不同压缩比米勒循环和低压废气再循环对增压直喷汽油机性能影响

通过对一台增压汽油直喷(gasoline direct injection,GDI)发动机活塞和凸轮型线的重新设计实现了高压缩比米勒循环,并在此基础上引入了废气再循环(EGR),研究了不同压缩比米勒循环和EGR综合作用对发动机的性能影响。结果表明:增大压缩比和采用米勒循环技术对爆震影响存在取舍(trade-off)关系,低速全负荷下高压缩比米勒循环相比原机油耗略有上升;而低压冷EGR技术由于缸内稀释冷却作用可以优化燃烧相位,对外特性工况有效燃油消耗率有明显的改善作用;在部分负荷工况下,压缩比的增加和米勒燃烧循环可使油耗较原机下降6.3%,在整合低压冷EGR技术后,油耗进一步下降3.1%。可以得出结论,合理地增加压缩比,采用米勒循环技术并匹配低压冷EGR技术,可以大幅改善发动机的燃油经济性。     

基于高压直喷航空煤油点燃式发动机燃烧性能优化的试验研究

为提高航空煤油在点然式发动机中的燃烧热效率,改善发动机爆震及拓宽发动机负荷范围,以3号航空煤油(RP-3)为基础燃料,基于一台单缸水冷、压缩比可调、4冲程点燃式发动机结合高压共轨缸内直喷技术,开展了单双点火、不同负荷、压缩比、喷射压力、喷射时刻和两次喷射策略下航空煤油燃烧特性的试验研究。结果表明,在原机压缩比为10的条件下,将直喷汽油改为直喷航空煤油后,由于航空煤油的抗爆性差、雾化困难、燃烧速率慢等理化特性,发动机的动力性损失约50.0%,油耗增加约60.0%,循环波动也大幅增加;相比于单点火,双点火可使缸内平均有效压力提高,燃烧相位提前,循环波动降低;为了抑制高压缩比下的爆震倾向,可通过降低压缩比来拓宽负荷范围,恢复原机功率。随着压缩比的降低,有效平均压力(BMEP)持续增大,当压缩比为6时,最大转矩可达39.5N·m,功率恢复至原机的88.0%。同时耦合高压及两次喷射策略,随着喷射压力的增大,有效燃油消耗率(BSFC)减小约30.0%,经济性有所提高。相比于单次喷射,采用两次喷射策略可降低油耗,提升缸内有效平均压力,提升燃烧效率,最终可实现发动机燃用航空煤油的性能接近原机水平,最大负荷达原机的90.0%且油耗增加量不超过15.0%。     

Atkinson循环汽油机热力学性能模拟开发及试验研究

对东风汽车某自然吸气发动机的仿真模型进行试验对标,根据阿特金森循环的原理对发动机模型进行压缩比和配气相位的优化计算,最后对原发动机进行改装并开展了性能验证试验。研究结果表明:改进后样机在满足动力性能要求的条件下,中低转速的燃油消耗率相比原机得到了较好的改善,其中部分负荷特征工况点的燃油消耗率降低约2%~4%。     

活塞头部形貌对汽油机爆震敏感性影响研究

基于台架试验和完整工作循环数值模拟,开展了汽油机活塞头部形貌特征对爆震的敏感性研究。以台架试验数据为基准校正了汽油机性能仿真模型,通过开展压缩比为9~16区间的外特性仿真模拟,得出压缩比为12时外特性最优。在压缩比为12的3 500r/min全负荷工况,采用化学反应动力学离子分析法,通过数值模拟分析3类基于活塞头部形貌方案的燃烧室,得出具有点火驻涡区域、气门避障区域、驻涡与避障区域之间的连通区域、后部连通区域的SABCD方案抗爆性最优,并指出活塞头部形貌特征中的连通区域对爆震敏感性具有显著影响。通过对方案SABCD的连通区域关键参数进行优化得出,当区域连通宽度和连通台阶高度均为4mm时爆震敏感性最低。研究结果表明,通过对活塞头部形貌特征的合理设计,能实现提升汽油机压缩比的同时有效抑制燃烧室对爆震的敏感性。     

压缩比对增压柴油机低负荷经济性的影响

以4108增压柴油机为研究对象,采用AVL-boost发动机性能仿真软件分析了压缩比对增压柴油机气缸压力和低负荷经济性能的影响。研究结果表明,提高增压柴油机的几何压缩比能够有效改善低负荷工况的经济性,压缩比由17.5提高到20.0时,在25%负荷下,指示燃油消耗率降低约2.2%,在15%负荷下,指示燃油消耗率降低约2.67%;提高增压柴油机几何压缩比的程度,受限于高负荷工况运行的可靠性。     

