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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]

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]