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Ammonia serves as an efficient hydrogen energy carrier, holding promise for achieving carbon neutrality in the internal combustion engine (ICE) industry. To boost the thermal efficiency of ammonia ICE, plasma-induced jet ignition (PIJI) is proposed with hydrogen as a combustion promoter. A zero-dimensional model of the PIJI ammonia-hydrogen engine, integrating SENKIN and ZDplaskin, reveals that plasma ignition consumes only 1/10 of the energy of spark ignition (SI) in the jet chamber. Active radicals resulting from the plasma reaction of O2 and NH3 expedite combustion in the jet chamber. By extending discharge duration from 3 ns to 4 ns, increasing discharge frequency from 30 kHz to 100 kHz, and strengthening the electric field from 350 Td to 450 Td, energy consumption is reduced by 80%, 50%, and 95% respectively. A low temperature exothermic phase is observed before the auto-ignition of the main chamber, irrespective of the ignition method. • A novel ignition method, plasma-induced jet ignition(PIJI), is proposed. • 0D plasma-induced ammonia-hydrogen jet ignition engine model is developed. • The ignition energy of plasma ignition is 1/10 of spark ignition. • Active radicals resulting from the collision of O 2 and NH 3 expedite combustion. [ABSTRACT FROM AUTHOR]

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 Re t and lower Da t in the Da t - Re t diagram, where the strong ignition is less pronounced. [ABSTRACT FROM AUTHOR]

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations. [ABSTRACT FROM AUTHOR]

• Zero-dimensional jet ignition engine model was developed based on SENKIN. • Low-temperature exothermic stage was observed before auto ignition of the jets. • The mechanism of jet ignition during the jet entrainment process was investigated. • The mechanisms of thermal and chemical effects of jets on ignition were compared. Jet ignition is a key ignition technology to improve the ignition and combustion of ammonia internal combustion engine. To achieve better design of jet ignition, the combustion mechanism of jet ignition is numerically studied. Firstly, a zero-dimensional jet ignition engine model is developed based on SENKIN. Then, the jet ignition mechanism of the stratified jets during the jet entrainment process is investigated through the dilution of the jet with various volumes of fresh mixture. Based on the validated model, the influence of the structural parameter of the jet chamber, key radicals, thermal and chemical effects on jet ignition are further revealed. The results show that a slow low-temperature exothermic stage is observed before auto ignition of the jets during the entrainment process. Through the quenching comparison between key radicals, O ̇ is the most crucial radical to jet ignition. The reaction path of consuming and producing OH ̇ could be affected by changing the parameter of the jet chamber, resulting a different ignition behavior. Successful jet ignition is the synergistic combination of thermal and chemical effects. The thermal environment established by the jets facilitates chemical reactions, while the heat released from these reactions further amplifies the thermal effect, ensuring the sustainability of the ignition process.. [ABSTRACT FROM AUTHOR]

The objective of multi-focus image fusion (MFIF) is to generate a fully focused image through integrating multiple partially focused source images. Most of the existing methods do not fully consider the local gradient variation rate of the source image, which makes it difficult to accurately distinguish the small defocused (focused) region covered by the large focused (defocused) region. In addition, these methods also cause edge blurring because they do not take into account misregistration of the source images. To address these issues, in this paper, we propose a simple and effective multi-focus image fusion framework based on multi-channel Rybak neural network (MCRYNN) model. Specifically, the proposed MCRYNN model is highly sensitive to local gradient changes of images based on input receptive fields, which can process multiple source images in parallel and extract the features of focused regions. Moreover, the decision maps can accurately be generate in proposed method based on the information interaction effect of parallel network structure for multi-focus image fusion task. Finally, we conduct qualitative and quantitative experiments on public datasets, and the results show that the performance of the proposed method outperforms the state-of-the-art methods. • A primary attempt to use Rybak's visual cortex theory to perform the MFIF. • A novel multi-channel Rybak neural network (MCRYNN) is proposed. • A simple and efficient MFIF framework is proposed based on MCRYNN model. • The proposed method has a high computational efficiency. [ABSTRACT FROM AUTHOR]

To avoid increasing the dilution gas to achieve higher temperatures for rapid compression machines (RCM) at the end of compression, this study introduced an oil-bath heating system designed to precisely improve the initial temperature of the mixture, thereby enabling ammonia-hydrogen ignition at practical fuel concentrations for internal combustion engines (ICEs). Using this oil-bath heated RCM, the ignition delay times (IDTs) of ammonia-hydrogen under conditions relevant to ICEs were measured. The ammonia-hydrogen combustion mechanism proposed by Stagni et al. (10.1016/j.proci.2022.08.024) was then validated against these measured IDTs and subsequently optimized. Guided by a comprehensive review of the existing literature on ammonia-hydrogen chemistry, the optimization involved updating 42 reaction rate coefficients and adding 9 new reactions. The optimized mechanism, comprising 31 species and 214 reactions, demonstrated accurate predictions of ignition delay time, laminar flame speed, and species mole fractions across a wide range of experimental setups and conditions for ammonia-hydrogen, confirming the mechanism's robust performance and its suitability for high-precision simulations in ammonia-hydrogen ICE applications. • An oil-bath heating system was developed to elevate the mixture temperatures of an RCM. • The NH 3 –H 2 ignition delay times (IDTs) at engine relevant conditions were measured. • An NH 3 –H 2 kinetic model was constructed with reliable reaction rate constants. • The model well predicts the NH 3 –H 2 IDTs, laminar flame speed, and species mole fractions. [ABSTRACT FROM AUTHOR]

The application of ammonia and hydrogen in internal combustion engines (ICE) is a promising zero-carbon technology. This paper investigates ammonia combustion in ICE using hydrogen multiple-injection jet-ignition (MIJI). The experiment was conducted on a single-cylinder heavy-duty engine with a compression ratio of 17 and a cylinder bore of 123 mm. In the passive jet ignition mode, the minimum hydrogen energy ratio for stable operation of the ammonia-hydrogen engine was 15 %. Using H 2 active jet ignition could improve the combustion stability significantly. It was found that engine performance was gradually improved by H 2 injection strategy in order of single-injection, double-injection to triple-injection. The triple-injection strategy could achieve a more ideal distribution of ammonia-hydrogen mixture: the first injection forms a homogeneous mixture to increase fuel reactivity; the second injection creates a stratified hydrogen mixture, facilitating rapid flame propagation; the third injection forms a relatively rich mixture in the jet chamber conducive to spark ignition. Through the optimization of the hydrogen injection strategy and spark timing, stable combustion (COV = 1 %) and high thermal efficiency (ITE = 42.5 %) in ammonia-hydrogen engines were achieved, with a minimum hydrogen energy ratio less than 3 %. • Both active and passive H2 jet ignition for NH3 combustion in ICE were tested. • NH 3 combustion using H 2 multiple-injection jet-ignition in ICE is achieved. • NH 3 –H 2 engine achieves high thermal efficiency at 3 % hydrogen energy ratio. • H 2 triple-injection strategy forms ideal distribution of ammonia-hydrogen mixture. [ABSTRACT FROM AUTHOR]

