Cycle-to-Cycle NO and NO_x Emissions From a HSDI Diesel Engine

Engine-out NO_x emissions from diesel engines continue to be a major topic of research interest. While substantial understanding has been obtained of engine-out (i.e., before any aftertreatment) NO_x formation and reduction techniques, not least exhaust gas recirculation (EGR) which is now well established and fitted to production vehicles, much less data are available on cycle resolved NO_x emissions. In this work, crank-angle resolved NO and NO_x measurements have been taken from a high-speed light duty diesel engine at test conditions both with and without EGR. These have been combined with 1D data of exhaust flow, and this used to form a mass average of NO and NO_x emissions per cycle. These results have been compared with combustion data and other emissions. The results show that there is a very strong correlation (R² > 0.95) between the NO_x emitted per cycle and the peak cylinder pressure of that cycle. In addition, the crank-angle resolved NO and NO_x measurements also reveal that there is a difference in NO : NO₂ ratio (where NO₂ is assumed to be the difference between NO and NO_x) during the exhaust period, with proportionally more NO₂ being emitted during the blowdown period compared to the rest of the exhaust stroke.