Reactivity Of Fuels Research Articles

The influence of Michael acceptors on the structural reactivity of renewable fuels

Decarbonising heavy-duty transport relies on understanding how chemical structures present in biomass may improve the combustion of otherwise overlooked biofuels.

The relationship between fuel reactivity and exergy features in combustion process

Fuel reactivity is a flexible and feasible method to control combustion, thus reactivity-controlled compression ignition (RCCI) has the potential to achieve lower emissions and higher thermal efficiency. However, the exergy loss during combustion hinders the further exergy-work transform, which limits the thermal efficiency further improvement. The present work focuses on the relationship between fuel reactivity and exergy which contains exergy loss (EL) and thermomechanical exergy (Exthermo due to the temperature and pressure imbalance between system and reference state). By varying the reactivity of primary reference fuels (PRFs) with different initial boundary conditions, it is found that different PRFs could not reduce EL and change Exthermo under same condition but could change the exergy loss rate as well as the EL structure by influencing low-temperature combustion. In terms of boundary conditions, both high temperature and lean mixture could improve Exthermo but with different mechanism: high temperature saving EL into Exthermo, lean mixture recovering incomplete combustion exergy (Exincom) into Exthermo. By contrast, equivalence ratio more easily affects EL and Exthermo. In three-dimensional situation, it is found that decreasing reactivity of PRFs leads more in-cylinder area below equivalence ratio 1.0 which benefits for the Exthermo improvement, but it would cause Exthermo structure mutation which is unfriendly for the smooth output of work near top dead center. The conversion of fuel exergy to final useful work is indirect, and there exist two sub-processes: fuel exergy to thermomechanical exergy then thermomechanical exergy to useful work. The final exergy-work transform efficiency is influenced by both processes. Therefore, using low reactivity fuel as PRF90 in engine is favor of the exergy from fuel transforming into Exthermo by more lean mixture, adopting high reactivity fuel as PRF0 is favor of the Exthermo sequential structure which is benefit for the combustion control and useful work extract near top dead center. Therefore, there is potential to further improve the thermal efficiency by using a dual-fuel combination, with the low reactivity fuel creating a low equivalence ratio region to improve the thermomechanical exergy, and the high reactivity fuel achieving precise phase control during the combustion.

A Review on Recent Developments of RCCI Engines Operated with Alternative Fuels

Environmental concerns over automotive exhaust emissions and consumer demand for higher fuel efficiency have led to the development of low-temperature combustion concepts. The reactivity-controlled compression ignition (RCCI) engine is one among them and has the potential to reduce NOx and smoke emissions simultaneously. In this concept, a low-reactivity fuel is injected into the intake port and another high-reactivity fuel is injected into the cylinder directly. This results in reactivity stratification and provides more control over the rate of heat release. However, operating parameters such as reactivity of fuels, premixing ratio, injection strategies, exhaust gas recirculation ratio, piston bowl geometry, and compression ratio influence emissions formation. The article reviews recent developments on the effect of the above operating parameters on the performance and emission characteristics of RCCI engines operated with alternative fuels. The combustion strategies used to extend the RCCI mode to higher loads are also reviewed. Applications of computational fluid dynamics (CFDs) to design the combustion chamber for RCCI engines are discussed. The need for further improvements in the CFD models for RCCI engines is explained. After presenting a thorough review of recent literature, directions for future research on RCCI engines are proposed.

The Dynamic Nature of Graphene Active Sites in the H2O Gasification process: A ReaxFF and DFT Study

