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Ammonia (NH 3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH 3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH 3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH 3 mixtures with NH 3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH 3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH 3 fraction. A blending mechanism of n-heptane/NH 3 was compiled based on the existing n-heptane mechanism and NH 3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH 3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH 3 /n-heptane blends and the interaction between n-heptane and NH 3. Suggestions are provided for the further development of the blending mechanism. [ABSTRACT FROM AUTHOR]

This paper shows that nanosecond repetitively pulsed discharges were able to mitigate the response of lean methane-air swirl flames to acoustic excitations, at pressures up to 3 bar. Flame transfer functions with and without plasma discharges were investigated at pressures of 1.2, 2.0, and 3.0 bar, in a frequency range 48–380 Hz. Results show that the plasma discharges decreased by up to 50% the gain of the flame transfer functions, regardless of the pressure. Mechanisms responsible for this effect are discussed. [ABSTRACT FROM AUTHOR]

Chemical composition difference widely exists in real fuels, and the composition difference will affect the fuel autoignition and heat release in HCCI-based advanced engines. This study aims to explore the composition effects on autoignition by comparing the autoignition characteristics of two commercial diesels (China Stage-V and Stage-VI). The main composition difference is that Stage-V diesel has a higher paraffin content and a lower naphthene content than Stage-VI diesel. Ignition delay times (IDTs) of the two diesels were measured in a heated rapid compression machine at equivalence ratios of 0.37–1.25, pressures of 10–20 bar, and temperatures of 687–865 K. It is found that the difference in the total IDTs of the two diesels varies with the temperature range, and the first-stage IDTs of Stage-V diesel are much shorter than those of Stage-VI diesel. The IDT discrepancies were appropriately explained using the composition difference between the two diesels. Model simulation was carried out using a ternary and a five-component diesel surrogate coupled with an updated kinetic model. Simulation results show that the composition effects on the autoignition of the two diesels can be well captured by the two surrogates, where the ternary and five-component surrogates agree well with Stage-V and Stage-VI diesels, respectively. To further reveal the intrinsic mechanism of the composition effects, low-temperature reactivity difference between the two surrogates was interpreted from a kinetic perspective. Rate of production (ROP) analysis on OH radical confirms that the addition of decalin in the five-component surrogate is primarily responsible for the longer first-stage IDT compared to the ternary surrogate. Since the two surrogates capture the composition difference and autoignition characteristics of the two diesels, the conclusion from the kinetic analysis will help understand the composition effects on the autoignition of the diesels. [ABSTRACT FROM AUTHOR]

Abstract Autoignition characteristics of RP-3 kerosene were investigated using a heated rapid compression machine and a heated shock tube over a wide range of conditions. Ignition delay times (IDTs) for RP-3 kerosene were measured at pressures of 10, 15 and 20 bar over a range of temperatures from 624 to 1437 K and for equivalence ratios from 0.5 to 1.5. A three-component surrogate fuel (49.8% dodecane, 21.6% iso-cetane and 28.6% toluene by mole) was proposed and a kinetic model was developed to describe the combustion chemistry of RP-3. Negative temperature coefficient (NTC) behavior was observed in the autoignition, of which the temperature range varied within 701−884 K depending on operating conditions. IDT correlations in low and high temperature regions were obtained and then the dependences of IDT on pressure, equivalence ratio, oxygen content and dilution ratio were systemically studied. Comparison between the predictions using the new model and the experimental data shows that this model can accurately describe the autoignition characteristics of RP-3. Brute force sensitivity analyses were carried out to identify the key reactions that govern the ignition event. The large experimental data set and kinetic model provided in the current work will provide insights into the understanding of RP-3 ignition. [ABSTRACT FROM AUTHOR]

Abstract Autoignition and oxidation characteristics of a three-component diesel surrogate fuel were studied in a heated rapid compression machine (RCM) and a flow reactor (FR) over a wide range of conditions. The surrogate components, n-cetane, 2,2,4,4,6,8,8-heptamethylnonane (HMN), and 1-methylnaphthalene (1-MN), all fall within the C10–C20 range of real diesel fuel. The RCM-measured ignition delay times (IDTs) and FR oxidation data of the surrogate fuel were then compared with those of the target diesel. It is found that the diesel surrogate fuel is in good agreement with the target diesel in total ignition delay time and FR speciation data over the whole temperature range, reflecting the success of the surrogate fuel construction. A semi-detailed kinetic model was used to predict the autoignition and oxidation behavior of the surrogate fuel in RCM and FR experiments. Results show that the kinetic model captures well the measured total IDTs and the mole fractions of O 2 , H 2 , CO 2 , and small hydrocarbons under all investigated conditions. To deepen the understanding of low-temperature reaction scheme, rate of production (ROP) analysis of OH and HO 2 radicals was carried out during the first-stage ignition to explore the causes of low-temperature and negative temperature coefficient (NTC) reactivity. A brute force sensitivity analysis of first-stage ignition delay time was also conducted to further identify the controlling reactions in the first-stage ignition of the surrogate fuel. [ABSTRACT FROM AUTHOR]

