Numerical investigation of cyclic variations in gasoline engines using a hybrid URANS/LES modeling approach

Cycle to cycle variations are an important aspect in the development and optimization process of internal combustion engines. In this study the feasibility of using a detached eddy simulation (DES) SST model, which is a hybrid URANS/LES model, to predict cycle to cycle variations is investigated. In the near wall region or in regions where the grid resolution is not sufficiently fine to resolve smaller structures, the two-equation RANS shear-stress transport (SST) model is used. In the other regions with higher grid resolution an LES model is applied. First, the numerical requirements associated with the hybrid URANS/LES and the employed solver are studied in detail. The numerical dissipation of the spatial scheme and the choice of the temporal scheme including the step size are evaluated. In addition, the accuracy of the solver for moving meshes, which are required for engine calculations, is assessed. The modeling constant linking the grid size to the DES filter length scale is determined by calculating a decaying homogeneous isotropic turbulence test case for different grid resolutions. The final applications of the model are two different engine cases with increasing complexity. The first case is the statistically stationary flow through an engine intake port. The time resolved flow structure predicted by the DES SST model is analyzed and the resulting time-averaged velocity fields are compared to experimental data at different locations. The second application is a motored multi-cycle simulation of a series production engine. The instantaneous flow development during the intake and compression stroke of one single cycle is studied and the ensemble-averaged and the instantaneous velocity fields as well as the resolved velocity fluctuations are compared to optical measurements. Special emphasis is placed on the cyclic differences of the velocity fluctuations at the time of ignition in the vicinity of the spark plug and the expected influence on the combustion process.     

内燃机中吸气和压缩过程的二维模拟

进行了发动机中空气吸气和压缩过程的二维计算机模拟．使用二维正交曲线坐标系以考虑燃烧室的复杂形状．采用SIMPLEC算法求解流体力学模型方程．论文中分别采用4个湍流模型：标准的k-ε模型,Realizable k-ε模型,雷诺应力模型和亚网格尺度模型．论文中对模拟计算的预测值进行了分析．计算出的流场与实验测定值进行了对比．通过模拟计算得到了内燃机中重要的流动图象．     

Robust oxygen fraction estimation for conventional and premixed charge compression ignition engines with variable valve actuation

In-cylinder oxygen fraction serves as a critical control input to advanced combustion strategies, but is extremely difficult to measure on production engines. Fortunately, the in-cylinder oxygen levels can be estimated based on accurate estimates or measurements of the oxygen fraction in the intake and exhaust manifolds, the in-cylinder charge mass, and the residual mass. This paper outlines such a physically based, generalizable strategy to estimate the in-cylinder oxygen fraction from only production viable measurements or estimates of exhaust oxygen fraction, fresh air flow, charge flow, fuel flow, turbine flow and EGR flow. While several of these flows are accurately measured or estimated, significant errors in the turbine and EGR flows are commonly observed and can highly degrade the accuracy of any calculations which utilize these flows. An EGR flow estimator was developed to improve the accuracy of this flow measurement over the stock engine control module (ECM) method and is detailed in this paper. Furthermore, the in-cylinder oxygen estimation algorithm is developed, and proven, to be robust to turbine flow errors. Regulation of in-cylinder oxygen levels is of interest for not only in conventional combustion modes but also in advanced combustion strategies such as premixed charge compression ignition. The proposed oxygen fraction estimator is designed such that its performance and stability is ensured in both conventional and advanced combustion modes. The model-based observer estimates the oxygen fractions to be within 0.5% O₂ and is shown to have exponential estimator error convergence with a time constant less than 0.05 s, even with turbine flow errors of up to 25%.     

Real-time control oriented HCCI engine cycle-to-cycle dynamic modelling

For homogeneous charge compression ignition (HCCI) combustion, the auto-ignition process is very sensitive to in-cylinder conditions, including in-cylinder temperature, in-cylinder components and concentrations. Therefore, accurate control is required for reliable and efficient HCCI combustion. This paper outlines a simplified gasoline-fueled HCCI engine model implemented in Simulink environment. The model is able to run in real-time and with fixed simulation steps with the aim of cycle-to-cycle control and hardware-in-the-loop simulation. With the aim of controlling the desired amount of the trapped exhaust gas recirculation (EGR) from the previous cycle, the phase of the intake and exhaust valves and the respective profiles are designed to vary in this model. The model is able to anticipate the auto-ignition timing and the in-cylinder pressure and temperature. The validation has been conducted using a comparison of the experimental results on Ricardo Hydro engine published in a research by Tianjin University and a JAGUAR V6 HCCI test engine at the University of Birmingham. The comparison shows the typical HCCI combustion and a fair agreement between the simulation and experimental results.     