A steam-rich environment is a more promising application scenario for future coal-fired processes, while active sites are the key factor that determines the reactivity of carbonaceous fuels. The steam gasification process of carbon surfaces with different numbers of active sites (0, 12, 24, 36) was simulated using reactive molecular dynamics in the present study. The temperature for the decomposition of H2O and the gasification of carbon is determined using temperature-increasing simulation. The decomposition of H2O was influenced by two driving forces, thermodynamics and active sites on the carbon surface, which dominated the different reaction stages, leading to the observed segmentation phenomenon of the H2 production rate. The existence and number of initial active sites have a positive correlation with both two stages of the reaction, greatly reducing the activation energy. Residual OH groups play an important role in the gasification of carbon surfaces. The supply of OH groups through the cleavage of OH bonds in H2O is the rate-limiting step in the carbon gasification reaction. The adsorption preference at carbon defect sites was calculated using density functional theory. Two stable configurations (ether & semiquinone groups) can be formed with O atoms adsorbed on the carbon surface according to the number of active sites. This study will provide further insights into the tuning of active sites for advanced carbonaceous fuels or materials. The large-scale atomic/molecule massively parallel simulator (LAMMPS) code combined with the reaction force-field method was used to carry out the ReaxFF molecular dynamics simulation, where the ReaxFF potentials were taken from Castro-Marcano, Weismiller and William. The initial configuration was built using Packmol, and the visualization of the calculation results was realized through Visual Molecular Dynamics (VMD). The timestep was set to 0.1 fs to detect the oxidation process with high precision. PWscf code in QUANTUM ESPRESSO (QE) package, was used to evaluate the relative stability of different possible intermediate configurations and the thermodynamic stability of gasification reactions. The projector augmented wave (PAW) and the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE-GGA) were adopted. Kinetic energy cutoffs of 50 Ry and 600 Ry, and a uniform mesh of 4 × 4 × 1 k-points were used.

On tert-butyl hydroperoxide in ignition intensification of n-heptane under engine-relevant condition

The higher reactivity of hydrocarbon fuels is necessary to match supersonic airflow in scramjet engines and micro-scale effect in micro-internal combustion engines. Tert-butyl hydroperoxide (TBHP) was selected in this study as an additive reactively enhancer to explore its capacity to intensify the auto-ignition of a typical hydrocarbon (n-heptane). Ignition delay times (IDTs) of n-heptane doped with TBHP were thus measured at engine-relevant conditions covering pressure of 20 atm, temperatures of 650 – 1250 K, and equivalence ratios of 0.5 and 1.0. A nonlinear promotion effect of TBHP on the auto-ignition of n-heptane was observed. Detonation caused by TBHP in region 2 has an impact on actual reflected shock temperature and pressure at high beginning temperatures (> 1000 K). With the addition of TBHP, the negative temperature coefficient (NTC) behavior of first-stage ignition eventually vanishes accompanied with a weakened main NTC of IDT. Analyses of exothermic reactions, reaction pathways, and species evolution indicated that the quick production of ȮH and ĊH3 radicals from TBHP enhances the reactivity of n-heptane and affects the main exothermic reaction in the early period. This study reveals the potential of TBHP to be used in the ignition regulation of the combustion of hydrocarbon fuels and provides a methodological approach for the efficient utilization of hydrocarbon fuel in scramjet engines and micro internal combustion engines.

Study on cool flame radical index and oxygen concentration dependence of oxygenated fuels

The growing utilization of alternative fuels has sparked an interest in understanding the combustion characteristics of oxygenated fuels at low temperature. Cool flame extinction limits provide direct measures of the low-temperature reactivity of fuels and critical information for advanced low-temperature combustion engine design. This paper investigates the effects of fuel and oxygen concentrations on the extinction limits of diffusion cool flames for oxygenated fuels, including dimethyl ether, methyl decanoate, and 1-dodecanol in an atmospheric counterflow burner. The cool flame radical indexes of these oxygenated fuels are developed by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The results show that the ranking of low-temperature reactivities of long carbon chain oxygenated fuels, compared with n-alkane, is ether > n-alkane > alcohol > ester for a similar carbon number. Furthermore, due to the critical role of multiple oxygen addition reactions in the low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is also explored. The results show that the cool flame extinction limits of the oxygenated fuels are proportional to a nth power of the oxygen concentration, [O2]n, due to the combined effects of multiple oxygen addition reactions and the negative temperature coefficient in low-temperature chemistry. Additionally, the measured n number of 1-dodecanol is found to be larger than 2, which suggests the existence of the third oxygen addition reactions in low-temperature chemistry for large alcohols.

