CNG Injector Nozzle Design and Flow Prediction

In the last few years, a great research effort has been made for developing and enhancing Direct Injection (DI) compressed natural gas (CNG) engines. A number of research projects has been promoted by the European Community (EC) in this field with the objectives of finding new solutions for the automotive market and also of encouraging a fruitful knowledge exchange among car manufacturers and technical universities. The present paper concerns part of the research activity that has been carried out at Politecnico di Torino (PT) within the EC VII Framework Program (FP) InGAS Integrated Project (IP). The target of the work was to support the design phase of a new injector for CNG direct injection, paying specific attention to the nozzle configuration and also to its behavior under different conditions and over runtime. The needle design was carried out with the aims of enhancing the injector reliability and reducing the injector internal friction, which usually causes injector wear due to the lack of lubrication effect with respect to liquid-fuel injectors. The new needle design concept which was considered in the present research project was oriented to maximize the contact area between the needle and its cartridge so as to reduce needle wear. For this reason, the injector feeding part was realized by means of two series of holes. The design was assisted by 3D numerical simulations which indicated the best feeding-hole number and geometry to obtain a maximum mass-flow rate. For this investigation, the needle was kept at its maximum lift and the feeding pressure was gradually increased up to the design rail pressure. The results indicated that the hole number remarkably influences the flow losses along the internal flow path and, in turn, the resultant mass-flow rate. These effects, along with the flow field characteristics inside the injector, are examined and discussed in detail throughout the paper.