进气中CO2浓度对预混合燃烧和排放影响的试验和模拟研究

本文研究了进气中CO2浓度对燃烧和排放特性的影响.研究表明在所有的预混合燃料比下,当CO2浓度增加时,NOx排放随之大幅减少,烟度排放有小的变化。利用KIVA3V和湍流与化学反应交互的燃烧模型对柴油机预混合燃烧进行了模拟研究,对缸内OH浓度的模拟计算表明,随着CO2浓度的增加,着火前期OH生成浓度明显向后推移,这表明燃料的氧化速率随CO2浓度的增加变慢,从而延长了着火滞燃期。进气中CO2浓度变大时,燃烧温度降低,有利于降低NOx的排放。     

Analysis of combustion regimes and conditional statistics of autoigniting turbulent n-heptane sprays

DNS is performed for a statistically one dimensional layer of a spray region resembling diesel engine conditions. The group and collective combustion regimes are identified according to the ratio of the chemical and transport time scales for a single droplet. The statistics in group combustion are similar with those in gas phase combustion. The collective combustion regime involves interspersed rich regions with different dissipation characteristics. Reasonable agreements are shown with the scaled AMC model and the linear evaporation model in the ranges of meaningful probability. Initially the evaporation terms are dominant in the budgets of the conditional enthalpy equation. After ignition the chemical reaction term becomes dominant to be balanced by the time rate of change term. For modeling turbulent spray combustion it may not be essential to consider detailed micro structures around each droplet, unless in the droplet combustion regime.     

超燃冲压发动机推进性能理论分析

超燃冲压发动机的正推力问题和超声速燃烧的稳定性问题是制约超燃冲压发动机发展的两个关键气动物理问题.虽然经过50多年的研究,但是目前国内外对这两个关键问题的机理还没有研究清楚.文章首次将CJ爆轰理论应用于超燃冲压发动机推进性能分析,给出了这两个关键气动问题的理论分析结果.分析结果表明,燃烧室入口空气静温对发动机的推进性能产生重要影响.当爆轰波的爆速大于隔离段内空气来流的速度时,会向隔离段上游传播,导致发动机不起动.飞行Mach数Ma=6~8是超燃发动机的临界不稳定范围,飞行Mach数Ma>9,超声速燃烧将变得稳定.      

Detection of NO in a spark-ignition research engine using degenerate four-wave mixing

We report the first application of degenerate four-wave mixing (DFWM) to combustion diagnostics in a methane-fuelled internal combustion research engine. Combustion-generated NO in the spark-ignited engine was detected using scanning narrowband DFWM in a modified forward folded BOXCARS geometry. The resulting spectra of the X²Π-A²Σ⁺(0,0) band at 226 nm display an excellent signal-to-noise ratio. Extension of the technique to different engine operating conditions and to time-resolved multiplex DFWM is discussed. Received: 3 May 2001 / Revised version: 1 October 2001 / Published online: 29 November 2001     

Quantitative high-speed burned gas temperature measurements in internal combustion engines using sodium and potassium fluorescence

When sodium- and potassium-containing fuel additives are used in internal combustion engines, the bright fluorescence that sodium and potassium atoms emit in the burned gas zone offers a large potential for spectroscopic combustion analysis. To utilize this potential quantitatively, it is crucial to fully understand all physical and chemical processes involved. This includes (1) the temperature dependence of the fluorescence intensity due to gas-phase collisions, (2) the pressure, temperature and equivalence ratio effects on thermodynamic equilibria in the burned gas zone and (3) pressure and temperature-dependent line shapes for quantitative correction of fluorescence reabsorption. High-speed imaging of sodium and potassium fluorescence in a spark-ignited, direct injection, single-cylinder research engine was conducted under well-controlled homogeneous operating conditions at equivalence ratios ranging from 0.71 to 1.43, cylinder pressure from 3 to 15 bar and burned gas temperatures from 1,700 to 2,600 K. This study demonstrates that the influence of pressure, temperature and equivalence ratio on the fluorescence signals of sodium and potassium is understood quantitatively and establishes the potentials and limitations of this tool for burned gas temperature measurements with high temporal and two-dimensional spatial resolution in a homogeneously operated internal combustion engine.     

