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In direct-injection gasoline (GDI) engines with charge stratification, minimizing engine-out nitrogen oxide (NOx) emission is crucial since exhaust-gas aftertreatment tolerates only limited amounts of NOx. Reduced NOx production directly lowers the frequency of energy-inefficient catalyst regeneration cycles. In this paper we investigate NO formation in a realistic GDI engine. Quantitative in-cylinder measurements of NO concentrations are carried out via laser-induced fluorescence imaging with excitation of NO (A-X(0,2) band at 248 nm), and subsequent fluorescence detection at 220-240 nm. Engine modifications were kept to a minimum in order to provide results that are representative of practical operating conditions. Optical access via a sapphire ring enabled identical engine geometry as a production line engine. The engine is operated with commercial gasoline (“Super-Plus”, RON 98).

The NO distribution in a directly-injected Diesel engine with realistic combustion chamber geometry was investigated with laser-induced fluorescence (LIF) imaging with KrF excimer laser excitation. The highest possible level of selectivity has been ensured using spectrally resolved LIF investigations inside the Diesel engine. To minimize interference from both, oxygen and polycyclic aromatic hydrocarbon (PAH) LIF the NO signal was detected around 237 nm, blue-shifted compared to the excitation wavelength resulting in a background contribution below 10% at the earliest detection timing possible in the engine under study (20°ca after top dead center, TDC). The in-cylinder NO LIF intensities were compared for different injection systems and operating conditions and correlated to variations in pressure traces and soot temperature measurements.

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.