In the last few years, a significant research effort has been made for developing and enhancing Direct Injection (DI) for compressed natural gas (CNG) engines. Several research projects have been promoted by the European Community (EC) in this field with the objective of finding new solutions for the automotive market and also of encouraging a fruitful knowledge exchange among car manufacturers, suppliers and technical universities. This paper concerns part of the research activity that has been carried out by the Politecnico di Torino, AVL List GmbH and Siemens AG within the EC VII Framework Program (FP) InGAS Collaborative Project (CP), aimed at optimizing the control phase of a new injector for CNG direct injection, paying specific attention to its behavior at small injected-fuel amounts, i.e., small energizing times. The CNG injector which was developed within the research project proved to be suitable to be used in a DI SI engine, featuring a pent-roof combustion chamber head and a bowl in piston, with reference to both homogeneous and stratified charge formation. Fuel flow measurements made by AVL on the four-cylinder engine revealed a good linearity between injection duration and fuel mass-flow rate for injection durations above a reference value. In order to improve the injector characterization at short injection durations, an experimental and numerical activity was designed. More specifically, a multidimensional CFD model of the actual injector geometry was built by Politecnico di Torino, and purposely-designed simulation cases were carried out, in which the needle-lift time-history was defined on the basis of experimental measurements made by Siemens. The numerical model was validated on the basis of experimental data concerning the total injected-fuel amount under different conditions. Then, the model was applied in order to evaluate the dynamic flow characteristic by taking also the inner geometry of the injector valve group into account, so as to establish a correlation to the needle lift measurements done by Siemens for injector characterization. In the paper this dynamic behavior of the injector is analyzed, under actual operating conditions, and its impact on the nozzle injection capability is discussed. The simulation results did not show significant oscillations of the stagnation pressure upstream of the nozzle throat section, and thus the resultant mass-flow rate profile is almost proportional to the needle-lift one. As a consequence, in order to characterize the injector flow behavior in the nonlinear region (short injection duration), the measurement of needle lift is sufficient.
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机译：在过去的几年里，已经进行了一个显着的研究努力，用于开发和增强用于压缩天然气（CNG）发动机的直接注射（DI）。欧洲共同体（EC）在这一领域促进了几个研究项目，目的是寻找汽车市场的新解决方案，也鼓励汽车制造商，供应商和技术大学之间的富有成效知识交换。本文的关注已经由都灵，AVL李斯特公司和西门子AG欧共体第七框架计划（FP）InGAS合作项目（CP）进行的研究活动的一部分，旨在优化新的控制阶段用于CNG直接注射的注射器，对其在小注射燃料量的特异性关注其行为，即小的激励时间。在研究项目中开发的CNG注射器被证明适用于DI Si发动机，其特征在于在活塞中具有浮雕屋顶燃烧室头和碗，参考均匀和分层电荷形成。通过AVL在四缸发动机上进行的燃料流量测量显示出注射持续时间和燃料质量流量之间的良好线性，用于注射持续时间以上参考值。为了改善短注射持续时间的注射器表征，设计了实验和数值活性。更具体地，通过PoliteCnico di Toino建造了实际喷射器几何的多维CFD模型，并进行了故意设计的模拟案例，其中基于由西门子制造的实验测量来定义针升时间历史。根据不同条件下的总注入燃料量的实验数据验证了数值模型。然后，应用该模型以便通过考虑喷射器阀组的内部几何形状来评估动态流动特性，以便与由西门子进行的针对喷射器表征的针升测量建立相关性。在本文中，在实际操作条件下分析了喷射器的动态行为，并讨论了对喷嘴注入能力的影响。仿真结果没有显示出喷嘴喉部上游的停滞压力的显着振荡，因此所得到的质量流量曲线曲线几乎与针升程度成比例。结果，为了表征非线性区域中的喷射器流动行为（短喷射持续时间），针升力的测量是足够的。
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