Intensification of shock-induced combustion by electric-discharge-excited oxygen molecules: numerical study

Mechanisms of combustion enhancement in a supersonic H₂–O₂ reactive flow behind an oblique shock wave front are investigated when vibrational and electronic states of O₂ molecule are excited by an electric discharge. The analysis is carried out on the base of updated thermally nonequilibrium kinetic model for the H₂–O₂ mixture combustion. The presence of vibrationally and electronically excited O₂ molecules in the discharge-activated oxygen flow allows to intensify the chain mechanism and to shorten significantly the induction zone length at shock-induced combustion. It makes possible, for example, to ignite the atmospheric pressure H₂–O₂ mixture at the distance shorter than 1 m behind the weak oblique shock wave at a small energy E_s = 1.2 × 10^–2 J · cm^–3 input to O₂ molecules. At higher pressure it is needed to put greater specific energy into the gas in order to ignite the mixture at appropriate distances. It is shown that excitation of O₂ molecules by electric discharge is much more effective for accelerating the hydrogen–oxygen mixture combustion than mere heating the gas.     

火花点火激发均质压燃燃烧过程的模拟研究

本文对火花点火激发均质压燃SICI燃烧过程进行了建模,利用发动机试验进行了模型验证,模型能较好地描述混合气被点燃压燃的过程。通过三维数值模拟与解析,对比了纯均质压燃HCCI燃烧模式和SICI燃烧模式下的燃烧过程,分析了SICI燃烧的特点。结果表明,SICI燃烧过程中存在多阶段着火,燃烧呈现出顺序放热。SICI燃烧热效率高,NO_x排放低,是一种汽油机有潜力的燃烧方式。     

The structure of the high-pressure gas jet from a spark plug fuel injector for direct fuel injection

Abstract  

A spark plug fuel injector (SPFI), which is a combination of a fuel injector and a spark plug was developed with the aim to convert any gasoline port injection spark ignition engine to gaseous fuel direct injection (Mohamad in Development of a spark plug fuel injector for direct injection of methane in spark ignition engine. PhD thesis, Cranfield University, 2006). A direct fuel injector is combined with a spark plug using specially fabricated bracket connected to a fuel pipe and a fuel path running along the periphery of a spark plug body to deliver the injected fuel to the combustion chamber. The injection nozzle of SPFI is significantly bigger than normal direct fuel injector nozzles. Therefore, it is important to understand the effect of such a configuration on the injection process and subsequently the air–fuel mixing behaviour inside the combustion chamber. The flow was visualized using the planar laser-induced fluorescent technique. For safety reasons, nitrogen was used as fuel substitute. Nitrogen at 50, 60 and 80 bar pressure was seeded with acetone as a flow tracer and injected into a bomb containing pressurised nitrogen. Bomb pressure was varied to simulate the pressure inside combustion cylinder during the compression stroke where actual injections in engine experiments will take place. The shape and depth of tip penetration of the gas jet were measured. Results show that the gas jet follows the behaviour suggested by vortex ball model (Turner in Mechanics 13:356–369, 1962). The cone angle and the maximum jet width of the fully developed gas jets from the SPFI injection are 23° and 25 mm, respectively regardless of the injection pressures. The penetration lengths of the fully developed jets are between 90 and 100 mm at 8–14 ms after the start of injection, depending on the bomb and injection pressure. Jet penetration is directly proportional to the injection pressure but inversely proportional to the cylinder or bomb pressure. The penetration lengths indicate that sufficient distance should be travelled by the gas jet for satisfactory air–fuel mixing in the engine.     

Numerical study of premixed HCCI engine combustion and its sensitivity to computational mesh and model uncertainties

This study used a numerical model to investigate the combustion process in a premixed iso-octane homogeneous charge compression ignition (HCCI) engine. The engine was a supercharged Cummins C engine operated under HCCI conditions. The CHEMKIN code was implemented into an updated KIVA-3V code so that the combustion could be modelled using detailed chemistry in the context of engine CFD simulations. The model was able to accurately simulate the ignition timing and combustion phasing for various engine conditions. The unburned hydrocarbon emissions were also well predicted while the carbon monoxide emissions were under predicted. Model results showed that the majority of unburned hydrocarbon is located in the piston-ring crevice region and the carbon monoxide resides in the vicinity of the cylinder walls. A sensitivity study of the computational grid resolution indicated that the combustion predictions were relatively insensitive to the grid density. However, the piston-ring crevice region needed to be simulated with high resolution to obtain accurate emissions predictions. The model results also indicated that HCCI combustion and emissions are very sensitive to the initial mixture temperature. The computations also show that the carbon monoxide emissions prediction can be significantly improved by modifying a key oxidation reaction rate constant.     

