Emissions Characteristics of Light-Duty Diesel Engine during Transient Operation

Abstract

As concerns about global environment issues have been becoming more serious, emission regulations, especially for automotive industries, have been continuously strengthened. For example, the current emission legislation is EURO-5b but the upcoming regulation, EURO-6, mandates reduction of NOx emissions by 55.6 % compared to the current standard while maintaining the same level of PM emissions. However, despite all the efforts to cut-down the emission level, the real world emission level has not dropped down much. This is due to the fact that the emission test cycle does not represent real-life driving conditions very well. As a result, emissions characteristics during transient operation are drawing more attention from automotive engineers. Therefore, in this research, emissions characteristics of light-duty Diesel engines during transient operation were studied. In order to measure NOx and PM emissions at both steady and transient states, Cambustions DMS-500 and CLD-500 were used along with Horibas exhaust gas analyzer. In addition, an EGR estimation model was adopted to measure EGR rates at transient states. For the first acceleration part of EUDC, transient NO emissions were lower than that of steady states due to increased EGR rate caused by higher boost pressure as a result of turbo-lag from a VGT. As EGR or boost pressure were matched, discrepancy in NO emissions between steady and transient states was disappeared. The opposite phenomenon was true for PM emissions considering NOx-PM trade-off. Also, an emission peak was observed for PM emissions due to instantaneously richer mixture yielded by delay in response of the amount of air. For deceleration, exactly opposite trend was found except that there was no emission peak. Furthermore, from post EURO-6 onward, harsher transient operation is going to be included in the emissions test cycle

hence, it is crucial to study emissions characteristics at sudden and rapid acceleration such as tip-in which frequently occurs at over-taking. As for tip-in, steady state NO emissions were higher than that of transient NO emissions. However, unlike normal transient operation, NO peak was observed for tip-in acceleration due to difference in the amount of air caused by turbo-lag of a VGT. Also, a PM emission peak was observed for tip-in operation but the order of magnitude was so small compared to the peak level for the conventional acceleration case. In addition, as vehicles operate under various environment temperature, intake temperature was varied to simulate both cold and hot conditions. When different surrounding temperatures rather than the ambient condition were applied, EGR was no longer supplied causing drastic increase in NO emissions while almost zero PM emissions were observed. Also, no emission peak was observed under non-ambient temperature.