Abstract Diesel/gasoline blended fuel, also named as dieseline, has attracted worldwide interests in varied advanced engine combustion modes dominated by mixing time scale and chemical reaction time scale. However, the research on ignition properties and chemical reaction mechanism of diesel/gasoline blends over wide operating conditions is still lacking. In current study, ignition delay time measurement of diesel/gasoline blends with different diesel proportion (30%, 50%, and 70% by volume) were conducted in a heated shock tube and a heated rapid compression machine (RCM) covering a broad range of temperatures between 636 and 1317 K at pressures between 6 and 20 bar for gas-phase dieseline/O 2 /N 2 mixture at varying equivalence ratios from 0.5 to 1.5. The experimental results revealed different dependence of ignition delay time on pressure, equivalence ratio and composition at different temperatures and varying negative-coefficient (NTC) behaviors were also observed at intermediate-temperature regime. The increase in proportion of diesel led to shorter ignition delay times under all conditions, though the ignition data at high temperature only showed marginal discrepancies. Besides, a crossover of ignition data occurred at the highest temperature for varying equivalence ratios. A tertiary surrogate for diesel and a tertiary surrogate for gasoline were chosen to calculate conditions after reflected shock in shock tube and compression in RCM. Then, simulations of ignition delay time for diesel/gasoline blends were conducted using a detailed model. Finally, kinetic analyses were performed to gain insight into the ignition characteristic of diesel/gasoline blends. [ABSTRACT FROM AUTHOR]

Abstract Autoignition characteristics of decalin, a bicyclic alkane, were investigated in a heated rapid compression machine (RCM) over a wide range of conditions. Ignition delay time (IDT) was measured at compressed pressures of 10, 15 and 20 bar, for equivalence ratios of 0.5, 1.0, 1.5 and 2.0, and at temperatures in the range 631−930 K. Negative temperature coefficient (NTC) behavior of decalin ignition delay time was observed within the temperatures of 750−860 K, in which the ignition delay time increases with rising temperature. The dependence of ignition delay time on compressed pressure, equivalence ratio, and oxygen concentration was systemically studied. A reasonable modification was made to a literature mechanism. The simulation results using the tuned mechanism are found to well capture the dependence of the measured ignition delay time on temperautre, pressure, equivalence ratio, and oxygen concentration over the entire temperature range. Correlation formulas of the simulated and measured ignition delay times were proposed to quantitatively reveal the ignition delay time dependence and to evaluate the mechanism performance. A reaction pathway analysis was carried out at low temperature (700 K), NTC temperature (850 K), and high temperature (1000 K), respectively, to identify the dominant reaction pathways consuming decalin and intermediate species. A sensitivity analysis was also performed at different temperatures and equivalence ratios to find out the important reactions that promote and inhibit decalin autoignition. [ABSTRACT FROM AUTHOR]

This work proposed a kinetic model for RP-3, the most widely used military-civilian aviation fuel in China. Four hydrocarbons within the typical size of major components in RP-3, i.e., n-dodecane, iso-dodecane (2,2,4,6,6-pentamethylheptane), decalin and n-butylbenzene, were included in the component palette to improve the ability of the surrogate to mimic the properties related to the molecular weight. Seven properties, cetane number, molecular weight, H/C ratio, threshold sooting index, lower heating value, the proportion of -CH 3 and -CH 2 in the total carbons were selected as targets aiming at the comprehensive emulation of the chemical and physical propensities of RP-3. By sequential use of the genetic algorithm and a local search method, a surrogate containing 27.44% n-dodecane, 28.81% iso-dodecane, 26.12% decalin and 17.63% n-butylbenzene by mole was formulated. The autoignition delay times of the surrogate were measured using a heated rapid compression machine at pressures of 10, 15, 20 bar and equivalence ratios of 0.5, 1.0 and 2.0 over low-to-intermediate temperature range. Results show that the surrogate can not only emulate the target properties but also the key non-targeted properties. A kinetic model of 3065 species and 11,898 reactions was then developed based on the proposed surrogate to describe the chemical process during the combustion of RP-3. Simulations show that the model can predict the fundamental combustion datasets in the present work and literature satisfactorily, suggesting the rationality and applicability of the model. Rate of production and evolution histories of OH and HO 2 were then conducted using the kinetic model to provide insight into the combustion of RP-3. Analysis suggests that negative temperature coefficient behavior also exists in the first stage ignition. [ABSTRACT FROM AUTHOR]