内燃机缸内复杂空间三维动态网格生成技术

将组合网格拆解为静态和动态2个区域,提出了在活塞和气门运动方向上拉伸和压缩网格空间的算法．实验结果表明：3D动态网格可以应用于内燃机缸内工作过程数值模拟．     

Engine for brain development: similarity between engine and brain

A theoretical model that simulates the developmental process of the human brain, including the blood vessels, is proposed. Observations of the development of the human brain with a high-speed camera show that the bones of the skull become increasingly large over the neck, and that a lot of soup-like fluid for generating brain cells enters the skull from the body. This process is essentially similar to the intake process of an internal combustion engine, because the volume of the engine cylinder, which increases according to the descent of the piston, corresponds geometrically to the development of the skull, and also because the human neck resembles the intake port that serves as the throat of the engine. A higher-order numerical computation of the Navier-Stokes equation reveals the similarity between the convexo-concave forms inside the brain and the flow structure in the internal combustion engine at very low Reynolds numbers. Further, we examine the similarity between the main blood vessels in the brain and the path lines in the engine.     

Numerical Simulation of Pulse Detonation Engine Phenomena

This computational study examines transient, reactive compressible flow phenomena associated with the pulse detonation wave engine. The PDWE is an intermittent combustion engine that relies on unsteady detonation wave propagation for combustion and compression elements of the propulsive cycle. The present computations focus on high order numerical simulations of the generic PDWE configuration with simplified reaction kinetics, so that rapid, straightforward estimates of engine performance may be made. Both one- and two-dimensional simulations of the high speed reactive flow phenomena are performed and compared to determine the applicability of 1D simulations for performance characterization. Examination of the effects of the combustion reaction mechanism and the use of a pressure relaxation length for 1D simulations is made. Characteristic engine performance parameters, in addition to engine noise estimates within and external to the detonation tube, are presented.     

在线式DPIV系统的开发与应用

介绍了一种自行研制的在线式数字粒子图像测速系统,首先说明这种系统的主要特点,然后从系统的硬件组成和软件结构两个方面进行了介绍.该系统的软件部分是整个系统的研究重点,包括图像采集模块,自相关图像分析模块、互相关图像分析模块以及数据后处理模块等.为了验证该系统的有效性,设计了水流流场分布的测量实验,实验结果表明系统能够提取出合理的水流流场速度分布.最后,利用该系统分析内燃机缸内流场的PIV图像,得到了内燃机缸内流场的流速分布.     

Three-dimensional computer analysis of flow and combustion in automotive internal combustion engines

The paper presents a method for calculating the three-dimensional flow-fields in reciprocating internal combustion engines, as a function of space and time, throughout the complete four-stroke cycle. The method is based on a computational procedure which solves the governing elliptic partial-differential equations on a finite-difference grid which expands and contracts with the motion of the piston, using a fully-implicit, iterative, finite-difference scheme. Results are presented for typical engines, under engine-motoring and spark-ignited conditions. It is concluded that careful physical experiments should now be conducted in parallel with the computer experiments to validate the predictions, before the model can be used directly in assisting engine design; but also that it is now practical, for the first time, to perform fully three-dimensional calculations of the flow within the engine.     

A comprehensive Lagrangian flame–kernel model to predict ignition in SI engines

A Lagrangian model to predict the first stages of the combustion process in SI engines, when the size of flame kernel is small compared with the mesh size, and flame development is influenced by heat transfer from the spark, local flow, turbulence and air/fuel mixture distribution is presented. The spark channel is initially represented by a set of Lagrangian particles that are convected by the mean flow. Flame kernels are launched locally for all the particles satisfying an ignition criterion based on the local Karlovitz number. For each of them, equations of energy and mass are solved accounting for electrical power transferred from the electrical circuit, local turbulence and flame speed. The proposed model has been validated with experimental data provided by Herweg et al.; a computational mesh reproducing the geometrical details of the optical, pre-chamber SI engine was built, including the electrodes. Initially, cold-flow simulations were carried out to verify the validity of the computed flow-field and turbulent distribution at ignition time. Then, the combustion process was simulated accounting for the effects of different engine speeds, air/fuel ratio and spark-plug position. Encouraging results were achieved for a wide range of operating conditions.     

