Numerical Investigation of Direct Gas Injection in an Optical Internal Combustion Engine 2018-01-0171
Direct injection (DI) of compressed natural gas (CNG) is a promising technology to increase the indicated thermal efficiency of internal combustion engines (ICE) while reducing exhaust emissions and using a relatively low-cost fuel. However, design and analysis of DI-CNG engines are challenging due to supersonic gas jet emerging from the DI injector, which results in a very complex in-cylinder flow field containing shocks and discontinuities eventually affecting the fuel-air mixing. In this paper, numerical simulations are used supported by experimental measurements to investigate the process of direct injection of helium and its influence on the flow field and mixture formation in an optically accessible ICE. The simulation approach involves computation of the in-nozzle flow with highly accurate Large-Eddy Simulations, which are then used to obtain a mapped boundary condition. This boundary condition is applied in Unsteady Reynolds Averaged Navier-Stokes simulations of the engine in order to investigate the in-cylinder velocity and mixing fields. The experimental measurement of the velocity field has been carried out using time-resolved stereoscopic Particle-Image Velocimetry (PIV) in a running engine. The scalar field indicating the injected gas has been measured with Holographic Tomographic Interferometry (HTI). The results of the simulations are compared with the measured velocity and scalar fields. The cycle-to-cycle fluctuations in the measured velocity field are found to be high, presumably due to shocks and their reflections from cylinder walls. The proposed simulation approach can reasonably predict the ensemble-averaged velocity field. Yet, it is not able to predict the wall-attached flow of the gas jet, which is visible in the scalar field measurements. The impact of the gas jet from a centrally-mounted injector on tumble motion and turbulent kinetic energy has been investigated. Gas injection at high injection pressures and lower engine speeds destroys the tumble flow quickly generating high turbulence levels, which in turn, result in a higher degree of mixing quantified by cumulative volume fraction over the mass fraction of the injected gas. However, this causes an overall reduction of turbulence levels near the combustion top dead center as compared with those without injection, which may lead to slower combustion in a fired engine.