Spark assistance for homogeneous charge compression ignition (HCCI) can control combustion phasing, improve thermal efficiency, and reduce emissions in gasoline engines. As the characteristics of flame propagation determine the control authority of ignition timing, it is important and necessary to investigate pressure dependence of flame speed in the lean-premixed mixture relative to engine operating conditions. Experimental study in an optical rapid compression machine (RCM) and simulation work were carried out using two fuels comprising n -heptane/iso-octane/ethanol with varied octane sensitivity (S). The effective pressure ranged from 10 to 35 bar, temperature from 715 to 860 K, and equivalence ratios between 0.3 and 0.7 to cover the region of lean flammability limits of low and high S fuels with ethanol blended. Based on pressure profiles, flame speed extracted from images, and sensitivity analysis of flame speed, the dependence of flame speed on the effective pressure in low and high S fuels was discovered and the fundamental mechanism behind this phenomena became to be understood in the negative temperature coefficient (NTC) and non-NTC regions, respectively. In the studied temperature conditions, the flame speed of high S fuel has stronger dependence on the pressure than that of low S fuel does. In the NTC region, this phenomenon is attributed to the dependence of H radical concentration on pressure in the unburned mixture and flame structure. In the non-NTC region, promoting effect of dominant reactions varied with pressure can significantly influence pressure dependence of flame speed. Although quite limited data of laminar burning velocity for studied fuels were obtained in high pressures (>15 bar), the trend of flame speed's dependence on pressure was well predicted by two models with different but well-accepted core mechanisms, showing consistent results with the experimental ones in the RCM. [ABSTRACT FROM AUTHOR]

The ignition phenomena of stoichiometric methanol/oxygen/argon mixture are comprehensively investigated at p = 12–24 bar, T = 840–1000 K, using a rapid compression machine (RCM). A strong tendency of stochastic ignition followed by non-forcible flame propagation at a speed of ∼11 m/s is demonstrated. Such an event may be responsible for the pre-ignition or super-knock issue in methanol engines. Under engine relevant spark ignition conditions, different ignition regimes are observed in the end-gas, including thermal explosion, supersonic deflagration, and detonation, characterized by the Chapman–Jouguet velocity criterion. All modes originate from a similar, early auto-ignition ahead of the spark-triggered flame. The auto-ignition process is proved to be dominated by chemical kinetics, where the Livengood–Wu correlation is applicable. In addition, the transition mechanism of different ignition regimes is thoroughly validated against previous ignition theories. Basically, the detonation onset is closely related to the initial thermodynamic condition of the mixture. At a given temperature, the decrease of pressure induces the gradual substitution of detonation by supersonic deflagration and thermal explosion due to a smaller reactivity gradient in the end-gas. The ε − ξ diagram proposed by Bradley is adopted to interpret the transition mechanism of methanol, which turns out to be different from previous results of isooctane. A moderate burned mass fraction range of 0.35–0.45 is found when detonation is initiated. [ABSTRACT FROM AUTHOR]

• A heat transfer model considered flame-wall interaction was constructed. • The mechanism of iso-octane with 24 species and 25 reactions was developed. • The heat transfer model and reaction mechanism are validated at different loads. • The peak-pressure difference between the experiment and simulation is only 0.1 %. With the advancements in hybrid vehicle technology and the increasing stringency of fuel consumption regulations, engines are operating more frequently in the high-efficiency zone of medium to high load, aiming to improve fuel economy and enhance vehicle endurance. However, this can lead to elevated engine thermal loads, and require numerous computational fluid dynamics (CFD) simulations to optimize the combustion system of gasoline engines. To address these challenges, it is crucial to accurately calculate the heat transfer process in CFD calculations. Furthermore, it is necessary to optimize and simplify the skeletal reaction mechanism of gasoline-like fuels to predict the combustion process effectively and reduce computational costs. This investigation presents a new heat transfer model and a new iso-octane skeletal reaction mechanism. The heat transfer model takes into account the boundary layer structure and considers the influence of the chemical source term on the heat transfer process of boundaries. Consequently, it enables more accurate predictions of in-cylinder pressure and heat release rate under different engine loads. Moreover, the new skeletal reaction mechanism with 24 species and 25 reactions has been validated against simulation results obtained using the detailed LLNL reaction mechanism, specifically at high pressures and over a wide range of temperatures and equivalence ratios. These validations demonstrate the promising potential of the proposed reaction mechanism for gasoline engine applications, particularly under higher engine load conditions. The difference of peak-pressure between the experiment and simulation coupled with the new heat transfer model and the new reaction mechanism is only about 0.1 % under the high engine load. [ABSTRACT FROM AUTHOR]

• Process of spark ignition to compression ignition with ethanol blends are investigated. • S can predict KI under 785 K and loses this function as temperature increases. • Dilution tolerance is determined by the intensity of HTHR not laminar speed itself. • The OID of 5 ms can well distinguish SI mode from CI mode for fuels with varied S. Spark assistance is an effective method to improve combustion stability of homogeneous charge compression ignition (HCCI) with lean mixture. Ethanol, a renewable energy, blended with commercial fuels is a promising method to deal with problems of energy safety and greenhouse gas (GHG) emission. However, the influence of ethanol blends on spark ignition to compression ignition is not clear. To this end, this study presents experimentally fundamental investigation on the compression ignition (with and without spark assistance) of ethanol-blended gasoline surrogate fuel in an optical rapid compression machine at the equivalence ratio of 0.5. Three fuels with different octane sensitivities (S) comprising n -heptane/ iso -octane/ethanol were formulated and S is the difference between research octane number (RON) and motor octane number (MON). Effective temperature of experiments ranges from 730 K to 860 K, overlapping most of the temperature region with negative temperature coefficient (NTC) of iso -octane at the corresponding equivalence ratio, and the pressure is consistence with engine-relevant operating conditions. The results show that more fuel reactivity of ethanol than iso -octane at 860 K is not attributed to the NTC behavior of iso -octane but to the reactive species generation in ethanol oxidation. There is always a positive correlation between effective pressure and knock intensity (KI). At lower temperature (<785 K), higher S results in the lower KI, while at higher temperature (860 K), the heat amount released by auto-ignition instead of S has a dominant impact on KI. Moreover, buffer gas dilution tolerance is not merely affected by laminar speed itself but determined by the intensity of heat release ahead of auto-ignition. The intensity of these heat release is proportional to flame speed and lower heating value (LHV). High S fuel is more sensitive to extra dilution at a fixed thermodynamic condition due to lower energy content, while the overall ignition delay (OID) requirement of no auto-ignition in the end gas is similar reflecting certain independence of fuel type. Medium S fuel has the better relationship between the timescale of flame propagation and auto-ignition, which is more suitable for spark-ignition to compression-ignition (SICI). This investigation provides a reference to fuel design for real engine applications. [ABSTRACT FROM AUTHOR]

