Combustion Period Research Articles

Surviving on the edge: the role of medieval and modern age charcoal production in the stand composition of colline beech woodlands in NE Hungary (EC Europe)

Abstract This study examines historical forest compositional changes in the Aggtelek Karst region, Hungary, focusing on the relationship between medieval and modern age charcoal burning sites (32 kilns) and the current vegetation. Our aim was to determine woodland composition during the intensive charcoal burning period using wood charcoal analysis and compare it with the present canopy composition to understand Fagus sylvatica (European beech) population dynamics at the margin of its range. Heat map visualization, cluster analysis and Procrustes analyses were used for comparison. The results show European beech as dominant taxon in charcoal spectra in some places, while Quercus, possibly Quercus petraea (sessile oak), and Carpinus betulus (European hornbeam) dominated in others. The findings suggest that beech had varying abundance in the potential beech habitats during the medieval and modern age coppice phases. We found multidirectional changes in canopy composition. Carpinus betulus and Q. petraea dominated the current stands in most cases with F. sylvatica being subordinate. These results suggest that historical forest use and management practices took place in already mixed canopy forests, where Fagus dominance was confined to a few stands only. We demonstrate that the forest stands plausibly represent different stages of secondary succession after last coppice management, leading to varying Carpinus abundance. We discuss the possible role of climate change (summer heat waves, increasing mean annual temperature) and conclude that F. sylvatica and Q. petraea compete with each other in the study region, confirming a transitional zone between the classical Fagus and Quercus climate.

Burn Period: A Use-Inspired Metric to Track Wildfire Risk across Arizona and New Mexico in the Southwest United States

Abstract Numerous tools and indices exist for wildland fire managers to anticipate and track changes in wildfire risk driven by variability in weather and climate conditions at hourly to seasonal scales. However, in working closely with Southwest U.S. managers, we learned of a simple meteorological metric being informally used, but not widely accessible in existing tools or information products, to gauge short-term changes in wildfire risk. This metric, termed “Burn Period” (BP), is the local count of hours per day with relative humidity values equal to or less than 20%. Our collaboration led to the development of an experimental tool called the “Burn Period Tracker” to ease access and promote the use of BP values for planning and response. This study is a climatological analysis of BP values at 124 fire weather stations across Arizona and New Mexico for the period 2000–22 to aid in interpretation and understanding of this use-inspired metric. BP values reflect the strong seasonality in temperature and moisture deficit–driven wildfire risk across the Southwest United States, with risk climbing through the arid spring season, peaking in June, and then falling rapidly with the onset of the summer monsoon in July. Regression analyses show that short-term variability in BP values is driven by variability in low-level atmospheric moisture in all months with strongest relationships during the summer after the onset of the monsoon. This study highlights the utility of BP as a short-term wildfire planning tool as well as an example of collaborative weather and climate services development.

A study on optical diagnostics and numerical simulation of dual fuel combustion using ammonia and n-heptane

As a zero-carbon fuel, ammonia is considered an ideal alternative for reducing carbon footprints. However, the use of pure ammonia is limited by its poor fuel properties, which can be mitigated by employing a dual fuel approach using high reactivity fuel and ammonia. Nonetheless, the evolution mechanisms of nitrogen oxides (NOx) and unburned ammonia (NH3) remain unclear. In this study, n-heptane was selected as the high-reactivity fuel. The characteristics of the dual fuel approach and the evolution mechanisms of NOx and unburned NH3 were investigated using optical diagnostics and numerical simulations, respectively. The results indicate that the combustion characteristics can be improved by adjusting the direct injection strategy. Unconsumed NH3 remains in the low-temperature regions of the late period of combustion, resulting in unburned NH3 emissions. The consumption of unburned NH3 is governed by the reaction NH3 + OH <=> H2O + NH2. The distribution of N2O is primarily concentrated in low-temperature regions. The generation of N2O is controlled by the reaction NH + NO <=> H + N2O. The improvement of the in-cylinder temperature can suppress the generation of N2O and promote the consumption of NH3, resulting in reduced emissions of both N2O and unburned NH3.

