Mass Fraction Burned Research Articles

Numerical Investigation of the Effect of Intake Closing Timing on Flow Field and Combustion Process in an Elliptical Rotary Engine

Abstract The aim of this research is to investigate the effect of intake closing timing (ICT) on the flow field and combustion process in elliptical rotary engines. The model that can accurately describe the working process of the elliptical rotary engine was established, five kinds of ICTs were designed, and the influence of ICT on the flow field and combustion process was studied. The results show that the advance of the ICT can increase the intake mass flowrate and reduce the back flowrate, the volumetric efficiency is 86.1% at a 145-deg crank angle (°CA) before top dead center (BTDC), which is 7.6% higher than 125 °CA BTDC. The advance of the ICT improves the consumption speed, makes the combustion reaction more intense, and shortens the combustion time. When the ICT is 145 °CA BTDC, the crank angle when the burned mass fraction is 90% (CA90) is 19.4 °CA earlier than 125 °CA BTDC, the peak mass of hydroxy in a cylinder is 41.6% higher, and the peak pressure in a cylinder is 25.9% higher. With the advance of the ICT, the pressure and heat release in the cylinder are significantly increased, the peak temperature in the cylinder is increased, the rate of carbon monoxide generation is accelerated, and the mass of nitrogen oxide emission is significantly increased. However, advancing the ICT cannot improve the indicated thermal efficiency of the elliptical rotary engine. This analysis provides a comprehensive understanding of the ICT of elliptical rotary engines.

A comprehensive combustion, performance, and environmental analyses of algae biofuel, hydrogen gas, and nano-sized particles (liquid-gas-solid mix) in agricultural CRDI engines

The present research encompasses the utilization of TiO2 nano particles with enrichment of H2 (10 lpm) with Chlorella Emersonii Methyl Ester (CEME) on an agricultural-based CRDI engine. CEME20 (20% chlorella emersonii methyl ester blended with 80% mineral diesel) is employed for the tests. The nanoparticle concentration is limited to 50 ppm which has been considered according to O2 concentration in biodiesel. Experimentation revealed that, with hydrogen gas and TiO2 nanoparticles, the BTE and BSFC were improved significantly. At full load conditions, BTE is enlarged by 7.3%. In addition, there is a simultaneous decrease in HC and CO emissions (by about 36.3 and 40.9% respectively) for algal biofuel blends, while NOx emissions increased by 27.3% with respect to mineral diesel. 5.1% and 6.5% increase in combustion characteristics (79.8 bar combustion pressure and 89.2 J/°CA heat release rate)were observed for the CEME/H2/TiO2 blend which could be attributed to effective combustion of hydrogen and catalytic combustion activity provided by TiO2 nano additives. CEME/H2/TiO2blend recorded maximum Mass Fraction burnt (MFB) (74.7%) as well as lowered PSD (Particle size Diameter) profile (34.8 nm @ 0.29 E+08 dN/dlogDp (#/cm3)) at full engine load condition indicating the presence of H2 and nano additives which has influence over the combustion.

A Novel Approach for Accurate Determination of Laminar Burning Velocity Using the Constant Volume Method in a Cylindrical Vessel with Full Optical Access

ABSTRACT The constant volume method (CVM) is a potential method for determining laminar burning velocity (LBV) at high pressures and temperatures. However, CVM using spherical vessels gives no optical access to the flame; in cylindrical vessels, with full optical access, the flame does not remain spherical. In either case, one has to rely on semi-analytical models, which have limitations in accuracy, to obtain burned gas volume (BGV) needed for LBV calculation. This study presents a novel approach for accurate determination of BGV and flame surface area (FSA) of non-spherical flames in cylindrical vessels from the measured end-view flame radius. Through CFD simulation, normalized relationships are developed to relate the measured flame end-view radius to the actual FSA and BGV. It is shown that these relationships are purely geometry-dependent and are independent of vessel size and combustion reactions in the vessel. This facilitates accurate determination of LBV using CVM in an optically accessible cylindrical vessel, eliminating the requirement to estimate burned gas mass fraction using burned gas mass fraction models. Experiments using CVM were performed for a stoichiometric methane-air mixture at 1.0 bar and 300 and 350 K. This new approach is demonstrated by using it to determine the LBV of stoichiometric methane-air mixture accurately. The present methodology is expected to provide a way to calculate accurate LBV without the need for additional thermodynamic models for burned gas mass fraction.

