Auto-ignition Process Research Articles

End-gas autoignition behaviors under pressure wave disturbance

The present study addresses effects of pressure wave disturbance on end-gas autoignition behavior and subsequent pressure wave development, which are of particular interest for understanding a fundamental process of knocking combustion in spark-assisted ignition engines. Such pressure wave disturbance is initiated by a compression wave generated from spark-like ignition and continuously exists in a combustion chamber. The investigation is conducted through a direct numerical simulation with a 1-D constant volume reactor, where the compressible Navier–Stokes equations are solved with detailed reaction mechanisms of n−C7H16 and n−C4H10/air mixtures. The width of an ignition kernel parametrically changes in order to control the strength of an initial compression wave and thus pressure wave disturbance. The result showed that stronger compression waves with larger kernel widths enhance the progress of the autoignition process at the wall through the transient increase in pressure and temperature due to the wave reflection. Consequently, stronger pressure waves such as a developing detonation are generated with the enhanced inhomogeneous end-gas autoignition starting from the wall. In contrast, weak pressure wave disturbance produces no strong pressure waves due to the lower inhomogeneity of autoignition timing in an end-gas region. An analysis with temperature variation in an end-gas region indicates that cool flame generation helps maintain the superiority of the wall for earlier autoignition through the heat release at an earlier stage. Therefore, a stronger pressure wave associated with inhomogeneous end-gas autoignition is likely generated around 650 K in the case of an n−C7H16/air mixture, which corresponds to a distinct peak generation of knocking intensity in the negative temperature coefficient regime. For lower temperature conditions such as 500 K, the end-gas autoignition first takes place in a region away from the wall. This is because the initially induced temperature inhomogeneity in the end-gas region is considerably weakened due to wave interactions and dissipation effects during longer ignition delay times. Thus, multiple end-gas points including the wall can become a preferred point for earlier end-gas autoignition, resulting in a variation of end-gas autoignition locations. The result for an n−C4H10/air mixture is finally shown and the difference from that for the n−C7H16/air mixture is highlighted.

Numerical simulation of the effects of evaporation on the n-heptane/air auto-ignition process under different initial air temperatures

Numerical simulations are carried out to study the effects of fuel evaporation and mixing on autoignition process of n-heptane/air with complex chemistry. By assuming that n-heptane spray evaporation and mixing are infinitely fast under initial air temperature Tair of 740–1200 K and pressure of 1–24 atm, homogeneous gaseous mixture of n-heptane/air with different equivalence ratio can be obtained. Three types of the most reactive mixture fraction ηMR are identified in mixture fraction space: ηMR,LTand ηMR,ig are associated with the shortest first-stage ignition by low temperature chemistry (LTC) and the total two-stage ignition delay time respectively, while ηMR,HT corresponds to the shortest single-stage ignition delay time by the high temperature chemistry (HTC). With the increasing initial air temperature Tair, ηMR,ig and ηMR,LT will increase, but the corresponding ignition delay times decrease. When Tair is high enough to make the temperature of gaseous mixture higher than the upper turnover temperature of NTC region in very lean mixture, ηMR,HT will appear with shortest ignition delay time. Moreover, the ignition delay time at ηMR,HT could be less than that at ηMR,LT in the spray autoignition process under high initial air temperature Tair=1200K and low initial pressure, 1 atm and 6 atm. This finding suggests that in spray autoignition process, due to the cooling effect of evaporation, LTC reaction and HTC reaction may coexist if the interval between the ignition delay times of ηMR,HT and ηMR,LT is quite small, and also that the high temperature ignition may precede the low temperature ignition. In addition, sensitivity and reaction pathway analysis are performed to disclose the controlling chemical kinetics at the three types of the most reactive mixture fraction ηMR of the case with high initial air temperature 1200 K and pressure 6 atm.