柴油机活塞碗参数化建模方法及在优化设计中的应用

为解决活塞碗排放优化过程中的参数驱动问题,提出一种柴油机活塞碗的参数化建模方法。该方法能够确保在活塞碗轮廓线变形过程中燃烧室的控制容积不变,以满足恒定压缩比的要求。采用CAESES建立活塞碗的参数化模型,在CONVERGE中对活塞碗内的燃烧进行分析,使用CAESES的遗传算法驱动优化流程,通过案例验证该方法的有效性。结果表明：遗传算法对活塞碗进行4代优化后得到的算例最佳,NOx浓度降低66%,Soot浓度降低78%。     

Effect of inlet and outlet boundary conditions on swirling flows

Numerical simulations are conducted for both three-dimensional, turbulent flow in a multi-channel swirler and axisymmetric, isothermal, turbulent flow in combustion chambers using the standard κ−ε turbulence model. Calculations are first carried out for three-dimensional, isothermal and turbulent flow inside the swirler channels in order to derive the velocity profiles of both air and gas at the swirler outlets, which are used as inlet boundary conditions of the model combustor and can also be used in future studies for different combustors with the same type of swirler. In order to study the sensitivity of swirling flow inside the chamber to the inlet and outlet boundary conditions, different inlet velocity profiles and outlet boundary conditions are also employed. The results show that in the cases considered, the flow behaviour in the chamber is not very sensitive to the actual shape of the inlet velocity profiles provided the averages of the inlet axial, radial and azimuthal velocity components are separately preserved. Other conditions being equal, we find that the swirling flow performance in the combustor depends not only on the inlet swirl number, but also strongly on the relative magnitude of the radial velocity component at inlet and introduce a new dimensionless number N_r, analogous to the swirl number, to measure the relative importance of this quantity. Outlet boundary conditions have some influence near the outlet, but nearly no effect further upstream for the cases investigated.     

Simple adaptive air-fuel ratio control of a port injection SI engine with a cylinder pressure sensor

The problem of air-fuel ratio(AFR) control of the port injection spark ignition(SI) engine is still of considerable importance because of stringent demands on emission control. In this paper, the static AFR calculation model based on in-cylinder pressure data and on the adaptive AFR control strategy is presented. The model utilises the intake manifold pressure, engine speed, total heat release, and the rapid burn angle, as input variables for the AFR computation. The combustion parameters, total heat release,and rapid burn angle, are calculated from in-cylinder pressure data. This proposed AFR model can be applied to the virtual lambda sensor for the feedback control system. In practical applications, simple adaptive control(SAC) is applied in conjunction with the AFR model for port-injected fuel control. The experimental results show that the proposed model can estimate the AFR, and the accuracy of the estimated value is applicable to the feedback control system. Additionally, the adaptive controller with the AFR model can be applied to regulate the AFR of the port injection SI engine.     

Engine for cerebral development

A deterministic theoretical model that simulates the developmental process of the human brain is proposed. Observations of the development of the human brain with a high-speed camera show that the bones of the skull become increasingly larger over the neck, and that a lot of soup-like fluid for generating brain cells enters the skull from the body. This process is essentially similar to the intake process of an internal combustion engine, because the volume of the engine’s cylinder, which increases according to the descent of the piston, geometrically corresponds to the development of the skull, and also because the human neck resembles the intake port that serves as the throat of the engine. A higher-order numerical computation of the Navier-Stokes equation reveals the similarity between the convexoconcave forms inside the brain and the flow structure in the internal combustion engine. We will show that the present computation also simulates the emergence of the eyeballs. Finally, we will clarify the reason why cerebral development is strongly influenced by fluid dynamics. This work was presented in part at the First European Workshop on Artificial Life and Robotics, Vienna, Austria, July 12–13, 2007     

液压发动机在市内公交车上的应用探究

简要介绍了液压自由活塞发动机相比于传统内燃机的优越性能，针对液压自由活塞发动机的特点以及公交车运行的特殊性，探究了液压自由活塞发动机在市内公交车上应用的可行性，展现了液压自由活塞发动机的广阔应用前景。     

