Under-Expanded Gaseous Jets Characterization for Application in Direct Injection Engines: Experimental and Numerical Approach

In the last years, increasing concerns about environmental pollution and fossil sources depletion led transport sectors research and development towards the study of new technologies capable to reduce vehicles emissions and fuel consumption. Direct-injection systems (DI) for internal combustion engines propose as an effective way to achieve these goals. This technology has already been adopted in Gasoline Direct Injection (GDI) engines and, lately, a great interest is growing for its use in natural gas fueling, so increasing efficiency with respect to port-fuel injection ones. Alone or in combination with other fuels, compressed natural gas (CNG) represents an attractive way to reduce exhaust emission (high H/C ratio), can be produced in renewable ways, and is more widespread and cheaper than gasoline or diesel fuels. Gas direct-injection process involves the occurrence of under-expanded jets in the combustion chamber. An accurate characterization of such phenomena is crucial for a consequent application in DI-CNG engines. In this paper an experimental and numerical analysis of methane under-expanded jets (as surrogate of CNG) has been carried out. The fuel has been injected into an optically-accessible constant-volume chamber by using a modified commercial injector at pressures up to 1.2 MPa. Schlieren imaging technique has been employed to evaluate the effects of the injection pressure and chamber thermodynamic conditions on jet macroscopic characteristics. Proper image post-processing has been performed to evaluate jet tip penetration, Mach disk position and spray cone-angle. Further, a numerical CFD model of the injection process has been developed using a large eddy simulation (LES) turbulence framework. The simulation reproduces both the fuels flow inside and outside the injector providing a better knowledge of the air-fuel mixing process.