Modeling the impacts of open biomass burning on regional O3 and PM2.5 in Southeast Asia considering light absorption and photochemical bleaching of Brown carbon

Open biomass burning in Southeast Asia has significant adverse impacts on air quality in the region and in downwind areas. These biomass burning events emit large amounts of light absorbing brown carbon (BrC). Once in the atmosphere, the light absorbing capacity of BrC is reduced by various oxidation processes. However, few modeling studies have been conducted to explicitly examine light absorption and bleaching on the prediction of ozone (O3) and fine particulate matter (PM2.5). In this study, a modified Community Multiscale Air Quality (CMAQ) model that explicitly tracks the concentrations of light absorbing and non-light absorbing organic aerosol components from different emission sources and the bleaching of BrC due to photooxidation and OH oxidation is applied to widespread open biomass burning events in March 2018 in Southeast Asia. Open biomass burning accounts for as much as 20–40 ppb (30–50%) of the maximum daily average 8-h ozone (MDA8 O3) and 40–120 μg m−3 (60–90%) of the daily average PM2.5 in the emission source regions. Compared to a simulation without BrC light absorption, the predicted MDA8 O3 and PM2.5 are as much as 16 ppb and 16 μg m−3 lower, respectively, than a simulation with light absorption. This confirms that neglecting the UV light absorption of BrC can lead to significant overpredictions of O3 and PM2.5 during the open biomass burning periods, which may lead to an overestimation of the adverse impacts of biomass burning on public health in Southeast Asia. The addition of BrC bleaching results in a 0.5–1% increase in MDA8 O3 and 1–5% increase in PM2.5 compared to the case without BrC bleaching. The results of this study indicate that light absorption by BrC needs to be considered in chemical transport modeling of large open biomass burning events. The BrC bleaching process is relatively slow and neglecting this process does not significantly change the predictions of MDA8 O3 and PM2.5 during open biomass burning.

A multi-site passive approach to studying the emissions and evolution of smoke from prescribed fires

Abstract. We conducted a 2-year study utilizing a network of fixed sites with sampling throughout an extended prescribed burning period to characterize the emissions and evolution of smoke from silvicultural prescribed burning at a military base in the southeastern USA. The measurement approach and an assessment of the instrument performance are described. Smoke sources, including those within and off the base, are identified, and plume ages are determined to quantify emissions and study the evolution of smoke PM2.5 (particulate matter with aerodynamic diameters 2.5 µm or smaller) mass, black carbon (BC), and brown carbon (BrC). Over the 2021 and 2022 prescribed burning seasons (nominally January to May), we identified 64 smoke events based on high levels of PM2.5 mass, BC, BrC, and carbon monoxide (CO), of which 61 were linked to a specific burning area. Smoke transport times were estimated in two ways: using the mean wind speed and the distance between the fire and the measurement site, and from Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back-trajectories. PM2.5 emission ratios based on ΔPM2.5 mass / ΔCO for fresh smoke (age ≤ 1 h) ranged between 0.04 and 0.18 µg m−3 ppb−1 with a mean of 0.117 µg m−3 ppb−1 (median of 0.121 µg m−3 ppb−1). Both the mean emission ratio and the variability were similar to findings from other prescribed fire studies but were lower than those from wildfires. The mean emission ratios of BC and BrC were 0.014 µg m−3 ppb−1 and 0.442 Mm−1 ppb−1, respectively. Ozone enhancements (ΔO3) were always observed in plumes detected in the afternoon. ΔPM2.5 mass / ΔCO was observed to increase with plume age in all of the ozone-enhanced plumes, suggesting photochemical secondary aerosol formation. In contrast, ΔBrC/ΔCO was not found to vary with plume ages less than 8 h during photochemically active periods.

A Nation-by-Nation Assessment of the Contribution of Southeast Asian Open Biomass Burning to PM2.5 in Thailand Using the Community Multiscale Air Quality-Integrated Source Apportionment Method Model

