涡轮增压柴油机EGR—VGT协调控制器的设计与仿真

针对柴油机废气再循环（EGR）和可变几何截面涡轮增压器（VGT）控制回路存在的强烈耦合作用,讨论了EGR—VGT的协调控制问题。根据不同控制器对排放的影响,基于平均值模型,通过对被控对象的控制特性分析,选择与排放密切相关的氧气／燃油比和EGR率作为反馈变量,建立了EGR—VGT协调控制器。在MATLAB／Simulink环境下,通过仿真验证了所建立的控制器的瞬态控制效果。结果表明,被控变量能够很好地跟踪其目标值,未出现明显的超调,稳态误差控制在5％以内,瞬态响应时间小于1S。     

不同EGR闭环控制方式应用研究

基于不同废气再循环(exhaust gas recirculation, EGR)闭环控制方式的测量原理和控制策略,通过台架试验,对比稳态新鲜空气测量精度和瞬态排放,分析不同控制方式的控制精度及瞬态排放差异。结果表明：基于新鲜空气量和EGR率的2种闭环控制方式精度均较高,新鲜空气测量相对偏差分别小于±3%、±5%;基于废气氧浓度的闭环控制在低负荷区域的测量相对偏差最大达-10%;基于新鲜空气量的闭环控制在瞬态测试循环中EGR阀关闭速率快,NO_x排放出现峰值;基于文丘里流量计的EGR率闭环控制可以有效兼顾新鲜空气和EGR废气流量,在满足柴油机动力性的同时有效控制排放污染物。     

增压柴油机瞬态排放仿真平台

提出了针对于欧洲瞬态测试循环(ETC)的典型工况建模思想,并引入复合瞬变率作为输入变量建立了瞬态排放神经网络模型,建立了增压柴油机瞬态工况全程仿真平台.通过对不同EGR率下十三工况排放和不同瞬变率变负荷工况排放的模拟,验证了该仿真平台的准确性和泛化能力,其稳、瞬态排放验证性模拟差异率分别低于3%和4%,瞬态排放泛化性模拟趋势相同,峰值差异率在8%以内.该平台完全适用于以控制策略开发为目的 ETC典型工况的瞬态模拟.     

EGR对涡轮增压柴油机瞬态特性的影响

通过一台电控单体泵柴油机进行定转速瞬态加载和定负荷瞬态加速试验,分析不同废气再循环(EGR)阀开度下柴油机重要性能参数在瞬态工况时的变化趋势.结果表明:在瞬态加载过程中,由于增压器响应滞后以及进/排气的相互影响,进气滞后现象严重,空燃比下降且碳烟(soot)排放值升高;在瞬态加速过程中,进气流量滞后现象不明显,由于油量超调引起过量空气系数减小,使得soot排放量增多;在整个瞬态试验过程中,氮氧化物(NOx)排放变化趋势与进气氧体积分数变化趋势的相关程度高于EGR率.在瞬态工况下,由于进气状态发生变化,传统的基于EGR率的EGR控制方法已不适用,需要研究新的柴油机瞬态控制策略.     

可变增量的EGR控制策略对柴油机瞬态排放的影响

针对涡轮增压柴油发动机在瞬态工况下因涡轮延迟现象所导致的碳氢化合物(THC)和颗粒物(PM)排放相对较高的问题,研究瞬态工况下废气再循环(EGR)变化率与发动机动力性能和排放性能之间的变化规律,搭建基于AVLPUMA系统的柴油发动机瞬态排放测试平台,监测瞬态工况下EGR变化率、进气质量流量与NOx、PM和THC排放物之间的变化关系,在此基础上提出一种可变增量的EGR控制策略.结果表明:通过该策略能够快速有效地降低发动机的喷油量,提高新鲜空气进入气缸的质量流量,提高瞬态空燃比,改善燃烧,最大程度地降低涡轮延迟现象对发动机造成的不利影响;在试验工况条件下,使用该策略可使累计颗粒物排放最高降低83.5%,.     

各缸废气再循环率不均匀性对柴油机排放影响的试验研究

针对一台由国五升级到国六的重型柴油机原始排放(无后处理系统)超过目标设定值的问题进行研究,提出了一种通过控制各缸废气再循环(exhaust gas recirculation,EGR)率不均匀性来降低柴油机原始排放的方法。建立了柴油机各缸EGR率不均匀性的排放测试试验台架和测试方法,通过分析排放万有特性曲线图确定所研究的进气总管EGR率和柴油机运行工况点,用CO2法在稳态工况下测量各缸EGR率,分析各缸EGR率不均匀对排放性能的影响,确定了达到企业原始排放目标设定值的各缸EGR率不均匀性范围,实现了通过控制各缸EGR率不均匀性来降低柴油机原始排放及降低满足国六排放法规后处理系统匹配难度和成本的目的。     