Autoignition characteristics of a four-component surrogate fuel(M2) were investigated in a heated rapid compression machine over a wide range of conditions. The near-term light surrogate M2 developed for marine diesel fuel was formulated in our previous study containing 17.3% n-hexadecane, 5.93% 2,2,4,4,6,8,8-heptamethylnonane, 30% decalin and 46.77% 1-methylnaphthalene by mole fraction. Ignition delay times (IDTs) of gas-phase M2/N2/O2 mixture were measured at pressures of 7, 10, 15 and 20 bar over the temperature range from 675 K to 914 K for the equivalence ratios (ER) of 0.5, 1 and 1.5. NTC behaviours within the temperature of 730–850 K and two-stage ignition phenomena were observed. The effects of operating conditions such as pressure, equivalence ratio and oxygen content were systematically studied, and correlations of total IDTs and first-stage IDTs were performed to further quantitatively reveal the IDTs dependence on the above parameters. Reasonable modifications were made to a literature mechanism to improve its predicting quality. The simulation results using the modified model could satisfactorily reproduce the IDTs under the test conditions, and the model is also able to capture the ignition delay dependence on pressure and fuel/oxygen content. A brute force sensitivity analysis at the low temperature (680 K) and the NTC temperature (780 K) was carried out to identify the key reactions that govern the ignition event of surrogate M2. [ABSTRACT FROM AUTHOR]

As the product of transesterification between various oils and alcohols, biodiesel exhibits characteristics of renewability and a wide range of raw materials. The ignition delay times (IDTs) of methyl oleate (MEOLE) and methyl linoleate (MLINO), the primary constituents of biodiesels, were measured in a heated rapid compression machine (RCM) to investigate the impact of C C double bond on the reactivity and autoignition characteristics. The experiment was conducted under two pressures of 8 bar and 10 bar at an equivalence ratio of 0.3–0.8 and a temperature range of 800–1000 K. The qualitative investigation unveiled the influence of compressed pressure, equivalence ratio, and oxygen mole fraction on the IDT of MEOLE and MLINO under varying compressed temperatures. Besides, a comparison of the IDTs for two fuels with different unsaturation degrees under identical conditions revealed that MEOLE exhibited greater reactivity at low temperatures compared to MLINO, while the increase in unsaturation degree can moderate and enhance reactivity at high temperatures. Moreover, three kinetic models in the literature were validated against the newly measured IDTs. The results indicate that the detailed lumped kinetic model exhibits better predictive capacity compared to other models, but it still underestimates the reactivity of MEOLE and MLINO within the investigated temperature range. Subsequently, the detailed kinetic model was optimized based on sensitivity analysis, resulting in excellent agreement between the predicted values of the optimized model and experimental data. Furthermore, accurate predictions were made regarding the relative reactivity changes of MEOLE and MLINO with temperature. Finally, different types of experimental data in the literature were employed to validate the optimized model, yielding satisfactory results. [ABSTRACT FROM AUTHOR]