This study utilized the Community Multiscale Air Quality (CMAQ) model to assess the impact of open biomass burning (OBB) in Thailand and neighboring countries—Myanmar, Laos, Cambodia, and Vietnam—on the PM2.5 concentrations in the Bangkok Metropolitan Region (BMR) and Upper Northern Region of Thailand. The Upper Northern Region was further divided into the west, central, and east sub-regions (WUN, CUN, and EUN) based on geographical borders. The CMAQ model was used to simulate the spatiotemporal variations in PM2.5 over a wide domain in Asia in 2019. The Integrated Source Apportionment Method (ISAM) was utilized to quantify the contributions from OBB from each country. The results showed that OBB had a minor impact on PM2.5 in the BMR, but transboundary transport from Myanmar contributed to an increase in PM2.5 levels during the peak burning period from March to April. In contrast, OBB substantially impacted PM2.5 in the Upper Northern Region, with Myanmar being the major contributor in WUN and CUN and domestic burning being the major contributor to EUN during the peak months. Despite Laos having the highest OBB emissions, meteorological conditions caused the spread of PM2.5 eastward rather than into Thailand. These findings highlight the critical impact of regional transboundary transport and emphasize the necessity for collaborative strategies for mitigating PM2.5 pollution across Southeast Asia.

Evaluation of burning detection using modified VGG19 for LULC classification changes

Abstract. This study aims to evaluate the effectiveness of a modified architecture of convolutional neural network (CNN) VGG19 for detecting fires and changes in land use and land cover classification (LULC). Remote sensing data from the Landsat 8 Operational Land Imagery (OLI) satellite was used to collect images from two distinct regions, one of which was used to obtain a dataset containing 1000 labeled images, and the other region was used to perform inference and verify the generalization of the model in an area with a high annual occurrence of fires. Analyses were conducted using a time series of normalized difference vegetation index (NDVI) and complementary cumulative distribution function (CCDF), to determine the potential for analysis in that area and define the periods of burning, pre-burning and post-burning. The VGG19 architecture was modified to maintain the input sizes of the images, resulting in a significant increase of 20.90 percentage points in the F1 score compared to the original architecture, as well as a 68.76% reduction in convergence time. In addition, the Gradient-weighted Class Activation Mapping (Grad-CAM) technique was used to improve the interpretability of the model at the moment of inference. The proposed methodology offers an approach for detecting burns by altering the LULC classification, and the modified VGG19 showed superior results.

Experimental study of ammonia energy ratio on combustion and emissions from ammonia-gasoline dual-fuel engine at various load conditions

Achieving carbon neutrality necessitates the adoption of zero-carbon fuels in engine applications, with ammonia emerging as an up-and-coming candidate due to its favorable safety profile and advantages in storage and transportation. This study experimentally investigated the feasibility of an ammonia-gasoline dual-fuel (AGDF) engine to achieve comparable power output and satisfactory carbon reduction without changing the main structural parameters of the engine. A four-cylinder, naturally aspirated, spark ignition engine was used to investigate the impact of ammonia energy ratio (AER), engine base torque and engine speed on the engine performance, combustion evolution and emission characteristics. The findings reveal that the brake thermal efficiency (BTE) in AGDF mode is lower than in gasoline-only mode, primarily due to the reduced combustion activity. However, this efficiency decline becomes noticeable only when the AER exceeds 15 %. Additionally, at high AERs and high engine base torques, the delayed effect of ammonia fuel on the main combustion period results in a double-peak pattern, which limits the energy output but presents opportunities for phase optimization. The study also examined three incomplete combustion emissions, each exhibiting distinct behaviors. Except for ammonia slip, adding ammonia fuel does not significantly affect carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions, particularly at AERs below 25 %. Nevertheless, nitrogen oxide (NOx) emissions under AGDF combustion are significantly higher than under gasoline alone in most instances. Crucially, the study demonstrates the carbon reduction potential of ammonia fuel across different engine loads, with a maximum carbon dioxide (CO2) reduction of 46.8 % at a 35 % AER. It is anticipated that further optimization of the combustion phase will improve the capability for carbon reduction.