废气再循环对二甲醚/甲醇均质压燃燃烧过程影响的试验研究

在单缸柴油机上进行了冷却废气再循环(EGR)对二甲醚(DME)/甲醇均质压燃(HCCI)燃烧过程影响的试验研究。结果表明,EGR对拓宽二甲醚/甲醇HCCI发动机的最大负荷作用不大;随着EGR率增大,主燃烧开始时刻和放热峰值明显后移,主燃烧持续期延长,放热峰值降低。EGR率为25%时的最大爆发压力比没有EGR时降低了近1.3 MPa,最大爆发压力出现的位置推迟了7°CA;EGR率增大,二甲醚/甲醇HCCI发动机的指示热效率升高。对应给定的EGR率,存在一个热效率较高的DME比例区间;HC和CO排放随EGR率的增大而增加,随DME比例的增加而降低,NOx排放接近于零。控制EGR率和DME比例是控制二甲醚/甲醇HCCI发动机燃烧过程、性能和排放的关键。     

采用VNT/EGR和DOC降低柴油机排放的试验

在一台高压共轨柴油机上进行了采用基于可变喷嘴环涡轮增压器(VNT)的废气再循环(EGR)系统(VNT/EGR)和柴油机氧化催化器(DOC)降低排放的试验研究。结果表明,通过调节VNT开度可以有效实现EGR循环和控制EGR率,显著降低NOx排放,并且在一定EGR率范围内对烟度影响较小。提高喷射压力可以较明显改善NOx和烟度的折衷关系,采用合理的后喷参数能够在不明显影响经济性和NOx排放的前提下显著降低烟度。对燃烧室进行降低压缩比的合理设计也可以明显改善NOx和烟度的折衷关系。在燃烧系统优化的基础上,最终加装DOC的ESC循环排放测试结果满足了国Ⅳ要求,其中PM为0.0191g/(kW·h),NOx为2.88g/(kW·h),HC和CO排放远低于国Ⅳ限值。     

柴油机EGR系统反馈控制参数研究

基于GT-Power软件建立了YC4E170-31型涡轮增压柴油机的仿真模型,并通过对比试验数据校核了仿真模型。在分析废气再循环(exhaust gas recirculation,EGR)对柴油机进气状态影响基础上,利用构建的仿真模型,研究了不同进气成分下热效应、稀释效应、化学效应对柴油机NOx排放的影响,对比分析了分别选择进气氧气质量分数和EGR率作为控制目标时,NOx排放随进气流量变化的趋势。结果表明:当进气流量一定时,氧气质量分数是影响NOx生成的主要参数,稀释效应对NOx排放的影响程度远高于热力及化学作用;瞬态工况下,选取EGR率作为EGR的控制参数是有局限性的,进气氧气质量分数与NOx排放的相关程度更高,更适合作为反映NOx排放生成的控制参数。     

国Ⅳ柴油机EGR的控制与匹配研究

废气再循环(EGR)技术,作为一种成熟的排放控制策略,在国Ⅳ排放柴油机上已有广泛的应用,以某电控共轨柴油机为例介绍了EGR阀的结构和工作原理,以及在应用该技术的过程中如何实现EGR率的控制和EGR脉谱图的标定,通过试验对EGR系统进行了验证,表明该电控共轨柴油机能够达到国Ⅳ排放要求。     

Matching of a DC motor to a photovoltaic generator using a step-upconverter with a current-locked loop

A photovoltaic (PV) generator is a nonlinear device having insolation-dependent volt-ampere characteristics. Because of its relatively high cost, the system designer is interested in optimum matching of the motor and its mechanical load to the PV generator so that maximum power is obtained during the entire operating period. However, since the maximum-power point varies with solar insolation, it is difficult to achieve an optimum matching that is valid for all insolation levels. In this paper it is shown that for maximum power, the generator current must be directly proportional to insolation. This remarkable property is utilized to achieve insolation-independent optimum matching. A shunt DC motor driving a centrifugal water pump is supplied from a PV generator via a step-up converter whose duty ratio is controlled using a current-locked feedback loop     

Natural convection of a non-Newtonian fluid about a horizontal cylinder and a sphere in a porous medium

The problem of natural convection of a non-Newtonian fluid about a horizontal isothermal cylinder and an isothermal sphere in the porous medium is considered. The present study is based on the boundary layer approximation and only suitable for a high Rayleigh number. Similarity solutions are obtained by using the fourth order Runge-Kutta method. The effects of the wall temperature T_W and the new power-law index n on the characteristics of heat transfer are discussed.     

Coupling a two-temperature model and a one-temperature model at a fluid-porous interface

We study a convective heat transfer problem in a fluid-porous domain in the case of the local thermal non-equilibrium assumption (LTNE). The issue of this study is to determine appropriate boundary conditions to model heat transfer, while using models with a different number of equations: a two-temperature model in the homogeneous porous region versus a one-temperature model in the free region. To proceed, a two-step up-scaling approach is used, which has the particularity to provide closed jump relations depending on intrinsic characteristic of the interface. Thus, the use of jump or continuity conditions depend only on the interface location inside the fluid-porous transition region. The pertinence of the approach is illustrated on a 2D convective heat transfer problem considering a solid heat source in the porous medium.     