Spark ignition induced super-knock was generated in a rapid compression machine using stoichiometric iso-octane/O 2 /N 2 mixture. In addition to the pressure traces, the combustion processes were also recorded using high-speed photography, from which the characteristics of the end-gas auto-ignition were analyzed. The effect of negative temperature coefficient (NTC) on the end-gas auto-ignition was investigated using detailed and reduced reaction mechanisms. The end-gas auto-ignition of diluted methanol/O 2 /Ar mixture without NTC behavior was also tested for comparison. The results showed that the end-gas auto-ignition of iso-octane/O 2 /N 2 mixture exhibited a two-stage ignition process. During the second ignition stage of the end-gas, two auto-ignition events with very short time interval were sequentially observed in the end-gas region. The first auto-ignition event generated a weak shock wave, and the second one initiated detonation. Both the two auto-ignition events occurred near the wall but at different sites. Due to the heat loss to the wall, the near-wall region is generally considered to be colder than the adiabatic core region, and thus the near-wall auto-ignition of end-gas was usually considered as a result of fuel's NTC behavior. However, in this study the chemical kinetic calculation showed that the evolution of the end-gas almost bypassed the NTC region in the ignition delay-temperature diagram. Furthermore, for the methanol/O 2 /Ar mixture the end-gas auto-ignition also started at the very near-wall region. Considering that methanol fuel does not has an NTC behavior, the near-wall auto-ignition of methanol/O 2 /Ar mixture should be a result of other factors than NTC. Therefore, it was concluded that NTC might not play a dominant role in the near-wall auto-ignition of the end-gas. [ABSTRACT FROM AUTHOR]

Rapid compression machines (RCMs) are intensively used in auto-ignition investigations for their high similarity to real engines and their convenience for optical visualization. However, previous works using RCMs usually visualized the combustion processes from only a single view, which is limited in capturing critical details regarding ignition in other directions due to line-of-sight integration, given that ignition is actually three-dimensional. This work presents a comprehensive investigation of auto-ignition behaviors in a novel dual-window optical RCM. By simultaneously providing fully-optical accesses into the entire combustion chamber from both the end- and lateral-view, the auto-ignition of primary reference fuels (PRFs) under various initial conditions were captured and the impacts of vortices were examined. Images obtained from dual perspectives demonstrated that auto-ignition under vortex-minimized conditions always initiated from the end-walls and then propagated in the chamber axial direction, exhibiting obvious "near-wall initiation, off-wall propagation" characteristic. Such feature was insignificantly impacted by fuel reactivity, initial thermodynamic condition, and the existence of initial spark-ignited flame. Under vortex-existing conditions, however, the initiation and development of auto-ignition was highly dependent on the interaction between the turbulent field and the fuel reactivity. Only for cases with Da >1, were vortices observed to have distinct impacts on auto-ignition characteristics. At last, the influences of ignition delay time gradient were also discussed. [ABSTRACT FROM AUTHOR]

• Hydrogen production from ammonia-rich combustion was conducted on an RCM. • Current mechanisms failed to predict hydrogen production under fuel-rich conditions. • Higher initial temperature yields higher hydrogen production. • Hydrogen production shows inverse V-shaped trend with increasing initial pressure. • Moderate equivalence ratio results in higher hydrogen production. Ammonia is a promising carbon-free fuel for internal combustion engines. However, the low reactivity and combustion sluggishness make ammonia difficult to be used in a single-fuel way. A small amount of hydrogen addition has been demonstrated beneficial to improving ammonia's reactivity and engine performance. To overcome the safety and cost issues of onboard hydrogen storage on vehicles, we propose to produce hydrogen by in-cylinder ammonia reforming through ammonia-rich combustion. This paper presents an investigation on the hydrogen production from ammonia-rich combustion on a rapid compression machine (RCM) over conditions with varying pressures (22–36 bar), temperatures (1200–1300 K), and equivalence ratios (1.75–2.25). Both major combustion reactants (NH 3) and products (H 2 and N 2) were analyzed using a fast sampling system and gas chromatography (GC). Chemical analysis was also conducted to interpret the experimental results. The results showed that current ammonia mechanisms were inadequate to predict the ignition delay time and the hydrogen production trend under engine-relevant and fuel-rich conditions. The experimental hydrogen production increased with the increasing initial temperature, which could be ascribed to the increase in reaction rate constants of key reactions. For the effect of pressure, the hydrogen production was found to first increase and then decrease as the initial pressure increased, which failed to be predicted by simulations. In the tested equivalence ratio range, a moderate fuel-rich equivalence ratio of two produced the highest hydrogen, in which the combined impact of key radical pool buildup (NH 3 and NH 2) and the change in reaction ratios of key reactions (NH 2 + H->NH + H 2) was responsible. [ABSTRACT FROM AUTHOR]

Super-knock is the main obstacle to improve power density and engine efficiency of modern gasoline engines. To understand the mechanism of super-knock, this study presents an investigation on the end gas combustion process of stoichiometric isooctane/oxygen/nitrogen mixture using a rapid compression machine (RCM), under the thermodynamic conditions close to those of production engines. The combustion process was captured by simultaneous high speed direct photography and pressure acquisition in the RCM. Three end gas combustion modes: no-auto-ignition, sequential auto-ignition, and detonation under different initial conditions were identified and characterized. The super-knock in engine was confirmed to be the result of detonation by comparing the pressure oscillation, thermodynamic state, and pressure rise relative to isochoric combustion with those of detonation observed in the RCM. The experimental results also indicate that the possibility of detonation occurrence increases with increasing initial pressure under the same compression ratio. However, comparing to the pressure, temperature has less effect on detonation formation. It was found that the end gas combustion mode is closely related to the mixture energy density. Generally, as the mixture energy density increases, the end gas combustion mode gradually transits from no-auto-ignition to sequential auto-ignition, and then to detonation. The first auto-ignition spots commonly appear in the mixture near the cylinder wall. The detonation was initiated by near-wall auto-ignition. [ABSTRACT FROM AUTHOR]

• Knock behaviors under jet ignition (JI) and spark ignition (SI) are investigated. • JI results in shorter combustion duration and lower knock intensity than SI. • Larger burned mass fraction under JI is responsible for its lower knock intensity. • Pressure impacts on ε and ξ in ε - ξ diagram are affected by initial temperature. • Ignition regime characterization with Da - Re diagram agree well with experiments. Jet ignition (JI) is increasingly considered a promising way to increase engine thermal efficiency due to its faster burning velocity than conventional spark ignition (SI). However, as an ignition method, JI does not change the nature of premixed combustion, and knock can still occur. This study investigated knock characteristics and end-gas auto-ignition behaviors of the stoichiometric iso-octane-air mixture under JI at an initial temperature ranging from 650 to 830 K and an initial pressure ranging from 10 to 20 bar. Experiments under SI were also conducted for comparison. The results showed that compared with SI, JI could reduce both combustion duration and knock intensity. The combustion mode of the end-gas transitioned from non-auto-ignition to mild auto-ignition and eventually to detonation as the initial pressure increased, which was similar to the situation under SI. Thermodynamic analysis indicated that compared with SI, the auto-ignition under JI tended to occur at higher thermodynamic states due to less heat loss, which led to higher burned mass fractions at the instant of auto-ignition. Further analysis using Bradley's ε - ξ diagram showed that, under low initial temperatures, the initial pressure had little impact on ε but could change ξ significantly, and the auto-ignition mode mainly depends on ξ rather than ε as the pressure changed. The effect of initial pressure on ε and ξ became inversed under high initial temperatures. Finally, the auto-ignition regime was analyzed using the Da t - Re t diagram. The result showed that the auto-ignition cases were all in the Mixed/DDT region, and the detonation cases were all in the strong ignition region. [ABSTRACT FROM AUTHOR]