Effect of Biomass Burnings on Population Exposure and Health Impact at the End of 2019 Dry Season in Southeast Asia

At the end of the dry season, from early March to early April each year, extensive agricultural biomass waste burnings occur throughout insular mainland Southeast Asia. During this biomass-burning period, smoke aerosols blanketed the whole region and were transported and dispersed by predominant westerly and southwesterly winds to southern China, Taiwan, and as far southern Japan and the Philippines. The extensive and intense burnings coincided with some wildfires in the forests due to high temperatures, making the region one of the global hot spots of biomass fires. In this study, we focus on the effect of pollutants emitted from biomass burnings in March 2019 at the height of the burning period on the exposed population and their health impact. The Weather Research Forecast-Chemistry (WRF-Chem) model was used to predict the PM2.5 concentration over the simulating domain, and health impacts were then assessed on the exposed population in the four countries of Southeast Asia, namely Thailand, Laos, Cambodia, and Vietnam. Using the health impact based on log-linear concentration-response function and Integrated Exposure Response (IER), the results show that at the peak period of the burnings from 13 to 20 March 2019, Thailand experienced the highest impact, with an estimated 2170 premature deaths. Laos, Vietnam, and Cambodia followed, with estimated mortalities of 277, 565, and 315 deaths, respectively. However, when considering the impact per head of population, Laos exhibited the highest impact, followed by Thailand, Cambodia, and Vietnam. The results highlight the significant health impact of agricultural waste burnings in Southeast Asia at the end of the dry season. Hence, policymakers should take these into account to design measures to reduce the negative impact of widespread burnings on the exposed population in the region.

Elucidating the impacts of aerosol radiative effects for mitigating surface O3 and PM2.5 in Delhi, India during crop residue burning period

Atmospheric aerosol radiative effects regulate surface air pollution (O3 and PM2.5) via both the aerosol–photolysis effect (APE) and the aerosol–radiation feedback (ARF) on meteorology. Here, we elucidate the roles of APE and ARF on surface O3 and PM2.5 in the heavily polluted megacity, Delhi, India by using a regional model (WRF-Chem) with constraints from limited surface observations. While APE reduces surface O3 (by 6.1%) and PM2.5 concentrations (by 2.4% via impeding the secondary aerosol formations), ARF contributes to a 2.5% and 17.5% increase in surface O3 and PM2.5, respectively. The ARF from smoke enhances PM2.5 (by 8%), black carbon (by 10%), and primary organic aerosol (by 18%) during late autumn when crop residue burning is significant. The synergistic APE and ARF have a negligible impact on the total concentrations of O3 and PM2.5. Hence, the reduction of PM2.5 may lead to O3 escalation due to weakened APE. Sensitivity experiments indicate the need and effectiveness of reducing VOC emission for the co-benefits of mitigating both O3 and PM2.5 concentrations in Delhi.

Effect of natural agents on the mechanical and flame retardant properties of cotton fabrics – finishing technique

The current study carefully examines the antibacterial finished cotton fabric materials in terms of enhanced flame-retardant capability by eco-friendly herbal extracts to cotton fabrics while concurrently analysing their tensile strength qualities. Because active compounds have been incorporated into the fabric’s structure, the treated fabrics’ tensile strength has changed less over time than untreated fabrics. The plain weave cotton fabric with a specification of 140 g/m2 and a 15% finished concentration had a tensile strength of 238.92 kg/cm2, while cotton fabrics with specifications of 240 g/m2, terry weave with a 15% finished concentration, had a tensile strength of 294.43 kg/cm2. The finishing process has betterment in comparison to the untreated cotton fabrics had tensile strengths of of 140 g/m2 plain and 240 g/m2 terry weaves fabric which has 238.38 kg/cm2 and 288.47 kg/cm2, indicating that the ashwagandha-finished cotton fabrics performed better. The burning period increases from 35 sec for untreated 140 g/m2 plain cotton fabric, according to the flame reaction studies. According to tests on flame reaction, fabrics treated with a combination of banana peels and casein have a burning time rise from 35 s for untreated 140 g/m2 plain cotton fabric to 100 s, and up to 60 s for Ashwaganda and Ginger completed fabrics. The findings of the Limited Oxygen Index (LOI) test revealed that the completed fabrics made from a mixture of banana peels and casein had significantly higher LOI values of 23 than the other treated samples, which had an average LOI of 19 and 20, compared to the untreated cotton fabrics’ LOI of 18.