Auto-ignition of single n-heptane droplets in low-temperature and high-pressure air using Lagrangian–Eulerian approach

The objective of this research is to investigate the evaporating, mixing, and auto-ignition processes of isolated fuel droplets using Lagrangian–Eulerian numerical approach. By using Peng–Robinson equation for the phase-equilibrium, the gas solubility at the droplet surface is considered at high-pressure ambient gases. Using perfectly stirred reactor combustion model and a low-temperature detailed mechanism for the n-heptane, the auto-ignition of the premixed fuel vapor–air mixture is verified at a constant pressure. Then, the auto-ignition of a fuel droplet with 60 µm diameter is studied at various low air temperatures. The analysis indicated that the results are strongly mesh dependent in this approach. Therefore, in order to predict the correct values of auto-ignition delay time and maximum gas temperature, the computational mesh size should be chosen in a specific range of cell sizes. Also, the auto-ignition of different size fuel droplets is studied at the air initial temperature of 800 K. It was observed that for accurate results, the computational mesh size should be chosen around seven to eight times the diameter of initial fuel droplets.

Autoignition-cascade in the windward mixing layer of a premixed jet in hot vitiated crossflow

This numerical study builds upon the experimental work of Wagner et al., e.g. [Combust. Flame 176 (2017)], and focuses on the flame stabilization mechanism of a reactive jet in crossflow (RJICF). A 300 K premixed ethylene-air jet (ϕj=1.2) is injected into a hot vitiated crossflow at 1500 K. A compressible three-dimensional (3-D) large eddy simulation (LES) was performed with an analytically reduced chemistry mechanism. Comparisons between LES and experiments are in good agreement in terms of jet trajectories and flow velocities. The LES captures the flame-flow interactions that were experimentally observed in the central plane of the RJICF by means of particle image velocimetry and OH planar laser-induced fluorescence. It gives access to the spatio-temporal evolution of the autoignition process along the windward mixing layer of the RJICF. Chemical explosive mode analysis shows that autoignition is the dominant flame stabilization mechanism on the windward side of the jet. “Low-heat-release” autoignition starts at the front root of the jet, at the “very lean” most reactive mixture fraction, which is the spot where the autoignition time of the mixture is the smallest. Immediately downstream, the released heat is advected and diffuses, contributing to a decrease of autoignition times in the neighboring richer inner side of the jet. Autoignition in this richer layer of the jet is characterized by higher heat release rate that again diffuses toward even richer regions until the stoichiometric layer ignites. This autoignition-cascade mechanism determines the position of the “visible” windward flame. Moreover, a 3-D flame analysis shows that the resulting “high-heat-release” autoignition patches expand within the stoichiometric layer and ultimately merge with the leeward propagating flame while being advected toward the flame tip. It is also shown that the latter layer, and therefore the growing autoignition patches, get wrinkled by the coherent structures that originate from the hydrodynamically unstable front root of the jet in crossflow.

Combustion of Hydrogen Enriched Methane and Biogases Containing Hydrogen in a Controlled Auto-Ignition Engine

The paper describes a numerical study of the combustion of hydrogen enriched methane and biogases containing hydrogen in a Controlled Auto Ignition engine (CAI). A single cylinder CAI engine is modelled with Chemkin to predict engine performance, comparing the fuels in terms of indicated mean effective pressure, engine efficiency, and pollutant emissions. The effects of hydrogen and carbon dioxide on the combustion process are evaluated using the GRI-Mech 3.0 detailed radical chain reactions mechanism. A parametric study, performed by varying the temperature at the start of compression and the equivalence ratio, allows evaluating the temperature requirements for all fuels; moreover, the effect of hydrogen enrichment on the auto-ignition process is investigated. The results show that, at constant initial temperature, hydrogen promotes the ignition, which then occurs earlier, as a consequence of higher chemical reactivity. At a fixed indicated mean effective pressure, hydrogen presence shifts the operating range towards lower initial gas temperature and lower equivalence ratio and reduces NOx emissions. Such reduction, somewhat counter-intuitive if compared with similar studies on spark-ignition engines, is the result of operating the engine at lower initial gas temperatures.

