发动机燃用天然气掺二氧化碳混合燃料的循环变动试验研究

在一台单缸火花点火发动机上开展了燃用不同组分配比的天然气掺二氧化碳混合气体燃烧循环变动的试验研究.研究结果表明:随着混合气中二氧化碳体积比的增加,燃烧稳定性下降,发动机循环中出现部分燃烧和失火等不正常燃烧现象.通过分析最高缸压和其对应曲轴转角的关系、平均指示压力与最高缸压对应曲轴转角的关系以及平均指示压力和最高缸压的关系等,考察了发动机快速燃烧循环与慢速燃烧循环在特征参数之间关系中的发展规律,混合气中二氧化碳体积比的增加,燃烧放热变慢,导致平均指示压力的循环变动系数增大.     

不同点火时刻取值方式对CNG发动机燃烧循环变动特性的影响

首先讨论点火时刻对平均指示压力变动系数的影响,同时对比了采用固定点火时刻和采用平均指示压力变动系数最小值的点火时刻时,不同因素对CNG发动机燃烧循环变动特性的影响规律.结果表明,在其他工况参数与控制参数不变时,存在平均指示压力变动系数最小的点火时刻.采用固定点火时刻研究循环变动特性规律时,点火时刻取值的差异会导致CNG发动机燃烧循环变动特性规律的差异;采用平均指示压力变动系数最小值点火时刻时,燃烧循环变动特性规律具有唯一性.     

火花点火天然气掺氢发动机结合EGR时的循环变动规律

在火花点火式天然气掺氢发动机上,开展天然气掺氢结合EGR时发动机循环变动的试验研究,分析了不同EGR率和掺氢比时发动机燃烧循环变动规律.结果表明,对于给定的燃料,随EGR率的增加,缸内最高压力和最大压力升高率下降,循环变动增加,缸内最高压力和最大压力升高率与其对应的曲轴转角之间的相关性减弱.平均指示压力下降且分布趋于分...     

火花点火发动机燃用天然气掺氢混合燃料循环变动研究

在火花点火天然气发动机上开展了不同掺氢比天然气掺氢混合燃料(氢气在混合燃料中的体积分数为0%、12%、23%、30%和40%)循环变动的试验研究,试验工况点对应于发动机中低负荷.分析了掺混氢气对天然气发动机循环变动的影响.研究结果表明:在稀燃条件下,随着掺氧比的增加,缸内最高压力、最大压力升高率以及平均指示压力均增加.随着掺氢比增加,缸内最高压力与其对应的曲轴转角之间和最大压力升高率与其对应的曲轴转角之间的相关性更强.在化学计量比或浓燃时,掺混氢气可以维持平均指示压力的循环变动系数在较低的水平.在稀燃时,平均指示压力的循环变动系数随掺氢比增加而降低.平均指示压力的循环变动系数达到10%所对应的过量空气系数随掺氢比增加而增加,表明天然气掺混氢气扩展了天然气发动机的稳定稀燃极限.     

四气门汽油机低速低负荷燃烧过程的试验研究

在发动机试验台上,用CB-466燃烧分析仪对四气门汽油机低速低负荷燃烧压力循环变动进行了试验研究。试验结果表明,转速不变时,随着负荷的增加,指示热效率逐渐增加,达到最大值后又逐渐减小,最高燃烧压力及其标准偏差和平均指示压力逐渐增加,最高燃烧压力循环变动率和平均指示压力循环变动率则随之减小;中等负荷与小负荷相比,最高燃烧压力循环变动率减少了28.3%,平均指示压力循环变动率减少了47.6%。在负荷不变的条件下,随着转速的增加,指示热效率逐渐增加,低转速时的指示热效率仅为中等转速时的指示热效率的57.4%,最高燃烧压力随之减小,最高燃烧压力循环变动率、平均指示压力及其标准偏差和平均指示压力循环变动率逐渐增加。平均指示压力循环变动率与最高燃烧压力循环变动率相比较小,平均指示压力循环变动率仅为最高燃烧压力循环变动率的37.1%。     

稀燃天然气发动机燃烧循环变动影响因素研究

通过对一台点燃式多点电喷稀燃天然气发动机进行试验,获得了不同工况下的平均指示压力循环变动系数,以此为基础研究了燃空当量比、节气门开度、转速及点火时刻对稀燃天然气发动机燃烧循环变动的影响趋势。结果表明:混合气燃空当量比越小,燃烧循环变动越明显,当燃空当量比降低到一定值时,平均指示压力循环变动系数的增长会突然加大;节气门开度越小燃烧循环变动越明显,节气门开度小于30%后,其对燃烧循环变动影响更加明显;燃烧循环变动量随转速上升有增加的趋势,在高转速工况下燃烧循环变动的加强尤其明显;在工况一定的条件下存在一个最优的点火时刻可使稀燃天然气发动机的燃烧循环变动最小。     

