Autoignition Of Mixtures Research Articles

Study of the engine configuration effect on the maximum achievable load in CAI using water injection

The approach of this research is to enlarge the knowledge about the methodologies to increase the maximum achievable load degree in the context of gasoline CAI engines. This work is the continuation of a previous work related to the study of the water injection effect on combustion, where this strategy was approached. The operating strategies to introduce the water and the interconnected settings were deeply analyzed in order to optimize combustion and to evaluate its potential to increase the maximum load degree when operating in CAI. During these initial tests, the engine was configured to enhance the mixture autoignition. The compression ratio was high compared to a standard gasoline engine, and suitable fuel injection strategies were selected based on previous studies from the authors to maximize the reactivity of the mixture, and get a stable CAI operation. Once water injection proved to provide encouraging results, the next step dealt in this work, was to go deeper and explore its effects when the engine configuration is more similar to a conventional gasoline engine, trying to get CAI combustion closer to production engines. This means, mainly, lower compression ratios and different fuel injection strategies, which hinders CAI operation. Finally, since all the previous works were performed at constant engine speed, the engine speed was also modified in order to see the applicability of the defined strategies to operate under CAI conditions at other operating conditions. The results obtained show that all these modifications are compatible with CAI operation: the required compression ratio can be reduced, in some cases the injection strategies can be simplified, and the increase of the engine speed leads to better conditions for CAI combustion. Thanks to the analysis of all this data, the different key parameters to manage this combustion mode are identified and shown in the paper.

Three-stage auto-ignition of n-heptane and methyl-cyclohexane mixtures at lean conditions in a flat piston rapid compression machine

One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture.

Measuring the Adiabatic Ignition Delay of n-Pentane Mixture using Rapid Compression Machine

Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines. Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines (RCMs) which are sensitive to the heat transfer characteristics of the experimental device. RCMs are widely used to measure autoignition data such as ignition delay and species concentration. Measured ignition delays from RCMs are typically reported at an adiabatic condition; however, this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process, e.g., a longer ignition delay has a greater pressure and temperature drops. RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process. In addition, depending on the diluent composition (e.g., helium versus nitrogen or argon), different heat transfer characteristics are expected. As a result, autoignition results at similar conditions may vary from facility to facility or depending on the used diluent. The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms. In this work, a new method is developed and utilized to eliminate heat transfer from the ignition delay data. To evaluate the new method, the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM. To vary the heat transfer, the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions. The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents. By applying the new method, the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined. A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay, which agreed well with the experimental data.

Analysis of Diesel Knock for High-Altitude Heavy-Duty Engines Using Optical Rapid Compression Machines

In high altitude regions, affected by the low-pressure and low-temperature atmosphere, diesel knock is likely to be encountered in heavy-duty engines operating at low-speed and high-load conditions. Pressure oscillations during diesel knock are commonly captured by pressure transducers, while there is a lack of direct evidence and visualization images, such that its fundamental formation mechanism is still unclear. In this study, optical experiments on diesel knock with destructive pressure oscillations were investigated in an optical rapid compression machine. High-speed direct photography and simultaneous pressure acquisition were synchronically performed, and different injection pressures and ambient pressures were considered. The results show that for the given ambient temperature and pressure, diesel knock becomes prevalent at higher injection pressures where fuel spray impingement becomes enhanced. Higher ambient pressure can reduce the tendency to diesel knock under critical conditions. For the given injection pressure satisfying knocking combustion, knock intensity is decreased as ambient pressure is increased. Further analysis of visualization images shows diesel knock is closely associated with the prolonged ignition delay time due to diesel spray impingement. High-frequency pressure oscillation is caused by the propagation of supersonic reaction-front originating from the second-stage autoignition of mixture. In addition, the oscillation frequencies are obtained through the fast Fourier transform (FFT) analysis.