Direct numerical simulation on auto-ignition characteristics of turbulent supercritical hydrothermal flames

Hydrothermal flames occur in aqueous environments at conditions above the critical point of water. Due to the lack of reliable diagnostics applicable to high-pressure, high density-gradient and corrosive environments in hydrothermal flames, the auto-ignition characteristics of hydrothermal flames are still unclear. The objective of this study is to provide a fundamental understanding of auto-ignition process in non-premixed hydrothermal flames in terms of homogeneous ignition calculations and a two-dimensional (2D) direct numerical simulation. To model the real-fluid effects, the Peng-Robinson cubic equation of state with consistent formulas for thermodynamic properties and Chung's high-pressure methods for transport properties are implemented. The homogeneous ignition results indicate that the correlation of the ignition delay time with the mixture fraction in the current study is quite different from those reported in canonical conditions and there is no well-defined “most reactive” mixture fraction. In the DNS, a hydrothermal flame in a 2D mixing layer with pseudo-turbulence is simulated. Auto-ignition occurs independently and ignition kernels form at locations where the local mixture fraction varies from 0.80 to 0.85, lower than the stoichiometric mixture fraction (0.911). The ignition kernels evolve to edge flames that propagate at a mean displacement speed of 1.286 m/s. A compact structure of edge flames is observed, which lacks a rich premixed branch in contrast with the classic triple flame structure. An analysis in terms of species profiles and flame index is carried out to reveal the dynamics of the edge flames.

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.

Deflagration/Autoignition/Detonation Transition Induced by Flame Propagation in an N-Decane/O2 /Ar Mixture

The aim of this study is to experimentally investigate the combustion regime transitions of a single-component kerosene surrogate (n-decane). For this purpose, a deflagration is generated by a spark in a constant-volume vessel with a length-to-width aspect ratio of 4.3. By consuming the unburnt gas, the flame behaves as a piston that compresses the end gas. The pressure and temperature of the end gas increase with the flame propagation until autoignition conditions are reached. Ultrafast schlieren visualizations are set up to monitor the dynamics of the reactive processes, whereas the time-pressure evolution associated with non-dimensional (0D) numerical models is used to characterize the unburnt gas thermodynamic conditions. Once the thermodynamic conditions needed to initiate the autoignition reactions are reached in the end gas, a transition between deflagration (flame speed of approximately ten m/s) and autoignition fronts is observed (propagation velocity ˜200 m/s). This transition occurs for various fuel equivalence ratio values (0.75 – 1). For the strongest thermodynamic conditions, once the velocity of this autoignition front reaches the speed of sound, a new reactive front propagating up to 1800 m/s is observed, thus indicating the transition to detonation combustion mode. Parametric studies indicated that the occurrence of both transitions was a function of the pressure, temperature and equivalence ratio for n-decane fuel. A small initial temperature variation (approximately 40 K) could change the phenomenology of the constant-volume combustion from deflagration to detonation through the autoignition process, even for lean mixtures. The results obtained using our experimental setup show that the transition between autoignition and detonation is due to the acceleration of the autoignition front towards the speed of sound in the unburnt gas.

On the Influence of Fuel Stratification and Its Control on the Efficiency of the Shockless Explosion Combustion Cycle

Constant volume combustion (CVC) cycles for gas turbines are considered a very promising alternative to the conventional Joule cycle and its variations. The reason is the considerably higher thermal efficiency of these cycles, at least for their ideal versions. Shockless explosion combustion (SEC) is a method to approximate CVC. It is a cyclic process that consists of four stages, namely wave propagation, fuel injection, homogeneous auto-ignition, and exhaust. A pressure wave in the combustion chamber is used to realize the filling and exhaust phases. During the fuel injection stage, the equivalence ratio is controlled in such a way that the ignition delay time of the mixture matches its residence time in the chamber before auto-ignition. This means that the fuel injected first must have the longest ignition delay time, and thus forms the leanest mixture with air. By the same token, fuel injected last must form the richest mixture with air (assuming that a rich mixture leads to a small ignition delay). The total injection time is equal to the time that the wave needs to reach the open combustor end and return as a pressure wave to the closed end. Up to date, fuel stratification has been neglected in thermodynamic simulations of the SEC cycle. The current work presents its effect on the thermal efficiency of the cycle and on the exhaust conditions (pressure, temperature, and Mach number) of shockless explosion combustion chambers. This is done by integrating a fuel injection control algorithm in an existing computational fluid dynamics code. The capability of this algorithm to homogenize the auto-ignition process by improving the injection process has been demonstrated in past experimental studies of the SEC. The numerical code used for the simulation of the combustion process is based on the time-resolved 1D-Euler equations with source terms obtained from a detailed chemistry model.

