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.     

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.     

混氢改善汽油机稀燃性能的试验研究

针对汽油机稀燃条件下循环变动大,燃料燃烧不充分的问题,本文在一台加装了电控氢气喷射系统的四缸汽油机上就混氢对改善汽油机稀燃条件下燃烧与排放性能的作用进行了试验研究。在发动机1400r/min,进气道绝对压力为61.5 kPa的条件下,就1%与3%两种进气混氢体积分数对稀燃汽油机燃烧与排放特性的影响进行了研究。试验结果表明,稀燃时发动机制动热效率随混氢分数增加而提高;滞燃期与速燃期随混氢分数增加而缩短;发动机稀燃极限所对应的过量空气系数由原机的1.45提高至混氢1%与3%时的1.55和1.96。混氢后发动机HC与CO排放降低,但NO_x排放有所升高。     

Influence of the in-cylinder flow on cycle-to-cycle variations in lean combustion DISI engines measured by high-speed scanning-PIV

In this study, the influence of the three-dimensional (3D) in-cylinder flow on engine's cycle-to-cycle variations (CCV) in a spray-guided direct-injection spark-ignition engine is investigated. The engine is operated at homogeneous lean air–fuel mixture which enhances the sensitivity to CCV due to reduced laminar flame speed. To compensate this, intake velocity is increased by a tumble-flap (TF) in the intake-port. To address the 3D-nature of the temporal evolution of the instantaneous in-cylinder flow for different TF-positions, time-resolved scanning particle image velocimetry (Scanning-PIV) is applied to the engine. The required scan-frequency is provided by an acousto-optical-deflector (AOD) to measure the flow field quasi-simultaneously in the central tumble-plane and both mid-valve-planes. The three planes are 18?mm displaced from each other to capture the variability of the large-scale tumble vortex. The in-cylinder flow measurements are combined with combustion analyses by the in-cylinder pressure-trace and the detection of the location of ignition through the evaluation of the luminous spark-plasma. A correlation-map analysis is conducted to identify coherent flow features responsible for CCV of the combustion parameters. This reveals a strong dependency of the spark position to variations of an upward directed flow pointing onto the spark plug. The variations of the upward flow are due to strong CCV of the bended tumble-axis position. An increased tumble motion caused by the TF results in favorable flow conditions by stabilizing the tumble-axis in the middle of the cylinder which decreases the CCV of the spark position significantly. Further correlation analysis including the combustion process exhibits that flow-structures moving the spark and early flame kernel towards the cylinder center reduces the crank angle of 5% heat release and the combustion duration considerably.     

Buck变换器内部本质安全性能评价判别式研究

为了建立起Buck变换器内部本质安全性能评价的相关判别式,首先以简单电感电路的电弧放电为研究对象,基于热引燃理论,采用持续发热点热源温度场模型,将初始燃烧容积的温度由最高值下降至气体混合物燃烧温度的时间是否大于化学反应的时间作为判断火花能否成功引燃气体混合物的临界条件,得到了相应的火花放电时间临界值的表达式。然后,基于爆炸性试验数据对采用等效电阻法和放电电流线性模型算得的Buck变换器电感开路电弧放电能量表达式进行了修正,进而建立了Buck变换器内部本质安全性能评价的能量判别式和放电时间判别式。验证结果表明了所求放电时间临界值的合理性和所建立判别式的正确性。     

雾化角对液体火箭发动机燃烧室压力振荡的影响

针对液氧/煤油火箭发动机模型燃烧室实现了三维非稳态两相燃烧过程的数值模拟,得到的燃烧室截面平均压力和平均速度与实验吻合。在初边值条件不施加任何扰动的情况下,得到了燃烧室压力自激振荡过程,并研究了液氧和煤油喷嘴雾化角对燃烧室压力振荡的影响。计算结果表明:当雾化角为40°或120°时,由于燃料与氧化剂喷雾锥重叠区域较小或较大,导致了推进剂混合很差或很好,不易在燃烧室头部出现局部爆炸性的可燃混气团,致使燃烧室压力振荡强度较弱;而当雾化角为中间值65°时,易于出现爆炸性的可燃气团并导致剧烈的压力振荡,使燃烧室中出现燃烧不稳定性。因此,雾化角的合理设计是抑制燃烧不稳定性的一种途径。     

Route to chaos for combustion instability in ducted laminar premixed flames

Complex thermoacoustic oscillations are observed experimentally in a simple laboratory combustor that burns lean premixed fuel-air mixture, as a result of nonlinear interaction between the acoustic field and the combustion processes. The application of nonlinear time series analysis, particularly techniques based on phase space reconstruction from acquired pressure data, reveals rich dynamical behavior and the existence of several complex states. A route to chaos for thermoacoustic instability is established experimentally for the first time. We show that, as the location of the heat source is gradually varied, self-excited periodic thermoacoustic oscillations undergo transition to chaos via the Ruelle-Takens scenario.     

