Study of combustion development and modeling of the pollutants formation process in diesel engines
In recent years, to improve the efficiency of diesel engines and, at same time, to reduce the pollutants emission (NOx and soot), the researchers and the automotive producers have focused on technologies that enable compliance with both requirements: devices for the exhaust after-treatment, exhaust gas recirculation (EGR) and new combustion models. Thus, it was necessary a deeper understanding of the combustion process in diesel engines with direct injection.
The research is related to the analysis of spray formation, combustion and pollutants emission processes in diesel engines. The study has focused its attention on various phenomenological multizone premixed – diffusive burning models: these models are based on the well-known Dec’s conceptual model for DI diesel combustion as well as spray models for steady fuel jets from Siebers et al. (liquid and vapor-phase fuel penetration, spray spreading angle and lift-off length).
Dec, with the use of innovative laser techniques, was successfully able to define the structure of diesel flames and their temporal evolution. The data obtained from the images and the optical analysis relate to the fuel zones in liquid and vapor phases, fuel/air mixing, reaction zones, IPA distribution, overall jet penetration, relative soot concentrations, relative soot particle-size distributions, diffusion flame structure, onset of NOX and natural-emission chemiluminescence of auto-ignition. These imaging diagnostics have been used to map out the processes occurring in the cylinder of an operating diesel engine from the start of injection, through auto-ignition, the premixed reaction, and the first part of the mixing-controlled burn (i.e. until the end of injection).
The combustion is considered as a two stage quasi steady process: all fuel particles undergo a first rich premixed combustion phase and the products complete their oxidation in close to stoichiometric conditions at the jet periphery, through a diffusion flame.
These new conceptual images (Figure 1 and 2) provide a spatial definition of liquid disappearance and zones where reactions occur in the fuel-vapor/air mixture. Also visible are the regions where specific reaction intermediaries, such as OH, are present that indicate the location of the vigorous diffusion burning processes.
These images allow the delineation of times and travel within this flame structure.
Figure 1 presents a temporal sequence of schematics showing the development of a diesel fuel jet from the start of injection, through the premixed burn and into the first part of the mixing-controlled burn. These schematics images depict the base operating condition in research diesel engine with a fuel loading that is sufficiently high for the injection duration to extend beyond the end of sequence.
In Fig. 1, the crank angle degree after the start of injection (ASI) is given at the side of each image (1° = 139 μs), the scale is approximately 1.5:1 and the color scheme is shown in the legend at the bottom.
The schematics show an idealized single cycle with all components of the reacting jet being shown with an average position and shape. The jet is shown as penetrating to an average (typical) length and the boundaries are drawn as smooth lines. In a real jet, there is always some cycle-to-cycle variation in the jet penetration and symmetry and the boundaries are always ragged in appearance due to small-scale turbulence.
Figure 2 present s a typical schematic of the conceptual model of DI diesel combustion during the mixing-controlled burn, prior to the end of fuel injection.
Temporally, the schematic in Fig. 2 follows the last one in the sequence (10° ASI) in Fig. 1 and it is representative of the remainder of the mixing-controlled burn up until the end of injection.
Going from the injector down the jet, Fig. 2 shows that turbulent air entrainment is sufficient to vaporize all the fuel by the time it has traveled about 18 or 19 mm from the injector.
The innovative aspect of this investigation is that the vaporization process in a DI diesel fuel jet is mixing-limited: the fuel jet entrainment and mixing processes, and not transport processes at droplet surfaces, control or limit vaporization.
Ambient gas density and fuel vaporization effects on penetration and dispersion of diesel sprays were examined by Siebers et al.
The results of their researches show that ambient gas density has significantly larger effect on spray penetration and a smaller effect on spray dispersion. In addition, the results show that vaporization decreases penetration and dispersion by as much as 20% relative to non-vaporizing sprays; however, the effects of vaporization decrease with increasing gas density.
Characteristic penetration time and lengths scales are presented in these researches that include a dispersion term that accounts for increased dependence of penetration on ambient density.
Siebers et al. also studied the penetration and vaporization of liquid-phase fuel within a fuel jet, flame lift-off and its impact on soot formation, scaling of many of these processes with engine and injector parameters and new insights on fuel jet development, soot formation, and combustion in a diesel fuel jet that were derived from the results. Conservation of mass, momentum, and energy principles, applied to an idealized model of a fuel jet, are used to provide a framework for presenting and analyzing the results.
In addition, Siebers et al. developed a penetration scaling law using an idealized model of a diesel fuel jet. The penetration scaling law and analysis were extended and utilized for analyzing liquid-phase fuel penetration and for helping interpret the impact of flame lift-off on soot formation in diesel fuel jets.
Finally, it was considered the possibility to integrate the multizone thermodynamic combustion model with a predictive non stationary variable–profile 1D spray model recently presented by Musculus and Kattle.
In particular, my research has focused on the design and development of a phenomenological model.
This model allows to evaluate:
- the process of formation of the charge and the combustion in the cylinder of a diesel engine;
- the processes of formation of pollutants inside the combustion chamber.
The research methodology has provided :
- the analysis and critique of the phenomenological approach proposed by various authors;
- to the design of a procedure for setting the work and development of algorithms for the characterization of the model;
- the implementation of the models for the formation and oxidation of the particulates and oxides of nitrogen.
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