丙烷发动机燃烧变动研究

测量了火花点火丙烷气体发动机在不同转速、涡流强度及混合比下的气缸压力,并对测量的气缸压力及由气缸压力求出的用曲轴转角表示的初期燃烧期间等进行了统计分析。试验结果表明,随着混合气变稀平均指示压力的变动迅速增大;转速相同时,平均指示压力的变动随着涡流比的增大而减小;在稀薄混合气条件下,随着初期燃烧期间平均值的增加平均指示压力的变动急剧上升。     

天然气火花点火发动机循环变动的分析方法

通过单点电喷天然气发动机上的实验研究,分析了利用实测缸内压力和放热率曲线来分析研究点火发动机循环变动的各种方法,并阐述了各种方法的适用性以及不同表征循环变动的参数之间的关系.分析的结果指出:正常燃烧循环峰值压力(pmax)与其出现转角(ψpmax)有良好得对应关系,混合气较浓时工况时pmax或ψpmax的变动率比稀混合气时小;而平均指示压力(IMEP)与pmax的关系却不明显,因此当以发动机的动力性为主要研究对象时,不宜采用pmax作为循环变动的分析参数.为了综合考虑整个循环压力变动的特征,提出了对每个曲轴转角处压力变动都进行分析的方法,另外为了分析燃烧等容度变动,还提出了放热率型心位置变动率的概念,并指出放热率型心靠近上止点有利于提高IMEP.     

天然气缸内直喷发动机燃烧循环变动特性研究

开展了天然气高压缸内直喷发动机的燃烧循环变动特性研究。研究结果表明,小当量比工况下(<0.4),会出现失火循环和部分燃烧循环,平均指示压力pmi值低,pmi的循环变动系数大,大当量比工况下pmi的循环变动系数小。最高气缸压力高的循环燃烧速率快、燃烧过程的等容度好。最高气缸压力与其对应的曲轴转角之间、最高气缸压力升高率与其对应的曲轴转角之间、最高气缸压力和pmi之间都存在着一定关系。最高气缸压力与火焰发展期、快速燃烧期也存在很好的对应关系。高的最高气缸压力对应着短的火焰发展期和短的快速燃烧期。循环的累积放热量越大则pmi越大,pmi的大小与累积放热量的大小存在对应关系。     

火花点火发动机压力循环变动的评价方法研究

分析比较了火花点发动机循环变动的评价方法,并提出了用各个曲轴转角下的压力变动率曲线表示压力循环变动的方法。结果表明,平均指示压力的标准差及变动率是评定内燃机压力循环变动的最佳参数;各个曲轴下的压力标准差及标准差及变动率曲线可以清楚地表示出发动机工作过程中各个曲轴转角下的压力变动情况;进气过程中也存在压力变动,其压力变动率高于燃烧过程;燃烧过程的压力变动率存在一个峰值。     

Determination of TDC in internal combustion engines by a newly developed thermodynamic approach

In-cylinder pressure analysis is nowadays an indispensable tool in internal combustion engine research & development. It allows the measure of some important performance related parameters, such as indicated mean effective pressure (IMEP), mean friction pressure, indicated fuel consumption, heat release rate, mass fraction burned, etc. Moreover, future automotive engine will probably be equipped with in-cylinder pressure sensors for continuous combustion monitoring and control, in order to fulfil the more and more strict emission limits. For these reasons, in-cylinder pressure analysis must be carried out with maximum accuracy, in order to minimize the effects of its characteristic measurement errors. The exact determination of crank position when the piston is at top dead centre (TDC) is of vital importance, since a 1° degrees error can cause up to a 10% evaluation error on IMEP and 25% error on the heat released by the combustion: the position of the crank shaft (and hence the volume inside the cylinder) should be known with the precision of at least 0.1 crank angle degrees, which is not an easy task, even if the engine dimensions are well known: it corresponds to a piston movement in the order of one tenth of micron, which is very difficult to estimate. A good determination of the TDC position can be pursued by means of a dedicated capacitive TDC sensor, which allows a dynamic measurement (i.e. while engine is running) within the required 0.1° precision [1], [2]. Such a sensor has a substantial cost and its use is not really fast, since it must be fitted in the spark plug or injector hole of the cylinder. A different approach can be followed using a thermodynamic method, whose input is in-cylinder pressure sampled during the compression and expansion strokes: some of these methods, more or less valid, can be found in literature [3], [4], [5], [6], [7], [8]. This paper will discuss a new thermodynamic approach to the problem of the right determination of the TDC position. The base theory of the method proposed is presented in the first part, while the second part deals with the assessment of the method and its robustness to the most common in-cylinder pressure measurement errors.     

