柴油机燃烧室的系统设计方法研究与应用

为使柴油机燃烧室设计走向系统化和正规化,提出了柴油机燃烧室系统设计的概念。通过对因子处理方法和响应分析方法的梳理总结了9种因子-响应组合方法,选取其中1个燃烧室设计方法进行方法展示。此方法以一款四气门直喷式柴油机作为研究对象,建立了缸内气体瞬态流动模型,以缸内气体流速和湍流动能作为评价标准,在压缩比基本保持不变的前提下,对比分析了缩口率分别为16.4%、6.1%、9.8%、9.8%且底面凸台形状不同的A、B、C、D 4种ω型燃烧室对缸内流场的影响。研究结果表明,燃烧室几何结构对柴油机进气阶段和压缩阶段前期的缸内气流运动影响较小,对压缩阶段后期缸内气流运动影响显著。在上止点前后20°曲轴转角区段,底面凸台呈锥形的C型燃烧室的平均挤流速度、逆挤流速度比底面凸台呈球形的D型燃烧室分别高25.2%、26.4%;缩口率为16.4%的A型燃烧室内气体平均湍流动能比缩口率为9.8%的D型燃烧室高25.4%。与底面凸台呈椭球形的A型和呈球形的D型燃烧室相比,底面凸台呈45°锥形的B、C型燃烧室在湍流动能强度和逆挤流强度方面的保持性更好。该文研究结果可为柴油机燃烧室结构设计和优化提供参考。

Investigation and application of systematic design method for combustion chamber of diesel engine

The effects of diesel engine combustion chamber design have important influences on the formation and combustion processes of the gas mixture, and greatly affect the power capability, fuel economy, and emissions of the engines.In order to make the design of the diesel engine combustion chamber more systematic and rigorous, the concept of diesel engine combustion chamber systematic design was proposed, which was elaborated from five aspects of design experience,design parameters, design criteria, factor processing methods, and response analysis methods. Nine design methods of combustion chamber were classified through combining three factor processing methods and three response analysis methods.The design method consisting of the factor sampling design method and the second type of the response analysis method was selected to illustrate its application process due to its effectiveness and convenience. A four-valve-head direct-injection diesel engine was analyzed, and a transient in-cylinder flow model was established. Under the assumption of an approximately constant compression ratio, the impacts of four different ω-shape combustion chamber structures on gas flow motions in cylinder were compared and analyzed. These four combustion chambers were named type A, B, C and D with shrinkage ratios of 16.4%, 6.1%, 9.8%, and 9.8%, respectively. The design evaluation criteria were gas flow velocity and turbulence kinetic energy. The results showed that the geometrical structures of the combustion chambers had little influence on the in-cylinder gas flow motions during the intake stroke and the early stage of the compression stroke, while they exhibited significant impacts during the late stage of the compression stroke. The average squish velocity and reverse squish velocity of the Type C combustion chamber, which had a conical bottom shape, was greater than that of the Type D combustion chamber having a spherical bottom by 25.2% and 26.4% respectively during the crank angle interval from 20° before the top dead center(BTDC) to 20° after the top dead center(ATDC). The average turbulence kinetic energy of the Type A combustion chamber with a shrinkage ratio of 16.4% was greater than that of the Type D combustion chamber with a shrinkage ratio of 9.8% by25.4% during the crank angle interval from 20° BTDC to 20° ATDC. Compared to the type A and D combustion chambers that had a elliptic bottom shape and a spherical bottom shape, respectively, the type B and C combustion chambers that had a 45°conical bottom shape exhibited stronger capabilities of maintaining turbulence kinetic energy and reverse squish intensity. The results in this paper can provide good guidance for the structural design and optimization of diesel engine combustion chamber.