Resumen de Radiation heat transfer in direct-injection diesel engines
In the last two decades engine research has been mainly focused on reducing pollutant emissions and increasing efficiency. These facts together with growing awareness about the impacts of climate change are leading to an increase in the importance of thermal efficiency over other criteria in the design of internal combustion engines (ICE). To achieve the objective, there are different strategies to apply. The heat transfer to the combustion chamber walls can be considered as one of the main sources of indicated efficiency diminution. In particular, in modern direct-injection diesel engines, the radiation emission from soot particles can constitute a significant component of the efficiency losses. In this context, the main objective of the thesis is framed: to contribute to the understanding of the radiation heat transfer in DI diesel combustion together with the improvement of the knowledge in the soot formation-oxidation processes. The work has been based on experimental results through the optical technique application in different types of engine and on simulated results from validated one-dimensional models.
In the first part of experimental results, the amount of energy lost to soot radiation relative to the input fuel chemical energy has been evaluated by means of the optoelectronic probe application (based on the 2-Color technique) in both an optical engine DI and a production 4-cylinder DI engine. In this study, the values of soot spectral intensity emitted have been obtained and later, the total radiation emitted by the soot particles in the whole spectrum.
As mentioned above, soot particles are the main responsible for the radiation heat transfer, in addition to one of the important concern in meeting emissions regulations. Soot emissions are the result of two competing processes: soot formation and soot oxidation. Mechanisms of soot formation are discussed extensively in the literature. However, there are deficiencies in the knowledge of soot oxidation. Therefore, the objective of this section has been to evaluate the impact of mixing process and bulk gas temperature on late-cycle soot oxidation process under real operating conditions.
Finally, based on the results and knowledge acquired, a model able to predict heat losses by radiation for a spray diesel has been developed. The model is based on three sub-models: spray model, which analyzes and characterizes the internal spray structure in terms of mixing and combustion process with temporal and spatial resolution. A soot model, in which the results have been justified according to soot formation and oxidation processes. The link of these two sub-models has been used to obtain the input values to the radiation model, which the radiation heat transfer values for a diesel flame are obtained.
