Local mixture injection to extend the lean limit of spark-ignition engines

 The effect of charge stratification by direct, in-cylinder injection of a small quantity of propane-air mixture in a single-cylinder, propane-fuelled, spart-ignition engine, has been determined in terms of in-cylinder pressure, flame visualisation and exhaust emissions. The operating conditions ranged from low load to wide open throttle at 1000 and 1500 rpm, and with quiescent and swirling in-cylinder flows. The effects of injection-driven flow and turbulence on combustion have been considered independently of mixture strength by assessing the consequences of injecting a local mixture of an equivalence ratio equal to that of the port-induced charge. The results provide further evidence to support the concept of direct in-cylinder mixture injection. At 1000 rpm and low load, faster development of the flame kernel and subsequent enflamed area were observed, together with a 60% increase in peak cylinder combustion pressure at an overall equivalence ratio of 0.7 (A/F of 22.5). The relative effect of local-charge stratification increased with reduction in equivalence ratio at this engine speed and without increase in emissions; at 1500 rpm, stable combustion was achieved even at an equivalence ratio of 0.61 (A/F of 25.8), which was below the flammability limit of the homogeneous port-only propane/ air mixture. Unthrottled engine operation produced similar results so that at an equivalence ratio of 0.55 (A/F of 28.5), for example, the Coefficient of variation of the indicated mean effective pressure was reduced from 0.4 to less than 0.1 by mixture local injection. With a shrouded inlet valve generating mean gas velocities of 6 m/s at the time of ignition in the vicinity of the spark plug, control of rich mixture injection both with and against the bulk in-cylinder flow, resulted in stable combustion with Coefficient of variation of the indicated mean effective pressure of less than 0.1 at an equivalence ratio of 0.55 (A/F of 28.5). Received: 28 October 1997/Accepted: 23 March 1998     

基于液体碳氢燃料的旋转爆轰燃烧特性研究

为了探索液体碳氢燃料参与旋转爆轰所产生的不完全燃烧现象,采用守恒元与求解元方法,开展柱坐标系下的汽油/空气两相旋转爆轰燃烧室三维数值模拟研究,针对燃料喷注压力和反应物当量比对旋转爆轰流场结构及燃烧室性能的影响进行分析。分析结果表明:保持总当量比为1.00,随着燃料喷注压力的上升,燃烧室内燃料不均匀分布增强,产生局部富燃区,燃料在燃烧室未能完全反应,导致燃烧室燃料比冲下降;保持喷注压力不变,减小当量比,在贫燃工况下依然存在局部富燃区,导致燃烧室内出现不完全燃烧现象,降低燃烧室比冲性能。由此可知,反应物喷注方案对气液两相旋转爆轰的不完全燃烧有显著影响。     

Effects of turbulence enhancement on combustion process using a double injection strategy in direct-injection spark-ignition (DISI) gasoline engines

Direct-injection spark-ignition (DISI) gasoline engines have been spotlighted due to their high thermal efficiency. Increase in the compression ratio that result from the heat absorption effect of fuel vaporization induces higher thermal efficiency than found in port fuel injection (PFI) engines. Since fuel is injected at the cylinder directly, various fuel injection strategies can be used. In this study, turbulent intensity was improved by a double injection strategy while maintaining mixture homogeneity. To analyze the turbulence enhancement effects using the double injection strategy, a side fuel injected, homogeneous-charge-type DISI gasoline engine with a multi-hole-type injector was utilized. The spray model was evaluated using experimental data for various injection pressures and the combustion model was evaluated for varied ignition timing. First and second injection timing was swept by 20 degree interval. The turbulent kinetic energy and mixture inhomogeneity index were mapped. First injection at the middle of the intake stroke and second injection early in the compression stroke showed improved turbulent characteristics that did not significantly decrease with mixture homogeneity. A double injection case that showed improved turbulent intensity while maintaining an adequate level of mixture homogeneity and another double injection case that showed significantly improved turbulent intensity with a remarkable decrease in mixture homogeneity were considered for combustion simulation. We found that the improved turbulent intensity increased the flame propagation speed. Also, the mixture homogeneity affected the pressure rise rate.     