An in-depth study of the chemical kinetic mechanism of biodiesels can facilitate accurate prediction of engine combustion and emission characteristics. Surrogate fuel is a feasible method for simulating the combustion chemistry of real biodiesel. Methyl myristate (MMY), one of the main components and a widely used surrogate for biodiesel, was investigated for the autoignition characteristics using a newly heated rapid compression machine (RCM). In this work, the ignition delay times (IDTs) of MMY were measured at compressed pressures of 10/15/20/30 bar with compressed temperatures ranging from 700 K to 900 K, and equivalence ratios varying from 0.3 to 1.25. The experimental observation successfully captured a typical negative temperature coefficient (NTC) phenomenon of IDT. The present experiment is the first one to measure the IDTs of MMY in a RCM, while also marking the first instance of observing the NTC phenomenon in macromolecular fatty acid methyl esters above C10. Besides, a skeletal model for methyl myristate has been developed based on the idea of decoupling methodology. In terms of fuel-dependent reactions, the kinetic model is divided into two parts: high-temperature reactions and low-temperature chain branching reactions. Combined with the experimental and simulation results, the dependence of the IDT of methyl myristate on compressed temperature, compressed pressure, equivalence ratio, and oxygen mole fraction at low-to-intermediate temperature was systematically investigated. The simulation results show that the developed skeletal model captures the ignition delay times well at low-to-intermediate temperatures and accurately predicts both the NTC phenomenon and the corresponding temperature range. Based on this, the key reactions in the combustion of MMY under different conditions were elucidated through sensitivity analysis. Ultimately, the model validation was conducted against the high-temperature IDT data from the literature. The chemical kinetic models of various biodiesels are essential for simulating the working process of internal combustion engines and accurately predicting the combustion and emission characteristics. Among these, the autoignition characteristic is one of the most important parameters for model validation. Besides, the carbon chain length of methyl myristate closely resembles that of real biodiesels, making it an ideal surrogate fuel for biodiesels. In this study, the IDTs of MMY were measured for the first time at low-to-intermediate temperatures in a heated RCM, thereby experimentally confirming the NTC phenomenon of methyl myristate. Additionally, a skeletal model of MMY was developed based on the idea of decoupling methodology, which can accurately predict the autoignition characteristics of MMY at low-to-intermediate temperatures. The simulated values demonstrate good agreement with experimental data. Overall, the present study provides an IDT database for the development and validation of the methyl myristate mechanism. [ABSTRACT FROM AUTHOR]

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650−1450 K, pressures of 6−20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (~1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (~940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates •OH, whereas •OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, •OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via •HO 2 while CH 3 OH consumes a large percentage of •OH. Consequently, the competition between methanol and diesel for •OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes. [ABSTRACT FROM AUTHOR]

The ignition delay times (IDTs) of n-alcohol/diesel blends were measured in a heated shock tube (ST) and a heated rapid compression machine (RCM). Three sets of blends were formulated to investigate the effect of n-alcohol (the volume ratio of n-alcohol/diesel being 20%/80% for four blends), cetane number (CN = 43, the volume ratio of n-alcohol/diesel being 20%/80% ethanol/diesel, 20.5%/79.5% n-propanol/diesel, 23.5%/76.6% n-butanol/diesel, 25.8%/74.2% n-pentanol/diesel), and oxygen content (mass fraction of oxygen w O2 = 6.65%, the volume ratio of n-alcohol/diesel being 20%/80% ethanol/diesel, 25.6%/74.4% n-propanol/diesel, 31.4%/68.6% n-butanol/diesel, 37.2%/62.8% n-pentanol/diesel) on the autoignition characteristics. In the RCM temperature region, the IDTs decrease with increasing n-alcohol chain length from ethanol to n-pentanol for all three sets of blends, while those in the ST temperature region show indiscernible variations. Interestingly, the intersection of IDTs for ethanol/diesel blend with other n-alcohol/diesel blends is observed when the temperature is below ~700 K. A recently-released detailed mechanism from the CRECK modeling group was used to predict the experimental results. The simulations show relatively good agreement with high-temperature (HT) ignition data, while the mechanism pronouncedly overpredicts the reactivity of blends at low-to-intermediate temperatures. Sensitivity analyses at three temperatures of 675 K, 800 K, and 1300 K were conducted to identify the dominant reactions during the autoignition of 20% n-alcohol/80% diesel blends and to kinetically elaborate and discuss the discrepancies between simulations and experiments. At the end, a preliminary mechanism tune-up was carried out grounded on the sensitivity analyses. The improved mechanism regains its predictive ability for IDTs of 20% n-butanol/80% diesel blend at RCM temperatures, while fails to precisely simulate the remaining three blends. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) blending combustion strategy has been regarded as a high-efficient form of NH 3 utilization. To understand the high-pressure gas-phase auto-ignition characteristics of NH 3 blending with diesel, the ignition delay times of NH 3 /diesel mixtures with high NH 3 energy fractions of 70%, 80% and 90% were measured in an ultrahigh-pressure rapid compression machine, at high pressures of 50–120 bar, temperatures of 755–996 K, and equivalence ratios of 0.5–1.5. As a consecutive study of Part 1 (Combust. Flame, 251 (2023), 112391), this study mainly focuses on the auto-ignition characteristics and oxidation mechanism of NH 3 /diesel mixtures at pressures higher than 50 bar. The ignition-promoting effect of increasing pressure from 50 to 120 bar was experimentally confirmed. Moreover, the increase of the diesel energy fraction, equivalence ratio, and oxygen concentration was found to decrease the ignition delay time of the NH 3 /diesel mixtures. The chemical reaction mechanism proposed in Part 1 was utilized to simulate the auto-ignition of the NH 3 /diesel mixtures. Simulation results show that the mechanism was able to quantitatively capture the ignition delay times of the NH 3 /diesel mixtures at the wide pressures. The cross-reactions between NH 3 and diesel, especially reaction RH + NH 2 = R + NH 3 , was found to play a key role in the correct prediction of auto-ignition at high pressure. In addition, it is revealed the effect of pressure is closely related to OH production reactions 2OH (+M) = H 2 O 2 (+M) and NO + HO 2 = NO 2 + OH, the rate of which increase with rising pressure. The reaction H 2 NO + O 2 = HNO + HO 2 is an important pathway where oxygen concentration affects NH 3 /diesel mixture auto-ignition. [ABSTRACT FROM AUTHOR]