Source attribution of carbon monoxide over Northern India during crop residue burning period over Punjab

National Capital Territory of Delhi and its satellite cities suffer from poor air quality during the post-monsoon months of October–November. In this study, a novel attempt is made to estimate the contribution of different emission sources (industrial, residential, power generation, transportation, biomass burning, photochemical production, lateral transport, etc.) towards the criteria air pollutant carbon monoxide (CO) concentration over North India. Multiple simulations of the WRF-Chem model with a tagged tracer approach with different inputs (6 anthropogenic emission inventories and 3 biomass burning emission inventories) were used. The model performance was evaluated against the MOPITT retrieved CO surface concentration. Analysis of model simulated CO over North India suggests that anthropogenic emissions contribute around 32–49% to surface CO concentration while crop residue burning contributes 27–44% of which 80% originates from Punjab. For Delhi, the contribution from anthropogenic sources is dominant (53–77%) of which 10–28% is from the domestic sector and 14–55% is from the transport sector. Agricultural waste burning contributes about 15–30% to Delhi's surface CO concentration (of which 75% originates from Punjab). Crop residue burning emission is a chief source of CO over Punjab with a contribution of about 56–76%. The results suggest that industrial, transport, and domestic sector activities are more responsible for increased CO levels over New Delhi and surrounding regions than crop residue burning over Punjab. Furthermore, critical meteorological parameters like 10 m wind speed, boundary layer height, 2 m temperature, total precipitation, and relative humidity were evaluated against CO concentration to understand their impact on CO distribution. Results conclude that deteriorating air quality over the North Indian region is caused by a combination of prevailing meteorological factors (such as slow winds, shallow mixing layer, and cold temperatures) and man-made emissions.

Understanding the role of methanol as a blended fuel on combustion behavior for rotary engine operations

Methanol, being one of the most promising carbon-neutral fuels, has the potential to enhance the fuel mixture homogeneity of gasoline rotary engines, thereby improving engine performance. This study, based on computational fluid dynamics model, investigates the in-cylinder flow, combustion, and emission characteristics under various methanol blending ratios, ignition timings, and equivalence ratios. At the methanol energy blending ratio of 10 %, a more extensive and efficient operating range is observed across different ignition timings and equivalence ratios, attributed to the higher diffusion and combustion rates. An increase in in-cylinder temperature enhances the complete conversion of HC. However, higher proportions of methanol are disadvantageous for the combustion process due to methanol's lower heat value and higher latent heat of vaporization. Thus, the optimal ignition timing was determined to be 30°EA BTDC. At this optimal ignition timing, the cylinder shows higher average turbulent kinetic energy and radical content compared to the ignition timing at 50° EA BTDC. Specifically, the concentrations of OH, H, and O free radicals increase by 12.40 %, 14.77 %, and 13.91 % respectively, leading to more complete initial flame kernel expansion. It is notable that emissions of CO, HC, and NOx are all reduced. The study further explores the influence of the equivalence ratio on the combustion process. At an equivalence ratio of 0.9, there is an earlier peak in the maximum cylinder pressure rise rate and the highest generation of O and OH, with a shorter main combustion period and concentrated heat release. This achieves economically viable operation for rotary engines, indicating a 2.122 % improvement in thermal efficiency and a reduction in fuel consumption to 375.537 g kW−1 h−1.

Numerical Study of the Influence of Regenerator Structure on the Performance of Hot Blast Stoves

The properties and arrangement of checker bricks in regenerators are crucial for the heat exchange process of hot blast stoves. In this study, a 3D fluid flow heat transfer model is established to analyze the influence of three regenerator layered structures on the combustion and air supply performance of hot blast stoves. The results show that a “silica bricks–high-alumina bricks–clay bricks” three-layer arrangement in regenerators produces a “thermal conduction hindrance effect” at the interface between silica and high-alumina bricks during the 2 h combustion period, which raises the local temperature and improves air supply performance. Compared to the “silica bricks–clay bricks” and “high-alumina bricks–silica bricks–clay bricks” structures, this setup increases the maximum air supply temperature difference to 23 and 64 K, respectively, and extends the effective air supply time by 80 and 320 s, respectively. However, the “thermal conduction hindrance effect” diminishes over longer combustion periods, and by 4 h, the performance across all structures becomes increasingly consistent. Additionally, the study suggests that the temperature level and distribution in the upper part of the regenerator are the key factors determining the air supply performance of hot blast stoves.