Effects of Unmixedness on Combustion Instabilities in a Lean-Premixed Gas Turbine Combustor

The present experimental study focuses on the effects of the degree of premixing and swirl strength on combustion instabilities occurring in a lean premixed gas turbine combustor burning natural gas and air. The combustor operated at pressurized conditions with heated air. Major measurements for the investigation of premixed combustion dynamics include pressure fluctuations, flame emissions in reacting flow, and acetone fluorescence in non-reacting flow to assess the degree of premixing between fuel and air. The acetone PLIF results revealed that the degree of premixing improves as mixing time increases. The first and second longitudinal acoustic modes were the dominant excited modes for most cases of interest. Combustion at a lean premixed condition becomes more susceptible to instabilities as the degree of premixing becomes poor, and self-excited pressure oscillations are obviously present under a fully premixed condition, even without equivalence ratio fluctuations in space. For incomplete premixing cases, local equivalence ratio fluctuations caused by poor premixing may initiate instabilities since reaction rate is sensitive to equivalence ratio fluctuations at lean conditions. Phase resolved chemiluminescence measurements show that pressure oscillations are strongly coupled with variations in flame structures.     

Central recirculation zone analysis in an unconfined tangential swirl burner with varying degrees of premixing

Swirl-stabilised combustion is one of the most widely used techniques for flame stabilisation, uses ranging from gas turbine combustors to pulverised coal-fired power stations. In gas turbines, lean premixed systems are of especial importance, giving the ability to produce low NOx systems coupled with wide stability limits. The common element is the swirl burner, which depends on the generation of an aerodynamically formed central recirculation zone (CRZ) and which serves to recycle heat and active chemical species to the root of the flame as well as providing low-velocity regions where the flame speed can match the local flow velocity. Enhanced mixing in and around the CRZ is another beneficial feature. The structure of the CRZ and hence that of the associated flames, stabilisation and mixing processes have shown to be extremely complex, three-dimensional and time dependent. The characteristics of the CRZ depend very strongly on the level of swirl (swirl number), burner configuration, type of flow expansion, Reynolds number (i.e. flowrate) and equivalence ratio. Although numerical methods have had some success when compared to experimental results, the models still have difficulties at medium to high swirl levels, with complex geometries and varied equivalence ratios. This study thus focuses on experimental results obtained to characterise the CRZ formed under varied combustion conditions with different geometries and some variation of swirl number in a generic swirl burner. CRZ behaviour has similarities to the equivalent isothermal state, but is strongly dependent on equivalence ratio, with interesting effects occurring with a high-velocity fuel injector. Partial premixing and combustion cause more substantive changes to the CRZ than pure diffusive combustion.     

煤油液滴直径对两相旋转爆轰发动机流场的影响

为探究煤油液滴不同初始直径对气液两相旋转爆轰发动机流场的影响,假设初始注入的煤油液滴具有均匀直径,考虑雾化破碎、蒸发等过程,建立了非定常两相爆轰的Eulerian-Lagrangian模型,进行了液态煤油/高温空气爆轰的非预混二维数值模拟。结果表明：在初始液滴直径为1～70μm的工况范围,燃烧室内均形成了单个稳定传播的旋转爆轰波;全局当量比为1时,爆轰波前的空气区域大于液滴煤油的蒸气区域,导致波前燃料空气混合不均匀,波前均存在富油区和贫油区,两相速度差导致分离出的空气形成低温条带;当煤油液滴的初始直径较小时,波前的反应物混合过程主要受蒸发的影响,爆轰波可稳定传播;当直径减小至1μm时,煤油液滴在入口处即蒸发,旋转爆轰波表现为气相传播的特性,爆轰波结构平整;当煤油液滴的初始直径较大时,波前的反应物混合过程主要受液滴破碎的影响;对于相同的燃料质量流量,在不同初始煤油液滴直径工况下,煤油液滴最大的停留时间均占爆轰波传播时间尺度的80%以上;爆轰波前燃料预蒸发为气相的占比越高,爆轰波的传播速度越高;初始液滴直径为10～70μm的工况范围内,爆轰波的速度随初始直径的增大先升高后降低。     

Experimental investigation of dynamics of premixed acetylene–air flames in a micro-combustor

