Onset Of Autoignition Research Articles

Study of water injection on suppressing knock in a high compression ratio and supercharged hybrid gasoline engine

This study investigated the effects of a direct water injection from both macroscopic characteristics and chemical kinetics aspects in high compression ratio engines, specifically regarding knocking concerns. On the one hand, −80 °CA is thought to be the ideal water injection timing (WIT) in terms of lowering mixture temperature, knock suppression, and water droplet dispersion. The mixture temperature is lowered to below 760 K by −80 °CA of WIT. When ignition timing (IT) is maintained at 2 oCA or −3 oCA, respectively, the water-fuel ratio (WFR) of 0.4 and 0.6 is the threshold for the impact of water droplet mass on combustion. The knock intensity decreases with increasing water-fuel ratio, and the pressure rise rate threshold of 0.6 bar/°CA is a knock-occurrence criterion. On the other hand, spontaneous combustion of end mixtures first occurs beneath the exhaust valve, transporting burned gases and raising temperatures. The IC4H7 concentration is higher in the mid-to-high temperature areas near the cylinder liner. Elevated concentrations of CH2O indicate high-temperature reactions, while increased levels of CH3 and HCO signify auto-ignition onset. Whether the IT is 2 oCA or −3 oCA, both IC4H7 concentration and OH concentration observe a declining trend with the increase of WFR.

Numerical study of controlling a lean mixture autoignition in the hybrid HCCI engine using high frequency corona discharges

A relationship has been established between the ignition timing of the region, activated by a high frequency corona discharge, and the autoignition timing of unburned gas in the hybrid HCCI engine in a propane-air mixture. This relationship depends on the equivalence ratio ϕ: the leaner the mixture, the weaker the dependence. For a richer mixture (ϕ = 0.7), autoignition is a sequential propagation of autoignition waves in unburned gas, in a leaner mixture (ϕ = 0.5) spontaneous ignition right away of the entire volume occurs just in front. It has been shown that autoignition event can be controlled by varying the discharge initiation timing, the configuration of discharge region, and the specific energy input into the streamer channel. These parameters are varied in wide range values, but occurrence of autoignition falls into suitable range of ignition angles corresponding to the optimal operating conditions of HCCI engine. It has been revealed that the controlling the onset of autoignition in a very lean mixture is less effective than in a richer one.

Experimental Investigation of Spark-Assisted Compression-Ignition with Ammonia-Hydrogen Blends

Carbon-free emissions and stable operation of ammonia-fueled internal combustion engines have been demonstrated in recent studies through combustion strategies suited to the fuel’s unique properties. High ignition energy and slow flame speed of ammonia are typically compensated for by hydrogen addition and/or enhanced ignition systems while the high octane rating of ammonia allows high compression ratio. The experimental study in this work investigates the performance of ammonia in spark-assisted compression-ignition (SACI) mode, where the cylinder charge is spark ignited and a subsequent auto-ignition event results in a faster burn and more ideal combustion phasing. The high-compression ratio engine was fueled by 100% anhydrous ammonia or blends with small quantities of hydrogen (2.5% and 5% by volume). Fuels were port-injected for a homogeneous intake charge mixture at stoichiometric equivalence ratio. Two different engine speeds were tested at several intake temperatures and spark timings. All cases showed an inflection point in the apparent heat release rate (AHRR) followed by more rapid rate of heat release, signaling the onset of auto-ignition. Blending hydrogen in the fuel benefited gross indicated mean effective pressure (gIMEP) at maximum brake torque (MBT) timing, but pure ammonia fuel was more stable across a wide range of spark timings and intake temperatures. Burn durations in the present study were similar to those of conventional SI engines fueled by gasoline indicating that the combustion enhancement of SACI compensated for the lower flame speed of ammonia. The control of SI percent, defined as the fraction of heat release before auto-ignition compared to the total heat release, is shown to be strongly related to the sensitivity of auto-ignition timing to spark timing. Unburned ammonia emissions are hypothesized to be primarily driven by unburned gases trapped in the crevices and are mitigated slightly through blending hydrogen in the fuel or by increasing intake temperature. However, the addition of hydrogen and increasing intake temperature led to increased nitric oxide (NO) emissions. Of primary concern because of its potent global warming potential, nitrous oxide (N2O) did not show a clear trend with fuel hydrogen content or intake temperature, but definitively increased for the higher engine speed.