Hygroscopic Properties of Water-Soluble Counterpart of Ultrafine Particles from Agriculture Crop-Residue Burning in Patiala, Northwestern India

To determine the link between hygroscopicity and the constituent chemical composition of real biomass-burning atmospheric particles, we collected and analyzed aerosols during wheat-straw (April–May), rice-straw (October–November), and no-burning periods (August–September) in 2008 and 2009 in Patiala, Punjab. A hygroscopicity tandem differential mobility analyzer (HTDMA) system was used to measure hygroscopicity at ~5 to ~95% relative humidity (RH) of aerosolized 100 nm particles generated from the water extracts of PM0.4 burning and no-burning aerosol samples. The chemical analyses of the extracts show that organic carbon and water-soluble inorganic-ion concentrations are 2 to 3 times higher in crop-residue burning aerosol samples compared to no-burning aerosols, suggesting the substantial contribution of biomass burning to the carbonaceous aerosols at the sampling site. We observed that aerosolized 100 nm particles collected during the crop-residue burning period show higher and more variable hygroscopic growth factor (g(RH)) ranging from 1.21 to 1.68 at 85% RH, compared to no-burning samples (1.27 to 1.33). Interestingly, crop-residue burning particles also show considerable shrinkage in their size (i.e., g(RH) &lt; 1) at lower RH (&lt;50%) in the dehumidification mode. The increased level of major inorganic ions in biomass-burning period aerosols is a possible reason for higher g(RH) as well as the observed particle shrinkage. Overall, the measured g(RH), together with the correlation observed between aerosol water content and ionic-species volume fraction, and the study of the abundance of individual constituent ionic species suggests that inorganic salts and their proportion in aerosol particles primarily governed the aerosol hygroscopicity.

Assessment of energy generation potential and mitigating greenhouse gas emissions from biogas from food waste: Insights from Jiangsu Province

Food waste is a significant issue in many developed and emerging countries and has significant environmental, social, and economic impacts. As China's urban population increases and restaurants expand, so does food wastage, resulting in many environmental and social issues. Therefore, this study investigates how the economic feasibility and environmental benefits of energy generation using biogas from food waste in Jiangsu Province can help achieve sustainable development goals and China's 14th Five-Year Plan. This study used Buswell's formulation, mathematical formulation, and economic metrics methods to analyze data from 2004 to 2020. The findings of the study are as follows: 1. The results show that 109.1 Mt. of food waste was produced in Jiangsu Province. 2. The total amount of electricity production potential from biogas yield during the 17 years is 16,126.0 GWh. 3. The study revealed that 3090.32 million tons (3.43% of the coal employed for energy production during the period) of coal combustion could be saved. 4. This prevented the release of 7262.26 million kilograms of carbon dioxide, 149.49 million kilograms of nitrous oxide, and 996.63 million kilograms of methane emissions. 5. The avoided coal consumption also helped reduce global warming by 76,727.33 kt of CO2 equivalent. 6. The project is economically beneficial with a shorter net present value (NPV) for 1 year, a positive NPV (US$34.7 Million), an internal rate of return (23%), a return on investment (33.0%), and a lower levelized cost of energy (US$ 0.039/kWh). Policy recommendations are also discussed.

Вивчення високотемпературного тепломасообміну двофракційних газозависів і поодиноких вуглецевих частинок в нагрітому повітрі

In the work, the regularities of high-temperature heat-mass exchange in two-fraction gas suspensions of carbon particles in heated air are studied. These studies are relevant for predicting high-temperature processes in combustion chambers and determining the main characteristics of these processes: induction period, time and temperature of combustion, critical parameters of ignition and extinction. Physico-mathematical modeling of the problem is carried out on the basis of the equations of heat and mass transfer and chemical kinetics for the components of the gas suspension, taking into account the molecular-convective and radiation mechanisms of heat transfer. The characteristics of ignition, burning, and extinction of a two-fraction gas suspension of carbon particles with equal mass concentrations of small (60 μm) and large (120 μm) fractions in the temperature range of 1100 ÷ 1500 K were studied; a comparison was made with the characteristics for single particles of the corresponding diameters. It has been proven that when the gas temperature decreases, the induction period of the fine fraction can exceed the induction period of the large fraction, while the combustion temperature of small particles becomes lower than the combustion temperature of large particles. At high gas temperatures (1400K, 1500K), the situation changes to the opposite. The critical ignition and extinction parameters of two-fraction gas suspensions (gas temperatures, particle diameters) were found.

