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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]

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]

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]