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.