The influence of thermochemistry on the reactivity of propane, the pentane isomers and n-heptane in the low temperature regime

The influence of thermochemistry on the reactivity of fuels at low temperatures (600–1000 K) is studied here. Specifically, the effect of different sets of thermochemistry on chemical model predictions is explored, where various sets are calculated at different levels of theory in addition to recently updated group additivity values. Experimentally measured ignition delay times for propane, the pentane isomers and n-heptane are simulated using NUIGMech1.2 and replacing the thermochemistry of the low-temperature species with the calculated values. For propane, three different thermochemistry sets were calculated, namely CCSD(T)-F12/TZ-F12//B2PLYP-D3/TZ//B2PLYP-D3/TZ (QM1), CCSD(T)-F12/TZ-F12//B2PLYP-D3/TZ//ωB97X-D/TZ (QM2) and B2PLYP-D3/TZ/ωB97X-D/6-31G*//ωB97X-D/6-31G* (QM3). The QM2 results provide parameters to optimize new group additivity (NGA) values which are used to calculate the fourth set of thermochemistry. The model predictions using these four sets are compared to those using NUIGMech1.2 for propane. As the QM1 and QM2 calculations are expensive, the thermochemistry calculated from the QM3 and NGA calculations are used in the pentane isomer and n-heptane models. For all of the models, it is found that the thermochemistry of the species involved in the low-temperature reaction sequence (RH, Ṙ, RO2H, RȮ2, Q˙OOH and Ȯ2QOOH species) significantly affect fuel reactivity. The NGA values were developed based on all of these species except Ȯ2QOOH radicals. The thermochemistry of Ȯ2QOOH species cannot be accurately calculated with the NGA representations due to the importance of non-next-nearest neighbor interactions of –OOH substitution. Further development of the NGA method to capture such interactions is in progress. Overall, the model developed using the NGA thermochemistry shows better agreement with experimental data than the model using thermochemistry from affordable and prominent QM methods, such as QM3. Based on the results presented for propane, the pentane isomers and n-heptane, the thermochemistry calculated using the NGA method can be used to model the oxidation of higher order hydrocarbons at low temperatures.

Effects of Carbon Chain Length on N-Alkane Counterflow Cool Flames: A Kinetic Analysis

An in-depth understanding of the low-temperature reactivity of hydrocarbon fuels is of practical relevance to developing advanced low-temperature combustion techniques. The present study aims to study the low-temperature chemistry of several large n-alkanes with different carbon chain lengths in counterflow cool diffusion flames by kinetic analysis. The large n-alkanes that were chosen are n-heptane (NC7H16), n-decane (NC10H22) and n-dodecane (NC12H26), which are important components of practical fuels. Firstly, the thermochemical structure of a typical cool diffusion flame is understood through its comparison with that of a hot diffusion flame. The boundary conditions, including the ozone concentration, fuel concentration and flow velocity—where cool flames can be established—are identified with a detailed chemical mechanism that evaluates the low-temperature reactivity of the investigated n-alkanes. The results show that the n-alkane with a longer carbon chain length is more reactive than the smaller one, thereby indicating the order of NC12H26 > NC10H22 > NC7H16. This trend is qualitatively similar to the findings from non-flame reactors. The reaction pathway and sensitivity analysis are performed to understand the effects of carbon chain length on the low-temperature reactivity. The contribution of an n-alkane with a longer carbon chain to the dehydrogenation reaction, oxidation reaction and isomerization reaction is greater than that of a smaller n-alkane, and abundant O and OH radicals are generated to promote the fuel low-temperature oxidation process, thereby resulting in an enhanced low-temperature reactivity. The effects of ozone addition on the low-temperature reactivity of n-alkanes are also highlighted. It is found that the addition of ozone could provide a large number of active O radicals, which dehydrogenate with the fuels to generate OH radicals and then promote fuel low-temperature oxidation. The present results are expected to enrich the understanding of the low-temperature characteristics of large n-alkanes.

Understanding the reactivity of magnesium powder subjected to various aging conditions

Metal fuels are emerging as one of the promising alternatives to fossil fuels by providing considerable heat energy without CO2 generation. In particular, magnesium (Mg) is utilized in rocket propellants, fuel cells, and energy storage systems by virtue of its high reactivity, great energy density, and low cost. While its performance degradation when exposed to both short-term and prolonged storage conditions is expected, the clear aging characteristics have not been explained in the past. Two products of hygrothermal aging process of Mg are identified as Mg(OH)2 and MgO, where Mg(OH)2 decomposes at a relatively low temperature for accelerating the oxidation process while MgO tends to desensitize the reactivity of Mg by the surface oxidation of particles. Here, a decreased on-set reaction temperature or enhanced reactivity is observed on relatively short-term aged samples, which is attributed to the formation of Mg(OH)2. For accelerated aging conditions beyond 25 weeks, the oxide film covers the entire particle surface, and thus only the core oxidation could occur, which results in an increase of activation energy. This observation is contrary to the decreased reactivity of metal fuels subjected to aging in general. Moreover, the reactivity based on the stoichiometry is examined by varying the amount of oxygen supplied. The activation energy for long-term aging periods tended to increase under oxygen-rich condition, which can be ascribed to the reaction between Mg(OH)2 and O2. Also, the effects of the generation or the decomposition of various intermediates of the oxidation process can be included. Thus, the present study identified the peculiar enhancement of reactivity of Mg subjected to hygrothermal aging and further clarified the roles of surface and core oxidations as they both affect the reactivity under the oxygen enrichment conditions.