Numerical study on the effects of spark strategies on knocking combustion in a downsized gasoline rotary engine

The application of dual-spark plug in rotary engines (REs) could be regarded as an effective way to improve combustion performance but there is lack of researches about influence of spark strategies on RE knocking. For this reason, the effects of varying dual-spark plug locations and asynchronous ignition phases on local auto-ignition and knocking characteristics of a downsized gasoline RE were studied numerically in this paper. Results showed that asynchronous ignition strategies formed by delaying the trailing-spark ignition timing significantly reduced the knocking intensity (KI) and delayed timing of auto-ignition onset. The appropriate asynchronous ignition phase could induce multiple auto-ignition hot-spots, and the KI caused by multiple hot-spots formed in sequence was higher than that caused by a single hot spot. The arrangement of the dual-spark plug determined the space for flame development, and it was favorable in reducing the KI for the trailing-spark plug to stand an offset from the minor axis of the engine. It should be noted that the KI of the RE was reduced by 87% when the offset distance between the trailing-spark plug location and the minor axis was double that between the leading-spark plug location and the minor axis.

Влияние химического состава матрицы на структуру и свойства монолитных СВС-композитов

Introduction. The development of new wear-resistant materials obtained by the method of self-propagating high-temperature synthesis (SHS) is an urgent problem in materials science. The SHS method is most widely used in the field of creating new powder materials. Much less attention has been paid to the production of monolithic non-porous composites. For monolithic composites, it is very important to identify the role of the metal matrix and phase transformations in the process of secondary structure formation after the completion of the synthesis process when the obtained material is cooled. The aim of this work was to carry out a comparative analysis of the structure and properties of SHS composites of the Fe-Ti-C-B, Fe-Ni-Ti-C-B, Fe-Ni-Cr-Ti-C-B, and Cu-TiC-B systems. Materials and research methods. Composites were obtained from powder mixtures consisting of thermoreactive components Ti, C, and B, as well as matrix Fe, Fe-Ni, Fe-Ni-Cr, and Cu. The initial powders were thoroughly mixed, loaded into a steel tube container, and the powder mixture was preliminary compacted. Then, the workpieces were heated in an electric furnace to the temperature of the onset of autoignition. After completion of the SHS, the workpieces were deformed with a force of 250 MPa in a hydraulic press at a temperature not lower than 1000 ° C. Samples were cut from the obtained sandwich plates for microstructural studies, density determination, hardness measurements, transverse bending tests and abrasive wear resistance tests. Results and discussion. All investigated composites were characterized by an uneven distribution of strengthening particles TiC and TiB2 over the volume. The use of the Fe-Ni matrix led to the formation of regions with the γ-Fe + Fe2B eutectic structure in the composite and an additional strengthening phase Ni3Ti. The use of Fe-Ni-Cr metal-matrix components led to the formation of two solid solutions in the matrix - austenite and ferrite, and Cr23C6 particles were formed along the boundaries of austenite grains. The maximum transverse bending strength was shown by SHS composites of the Fe-Ti-C-B and Cu-Ti-C-B systems with a matrix of FCC solid solutions. All composites had a hardness of 66 -72 HRC and showed the same abrasion resistance.

The Effect of EGR Dilution on the Heat Release Rates in Boosted Spark-Assisted Compression Ignition (SACI) Engines