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε - ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε - ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture. [ABSTRACT FROM AUTHOR]

• An Adaptive Receptive Field Network for mass segmentation in whole mammograms. • The multi-scale attention contributes to the final segmentation results. • The diversity of receptive fields contributes to the segmentation of small masses. UNet adopting an encoder-decoder structure has been used widely in medical image segmentation tasks for its outstanding performance. However, in our work, we find that UNet has the worse segmentation performance of small masses. The reason behind this is that the sizes of receptive fields are limited. In this work, to address this issue, we develop a novel end-to-end model, Adaptive Receptive Field Network (ARF-Net), for the precise breast mass segmentation in whole mammographic images and ultrasound images. ARF-Net composes of an encoder network and a corresponding decoder network, followed by a pixel-wise classifier. In ARF-Net, a Selective Receptive Filed Module (SRFM) is proposed to allocate the suitable sizes of receptive fields to the breast masses of different sizes. SRFM consists of a Multiple Receptive Field Module (MRFM) for generating multiple receptive fields of different sizes and a Multi-Scale Selection Module (MSSM) for selecting the suitable sizes of receptive fields based on the objects' size. The proposed ARF-Net achieves the dice index of 86.1 % , 85.75 % , and 88.12 % on the two mammographic databases (INbreast and CBIS-DDSM) and one ultrasonic database (UDIAT), respectively. Moreover, extensive ablation experiments show that ARF-Net transcends several state-of-the-art segmentation networks, and the developed MSSM exceeds several counterparts. [ABSTRACT FROM AUTHOR]

• An automatic end-to-end convolutional neural network model is designed for breast density classification in mammograms. • By simulating the doctor's reading mechanism, we combine the information of bilateral breasts to classify breast density. • Adaptive spatial attention module (ASAM) and adaptive channel attention module (ACAM), are employed to explore discriminant information for breast density classification. • Proposed BASCNet has been verified on the DDSM and INbreast datasets, and both have achieved state-of-the-art results. Breast density is a significant element for breast cancer precaution. The existing mammographic density classification methods cannot achieve satisfactory classification accuracy while achieving end-to-end. In this paper, we present a novel bilateral adaptive spatial and channel attention network (BASCNet) which integrates the information of the left and right breasts and adaptively pays attention to the discriminative features in spatial and channel dimensions. The proposed BASCNet has been fully proved on the public Digital Database for Screening Mammography (DDSM) and INbreast dataset, and the classification accuracies of 85.10% and 90.51% were achieved with fivefold cross-validation, respectively. Our method is fully automatic and has achieved the classification performance superior to the existing breast density classification methods. Massive ablation experiments were conducted to demonstrate the effectiveness of the network structure. Moreover, we compared the effects of different views (CC and MLO) on breast density classification and verified the effectiveness of the contralateral breast information integration. Overall, the proposed BASCNet has the potential to be applied to clinical diagnosis. [ABSTRACT FROM AUTHOR]

The decarbonization movement has sparked interests in ammonia (NH 3) as a fuel for internal combustion engines in the transportation sector. Despite its promise, the poor combustion and emission performance are the main obstacles to widespread application. The purpose of this paper is to investigate the combustion and emission performance of a single-cylinder spark ignition engine fueled with NH 3 /gasoline blends under different operating conditions. The effects of NH 3 blending ratio, engine load, EGR ratio, engine speed, intake variable valve timing (VVT) phase and the composition and octane of the fuel were examined. The results indicated that blending NH 3 could effectively suppress knock, optimize combustion phase and improve thermal efficiency. Blending NH 3 had minimal impact on flame propagation in the main combustion duration (denoted by CA50 - CA10 and CA90 - CA50) under knock-free conditions but negative effects were exhibited on the development of initial flame kernel. The combustion stability under high NH 3 blending ratios were improved by increasing engine load, decreasing EGR ratio, etc. Furthermore, stable and efficient combustion of pure NH 3 with COV of 1.3% and indicated thermal efficiency of 41% was realized with a commercial ignition coil and without intake-charge heating. The fuel-type NO x exhibited a positive temperature correlation and negative pressure correlation. Accordingly, NO x emissions were decreased with spark timing (ST) advancing within a specific range of NH 3 blending ratios. However, it remained essentially unchanged or showed a slight increase with advanced ST at high NH 3 blending ratio. NH 3 emission was simultaneously affected by both the "crevice mechanism" and the "flame quenching mechanism". Particularly, the influence of the "flame quenching mechanism" intensified under higher NH 3 blending ratio and lower engine load conditions. N 2 O emission was regulated by the low-temperature oxidation path of NH 3. Increasing the combustion temperature could effectively decrease N 2 O emission. • Performance of SI engine fueled with NH 3 /gasoline was systematically investigated. • The fuel economy was improved through NH 3 addition under high load conditions. • Stable and efficient combustion of pure NH 3 with COV = 1.3% and ITE = 41.0% was achieved. • Factors influencing NO x , NH 3 and N 2 O emissions were identified. [ABSTRACT FROM AUTHOR]

• A Multi-Scale Attention-Guided Network for recognizing abnormalities in mammograms. • The self-adaption and multi-scale contribute to the final classification results. • Multiple receptive fields are beneficial for recognizing objects of different sizes. For the breast mass segmentation in whole mammograms, in our studies, we observe that there is an enormous performance reduction in the case of considering the normal data during training. Therefore, the mammogram classification (normal vs. abnormal) is essential for boosting the breast mass segmentation performance in whole mammograms and is our research topic in this paper. Due to the breast lesions with a variety of sizes, the mammogram classification (normal vs. abnormal) is a challenging task. To improve the mammogram classification performance, we propose an end-to-end convolutional neural network, namely Multi-Scale Attention-Guided Network (MSANet). Specifically, MSANet can be constructed by stacking several Multi-Scale Attention (MSA) bottlenecks. Each MAS bottleneck consists of a Scale Aggregation (SA) unit and a Multi-Scale Attention Module (MSAM). The SA unit is used to generate multiple feature maps of different scales, and the MSAM is used to allocate the suitable size of receptive field for objects of different sizes. According to the extensive experiments, our proposed MSANet-50 achieves a fully automated classification AUC of 0.942 on the DDSM database, which outperforms several approaches. [ABSTRACT FROM AUTHOR]