The Influence of Ozone on the Flame Propagation Behavior of n-Heptane/air and i-Octane/air Premixed Flame in an Engine-Relevant Environment

ABSTRACT The promotional effect of ozone on lean premixed spherical flame propagation in an engine-relevant, high-temperature, high-pressure environment was investigated using an optically accessible rapid compression and expansion machine (RCEM). The laminar burning velocity, burned gas Markstein length, and initial flame development time based on mass fraction burned (MFB), were examined for n-heptane/air and i-octane/air premixture (φ = 0.55) with 15 [ppm] ozone, through self-emission observation of a propagating flame with a high-speed camera and combustion pressure analysis. The results showed that ozone reduced the initial flame development time and increased the laminar burning velocity for n-heptane/air premixture under temperature conditions in which the low-temperature oxidation (LTO) reaction was more clearly pronounced; but ozone did not enhance either the initial flame development or the laminar burning velocity in the case of i-octane/air premixture, where the LTO reaction was weak owing to the fuel molecular structure, even under the same temperature conditions. These results suggest that the enhancement of the LTO reaction by ozone, and the subsequent generation of thermal energy and CO radicals through an H2O2 loop reaction in the pre-flame region, were responsible for the increase in the laminar burning velocity. Furthermore, ozone increased the burned gas Markstein length, flame thickness, and Zeldovich number of the n-heptane/air mixture under the temperature conditions in which the laminar burning velocity was enhanced by ozone. This suggests that the chemical reaction in the reaction zone was more active in response to the increase in flame temperature.

Investigating the influence of varying split injection profiles on stability of diesel engine operated under partially premixed mode with methanol

The present study explores the prospects of operational stability of dual fuel operation comprehensively, considering that; methanol and diesel are of two different kinds which exhibit different combustion processes. To this effect, investigating the stability of engine operation under such condition becomes important. In this investigation, the harshness of the operation and the non-repeatability of the combustion cycles are examined, which have been identified as the major stability indicator in many studies. Such stability indices are characterized in this study through a comprehensive set of parameters, including of maximum pressure rise rate (ROPRmax) and coefficient of variation of indicated mean effective pressure (COVIMEP); and of peak pressure (COVPP) and crank angle of 50% mass fraction burn (COVCA50). The experimental investigation is carried out under a split injection strategy by varying injection timings as well as injection mass percentages at predefined methanol injection durations. It is shown that the partially premixed mode under the split injection strategy exhibits significant potential in reducing the harshness of operation indicated by lower peak pressure rise and increasing the repeatability of the combustion cycles implied by the substantially lower scores of the considered parameters for mapping stability.

Study on the Effect of High-Concentration Oxygen Enrichment on Engine Performance and Exhaust Emissions Using Diesel Fuel and Palm Biodiesel Substitute Fuel

