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Andre L Oehma – One of the best experts on this subject based on the ideXlab platform.

  • influence of intermediate temperature heat release on Autoignition reactivity of single stage ignition fuels with varying octane sensitivity
    Proceedings of the Combustion Institute, 2020
    Co-Authors: Kwang Hee Yoo, Alexande K Voice, Andre L Oehma


    Abstract This study investigates the effects of intermediate temperature heat release (ITHR) on Autoignition reactivity of full boiling range gasolines with different octane sensitivity through intake temperature and simulated exhaust gas recirculation (EGR) sweeps in a homogenous charge compression ignition (HCCI) engine. To isolate the ITHR effects, low temperature reactivity was suppressed through the use of high intake temperature and low intake oxygen mole fraction. For quantification of ITHR, a new method was applied to the engine data by examining the maximum value of the second derivative of heat release rate. Combustion phasing comparisons of fuels with octane sensitivity showed that fuel with less octane sensitivity became more reactive as intake temperature and simulated EGR ratio decreased, while fuel with higher octane sensitivity had a reverse trend. For all of the fuels that were tested, the amount of ITHR increased as the intake temperature and oxygen mole fraction increased. These ITHR trends, depending on octane sensitivity, were almost identical with the trends of combustion phasing, showing that ITHR significantly affects fuel Autoignition reactivity and determines octane sensitivity.

  • impact of fuel composition and intake pressure on lean Autoignition of surrogate gasoline fuels in a cfr engine
    Energy & Fuels, 2017
    Co-Authors: Vickey Kalaska, Dongil Kang, Andre L Oehma


    The critical compression ratio (CCR) criterion (defined as the minimum compression ratio at which the fuel shows initial signs of Autoignition) was examined for various gasoline surrogate fuels in a motored engine. This investigation builds on the concept of CCR which is a good indicator of a fuel’s Autoignition characteristics, to study the fuel compositional effects with increasing intake manifold pressure. The blends consisted of binary and ternary mixtures of n-heptane and/or iso-octane, and a fuel of interest. These fuels of interest were higher octane components; toluene, ethanol, and iso-butanol. A lean condition (Φ = 0.25) with varying intake pressure (atmospheric to 3 bar, abs) and at a constant intake temperature of 155 °C was used to investigate the ignition behavior of all the blends. Two sets of blends consisted of varying percentages of fuels of interest, formulated to approximately have research octane numbers (RON) at 80 and 100. For comparison, neat iso-octane was selected as the represen…

  • Autoignition of pentane isomers in a spark ignition engine
    Proceedings of the Combustion Institute, 2017
    Co-Authors: Song Cheng, Yi Yang, Dongil Kang, Stanislav V Ohac, Andre L Oehma


    Abstract This paper describes a study on the Autoignition of three pentane isomers (n-, neo- and iso-pentane) in a Cooperative Fuel Research (CFR) engine operating at standard, ASTM knocking conditions. The Research Octane Numbers (RONs) of these three fuels are first measured and compared to historical data. Autoignition of pentane/air mixtures in the CFR engine are then simulated using a two-zone model with detailed chemical kinetics. Initial and boundary conditions for these kinetic simulations are systematically calibrated using engine simulation software. Two published, detailed kinetic mechanisms for these fuels are tested with a published NO sub-mechanism incorporated into them. Simulations using both of these mechanisms demonstrate Autoignition in the engine for all three pentanes, and that residual NO promotes Autoignition, as found in previous studies. Differences between these two mechanisms and the engine experiments are nonetheless observed, and these differences are consistent with those observed in simulations of published rapid compression machine (RCM) data. Comparison of the RCM and the CFR engine modelling also suggests the need for high accuracy experiments and high-fidelity models due to the significant impact that small differences in Autoignition timing can potentially produce in real engines.

Chihje Sung – One of the best experts on this subject based on the ideXlab platform.