• Effect of octane number on SICI combustion is investigated with ethanol blends. • Low- T oxidation acceleration of ethanol is caused by OH radical not by fuel interaction. • Contribution to KI from reactivity, AHRA and flame compression decreases in turn. • Transition temperature and pressure from SI to CI is dropped with RON decreased. • The fuel with medium RON and S is preferable for lean SICI combustion. Spark assistance in homogeneous charge compression ignition (HCCI) is a promising method to improve combustion stability. Fundamental experiments were carried out in a rapid compression machine along with chemical kinetics analysis to investigate the complete combustion process of spark ignition to compression ignition (SICI) using ethanol-blended fuels. Five fuels, consisting of n -heptane, iso -octane and ethanol with different fractions, are divided into two groups. The fuels with different research octane number (RON) and motor octane number (MON) but identical octane sensitivity (S) are in the same group. The equivalence ratio is fixed at 0.5, and the experimental pressure covers the engine-relevant conditions (10–35 bar) while the target temperature ranges from 735 K to 860 K, overlapping most regions with negative temperature coefficient (NTC) of n -heptane and iso -octane. Results show that octane sensitivity with low RON has poor ability to evaluate fuel reactivity especially in the vicinity of "beyond MON" area due to low-temperature oxidation acceleration of ethanol. The influence of fuel reactivity, auto-ignition heat release amount and flame compression effect on knock intensity enhancement decreases in turn. The lower transition temperature and pressure at the time of auto-ignition is observed in the fuel with lower RON regardless of S, resulted from stronger LTHR and greater temperature rise from cool flame. The fuel with medium RON and S based on ethanol blending is more suitable for SICI combustion since it can make a better balance among knock intensity, dilution tolerance and control authority from flame in the conditions studied, which gives an insight into the effect of ethanol blends on combustion process and provides a reference for fuel design aimed at lean SICI combustion. [ABSTRACT FROM AUTHOR]

• Ignition delay time and species sampling data of NH 3 /CH 3 OH blends were obtained. • Methanol addition enhanced the overall reactivity of the mixture. • Two patterns of ammonia consumption were observed at different methanol proportion. • Methanol inhibited ammonia consumption, ammonia promoted methanol consumption. Ammonia has attracted wide attention in recent years as a carbon-free fuel. However, the low reactivity and combustion inertness of ammonia pose challenges to its applications in engines. Adding highly reactive fuels, such as renewable carbon–neutral methanol, offers a potential solution. To investigate the ignition characteristics of ammonia-methanol blended fuel, ignition delay time measurement and fast gas sampling under stoichiometric conditions with four ammonia blending ratios (20 % ammonia, A20; 40 % ammonia, A40; 80 % ammonia, A80; 95 % ammonia, A95) were carried out on a rapid compression machine under engine-relevant conditions. The effective thermodynamic conditions were 15–25 bar and 810–970 K. The concentrations of fuels NH 3 , CH 3 OH, and intermediate species N 2 O, CO, as well as products N 2 , CO 2 were detected using gas chromatography. Chemical analysis was performed based on simulations using five ammonia-methanol reaction mechanisms. The ignition delay time results showed that the addition of methanol shortened the ignition delay time, with only a little change in ignition delay time beyond 20 % methanol content. Methanol significantly promoted the production of OH radical, leading to the enhancement of the overall mixture reactivity. The sampling results showed different consumption patterns of ammonia and methanol during the ignition process at different mixing ratios. For A80, methanol was consumed from the early stage of the ignition process, while ammonia consumption was negligible in the early stage. Conversely, for A95, ammonia consumption began in the early stage. This suggests that methanol and its intermediate species inhibited the consumption of ammonia, however, ammonia promoted the consumption of methanol. Chemical analysis further revealed that the inhibitory effect of methanol on ammonia consumption was weakened with the decrease of methanol proportion. [ABSTRACT FROM AUTHOR]

• Effect of NH 3 blending was investigated in a gasoline engine. • Optimized combustion performance was obtained with NH 3 blending. • Emissions from the gasoline/NH 3 were analyzed in detail. • The NO x emission laws were revealed by the chemical reaction analysis. Mitigating climate change involves the greater adoption of carbon–neutral and renewable energy sources within the transportation sector. Ammonia (NH 3), as a carbon-free and sustainable fuel, has garnered growing interest in recent years. The present study aims to investigate the impact of NH 3 blending on combustion and emission characteristics of a stoichiometric spark-ignition gasoline engine, with a particular emphasis on nitrogen-based emissions. The experimental investigation was complemented by chemical kinetic calculations. The results showed that NH 3 blending could effectively suppress engine knock, optimize combustion phase and improve thermal efficiency. For pure gasoline, advancing the spark timing resulted in increased NO x emissions. However, when NH 3 was blended, NO x emissions decreased with advancing spark timing, indicating a negative correlation with pressure. The NH 3 emission was attributed to the 'crevice mechanism' as well as the absorption/desorption in the lubricant oil film on the cylinder wall. Chemical kinetic analysis revealed that the NO x emission from NH 3 blended combustion is closely related to reactive radicals such as OH, H and O. The reduction in NO x emissions under high-pressure conditions was primarily attributed to the consumption of these reactive radicals via three-body reactions. Interestingly, NO x emissions initially increased with increasing NH 3 blending ratio but eventually followed a decreasing trend. This can be attributed to the lower combustion temperature, lower concentration of reactive radicals, and enhanced de-NO x reactions. [ABSTRACT FROM AUTHOR]

Ammonia-hydrogen engines are promising for heavy-duty, long-distance transportation, while the lower hydrogen energy share (X H2) in ammonia-hydrogen fuels facilitates hydrogen storage and use. In this study, a numerical investigation of engine performance based on an engine with ammonia port injection and different hydrogen jet ignition strategies at a fixed X H2 of 3% was conducted. The results show that too-early hydrogen injection results in lower hydrogen concentration and weak ignition in the pre-chamber, while too-late injection results in less hydrogen entering the main chamber, making more ammonia not mix well with the hydrogen, both of which increase the combustion duration. Hydrogen injection at the bottom dead center before the compression stroke mixes the hydrogen with the ammonia preliminary and leads to the formation of a high-temperature, slightly lean-burn region during the combustion, which worsens NO emission but reduces N 2 O emission. Compared with hydrogen single injection, splitting the same mass into two equal-mass injections favors higher peak pressures, shorter combustion durations, and higher indicated thermal efficiency (ITE). Injections at 360 and 90 °CA before the top dead center, respectively, achieve at least 1% relatively higher ITE with similar NO, NH 3 , and N 2 O emission performance compared with the single injection. • Study of a large bore H 2 active jet ignition NH 3 –H 2 engine at 3% H 2 energy share. • Multiple injection favors shorter combustion durations and higher engine efficiency. • Injection at the bottom dead center likely to lead to high NO and low N 2 O emissions. • Injecting hydrogen too early or too late both prolongs the combustion duration. [ABSTRACT FROM AUTHOR]

