凸轮轴负载扭矩计算与虚拟止回控制策略研究

当代汽油机普遍配置了双顶置凸轮轴(DOHC)的可变气门正时(VVT)系统,发动机工作时配气机构对凸轮轴产生的负载扭矩会影响VVT相位器的调节速度。本文通过分析凸轮轴的受力状况,建立了凸轮轴负载扭矩的数学模型,基于Matlab/Simulink的仿真计算,确定了凸轮轴负载扭矩随转速的变化规律。针对发动机低速时凸轮轴负载扭矩对相位器调节的阻碍作用,开发了虚拟止回控制策略,基于凸轮轴负载扭矩正、负切换的基准角和机油流量特性,确定占空比切换时刻和虚拟止回占空比数值。目标相位提前和滞后的仿真结果表明,设定值从5°CA到45°CA和从45°CA到5°CA阶跃变化时,相位器调节速率分别提高25%和34%。     

141F小型汽油机数值模拟及性能优化

针对141F汽油机性能提升的研究,采用发动机循环模拟数值方法研究并预测了配气相位优化前后发动机扭矩的变化规律。分别设计了进排气整体气门正时、进气气门正时和排气气门正时三种优化方案。数值模拟结果表明:141F汽油机整体气门正时提前15℃A时,扭矩整体最优,扭矩在低速有6%的提高,高速有2%~4%的提升;进气气门正时提前20℃A扭矩整体最优,扭矩低速有7%~8%左右的提高,高速有2%的提升。     

发动机可变长度进气歧管系统的优化设计研究

以发动机进气歧管长度、直径和不同长度的进气歧管切换的发动机转速等为设计变量,以发动机进气系统的充气效率、低速扭矩及其转速、最大扭矩及其转速和最大功率等为优化目标,应用发动机热力学和性能分析商用软件GT-POWER进行多目标优化仿真计算,确定优化设计方案;然后制作快速成型样件,通过试验验证计算结果,从而完成发动机可变进气歧管的优化设计。     

利用可变气门相位提高车用发动机性能的研究

应用WAVE整机性能分析软件研究了某汽油机的可变气门相位对发动机性能的影响,提出了优化的进、排气相位.在此基础上,进行了不同进、排气相位策略对发动机动力性、经济性和排放影响的试验研究.研究着重分析在外特性及2 000 r/min负荷特性下采用可变气门相位技术对发动机性能的影响.研究结果表明,采用可变气门相位技术,不但提高了低速扭矩和最大功率,还改善了低速、部分负荷时的燃油经济性.     

基于BOOST的摩托车发动机进气系统优化

针对某款摩托车发动机在特定市场需求高速性能的情况下,运用性能先进模拟软件BOOST对该机进行了分析,找出了影响原机高速性能的主要原因。通过优化进气系统的结构,提高了发动机高速性能。对优化后方案进行了台架试验,高速性能提高11％,中低速性能基本不变,达到预期效果。     

气门间隙对发动机性能的影响研究

某四缸自然吸气汽油发动机怠速时燃烧不稳定,经热力学性能仿真计算发现怠速时气门重叠角偏大,导致残余废气率较高,从而引起燃烧不稳定及循环变动大。可通过调整进气凸轮轴安装相位或增大气门间隙来推迟进气门开启时刻,减小气门重叠角。本文对加大气门间隙方案做换气过程分析计算,加大气门间隙,气门重叠角减小。在低速时,小的气门重叠角有利于减小废气倒流,提高进气量;但在高速时进气惯性大,由于加大气门间隙,减小了进气迟闭角,造成高速时进气量减小。试验结果验证仿真分析的正确性。试验结果表明,减小气门重叠角可显著改善怠速工况的循环变动和排放,中低速工况的扭矩也有所提升,但高速段功率稍有下降。     

非同步气门正时对高速汽油机进气及燃烧过程的影响

结合某高速进气道喷射五气门汽油机,采用CFD技术和台架试验手段研究了同步与非同步气门正时对高速汽油机进气和燃烧过程的影响。CFD分析结果表明:非同步气门正时能够加强缸内气体流动,提高点火时刻的缸内湍动能,有利于火焰的快速传播,提高燃烧速率。试验结果表明:进气门2提前20°开启,比其他进气门开启时刻的扭矩提高了约3.1%,燃油消耗率降低了约4.2%;进气门2推迟20°开启燃油消耗率降低了约2.6%,发动机扭矩略微降低。非同步气门正时下,发动机HC和CO排放量均降低,进气门2提前20°开启由于缸内温度较高,NO_x排放量增加了约11.3%。进气门2推迟20°开启由于缸内进气量较低,NO_x排放量降低了约6.7%。     