  • Autoignition of gasoline surrogates at low temperature combustion conditions
    Combustion and Flame, 2015
    Co-Authors: Goutham Kukkadapu, Kamal Kuma, Chihje Sung, Marco Mehl, William J. Pitz


    Abstract Understanding the Autoignition characteristics of gasoline is essential for the development and design of advanced combustion engines based on low temperature combustion (LTC) technology. Formulation of an appropriate gasoline surrogate and advances in its comprehensive chemical kinetic model are required to model Autoignition of gasoline under LTC conditions. Ignition delays of two surrogates proposed in literature for a research grade gasoline (RD387), including a three-component mixture of iso -octane, n -heptane, and toluene and a four-component mixture with the addition of an olefin (2-pentene), were measured in this study using a rapid compression machine (RCM). The present RCM experiments focused on two fuel lean conditions in air corresponding to equivalence ratios of ϕ  = 0.3 and 0.5, at two compressed pressures of P C  = 20 bar and 40 bar in the compressed temperature range of T C  = 665–950 K. Comparison of the measured ignition delays of two gasoline surrogates with those of RD387 reported in our previous study shows that the four-component surrogate performs better in emulating the Autoignition characteristics of RD387. In addition, numerical simulations were carried out to assess the comprehensiveness of the corresponding gasoline surrogate model from Lawrence Livermore National Laboratory. The performance of the chemical kinetic model was noted to be pressure dependent, and the agreement between the experimental and simulated results was found to depend on the operating conditions. A good agreement was observed at a compressed pressure of 20 bar, while a reduced reactivity was predicted by the chemical kinetic model at 40 bar. Brute force sensitivity analysis was also conducted at varying pressures, temperatures, and equivalence ratios to identify the reactions that influence simulated ignition delay times. Finally, further studies for improving the surrogate kinetic model were discussed and suggested.

  • comparative Autoignition trends in butanol isomers at elevated pressure
    Energy & Fuels, 2013
    Co-Authors: Ya W Webe, Chihje Sung


    Autoignition experiments of stoichiometric mixtures of s-, t-, and i-butanol in air have been performed using a heated rapid compression machine (RCM). At compressed pressures of 15 and 30 bar and …

  • a comparative experimental study of the Autoignition characteristics of alternative and conventional jet fuel oxidizer mixtures
    Fuel, 2010
    Co-Authors: Kamal Kuma, Chihje Sung


    Abstract Autoignition characteristics of an alternative (non-petroleum) and two conventional jet fuels are investigated and compared using a heated rapid compression machine. The alternative jet fuel studied is known as “S-8”, which is a hydrocarbon mixture rich in C 7 –C 18 linear and branched alkanes and is produced by Syntroleum via the Fischer–Tropsch process using synthesis gas derived from natural gas. Specifically, ignition delay times for S-8/oxidizer mixtures are measured at compressed charge pressures corresponding to 7, 15, and 30 bar, in the low-to-intermediate temperature region ranging from 615 to 933 K, and for equivalence ratios varying from 0.43 to 2.29. For the conditions investigated for S-8, two-stage ignition response is observed. The negative temperature coefficient (NTC) behavior of the ignition delay time, typical of higher order hydrocarbons, is also noted. Further, the dependences of both the first-stage and the overall ignition delays on parameters such as pressure, temperature, and mixture composition are reported. A comparison between the Autoignition responses obtained using S-8 and two petroleum-derived jet fuels, Jet-A and JP-8, is also conducted to establish an understanding of the relative reactivity of the three jet fuels. It is found that under the same operating conditions, while the three jet fuels share the common features of two-stage ignition characteristics and a NTC trend for ignition delays over a similar temperature range, S-8 has the shortest overall ignition delay times, followed by Jet-A and JP-8. The difference in ignition propensity signifies the effect of fuel composition and structure on Autoignition characteristics.

E Mastorakos – One of the best experts on this subject based on the ideXlab platform.