Verification of RANS solvers with manufactured solutions

This paper discusses code verification of Reynolds-Averaged Navier Stokes (RANS) solvers with the method of manufactured solutions (MMS). Examples of manufactured solutions (MSs) for a two-dimensional, steady, wall-bounded, incompressible, turbulent flow are presented including the specification of the turbulence quantities incorporated in several popular eddy-viscosity turbulence models. A wall-function approach for the MMS is also described. The flexiblity and usefulness of the MS is illustrated with calculations performed in three different exercises: the calculation of the flow field using the manufactured eddy-viscosity; the calculation of the eddy-viscosity using the manufactured velocity field; the calculation of the complete flow field coupling flow and turbulence variables. The results show that the numerical performance of the flow solvers is model dependent and that the solution of the complete problem may exhibit different orders of accuracy than in the exercises with no coupling between the flow and turbulence variables.     

Engine performance and optimum injection timing for 4-cylinder direct injection hydrogen fueled engine

This paper presents the engine performance and optimum injection timing for 4-cylinder direct injection hydrogen fueled engine. The 4-cylinder direct injection hydrogen engine model was developed utilizing the GT-Power commercial software. This model employed one dimensional gas dynamics to represent the flow and heat transfer in the components of engine model. Sequential pulse injectors are adopted to inject hydrogen gas fuel within the compression stroke. Injection timing was varied from 110° before top dead center (BTDC) until top dead center (TDC) timing. Engine speed was varied from 2000 rpm to 6000 rpm, while the equivalence ratio was varied from 0.2 to 1.0. The validation was performed with the existing previous experimental results. The negative effects of the interaction between ignition timing and injection duration was highlighted and clarified. The results showed that optimum injection timing and engine performance are related strongly to the air fuel ratio and engine speed. The acquired results show that the air fuel ratio and engine speed are strongly influence on the optimum injection timing and engine performance. It can be seen that the indicated efficiency increases with increases of AFR while decreases of engine speed. The power and torque increases with the decreases of AFR and engine speed. The indicated specific fuel consumption (ISFC) decreases with increases of AFR from rich conditions to lean while decreases of engine speed. The injection timing of 60° BTDC was the overall optimum injection timing with a compromise.     

基于 AMESim 的航空燃油柱塞泵特性仿真

针对柱塞倾斜安装的某型航空燃油柱塞泵进行运动学分析,首先基于坐标互换法建立柱塞运动方程,然后采用 AMESim 软件构建柱塞泵模型,最后仿真分析柱塞倾角和转子转速对柱塞泵运动特性、流量特性及压力特性的影响。仿真结果表明,相同工况下柱塞倾角对流量的影响小于转子转速,但斜柱塞结构能增大柱塞行程,增加泵的供油量,适度改善柱塞泵的流量脉动情况。     

Stability analysis of residual-affected HCCI using convex optimization

Residual-affected homogeneous charge compression ignition (HCCI) is a promising methodology for simultaneously reducing emissions and fuel consumption. However, the process relies on cycle-to-cycle coupling between subsequent engine cycles through the exhaust gas temperature, resulting in sections of the state space which are unstable. This paper exploits a previously validated control model of HCCI to analytically determine the area of the state space which is stable to perturbation of either combustion timing or in-cylinder pressure. As efforts to control and expand the operating range of HCCI continue, analytical stability tools like that developed here will likely play an increasingly important role.     

Precise piston trajectory control for a free piston engine

A free piston engine removes the mechanical constraint on the piston motion by eliminating the crankshaft. The extra degree of freedom offers many advantages for reducing fuel consumption and emissions. Nevertheless, stability and robustness of the engine operation has been affected in the meantime. To ensure smooth engine operation, an active motion controller, which utilizes robust repetitive control, was developed previously to regulate the piston motion of a hydraulic free piston engine to track pre-defined trajectories. However, the long piston stroke length, high operating frequency and system nonlinearity impose challenges to precise piston motion control. Therefore, feedforward controllers are investigated in this paper to complement the repetitive control to further improve the tracking performance. The first feedforward design involves the inversion of a linear plant model that describes the dynamics of the engine operation, and the second design is based on the flatness approach, which involves the inversion of a nonlinear model of the system. The two feedforward controllers are designed and implemented on the free piston engine. The experimental and simulation results demonstrate the effectiveness of the proposed control under various operating conditions and reference piston trajectories.