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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