The nature of early flame development in a lean-burn stratified-charge spark-ignition engine

The operating range of lean-burn SI engines is limited by the level of cycle-by-cycle variability in the early flame development, which typically corresponds to the 0-5% mass fraction burned. An experimental investigation was undertaken to study this flame variability in an optical, stratified-charge, SI engine close to the lean limit of stable operation (A/F=22). Double-exposed flame images acquired through either a pentroof window (“tumble plane” of view) or the piston crown (“swirl plane” of view) were processed to calculate the intra-cycle flame growth and convection rates under 1500 RPM low-load conditions. Projected flame-boundary analysis was also performed to investigate the effect of flame shape/wrinkling on the subsequent timing of 5% mass fraction burned on a cycle-by-cycle basis. The images showed that the flame always preserved its shape while growing in size (even when it had been initiated with a highly convoluted shape); image processing demonstrated the manner with which the flame-growth speed varied as the flame propagated and approached the pentroof and piston-crown walls for slow, “typical” or fast burning cycles. It was found that it was beneficial to have a high convection velocity in the swirl plane of flow during the first 10° CA after ignition timing (corresponding to less than 0.1% mass fraction burned), but after this stage it was beneficial to have a moderate convection velocity for the flame. However, on the tumble plane of flow, a high convection velocity was preferable up to 30° CA after ignition timing (corresponding, typically, to 1% mass fraction burned). Slow development of a flame was associated with higher stretch rates for the same flame radius than fast-developing flames during the period of growth from 3 to 6 mm in radius (about 0.1-1% mass fraction burned). Extended analysis of the projected flame front's shape and its wrinkling showed that the fastest lean-condition flames had contour characteristics similar to those of the flames recorded for stoichiometric conditions. This suggested that the fastest lean flames on a cycle-by-cycle basis might have been richer than the average in the vicinity of the spark plug at ignition.     

Direct injection of hydrogen main fuel and diesel pilot fuel in a retrofitted single-cylinder compression ignition engine

Up to 90% hydrogen energy fraction was achieved in a hydrogen diesel dual-fuel direct injection (H2DDI) light-duty single-cylinder compression ignition engine. An automotive-size inline single-cylinder diesel engine was modified to install an additional hydrogen direct injector. The engine was operated at a constant speed of 2000 revolutions per minute and fixed combustion phasing of ?10 crank angle degrees before top dead centre (°CA bTDC) while evaluating the power output, efficiency, combustion and engine-out emissions. A parametric study was conducted at an intermediate load with 20–90% hydrogen energy fraction and 180-0 °CA bTDC injection timing. High indicated mean effective pressure (IMEP) of up to 943 kPa and 57.2% indicated efficiency was achieved at 90% hydrogen energy fraction, at the expense of NO_x emissions. The hydrogen injection timing directly controls the mixture condition and combustion mode. Early hydrogen injection timings exhibited premixed combustion behaviour while late injection timings produced mixing-controlled combustion, with an intermediate point reached at 40 °CA bTDC hydrogen injection timing. At 90% hydrogen energy fraction, the earlier injection timing leads to higher IMEP/efficiency but the NO_x increase is inevitable due to enhanced premixed combustion. To keep the NO_x increase minimal and achieve the same combustion phasing of a diesel baseline, the 40 °CA bTDC hydrogen injection timing shows the best performance at which 85.9% CO₂ reduction and 13.3% IMEP/efficiency increase are achieved.     

Further refinement and validation of a turbulent flame propagation model for spark-ignition engines

A turbulent flame propagation model that is dependent on the structure of the turbulent flow field is formulated and applied to combustion in a spark-ignition engine. The turbulence structure is modeled after the work of Tennekes and assumes that the flow is composed of vortex tubes the diameter of the Kolmogorov scale and the spacing of the Taylor mircoscale. Combustion is assumed instantaneous over the Kolmogorov scale and the burned gases in the vortex tubes are assumed to propagate at a rate equal to U′ + S_L. Combustion is assumed to proceed in a laminar fashion across the microscale. In applying the model to a spark-ignition engine, we conserve the turbulent kinetic energy and angular momemtum in the unburned gases. Validation of the model is presented in the form of mass fraction burned versus crank angle curves. Comparisons of predicted versus experimental data show good agreement for variations in equivalence ratio, dilution, speed, load, and spark advance.     

