ABSTRACT

Pollutant emissions to the atmosphere, partly produced by the internal combustion engines used in transportation, is one of the main concerns in our society. Concerning diesel engines, their pollutant emissions are carbon monoxide, unburned hydrocarbons and, mainly, nitrogen oxides and soot particulates. The latter are the product of a complex balance between formation and oxidation. A promising path for the future is to achieve a low temperature diffusion combustion, which intrinsically produces nearly no soot. Both the soot formation and oxidation processes directly depend on the mixing process. In fact, the main parameter that seems to control the soot formation process is the oxygen entrained by the fuel spray between the injector and the beginning of the flame (lift-off). It was observed that a fast, non-sooting diffusion combustion can be achieved if the mixing process is enhanced by increasing the injection pressure and strongly reducing the injector orifices diameter. However no many things are known about the behavior of the lift-off and the other items affecting soot formation at low temperature combustion conditions with low oxygen mass fraction.

The main objective of this work is to improve the knowledge of these processes. To achieve this objective, four other specific objectives were defined:
* To characterize the effect of oxygen mass fraction on the lift-off length.
* To characterize its consequences on the soot formation process.
* To quantify the effect of increasing the injection pressure and strongly reducing the orifice diameter on the two previous aspects.
* For this purpose, it is necessary to develop:
o A methodology to simultaneously measure the lift-off length and the soot content in the flame based on different optical techniques applied in an optically accessible engine.
o A methodology that allows quantifying the studied physico-chemical mechanisms based on the spray mixing process analysis.

As a result of this last point, the main originality of this work is the definition and use of a parameter (Fsoot) that allows characterizing the soot formation process in a more quantitative way compared to other studies already published in the bibliography. Thanks to this parameter, the behavior of the flame at low oxygen mass fraction conditions was better characterized, and some criteria to achieve non-sooting conditions were defined.