NOx emissions in direct injection diesel engines – part 1: Development of a phenomenological NOx model using experiments and three-dimensional computational fluid dynamics

Abstract

There exists a well-established correlation of exhaust NOx emissions arising from diesel engines with the adiabatic flame temperature, in particular for conventional (i.e. short ignition delay, diffusion combustion-dominated) operating conditions. Most published NOx emission models rely on this correlation. However, numerous experimental studies have identified operating conditions where this correlation fails to capture the exhaust NOx trend. In this work, a novel phenomenological NOx model concept is introduced, including a first successful validation against experimental data. The model development is based on experimental observations and is supported by three-dimensional computational fluid dynamics computations, strengthening the understanding of the underlying mechanisms leading to the discrepancy between the adiabatic flame temperature and exhaust NOx trend. For long ignition delay operating conditions, the improved mixture preparation before ignition leads to reduced mixing rates during and after combustion. Both the improved mixture preparation before ignition and the instantaneous increase of mass observed above 2000 K after start of combustion are due to compression heating of the burned gases. Key features of the model are improved description of mixture distribution at start of combustion, NOx formed in products of premixed burn, different physical treatments of premixed and diffusion sourced products, and inherent consideration of burned gas compression heating. Model results capture the NOx emissions for conventional diesel combustion, as well as for operating conditions where the NOx emissions do not follow the adiabatic flame temperature trend. Moreover, the results show that the contribution of NOx from products from premixed burn and the consideration of compression heating effects on burned (post-flame) gases are essential to capture the NOx emissions under the latter conditions.