  • direct numerical simulations of premixed methane flame initiation by pilot n heptane spray Autoignition
    Combustion and Flame, 2016
    Co-Authors: Elena Demosthenous, Giulio Orghesi, E Mastorakos


    Abstract Autoignition of n-heptane sprays in a methane/air mixture and the subsequent methane premixed flame ignition, a constant volume configuration relevant to pilot-ignited dual fuel engines, was investigated by DNS. It was found that reducing the pilot fuel quantity, increases its Autoignition time. This is attributed to the faster disappearance of the most reactive mixture fraction (predicted from homogeneous reactor calculations) which is quite rich. Consequently, ignition of the n-heptane occurs at leaner mixtures. The premixed methane flame is eventually ignited due to heating gained by the pressure rise caused by the n-heptane oxidation, and heat and mass transfer of intermediates from the n-heptane Autoignition kernels. For large amounts of the pilot fuel, the combustion of the n-heptane results in significant adiabatic compression of the methane–air mixture. Hence the slow methane oxidation is accelerated and is further promoted by the presence of species in the oxidizer stream originating from the already ignited regions. For small amounts of the pilot fuel intermediates reach the oxidizer stream faster due to the very lean mixtures surrounding the n-heptane ignition kernels. Therefore, the premixed methane oxidation is initiated at intermediate temperatures. Depending on the amount of n-heptane, different statistical behaviour of the methane oxidation is observed when this is investigated in a reaction progress variable space. In particular for large amounts of n-heptane the methane oxidation follows roughly an Autoignition regime, whereas for small amounts of n-heptane methane oxidation is similar to a canonical premixed flame. The data can be used for validation of various turbulent combustion models for dual-fuel combustion.

  • simulations of Autoignition and laminar premixed flames in methane air mixtures diluted with hot products
    Combustion Science and Technology, 2014
    Co-Authors: Jennife A Sidey, E Mastorakos, Robe L Gordo


    This article considers constant-pressure Autoignition and freely propagating premixed flames of cold methane/air mixtures mixed with equilibrium hot products at high enough dilution levels to burn within the moderate to intense low oxygen dilution (MILD) combustion regime. The analysis is meant to provide further insight on MILD regime boundaries and to identify the effect of hot products speciation. As the mass fraction of hot products in the reactants mixture increases, Autoignition occurs earlier. Species profiles show that the products/reactants mixture approximately equilibrates to a new state over a quick transient well before the main Autoignition event, but as dilution becomes very high, this equilibration transient becomes more prominent and eventually merges with the primary ignition event. The dilution level at which these two reactive zones merge corresponds well with that marking the transition into the MILD regime, as defined according to conventional criteria. Similarly, premixed flame simu…

  • experimental investigation of the effects of turbulence and mixing on Autoignition chemistry
    Flow Turbulence and Combustion, 2011
    Co-Authors: Christos N Markides, E Mastorakos


    The Autoignition of acetylene, released from a finite-sized circular nozzle into a turbulent coflow of hot air confined in a pipe, has been the subject of a recent experimental study to supplement previous work for hydrogen and n-heptane. As with hydrogen and n-heptane, Autoignition appears in the form of well-defined localized spots. Quantitative information is presented concerning the effects of turbulence intensity, turbulent lengthscale and injector diameter on the location of Autoignition. The effects of these parameters on inhomogeneous Autoignition have not been investigated experimentally before. The present study establishes that increasing the bulk velocity increases the Autoignition length, as was reported for hydrogen and n-heptane. For the same turbulence intensity, the Autoignition length increases as the injector diameter increases and as the turbulent lengthscale decreases. A simultaneous decrease in turbulence intensity and increase in lengthscale causes a reduction in Autoignition length. Further, the frequency of appearance of the Autoignition spots has also been measured. It is found to increase when Autoignition occurs closer to the injector, and also at higher velocities. The observed trends are consistent with expectations arising from the dependence of the mixture fraction and the scalar dissipation rate on the geometrical and flow parameters. The data can be used for the validation of turbulent combustion models.