Cycle-Resolved NO Measurements in a Spray-Guided SIDI Engine using Fast Exhaust Measurements and High-Speed OH* Chemiluminescence Imaging

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Cycle-Resolved NO Measurements in a Spray-Guided SIDI Engine using Fast Exhaust Measurements and High-Speed OH* Chemiluminescence Imaging

Smith, James D. / Chin, Michael K. / Sick, Volker

A potential correlation between OH* chemiluminescence and exhaust NO concentration is investigated to pursue a simple diagnostic technique for measurements of NO cycle-to-cycle fluctuations. Previous investigations of NO formation in a direct-injection gasoline engine have indicated that there may be a correlation between the concentration of NO and OH* chemiluminescence. Shortcomings of this work, namely phase-locked measurements, were overcome in the present study by using highspeed imaging capability to obtain chemiluminescence within the entire engine cycle and from entire engine cylinder volume. Cycle-resolved NO exhaust gas detection were performed synchronously with the chemiluminescence measurements on an optical spark-ignited engine with spray-guided direct-injection. A quartz cylinder liner, head and piston windows provide optical access for a highspeed CMOS camera and image intensifier to capture OH* images. Much of the combustion occurs within the piston bowl, requiring side windows for a complete visualization of the combustion process.A moderate correlation between integrated OH* signal and exhaust NO concentration was found, demonstrating that OH* could in fact be used to indirectly measure NO concentration of individual engine cycles. Variations from the fitted correlation line were found to be heteroscedastic and increase with OH* or NO. If these variations can be accepted, then in-cylinder OH* signals can approximate NO concentrations in the exhaust. It is also important to observe that the correlation between OH* signal and exhaust NO concentration follows different proportionalities depending on the operating mode of the engine, i. e. stratified vs. homogenous fuel distribution.