<div class="section abstract"><div class="htmlview paragraph">This paper presents an experimental investigation of the impact of EGR dilution on the tradeoff between flame and end-gas autoignition heat release in a Spark-Assisted Compression Ignition (SACI) combustion engine. The mixture was maintained stoichiometric and fuel-to-charge equivalence ratio (<i>ϕ</i><sup>′</sup>) was controlled by varying the EGR dilution level at constant engine speed. Under all conditions investigated, end-gas autoignition timing was maintained constant by modulating the mixture temperature and spark timing. Experiments at constant intake pressure and constant spark timing showed that as <i>ϕ</i><sup>′</sup> is increased, lower mixture temperatures are required to match end-gas autoignition timing. Higher <i>ϕ</i><sup>′</sup> mixtures exhibited faster initial flame burn rates, which were attributed to the higher laminar flame speeds immediately after spark timing and their effect on the overall turbulent burning velocity. The increasing trends in peak heat release rate and peak autoignition rate as a function of <i>ϕ</i><sup>′</sup> were consistent for all intake pressures ranging from 80 kPa to 150 kPa. For a constant spark timing, the mass fraction burned at the onset of autoignition correlated well with <i>ϕ</i><sup>′</sup>, regardless of intake pressure. Reducing the unburned mass fraction at the onset of autoignition was effective at limiting the peak heat release rates. This behavior was attributed to the reduced energy content of the end-gas and also to the reduced end-gas burn rates. Possible explanations for the latter observation were the greater buffer gas effect due to a larger burned gas mass, and the higher reactivity stratification of the end-gas being closer to the walls.</div></div>

Understanding the correlation between auto-ignition, heat release and knocking characteristics through optical engines with high compression ratio

Improving compression ratio is an effective way to meet higher thermal efficiency and lower CO2 emissions for internal combustion engines. However, the severe thermodynamic condition under low speed and high road may promote the possibility of auto-ignition and damage engine body if it is uncontrollable. Therefore, further study on the correlation between auto-ignition and knocking characteristics are necessary for modern engines with high compression ratio. In this study, combined with high-speed direct photography and simultaneous pressure acquisition, flame images and in-cylinder pressures characterizing auto-ignition, heat release rate and knocking combustion were studied based on optical engine experiments. The results show that auto-ignition occurs in all engine cycles under current critical knocking conditions, and fast SI flame speed cycles is more likely to facilitate end-gas auto-ignition. However, the auto-ignition flame speed is comparable to the SI flame and auto-ignition is not a sufficient condition for knock: for some cycles multipoint auto-ignitions occur near the SI flame front and result in two peak heat release rate, which is beneficial to engine thermal efficiency. Further analysis shows that there is no obvious pertinence between knock intensity and auto-ignition onset while knock intensity strongly depends on the peak HRR per unit volume. High peak HRR per unit volume always results in knocking combustion. The current study shall give insights into utilizing auto-ignition in high compression ratio SI engines.

Numerical Analysis of End-Gas Autoignition and Pressure Oscillation in a Downsized SI Engine Using Large Eddy Simulation

Knock and super-knock are abnormal combustion phenomena in engines, however, they are hard to study comprehensively through optical experimental methods due to their inherent destructive nature. In present work, the methodology of large eddy simulation (LES) coupled with G equations and a detailed mechanism of primary reference fuel (PRF) combustion is utilized to address the mechanisms of knock and super-knock phenomena in a downsized spark ignition gasoline engine. The knock and super-knock with pressure oscillation are qualitatively duplicated through present numerical models. As a result, the combustion and onset of autoignition is more likely to occur at top dead center (TDC), which causes end gas at a higher temperature and pressure. It is reasonable to conclude that the intensity of knock is not only proportional to the mass fraction of mixtures burned by the autoignition flame but the thermodynamics of the unburned end-gas mixture, and the effect of thermodynamics is more important. It also turns out that two auto-ignitions occur in conventional knock conditions, while only one auto-ignition takes place in super-knock conditions. However, the single autoignition couples with the pressure wave and they reinforce each other, which eventually evolves into detonation combustion. This work gives the valuable insights into knock phenomena in spark ignition gasoline engines.

Chemical Ignition Characteristics of Ethanol Blending with Primary Reference Fuels