Methodological specifics of the study of micro HPP based on internal combustion engines with air cooling and cogeneration

The article considers some aspects of the research methodology of micro heat power plants based on internal combustion engines with air cooling and cogeneration based on energy balance equations and the laws of heat transfer. The research is conducted for such a setup based on the Hitachi internal combustion engine with 2.4 kW capacity. It has shown the efficiency of cogeneration use in the form of useful heat flow from air, cooling the cylinder head, with its further heating by utilizing the heat of flue gases in an additional plate heat exchanger. It has been shown that the cogeneration can save fuel costs 3–10 times compared with heat guns, depending on the duration of the setup use.     

The Rayleigh integral is always positive in steadily operated combustors

The Rayleigh index has been used for decades by a large number of researchers as an indicator to determine if a flame is driving or damping thermoacoustic interaction mechanisms. The use of the Rayleigh criterion has found applications in rocket combustors, gas turbine combustion technology and basic combustion research. The global Rayleigh index or integral is obtained by integrating the product of heat release rate and pressure fluctuations over space and time. Depending on the phase between pressure oscillations and heat release rate response, the oscillations can be enhanced or damped. It is commonly assumed in literature that the sign of the Rayleigh index from steady state data can be used to determine if the thermoacoustic feedback loop is stabilizing or destabilizing. However, we show in this paper that under fairly general conditions, a correctly measured Rayleigh index is always positive if evaluated from statistically stationary data. This proves to be true even if the heat release rate response to pressure fluctuations is in phase opposition to those pressure fluctuations. This is shown in a straightforward manner by substituting the wave equation with a heat release rate source term into the Rayleigh index. This was verified experimentally on a fully premixed combustion system by measuring the flame chemiluminescence using a photo multiplier and pressure fluctuations using a microphone placed sufficiently close to the flame to ensure acoustic compactness for the frequency range of interest. A large range of operating conditions have been tested, spanning linearly stable and unstable stationary thermoacoustic states, respectively corresponding to resonance or a limit cycle driven by the inherent stochastic forcing from the turbulent combustion noise. The experimental results corroborated the analytic finding: the Rayleigh index is found to be positive for all frequencies and all operating conditions.     

Effects of pre-spark heat release on engine knock limit

It is well known that spark ignited engine efficiency is limited by end gas autoignition, commonly known as knock. This study focuses on a recently discovered phenomena, pre-spark heat release (PSHR) due to low-temperature chemistry, and its impact on knock behavior. Boosted operating conditions are more common as engines are downsizing and downspeeding in efforts to increase fuel economy and prone to PSHR. Experiments were prone at fixed fueling and air fuel ratio for a range of intake temperature that spanned the threshold for PSHR. It was found that when PSHR occurred, the knock-limited combustion phasing was insensitive to intake temperature; higher intake temperatures did not require retarded timings as it is usual. Inspection of the temperature–pressure history overlaid on ignition delay contours allow the results to be explained. The temperature rise from the low-temperature reactions moves the end gas state into the negative temperature coefficient (NTC) region, which terminates the heat release reactions. The end gas then resides in the long ignition delay peninsula, which inhibits knock.     

Visualization of Crevice Flow in an Engine Using Laser-Induced Fluorescence

A simultaneous visualization technique of reacting and unburned zones using laser-induced fluorescence (LIF) was applied to a high-pressure combustion field in an engine cylinder. Crevice flow from a crevice between a piston and a cylinder wall of a spark ignition gas engine was visualized by LIF of OH and acetone. OH was excited simultaneously with acetone that was seeded into fuel as a tracer by an excitation light at 283.92 nm. Fluorescence signals from each species were detected individually by two intensified CCD cameras using optical band-pass filters which transmit fluorescence wavelength of OH and acetone, respectively. Pressure- and temperature-dependence of LIF signals from each species were evaluated. From the visualized images, it was clarified that oxidation of the crevice flow is stopped at the time of exhaust valve opening. Effects of exhaust port pressure on the oxidation process were investigated.     

Investigation of lubricant induced pre-ignition and knocking combustion in an optical spark ignition engine

An experimental study was performed to investigate lubricant oil induced pre-ignition and knocking combustion process in a single cylinder spark ignition (SI) engine with full bore overhead optical access. Lubricant oil was deliberately injected to the exhaust area through a specially modified direct injector to trigger the stochastic pre-ignition in a premixed air and fuel mixture. Simultaneous heat release analysis and high speed combustion imaging were used to study the pre-ignition and combustion processes. Outlier detection based on robust statistical methods was validated as an effective and efficient approach to identify sporadic pre-ignition. When pre-ignition occurred, the pre-ignited flame-front exhibited much faster propagating speed than that of the normal spark-ignited flame-front in the first stage of flame development. In several cycles, pre-ignition was followed by the pre-ignited propagating flame-front and then a separate spark-ignited flame-front before they subsequently merged together. In a few other cycles, pre-ignition led to heavy knocking combustion caused either by the auto-ignition close to the flame-front or near the cylinder wall, or both. The ultimate knock intensity of such cycles was determined by the timing, size, and location of end-gas auto-ignition of the unburned gas. Furthermore, optical detection of the oil droplet entrained combustion in the cycle subsequent to the knocking combustion cycle implied that high frequency oscillation pressure waves ejected lubricant from the piston-ring crevice.     

