Investigation of the mixture formation inside a gasoline direct injection engine by means of linear Raman spectroscopy

Abstract

Linear Raman scattering has been used for the investigation of the mixture formation inside an optically accessible gasoline direct injection spark ignition engine. The concentrations of O 2 , N 2 , H 2 O, and isooctane have been measured simultaneously and cycle resolved along a line of nearly 1 cm at three different locations inside the combustion chamber. By means of polarization-resolved detection optics, it was possible to separate the highly polarized Raman signals from unpolarized contributions from light emissions by stray light from surfaces, background luminescence, or laser-induced fluorescence. A separation between pure air and air/fuel mixture spectra was possible, indicating the influence of air entrainment on mixture formation. As all droplet influences on the taken spectra could be suppressed successfully, only pure gasphase spectra enter the air/fuel ratios evaluated. At the three selected locations (underneath the spark plug and displaced about 1.6 cm toward the intake and toward the outlet valve), the concentrations of all major species have been detected with high spatial and temporal resolution to observe the mixture formation process in a wide area inside the cylinder starting at fuel injection until shortly before ignition. Comparison with two-dimensional laser-induced exciplex fluorescence (LIEF) images allows improved interpretation of the phenomena taking place during mixture formation.