Air pollution remains a big issue in many countries. One form of air pollution comes from the use of fossil fuels as the primary fuel in the power generating and transportation sectors. Diesel engines are employed in a variety of industries due to their dependability, durability, and efficiency. Enhancing the availability of oxygen within the combustion chamber is one technique for reducing exhaust gas emissions and optimizing diesel engine combustion. The aim of this study is to investigate how oxygen enrichment in diesel engines with diesel fuel and biodiesel affects their performance and emissions. The modeling in this research was carried out using AVL BOOST version 2011 software based on experimental results of the YANMAR TF 155 R-DI diesel engine at 1200 rpm with and without oxygen enrichment. Modeling was performed based on the baseline parameter of a diesel engine with gradual loads at 50%, 75%, and 100%. The oxygen concentration was increased to 30.6%, 37.8%, 45%, and 54% by mass. The results show an increase in the maximum heat release rate (HRR) and the mass fraction burned (MFB) up to 90% for both fuels. The peak heat release rate of biodiesel shifts around 6 J/degree and the brake-specific fuel consumption (BSFC) is up to 0.0035 kg/kWh higher than that of diesel fuel. When compared to diesel fuel, the thermal efficiency and BSFC of biodiesel usage are around 0.3% and 0.028 kg/kWh, respectively. NOx emissions increase due to higher combustion temperatures and more oxygen availability. Biodiesel emits 50% less NOx than diesel fuel, presumably due to a lower combustion temperature. As a result, while high-concentration oxygen enrichment improves combustion and lowers soot emissions, it raises NOx emissions. Soot emissions were reduced as a result of the enhanced combustion process, while NOx emissions rose due to higher combustion temperatures and increased oxygen availability.

Early flame development characterization of ultra-lean hydrogen–air flames in an optical spark-ignition engine

Hydrogen-fueled internal combustion engines (H2ICEs) represent a promising technology for decarbonizing the transport sector. In pursuit of this goal, research is needed on lean hydrogen–air combustion under engine conditions aiming to mitigate NOx emissions. However, there is a limited amount of experimental data focusing on flame development at high pressure and temperature, especially in ultra-lean mixtures, which are recently considered to play an important role for future H2ICEs. This study investigates the behavior of very-lean to ultra-lean hydrogen–air flames (0.20 <ϕ< 0.55) in an optical spark-ignition engine, focusing on the early flame development phase. Fuel–air equivalence ratio and engine speed (900 <N< 1500 rpm) effects on hydrogen combustion are addressed with pressure-based heat release analysis and chemiluminescence flame imaging techniques. The findings revealed an intrinsic association between these two approaches. Hydrogen flames development measured before 2% of Mass Fraction Burned (MFB) strongly correlates with pressure-based quantities up to 50% MFB, underscoring the critical role of understanding the early flame phase for predicting total combustion duration and in-cylinder pressure traces. A model for hydrogen turbulent flames in the initial combustion stages is formulated based on the Zimont equation and the experimental data. An equation for 0D/1D combustion duration prediction up to 10% MFB is proposed, demonstrating its validity from the wrinkled flame to the well-stirred reactor premixed combustion regimes. Additionally, flame chemiluminescence images for each zone within the Peters–Borghi diagram are presented, offering valuable insights into the diverse characteristics of these flames.

Computational fluid dynamics and Machine learning-based Piston-Bowl optimization for Energy-Assisted compression ignition of low cetane number sustainable aviation fuel blends

Hybrid-electric powertrains have been gaining tremendous interest to propel unmanned aerial vehicles (UAVs) for Advanced Air Mobility. Present UAV powertrains are designed with internal combustion engines capable of combusting single fuel types, which have significant restrictions on fuel ignition quality. However, sustainable aviation fuels (SAFs) can exhibit wide variance in ignitability with cetane numbers (CNs) ranging from 15 to greater than 50. Coupled with high-speed and high-altitude engine operating conditions, low CN jet fuels can pose a serious challenge to achieving reliable, clean, and fuel-flexible ignition. Energy-Assisted Compression Ignition (EACI) has demonstrated superior ignition-controlling capability with jet fuels due to the precise thermal energy deposition achieved from the continuously operating high-temperature Ignition-Assistant (IA). However, the current literature highlights enhancement needs in the operational capabilities of EACI strategy with low CN jet fuels. Therefore, this paper aims to demonstrate improved EACI characteristics of low and varying ignition-quality SAF blends used in high-altitude aerial conditions through a combined computational fluid dynamic (CFD) and machine learning (ML)-based piston-bowl optimization methodology. In this work, a CFD-enabled Design-of-experiments approach, covering nine piston-bowl and three operational parameters, is used to develop a Gaussian Process Regression (GPR)-based ML modeling routine. Further, a merit-function-driven piston-bowl optimization is performed using the GPR model for improved EACI operations with CN 30 SAF blend. The combined CFD and GPR methodology helped screen more than 20 million piston designs to find the optimum. The study found that the piston optimization allowed consistent improvements in EACI operability for CN 30 to CN 48 jet fuels with maximum reductions in ignition delays of 17% and CA50 (50% mass-burn fraction crank angle) of 22% for CN 30 SAF blend compared to the stock piston. The optimized piston-bowl also helped reduce IA input power requirements by 20–28%, thus improving the reliability and capabilities of EACI to achieve multi-jet fuel operation.

