Mechanism of the Smokeless Rich Diesel Combustion by Reducing Temperature 2001-01-0655
Recently, the smokeless rich diesel combustion had been demonstrated . This can realize smokeless and NOx-less combustion by using a large amount of cooled EGR under a near stoichiometric and even in a rich operating condition. We focus on the effects of reducing diesel combustion temperature on soot reduction. In this paper, the smoke suppression mechanism in the smokeless rich combustion, where the temperature is reduced by higher EGR rate, is analyzed by the following procedure.
ϕ (equivalence ratio) - T (temperature) map, which shows soot formation tendencies as a function of ϕ and T, was made using zero dimensional calculations with a detailed chemical kinetic model including PAH (polycyclic aromatic hydrocarbons) formation, soot particle nucleation, growth and surface oxidation.
The combustion processes of the smokeless and conventional diesel combustion were simulated by the 3D-CFD KIVA2 code.
In-cylinder conditions of smokeless and conventional combustion predicted by 3D-CFD were plotted on the ϕ -T map to investigate their behaviors and differences on the map.
The following results were obtained.
According to 3D-CFD, there is little difference in mixture formation between the smokeless and conventional combustion. It is calculated that the temperature of the smokeless combustion with higher EGR rate is significantly reduced compared with that of conventional one.
The smokeless combustion proceeds so as to avoid soot formation regions on the ϕ -T map in contrast to the conventional one, due to this significant temperature reduction.
The smoke suppression is realized by the combustion taking place at temperatures below that needed to form soot. In such lower temperature region, the soot formation itself can be suppressed because the reactions forming soot particles from PAH do not progress even if the rich combustion occurs. This shows the reason why the smokeless rich combustion was realized without regard to the improvement of the mixture formation.