The influence of exhaust gas recirculation (EGR) on the formation of nitric oxide (NO) was studied experimentally in a transparent gasoline direct injection engine by quantitative laser-induced fluorescence imaging. Spectral properties of the excited transition within the NO A2∑+-X2∏(0,2) band are well known from previous studies. The excitation scheme allows quantitative NO concentration measurements without detailed knowledge of the gas phase temperature. Good agreement was found with exhaust gas NOx chemi-luminescence (CLD) measurements.
The experiments were carried out in an optically accessible gasoline engine featuring a direct injection cylinder head (BMW) and a Bosch injection system, based on a serial inline six-cylinder engine with an enlarged crankcase. The measurements were performed in the pentroof section of the combustion chamber.
Various concentrations (0, 10 and 16%) and compositions (pure nitrogen, nitrogen / carbon dioxide and nitrogen / carbon dioxide / water) of synthetic exhaust gas were investigated. NO formation could be significantly reduced by EGR, from an average of about 1200 ppm (without EGR) to about 200 ppm (16% EGR-rate). The NO formation decreased steadily with increasing heat capacity of the inert gases present, independent of their chemical composition. In the temporal evolution of the NO formation, a retarded onset of up to 5° CA was found with 16% EGR. The two-dimensional NO concentration fields appeared to be very homogeneous when adding 16% EGR, whereas without EGR, strong inhomogeneities with local NO peak concentrations of up to 3400 ppm were detected.