Evaluation of engine performance and emissions in an optical DISI engine with various spark plug designs and gaps

This study aims to assess how spark-ignition engine performance and emissions are affected by both the design of the spark plug and the size of its electrode gap. Two distinct spark plug configurations (BUGAETS and NGK) with varying spark gap sizes of 1, 1.2, and 1.4 mm were used. Experiments were performed in an optical constant volume chamber as well as an optical direct injection spark ignition engine. At various equivalence ratios, the effects of both spark plugs on flame propagation, in-cylinder pressure, mass fraction burnt, and emissions for gasoline fuel were explored. For all spark plug gaps and equivalence ratios tested, the constant volume experimental results showed that the flame propagation area with the BUGAETS spark plug is faster than that with the NGK spark plug. According to the engine results, BUGAETS can produce an IMEP up to 2.4 % higher with a significant reduction in both cyclic variability and fuel consumption when compared to NGK. BUGAETS electrode consistently produces higher in-cylinder pressure and heat release rate than the NGK spark plug. When compared to NGK, the BUGAETS spark plug emits more NOx while emitting less hydrocarbon and particulate emissions. In terms of NGK electrode gaps, a comparison revealed that a gap size of 1.4 mm resulted in the most optimal engine performance and the lowest exhaust emissions. On the other hand, when it came to BUGAETS' spark plug, a gap size of 1.2 mm delivered the best engine performance and emissions out of all the tested gap sizes for the BUGAETS electrode.

Biodiesel from Biomass Waste Feedstock Prosopis Juliflora as a Fuel Substitute for Diesel and Enhancement of Its Usability in Diesel Engines Using Decanol

Biomass‐based biofuel production is a promising solution to the decline of fossil fuels. Prosopis juliflora seed‐derived vegetable oil, known as Prosopis juliflora methyl ester (JFME), offers a potential feedstock for biodiesel. To enhance its properties, the addition of Decanol is investigated, a higher‐order alcohol similar to Diesel. Experiments are conducted on a 5.2 kW compression ignition (CI) engine using JFME blended with different decanol concentrations (5%, 10%, 15%, and 20%). Fourier‐transform infrared spectroscopy and gas chromatography‐mass spectrometry analysis confirm its compliance with fuel standards. The findings reveal that the 20% decanol blend (D20) achieves a brake thermal efficiency of 29.9% at full load, with reduced NO, smoke, and hydrocarbon (HC) emissions compared to diesel. D20 shows NO emissions of 1265 ppm, smoke opacity of 53%, and HC emissions of 69 ppm, while diesel records 1320 ppm, 69%, and 75 ppm, respectively. The CO emissions for D20 are 0.359 vol%, slightly higher due to decanol's higher latent heat of evaporation. Moreover, D20 exhibits improved combustion with a higher mass fraction burnt and faster heat release rates. These results indicate the potential of using JFME blended with 20% decanol as an alternative fuel for CI engines, offering higher performance and reduced emissions.