Experimental observation on the end-gas autoignition and detonation affected by chemical reactivity in confined space

The deflagration-to-detonation transition remains one of the most interesting and mysterious physical phenomena in the combustion of energetic materials, which contains substantial complicated and nonlinear characteristics. In the present work, the effect of the chemical reactivity of different fuels and diluent gases on the end-gas autoignition and detonation development in a confined space was investigated. Five fuels (hydrogen, methane, iso-octane, n-heptane, and PRF50) and three diluent gases (argon, nitrogen, and carbon dioxide) were used to change the chemical reactivity. The results showed that both the chemical reactivity and shock wave had a significant influence on the end-gas autoignition and detonation development. For mixtures with different diluent gases, it was observed that the transition thresholds (denoted by critical oxygen fraction) increased in the order of argon, nitrogen, and carbon dioxide. Different detonation modes with varying shock compressions were observed under different diluents for n-heptane. Although the flame propagation of different fuels differs at 21% oxygen fraction, end-gas autoignition and detonation development processes can still be observed in all kinds of fuels when the oxygen fraction was elevated to a certain value. The transition thresholds increased in the order of hydrogen, n-heptane, PRF50, iso-octane, and methane. Further analysis revealed that the fuel with a shorter ignition delay usually required a lower flame tip velocity, accomplished with a delayed occurrence of detonation. In addition, the transition threshold was determined by the chemical reactivity and flame speed.

Investigation on the autoignition characteristics of propanol and butanol isomers under diluted lean conditions for stratified low temperature combustion

Low-temperature combustion (LTC), which can be achieved through either fuel-lean mixtures or high dilution, is an effective way to reduce NOx and soot emission for an internal combustion engine. Alcohols are considered suitable fuels for LTC as they have the advantage of better fuel-air mixing, such as high autoignition resistance and fast vaporization. Propanol and butanol can be produced as bio- or electro-fuels, which are carbon-neutral and become candidates as transportation fuels. This work focuses on investigating the autoignition and emission behavior of these alcohols regarding the application in stratified LTC. The autoignition properties of propanol and butanol isomers have been investigated in a rapid compression machine (RCM) under 90% dilution, at temperatures from 800 K to 1100 K, pressures of 20 and 40 bar and equivalence ratios of 0.25, 0.5 and 0.9. Under all investigated conditions, the order of reactivity among butanol isomers is consistent, namely n-butanol > isobutanol ≅ 2-butanol > tert‑butanol, while n-propanol is always more reactive than isopropanol. The reactivity of propanol isomers is similar to that of iso and 2-butanol. It was observed in the measurement that the difference between the reactivity of different fuels was not constant in the investigated temperature range, which was explained by the autoignition chemistry transition between 800 K and 950 K. According to the modeling, emissions of all butanol and propanol isomers are similar. The non-diluted mixtures can only achieve low-temperature combustion (LTC) in extreme lean conditions, while the diluted mixtures can achieve LTC in a wider range of equivalence ratios and have both low NOx and hydrocarbon emissions.

