Contour plots and image sequences pertaining to smalls-scale free-piston homogeneous charge compression ignition engines

The contour plots and image sequences are presented for illustrating the ignition and combustion processes in smalls-scale free-piston homogeneous charge compression ignition engines. In contrast to transport-limited engine combustion modes, homogeneous charge compression ignition depends primarily upon the compression process and fuel oxidation kinetics. Therefore, matching engine operating conditions and homogeneous charge compression ignition combustion is essentially a reaction engineering problem. The governing equations for a non-isothermal and non-adiabatic system are mass conservation. To model heat transfer in a smalls-scale engine, several approaches with varying levels of complexity may be taken. However, the heat transfer model should be relatively simple but capture the surface-area-to-volume ratio dependence. Moreover, conduction from the charge to an isothermal wall represents a worst-case scenario in terms of heat loss. Hence, conduction is assumed to be the dominant heat transfer mode. The non-dimensional heat transfer rate is proportional to the non-dimensional surface-area-to-volume ratio and depends exclusively upon the compression ratio and the aspect ratio. Evidently, large aspect ratios tend to minimize heat transfer. Diffusion fluxes also increase with surface-area-to-volume ratio. Therefore, assuming that radical recombination reactions on the combustion chamber walls are mass transfer-limited, it is hypothesized that the frequency of these events will increase with the surface-area-to-volume ratio. Thus, this effect is taken into account by assuming the walls to be perfect radical sinks and that diffusion dominates. Wall species are assumed to have thermodynamic properties identical to their authentic counterparts. Temperature-dependent terms of the reaction rates are included in the kinetic mechanism while pressure and geometry-dependent parts are incorporated in the source code. Transport properties of the charge are assumed identical to air. To achieve homogeneous charge compression ignition, the charge must be brought by compression to a thermodynamic state such that the ignition delay time is short relative to the residence time. Therefore, in contrast to typical reaction engineering problems, the bulk temperature, chemical time, and residence time are variables. Additionally, homogeneous charge compression ignition depends strongly upon the initial conditions and the compression ratio. Hence, the relationships between the various parameters and homogeneous charge compression ignition are seldom obvious. Contributor: Junjie Chen, E-mail address: [e-mail], ORCID: 0000-0002-5022-6863, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China本数据集通过等高线图与图像序列，展示了小型自由活塞式均质充量压缩点火（homogeneous charge compression ignition, HCCI）发动机的着火与燃烧过程。与传质限制型发动机燃烧模式不同，均质充量压缩点火主要依赖压缩过程与燃料氧化动力学。因此，匹配发动机运行工况与均质充量压缩点火燃烧本质上属于反应工程问题。非等温非绝热系统的控制方程为质量守恒方程。为模拟小型发动机内的传热过程，可采用多种复杂度各异的方法。然而，传热模型应相对简洁，同时需体现表面积与体积比的依赖关系。此外，从充量到等温壁面的导热，在热损失层面属于最严苛的工况。因此，假设导热为主要的传热模式。无量纲传热速率与无量纲表面积体积比成正比，且仅取决于压缩比与长径比（aspect ratio）。显然，较大的长径比可最大限度降低传热损失。扩散通量也随表面积体积比的增大而提升。因此，假设燃烧室壁面的自由基复合反应受传质限制，可推测这类反应的发生频率会随表面积体积比的升高而增加。据此，我们通过假设壁面为完全的自由基阱且扩散过程占主导，来考量这一效应。壁面物种的热力学性质与其本体完全一致。反应速率的温度相关项已纳入动力学机理中，而压力与几何相关项则整合至源代码内。充量的输运性质假设与空气一致。为实现均质充量压缩点火，需通过压缩将充量提升至某一热力学状态，使得着火延迟时间相对于驻留时间足够短。因此，与典型的反应工程问题不同，整体温度、化学时间与驻留时间均为变量。此外，均质充量压缩点火强烈依赖初始条件与压缩比。故而，各类参数与均质充量压缩点火之间的关联往往并不直观。贡献者：陈俊杰，电子邮箱：[e-mail]，ORCID：0000-0002-5022-6863，河南理工大学机械与动力工程学院能源与动力工程系，河南省焦作市世纪大道2000号，454000，中华人民共和国。