Effects of Scenedesmus dimorphus, spirulina biodiesel, hydrogen and nanoparticles fuel blends on mass burn fraction, emission, noise and vibration characteristics

Severe environmental implications, such as those connected to greenhouse gas emissions and widespread pollution, have resulted from the unsustainable demand for fossil fuels. In order to find and implement technological, sustainable, and cost-effective alternatives to fossil fuel production and use, it is necessary to first identify the primary challenges originating from these activities. The significant potential for their production from various types of biomass has led to a rise in their use in recent years, and this has coincided with the advent of current technological breakthroughs in the field. Scenedesmus dimorphus The energy sector has a strong interest in the process of making biodiesel from and spirulina oil because of its potential to provide clean and sustainable energy.The combustion, emission, vibration and noise characteristics of Scenedesmus Dimorphus and spirulina based microalgae biodiesel were studied in this study, both with and without the addition of nanoparticles and hydrogen. Hydrogen and nanoparticles were employed at a rate of 5 L/min and 100 ppm respectively. Biodiesel blends B10 (5 % spirulina + 5 % Scenedesmus Dimorphus + 90 % diesel) and B20 (10 % spirulina + 10 % Scenedesmus Dimorphus + 90 % diesel) were used in this study. The test fuels prepared from biodiesel, hydrogen and nanoparticles has been compared with conventional diesel.The experiment was carried out on four stroke diesel engine at varying load conditions ranging from 0 % to 100 % at an interval of 25 %. Incorporation of nanoparticles and hydrogen on microalgae based biodiesel reduces the emission, vibration and noise generated by the engine.

Effects of turbulence–heat loss interactions on detonation development in end gas and its resulting knock intensity

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

Optimizing biodiesel blends with green hydrogen fuel: A study on combustion duration, fuel mass burnt, engine performance and emissions

Although the combustion of petroleum-based fuels currently satisfies the majority of our energy needs, rapid industrialization has spurred the utilization of alternative energy sources to meet the growing demand for electricity. With the global population on the rise and increasing awareness of the dangers of climate change, there is a greater need to find sustainable and eco-friendly fuels. Utilizing biodiesel derived from non-edible sources is the most efficient and appealing option for meeting energy needs while mitigating environmental impacts. This paper focuses on the preparation and experimental analysis of biodiesel from Karanja oil. Various blends of Karanja biodiesel and regular diesel were produced and stirred to create KB10 (Diesel 90% + Karanja Biodiesel 10%) and KB20 (Diesel 80 % + Karanja Biodiesel 20%). Hydrogen was incorporated into the biodiesel to enhance fuel characteristics, delivered at a rate of 5 L per minute. The emissions and combustion results of the biodiesel blends were compared to those of pure diesel. At full load condition, KB10H5 reported the minimum combustion duration of 25.8°CA with mass fraction burnt of 17.8%. However, the biodiesel blend with 10% and 20% reported the reduced BTE. Highest brake power has been observed at 4.95 kW. The combination of Karanja biodiesel and hydrogen improves fuel quality and reduces the harmful gases emissions such as carbon monoxide and carbon dioxide. It is crucial to ensure that the operation is both efficient and comfortable, and this approach achieved both objectives. In addition, hydrogen energy can also be applied to the railway transportation industry, as a new direction for the green and low-carbon development of intercity transportation, driving the virtuous cycle and innovative development of the hydrogen energy-related industrial chain.

Analysis of combustion cyclic variation in a lean burn spark-ignited engine using large eddy simulation

The low-temperature combustion technique improves engine efficiency by reducing the thermal and exhaust losses. The representative method of low-temperature combustion technique is lean burn. However, lean burn has been limited because it increases the cycle-to-cycle variation (CCV) owing to poor initial ignition stability. To overcome the low initial ignition stability, increasing the spark energy intensity and strengthening the tumble motion have been proposed. Therefore, lean burn engines with ignition stability are being rapidly developed recently. In this study, the result of the LES simulation of a lean burn engine showed the importance of ignition delay increased more than that of a conventional gasoline engine. With the importance of ignition delay in lean burn condition, the sources of the ignition delay were analyzed. The primary ignition delay sources were the flow velocity near the spark plug and the subgrid-scale turbulent kinetic energy. Next, the parameters affecting the 5% mass fraction burned (MFB) duration were analyzed by comparing five characteristic cycles. Also, factors that decreased the correlation coefficient were analyzed by comparing with the outlying cycle. As a result of analyzing the cycle with a significant change in MFB 5 and MFB 50 ranking, the burn duration was found to be affected by curl TKE, SGS TKE, large-scale flow motion, and air–fuel ratio. Finally, methods to reduce CCV were proposed. The first method was to increase tumble X+ and tumble X- using pistons that enhance the wall flow. Consequently, the coefficient of variance (COV) during the ignition delay was reduced by 27%. The second method involved changing the injector target to enhance tumble Y. The enhanced tumble Y reduced the COV of burn duration by 11%.