Butanol/diesel blend is considered as a very promising alternative fuel with agreeable combustion and emission performance in engines. This paper intends to further investigate its autoignition characteristics with the combination of a heated rapid compression machine (RCM) and a heated Shock Tube (ST) to acquire a wide temperature region view. The ignition delay times (IDTs) of mixtures of 20% blending (by vol.) of n-butanol to China Stage-VI diesel, referred to as N20, were studied under the pressures of 6/10/15 bar, the equivalence ratios of 0.5–2.0, and a broad temperature range of 670–1300 K. Typical two-stage ignition and NTC behavior of the fuel were identified, and the IDT dependences on the compressed pressure, fuel/oxygen fraction, and the blending ratio were explained thoroughly and quantitively with correlation formulas. Furthermore, this paper expands the study to the effects of butanol structures. Autoignition characteristics of mixtures of 20% blending (by Vol.) of each of the four butanol isomers with neat diesel were investigated experimentally and numerically. The CRECK model was validated against the experimental results and critical reactions controlling fuel ignition were identified for further optimization. [ABSTRACT FROM AUTHOR]

Blending diesel and gasoline is a promising way to stabilize combustion process and reduce emissions of NOx and soot in the Homogeneous Charge Compression Ignition (HCCI) engine. In order to computationally study diesel/gasoline blends for their autoignition characteristics, a five-component diesel surrogate (21.6% n-hexadecane, 15.5% n-octadecane, 26.0% isocetane, 20.7% 1-methylnaphthalene, 16.2% decalin, by mol.) and a five-component gasoline surrogate (11.0% n-heptane, 30.8% isooctane, 38.2% toluene, 10.3% diisobutylene, 9.7% cyclohexane, by mol.) are blended in three volume ratios of 30%/70%, 50%/50%, and 70%/30% (denoted as SD30, SD50, and SD70) to compare measured ignition delay with published data of diesel/gasoline blends. A heated shock tube and a heated rapid compression machine (RCM) are used to measure the ignition delay of three surrogate blends under compressed pressures of 6, 10, and 20 bar for fuel-lean (ϕ = 0.5), fuel-rich (ϕ = 1.5), and stoichiometric mixtures. Experimental results indicate that surrogate blends match well with fuel-lean and stoichiometric diesel/gasoline/air mixtures while get longer ignition delays for the fuel-rich case, which is thought of as the consequence of fitting the surrogate to fuel-lean conditions in the engine. The kinetic simulation for ignition delays is performed with a comprehensive mechanism. Due to the mechanism overpredicting ignition delays in the intermediate-to-high temperature region, proper refinement is done to the original mechanism based on sensitivity analyses at 1000 and 660 K. A further comparison of ignition delay predictive ability for two mechanisms is conducted subsequently. The modified mechanism not only simulates ignition delays in the intermediate-to-high temperature region successfully but also maintains the good performance of the original mechanism in the negative-temperature-coefficient (NTC) and low-temperature region. Besides, species and temperature profiles after the compression stroke in RCM are simulated using the modified mechanism for three surrogate blends at equal equivalence ratio, compressed pressure, and compressed temperature. Sensitivity analysis is also conducted to identify reactions dominating the system reactivity. [ABSTRACT FROM AUTHOR]