Experimental and kinetic modeling study of di-n-propyl ether and diisopropyl ether combustion: Pyrolysis and laminar flame propagation velocity

To explore the fuel isomeric effect on ether combustion characteristics, pyrolysis, and laminar flame propagation of di-n-propyl ether (DPE) and diisopropyl ether (DIPE) were investigated. A new kinetic model of DIPE was developed based on our recent DPE model [Fuel 298 (2021) 120797]. The pyrolysis experiments were carried out in two jet-stirred reactors at near-atmospheric pressure with good agreement on the measurements using synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography/mass spectrometry. The decomposition profile of DIPE showed a faster trend than that of DPE, which can be attributed to the faster alcohol elimination reaction of DIPE. The dominant roles of alcohol elimination reaction and H-abstraction reactions in fuel consumption explain the production of fuel-specific oxygenated species, i.e. n-propanol and propanal in DPE and i-propanol, acetaldehyde and acetone in DIPE. The laminar burning velocities of DPE and DIPE were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 373 K and pressures of 1–10 atm. It was found that the linear DPE propagated faster than DIPE with the branched structure under all investigated conditions. Rate of production analysis and sensitivity analysis were also performed to elucidate the key radicals and reactions responsible for the remarkable reactivity of isomeric fuels. Fuel structures have a great influence on the distribution of radical pools, resulting in the easy formation of active radicals in DPE flames such as vinyl and ethyl and stable radicals in DIPE flames like methyl and allyl. The successive decomposition reactions of these dominant radicals promote the DPE flame and inhibit the DIPE flame propagation respectively, which explains the higher laminar burning velocities and reactivity of DPE than that of DIPE. Furthermore, the present model was also examined against the literature data, including pyrolysis and oxidation in the jet-stirred reactor and flow reactor.

Combustion performance and kinetic modeling of lignite blended with torrefied biomass of different origin

ABSTRACT Biomass materials of different origin were upgraded by torrefaction and blended with lignite. Combustion characteristics, reactivity, and synergistic effects were determined through thermal analysis tests. An independent parallel reactions model was developed to describe combustion kinetics. According to the results, torrefaction delayed ignition and reduced the reactivity of biomass fuels. Blending of lignite with either raw or torrefied materials exhibited some synergy and improved the reactivity of lignite up to two times/fold. The model applied fitted the experimental results accurately. The kinetic parameters of lignite/torrefied biomass blends were closer to the values corresponding to lignite (ΔΕl = 74–235 kJ/mol, ΔΕl/t biomass = 80–223 kJ/mol), pointing to a higher similarity in mechanism of combustion and thermal behavior.

ReaxFF Molecular Dynamics Simulations of Thermal Reactivity of Various Fuels in Pyrolysis and Combustion

The methodology development and applications of ReaxFF molecular dynamics (ReaxFF MD) in unraveling the complex reactions and kinetics for pyrolysis and oxidation of organic systems are reviewed. Particular attention is given to the large-scale ReaxFF MD simulation method of similar to 10,000 atoms and practical simulation strategies to overcome the temporal-spatial limits as much as possible. High-performance computing codes running on CPU cluster/supercomputers and on GPU were overviewed. GPU-enabled code like GMD-Reax is revolutionizing large-scale ReaxFF MD simulations to run mainly on a single GPU. A step-forward for reaction analysis was achieved in the code of VARxMD to reveal detailed reactions based on the identification of bond types and unique species that allows for categorization of reaction species and pathways through structure searching of reaction sites and reactants/products. Efforts for extracting rate constants and kinetics modeling from unperturbed kinetics of ReaxFF MD simulations are reviewed, and challenges remain. The important factors of model scale effects, elevated simulation temperature, and possible validation of ReaxFF MD reaction simulation results theoretically and experimentally are illustrated and discussed. The novel hybrid simulation strategies proposed recently that may expand applications of ReaxFF MD to connect with realistic scenarios of engine combustion and relevant chemical effects on aerospace vehicle materials are briefed. The simulation practices of large-scale ReaxFF MD in understanding the similarities and differences of reactivity and reaction mechanisms in pyrolysis of liquid hydrocarbon fuels, solid fuels (coal, biomass, polymer), and energetic materials of CL-20 and its cocrystals are briefly described, which indicates that large-scale ReaxFF MD simulations are a feasible and straight means to capture the dynamics of an almost entire reaction process and to evaluate reactivity of organic systems computationally. There is still a great deal of work to be done to push the boundaries of ReaxFF MD simulations forward. With the constant improvement of the ReaxFF force field and reaction analysis capability, ReaxFF MD playing a key role in understanding reaction mechanisms and its potential in kinetics modeling of pyrolysis and oxidation of various fuels can be expected.

