Role of the Chemical Kinetics on Modeling NO_x Emissions in Diesel Engines

Abstract

New diesel engine strategies (involving high injection pressure and multiple injections) have been proposed in recent years aiming to reduce pollutant emissions (mainly NO x 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 NO x 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), modern 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 NO x 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.

Cycle by cycle NOx model for diesel engine control

“…Figure 4 represents the re-entrant flows and Eq. 14determines the final NOx concentration at the exhaust: Although thermal formation mechanism is the most representative of NOx formation on diesel combustion, it is not the only one [55]. Prompt [56] and via N 2 O mechanisms [36] are other important mechanisms of NOx production and they are responsible of an important percentage of NOx under LTC combustion in diesel engines.…”

NOx emissions in direct injection diesel engines: Part 2: model performance for conventional, prolonged ignition delay, and premixed charge compression ignition operating conditions

“…Similarly, Musculus 9 concludes that while the importance of prompt NO chemistry in diesel flames is apparent, data available confirming its importance are insufficient. In a recent numerical study on NOx formation in diesel engines, Hernández et al 34 state that the extended Zel’dovich (thermal) mechanism and NOx formation via a N 2 O intermediate route are the most relevant reactions on the lean side of the diffusion flame.…”

Development of multi-component diesel surrogate fuel models – Part II: Validation of the integrated mechanisms in 0-D kinetic and 2-D CFD spray combustion simulations

“…However, it should also be highlighted that the computation of thermal NOx emissions also depends on the associated Arrhenius rate constants [67]. Hence, the surrogate models can be useful for thermal NOx simulations only when they are integrated with the extended Zeldovich reaction rates which are validated for engine applications.…”