Quantitative In-Cylinder NO LIF Measurements with a KrF Excimer Laser Applied to a Mass-Production SI Engine Fueled with Isooctane and Regular Gasoline

Quantitative 1-D spatially-resolved NO LIF measurements in the combustion chamber of a mass-production SI engine with port-fuel injection using a tunable KrF excimer laser are presented. One of the main advantages of this approach is that KrF laser radiation at 248 nm is only slightly absorbed by the in-cylinder gases during engine combustion and therefore it allows measurements at all crank angles. Multispecies detection turned out to be crucial for this approach since it is possible to calculate the in-cylinder temperature from the detected Rayleigh scattering and the simultaneously acquired pressure traces. Additionally, it allows the monitoring of interfering emissions and spectroscopic effects like fluorescence trapping which turned out to take place. Excitation with 248 nm yields LIF emissions at shorter wavelengths than the laser wavelength (at 237 and 226 nm). This is advantageous since the shorter wavelength region shows less interferences by Raman scattered light or (broadband) LIF. However, the present study shows that nearly interference free NO LIF detection in the burnt gases is also possible at the longer wavelength of emission, i.e., at 272 nm. For the present experiments severe trapping of NO fluorescence at 226 nm and 237 nm is observed. Therefore, the spectral detection window must exclude these emissions if quantification is carried out. Measurements were performed for different engine operating conditions and different fuels (isooctane, regular gasoline). They revealed a reproducible in-cylinder spatial gradient of NO density in the burnt gases. This inhomogeneity can be explained by the combustion chamber geometry (spark plug position) and the Zeldovich mechanism.