军用装备发动机最佳供油提前角的试验研究

讨论了供油提前角对发动机动力性、经济性和排放性能的重要性,利用外特性试验、负荷特性试验和转速稳定性试验,考察了供油提前角对发动机工况的影响。研究结果表明,对于12150ZLD发动机来讲,供油提前角对动力性影响较大,功率和扭矩的最大差距分别为7.6%和7.5%,油耗率最大相差6.6%。其中供油提前角为26°时,发动机运转正常,功率、扭矩最高,油耗率最低,气缸的最大爆发压力符合要求,为该型发动机的最佳供油提前角。为保证装备长期运行或更换燃料后仍发挥最优性能,应及时调整供油提前角。     

天然气发动机增压器性能分析与匹配优化研究

针对目前天然气发动机增压器低速扭矩不足,低速和部分负荷下经济性差,瞬态响应迟钝的不良现状,基于一台10L采用混合进气的天然气发动机,研究分析了其废气涡轮增压器性能,并结合试验数据分析结果,采用软件分析手段,提出了优化匹配方案。     

凸轮轴型线设计对PFI汽油机低速性能影响的试验研究

凸轮轴是发动机进气系统的重要组成部分,不同型线直接影响发动机的性能和排放水平.本文通过对PFI增压汽油机进行不同型线的凸轮轴计算分析和试验验证,研究了凸轮轴型线对发动机性能及燃烧特性的影响.试验表明,适当减小气门重叠角可以有效提高低速扭矩,但是燃烧变差,适当增大进气门升程及包角,可以增加扫气的同时提高燃油雾化效果,改善燃烧降低油耗,但对扭矩无明显影响.     

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.     

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%.     

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.     

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.     

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.     

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.     

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.     

Assessment methods of material ageing using Sdobyrev''s creep rupture model

The physical aspects of the activation energy, in higher and high temperatures, of the metal creep process were examined. The research results of creep-rupture in a uniaxial stress state and the criterion of creep-rupture in biaxial stress states, at two temperatures, are then presented. For these studies creep-rupture, taking case iron as an example the energy and pseudoenergy activation was determined. For complex stress states the criterion of creep-rupture was taken to be Sdobyrev's, i.e. σ_red = σ₁ β + (1 − β)σ_i, where: σ₁-maximal principal stress, σ_i-stress intensity, β-material constant (at variable temperature β = β(T)). The methods of assessment of the material ageing grade are given in percentages of ageing of new material in the following mechanical properties: 1) creep strength in uniaxial stress state, 2) activation energy in uniaxial stress state, 3) criterion creep strength in complex stress states, 4) activation pseudoenergy in complex stress states. The methods 1) and 3) are the relatively simplest because they result from experimental investigations only at nominal temperature of the structure work, however, for methods 2) and 4) it is necessary to perform the experimental investigations at least at two temperatures.     

Production of hydrogen from sewage biosolids in a continuously fed bioreactor: Effect of hydraulic retention time and sparging

Hydrogen was produced from primary sewage biosolids via mesophilic anaerobic fermentation in a continuously fed bioreactor. Prior to fermentation the sewage biosolids were heated to 70 °C for 1 h to inactivate methanogens and during fermentation a cellulose degrading enzyme was added to improve substrate availability. Hydraulic retention times (HRT) of 18, 24, 36 and 48 h were evaluated for the duration of hydrogen production. Without sparging a hydraulic retention time of 24 h resulted in the longest period of hydrogen production (3 days), during which a hydrogen yield of 21.9 L H₂ kg⁻¹ VS added to the bioreactor was achieved. Methods of preventing the decline of hydrogen production during continuous fermentation were evaluated. Of the techniques evaluated using nitrogen gas to sparge the bioreactor contents proved to be more effective than flushing just the headspace of the bioreactor. Sparging at 0.06 L L min⁻¹ successfully prevented a decline in hydrogen production and resulted in a yield of 27.0  L H₂ kg⁻¹ VS added, over a period of greater than 12 days or 12 HRT. The use of sparging also delayed the build up of acetic acid in the bioreactor, suggesting that it serves to inhibit homoacetogenesis and thus maintain hydrogen production.     

汽轮机数字电液调节系统挂闸异常的技术完善

分析了200MW汽轮机数字电液调节系统在运行中存在的挂闸异常问题,采取了相应的技术处理措施,且运行实践效果良好。