NO_X -Particulate Emissions Trade Off

Nitrogen oxides and particulates are the main pollutants emitted by the diesel engines although relatively small amounts of carbon monoxide and unburned hydrocarbons are also emitted.  The combustion temperature as discussed earlier has a strong influence on the formation of both the nitrogen oxides as well as the soot. In the diesel engines, it is observed that when an engine design or operational parameter is adjusted to decrease combustion temperature to obtain reduction in nitrogen oxides emissions, soot and hence the particulate emissions increase.  For example, when the injection timing is advanced it increases combustion temperatures. A higher injection advance thus, results in reduction of soot emissions but increase the NO_x. Lower soot emissions at higher combustion temperatures are attributed to higher soot oxidation rates.   A typical effect of injection timing on soot and NO_x emissions is shown qualitatively in Fig.2.26. The selection of engine design parameters therefore, has to be optimized to obtain reduction in NO_x or PM without causing an excessive increase in the other. This process of adjustment of engine parameters is known as ‘NO_x-PM trade off’.