This paper describes a detailed experimental study performed to investigate the flame propagation behaviour of premixed flames in micro-channels. A novel, modular, stackable micro-combustor was developed for this purpose. For a chosen planar channel geometry, the flow condition and the mixture equivalence ratio of premixed acetylene–air were varied to investigate various modes of operation. Three different modes of operation were observed; they were (i) stable periodic operation – consisting of ignition, flame propagation, flame extinction, and re-ignition, (ii) a-periodic operation, and (iii) anchored flame condition. The present work also aims to provide quantitative information on the dynamics of premixed acetylene–air flames propagating inside micro-channels. A novel measurement approach based on OH^* chemiluminescence measurements employing a single photomultiplier unit was developed for this purpose. The data recorded were post processed using an in-house developed MATLAB code to evaluate the mean flame propagation speed measured between three different spatial locations along the length of the micro-channel. The results from the flame propagation speed measurements performed during ‘periodic’ mode of operation indicated that the flame travelled at higher propagation speed in the mid-length region of the channel compared to that at the initial entry point, suggesting flame acceleration. This flame acceleration could be attributed to a situation where the flame experienced different local equivalence ratio conditions at different upstream locations. The results suggest that after completion of a cycle of operation consisting of ignition, flame propagation and flame extinction, the fresh mixture that filled the channel was diluted with the exhaust gas from the previous cycle. This pocket of diluted mixture convected downstream with time, thus enabling the spatial variation in local equivalence ratio along the micro-channel.     

液体碳氢燃料云雾爆轰特性的实验研究

采用升降法和烟迹技术在立式激波管中分别实测了液态燃料（环氧丙烷、硝酸异丙酯、己烷、C5~C6、庚烷、癸烷）与空气混合物直接起爆的临界起爆能和胞格尺寸。数据表明,气液两相云雾爆轰的临界起爆能与当量比呈U形曲线关系,这与气相爆轰得到的结论是一致的;临界起爆能的最小值并不是对应于等化学当量的混合物而是偏向于富燃料;根据三波点运动的烟迹记录,分析了云雾爆轰作用机制,认为液滴的碎解、汽化过程以及燃烧区前导是控制气液两相云雾爆轰的主要步骤。此外,还测定了无限空间下可燃气云的临界起爆能,并将激波管内得到的临界起爆能数据外推到无约束气云的临界起爆能,理论推算结果与实验值吻合较好。     

Performance of supersonic model combustors with staged injections of supercritical aviation kerosene

Supersonic model combustors using two-stage injections of supercritical kerosene were experimentally investigated in both Mach 2.5 and 3.0 model combustors with stagnation temperatures of approximately 1,750 K. Supercritical kerosene of approximately 760 K was prepared and injected in the overall equivalence ratio range of 0.5-1.46. Two pairs of integrated injector/flameholder cavity modules in tandem were used to facilitate fuel-air mixing and stable combustion. For single-stage fuel injection at an upstream location, it was found that the boundary layer separation could propagate into the isolator with increasing fuel equivalence ratio due to excessive local heat release, which in turns changed the entry airflow conditions. Moving the fuel injection to a further downstream location could alleviate the problem, while it would result in a decrease in combustion efficiency due to shorter fuel residence time. With two-stage fuel injections the overall combustor performance was shown to be improved and kerosene injections at fuel rich conditions could be reached without the upstream propagation of the boundary layer separation into the isolator. Furthermore, effects of the entry Mach number and pilot hydrogen on combustion performance were also studied.     

Enhancement of evaporation and combustion of liquid fuels through porous media

Existing designs of most conventional liquid fuel burners have relied solely on spray atomizers, with a large amount of very fine droplets forming in a relatively large combustion chamber, resulting in a relatively low combustion intensity. Against this background, a novel down-flow compact porous burner system was developed for burning kerosene without the need of using a spray atomizer. Successive development on this burner research is important in view of the need to create energy by an efficient device based on simple technology. The focus has been on the introduction of the packed bed emitter installed downstream of the porous burner. The evaporation process and combustion phenomena that occurred are described through the coupled interaction of the solid phase (porous burner), the liquid phase (kerosene) and the gas phase. Enhancement of evaporation and combustion are evaluated through the measured thermal structures in terms of temperature distribution along the burner length and emission characteristics at the burner exit. Stable combustion with low emission of pollutants was realized even though the combustion flame was confined in-between the porous burner and the packed bed emitter with an increase in the back-pressure. The effects of various parameters including heat input and equivalence ratio on the combustion characteristics were clarified to confirm improvement in mixing of the fuel vapor/air mixture and turn-down ratio of the burner. The effect of the introduced packed bed emitter with suitable bed length and its installation location is investigated as an efficient method for enhancement of evaporation and combustion of the liquid fuels without a spray atomizer. Future applications of this type of burner system are suggested.     