Differences between PREMIER combustion in a natural gas spark-ignition engine and knocking with pressure oscillations

PREMIER (PREmixed Mixture Ignition in the End-gas Region) combustion occurs with auto-ignition in the end-gas region when the main combustion flame propagation is nearly finished. Auto-ignition is triggered by the increases in pressure and temperature induced by the main combustion flame. Similarly to engine knocking, heat is released in two stages when engines undergo this type of combustion. This pattern of heat release does not occur during normal combustion. However, engine knocking induces pressure oscillations that cause fatal damage to engines, whereas PREMIER combustion does not. The purpose of this study was to elucidate PREMIER combustion in natural gas spark-ignition engines, and differentiate the causes of knocking and PREMIER combustion. We applied combustion visualization and in-cylinder pressure analysis using a compression–expansion machine (CEM) to investigate the auto-ignition characteristics in the end-gas region of a natural gas spark-ignition engine. We occasionally observed knocking accompanied by pressure oscillations under the spark timings and initial gas conditions used to generate PREMIER combustion. No pressure oscillations were observed during normal and PREMIER combustion. Auto-ignition in the end-gas region was found to induce a secondary increase in pressure before the combustion flame reached the cylinder wall, during both knocking and PREMIER combustion. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. This caused a sudden pressure difference and imbalance between the flame propagation region and the end-gas region, followed by a pressure oscillation.     

Implementation of the Effective Combustion of Gas Mixtures with a Low Emission of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and CO

The method for burning stoichiometric or near-stoichiometric gas mixtures in a burner device with a volumetric matrix is proposed, which provides low concentrations of harmful substances in the combustion products. It is shown that the concentrations of nitrogen oxide and carbon monoxide in the combustion products can be reduced to 10 ppm at an air excess ratio of ~1 and an output firing rate of ~300 W/cm².     

Analysis of combustion chamber insulation effects on the performance and exhaust emissions of a DI diesel engine using a multi-zone model

A comprehensive two-dimensional multi-zone model of a diesel engine cycle is presented in this study, in order to examine the influence of insulating the combustion chamber on the performance and exhaust pollutants emissions of a naturally-aspirated, direct injection (DI), four-stroke, water-cooled diesel engine. The heat insulation is taken into account by the corresponding rise of wall temperature, since this is the final result of insulation useful for the study. It is found that there is no remarkable improvement of engine efficiency, since the decrease of volumetric efficiency has a greater influence on it than the decrease of heat loss to the coolant, which is converted mainly to exhaust gas enthalpy (significant rise of the exhaust gas temperature). As far as the concentration of exhaust pollutant emissions is concerned, it is found that the rising heat insulation leads to a significant increase of the exhaust nitric oxide (NO) and to a moderate increase of the exhaust soot concentration. Plots of temperature, equivalence ratio, NO and soot distributions at various instants of time inside the combustion chamber, emanating from the application of the multi-zone model, aid the correct interpretation of the insulation effects gaining insight into the underlying mechanisms involved.     

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.     

External combustion of high-speed multicomponent hydrocarbon-air flow under conditions of low-temperature plasma

The stabilization of external combustion (at the plate surface) of high-speed multicomponent (air, alcohol, and propane) flows is realized experimentally. It is shown that heat fluxes during alcohol combustion rise by a factor of about 7 and during propane combustion by a factor of 15, in comparison with the heat flux from a discharge in a high-speed air flow. The electron concentration measured at a distance of 10 cm down-stream from the electrode ends is approximately 10⁹ cm^?3 in the case of the discharge in an air flow, whereas during alcohol combustion it attains 2 × 10¹¹ cm^?3 and during propane combustion it is 3 × 10¹¹ cm^?3. The flame temperature in the area of the discharge existence varies from 2000 up to 2500 K and outside the discharge at the distance of z = 20 cm from electrodes is 1800 K, gradually decreasing downstream. It is shown that the combined discharge in the subsonic flow allows for complete combustion of liquid and gaseous hydrocarbons. The completeness of combustion in supersonic flows attains 95%, depending on the flow velocity.     

Soot particle size modelling in 3D simulations of diesel engine combustion

The present work is focused on multi-dimensional simulations of combustion in diesel engines. The primary objective was to test, in a diesel engine framework, a soot particle size model to represent the carbon particle formation and calculate the corresponding size distribution function. Simulations are performed by means of a parallel version of the KIVA3V numerical code, modified to adopt detailed kinetics reaction mechanisms. A skeletal reaction scheme for n-heptane autoignition has been extended, to include PAH kinetics and carbonaceous particle formation and consumption rates: the full reaction set is made up of 82 gas species and 50 species accounting for the particles, thus the complete reaction scheme comprises 132 species and 2206 reaction steps. Four different engine operative conditions, varying engine speed and load, are taken into account and experimentally tested on a single cylinder diesel engine fuelling pure n-heptane. Computed particle size distribution functions are compared with corresponding measurements at the exhaust, performed by a differential mobility spectrometer.

