Development and Validation of Small-Size Mechanism for RP-3 Aviation Fuel with High Precision.

In this study, a kerosene surrogate model fuel containing 73% n-dodecane, 14.7% 1,3,5-trimethylcyclohexane, and 12.3% n-propylbenzene (percentage in mass) is developed by considering both the physical and chemical characteristics of practical aviation kerosene. By combining the small-size C0-C4 (carbon number) core mechanism and the large hydrocarbon submechanisms, a low- and high-temperature chemical kinetic mechanism including 43 species and 136 reactions is constructed for the kerosene surrogate model fuel. The performance of the 43-species mechanism is validated by examining various experimental ignition delay times and laminar flame speeds of single component of n-dodecane and practical kerosene. The predicted main species concentrations during the oxidation process in the jet-stirred reactor by this small-size mechanism exhibit generally acceptable performance with the corresponding experimental data of RP-3 kerosene. The results of brute force sensitivity analysis indicate that the mechanism retains key reaction paths. This relatively small size can be applied to the simulation of computational fluid dynamics to further explore the practical problems of aviation fuel application in engine.

Comparative Analysis of Combustion Behaviors and Emission Characteristics of Diesel Engines Fueled with Biodiesel or Biodiesel Blends.

In this study, we carried out an experimental analysis of combustion behaviors and emission characteristics of pure biodiesel and biodiesel-dimethyl ether (DME) blends and determined the impacts of the biodiesel ratio and the nozzle parameter on the combustion pressure characteristics in the time domain/frequency domain and emission characteristics. The findings show that with a decrease in the biodiesel proportion in biodiesel-DME blends, the maximum combustion pressure and fuels in the premixed combustion stage decrease with a retarded maximum value phase, but the maximum heat release in diffusive combustion increases and the maximum amplitude of pressure rise acceleration decreases. All of the pressure level curves of BD100, BD80, and BD50 contain a rapid decrease stage 1, a slow decrease stage 2, and a fluctuating stage 3. With a decrease in the biodiesel proportion, the exhaust gas temperature, NO x emissions, and smoke emissions of BD100, BD80, and BD50 decrease gradually. Compared with a 5 × 0.43 mm nozzle, the maximum combustion pressure and maximum heat release rate of BD50 for a 4 × 0.35 mm nozzle were higher and the phase of the maximum value was advanced. Soot emissions for the 4 × 0.35 mm nozzle were lower, which is especially obvious under a high brake mean effective pressure (BMEP).

Effects of Ethanol and Methanol on the Combustion Characteristics of Gasoline with the Revised Variation Disturbance Method.

This paper proposes a revised variation disturbance method to provide valuable information and reference for fuel design or optimization of internal combustion engines to realize the comprehensive and quantitative evaluation of the effects of blending agents on the combustion performance of primary fuels. In this method, methanol and ethanol are blended into gasoline to form six kinds of alcohol-gasoline (E10, E20, E30, M10, M20, and M30). Then, the ignition delay, adiabatic flame temperature, component concentration, fuel-burning rate, extinction strain rate, and CO emission of gasoline and alcohol-gasoline are studied by system simulation in a wide range of operating conditions. Based on the new variation disturbance method, the effects of methanol and ethanol on the combustion performance of gasoline are next analyzed globally and characterized quantitatively. The comprehensive results of ethanol and methanol on the gasoline's combustion are visually presented. The method proposed in this paper is preliminarily validated based on the analysis of the microscopic mechanism of combustion. The results show that the blending of ethanol and methanol has positive effects on gasoline combustion, and ethanol can rapidly ignite the gasoline in a wide range of operating conditions and is superior to methanol in terms of fuel combustion, stability, and pollutant discharge. Based on the treatment of simulated values of six combustion characteristics selected in this paper and the calculations of the variation disturbance method, the total disturbance values of ethanol and methanol to gasoline combustion are obtained as 0.8493 and 0.2605, respectively. That is, ethanol has a more significant effect on improving the combustion performance of gasoline than methanol. In addition, based on the analysis results of the combustion, it is found that the blending of ethanol enlarges the reaction of notable components in gasoline. This finding also proves the effectiveness and validity of the scientific method utilized in this paper.

Theoretical Study on Reactions of α-Site Hydroxyethyl and Hydroxypropyl Radicals with O2.

α-Site alcohol radicals are the most important products of H-abstract reactions from alcohols since the hydroxyl moiety weakens the α-site C-H bond. Reactions between α-site alcohol radicals and O2 play an important role in combustion of alcohols, especially at relatively low temperatures. However, reliable reaction pathways and rate constants for these reactions are still lacking. Theoretical studies on reactions in α-hydroxyethyl radical (CH3C•HOH) + O2 and α-hydroxypropyl radical (C2H5C•HOH and CH3C•OHCH3) + O2 reaction systems are performed in this work. Pressure-dependent rate constants for the involved reactions in a wide range of temperatures are determined using the Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) method. Our results show that rate constants for reactions in the α-hydroxypropyl radical + O2 system are quite different from those in the CH3C•HOH + O2 system. Detailed reaction pathways for these reaction systems are clarified, although combustion characteristics of ethanol and propanol do not change much with the obtained rate constants for these reactions. Important reaction channels in producing enols, especially in the combustion of propanol, are also provided. The obtained rate constants for these reactions over a wide range of temperatures and pressures are helpful in developing combustion mechanisms for ethanol and propanol.

Theoretical Study on Reactions of Alkylperoxy Radicals.

We carried out a theoretical study on geometries, relative energies of stationary points, and reaction rate constants for ethyl + O2, propyl + O2, and butyl + O2 reactions, which are important reactions in the low-temperature oxidation of corresponding alkanes. Geometries with CCSD(T)/aug-cc-pVTZ for the ethyl + O2 system are adopted as the benchmark to choose a proper exchange-correlation functional for geometry optimization. Our results show that B3LYP with 6-311+G(d,p) can provide reliable structures for this system, and structures of the other two systems are determined with this functional. The performances of the explicitly correlated CCSD(T)-F12a and the locally correlated DLPNO-CCSD(T) methods on barrier heights and reaction energies are evaluated by comparing their results with those of CCSD(T)/aug-cc-pVQZ for the ethyl + O2 system. Our results indicate that reliable energy differences for this system are achieved with CCSD(T)-F12a using the cc-pVDZ-F12 basis set, and this method is employed in calculating single-point energies for the other two systems. The single-reference equation-of-motion spin-flip coupled-cluster method is adopted to obtain the potential energy surface of the barrierless reaction C2H5· + O2 → CH3CH2OO·, and the results are compared with those using broken-symmetry density functional theory and the Morse potential. Differences between energies with these methods are <1.6 kcal/mol, but the difference in the rate constants could be sizable at temperatures <500 K, and rate constants obtained in this work are reliable only for temperatures >500 K. Pressure-dependent rate constants for these reactions are determined using the Rice-Ramsperger-Kassel-Marcus/Master equation method. The obtained reaction energies, barrier heights, and rate constants could be valuable for reactions between the large alkane radical and O2, which are important in the low-temperature combustion of fuels such as kerosene and gasoline.