Mixing Measurements in a Supersonic Expansion-Ramp Combustor

This paper reports results on molecular mixing for injection via an expansion-ramp into a supersonic freestream with M ₁ = 1.5. This geometry produces a compressible turbulent shear layer between an upper, high-speed “air” stream and a lower, low-speed “fuel” stream, injected through an expansion-ramp at α = 30° to the high-speed freestream. Mass injection is chosen to force the shear layer to attach to the lower guide wall. This results in part of the flow being directed upstream, forming a recirculation zone. Employing the hypergolic hydrogen-fluorine chemical reaction and pairs of “flip” experiments, molecular mixing is quantified by measuring the resulting temperature rise. Initial experiments established the fast-chemistry limit for this flow in terms of a Damköhler number (Da). For Da ≥ 1.4, molecularly mixed fluid effectively reacts to completion. Parameters varied in these experiments were the measurement station location, the injection velocity of the (lower) “fuel” stream, the stoichiometry for the flip experiments, and the density ratio of the fuel and air streams. As expected, mixing increases with increasing distance from the injection surface. The mixed fluid fraction increases by 12% when changing the fuel-to-air stream density ratio from 1 to 0.2. Comparisons with measurements at subsonic (high-speed) “air” stream velocities show that the trend of decreasing mixing with increasing speed documented in free-shear layer flows is also encountered in these flows. The current geometry produces higher mixing levels than do free shear layers.     

Development of a local continuous sampling probe for the equivalence air–fuel ratio measurement. Application to spark ignition engine

This paper is a contribution to the development of an original technique for measuring the in-cylinder equivalence air–fuel ratio. The main objective was to construct an instrument able to furnish instantaneous values of hydrocarbon concentration for many consecutive cycles at a definite location, especially at the spark plug location. The probe is based on a hot-wire-like apparatus, but involves catalytic oxidation on the wire surface in order to be sensitive to the hydrocarbon concentration. In this paper, we present the different steps needed to develop and validate the probe. The first step focuses on the geometric configuration to simplify as much as possible the mass transfer phenomena on the wire. The second step is a parametric study to evaluate the sensitivity, confidence and lifetime of the wire. By physical analysis, we propose a relationship between the electrical signal and the air–fuel equivalence ratio of the sampled gases. The third step is the application of the probe to in-cylinder motored engine measurements, which confirms the ability of the technique to characterise, quantitatively, the homogeneity of the air–fuel mixture, especially during the compression stroke. This work points out that the global sensitivity is estimated at 4 V per unit of equivalence air–fuel ratio and the response time is estimated at about 400 μs. The equivalence air–fuel ratio range is from pure air to 1.2. Experiments show that it is necessary to calibrate the system before use because of the existence of multiple catalysis states. The probe presents advantages associated with its simplicity, its low cost and its direct engine application without any modifications. Received: 1 November 2000 / Accepted: 30 May 2001     

In-cylinder fuel distribution in a port-injected model engine using Rayleigh scattering

The spatial and temporal variation of the fuel concentration (air/fuel ratio) in a model engine was quantified by laser Rayleigh scattering. Fuel was simulated by gaseous Freon-12 injected at various timings and quantities into the intake port. The results showed that the fuel concentration in the engine cylinder was strongly dependent on the injection timing and duration and that the Rayleigh system was able to identify spatial variations of the order of one air/fuel ratio at realistic mixture strengths.     

Investigation of the Syngas Flame Characteristics at Elevated Pressures Using Optical and Laser Diagnostic Methods

The effect of pressure on the characteristics of syngas flames is investigated under gas turbine relevant conditions using planar laser induced fluorescence of OH radicals and OH^* chemiluminescence imaging. An optically accessible combustor fitted with a swirl burner was operated with two different syngas mixtures, preheated air at 700?K, and pressures ranging from 5 to 20?bars. The thermal load varied from 15 to 25?kW/bar at an equivalence ratios 0.5. The OH-PLIF measurements show that the flames under all conditions exhibited two reaction fronts, one at the shear layer between the inner recirculation zone and the fuel inlet, and one between the fuel inlet and the air nozzle. The more or less continuous reaction front at low pressure turned into a highly corrugated flame front at higher pressures, with isolated regions of ignition and extinction. The probability density distribution of the flame curvature for the mixtures studied showed that the inner and outer flame responded differently to the pressure increase, with the mean curvature magnitude also depending on the mixture composition and thermal load. The measurements clearly shows the limitations associated with the use of OH^* chemiluminescence images as a marker for the heat release rate especially in case of syngas mixtures.     