Combustion kinetics of alternative jet fuels, Part-III: Fuel modeling and surrogate strategy

In Part III of our study on alternative aviation fuels, we present a comprehensive database of modeled speciation data consisting of seven hydrocarbons of varying molecular structure and 26 alternative and conventional aviation fuels. The speciation data is obtained from the DLR atmospheric high temperature flow reactor with a coupled molecular beam mass spectrometry (MBMS) detection system (Part-I). The chemical reactivity of these real liquid fuels is investigated both experimentally and numerically. For modeling, detailed fuel surrogates (up to 14 components) are employed for characterizing the fuels. The surrogate formulation strategy is defined based on the fuels’ compositional analysis.This work employs high temperature reaction kinetic mechanism for the combustion of wide variety of hydrocarbons of varying molecular classes from n-, iso-, cy-paraffins, to five-ring aromatics presented in Part-II. The reaction mechanism is applied to model 26 aviation fuels using 21 different validated fuel surrogate components to predict the fuel intermediates and soot precursors measured in the reactor.This work aims to identify the effect of fuel surrogate components on the intermediates’ formation and its influence on the emissions. Through compilation of many fuels with composition of wide range of chemical classes, we provide a systematic evaluation of how the fuel composition can be used to extract information of specific fuel intermediates and emissions formed.

Chemical looping combustion-adsorption of HCl-containing syngas using alkaline-earth coated iron ore composites for simultaneous purification and combustion enhancement

• A novel three-stage CLCA process for HCl-containing syngas is proposed. • IO-Ba and IO-Sr can be used as oxygen carrier-HCl sorbent composites for CLCA. • Ba/Sr/Ca sorbents improve reactivity of iron ore in syngas combustion. • High temperature (950 °C) and H 2 O are required for the sorbent regeneration. • SR stage enhances syngas combustion owing to mitigation of OCS sintering. For high temperature purification of HCl-containing syngas, chemical looping combustion-adsorption (CLCA) was developed using Ba-, Sr- and Ca-coated iron ore (IO) as bifunctional oxygen carrier-HCl sorbent (OCS) composites. The behavior of OCS at fuel reaction (FR), air reaction (AR) and sorbent regeneration (SR) stages was studied. It was found that compared with conventional chemical looping combustion (CLC), CLCA could maintain high dechlorination performance at FR stage, without any deterioration after multiple cycles. Among the OCS composites, Ba and Sr-coated IO showed extraordinary performance as OCS for CLCA process of HCl-containing syngas, whereas Ca-coated IO exhibited low HCl removal ability. During 5-cycle test of Ba and Sr-coated IO, the HCl concentration in flue gas from fuel reactor was 20–30 ppmv, corresponding to approx. 95% dechlorination efficiency. For the selection of SR conditions, high temperature (950 °C) was required to achieve stable and high regeneration performance, while H 2 O was the effective regenerating gas for Ba-coated IO, instead of CO 2 . Regarding syngas combustibility, the coating with alkaline-earth metal compounds improved the oxidizing ability of IO leading to higher combustion efficiency compared to pure IO. Ba- and Sr-coated IO showed greater performance in syngas combustion than Ca-coated IO due to the greater electron donating ability and base strength. The addition of SR stage was also found to promote the syngas combustion performance, due to the mitigation of sintering problem from molten metal chlorides. The results suggest that the CLCA process could be an efficient solution for processing HCl-containing syngas.

Evaluation of kinetics mechanisms ability to predict natural gas reactivity using a fuel substitution method

Previous homogeneous-charge compression-ignition engine fuel substitution experiments1 were used to test the performance of five different chemical kinetics mechanisms for natural gas fuels. A two-zone model was used to simulate the engine experiments using the five mechanisms tested. Simulations of engine experiments for six distinct operating conditions using based fuel mixtures of methane/hydrogen and methane/propane were performed along with simulations of fuel substitution sweeps with fixed boundary conditions using hydrogen, ethane, ethylene, propane, and n-butane as test fuels. The simulation results were used to quantify the ability of mechanisms to predict both the absolute reactivity of the reference fuels and the reactivity of the test fuels relative to the reference fuels using two metrics: required temperature at intake valve closure (IVC) with fixed fuel concentrations and required reference fuel concentrations with fixed IVC temperature. The five mechanisms used for this study showed substantially different results, most notably related to their propane chemistry. Simulations using the LLNL mechanism resulted in the closest match to propane engine test results, while the Aramco and C5-49 mechanisms showed some disagreement, and the GRI and UBC mechanisms showed much larger discrepancies. Simulations with all five mechanisms showed good agreement with hydrogen engine test results.

