In-Cylinder LIF Imaging, IR-Absorption Point Measurements, and a CFD Simulation to Evaluate Mixture Formation in a CNG-Fueled Engine

Two optical techniques were developed and combined with a CFD simulation to obtain spatio-temporally resolved information on air/fuel mixing in the cylinder of a methane-fueled, fired, optically accessible engine. Laser-induced fluorescence (LIF) of anisole (methoxybenzene), vaporized in trace amounts into the gaseous fuel upstream of the injector, was captured by a two-camera system, providing one instantaneous image of the air/fuel ratio per cycle. Broadband infrared (IR) absorption by the methane fuel itself was measured in a small probe volume via a spark-plug integrated sensor, yielding time-resolved quasi-point information at kHz-rates. The simulation was based on the Reynolds-averaged Navier-Stokes (RANS) approach with the two-equation k-epsilon turbulence model in a finite volume discretization scheme and included the port-fuel injection event. Commercial CFD software was used to perform engine simulations close to the experimental conditions. Experimentally, the local gas temperature influences both LIF and IR measurements through the photophysics of fluorescence and IR absorption, respectively. Thus, in advances over previous implementations, both techniques also measured temperature and used this information to improve the accuracy of the measured air/fuel ratio. In the vicinity of the IR sensor, the local temperature deviated significantly from the bulk-gas temperature due to heat transfer. This was consistent with results of LIF measurements and CFD simulation. The simultaneous application of the two different, but complementary optical techniques together with a simulation gave detailed insight into mixture formation in the port-fueled engine. It also allowed for a cross-check of the uncertainties associated with the experiments as well as the simulation.