Spark-assisted compression ignition (SACI) has a promising potential to substantially improve engine's fuel efficiency. To this end, two exothermic stages in SACI combustion, flame propagation and auto-ignition, need to be well organized to increase control authority of bulk ignition timing especially in lean burn. In this study, three gasoline surrogates, namely EPRF, ETPRF and TPRF, formulated through blending ethanol/toluene with primary reference fuel (PRF) and having the same research octane number (RON) and octane sensitivity (S), were used to conduct experiments in a rapid compression machine (RCM) under lean engine-relevant conditions (10–30 bar and 722–862 K). Under different ethanol blending ratios, both ethanol's synergistic effect during auto-ignition and its stronger pressure dependence of flame speed (S Flame) than toluene were observed. The ethanol's synergistic effect is mainly attributed to its more HO 2 production and then faster consumption by benzyl which results in more OH radical production. As for the stronger pressure dependence of S Flame of ethanol, at 722 K, it is primarily determined by the stronger pressure dependence of H radical in EPRF's flame structure rather than the promotion effect from critical reactions on S Flame ; while at 862 K, these two factors influence the pressure dependence of S Flame simultaneously. Whatever the temperature is, third-body reactions have larger impacts on ethanol's S Flame than on toluene's. In this study, the relative magnitude of S Flame 's pressure dependence between ethanol and toluene shows rationality at lower φ and higher T , which is in line with the pressure exponents extracted from the existing high- p laminar burning velocities of ethanol and toluene. Further verification was made in a spark-ignition engine, which showed that low-carbon alcohols, exhibited stronger pressure dependence of S Flame than monophenyl aromatics in commercial gasoline, represented by toluene. The aforementioned characteristics of ethanol can be utilized under different engine loads and provide a reference in fuel design for lean SACI combustion. [ABSTRACT FROM AUTHOR]

Ammonia, ethanol, and methanol are promising carbon-neutral fuels in the future. Therefore, the use of alcohol fuels to enhance ammonia combustion in engines deserves further study. The performance of a spark ignition engine with high compression ratio at various loads was experimentally investigated to determine the impact of fuel composition. Specifically, ethanol and methanol were chosen as the primary fuels, while gasoline was assigned as the control group. Gaseous ammonia was introduced through the intake port. The results show that ammonia increases indicated thermal efficiency (ITE) in different ways when blended with different fuels, with the gasoline group obtaining higher ITE (2.8 % relative increase) by optimizing combustion phase, and the ethanol and methanol groups directly enhancing ITE (1.2 % and 0.8 % relative increase) with the same combustion phase due to reduced heat transfer loss. Methanol group shows a more ideal combustion duration under low loads and large ammonia blending ratios and achieves higher ITE due to higher oxygen content. Replacing gasoline with ethanol and methanol significantly reduces greenhouse gas (GHG) under similar NO x emissions, and the advantage of blending carbon-containing fuels with ammonia to reduce GHG is more significant under high loads and large ammonia blending ratios. • NH 3 raises fuel economy by optimizing combustion phase when blending with gasoline. • NH 3 raises fuel economy by reducing heat transfer loss when blending with alcohol. • Methanol performs better than ethanol for low loads and large NH 3 blending ratios. • NH 3 blending may raise greenhouse gas emissions due to worsened N 2 O emissions. [ABSTRACT FROM AUTHOR]

Ammonia is a carbon-free fuel with significant potential to minimize carbon emissions. However, ammonia has weak combustion properties, necessitating more study to improve its combustion performance in engines. A numerical simulation was conducted to evaluate the impact of fuel composition and injection-ignition synergy strategy on the performance of an ammonia-hydrogen spark ignition engine with liquid ammonia direct injection and hydrogen port injection. Specifically, two distinct injection modes were investigated: injection after intake valve close (IAIVC) and injection before top dead center (IBTDC). The outcomes reveal that the IBTDC mode generates a strong stratification of ammonia near the top dead center, resulting in ammonia-rich combustion, then leading to enriched hydrogen production and finally enhancing ammonia combustion and shorting the combustion duration. Liquid ammonia in-cylinder direct injection reduces the combustion temperature and decreases NO emissions. Optimizing the injection timing and spark timing based on a split injection strategy results in lower fuel consumption and emissions. Specifically, NO emissions decrease from 30.5 g/kWh to 21.7 g/kWh at a similar ITE (≈43.5%), and ITE increased from 43.3% to 44.3% for similar NO emission (≈30.0 g/kWh), respectively, with the reduction in both NH 3 and N 2 O emissions. • Ammonia-rich combustion in engines can produce hydrogen. • Ammonia pyrolysis to produce hydrogen shortens combustion duration. • Late injection of ammonia improves thermal efficiency and reduces emissions. • Liquid ammonia in-cylinder direct injection reduces NO emissions. [ABSTRACT FROM AUTHOR]

Super-knock has been the main obstacle to improve power density and engine efficiency of modern highly turbocharged gasoline engines. Previous researches show that pre-ignition is the inducement of super-knock, while detonation is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted to eliminate pre-ignition for suppressing super-knock indirectly. This work applies the fuel of methane to suppress detonation and then to suppress super-knock directly using a rapid compression machine (RCM). Furthermore, 1-D simulation model was set up to investigate the mechanism why methane could suppress detonation and super-knock. Finally, through single cylinder engine tests, this suppression strategy in engine practical usage was validated. The experiment and simulation results show that replacing the fuel from iso-octane or gasoline to methane while keeping other conditions identically could transfer detonation combustion mode to flame propagation. The peak pressure could be reduced dramatically and there is no pressure oscillation. Therefore, methane mixture could suppress detonation and then to suppress super-knock effectively, even if pre-ignition already exists. It could be an effective and practical control strategy to protect modern highly turbocharged engines. • First study concerning using methane to eliminate detonation for suppressing super-knock directly. • Replacing the fuel from iso-octane to methane could transfer the combustion mode from detonation to flame propagation. • 1-D direct simulation, RCM and engine experiments show that methane is an effective super-knock suppression strategy. [ABSTRACT FROM AUTHOR]

Highlights • RON/MON and S are decoupled to investigate effect on ignition delay times. • NTC is dominant by the timing of the first peak HO 2 concentration not peak value itself. • Ethanol can extend the beneficial area of high S contents compared to PRFs. • Region of NTC anti-knock benefit exists for fuels with high RON and low S. • Lower temperature of NTC influences higher S components utilization. Abstract Overall ignition delay time (OID) is used as indicator to characterize reactivity and anti-knock quality of fuel. In this work, both experimental measurement and modeling work have been conducted to evaluate OID properties for six ternary blends comprising n -heptane/ iso -octane/ethanol. Experimental measurement was performed in a rapid compression machine, while modeling work was based on the gasoline surrogate mechanism. The decoupling study on research octane number (RON)/motor octane number (MON) and octane sensitivity (S) has been carried out by adjusting fuel composition. The thermal conditions of experiments cover low-to-medium temperatures from 640 K to 740 K and pressures from 9.5 bar to 21 bar. Oxygen is utilized as the oxidizer while nitrogen and argon are regarded as the buffer gas to adjust thermal conditions. Combined with chemical kinetics analysis, negative temperature coefficient (NTC) behavior in the experiment is entirely attributed to iso -octane and the timing of the first peak HO 2 radical concentration has more significant influence on NTC behavior than peak value itself. Octane number and/or S cannot fully revealed fuel anti-knock quality, which is substantially influence by the property of fuels and thermal conditions. At low temperature (640 K), fuels with higher S exhibit better anti-knock performance. However, this advantage will disappear when temperature falls into NTC region. For engine operating in this area, high RON but low S fuels are recommended. Ethanol extends the beneficial area of high S contents due to its low temperature chemical inertness. The low temperature boundary of NTC for iso -octane can be regarded as the demarcation line to determine whether high S contents should be utilized, which is an important reference for fuel-engine co-optima research. [ABSTRACT FROM AUTHOR]