Fuel and thermal load flexibility of a MILD burner

The strategy to stabilize distributed combustion regimes adopting cyclonic flow fields has been proven to be challenging. In fact, the establishment of a toroidal flow field within a combustion chamber may ensure the recirculation of mass and sensible enthalpy required to simultaneously dilute the fresh reactants and increase the temperature above the autoignition one. The combination of reactants dilution and preheating may greatly increase system energy efficiency and lower pollutants production producing very peculiar combustion regime (named MILD Combustion). At the same time this strategy can be compromised if the sensible enthalpy is not high enough to promote the auto-ignition process of diluted mixtures. This may happen either because of an inefficient recirculation system or due to a too low calorific fuel. The paper aims at exploiting the performance of a small-size cyclonic burner for a conventional fuel (CH4) and a low calorific fuel (synthetic biogas) through the characterization of the process stabilization and pollutant emissions as a function of the mixture equivalence ratio and the nominal thermal power of the inlet mixture (from 2 to 10 kW), with the aim of identifying the optimal operating condition of the system. Results suggest that the system has to be exercised with mixtures with compositions slightly under the stoichiometric conditions and in a well identified temperature range to minimize both NOx and CO emission. The burner can be easily exercised also with low calorific fuels for higher thermal powers according to the low LHV. However, it results that an efficient recirculation of the exhausts produces a robust MILD combustion condition also when low calorific fuels are used.

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel, CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36 kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH2O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH2O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.

Investigations on the effects of fuel stratification on auto-ignition and combustion process of an ethanol/diesel dual-fuel engine

In dual-fuel engines, the fuel stratification produced by direct in-cylinder blending of port injected and direct injected fuels is important for combustion and emission control, while the fuel stratification could be influenced by port fuel proportion, direct injection strategy as well as geometry of direct injector. In this paper, based on an ethanol/diesel dual-fuel engine, experiments and simulations have been conducted to study how these factors influence in-cylinder fuel stratification as well as auto-ignition and combustion process. The equivalence ratio of premixed ethanol, maximum local equivalence ratio and fuel concentration gradients in radial and circumferential directions of the cylinder were used to evaluate the fuel stratification. Under the tested dual-fuel conditions, the results showed that auto-ignition was more likely to first occur in regions with mixture near stoichiometric ratio, rather than regions with mixtures exhibiting highest fuel reactivity. As a consequence, larger equivalence ratio of premixed ethanol could prolong ignition delay of in-cylinder charge by reducing the fuel reactivity of regions where ignition kernels first occurred. Meanwhile, as the fuel rich regions produced by retarded diesel injection could not further extend the ignition delay gradient, the improved fuel reactivity and higher local equivalence ratio consequently resulted in steeper heat release rate. Additionally, the experimental results showed that with reduced nozzle holes, the peak pressure and heat release rate of dual-fuel operation could be decreased. This is mainly because fewer nozzle holes could result in fuel stratification in both circumferential and radial directions, thus the development of combustion zone was slowed down and consequently the heat release rate was decreased.

