Chemical-Kinetic Characterization of Autoignition and Combustion of Surrogate Diesel (original) (raw)
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Proceedings of the Combustion Institute, 2015
Aromatic species represent a significant fraction (about one third by weight) of both diesel and gasoline fuels. Much of the aromatics in diesel and gasoline are alkyl-benzene species. Although toluene, the lightest of the alkyl-benzenes, has been the subject of extensive literature investigations, very little experimental data are available for heavier alkyl-benzenes (9-20 carbon atoms) relevant to diesel fuel. In this work, the burning velocity of ethyl-, n-propyl-and n-butyl-benzenes were measured in a premixed flat-flame burner using the heat flux method. The burning velocities were measured as a function of the equivalence ratio at atmospheric pressure and for two unburned gas temperatures (358 and 398K). These new experiments are compared with burning velocities for toluene previously measured by the authors. The comparisons showed that ethyl-benzene has the highest flame speed, followed by n-propyl-and n-butyl-benzenes which have similar burning velocities. Toluene has the lowest flame speed. Excellent agreement was observed between the new measurements and simulations using a mechanism for alkyl-benzenes recently published by Lawrence Livermore National Laboratory (LLNL) and National University of Ireland.
Computational and statistical analysis of diesel homogenous
This paper explores the auto-ignition chemistry of n-heptane and toluene mixture to represent diesel-homogenous charge compression ignition (HCCI) combustion by means of a reduced chemical kinetic mechanism using CHEMKIN-PRO. In particular, pressure and temperature histories of n-heptane–toluene mixture are demonstrated at the stoichiometric condition and at the lean operating condition with 0%, 10%, and 20% exhaust gas recirculation (EGR) rates, respectively. Since pressure and temperature profiles play a vital role in a reaction path at a certain operating condition, an endeavour has been made here to present a comprehensive reaction path analysis on the formation/destruction of chemical species at a peak operating condition. Also, validation of the proposed model was done by comparing the results with the experimental analysis carried out by prior researchers. Besides, multiple linear regression analysis was performed to compute the pressure and temperature histories of various lean operating conditions ( = 0.5–0.9) with and without EGR through the fresh intake stream.
Combustion and Flame, 2019
The aim of the work is to evaluate the influence of the n-heptane/toluene ratio on the reactivity of binary toluene Toluene reference Reference fuels Fuels (TRF)(TRF), through a combined experimental and numerical work. Novel experimental ignition delay time (IDT) data of three binary TRFs of varying n-heptane/toluene ratios are obtained in a high-pressure shock tube (HPST) and a rapid compression machine (RCM) at conditions relevant to novel engine operation. Measurements have been performed at two pressures (10 and 30 bar), and three fuel/air equivalence ratios (0.5, 1.0 and 2.0) for TRF mixtures of 50%, 75% and 90% vol. toluene concentration, over the temperature range of 650-1450 K. It was found that, increasing the n-heptane content, an increase in the reactivity and shorter IDTs occur. Reduced sensitivity to the equivalence ratio was observed at high temperatures, especially for high toluene content mixtures. A well validatedn accredited detailed kinetic mechanism for TRF oxidation was utilized to provide further insight into the experimental evidence. The mechanism, which has recently been updated, was also assessed in terms of its validity, contributing thus to its continuous development. Reaction path analysis was performed to delineate critical aspects of toluene oxidation under 2 the considered conditions. Further, sensitivity analysis highlighted the interactions between the chemistry of the two TRF components, revealing toluene's character as a reactivity inhibitor mainly through the consumption of OH radicals.