Influence of intermediate temperature heat release on autoignition reactivity of single-stage ignition fuels with varying octane sensitivity

This study investigates the effects of intermediate temperature heat release (ITHR) on autoignition reactivity of full boiling range gasolines with different octane sensitivity through intake temperature and simulated exhaust gas recirculation (EGR) sweeps in a homogenous charge compression ignition (HCCI) engine. To isolate the ITHR effects, low temperature reactivity was suppressed through the use of high intake temperature and low intake oxygen mole fraction. For quantification of ITHR, a new method was applied to the engine data by examining the maximum value of the second derivative of heat release rate. Combustion phasing comparisons of fuels with octane sensitivity showed that fuel with less octane sensitivity became more reactive as intake temperature and simulated EGR ratio decreased, while fuel with higher octane sensitivity had a reverse trend. For all of the fuels that were tested, the amount of ITHR increased as the intake temperature and oxygen mole fraction increased. These ITHR trends, depending on octane sensitivity, were almost identical with the trends of combustion phasing, showing that ITHR significantly affects fuel autoignition reactivity and determines octane sensitivity.

Energizing Fuel Cells with an Electrically Rechargeable Liquid Fuel

Direct liquid fuel cells with high energy density and facile fuel storage have received increasing attention. Owing to the poor reactivity of conventional liquid fuels, they not only require noble metal catalysts for their oxidation but also exhibit limited performance. Here, we report a power-generation system, the direct liquid e-fuel cell, where “e-fuel” stands for “electrically rechargeable fuel.” This e-fuel cell consists of a catalyst-free graphite-felt anode and a conventional oxygen cathode separated by a proton exchange membrane, producing a maximum current density of 750 mA cm−2, a peak power density of 293 mW cm−2, and an energy efficiency of 42.3% at room temperature, which is much higher than the performances achieved by conventional direct liquid fuel cells. This emerging technology, capable of fast recharging, could be a powerful, efficient, cost-effective, and durable power-generation device, showing great potential for commercialization in the fuel cell electric vehicle industry.

Effects of branch structure of alkylbenzenes on spray auto-ignition of n-decane and alkylbenzenes blends

Spray auto-ignition characteristics of the blends of n-decane and several alkylbenzenes were carried out on a heated constant-volume spray combustion chamber. The derived cetane numbers of the fuel blends were determined, and the temperature-dependent ignition delay times and combustion durations were measured across a range of temperatures from 808 to 911 K. The results reveal that blending alkylbenzene to n-decane inhibits fuel spray auto-ignition propensity. For mono-alkylbenzenes, the fuel blend containing toluene has a higher derived cetane number than those with ethylbenzene and n-propylbenzene, but has a lower derived cetane number than the fuel blend containing n-butylbenzene. For those binary fuels containing ethylbenzene, n-propylbenzene and n-butylbenzene, their derived cetane numbers increase with the side alkyl chain length. The derived cetane numbers of the fuel blends with C8H10 isomers follow the trend of n-decane/ o-xylene > n-decane/ethylbenzene > n-decane/ m-xylene ∼ n-decane/ p-xylene, given the alkylbenzene blending fraction. For the blends with C9H12 isomers, those containing 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene have the highest and lowest derived cetane numbers, respectively, while the fuel blends containing 1,2,4-trimethylbenzene, n-propylbenzene and i-propylbenzene have comparatively intermediate derived cetane numbers. The blending effects of alkylbenzenes on ignition delay time are consistent with the observation on fuel derived cetane numbers. Both the number and proximity of substituted methyl groups significantly affect fuel auto-ignition propensity, and the adjacent methyl groups could increase the auto-ignition propensity. The combustion duration for the test fuels, except for n-decane and the n-decane/ n-butylbenzene blend, monotonically decreases with increased temperature. The non-monotonic dependence of combustion duration on temperature, for neat n-decane and the n-decane/ n-butylbenzene blend, may result from the increased diffusive burnt fraction. Finally, the comparison between gas-phase and spray auto-ignition reactivity of the test fuels highlights the contribution of both fuel physics and chemistry in spray auto-ignition.