Cycle-Resolved Investigation of In-Cylinder and Exhaust NO in a Spray-Guided Gasoline Direct-Injection Engine: Effect of Intake Temperature and Simulated Exhaust Gas Recirculation

The formation of NO was investigated in a spray-guided spark-ignition direct-injection gasoline engine. The influence of variations in intake air temperature and simulated exhaust gas recirculation was examined in an optical single-cylinder engine, fueled with iso-octane. Cycle-resolved simultaneous measurements of OH-chemiluminescence, NO laser induced fluorescence, and fast NO exhaust gas sampling allowed a detailed view of the formation process of NO in this engine. Overall, it was found that cycle-resolved information is needed to explain the differences found between operating conditions, since the initial high stratification of fuel leads to large spatial gradients in the NO concentration. Averaged in-cylinder NO distributions do not adequately reflect the formation process rather than show a smoothed distribution that may even be counter-intuitive based on averaged chemiluminescence data. The strong impact of the high level of fuel stratification is also reflected in the temporal evolution of the in-cylinder NO concentrations. Spatially averaged peak concentrations can reach 2700 ppm for engine operation with 90 °C intake air temperature and no EGR. This compares to approximately 300 ppm as measured in the exhaust gas. This ratio is high for spark-ignition engines. However, given the high level of fuel stratification in this spray-guided engine, the observations are plausible and are supported by the measured high local concentrations of NO.