Abstract Super-knock is the main obstacle to improve power density and fuel efficiency of highly boosted gasoline engines. Previous investigations show that pre-ignition and detonation are the two key combustion processes of super-knock. The former is the inducement and the latter is the root reason of how super-knock could damage engines dramatically. Lots of studies have been conducted for suppressing super-knock through eliminating pre-ignition. Using a rapid compression machine, this study explores the stoichiometric propane or methanol mixture to suppress super-knock by eliminating detonation. Under the same pressure at the end of compression, the same fuel energy density and the same charged fresh air, the peak pressure of propane mixture could be reduced dramatically and the pressure oscillation could be eliminated, when compared to iso-octane mixture. The combustion could be transferred from detonation to flame propagation. For methanol mixture, the combustion process could be transferred from detonation to weak auto-ignition with low peak pressure and negligible pressure oscillation. These indicate that both propane and methanol mixture could suppress detonation and thus suppress super-knock effectively, even if pre-ignition occurs. It could be an effective and practical control strategy to protect modern highly turbocharged spark ignition engines. Highlights • First study using propane or methanol mixture to eliminate detonation for suppressing super-knock directly. • Using propane mixture could transfer the combustion mode from detonation to flame propagation. • Using methanol mixture could transfer the combustion mode from detonation to weak sequential auto-ignition. • Eliminating detonation with the propane or methanol mixture is conducted by preventing the generation of shock wave. [ABSTRACT FROM AUTHOR]

High-efficiency, low-emission ammonia-hydrogen engines with low hydrogen energy share (X H2) help promote a carbon-neutral transition in heavy-duty, long-distance transportation. In this study, numerical investigations were carried out on a hydrogen jet ignition (HJI) ammonia engine to study the effects of ammonia port fuel injection (PFI) and ammonia direct injection (ADI) on the engine performance, including combustion and emission characteristics, at X H2 = 3 % with varied spark timing (ST). The results show that the ADI mode has higher volumetric efficiency, lower in-cylinder temperature, and lower heat transfer loss compared with the PFI mode, and more retarded injection can further increase the volumetric efficiency and reduce the temperature. ADI mode results in a more inhomogeneous fuel distribution and a lower local equivalence ratio, with hydrogen consumed earlier than ammonia, potentially resulting in more rapid early-stage combustion. ADI mode with multiple injections is more conducive to engine performance than single injection and can achieve the higher indicated mean effective pressure (IMEP) and indicated thermal efficiency (ITE) than PFI mode. By further optimizing ST, the ADI mode improves ITE by 2.8 % and reduces NO emission by 70 % compared with the PFI mode under acceptable NH 3 and N 2 O emission conditions. • NH 3 direct injection and H 2 jet ignition are used together on a large bore engine. • NH 3 direct injection brings higher volume efficiency and lower heat transfer loss. • Direct injection raises thermal efficiency by up to 2.8 % over port fuel injection. • NH 3 direct injection reduces NO emission with acceptable NH 3 and N 2 O emission. [ABSTRACT FROM AUTHOR]

Spark assistance along with oxygenated components addition is a promising method to achieve stable compression ignition, high thermal efficiency and low particle emission. To this end, ethanol blended with non-oxygenated gasoline was fueled to an engine working with spark-ignition to compression-ignition (SICI) mode under air dilution and exhaust-diluted conditions. The effects of ethanol addition on engine performance including combustion characteristics, fuel economy, particle number (PN) emissions, were studied in two categories: changing research octane number (RON) by varying ethanol content and maintaining RON by changing fuel type. The results showed that ethanol addition by splash blending suppressed knock tendency, and the knock intensity could be lowered by up to 65–75% with increasing ethanol content. However, when maintaining the same RON, the ethanol-gasoline blend exhibited higher knock intensity than pure gasoline due to synergistic effects between ethanol and aromatics on auto-ignition. Compared to pure spark ignition with high-RON gasolines, using ethanol-gasoline blends under SICI could reduce the minimum fuel consumption rate by up to 25 g/(kW·h). To characterize the high-efficiency cycles under SICI, two dimensionless parameters were proposed by considering the ratios of heat release amount and duration between the flame propagation stage and auto-ignition stage. The two parameters showed good exponential correlation. As for emissions, blending ethanol could basically reduce PN emissions under SICI mode except for the cases with significant increase in nucleation particles, such as those with high knock intensity under stoichiometric condition and poor combustion quality under heavily exhaust-diluted conditions. The total PN reduction by blending ethanol is mainly due to the decrease of accumulation mode particles, during the stage of flame propagation rather than auto-ignition. Blending ethanol into non-oxygenated gasoline will directly increase unburned hydrocarbons and nitrogen oxides due to the low auto-ignition propensity of ethanol under stoichiometric or moderately lean conditions according to the temperature-pressure trajectory. Therefore, a dedicated combustion system with higher compression ratio and lean-boosted mixture is required to enhance ethanol's reactivity and achieve better fuel economy along with low PN emission for diluted SICI combustion. • Ethanol's effects under spark-ignition to compression-ignition mode is investigated. • Synergistic effect between ethanol and aromatics on auto-ignition was verified. • Two dimensionless parameters are proposed to characterize high-efficiency cycles. • Accumulation mode particles are lowered during flame propagation rather than auto-ignition. [ABSTRACT FROM AUTHOR]

• The effects of ammonia addition on knocking combustion of iso -octane were investigated. • Ammonia addition inhibited two-stage auto-ignition and decreased knock intensity. • The inhibitory effects were less pronounced at higher initial temperature conditions. • Ammonia addition elevated the energy density threshold for detonation occurrence. Ammonia has attracted much attention because of its carbon-free nature. Ammonia also has a high research octane number (RON), making it a promising anti-knock additive for conventional engine fuels. In this study, to investigate the effects of ammonia addition on the knocking combustion of iso -octane, a series of spark ignition experiments were conducted in an optical rapid compression machine. And compression ignition experiments were also carried out to measure the ignition delay times. The molar fractions of ammonia in iso -octane-ammonia blends were set as 80% (A80), 40% (A40), and 0% (A0), and the initial thermodynamic conditions were 630–840 K and 8–25 bar. Chemical kinetics analysis of the end-gas was also performed with a blended reaction mechanism obtained by directly merging the LLNL iso -octane mechanism and the Glarborg ammonia mechanism. The experimental results showed that the ignition delay time increased with increasing ammonia fraction, but this trend could only be qualitatively predicted by the blended mechanism. At the same initial pressure and temperature conditions, the knock intensity (KI) decreased with increasing ammonia fraction, but the inhibitory effect was less pronounced at high initial temperature conditions. At the same initial energy density and high-temperature conditions, the KI of A40 and A80 was higher than A0, which was ascribed to the higher pressure requirement for the blended fuels with higher ammonia fractions to achieve the same energy density as those with lower ammonia fractions. At low temperature and pressure conditions, the auto-ignition showed a two-stage characteristic. With the increasing fraction of ammonia, the first-stage auto-ignition shifted from supersonic to subsonic, and the second-stage detonative auto-ignition gradually disappeared. The weakened auto-ignition with increasing ammonia fraction was attributed to the less reactive end-gas, as evidenced by the lower mole fraction of OH radical and lower heat release rate in the end-gas at the auto-ignition timing. When using the ε- ξ diagram to examine the auto-ignition mode, the experimental detonation cases were well distributed in the detonation peninsula, but the non-detonation and critical detonation cases were scattered irregularly. [ABSTRACT FROM AUTHOR]