This work presents new fundamental combustion experimental datasets and propose an improved kinetic model for n-dodecane, a widely used surrogate component for jet fuels and diesel. In the experiments, ignition delay times for n-dodecane, were measured over low-to-high temperature range by combining a heated rapid compression machine (RCM) and a heated shock tube (ST). The measurements cover a wide range of temperatures (621–1320 K), pressures (8, 15 bar) and equivalence ratios (0.5–1.5). Flow reactor oxidation was also carried out over temperature range of 600–1100 K and at equivalence ratios of 0.5, 1.0 and 1.5 under atmospheric pressure to provide species evolution profiles of O 2 , CO, CO 2 CH 4 , C 2 H 4 etc. These datasets were used to develop a mechanism for n-dodecane. New rate coefficients were adopted from recent literature to improve the performance of the model, leading to a model composed of 737 species and 3629 reactions. The present experimental data, as well as a wide range of datasets in literature, were used to evaluate the performance of the model. An overall good agreement of the predictions with the experimental results was observed. This model is anticipated to facilitate the development of kinetic models for jet fuel surrogate. [ABSTRACT FROM AUTHOR]

A good knowledge of the reaction kinetics of n-butylcyclohexane, a widely used surrogate component for jet fuels and diesel, is crucial for understanding the combustion chemistry of practical fuels. This paper proposes a detailed kinetic model consisting of 1802 species and 7246 reactions to describe from low to high temperature chemistry of n-butylcyclohexane. The autoignition and oxidation of this fuel were investigated over a wide range of conditions. Ignition delay times were measured at 10, 15 and 20 bar for equivalence ratios of 1 and 1.5 under highly diluted conditions in a heated rapid compression machine. Oxidation experiments were performed in a flow reactor for equivalence ratios of 1 and 1.5 over the temperature range of 650–1075 K at 1 atm. Several oxidation products were identified and their mole fraction profiles were measured as a function of temperature. The model was evaluated with respect to the new experimental data. Additionally, the model was compared to experimental data in the literature, including ignition delay times in both rapid compression machine and shock tube, speciation profiles in jet-stirred reactor. Good agreement was achieved with all the data. Reaction flux analyses and sensitivity analyses were performed in order to provide insight into the combustion kinetics of n-butylcyclohexane. The present model can be used to construct the kinetic models for surrogates of jet fuels or diesel, as well as to work as the starting mechanism for the development of kinetic models of larger alkylcyclohexane and bicyclic cyclohexane fuels. [ABSTRACT FROM AUTHOR]

2,2,4,6,6-pentamethylheptane, a highly branched alkane, is a promising component candidate of branched alkanes in the surrogates for jet fuels and a major component in alternative fuels. In spite of its great relevance and importance in real fuels, it has only attracted very little interest. This work provides a new set of experimental data and a newly proposed detailed kinetic mechanism for this hydrocarbon. In the experiments, the autoignition characteristics of 2,2,4,6,6-pentamethylheptane, was measured in a rapid compression machine and a shock tube spanning over the equivalence ratios of 0.5, 1 and 1.5, pressures of 15 and 20 bar and temperature range of 603–1376 K. The dependence of ignition delay times on pressure, mole fraction of fuel, mole fraction of oxygen and dilution ratio were systematically investigated. Negative temperature coefficient (NTC) behavior was observed during the autoignition event, but the NTC temperature window was found to be much lower than the counterpart normal paraffin. A detailed kinetic model containing 729 species and 3390 reactions was proposed to describe the combustion behavior of this compound over low-to-high temperature range. The model was validated against the present experimental data, as well as the limited datasets in literature. Good agreements were observed between the experimental data and the predictions from the proposed model. Kinetic analyses, including sensitivity analysis and reaction pathway analysis, were carried out to provide insight into the combustion characteristics from the kinetic perspective. Comparisons were also carried out with the datasets for other branched alkanes and normal-paraffin with same carbon numbers over the same conditions to reveal the effects of the molecular structure. [ABSTRACT FROM AUTHOR]