Detailed measurements of equivalence ratio modulations in premixed flames using laser Rayleigh scattering and absorption spectroscopy

Equivalence ratio non-uniformities may give rise to some of the instabilities observed in modern lean premixed combustion systems. The present work intends to investigate the influence of equivalence ratio perturbations on the dynamics of premixed flames. A burner equipped with a secondary injection system is used to generate equivalence ratio perturbations which are convected by the flow and impinge on a conical flame. Two laser-diagnostics, based on Rayleigh scattering and hydrocarbon infrared absorption, respectively, are employed to give insight into the spatial and temporal evolution of the mixture composition field. Rayleigh scattering images also reveal the flame front dynamics providing an indication on the response of a weakly turbulent flame subject to mixture composition inhomogeneities. Laser light absorption provides a time resolved signal which is used to estimate the equivalence ratio perturbation level. A theoretical model based on the G-equation is used to interpret the experimental data and compare the relative effects of velocity and equivalence ratio perturbations.     

An experimental study of fuel injection strategies in CAI gasoline engine

Combustion of gasoline in a direct injection controlled auto-ignition (CAI) single-cylinder research engine was studied. CAI operation was achieved with the use of the negative valve overlap (NVO) technique and internal exhaust gas re-circulation (EGR). Experiments were performed at single injection and split injection, where some amount of fuel was injected close to top dead centre (TDC) during NVO interval, and the second injection was applied with variable timing. Additionally, combustion at variable fuel-rail pressure was examined.Investigation showed that at fuel injection into recompressed exhaust fuel reforming took place. This process was identified via an analysis of the exhaust-fuel mixture composition after NVO interval. It was found that at single fuel injection in NVO phase, its advance determined the heat release rate and auto-ignition timing, and had a strong influence on NO_X emission. However, a delay of single injection to intake stroke resulted in deterioration of cycle-to-cycle variability. Application of split injection showed benefits of this strategy versus single injection. Examinations of different fuel mass split ratios and variable second injection timing resulted in further optimisation of mixture formation. At equal share of the fuel mass injected in the first injection during NVO and in the second injection at the beginning of compression, the lowest emission level and cyclic variability improvement were observed.     

超燃冲压发动机燃烧模态转换试验研究

在模拟飞行高度为25 km、来流马赫数为6的情况下,采用试验研究的方法对超燃冲压发动机燃烧模态转换进行了直连式试验。根据燃烧室壁面压力分布和一维模型分析表明,燃料喷射位置和当量比的动态改变,实现了燃烧室内燃烧模态的动态转换。不同燃料喷射位置切换顺序比较表明,燃烧室内燃烧状态的改变受燃料分布所决定,但是燃烧室自身具有一定的抗波动能力。     

LES of Chemical and Acoustic Forcing of a Premixed Dump Combustor

This paper describes the first steps in the development of a large eddy simulation (LES) code able to compute combustion instabilities in gas turbines. This code was used to compute the forcing of an experimentally investigated premixed dump combustor. It is shown that the main effect of acoustic waves entering the combustion chamber is to create large vortices and unsteady heat release when these vortices burn. Another effect of waves entering the combustor is to modulate the fuel and air flow rates produced by the feeding lines. In this case the equivalence ratio of the mixture entering the combustor may also vary. This was investigated in a “chemical effect” simulation where the inlet equivalence ratio fluctuates but the total flow rate remains constant. For perturbations from stoichiometric burning, this mechanism was shown to induce less destabilizing effects than the purely aerodynamical mechanism due to vortex formation and combustion. It is shown that the LES methodology developed is able to reproduce the experimentally observed phase shift between acoustic excitation and total reaction rate in the chamber. This revised version was published online in July 2006 with corrections to the Cover Date.     

液态燃料对连续旋转爆轰发动机爆轰特性的影响

为了研究液态燃料对连续旋转爆轰发动机爆轰特性的影响,采用CE/SE方法对以汽油/富氧空气为燃料的CRDE进行数值模拟,分析了不同液滴半径、当量比对爆轰性能参数的影响。计算结果表明:随着液滴半径增大,爆轰压力峰值、温度峰值以及爆轰波速度均降低,且压力峰值与温度峰值在爆轰波传播过程中出现不稳定现象;当增大到70 μm时,爆轰波将无法成功起爆。随着当量比的增大,CRDE爆轰波速度及平均推力增大,爆轰压力、温度以及气相周向速度的峰值均先增大后减小。在当量比1.1附近,爆轰压力与温度的峰值出现极大值;而气相周向速度峰值的极大值出现在当量比0.9附近。基于燃料的比冲随着当量比增大而减小。