Based on the fact that actual cerebral cortex has different structure, a new heterogeneous simplified pulse coupled neural network (HSPCNN) model is proposed in this paper for image segmentation. HSPCNN is constructed with several simplified pulse coupled neural network (SPCNN) models, which have different parameters corresponding to different neurons. An image is segmented by HSPCNN into several regions according to their gray levels. Moreover, the parameter of HSPCNN is set automatically in this paper, the experimental segmentation results of the gray natural images from the Berkeley Segmentation Dataset (BSD 300) show the validity and efficiency of the proposed segmentation method. Finally, an evaluation index is proposed to measure the segmentation result. [ABSTRACT FROM AUTHOR]

Dedicated exhaust gas recirculation (D-EGR) is to generate H 2 via fuel-rich combustion and viewed as a potential technique to meet future emission regulations without further after-treatment. In this study, firstly, the H 2 /CO formation through fuel-rich combustion in a single-cylinder natural gas spark ignition engine was quantitatively characterized by gas chromatography. Then, the effect of H 2 /CO addition on stoichiometric natural gas combustion performance and emission characteristics at 15% and 20% EGR levels was investigated. Finally, reaction path analysis and the brute-force sensitivity of ignition delay were conducted to evaluate the effect of H 2 addition on reaction process. The yields of H 2 and CO approximately linearly increased from ∼2% to ∼10% as the equivalence ratio varied from 1.1 to 1.5. The H 2 /CO addition accelerated the flame speed of mixture and significantly shortened the combustion duration, significantly improving the indicated thermal efficiency and the total unburned hydrocarbon with the acceptable penalty of increased NOx and CO emissions. Numerical results revealed that OH + H 2 = H + H 2 O and H + O 2 = O + OH were the most sensitive reactions with the presence of H 2 . This study delivered a quantitative basis for the optimization of D-EGR fueling strategies in natural gas engines. [ABSTRACT FROM AUTHOR]

Increasing the compression ratio of gasoline engines is a promising method to increase the engine's fuel economy. However, engine knock caused by auto-ignition is still a large obstacle to improving thermal efficiency and engine load for high compression ratio hybrid engines. Spark induced compression ignition (SICI) is an effective way to utilize auto-ignition to solve the aforementioned problems. Meanwhile, ammonia, a carbon-free fuel, with an outstanding antiknock property, has the great potential to be used in SICI mode. In this study, the effects of ammonia addition on knock suppression, combustion characteristics, thermal efficiency, and emission performance were investigated in a high compression ratio (15.5), four-valve, single-cylinder gasoline engine under SICI combustion mode. In experiments, gasoline was directly injected into the cylinder while ammonia was injected into the intake port. The results show that blending ammonia could resist engine knock and improve thermal efficiency. Within the knock limitation, the duration of flame propagation under ammonia blending conditions could be shortened and meanwhile, the auto-ignition becomes weakened compared with pure gasoline. Benefiting from combustion phase optimization, the thermal efficiency and engine load could be increased or maintained at optimal ammonia blending ratio. The maximum increase of thermal efficiency and engine load is 2.46% and 0.2 MPa respectively. Moreover, the increased engine load can extend the limit of the ammonia blending ratio. For nitrogen emissions, blending ammonia results in NO x emission deterioration due to the formation of fuel-type NO x. NO x emission has a weak dependence on the ammonia blending ratio, and the trend of NO x emission varied with spark timing is opposite to pure gasoline conditions, which is closely related to the pressure sensitivity of fuel-type NO x. Ammonia slip was also detected in the engine exhaust because of the incomplete combustion. • This is the first study revealing the effect of NH 3 addition on SICI mode. • NH 3 addition with spark timing adjustment could suppress knock without reducing flame speed. • Thermal efficiency increases through combustion phase optimization when blending NH 3. • There is a negative correlation between pressure and fuel-type NO x. [ABSTRACT FROM AUTHOR]

• Combustion and particle number (PN) emissions characteristics of spark induced compression ignition (SICI) were investigated. • Ethanol blending accelerated flame propagation, optimized CA50 and suppressed autoignition. • Knock could be effectively suppressed by split injection with appropriate split ratio and injection timing. • PN emissions of ethanol was much lower than that of the hydrocarbon fuel (93#) or the ethanol-blended fuel (80#E15). Spark induced compression ignition (SICI) is a promising method to achieve high thermal efficiency as well as a robust control of combustion. With the increase of compression ratio, the popularization of bioethanol and the development of NO x emission control technologies for lean combustion, it is easier to realize high-efficiency and clean SICI combustion under medium-to-high load. Combining split injection with various excess air ratio (λ), the effects of ethanol in fuels with the same RON value (93# and 80#E15) on combustion characteristics, indicated thermal efficiency and PN emissions were investigated in a high compression ratio (15.5), single-cylinder, four-stroke engine. For comparison, the performance of pure anhydrous ethanol (E100) was also studied in this engine. The results show that ethanol-contained fuels can accelerate the flame propagation but suppress the heat release rate (HRR) of autoignition, and autoignition does not even exist in E100 combustion. Consequently, CA50 is advanced but combustion duration is prolonged for ethanol-contained fuels. It is highlighted that the predominant contribution of ethanol in improving the indicated thermal efficiency is the optimization of CA50 rather than the combustion iso-volume. In addition, the knock intensity is effectively suppressed, and the combustion process can be optimized by split injection with appropriate EOI2 (the end of the second injection) and split ratio for gasoline and ethanol-blended fuel, which further improve the indicated thermal efficiency. PN emissions exhibit a generally positive correlation with knock intensity in SICI mode and SICI mode can reduce the particles generated by the spray-wall impingement and ultra-rich mixture near the wall. Lean combustion can simultaneously suppress flame propagation and autoignition and is more effective in PN emissions reduction for fuels of 93# and 80#E15 within the cycle-to-cycle variation (COV) limitation rather than E100. However, E100 demonstrates two orders of PN emissions smaller than 93# and 80#E15 due to its high oxygen fraction content. [ABSTRACT FROM AUTHOR]