Effects of partitioned fuel distribution on auto-ignition and knocking under spark assisted compression ignition conditions

Spark-assisted compression ignition (SACI) is a potential way to enhance ignition-control robustness and extend the load range of homogeneous charge compression ignition (HCCI) engines. However, the mechanism underlying combustion mode transition without knocking is not completely elucidated. Using direct high-speed photography and simultaneous pressure acquisition, the proposed study examines different auto-ignition and engine knock scenarios under SACI conditions. A small amount of heptane was directly injected into premixed methane-air mixture to modify local mixture reactivity. Additionally, late side injection was adopted to achieve stable SACI combustion whilst suppressing knocking combustion. Results obtained demonstrate that SACI combustion is essentially determined by the stochastic auto-ignition of the unburned end-gas mixture. End-gas mixture auto-ignition becomes stable thus normal SACI under conditions of high mixture reactivity, but a further rise in mixture reactivity causes engine knock prevail. To suppress SACI knocking, partitioned distribution of heptane via side injection has been found to be significantly effective. Results demonstrate that partitioned fuel distribution via late injection can greatly reduce the knock intensity, thereby implying a transition from SACI knocking to normal SACI combustion. Further analysis demonstrates that knock intensity is determined by the peak heat release rate of auto-ignition and there exists distinct boundaries between different combustion modes. The corresponding threshold value characterizing knocking combustion is approximately 202.30 J/CAD under current conditions. Besides, the peak heat release rate is related to the auto-ignition propagation speed and can be adjusted by the partitioned fuel distribution. The proposed study provides great insight into realizing and controlling SACI combustion to improve engine efficiency.

Tailoring the Temperature Sensitivity of Ignition Delay Times in Hot Spots Using Fuel Blends of Dimethyl Ether, Methane, and Hydrogen

Advanced combustion concepts based on autoignition of combustible mixture for internal combustion engine and gas turbine applications are sensitive to reactivity gradients arising from temperature ...

Experimental study of pre-chamber jet ignition in a rapid compression machine and single-cylinder natural gas engine

Pre-chamber jet ignition is a promising combustion technology to achieve fast combustion in natural gas engines. First, the ignition and combustion characteristics of mixtures in a pre-chamber system with different diameter orifices were studied under engine-relevant pressures and temperatures in a rapid compression machine. The tested fuels, CH4/air stoichiometric mixtures, were diluted by different proportions of CO2/N2 to simulate the corresponding exhaust gas recirculation conditions in engines. High-speed photography was applied to visualize the jet ignition and combustion processes. The experimental results revealed that two ignition patterns existed in the pre-chambers depending on the diameter of orifices. Pre-chamber jet flame ignition pattern appeared when the orifice diameter of the pre-chamber exceeded a critical value, which produced jet flame and ignited the mixtures in the main chamber directly. Pre-chamber jet auto-ignition pattern produced jet which promoted the auto-ignition of mixture in the main chamber when the orifice diameter was smaller and presented much shorter combustion durations. Based on the experimental results in the rapid compression machine, a practical pre-chamber system was designed in a single-cylinder natural gas engine to investigate the combustion performance and emission characteristics. The experimental results indicated appropriate 0.8%–1.4% increases of indicated thermal efficiency were achieved by pre-chamber jet ignition due to the higher combustion efficiency and shorter combustion duration compared to conventional spark ignition. Lower total hydrocarbon and CO emissions but higher NO x emissions were produced by pre-chamber jet ignition due to the faster burning velocity and higher combustion temperature.

Numerical analysis of knocking characteristics and heat release under different turbulence intensities in a gasoline engine

In this work, different knocking characteristics and heat release have been numerically studied in a downsized spark-ignition engine, with addressing turbulence intensity and spark-ignition timing. The results show that knock intensity mainly depends on the heat release of hot-spots, and the heat release is not only associated with unburned mass fraction but also closely related to subsequent autoignition development processes, e.g. end-gas autoignition mode. Under low turbulence intensity, local mixture autoignition is induced by a single hot-spot, accompanied by an autoignition reaction wave propagation; whereas under high turbulence intensity, a nearly homogeneous autoignition event characterizing quasi constant-volume combustion is observed. Meanwhile, compared with the single hot-spot autoignition, the homogeneous one can lead to more intensive local overpressure and thus stronger knock intensity. Further analysis on the underlying reasons shows that the higher heat release rate under homogeneous autoignition conditions is the main reason for the stronger knock intensity. Besides, the dependency of knock intensity and heat release on unburned mass fraction is found effective only for the same autoignition scenarios. Current study can provide insights into knocking suppression with consideration of intake system optimization in modern downsized gasoline engines.

Auto-ignition behaviors of nitromethane in diluted oxygen in a rapid compression machine: Critical conditions for ignition, ignition delay times measurements, and kinetic modeling interpretation.

