Role of the chemical kinetics on modeling NOx emissions in diesel engines

New diesel engine strategies (involving high injection pressure and multiple injections) have been proposed in recent years aiming to reduce pollutant emissions (mainly NOx and particulate matter). These strategies have led to very fast combustion processes as a consequence of the improvement on the fuel atomization, evaporation, and air entrainment phenomena. Although NOx emissions models for diesel engines usually assume equilibrium and/or the steady state hypothesis together with the consideration of very simplified kinetic reaction mechanisms (such as the Lavoie method based on the extended Zeldovich reaction mechanism), modem diesel engine combustion models require more complex chemical kinetic approaches due mainly to the lack of time to reach the equilibrium state. These kinetic considerations are even more important for simulating new diesel combustion concepts (such as homogeneous charge compression ignition (HCCI) and low temperature combustion (LTC)), which are well-known to be kinetically controlled. In such a frame, this work shows, through a reaction mechanism which considers 83 reactions and 38 chemical compounds, the role of the kinetics on the local NO formation/destruction paths under typical diesel engine conditions. The analysis has been carried out for different combustion and dilution rates, showing that equilibrium assumptions overestimate the engine NO emissions. Reactions controlling NO formation during both the combustion and dilution processes have been identified through a sensitivity analysis carried out by using the chemical package CHEMKIN 4.0. This analysis suggests a coupling between the fuel oxidation process and the thermal NO mechanism, which is not considered in most of the diesel engine NOx models. Finally, the differences in modeled engine NO emissions caused by the uncertainties derived from the different kinetic rate values used in the literature are also presented, showing a significant need for more reliable experimental kinetic data obtained under engine pressure and temperature conditions.