增压直喷汽油机进气歧管喷水技术试验研究

针对1台增压直喷汽油机在低速全负荷工况进行了台架试验,并利用自行搭建的供水系统向发动机进气歧管内喷入一定压力的水量,通过降低缸内燃烧温度抑制爆震强度,最终探究了进气歧管喷水技术对改善燃油经济性和排放的潜力。结果表明:进气歧管喷水的最佳喷水时刻位于进气门打开前后一小段区间内,此时喷水使得同一爆震强度的点火角提前、燃烧相位改善、燃烧持续期延长、排温降低、有效燃油消耗率减小、颗粒物数量(PN)与NOx的排放降低,并且在20%～80%喷水比例范围内,喷水量越大上述影响越显著。     

米勒循环在小型增压汽油机典型工况的应用研究

针对某3缸增压小型化汽油机,基于一维和三维发动机循环仿真,分别研究了进气门早关(EIVC)和进气门晚关(LIVC)两种米勒循环形式对发动机油耗、爆震及燃烧特性的影响。结果表明:两种米勒循环形式均能有效降低全工况范围的燃油消耗率,平均降幅约为8%。与LIVC相比,低负荷时EIVC具有更大的节气门开度和进气压力,故泵气损失相对较小,具有更好的燃油经济性;而在高负荷区域,EIVC会显著降低缸内滚流和压缩终了时的湍动能,导致燃烧变慢,CA50相比LIVC滞后2.5°曲轴转角,燃油经济性和爆震抑制作用较差。     

ATKINSON循环汽油机热力学性能模拟分析

重庆凯特动力某款自然吸气发动机利用RICARDO-WAVE软件建立发动机仿真模型,用实验数据验证发动机仿真模型的合理性。根据Atkinson循环原理,以原机模型为基础对发动机的配气相位及压缩比优化计算。计算结果表明:优化后的发动机满足外特性扭矩降低不超过原机9%的要求,特征点部分负荷燃油消耗率最大下降10.5%左右。     

高滚流Atkinson循环燃烧系统研究

为进一步提高发动机热效率,提出高滚流Atkinson循环燃烧系统概念。其特征是采用高滚流气道和活塞组合,配合Atkinson循环和废气再循环(EGR)技术,提高缸内的滚流和湍流水平,加快燃烧速度,同时降低汽油机爆震倾向。利用GT-Power和AVL FIRE软件针对某型汽油机进行了一维整机工作过程和三维计算流体动力学(CFD)模拟分析。结果表明:高滚流气道有利于促进缸内滚流运动,滚流比由原机的0.5提高到2.6;配合高滚流活塞后,使进气过程中产生的缸内初始滚流和压缩过程中的滚流维持作用都比原机有所增强,湍动能水平提升6.3%,瞬时放热率与原机相比平均提高15%;在此基础上,采用进气门晚关的方式实现Atkinson循环,并增加EGR系统,降低高压缩比带来的爆震倾向,比油耗在整个万有特性中均呈下降趋势,最低比油耗区明显变大,最低比油耗相比原机下降11.3g/(kW·h)。     

基于虚拟标定技术研究柴油机燃油消耗率和NOx排放的鲁棒性

在虚拟标定台架上进行了涡轮增压器废气旁通阀门开启特性和文丘里管压差信号偏移对柴油机燃油消耗率和氮氧化物排放影响的研究,揭示了上述两种因素分别作用下的发动机性能排放的鲁棒性.结果表明:大负荷工况下,废气旁通阀门开启特性正向偏移使泵气损失增大,燃油消耗率显著恶化,在偏移量为10 kPa时,全球统一重型发动机瞬态试验循环(WHTC)平均燃油消耗率相比标准状态升高0.68%;文丘里管压差信号偏移导致实际再循环废气流量变化,发动机原机(不加装后处理系统)氮氧排放变化显著.压差信号偏移量与瞬态测试循环平均燃油消耗率与氮氧排放呈线性变化规律,其中燃油消耗率变化随偏移量正向增大而降低,在偏移量为4 kPa时燃油消耗率相比标准状态降低0.23%;氮氧化物排放与压差信号偏移量呈线性正相关关系,偏移量为4 kPa时循环平均氮氧化物排放升高8.61%.     

Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.     

Contents in Volume 23,2014

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Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

NEXT GENERATION ENGINEERED MATERIALS FOR ULTRA SUPERCRITICAL STEAM TURBINES

正1 ABSTRACT To reduce the effect of global warming on our climate,the levels of CO2emissions should be reduced.One way to do this is to increase the efficiency of electricity production from fossil fuels.This will in turn reduce the amount of CO2emissions for a given power output.Using US practice for efficiency calculations,then a move from a typical US plant running at 37%efficiency to a 760℃/38.5 MPa(1 400/5 580 psi)plant running at 48%efficiency would reduce CO2emissions by 170kg/MW.hr or 25%.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Aerodynamic performance effects of leading-edge geometry in gas-turbine blades

The purpose of this paper is to illustrate the advantages of the direct surface-curvature distribution blade-design method, originally proposed by Korakianitis, for the leading-edge design of turbine blades, and by extension for other types of airfoil shapes. The leading edge shape is critical in the blade design process, and it is quite difficult to completely control with inverse, semi-inverse or other direct-design methods. The blade-design method is briefly reviewed, and then the effort is concentrated on smoothly blending the leading edge shape (circle or ellipse, etc.) with the main part of the blade surface, in a manner that avoids leading-edge flow-disturbance and flow-separation regions. Specifically in the leading edge region we return to the second-order (parabolic) construction line coupled with a revised smoothing equation between the leading-edge shape and the main part of the blade. The Hodson–Dominy blade has been used as an example to show the ability of this blade-design method to remove leading-edge separation bubbles in gas turbine blades and other airfoil shapes that have very sharp changes in curvature near the leading edge. An additional gas turbine blade example has been used to illustrate the ability of this method to design leading edge shapes that avoid leading-edge separation bubbles at off-design conditions. This gas turbine blade example has inlet flow angle 0°, outlet flow angle −64.3°, and tangential lift coefficient 1.045, in a region of parameters where the leading edge shape is critical for the overall blade performance. Computed results at incidences of −10°,   −5°,   +5°,   +10° are used to illustrate the complete removal of leading edge flow-disturbance regions, thus minimizing the possibility of leading-edge separation bubbles, while concurrently minimizing the stagnation pressure drop from inlet to outlet. These results using two difficult example cases of leading edge geometries illustrate the superiority and utility of this blade-design method when compared with other direct or inverse blade-design methods.     

Exergetic evaluation of 5 biowastes-to-biofuels routes via gasification

This paper presents the exergy analysis results for the production of several biofuels, i.e., SNG (synthetic natural gas), methanol, Fischer–Tropsch fuels, hydrogen, as well as heat and electricity, from several biowastes generated in the Dutch province of Friesland, selected as one of the typical European regions. Biowastes have been classified in 5 virtual streams according to their ultimate and proximate analysis. All production chains have been modeled in Aspen Plus in order to analyze their technical performance. The common steps for all the production chains are: pre-treatment, gasification, gas cleaning, water–gas-shift reactions, catalytic reactors, final gas separation and upgrading. Optionally a gas turbine and steam turbines are used to produce heat and electricity from unconverted gas and heat removal, respectively. The results show that, in terms of mass conversion, methanol production seems to be the most efficient process for all the biowastes. SNG synthesis is preferred when exergetic efficiency is the objective parameter, but hydrogen process is more efficient when the performance is analyzed by means of the 1st Law of Thermodynamics. The main exergy losses account for the gasification section, except in the electricity and heat production chain, where the combined cycle is less efficient.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO_x) emissions, while producing lower emissions of carbon dioxide (CO₂), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO_x emissions. High NO_x emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas-hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO_x and CO₂ emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy – V. Reactor modeling,optimization, and scale-up

A chemical reactor for the steam-gasification of carbonaceous particles (e.g. coal, coke) is considered for using concentrated solar radiation as the energy source of high-temperature process heat. A two-phase reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is applied to optimize the reactor geometrical configuration and operational parameters (feedstock's initial particle size, feeding rates, and solar power input) for maximum reaction extent and solar-to-chemical energy conversion efficiency of a 5 kW prototype reactor and its scale-up to 300 kW. For the 300 kW reactor, complete reaction extent is predicted for an initial feedstock particle size up to 35 μm at residence times of less than 10 s and peak temperatures of 1818 K, yielding high-quality syngas with a calorific content that has been solar-upgraded by 19% over that of the petcoke gasified.