The objective of the current study was to formulate RP-3 kerosene surrogate by emulating fuel properties affecting the physical and chemical processes of the target fuel under the engine relevant conditions. This study utilized two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC-TOFMS) and 13C and 1H nuclear magnetic resonance (NMR) spectroscopy to characterize the compositional characteristics of RP-3 fuel, and various standard test methods were applied to measure the physical and chemical properties of the target fuel. Two surrogate fuels (K1, a mixture of five components and K2, a mixture of seven components) were optimally determined through a multi-property regression algorithm by matching carbon types (CTs), distillation curve, cetane number (CN), density, and threshold sooting index (TSI) of the target fuel. The measured and estimated values of both target properties and non-target properties of surrogates were validated against the experimental data of RP-3 kerosene. Ignition delay times (IDTs) of both surrogates were investigated in a heated shock tube and a heated rapid compression machine under engine relevant conditions and validated against the measured results of RP-3. Overall, K1 and K2 both exhibited good matching on the compositional characteristics, physical-chemical properties, and gas phase ignition behaviors with the target fuel. In contrast, the seven-component K2 was more competitive and more comprehensive. [ABSTRACT FROM AUTHOR]

Cycloalkanes with long alkylic side chains are important chemical components in diesel and jet fuels. Although with the increasing interest in their combustion chemistry, related studies are very limited. In the present study, ignition delay times were measured for n-butylcyclohexane by combining a heated rapid compression machine and a heated shock tube. The data was obtained for equivalence ratios of 0.5, 1 and 1.5, at pressures of 10, 15 and 20 bar and over the temperature range of 612–1374 K. It was found that n-butylcyclohexane displayed marked low temperature reactivity with a typical negative temperature coefficient behavior. A correlation was developed to reproduce the measured ignition delay times over the entire temperature ranges. The results showed that it can well estimate the ignition delay times in the testing conditions. The effects of operating parameters on ignition delay times, including temperature, pressure, equivalence ratio, oxygen concentration and dilution ratio, were systematically presented and discussed. In the present work, a published mechanism was tuned with some modifications and was used to simulate the autoignition of n-butylcyclohexane. This model accurately captured the effect of the operating parameters over a wide range of conditions, and the simulations were in satisfactory agreement with the experimental results. For a better understanding of the chemistry involved during the autoignition events, kinetic analyses were performed using the tuned mechanism to provide insight to the experimental results. Comparisons were also carried out with the datasets for other cycloalkanes over the same conditions to reveal the effect of the side chain, and the trends, i.e., the longer side chain contributes to the stronger low temperature reactivity, were highlighted and discussed. [ABSTRACT FROM AUTHOR]

With the extensive exploration of advanced combustion modes and the increasing demand for engines with extremely high efficiency and low emissions, biodiesel has garnered significant attention and research as a renewable fuel source that boasts a wide range of raw materials. The ignition delay times (IDT) of methyl stearate, one of the main components in biodiesel, were measured in a new heated rapid compression machine (RCM). The data were obtained for the equivalence ratio of 0.3–0.8 and the compressed temperatures ranging from 820 K to 1000 K under three different pressures of 8, 10, and 12 bar. The influence of compressed temperature, compressed pressure, equivalence ratio, and oxygen mole fraction on ignition delay times was systematically discussed under experimental conditions, revealing the autoignition characteristics of methyl stearate in intermediate temperatures. Besides, the CRECK model and a skeletal model were verified against the experimental results, revealing that the model obtained by analogy significantly overestimates the low-temperature reactivity of methyl stearate. Furthermore, appropriate adjustments were made to the pre-exponential factors for the low-temperature elementary reactions in the CRECK model based on sensitivity analysis. The results indicate that the present model with optimization can well capture the dependence of the IDTs on the operating parameters under the investigated conditions, and the simulation results are in satisfactory agreement with the experimental data. Finally, the optimized model was verified against the experimental data from both the shock tube and jet-stirred reactor. To the authors' knowledge, this study represents the first gas-phase RCM experiment conducted on pure methyl stearate, which is a waxy solid at room temperature. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is considered as a good carbon-free hydrogen-carrier fuel and has gained extensive attention as a promising alternative fuel for internal combustion engine in recent years. To explore the application potential of the NH 3 burned with diesel fuel in internal combustion engine, the auto-ignition delay times of NH 3 /diesel fuel blends were measured in a rapid compression machine at a wide NH 3 energy fraction of 20%, 40%, 60%, 70%, 80%, and 90%, temperature range of 675–995 K, pressures of 20–50 bar, and equivalence ratios of 0.5–1.5. According to the variation of ignition delay times with temperature, three different combustion regimes for NH 3 /diesel mixtures can be determined. It is found that the typical NTC behavior and two-stage ignition process were only observed at the regime where diesel chemistry dominates. Both the first-stage and the total ignition delay times increase with rising NH 3 energy fraction, but decrease with the increase of equivalence ratio. Then, an updated NH 3 /diesel kinetic mechanism was proposed by adding new cross reactions between diesel and NH 3. Results show that the current mechanism exhibits an obvious improvement for ignition delay time prediction compared to the original mechanism, though it still fails to reproduce the ignition delay times of NTC range for the mixture containing 20% NH 3. Further sensitivity analysis and the OH radical rate of production analysis indicate that the overestimation of the reactivity for the NH 3 /diesel fuel blends at NTC range is closely related to reaction NO+HO 2 NO 2 +OH. The promoting effect of NO species on the conversion of inactive HO 2 radical into active OH radical at low temperature greatly accelerates the autoignition. In addition, the current mechanism does not include the cross reaction between the large NTC-related species (QOOH, OOQOOH, OQOOH, ROO) and the N-containing species (NO, NO 2 , NH 2), especially for the NO related reactions, which may have a great impact on the IDT simulation of the NH 3 /diesel mixtures. Consequently, this work presents new ignition delay time measurements and mechanism optimization for NH 3 /diesel mixtures. Continuous works should be emphasized on these unclear reactions to further understand the combustion behavior of NH 3 /diesel fuel blends. [ABSTRACT FROM AUTHOR]