A satisfying agreement between computed and measured combustion profiles is obtained in all the conditions.

A reasonable aerosol evolution can be obtained, yet in all the cases the model exhibits the tendency to overestimate the number of particles within the range 5–160 nm. Moreover calculations predict a nucleation mode not detected by the available instrument. According to the simulations, the total number and size of the nascent particles would not depend on the operative conditions, while the features of the larger aggregates distinctly vary with the engine functioning.     

Parameters of the flame due to surface-microwave discharge-initiated inflammation of thin alcohol films

Nonthermal plasma-stimulated inflammation of thin alcohol films under the conditions of a microwave surface discharge initiated in quiescent air under atmospheric pressure is realized. The main parameters and properties of the flame due to alcohol inflammation and combustion are studied. It is shown that inflammation occurs when the gas temperature near the antenna is no higher than 1000 K. When the reduced electric field is high, the flame temperature near the antenna reaches 3300 K and the electron concentration equals 2 × 10¹² cm⁻³. The electron temperature during alcohol combustion varies from 0.8 eV at distance y = 10 mm from the antenna surface to 0.3 eV for y = 40 mm.     

Neural network prediction of cycle-to-cycle power variability in a spark-ignited internal combustion engine

Cycle-to-cycle variation (CCV) limits how lean a spark-ignited (SI) internal combustion engine (ICE) can stably operate at, restricts efficiency, and increases emissions through incomplete combustion. Therefore, a way to cleaner, more efficient SI ICEs is to minimize the CCV. Current methods to study CCV include experimental investigations and CFD-based numerical simulations. This study, in contrast, investigates the ability of neural networks to accurately model the indicated mean effective pressure (IMEP) and its coefficient of variation (COV of IMEP). Experimental data from a previous study of spark-ignited propane/air combustion in the TCC-III engine was used to train and evaluate a neural network. An optimized network was generated that utilizes 109 experimental inputs and is operated with 15 neurons in one hidden layer to determine IMEP for 18 engine operating conditions, with 625 individual consecutive engine cycles for each condition. The impact of training set size and the number of input parameters was also investigated. The average deviation for IMEP from the experimental measurements is 0.7–2.2% for the training data set and less than 12% for the entire predicted range of operating conditions. Data sets consisted of tests under rich, lean, and stoichiometric conditions without and with 9% nitrogen dilution. Predicted COV of IMEP strongly correlates with experimental data (R²?=?0.8453). However, a systematic over prediction of COV of IMEP for low COVs was observed while higher COVs were under-predicted by the neural network. The cause for this systematic behavior has not yet been identified but histograms of the predicted IMEP data indicate that this could be related to missing physical parameters that have a significant impact on combustion variability.     

IR spectroscopic investigation of the structure of water–fuel microemulsion for diesel engines

The structures of a microemulsion formed by a surfactant (ammonium oleate), water drops of a linear size of 1–3 µm, and a diesel fuel has been investigated using IR spectroscopy. It has been found that ammonium oleate molecules in the microemulsion are dissociated on the positive NH₄⁺ ion and the negative ion of the remaining part of the molecule, which forms the hydrogen bond with water molecules. This increases the rate of water, evaporation and leads to the more complete combustion of the diesel fuel. As a result, the concentration of harmful nitrogen oxides and soot particles in the exhaust gas of the diesel engine decreases.     

Combustion response of an aluminum droplet burning in air

The combustion in air of a 100 μm-diameter aluminum droplet is studied by direct Navier–Stokes simulations. The model only considers the gas phase and includes a reduced Al/O₂ kinetic scheme with 8 species and 10 reactions. The model is validated against experimental burn time data and appears to be fairly correct despite its simplicity. The unsteady combustion is then investigated by superimposing an acoustic disturbance to the mean flow. The velocity-coupled response is computed for different frequencies and slip Reynolds numbers. A resonance peak is found to occur when the acoustic time scale matches the gas diffusion time scale. For lower frequencies however (typically below a few kHz), a quasi-steady regime seems to hold out which means that assuming quasi-steady combustion (e.g., given by a D² model) is valid in this case. In this regime, the computed response corresponds with a theoretical expression obtained by a linearization of the Ranz–Marshall correction term. This implies that unsteady aluminum combustion is strongly dependent on convection effects.     

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.