In this work, the weakest thermodynamic conditions for the auto-ignition of mixtures containing nitromethane were experimentally determined by using the rapid compression machine facility. Results show there is a narrow weak ignition region between ignition and non-ignition. The weak ignition region would disappear with the increase of the EOC (end of compression) pressure and nitromethane concentration. In addition, the ignition delay times for successful auto-ignition for different nitromethane concentrations and equivalence ratio mixtures were measured and compared. Results show that the dependence of nitromethane ignition on the equivalence ratio is weak. Subsequently, the measured ignition delay time data were employed to validate several kinetic models in literature and our previous model shows better agreement with experimental results, as well as other available literature data. Sensitivity analysis for the model reveals the importance of unimolecular decomposition and H-abstraction reactions for the ignition delay times in the temperature range studied herein. Finally, critical conditions for nitromethane ignition under extended conditions that are beyond the ability of the experimental facility were predicted.

대형 천연가스/디젤 혼소 엔진에서 압축비 및 피스톤 보울 형상에 따른 디젤 분사시기 별 연소 및 배기 특성 비교 분석

The governing combustion regimes were slightly different between the neat diesel and dual-fuel combustion conditions. Neat diesel combustion was based on the auto-ignition of heterogeneous mixture of air and diesel fuel, whereas dual-fuel combustion mainly consisted of pre-mixed combustion and some local flame propagation. Therefore, the condition of the combustion chamber should be modified so as to adjust to the dual-fuel combustion instead of the neat diesel combustion. For this reason, dual-fuel combustion characteristics were investigated in this research under two different pistons(compression ratio and bowl shape together) as varying diesel injection timings. Under 1,200 rpm and low load condition, the diesel injection timing varied from TDC to advance limit(CoV of gIMEP < 5 %), which differed as natural gas fractions(60 % and 80 %).

Autoignition and flame spectroscopy of propane mixture in a rapid compression machine

The autoignition process of propane has been studied using an optically accessible rapid compression machine (RCM). The ignition delay times of the fuel were measured at two compressed pressures of approximately 21 and 30 bar, equivalence ratios of 0.5, 0.8, 1.0 and 1.2, and at various compressed gas temperatures. A detailed kinetic model was also used to simulate the autoignition. A new methodology was also developed and discussed to include the effect of heat transfer in the mechanism model simulation. The combustion process imaging was acquired using high-speed camera and the deflagration like combustion with blue continuum spectrum was observed close to the end of ignition delay at all of the studied conditions, excluding the condition with very long ignition delay (e.g., 90 ms). The deflagration concluded with detonation for most of the studied conditions. The excited radicals’ chemiluminescence emissions were acquired using flame spectrometer and Electron-Multiplying Charge-Coupled Device (EMCCD) with wavelength ranging from 280 nm to 700 nm. Several excited radicals were detected during autoignition process and their time-resolved intensity as a function of mixture conditions (pressure, temperature and equivalence ratio) were studied.

Classification of Similarity Criteria in Combustion Theory and the Problem of Autoignition of a Combustible Mixture by Compression

The dynamic problems of the theory of combustion and explosion are classified from the point of view of competition of the characteristic times of bulk chemical reaction, frontal combustion, heat transfer, gas flow from the vessel, compression (motion of the piston), and heating (cooling) rate. We consider dynamic similarity criteria such as the ratio of the characteristic times in problems of classical thermal explosion, dynamic thermal explosion, flammability limits, combustion in communicating vessels, competition of frontal and volumetric combustion, autoignition by adiabatic compression, competition of frontal combustion and piston motion, as well as compound similarity criteria (such as functions of simple dynamic similarity criteria). The problem of autoignition of a mixture by compression is considered, and a method for obtaining an analytical solution of the problem and an algorithm for the approximate solution based on a special differential criterion are proposed.

