A mathematical model has been used to evaluate the effect of diesel engine operating conditions on nitric oxide formation and emissions. Although there are several formation pathways during diesel combustion, only the supposed most important one was considered in the present work: the thermal mechanism. The effect of the engine operating conditions was simulated by means of schematical variations of local temperature and fuel–air ratio of the mixture. The evolution of these local variables is described by three characteristic time intervals, which define the timing of the following items: the temperature and pressure peaks; the dilution of burnt products with surrounding air; and the decay of temperature and pressure due to expansion. In addition, the temperature peak was related to the adiabatic flame temperature, which was previously calculated for each mixture condition. A computational parametric study was carried out by varying each of the local mixture parameters and characteristic times, either separately or in combinations. The trends obtained show a qualitative agreement with those obtained by other authors from experimental tests on different diesel engines. The analysis of these trends contributes to a better understanding of the sources of NO diesel emissions and could provide guidelines for cleaner diesel engine design.
The natural thermostat of nitric oxide emission at 5.3 μm in the thermosphere observed during the solar storms of April 2002