Synergistic octane blending behavior of ethanol with gasoline and its surrogates has been observed by many researchers. The nonlinear octane boosting tendency is observed at mid and high molar blends of ethanol in primary reference fuels. The present work aims to provide chemical insight into this nonlinear blending behavior of ignition processes when ethanol is blended with primary reference fuels. To this end, ignition delay time (IDT) calculations, using a well-validated mechanism, were performed for several fuel blends of iso-octane, n-heptane, and ethanol. Temperature and pressure values were found, correlating experimentally measured octane numbers and simulated homogenous batch reactor IDTs. The temperature and pressure conditions obtained, were then used to study the evolution of heat release and reactivity before the onset of auto-ignition in a homogeneous premixed reactor. Markers of low- and high-temperature reactivity (OH and HO2) were analyzed for various molar blends of n-heptane with ethanol–iso-octane. Ethanol was observed to be better at radical scavenging than iso-octane at a higher mole fraction. A computational singular perturbation analysis was conducted for a selection of blends to clarify the reactions responsible for the synergistic blending behavior of ethanol in n-heptane. The role of the H-abstraction reactions was highlighted during the first ignition stage; reactions related to n-heptane were found to compete with the H-abstraction reactions of iso-octane or ethanol. Notably, the H-abstraction path of ethanol was more favored than that of the iso-octane, as a result of the smaller activation energies of the related reactions in ethanol. The competition of the H-abstraction paths resulted in a smaller radical pool in the n-heptane–iso-octane–air case, and an even smaller pool in n-heptane–ethanol–air. In all the cases considered, the second stage was dominated mainly by hydrogen-related reactions, regardless of the initial mixture, with H2O2 (+M) → 2OH (+M) and H + O2 → O + OH playing the most important roles. This work employed a novel approach to examine specific reactions responsible for auto-ignition in ethanol blends, which can be used for fuel design, primarily around the generation/consumption of radical pool intermediates by interaction with fuel components.

RANS and LES of a Turbulent Jet Ignition System Fueled with Iso-Octane

The behaviour of a homogeneously charged Turbulent Jet Ignition (TJI) system, fueled with iso-octane was investigated numerically using Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes (RANS) turbulence models. This study was an attempt to capture the start of autoignition in a lean charge TJI system, numerically, and, validate the results with experimental pressure measurements and OH chemilominescence images recorded during high speed imaging. Experiments were performed in an optically acessible Rapid Compression Machine (RCM) and the effect of auxiliary fuel injection in the prechamber and ignition distribution through various orrifices was investigated in depth through jet induced autoignition analysis. Heat release and pressure trace analysis were completed to capture the onset of autoignition, as well as comparing LES and RANS capabilities in this regard. Results showed that enhanced prechamber fueling leads to an increase in combustion stability while reducing the ignition delay. It was determined that keeping the prechamber mixture near stoichiometric is essential in order to have a more powerful turbulent jet discharged into the main chamber and to enhance ultra-lean main chamber combustion. The predicted flame propagation speed in the lean iso-octane mixtures was found to be slower than that observed in the experimental measurements. Results showed that the LES turbulence model is capable of predicting the start of autoignition more accurately and enhances the accuracy of the calculated peak pressure and burn rates relative to the RANS model. Two detailed iso-octane mechanisms were tested and based on the ignition delay comparisons, the LLNL-v3 mechanism was used in the current study.

The LES and LEM Study of End-Gas Auto-Ignition Mechanism in a Downsized Spark Ignition Engine: Effect of Turbulence

ABSTRACT The effect of turbulence on the end-gas auto-ignition process of different primary reference fuels (PRFs) under engine-relevant conditions is numerically investigated through 3-D LES (large eddy simulation) and 1-D LEM (linear eddy model) simulations. Spontaneous ignition in the end-gas and pressure oscillations under the knock condition of a downsized spark ignition engine are qualitatively captured through the LES simulation. In order to take a further insight into the effect of full-scale turbulence-chemistry interaction on the auto-ignition process of the end-gas, 1-D LEM simulations are performed in a 1-D domain composed of a homogeneous core region and a linearly thermal distributed boundary layer. The effects of turbulent fluctuating velocity, fuel composition, turbulence length scale and initial temperature on the ignition process are investigated in the present study. It is found that the turbulence has a non-monotonic influence in the auto-ignition (AI) formations and expansion process of the PRF/air mixtures. Increased turbulence intensity delays the formation of the initial AI and shrinks the kernel size at the onset of AI due to the enhanced heat and radicals dissipation. However, after the first AI kernel forms, the intensified turbulence accelerates the overall combustion as a result of expanded preheating zone and more homogeneous unburned mixtures caused by turbulent stirring. In addition, the dissipation and homogenization effects of turbulence on the high-octane fuel are more prominent than on the low-octane fuel. The analysis of cases with different turbulent integral length scales reveals that the dissipation effect of the turbulence with the similar length scale of the nascent AI kernel is more prominent while the small-scale turbulence is more apt to shift the combustion mode after the first auto-ignition flame forms. At a high initial temperature of 900 K falling in the NTC regime, the first AI kernel of PRF10 appears in the boundary layer with a lower temperature instead of the core region. As the turbulence intensifies, the effect of NTC on the end-gas auto-ignition vanishes.