Experimental investigation on the performance and emissions characteristics of ethanol/diesel dual-fuel combustion

The diesel compression ignition (CI) engine has higher durability and thermal efficiency than the gasoline spark ignition (SI) engine. However, the high levels of nitrogen oxides (NOx) and particulate matter (PM) emissions are major problems of diesel engines due to the auto-ignition of heterogeneous mixtures. The dual-fuel combustion concept could be solution to environmental concerns. Dual-fuel combustion can be implemented by substituting some of the diesel fuel with high-volatility fuels, such as gasoline and natural gas. The premixed mixture condition can be improved to diminish localized rich and stoichiometric regions. Notably, ethanol has greater potential to reduce PM emissions because it is highly volatile and readily oxidized.For these reasons, in this research, the effects of varying the ethanol substitution ratio on engine performance and emissions under the dual-fuel combustion condition were experimentally investigated under various load conditions. The test engine was a heavy-duty single-cylinder diesel engine with two direct injectors. Engine speed was fixed at 1000 rpm and the load condition was varied for an indicated mean effective pressure (IMEP) ranging from 0.2 to 0.8 MPa. The ratio of ethanol to the total input energy was controlled from zero to nearly 50% of the input energy. The NOx and PM emissions decreased with increasing ethanol substitution and the mean size of the PM emissions decreased. For the mid-load condition (IMEP 0.6 MPa), the substitution was increased to 63%, but for low and high loads, higher ethanol fractions could not be used because of insufficient ignition energy at low loads and sharp increment of the in-cylinder pressure under high loads.

Pressure wave evolution during two hotspots autoignition within end-gas region under internal combustion engine-relevant conditions

To further understand the intense pressure wave formation mechanism induced by multiple spontaneous ignition kernels under internal combustion engine-relevant conditions, a one dimensional code to solve the Navier–Stokes equations for reactive flows is adopted in this parametric study. The multiple autoignition kernels occurring in the end-gas region are simplified into two neighboring hotspots. Stoichiometric mixtures of PRF40/air (without an obvious Negative Temperature Coefficient (NTC) behavior, and showing a certain degree of anti-knock property) and PRF0/air (exhibiting a strong NTC behavior and weak anti-knock property) are both used to explore the effect of fuel characteristic. The initial pressure is 30.0 atm. Unburnt gas temperatures of 800.0 K and 890.0 K are below and within the NTC regime of stoichiometric PRF0/air under the initial pressure. The hotspots are modeled as sine waves in the initial temperature field. Different sine wave amplitudes are adopted to examine the effect of temperature inhomogeneity. Within a limited computational domain, a detonation wave tends to form under the initial conditions with a small interval between hotspots, a low initial temperature of the unburnt mixture and a large temperature inhomogeneity. During the formation of detonation wave, a process similar to the “explosion in the explosion” phenomenon that are found in previous experiments has been detected. Moreover, at the low initial temperature, a large interval between hotspots reduces the maximum intensity of pressure wave. In contrast, a wide interval increases the maximum pressure intensity at a higher initial temperature. The strongest pressure intensity induced by PRF0/air mixture autoignition is generally higher than that during the autoignition of PRF40/air mixture under the same initial condition, but a common intense pressure wave generation mechanism is shared. Furthermore, compared with the autoignition of the PRF40/air mixture, the distance for detonation formation within PRF0/air mixture becomes shorter.

On Relative Effectiveness of Halogenated Hydrocarbons for Suppression of Hydrogen-Oxygen Mixture Autoignition

ABSTRACTIn a recent paper (Eremin et al., 2017), the authors investigated the opposing influence of various fire suppressants based on the differences in the effects of halogenated hydrocarbons on the ignition of various combustible mixtures. It was shown that all of the additives do not suppress, but some accelerate the ignition of methane/oxygen mixtures. At the same time, these additives suppress the ignition of hydrogen/oxygen mixtures, and the suppression efficiency increases from fluorine to chlorine and bromine compounds. The purpose of this work is to further study the effectiveness of suppressing the ignition of hydrogen/oxygen mixtures by iodine-based additives, namely, CF3I and C3F7I. The results of the experiments show that these additives are the most effective suppressors of hydrogen ignition.

Diesel/CNG Mixture Autoignition Control Using Fuel Composition and Injection Gap

Combustion phasing is the main obstacle to the development of controlled auto-ignition based (CAI) engines to achieve low emissions and low fuel consumption operation. Fuel combinations with substantial differences in reactivity, such as diesel/compressed natural gas (CNG), show desirable combustion outputs and demonstrate great possibility in controlling the combustion. This paper discusses a control method for diesel/CNG mixture combustion with a variation of fuel composition and fuel stratification levels. The experiments were carried out in a constant volume combustion chamber with both fuels directly injected into the chamber. The mixture composition was varied from 0 to 100% CNG/diesel at lambda 1 while the fuel stratification level was controlled by the injection phasing between the two fuels, with gaps between injections ranging from 0 to 20 ms. The results demonstrated the suppressing effect of CNG on the diesel combustion, especially at the early combustion stages. However, CNG significantly enhanced the combustion performance of the diesel in the later stages. Injection gaps, on the other hand, showed particular behavior depending on mixture composition. Injection gaps show less effect on combustion phasing but a significant effect on the combustion output for higher diesel percentage (≥70%), while it is contradictive for lower diesel percentage (&lt;70%).

