The Influence of Cycle-to-Cycle Hydrocarbon Emissions on Cyclic NO:NO2 Ratio From a HSDI Diesel Engine

Abstract

Knowledge of the NO:NO2 ratio emitted from a diesel engine is particularly important for ensuring the highest performance of selective catalytic reduction (SCR) NOx after-treatment systems. As real driving emissions from vehicles increase in importance, the need to understand the NO:NO2 ratio emitted from a diesel engine during transient operation similarly increases. Previous work by the authors identified significant differences in NO:NO2 ratio throughout the exhaust period of a single-engine cycle, with proportionally more NO2 being emitted during the blowdown period compared to the rest of the exhaust stroke. At the time it was not known what caused this effect. In this study, crank-angle resolved NO and NO2 measurements using fast response chemiluminescence detector (CLD) (for NO) and a new fast laser-induced fluorescence (LIF) instrument (for NO2) have been taken from a single-cylinder high-speed light-duty diesel engine at three different speed and load points including a point with and without exhaust gas recirculation (EGR). In addition, crank-angle resolved unburned hydrocarbon (UHC) measurements have been taken simultaneously using a fast flame ionization detection (FID). The NOx emitted per cycle and the peak cylinder pressure of that cycle have shown high correlation coefficients (R2 < 0.97 at all test points) in this work. In addition, a variation of the NO:NO2 ratio through the engine's exhaust stroke is also observed indicative of in-cylinder stratification of NO and NO2. A new link between the NO:NO2 ratio and the UHC emissions from an individual engine cycle is observed - the results show that where there are higher levels of UHC emissions in the first part of the exhaust stroke (blowdown), perhaps caused by injector dribble or release from crevices, the proportion of NO2 emitted from that cycle is increased. This effect is observed and analyzed across all test points and with and without EGR. The performance of the new fast LIF analyzer has also been evaluated, in comparison with the previous state-of-the-art and standard “slow” emissions measurement apparatus showing a reduction in the noise of the measurement by an order of magnitude.