A Comprehensive Model for the Auto-Ignition Prediction in Spark Ignition Engines Fueled With Mixtures of Gasoline and Methane-Based Fuel

The introduction of natural gas (NG) in the road transport market is proceeding through bifuel vehicles, which, endowed of a double-injection system, can run either with gasoline or with NG. A third possibility is the simultaneous combustion of NG and gasoline, called double-fuel (DF) combustion: the addition of methane to gasoline allows to run the engine with stoichiometric air even at full load, without knocking phenomena, increasing engine efficiency of about 26% and cutting pollutant emissions by 90%. The introduction of DF combustion into series production vehicles requires, however, proper engine calibration (i.e., determination of DF injection and spark timing maps), a process which is drastically shortened by the use of computer simulations (with a 0D two zone approach for in-cylinder processes). An original knock onset prediction model is here proposed to be employed in zero-dimensional simulations for knock-safe performances optimization of engines fueled by gasoline-NG mixtures or gasoline-methane mixtures. The model takes into account the negative temperature coefficient (NTC) behavior of fuels and has been calibrated using a considerable amount of knocking in-cylinder pressure cycles acquired on a Cooperative Fuel Research (CFR) engine widely varying compression ratio (CR), inlet temperature, spark advance (SA), and fuel mixture composition, thus giving the model a general validity for the simulation of naturally aspirated or supercharged engines. As a result, the auto-ignition onset is predicted with maximum and mean error of 4.5 and 1.4 crank angle degrees (CAD), respectively, which is a negligible quantity from an engine control standpoint.

Experimental investigation of the auto-ignition of a transient propane Jet-in-Hot-Coflow

Auto-ignition is a complex process which is extremely sensitive to boundary conditions such as local temperature, mixture or strain rate and occurs on very short time-scales. Therefore, measurement techniques with high spatio-temporal resolution have to be applied to test cases with well-defined boundary conditions in order to generate high-quality validation data for numerical simulations. In the current paper, the auto-ignition of a transient propane jet-in-hot coflow was studied with high-speed OH* chemiluminescence imaging and high-speed Rayleigh scattering for the simultaneous determination of mixture fraction, mixture temperature and scalar dissipation rate immediately prior to the onset of auto-ignition. A variation of the coflow temperature showed a pronounced temperature dependence of the auto-ignition location and time, and the temperature sensitivity was higher than for a comparable methane test case from the literature. This is explained by the lower sensitivity of propane ignition delay times to the local strain rate in comparison to methane. The Rayleigh measurements however showed that the formation mechanism of auto-ignition kernels is similar for propane and methane. Ignition kernels were found to form upstream of bulges of the inflowing jet at locations with locally low scalar dissipation rate.

A thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part I — Adiabatic core ignition model

This two-part article presents a model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two components: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development and validation of the homogeneous charge compression ignition model for use under a broad range of operating conditions. Using computational fluid dynamics simulations of the negative valve overlap valve events typical of homogeneous charge compression ignition operation, it is shown that there is no noticeable reaction progress from low-temperature heat release, and that ignition is within the high-temperature regime ( T > 1000 K), starting within the highest temperature cells of the computational fluid dynamics domain. Additional parametric sweeps from the computational fluid dynamics simulations, including sweeps of speed, load, intake manifold pressures and temperature, dilution level and valve and direct injection timings, showed that the assumption of a homogeneous charge (equivalence ratio and residuals) is appropriate for ignition modelling under the conditions studied, considering the strong sensitivity of ignition timing to temperature and its weak compositional dependence. Use of the adiabatic core temperature predicted from the adiabatic core model resulted in temperatures within ±1% of the peak temperatures of the computational fluid dynamics domain near the time of ignition. Thus, the adiabatic core temperature can be used within an auto-ignition integral as a simple and effective method for estimating the onset of homogeneous charge compression ignition auto-ignition. The ignition model is then validated with an experimental 92.6 anti-knock index gasoline-fuelled homogeneous charge compression ignition dataset consisting of 290 data points covering a wide range of operating conditions. The tuned ignition model predictions of [Formula: see text] have a root mean square error of 1.7° crank angle and R2 = 0.63 compared to the experiments.