Estimation of Wiebe Function Parameters for Syngas and Anode Off-Gas Combustion in Spark-Ignition Engines

Abstract Wiebe functions, analytical equations that estimate the fuel mass fraction burned (MFB) during combustion, have been effective at describing spark-ignition (SI) engine combustion using gasoline fuels. This study explores if the same methodology can be extended for SI combustion with syngas, a gaseous fuel mixture composed of H2, CO, and CO2, and anode-off gas; the latter is an exhaust gas mixture emitted from the anode of a Solid Oxide Fuel Cell, containing H2, CO, H2O, and CO2. For this study, anode off-gas is treated as a syngas fuel diluted with CO2 and vaporized water. Combustion experiments were run on a single-cylinder, research engine using syngas and anode-off gas as fuels. One single Wiebe function and three double Wiebe functions were fitted and compared with the MFB profile calculated from the experimental data. It was determined that the SI combustion process of both the syngas and the anode-off gas could be estimated using a governing Wiebe function. While the detailed double Wiebe function had the highest accuracy, a reduced double Wiebe function is capable of achieving comparable accuracy. On the other hand, a single Wiebe function is not able to fully capture the combustion process of a SI engine using syngas and anode off-gas.

Determination of Unstretched Laminar Burning Velocity by Simultaneous Measurements of Flame Radius and Pressure-Time Trace Using Constant Volume Method

ABSTRACT Values of laminar burning velocity obtained by the constant volume method suffer from the inaccuracies introduced by the semi-analytical burned gas mass fraction model and flame stretch. A cylindrical combustion chamber with full optical access was developed in the present study, enabling the photography of the flame until the flame touches the inner wall of the combustion chamber. This enables direct visualization of the flame, thus eliminating the dependency on burned gas mass fraction models. The values of laminar burning velocity obtained by simultaneous measurement of flame radius and pressure-time data using the constant volume method are not stretch-free. This paper also develops a methodology for stretch correction based on the determined laminar burning velocity. Experiments were performed on mixtures of methane and air at initial pressures (0.8–1.05 bar), initial temperatures (293–320 K) and equivalence ratios (0.8–1.2). Stretch-corrected laminar burning velocity for lean, stoichiometric, and rich methane-air flames has been obtained in the present study, and the results obtained are validated against available experimental data and 1-D flame computations. It is expected that the proposed method significantly reduces the inaccuracies in laminar burning velocity obtained by the constant volume method.

Sustainable Biofuels from First Three Alcohol Families: A Critical Review

With its unique qualities, such as infinite supply, high octane number, and capacity to cut greenhouse gas emissions, alcohol is a viable alternative fuel for SI engines. This review article aims to reveal to readers the effects of alcohol on the performance, combustion behavior, and emission characteristics of SI engines by collecting the outcomes from previous research. This article looks at methanol, ethanol, and butanol fuel qualities. The performance of SI engines with butanol, ethanol, and methanol combined with gasoline is investigated in terms of brake torque, brake power, fuel consumption, thermal efficiency, volumetric efficiency, mean effective pressure, and coefficient of variation under various conditions. Second, in-cylinder pressure, mass fraction burnt, ignition delay, pressure increases, and heat release rates are also used to evaluate the combustion characteristic. Finally, the article discusses pollutant emissions such as CO, CO2, NOx, UHC, and exhaust gas temperature. Methanol, ethanol, and butanol mixed with gasoline increased fuel consumption and lowered spark-ignition engines’ thermal efficiency. When alcohol was combined with gasoline, most research found that CO, NOx, and UHC emissions were reduced due to improved combustion.