Studies on a two-staged micro-combustor for a micro-reformer integrated with a micro-evaporator

A new micro-combustor configuration for a micro fuel-cell reformer integrated with a micro-evaporator is studied experimentally and computationally. The micro-combustor as a heat source is designed for a 10–15 W micro-reformer using the steam reforming method. In order to satisfy the primary requirements for designing a micro-combustor integrated with a micro-evaporator, i.e., stable burning in a small confinement and maximum heat transfer through a wall, the present micro-combustor is a simply cylinder, which is easy to fabricate, but is two-staged (expanding downstream) to control ignition and stable burning. The aspect ratio and wall thickness of the micro-combustor substantially affect ignition and thermal characteristics. For optimized design conditions, a pre-mixed micro-flame is easily ignited in the expanded second-stage combustor, moves into the smaller first-stage combustor, and finally is stabilized therein. The measured and predicted temperature distributions across the micro-combustor walls indicate that heat generated in the micro-combustor is well transferred. Thus, the present micro-combustor configuration can be applied to practical micro-reformers integrated with a micro-evaporator for use with fuel cells.     

Using a fin with a parabolic concentrator

The consequences of using a fin collector in focusing solar collectors is examined and is found to have merits.     

Bioconvection in a squeezing flow of a micropolar fluid in a horizontal channel

The bioconvection flow of an incompressible micropolar fluid containing microorganisms between two infinite stretchable parallel plates is considered. A mathematical model, with a fully coupled nonlinear system of equations describing the total mass, momentum, thermal energy, mass diffusion, and microorganisms is presented. The governing equations are reduced to a set of nonlinear ordinary differential equations with the help of suitable transformations. The resulting nonlinear ordinary differential equations are linearized using successive linearization method, and the resulting system of linear equations is solved using the Chebyshev collocation method. The detailed analysis illustrating the influences of various physical parameters, such as the micropolar coupling number, squeezing parameter, the bioconvection Schmidt number, Prandtl numbers, Lewis number, and bioconvection Peclet number on the velocity, microrotation, temperature, concentration and motile microorganism distributions, skin friction coefficient, Nusselt number, Sherwood number, and density number of motile microorganism, is examined. The influence of the squeezing parameter is to increase the dimensionless velocities and temperature and to decrease the local Nusselt number and local Sherwood number. The density number of motile microorganism is decreasing with squeezing parameter, bioconvection Lewis number, bioconvection Peclet number, and bioconvection Schmidt number.     

Exploration of heat transfer effect in a magnetohydrodynamics flow of a micropolar fluid between a porous and a nonporous disk

An analysis is carried out for the flow characteristics of a conducting micropolar fluid. The fluid was passed in between two parallel disks of infinite radii. The novelty of the study is to consider one of the disks as porous and the other one as nonporous, and the external magnetic field is applied along the transverse direction of the flow. The flow phenomena for the polar fluid characterized by the magnetic effect in conjunction with the temperature equation reduce to a set of coupled nonlinear ordinary differential equations using the requisite transformations and nondimensionalization. An analytical approach such as the variation parameter method is employed to tackle the system efficiently. To emphasize the effect of various physical parameters contributing to the flow phenomena, that is, non-zero tangential slip, Reynolds number, Prandtl number, magnetic parameter, and material parameter on the flow profiles of axial and radial velocities, the microrotation and temperature profiles are presented graphically. To validate the simulated results, a comparison with established results is made, and it is concluded that both are in good correlation.     

Development of a system model for a hydrogen production process on a solar tower

An attractive path to the production of hydrogen from water is a two-step thermo chemical cycle powered by concentrated sunlight from a solar tower system. In the first process step the redox system, a ferrite coated on a monolithic honeycomb absorber, is present in its reduced form while the concentrated solar energy hits the ceramic absorber. When water vapour is fed to the honeycomb at 800 °C, oxygen is abstracted from the water molecules, bond in the redox system and hydrogen is produced. When the metal oxide system is completely oxidised it is heated up for regeneration at 1100–1200 °C in an oxygen-lean atmosphere. Under those conditions and in the second process step, oxygen is set free from the redox system, so the metal oxide is being reduced and after completion of the reaction again capable for water splitting.Since the overall process consists of two core reaction steps, which need to be carried out sequentially in a reactor unit at two different temperature steps, a special process and plant concept had to be developed enabling the continuous supply of product regardless of the alternating nature of the solar reactor operation. The challenge of the process control is to keep the two core reaction temperatures constant and to ensure regular temperature switches after completion of the individual process steps, independent of the weather conditions, like DNI fluctuation, clouds and wind speed. Also start-up, the fast switching after completion of half-cycles and the shutdown must be controlled. State of the art is the manual switching of heliostats to fulfil those control tasks.This paper describes the development and use of a system model of this process. The model consists of three main parts: the simulation of the solar flux distribution at the receiver aperture, the simulation of the temperatures in the reactor modules and the simulation of the hydrogen generation. It can be used for the analysis of the operational behaviour. The model is intended to be used in the future for the control of the whole process.