Autoignition characteristics of methyl palmitate (C 17 H 34 O 2), a waxy solid at room temperature, were investigated in a newly developed heated rapid compression machine. Ignition delay times (IDT) of methyl palmitate were firstly measured at the low-to-intermediate temperatures of 780–980 K and equivalence ratios of 0.3–1.0 under three pressures of 10, 15, and 20 bar. A negative effect of the increase in compressed pressure, equivalence ratio, and oxygen mole fraction on IDTs at different compressed temperatures was qualitatively and quantitatively revealed. Besides, the dependence of IDTs on compressed pressure and equivalence ratio decreases with increasing temperature. A recently optimized mechanism was used to predict the experimental results. The simulations show an obvious negative temperature coefficient (NTC) behavior at the compressed temperatures of 780–830 K which was not observed in the experiment. Meanwhile, the mechanism pronouncedly underestimated the reactivity of methyl palmitate at higher temperatures in this study. Sensitivity analyses at different temperatures and pressures for equivalence ratios of 0.5 were conducted to determine the crucial reactions that caused the difference between the simulation and experiments. On this basis, further optimization was carried out and the simulation results of the latest mechanism show a good agreement with the experimental data. It is verified against shock tube experimental data that the optimization adjustment in this paper has a small positive impact on the prediction ability of the originally documented mechanism under temperatures above 1000 K. In the end, validation for the optimized mechanism against experimental data in a jet-stirred reactor (JSR) was performed. Overall, the experimental results fill in gaps in IDT data on methyl palmitate at low-to-intermediate temperatures and provide a reference IDT database for the development and validation of methyl palmitate mechanisms. [ABSTRACT FROM AUTHOR]

Burning oxygenated hydrocarbon biofuels in engines is a viable path for saving energy and reducing carbon emissions. N-butanol and biodiesel are two representative biofuels and have attracted widespread interest. In this study, the blends of n-butanol and biodiesel (a waste cooking oil) with different n-butanol ratios (40%, 60%, 80% by volume) were adopted to study their autoignition characteristics in a newly developed ultrahigh-pressure rapid compression machine. The ignition delay times of the blends were precisely measured under wide pressures of 10/20/40/60 bar, equivalence ratios of 0.3/0.5/1.0, and a temperature range of 700–970 K. Experimental results show that the ignition delay time decreases with the increase of pressure and equivalence ratio at the investigated conditions regardless the blending ratios. It is also found that the ignition delay time becomes slightly longer with the increasing n-butanol ratio in the blends at temperatures below 820 K. However, as the temperature further increases, the ignition delay times of different blends get closer and has a crossover tendency. The composition of the biodiesel was quantitively analyzed and a surrogate fuel was developed. An optimized mechanism was proposed based on a documented detailed mechanism with 461 species and 18,217 reactions. Simulation results show that the optimized mechanism better captures the dependence of the measured ignition delay times on temperature, pressure, and blending ratios over the entire temperature range compared to the original mechanism. In the end, species evolution and sensitivity analysis were performed sequentially with the optimized mechanism to give kinetics insight into the chemical interaction between biodiesel and n-butanol. The experimental data and modeling results reported here provide a basis for understanding the combustion reaction kinetics of biodiesel/n-butanol blending fuels. [ABSTRACT FROM AUTHOR]

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in 'air' mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness. [ABSTRACT FROM AUTHOR]