.

Fuel ignition using remote generation of microwave plasma in air at atmospheric pressure

The high demand for a new ignition device for aeronautical engines has motivated the study on an innovative microwave approach, presented in this paper. A dedicated experimental set-up is presented in which we demonstrate the successful ignition of a kerosene spray by a remotely excited microwave plasma. This plasma is created in a Split Ring Resonator (SRR) gap, placed in a copper cavity, in air at atmospheric conditions at the frequency of f = 2.8467 GHz and a pulsed power injected into the cavity of Pcav ≈ 1500 W.

Ignition and thermal decomposition of solid organic fuel: The influence of activation, deactivation, and composite formation

The article discusses research on the thermal decomposition and ignition of coal, pine sawdust, and composite fuels derived from them. Through simultaneous thermal analysis, it was observed that the combustion process occurs in two primary stages. In the first stage, gaseous products are released and burned, while in the second stage, carbon produced in the first stage undergoes oxidation. The study also revealed the impact of mechanical activation and deactivation of ground fuel. Interestingly, the properties of coal and sawdust in the composite fuel formation are not simply additive. When the composite contains more carbon, there is a shift in the maximum decomposition rates towards higher temperatures in the first stage. Additionally, there is a decrease in decomposition magnitude during the first stage and an increase during the second stage. Moreover, it was found that the combustion temperatures of the composite fuel and mixture are higher than those of sawdust alone. Furthermore, the composite fuel exhibits superior burning characteristics as compared to the mixture. It ignites earlier, and the combustion zone shifts towards the center, resulting in enhanced fuel combustion efficiency.

Hot surface ignition of H2-air and CH4– H2-air mixtures for various equivalence ratios and heating Rates

Gaseous mixtures ignite when they come in contact with a surface with the ignition threshold temperature of the mixture. The hot surface ignition of fuels like hydrogen poses a safety threat due to its wide flammability range and high diffusivity. The present study numerically investigates hot surface ignition of H2-air and CH4/H2/air mixtures using detailed chemistry. The effects of the equivalence ratio (φ), composition and heating rate on the ignition threshold temperature of the mixture are studied. At rich mixture conditions, the ignition occurs at the side of the heated surface (glow plug). On the other hand, the lean mixtures ignite at the top of the heated surface. The variations in heating rates significantly affect the ignition temperature of rich mixtures. In contrast, the ignition temperature of the lean mixture does not show any considerable deviation concerning the variation in heating rates. The phenomenon of puffing flame is observed at lean extinction limits of H2-air mixtures. Lower ignition thresholds are observed in CH4/H2/air mixtures with high concentrations of H2. Mixtures having higher CH4 concentrations show higher ignition thresholds. The role of local equivalence ratio and heat release rate on the ignition characteristics has been studied to predict the behaviour of ignition kernel at various mixture conditions.

On the application of principal component transport for compression ignition of lean fuel/air mixtures under engine relevant conditions

Principal component transport-based data-driven reduced-order models (PC-transport ROM) are being increasingly adopted as a combustion model of turbulent reactive flows to mitigate the computational cost associated with incorporating detailed chemical kinetics. Previous studies were mainly limited to replicating relatively-simple chemistry in canonical configurations. The objective of the present study, therefore, is to further explore the accuracy of PC-transport ROM on more complex combustion phenomenon where, for example, large hydrocarbon fuel chemistry spanning a broad range of thermochemical space governs sequential multi-stage compression ignition processes. The cumulative error of PC-transport for this problem, and for others that depend upon sequential highly nonlinear physics, has to be minimal as the combustion phasing and heat release rate in internal combustion engines depends upon accurate predictions of minor ignition species whose concentrations start from ashes and grow orders of magnitude over the course of low- and high-temperature autoignition. Specifically, the PC-transport ROM is applied to predict the compression ignition characteristics of lean n-heptane/air and primary reference fuel (PRF)/air mixtures in a two-dimensional (2-D) constant volume computational domain initialized with a two-dimensional isotropic turbulence spectrum and temperature inhomogeneities. PCA is used to define the low-dimensional manifold that represents the original thermochemical state vector, and artificial neural network (ANN) models are adopted to tabulate chemical kinetics, transport, and thermodynamic properties. A series of 2-D pseudo-turbulent simulations are performed at engine pressures by varying the initial mean and r.m.s. of temperature, turbulence intensity, and the composition of fuel/air mixture. The results show that the PC-transport ROM accurately reproduces the instantaneous and statistical ignition characteristics of the fuel/air mixture, aided by pre-processing techniques including species subsetting, data clustering, and data transformation. It is found that PCs are not properly scaled with a power transformer if reactants are included in the species subset, which leads to a decrease in the accuracy of the PC-transport ROM. A separation of the reactants from the species subset ensures that the temporal evolution of the PCs starts from zero and spans orders of magnitude with time, and as such, this approach is found to effectively redistribute both PCs and their source terms with a power transformer. The computational speed-up factor of the PC-transport ROM ranges between 5.1 and 15.0 for the cases with n-heptane/air mixture and PRF/air mixture, respectively. Moreover, a potential further speed-up is anticipated through a combination of reduction in grid resolution requirements and in the stiffness of the chemical system. As an example, many of the pre-processing methods for inhomogeneous compression ignition may also apply to other complex intermittent combustion phenomena.Novelty and significance statement• The PCA-based reduced-order model (PC-transport ROM) has been applied to the multi-stage compression ignition of large hydrocarbon fuels under HCCI-relevant conditions. The present work presents a systematic procedure to accurately capture the two-stage ignition behavior of lean n-heptane/air or PRF50/air mixture.• The present work demonstrates an advantage of the PC-transport ROM in terms of computational speed-up. The computational speed-up factor for the ROM is up to 15, and moreover, a potential additional speed-up is anticipated through the reduction in the spatial and temporal resolution required.• A series of 2-D PC-transport ROMs are conducted to demonstrate the robustness of the ROM. A limitation of the ROM against different operating conditions is also discussed.

Understanding fundamental effects of biofuel structure on ignition and physical fuel properties

The development of biochemicals and biofuels with advantaged properties of higher combustion efficiency at lower emission levels can be aided by a priori prediction of critical global characteristics based on molecular structure. Existing predictive techniques are incomplete for comprehensive ignition and physical fuel property prediction. This investigation focuses on filling this knowledge gap by developing a priori prediction techniques for predicting important ignition as well as physical fuel properties of promising advantaged biofuels. We demonstrate the utility of this a priori prediction technique on saturated (≤C5) alcohols, both linear and branched in nature by correlating their molecular structure with both important ignition and physical properties of the fuels. This work utilizes a semi-automated method of predicting ignition characteristics via accurate fundamental-based calculations of the fuel radical cascades that govern low-temperature combustion. This numerical methodology is supported by the relevant fuel ignition delay time (IDT) and research octane number (RON) data experimentally measured by the Advanced Fuel Ignition Delay Analyzer (AFIDA). Phyiscal fuel property trends are also analyzed by correlating the molecular structure of the fuel to its physical characteristics. Based on their molecular structure, recommendations are made on an a priori method for selecting biofuels that have advantaged properties for the selected fuel application. Lastly, the utility of this investigation is extended beyond the subset of alcohols considered by analyzing the IDT trends of i-Pentanol and n-Hexanol, revealing the fidelity of the novel kinetic modelling approach and the accuracy of the predicted trends in this study.