Effects of Exhaust Gas Recirculation Constituents on Methyl Decanoate Auto-Ignition: A Kinetic Study

Biodiesel engines are found to have improved soot, hydrocarbon (HC), and carbon monoxide (CO) emissions, with modestly increased nitrogen oxides (NOx) emissions. Exhaust gas recirculation (EGR) could be used for the NOx emissions control, especially in the fuel-kinetics-dominated engine combustion concepts. A detailed chemical kinetic model of methyl decanoate (MD), a biodiesel surrogate fuel, was used here to simulate the two-stage auto-ignition process of biodiesel with EGR addition. The effects of EGR constituents, including carbon dioxide (CO2), water vapor (H2O), CO and H2, were identified in a constant-pressure ignition process and in a variable pressure, variable volume process. Firstly, numerical methods were used to isolate the dilution, thermal, and chemical effects of CO2 and H2O at a constant pressure. It was found that in the biodiesel auto-ignition processes, the dilution effects of CO2 and H2O always played the primary role. Their thermal and chemical effects mainly influenced the second-stage ignition, and the chemical effect of H2O was more significant than CO2. The triple effects of CO and H2 were also analyzed at the same temperature and pressure conditions. Additionally, the sensitivity analysis and reaction pathway analysis were conducted to elucidate the chemical effects of CO and H2 on the ignition processes at different temperatures. Finally, based on a variable pressure, variable volume model simulating the engine compression stroke, the effects of CO2, H2O, CO and H2 addition under the engine operational conditions were studied and compared to those under the constant pressure conditions.

Analysis of auto-ignition characteristics of low-alcohol/iso-octane blends using combined chemical kinetics mechanisms

Knocking combustion is mainly caused by auto-ignition of the end gas in spark-ignition (SI) engines. It not only leads to severe engine damage, but also can reduce thermal efficiency and constrain engine performance. As high-octane gasoline additives, the auto-ignition characteristics of low-alcohols, such as methanol, ethanol and n-butanol, and their blends with iso-octane fuels are analyzed under the conditions of the end gas. Moreover, the effects of what mechanism that promots and inhibits low-alcohols effects on auto-ignition characteristics of iso-octane is studied based on detailed chemical kinetics mechanisms. First, the low-alcohols mechanisms are validated and selected by comparing the predicted ignition delay times of the end gas with the corresponding experimental data under different target conditions of lower temperature and high pressure. From there, the auto-ignition characteristics of low-alcohol are analyzed to predict the knock tendency of fuels. In addition, the reactivity of low-alcohol/iso-octane blends is analyzed by comparing the CA50 contours of different fuels under various intake temperature and pressure conditions. It is found that the n-butanol and iso-octane have stronger auto-ignition characteristics than methanol and ethanol, revealing that methanol and ethanol have higher anti-knock properties. For further explaining the effect of low-alcohols on the anti-knock performance of low-alcohol/iso-octane fuels, the key reaction pathways for the low-temperature reaction of low-alcohol/iso-octane fuels is studied. Studies show that metastable products, such as aldehydes and ketones, produced by the chain propagation can delay the auto-ignition process and reduce the low temperature reactivity of blended fuels, when methanol and ethanol fuels are added to iso-octane. But the reaction rate of the system is accelerated, and the ignition time of the blended fuel is significantly earlier than that of iso-octane after the addition of n-butanol in iso-octane.

Evolution of sensitivity directions during autoignition

Abstract Sensitivity analysis of the ignition delay time and species profiles to kinetic parameters has been widely used to identify the rate-limiting steps during the autoignition process, providing insights for the optimization of the reaction mechanism. This work studies the time evolution of the sensitivity directions of the temperature and species concentration during autoignition. The direction is represented by a unit vector along the gradient of the simulation output to the kinetic parameters, and the alignment between the two directions are measured by the inner product between the corresponding unit vectors. We use evolution of the sensitivity directions to reveal changes in the rate-limiting steps and the correlation among species at different phases of the ignition delay period. It is found that the sensitivity directions of temperature and the concentrations of the major intermediate species are similar to each other during the entire ignition delay period. In particular, they converge to the same direction when approaching the ignition state, and the direction is the same as the one for the ignition delay time. The correlation is validated for various fuels across a wide range of pressures and temperatures, and works for both single-stage ignition and two-stage ignition. Consequently the sensitivity of the ignition delay time can be efficiently evaluated based on the temperature sensitivity at the ignition point, for which a single run of the simulation can produce the sensitivity to all parameters, as otherwise the sensitivity of the ignition delay time has to be evaluated through finite difference, in which the number of runs equals to the number of parameters. It can also significantly reduce the computation cost of gradient-based algorithms for the purpose of mechanism optimization and uncertainty quantification.