Numerical prediction of effects of CO2 or H2 content on combustion characteristics and generation efficiency of biogas-fueled engine generator

The fundamental effect of additives and the lean burn capability of methane combustion are investigated using cycle simulation and Latin hypercube sampling. The target engine is a spark-ignition engine fueled by methane and biogas mixed with hydrogen. The dominant variables were CO₂ and H₂ content, spark timing, and excess air ratio (EAR). Varying the amount of additives (CO₂ and H₂), spark timing, and EAR demonstrated that hydrogen plays an important role in extending the lean operating limit. The fractional factorial design of the experiment, Latin hypercube sampling, was applied to obtain the maximum brake torque (MBT) spark timing with variants in excess air ratios and additive content. When MBT spark timing is employed, the maximum mass fraction burned can be enhanced in the lean-burn region by increasing the hydrogen content with improved generating efficiencies under lean operating conditions.     

Theoretical investigation of flame propagation process in an SI engine running on gasoline–ethanol blends

Turbulent flame propagation process in a spark-ignition (SI) engine is theoretically investigated. Fueling with gasoline, ethanol and different gasoline–ethanol blends is considered. A quasi-dimensional SI engine cycle model previously developed by the author is used to predict the thermodynamic state of the cylinder charge during the cycle. The flame is assumed to be spherical in shape and centered at the spark plug. Computations are carried out for an automobile SI engine having a disc-shaped combustion chamber, for which the compression ratio and the nominal speed are 9.2 and 5800 rpm, respectively. Geometrical features (flame radius, flame front area and enflamed volume) of the flame, combustion characteristics (mass fraction burned and burn duration), and cylinder pressure and temperature are predicted as a function of the crank angle. Three different positions of the crank angle are studied: −10°, TC and +10°. It was concluded that ethanol addition to gasoline up to 25 vol% accelerated the flame propagation process.     

Comparison of cycle-by-cycle variation of measured exhaust-gas temperature and in-cylinder pressure measurements

Closed-loop control of the combustion process in the internal-combustion engine on a cycle-to-cycle basis is desirable to achieve better fuel economy and reduce pollutant emissions. This work explores the possibility of monitoring the in-cylinder combustion process of a spark-ignition engine by measuring exhaust-gas temperature. A small-diameter thermocouple (25.4 μm or 0.001 in.) is inserted into the exhaust manifold to measure exhaust-gas temperature, while in-cylinder pressure is monitored with a pressure transducer. Cycle-by-cycle variations of the measured exhaust-gas temperature are compared to IMEP variation for groups of cycles. The experimental results show that coefficient of variation of maximum temperature follows the same trend as the coefficient of variation of indicated mean effective pressure. Cycle-by-cycle variation of IMEP is reflected in cyclic variation of measured exhaust-gas temperatures for operating conditions under which the engine is working at a fixed speed or load and when cyclic variability is caused by changing engine parameters such as spark timing, injection timing, and air/fuel ratio. Finally, by using a moving window covering a small number of cycles, it is possible to show that the COV trend of maximum exhaust temperature mirrors that of IMEP.     

S.I. engine fuelled with gasoline,methane and methane/hydrogen blends: Heat release and loss analysis

Experiments were carried out to investigate the performance of different fuels used in a internal combustion engine: gasoline, methane and fuel blends containing methane with 5%, 10% and 15% hydrogen by volume, respectively. A two-litre naturally aspirated bi-fuel engine with port fuel injection was used. The engine was operated stoichiometrically. For each fuel the spark advance for best efficiency was determined. Experiments were conducted at 2000 rpm and 2 bar brake mean effective pressure. A heat release analysis and a loss analysis were performed for all fuels. The main findings are that increasing the hydrogen fraction of the methane hydrogen fuel blend decreases the overall burn duration. This decrease is predominantly achieved by a shortened duration of the fist stage of combustion (ignition to 5% mass fraction burned). The faster combustion comes along with an increase in fuel conversion efficiency. The different losses for gasoline and pure methane operation interact such that equal fuel conversion efficiencies result. However, care has to be taken when comparing fuel conversion efficiencies among the different fuels as the relative error in fuel conversion efficiency for the gaseous fuels is 0.2% at most, whereas it is about 1% for gasoline.