Ignition of Hydrogen Peroxide with Gel Hydrocarbon Fuels

Experimental counterflow and impinging jet studies and modeling analysis of hypergolic hydrogen peroxide () and gel hydrocarbon fuel/particle mixtures were conducted to characterize condensed phase reaction rates and ignition delay times. The mixtures consisted of -dodecane, -heptane, and kerosene containing fumed silica and sodium borohydride () particles. Scanning electron microscopy, x-ray photoelectron spectroscopy, and simultaneous thermogravimetric and differential scanning calorimetry analysis of the particles were performed to characterize particle size, size distribution, geometry, surface composition, and thermal decomposition. Liquid-phase counterflow experiments were used to derive a global rate constant for the condensed phase reaction between and . Chemical kinetics calculations were performed using the condensed phase global reaction coupled with a detailed gas phase mechanism for hydrocarbon oxidation to phenomenologically study the ignition process. Shorter ignition delays were achieved when fuel flow was established before oxidizer injection. Ignition delay decreased with addition until a limiting loading was achieved, after which ignition delay remained nearly constant. Elevating the reactant temperature reduced ignition delay, consistent with fuel volatility trends. Modeling results show that the ignition process relies upon the reaction between and to gasify and heat an ignition kernel to the mixture autoignition temperature.

潤滑油が混合したガソリンの自着火に関する反応論的考察

潤滑油が混合したガソリンの自着火に関する反応論的考察

Numerical simulation of ignition in pulverized coal combustion with detailed chemical reaction mechanism

A two-dimensional direct numerical simulation with a detailed chemical reaction mechanism that considers 158 species and 1804 reactions is applied to pulverized coal combustion occurring in a mixing layer and the ignition phenomena is investigated in detail. The pulverized coal particles which are initially heated up to 2000K are distributed in a fully developed mixing layer. The distributions of temperature and various chemical species concentrations in terms of the mixture fraction are analyzed during ignition. The results show that the ignition occurs in the rich mixture fraction condition (ξ∼0.2), as the gas temperature is also high in this condition because of the high initial temperature set for the central coal particles. However, once ignition occurs, the combustion reaction dramatically takes place in the condition indicating ξ=0.03–0.06, which is a slightly lean value compared with the stoichiometric value and shows the shortest ignition delay time in the calculation for autoignition of homogeneous mixtures of volatile matter and air (zero-dimensional calculation). In addition, in the combustion reaction zone, high OH radical mass fraction is observed around the conditions of stoichiometric mixture fraction, whereas high CH radical mass fraction is observed in the rich mixture fraction conditions.

Direct numerical simulation of H2/air combustion with composition stratification in a constant volume enclosure relevant to HCCI engines

Two-dimensional direct numerical simulation (2-D DNS) is used to investigate the effect of turbulence intensity and composition stratification on H2/air mixture auto-ignition in a constant volume enclosure relevant to homogeneous charge compression ignition (HCCI) engines. Different turbulence levels, composition fluctuations, EGR (Exhaust Gas Recirculation) ratios, initial pressure, domain lengths and energy spectra are simulated with detailed analysis in ten 2-D DNS cases. The results show that the ignition delay time tends to be prolonged and the heat release rate increased under higher turbulence intensity. Turbulence can affect the reaction zone, e.g., through wrinkling of reaction front and enhancement of mixing and heat transfer. Higher composition stratification can smoothen the overall heat release rate and shorten the ignition delay time. Budgets terms and Probability Density Function (PDF) of density weighted displacement speed show that in HCCI engines flame propagation can co-exist with volumetric auto-ignition. As expected, lower pressure leads to thicker flame thickness and longer ignition delay time. Increasing EGR ratio has a negative influence on the formation of OH reaction, resulting in a longer ignition delay time. Two energy spectra with respect to low and high Reynolds number are compared to show a discrepancy on ignition delay time due to different kinetic energy dissipation rates.