Effect of turbulent mixing on the end gas auto-ignition of n-heptane/air mixtures under IC engine-relevant conditions

The effect of turbulence on the end-gas auto-ignition (AI) of stoichiometric n-heptane/air mixtures under engine relevant conditions is numerically studied through a combined methodology of Large Eddy Simulation (LES) and one-dimensional stand-alone Linear Eddy Model (LEM). Similar end-gas auto-ignition process in super rapid compression machine experiments is first been qualitatively reproduced through LES. To further investigate the effect of small-scale turbulence-chemistry interaction on the end-gas auto-ignition in the near-wall region which consists of a core region and a boundary layer, 1-D LEM simulations are performed by extracting the thermal-chemical and turbulent parameters at top dead center from LES. The parametric study covers a range of representative fluctuation velocity values under engine-like conditions at two initial temperatures of 700K and 900K, which are below and within the Negative Temperature Coefficient regime respectively. It is generally found that, increased turbulence intensity delays AI formation and also reduces the kernel size at the onset of AI. At the initial temperature of 700K, intense turbulence can change the combustion mode of the end-gas following the initial spontaneous auto-ignition in the core region. Diffusive - reactive flame structures are observed and the kernel expansion is governed by turbulent flame propagation. At a high initial temperature of 900K, the formation of “cold” AI kernels is inhibited with increased turbulence intensity. A budget analysis reveals that the expansion processes of nascent kernels mainly are spontaneous ignitions and turbulence may enhance the local scalar mixing. However, turbulence won't change the nature of auto-ignition propagation wave driven dominantly by exothermicity during the end-gas combustion process.

Thermodynamic model for homogeneous charge compression ignition combustion with recompression valve events and direct injection: Part II—Combustion model and evaluation against transient experiments

This two-part article presents a combustion model for boosted and moderately stratified homogeneous charge compression ignition combustion for use in thermodynamic engine cycle simulations. The model consists of two parts: one an ignition model for the prediction of auto-ignition onset and the other an empirical combustion rate model. This article focuses on the development of the combustion model which is algebraic in form and is based on the key physical variables affecting the combustion process. The model is fit with experimental data collected from 290 discrete automotive homogeneous charge compression ignition operating conditions with moderate stratification resulting from both the direct injection and negative valve overlap valve events. Both the ignition model from part 1 and the combustion model from this article are implemented in GT-Power and validated against experimental homogeneous charge compression ignition data under steady-state and transient conditions. The ignition and combustion model are then exercised to identify the dominant variables affecting the homogeneous charge compression ignition and combustion processes. Sensitivity analysis reveals that ignition timing is primarily a function of the charge temperature, and that combustion duration is largely a function of ignition timing.

Auto-Ignition Characteristics Study of Gas-to-Liquid Fuel at High Pressures and Low Temperatures

Onset of auto-ignition of premixed gas-to-liquid (GTL)/air mixture has been determined at high pressures and low temperatures over a wide range of equivalence ratios. The GTL fuel used in this study was provided by Air Force Research Laboratory (AFRL), designated by Syntroleum S-8, which is derived from natural gas via the Fischer–Tropsch (F–T) process. A blend of 32% iso-octane, 25% n-decane, and 43% n-dodecane is employed as the surrogates of GTL fuel for chemical kinetics study. A spherical chamber, which can withstand high pressures up to 400 atm and can be heated up to 500 K, was used to collect pressure rise data, due to combustion, to determine the onset of auto-ignition. A gas chromatograph (GC) system working in conjunction with specialized heated lines was used to verify the filling process. A liquid supply manifold was used to allow the fuel to enter and evaporate in a temperature-controlled portion of the manifold using two cartridge heaters. An accurate high-temperature pressure transducer was used to measure the partial pressure of the vaporized fuel. Pressure rise due to combustion process was collected using a high-speed pressure sensor and was stored in a local desktop via a data acquisition system. Measurements for the onset of auto-ignition were done in the spherical chamber for different equivalence ratios of 0.8–1.2 and different initial pressures of 8.6, 10, and 12 atm at initial temperature of 450 K. Critical pressures and temperatures of GTL/air mixture at which auto-ignition takes place have been identified by detecting aggressive oscillation of pressure data during the spontaneous combustion process throughout the unburned gas mixture. To interpret the auto-ignition conditions effectively, several available chemical kinetics mechanisms were used in modeling auto-ignition of GTL/air mixtures. For low-temperature mixtures, it was shown that auto-ignition of GTL fuel is a strong function of unburned gas temperature, and propensity of auto-ignition was increased as initial temperature and pressure increased.