The impact of adding an olefin to ternary mixtures of toluene and primary reference fuels to mimic the oxidation of a fully-blended gasoline was examined with kinetic modeling. Reactions for the oxidation of 2,4,4-trimethyl-1-pentene (DIB-1), which is the major constituent in diisobutylene (DIB), were added to a previously developed semidetailed mechanism for ternary mixtures. The merged kinetic mechanism was revised and successfully checked for validity against data for neat fuel components as well as fuel mixtures at conditions relevant to engine combustion. The validated kinetic model was then used to model a fully-blended research gasoline. By using a nonlinear-by-volume blending model for octane numbers, a four-component surrogate fuel was formulated which consisted of 51% isooctane, 18% n-heptane, 26.4% toluene, and 4.6% DIB-1 by liquid volume. The surrogate fuel reflected molecular-structure class composition, research octane number, motor octane number, density, and H/C ratio of the target gasoline. Ignition delay times for gasoline measured in a shock tube, rapid compression machine, and an HCCI engine were then compared to simulated results using the quaternary mixture and ternary mixtures with similar octane numbers and H/C ratio as the target gasoline. Adding DIB-1 to a ternary mixture had a small but significant effect on the autoignition of gasoline surrogate fuels. The quaternary mixture showed better agreement when compared to measurements, especially at higher temperatures. The simulated ignition delays at shock tube and rapid compression machine conditions were also well-correlated with the combustion phasing in an HCCI engine defined as the temperature required at bottom dead center to achieve 50% heat release (CA50) at top dead center. Similar results were achieved when comparing with other published mechanisms. Simulations with neat and binary mixtures combined with a rate-of-production and sensitivity analysis with multicomponent mixtures show that the reason for the increased reactivity and shorter ignition delay when adding DIB-1 to the ternary mixture is that DIB-1 promotes toluene ignition more than isooctane at these conditions. 65 chemistry has significant effects on engine performance under 66 HCCI conditions. 21 Having n-heptane and the olefin 1-hexene in 67 the gasoline surrogate fuel was beneficial to achieve satisfactory 68 combustion phasing (CA50) but detrimental to thermal
The Proceedings of the International symposium on diagnostics and modeling of combustion in internal combustion engines
Recently, many development efforts have been made to enhance the diesel spray mixing by injecting fuel at high pressure and with micro-hole nozzles in order to achieve the best compromise between the premixed and the nonpremixed stage of the combustion process. The knowledge of the instant of fuel auto-ignition under different operating conditions and the individuation of the most affecting working parameters represents, therefore, a key point for the optimisation of diesel engines design, as well as for the improvement of predictive numerical models. The auto-ignition and the first stage of combustion are some of the most complex phenomena of diesel combustion because of the occurrence of chemical reactions in a heterogeneous environment, for the characterisation of which both experimental and numerical techniques are often needed. Optical diagnostics, in fact, allows detecting and following the evolution of a limited number of chemical species representative of the auto-ignition process, whereas multidimensional numerical modelling involves, for the most, reduced kinetic scheme, in which intermediate species are not always suitable of a proper identification. In this work attention was focused on a diesel engine with an optically accessible external combustion chamber, in which a strong anticlockwise swirl flow contributed to enhance both the spray penetration and the vapour distribution. Location and timing of auto-ignition and the first stage of diesel combustion were highlighted by flame intensity measurements from ultraviolet to visible performed in fixed positions of main interest within the combustion chamber. Chemiluminescence signals of HCO and CH radicals were detected before the indicated auto-ignition corresponding to the first combustion-induced pressure rise. OH emission was instead revealed as synchronised with auto-ignition. Experimental data relative to different air/fuel ratios were compared with results of optimised numerical simulations realised by means of a customised version of the KIVA-3 code. The concentration of the intermediate species participating the employed kinetic scheme for auto-ignition was followed with respect to time at the same spatial locations considered in the experiments. A sort of identification of these species was achieved and, at the same time, a deeper understanding of the cool flame less exothermic reactions was reached. Some insight into the transition to the hot temperature regime was derived by comparing the behaviour of both experimental and numerical data integrated over the whole combustion chamber.
The Proceedings of the International symposium on diagnostics and modeling of combustion in internal combustion engines, 2001
The aim of this paper is to study numerically the detailed flame zone structure of DI diesel sprays during combustion at engine-like conditions. To address this issue, the KIVA-3 code was modified to include complex chemical mechanisms. A "subgrid", partially stirred reactor model was applied to handle turbulence-chemistry interaction. Diesel fuel is assumed to be single-component and its oxidation chemistry is represented by the n-heptane kinetics. The chemical mechanism, reduced to a size of 65 species and 273 elementary reactions, retains the important low/intermediate temperature ignition reactions for n-heptane, the low hydrocarbon oxidation chemistry, the formation reactions of polycyclic aromatic hydrocarbons (PAHs) (up to two aromatic rings), and the NOx formation kinetics. Numerical simulation of the transient diesel combustion process at a specific injection condition was performed. The numerical prediction shows that the current approach is capable of capturing the essential features of the diesel process such as auto-ignition and liftoff phenomena. The simulation illustrates that the lifted flame is stabilized as a triple flame. The simulated spatial soot and NO distributions are similar to those described in Dec's "conceptual diesel model". Analysis of the flame zone shows the molecular precursors of soot (e.g. PAHs and acetylene) produced during the rich burning of the sprays contributing to soot formation, whereas NO is formed closer to the oxygen diffusion layer on the lean side of the flame. The simulation was also extended to investigate the effects of charge composition variation on diesel auto-ignition and combustion. The results demonstrate that variation of oxygen molar concentration in the charge can substantially affect the auto-ignition and combustion pattern of diesel sprays.