Effect of methane on pilot-fuel auto-ignition in dual-fuel engines

The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100 µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH2O-PLIF (10 kHz) and Schlieren (80 kHz) imaging for detection of the first-stage ignition, and simultaneous high-speed OH* chemiluminescence (40 kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH2O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low T and high T chemistry and micro-mixing for dual-fuel auto-ignition processes for which the governing processes were identified.

A CFD Investigation of the Effects of Fuel Split Fraction on Advanced Low Temperature Combustion: Comparing a Primary Reference Fuel Blend and Ethanol

There is continuously growing interest in renewable biofuels for combustion engines to help reduce transportation energy consumption. In the present work, ethanol and a Primary Reference Fuel (PRF) were studied in an advanced LTC concept using CFD. A split injection strategy was used where the majority of the fuel was injected early during the intake stroke to create a well-mixed charge, while a portion of the charge was direct injected closer to ignition to induce forced thermal and equivalence ratio stratification in a strategy similar to partial fuel stratification (PFS). This way, the combustion process in LTC can be better controlled by staggering the autoignition process through mixture stratification. The unique characteristics of ethanol, such as its high latent heat of vaporization and reduced ϕ-sensitivity, result in unique features for a PFS-style advanced LTC mode, explored in this paper. A 3D CFD model with detailed chemistry was implemented in CONVERGE. The results showed that for both ethanol and PRF fuels, a split direct injection strategy lowers the peak heat release rate and elongates the combustion process compared to a single early direct injection due to the increased stratification. However, this effect was more pronounced for ethanol compared to PRF90 due to its higher latent heat of vaporization and reduced ϕ-sensitivity. For a 60%-40% split injection, the burn duration increased by 118% for ethanol and 91.6% for PRF90. The temperature, equivalence ratio, and OH mass fraction distributions illustrated that ethanol is primarily temperature-sensitive, while PRF90 shows a degree of ϕ-sensitivity in conjunction with temperature-sensitivity. The split direct injection of fuel creates an equivalence ratio and temperature distribution that are coupled due to the latent heat of vaporization of the fuel. For the PRF, these two effects are competing; whereas for the ethanol, the autoignition event is dictated by the thermal gradients since the fuel has a higher latent heat of vaporization and is not as ϕ-sensitive. Therefore, a PFS-style injection strategy is able to elongate the heat release rates more significantly with ethanol compared to a PRF.

Autoignition Process in Compression Ignition Engine Fueled by Diesel Fuel and Biodiesel with 20% Rapeseed Biofuel in Diesel Fuel

AbstractControlled autoignition delay is considered a very promising new technology, having the potential of combining low fuel consumption with low pollutant emissions. In this study, the effect o...

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH4/H2 flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.

Large-eddy simulations of transcritical injection and auto-ignition using diffuse-interface method and finite-rate chemistry

The need for improved engine efficiencies has motivated the development of high-pressure combustion systems, in which operating conditions achieve and exceed critical conditions. Associated with these conditions are strong variations in thermo-transport properties as the fluid undergoes mixing and phase transition, and two-stage ignition with low-temperature combustion. Accurately simulating these physical phenomena at real-fluid environments remains a challenge. This study examines a diffuse-interface method for simulating the injection and ignition of n-dodecane at transcritical conditions. To this end, a compressible solver with a real-fluid state equation and finite-rate chemistry is employed. Simulations of an ECN-relevant diesel-fuel injector are performed for both inert and reacting conditions. For the spray ignition, four specific operating points (corresponding to ambient temperatures between 900 K and 1200 K) are investigated to examine effects of the real-fluid environment and low-temperature chemistry. Comparisons with available experimental data demonstrate that the presented numerical method adequately captures the diesel fuel injection and auto-ignition processes under transcritical conditions.