The role of temperature, mixture fraction, and scalar dissipation rate on transient methane injection and auto-ignition in a jet in hot coflow burner

The transient injection and subsequent auto-ignition of a methane jet issuing into a laminar coflow of hot exhaust gas from a lean premixed hydrogen air flame was studied using high-speed planar Rayleigh scattering, yielding two-dimensional measurements of mixture fraction, temperature and scalar dissipation rate with high spatio-temporal resolution. The temporal development of the mixing field between the transient fuel jet and the surrounding coflow prior to the occurrence of auto-ignition was examined at a sampling rate of 10kHz. The impact of the transient jet development on numerical modeling of this test case is discussed. It was found that auto-ignition occurred after the jet transitioned from a transient state into the steady state, thus eliminating the need to model the complete transient fuel injection when the primary focus is on the onset of auto-ignition.Simultaneous high-speed OH* chemiluminescence from two viewing angles was applied to gain 3D-information of the ignition kernel location. This information allowed the selection and analysis of ignition events where the initial kernel formed inside the laser light sheet. Detailed analysis of the dynamics of a single ignition event, as well as statistical analysis of multiple ignition events based on a joint probability density approach, indicated that the ignition kernels occurred at very lean mixture fractions and at locations with low scalar dissipation rates.

Autoignition of n-heptane in a turbulent co-flowing jet

N-heptane autoignition in turbulent co-flowing jets with preheated air is studied using the one-dimensional turbulence (ODT) model. The simulations are designed to investigate the effects of molecular and turbulent transports on the process of autoignition. Both homogeneous and jet configuration simulations are carried out. The jet configurations are implemented at different jet inlet Reynolds numbers and for two air preheat conditions. Statistics for the cases considered show that, while the onset of autoignition may be delayed by turbulence, the eventual evolution of the volumetric heat release rate indicates that turbulence enhances the post-ignition stages. Since different regions of the mixture can have different ignition delays and may be characterized by one- or two-stage ignition, the autoignition process can be accelerated by ignition kernel propagation or the role of heat dissipation may be reduced through the prevalence of one-stage and two-stage ignitions in different regions of the mixture.

Modeling of combustion phasing of a reactivity-controlled compression ignition engine for control applications

Reactivity-controlled compression ignition (RCCI) is a promising combustion strategy to achieve near-zero NOx and soot emissions and diesel-like efficiencies. Model-based control of RCCI combustion phasing requires a computationally efficient combustion model that encompasses factors such as injection timings, fuel blend composition, and reactivity. In this work, physics-based models are developed to predict the onset of auto-ignition in RCCI and to estimate the burn duration based on an approximation of the spontaneous ignition front speed. A mean value control-oriented model of RCCI is then developed by combining the auto-ignition model, the burn duration model, and a Wiebe function to predict combustion phasing. The control-oriented model is parameterized and validated using simulation data from an experimentally validated, detailed computational fluid dynamics combustion model developed using the KIVA-3V code. The validation results show that the control-oriented model can predict the start of combustion, burn duration, and crank angle of 50% burnt fuel with an average error of less than 2 crank angle degrees. Thus, the control-oriented model demonstrates sufficient accuracy in predicting RCCI combustion phasing for control applications. The control-oriented model is an integral part of designing a model-based controller, which in the case of RCCI is of paramount importance due to various attributes concerning combustion, particularly for transient engine operation.