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

Knowledge of the NO:NO₂ ratio emitted from a diesel engine is particularly important for ensuring the highest performance of SCR NO_x aftertreatment 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 CLD (for NO) and a new fast 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 EGR. In addition, crank-angle resolved unburned hydrocarbon (UHC) measurements have been taken simultaneously using a fast FID.

The NO_x emitted per cycle and the peak cylinder pressure of that cycle have showed 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 analysed across all test points and with and without EGR.

The performance of the new fast LIF analyser 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.