Ignition behaviors of primary reference fuels in a rapid compression machine under vortex-existing/minimized conditions

Knock is one of the main obstacles to improving the thermal efficiency of spark-ignition internal combustion engines. Although knock has been widely studied and accepted as a result of end-gas auto-ignition, the fundamentals regarding auto-ignition behaviors are still not fully revealed. In this study, the ignition behaviors of primary reference fuels were investigated in an optical rapid compression machine equipped with a quartz combustion chamber allowing for visualizing the combustion process from the lateral view. By combining both the lateral view and the top view photography, ignition behaviors under vortex-existing conditions with a creviced piston and vortex-minimized conditions with a flat piston were comprehensively analyzed to reveal the impact of the vortex on the ignition behaviors of PRF fuels. The influence of fuel reactivity was also investigated. The results showed that mild ignition was prevalent under large Da* numbers. The occurrence of mild ignition was closely related to the ignition delay time of the mixture, and the critical ignition delay time was not fixed but decreased with increasing initial temperature. The propensity of mild ignition could be boosted under vortex-existing conditions due to the increasing hotspot formation probability. Vortices were demonstrated to be capable of mitigating knock intensity via 1) the buffer effect of surrounding burned regions on the shock waves generated from surrounded unburnt pockets; 2) a larger burned mass fraction at the instant of the final ignition under vortex-existing conditions. The results also showed that strong ignition was more likely to occur under vortex-minimized conditions. Besides, higher fuel reactivity also could increase the probability of strong ignition occurrence. Compared with the creviced piston, the use of the flat piston could shift the ignition regime towards regions with higher Ret and lower Dat in the Dat-Ret diagram, where the strong ignition is less pronounced.

Large-eddy simulation of a natural gas direct injection spark ignition engine with different injection timings

This paper presents a large-eddy simulation (LES) study of a compressed natural gas (CNG) direct injection spark ignition (DISI) engine with different injection timings. Three cases with late, moderate and advanced injection timings are investigated to examine fuel/air mixing and combustion characteristics of a CNG DISI engine. A transition flame speed approach is used in which the flame propagates like a laminar flame until it reaches a specified kernel size and then develops as a turbulent flame. Changing the transition kernel size significantly affects both the in-cylinder pressure trace and the mass fraction burned (MFB) profile, highlighting the importance of correctly capturing the early stage of flame development. Different injection timings result in varying levels of mixing and homogeneity within the cylinder. Although more time is available for mixing in the most advanced injection case, the jet impingement on the piston crown increases the level of mixture in-homogeneity. It is shown that the variation of the laminar flame speed as a result of mixture in-homogeneity is the primary factor affecting flame propagation for different injection timings.

Clean smoldering biowaste process: Effect of burning direction on smoke purification by self-sustained flame

Smoldering is an attractive biowaste removal technology, but its toxic pollutions are significant concerns. This work explores smoldering biowaste removal with the emissions purified by a self-sustained flame and the effect of smoldering direction. The smoldering direction and intensity are controlled by the ignition position and upward internal airflow velocity (5–24 mm/s). We found that the flame can be sustained above the smoldering fronts to purify the smoldering emissions in different burning directions, and the flame duration and burnt mass fraction are much larger for the opposed smoldering. To sustain a flame above an opposed smoldering front, the minimum airflow velocity is 6 mm/s, and the minimum smoldering burning flux is 10 ± 1 g/m2∙s. Comparatively, sustaining a flame above a forward smoldering requires a larger airflow velocity (~11 mm/s) and smoldering intensity (>16 g/m2∙s). Therefore, the flame is easier to remove more emissions from opposed smoldering which is recommended for the waste removal process. The critical smoldering burning flux at flame extinction is constant at 9 ± 1 g/m2∙s, regardless of the smoldering direction and airflow velocity. This work deepens the understanding of the co-existence of smoldering and flaming and enriches strategies for environmentally friendly smoldering waste removal technologies.