Impact of pilot diesel injection timing on performance and emission characteristics of marine natural gas/diesel dual-fuel engine

In diesel-ignited natural gas marine dual-fuel engines, the pilot diesel injection timing (PDIT) determines the premixing time and ignition moment of the combustible mixture in the cylinder. The PDIT plays a crucial role in the subsequent development of natural gas flame combustion. In this paper, four PDITs (− 8 °CA, − 6 °CA, − 4 °CA, and − 2 °CA) were studied. The results show that the advancement of PDIT increased the engine's power, thermal efficiency, and natural gas flame spread velocity, and increased NO emissions and CH4 emissions of the marine engine. The PDIT affected the ignition delay period and the rapid combustion period to a greater extent than the slow combustion period and the post combustion period. With each 2 °CA advancement of PDIT, the engine's power increased by 69.87 kW, thermal efficiency increased by 0.42%, radial flame spread velocity increased by 2 m/s, axial flame spread velocity increased by 1.7 m/s, NO emissions increased by 6.1%, and CH4 emissions increased by 3.75%.

LES and RANS Spray Combustion Analysis of OME3-5 and n-Dodecane

Clean-burning oxygenated and synthetic fuels derived from renewable power, so-called e-fuels, are a promising pathway to decarbonize compression–ignition engines. Polyoxymethylene dimethyl ethers (PODEs or OMEs) are one candidate of such fuels with good prospects. Their lack of carbon-to-carbon bonds and high concentration of chemically bound oxygen effectively negate the emergence of polycyclic aromatic hydrocarbons (PAHs) and even their precursors like acetylene (C2H2), enabling soot-free combustion without the soot-NOx trade-off common for diesel engines. The differences in the spray combustion process for OMEs and diesel-like reference fuels like n-dodecane and their potential implications on engine applications include discrepancies in the observed ignition delay, the stabilized flame lift-off location, and significant deviations in high-temperature flame morphology. For CFD simulations, the accurate modeling and prediction of these differences between OMEs and n-dodecane proved challenging. This study investigates the spray combustion process of an OME3 − 5 mixture and n-dodecane with advanced optical diagnostics, Reynolds-Averaged Navier–Stokes (RANS), and Large-Eddy Simulations (LESs) within a constant-volume vessel. Cool-flame and high-temperature combustion were measured simultaneously via high-speed (50 kHz) imaging with formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) representing the former and line-of-sight OH* chemiluminescence the latter. Both RANS and LES simulations accurately describe the cool-flame development process with the formation of CH2O. However, CH2O consumption and the onset of high-temperature reactions, signaled by the rise of OH* levels, show significant deviations between RANS, LES, and experiments as well as between n-dodecane and OME. A focus is set on the quality of the simulated results compared to the experimentally observed spatial distribution of OH*, especially in OME fuel-rich regions. The influence of the turbulence modeling is investigated for the two distinct ambient temperatures of 900 K and 1200 K within the Engine Combustion Network Spray A setup. The capabilities and limitations of the RANS simulations are demonstrated with the initial cool-flame propagation and periodic oscillations of CH2O formation/consumption during the quasi-steady combustion period captured by the LES.

Research on the impacts of operating frequency at combustion process for opposed single-cylinder free piston generator under direct injection

The opposed single-cylinder free piston generator（OSFPG）has the characteristics of compact structure, variable compression ratio and low emission characteristics of common free piston generator, etc. More, it also has unique features such as high unit power, low vibration noise, and good motion stability, etc. So, in this research, a physical prototype platform based on OSFPG is built. For the key factors, both experiment and simulation have been combined to investigate the changes in OSFPG's work performance under different operating frequencies. It was found that operating frequency affected OSFPG's working characteristics through the entire combustion period. More, the combustion period of each combustion stage increased with the increase of the operating frequency, and the increase amplitude was gradually reduced with the advancing of the ignition advance angle, but the operating frequency affected the after burning period most. When the operating frequency changed from 15 Hz to 35 Hz, the after burning period was extended by at least 100 %. The important thing was that the indicated thermal efficiency of the OSFPG increased first and then decreased with the increase of operating frequency. When the operating frequency was 25 Hz, the OSFPG's indicated thermal efficiency could reach 40.4 %.