Evaluation of the flame propagation within an SI engine using flame imaging and LES

This work shows experiments and simulations of the fired operation of a spark ignition engine with port-fuelled injection. The test rig considered is an optically accessible single cylinder engine specifically designed at TU Darmstadt for the detailed investigation of in-cylinder processes and model validation. The engine was operated under lean conditions using iso-octane as a substitute for gasoline. Experiments have been conducted to provide a sound database of the combustion process. A planar flame imaging technique has been applied within the swirl- and tumble-planes to provide statistical information on the combustion process to complement a pressure-based comparison between simulation and experiments. This data is then analysed and used to assess the large eddy simulation performed within this work. For the simulation, the engine code KIVA has been extended by the dynamically thickened flame model combined with chemistry reduction by means of pressure dependent tabulation. Sixty cycles have been simulated to perform a statistical evaluation. Based on a detailed comparison with the experimental data, a systematic study has been conducted to obtain insight into the most crucial modelling uncertainties.     

A mathematical model of Helmholtz resonator type gas oscillation discharges of two-stroke cycle engines

For the example of a one cylinder, 3·7 in³ two-stroke cycle engine, it is shown that combustion engines discharge combusted gas in oscillatory bursts whose primary frequency is independent of engine speed. The superposition of a Helmholtz resonator model and a thermodynamic model of the combustion process is used to predict pressure oscillations and sound radiation. It is shown that the elastic and inertial characteristics of the gas in the combustion chamber and the exhaust port have to be considered in the model. Theoretical and experimental results compare well.     

Mesh refinement algorithms in an unstructured solver for multiphase flow simulation using discrete particles

This study developed spray-adaptive mesh refinement algorithms with directional sensitivity in an unstructured solver to improve spray simulation for internal combustion engine application. Inadequate spatial resolution is often found to cause inaccuracies in spray simulation using the Lagrangian–Eulerian approach due to the over-estimated diffusion and inappropriate liquid–gas phase coupling. Dynamic mesh refinement algorithms adaptive to fuel sprays and vapor gradients were developed in order to increase the grid resolution in the spray region to improve simulation accuracy. The local refinement introduced the coarse-fine face interface that requires advanced numerical schemes for flux calculation and grid rezoning with moving boundaries. To resolve the issue in flux calculation, this work implemented the refinement/coarsening algorithms into a collocated solver to avoid tedious interpolations in solving the momentum equations. A pressure correction method was applied to address unphysical pressure oscillations due to the collocation of pressure and velocity. An edge-based algorithm was used to evaluate the edge-centered quantities in order to account for the contributions from all the cells around an edge at the coarse-fine interface. A quasi-second-order upwind scheme with strong monotonicity was also modified to accommodate the coarse-fine interface for convective fluxes. To resolve the issue related to grid rezoning, rezoning was applied to the initial baseline mesh only and the new locations of the refined grids were obtained by interpolating the updated baseline mesh. The time step constraints were also re-evaluated to account for the change resulting from mesh refinement. The present refinement algorithm was used in simulating fuel sprays in an engine combustion chamber. It was found that the present approach could produce the same level of results as those using the uniformly fine mesh with substantially reduced computer time. Results also showed that this approach could alleviate the artifacts related to the Lagrangian discrete modeling of spray drops due to insufficient spatial resolution.     

多孔介质(PM)发动机理想循环热力学分析

本文论述了基于多孔介质燃烧技术的超绝热发动机的原理及其工作过程,建立了PM发动机的热力学模型,对 PM发动机内的PM回热循环进行热力学分析,列出了循环参数如压缩比、预胀比、预压比等对发动机效率、循环功的影响,确定了PM回热循环的两种极限状态。将PM回热循环与发动机的Otto循环、Diesel循环进行比较,结果表明: PM回热循环在保证效率的同时,可以大幅度提高循环功。     

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.     

Analysis of the temporal flame kernel development in an optically accessible IC engine using high-speed OH-PLIF

The investigation of the combustion process of a direct injection spark-ignition internal combustion (IC) engine is crucial in modern engine development. The present study is aimed at inspecting the temporal development of the spark induced flame kernel within single combustion cycles using high-speed planar laser-induced fluorescence (PLIF). The analysis is based upon the excitation of OH radicals, which are an indicator of the transient flame front. To achieve an adequate temporal resolution of the early combustion phase, the image sampling rate was set to 6 kHz, recording one image per crank-angle (CA) degree at 1000 rpm. A further feature of the technique is a large field of view spanning ∼54×53 mm. The performance of the transient combustion process is characterized by temporally tracking subsequential engine cycles individually. Flame front dynamics with different dilution levels of the intake air, simulating exhaust gas recirculation (EGR) are investigated. Resolving flame front dynamics especially with varying EGR is an important step towards an improved understanding of cyclic variations and pollutant formation.