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ABSTRACT
HCCI (Homogeneous Charge Compression Ignition) combustion takes place in homogeneous mixtures once that spontaneous combustion temperature and latency time are exceeded. However, differently from simultaneous combustion, the reactions take place gradually, since lean mixture is used and thanks to suitable solutions, commonly consisting in high levels of charge dilution by exhaust gas.
The most important advantage of HCCI combustion in respect to the diffusive one is the possibility of minimizing soot and NOx emissions. This occurs because HCCI combustion avoids the presence, in combustion chamber, both of excessively fuel- rich zones and of local temperature peaks, responsible of soot and NOx formation respectively. However, this combustion strategy still does not cover the whole engine operating field, thus the engine must be built to operate basically as a conventional engine in transient operations and with high engine loads. The homogeneous charge formation represents a critical factor for the obtaining of HCCI combustion. Another limitation is that, being triggered by homogenous charge spontaneous ignition during compression, HCCI combustion timing control and optimization are not easy.
The research activity can be divided in three sections, in which possible solutions to HCCI combustion limitations have been analyzed.
In the first section a numerical and experimental analysis of gasoline HCCI and PCCI (partially Premixed Charge Compression Ignition) combustion has been made, to understand the influence of some operating parameter on combustion behavior and emission production. After a first spray model validation, CFD simulations of HCCI and PCCI combustion have been performed with various engine speed, injection timings and equivalence ratios. Two combustion models (Shell-CTC and CHEMKIN) have been adopted to evaluate the importance of detailed chemical kinetics on low temperature combustions. From the results it emerges that increasing the in-cylinder in-homogeneity at ignition time the combustion efficiency improves. However, due to the higher temperatures, NOx emissions increase as well.
In the second section of this activity a numerical analysis of dual fuel combustion through advanced combustion models has been performed, to avoid ignition phasing control uncertainties of gasoline HCCI. A small amount of diesel fuel is injected as a pilot injection to ignite a pre-mixture of gasoline (or other high- octane fuel) and air. Several injection timings and mixture compositions have been evaluated in two engine operating points using two different injectors and four different combustion strategies. Initially a validation for a newly proposed combustion strategy was necessary. From the results it emerges that even a very small injection of diesel fuel plays a successful role as an ignition source for the pre-mixed gasoline, and the combustion initially takes place by a flame propagation mechanism. Then, due to the high in-cylinder pressure and temperature conditions, auto-ignition spreads in the whole combustion chamber.
In the last part of this research activity a preliminary study of an innovative concept to control HCCI combustion in diesel-fuelled engines is proposed. The
iv main purpose of the research is the obtaining of diesel HCCI combustion also with high mean effective pressures rendering the combustion behavior more controllable as well. The concept consists in forming a pre-compressed homogenous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the flow rate transfer and high pressure gradients, typical of common HCCI combustion, can be limited as well. A first analysis has been done, only to test the validity of the concept then constructive two and four stroke solutions have been proposed. Results have been compared with a conventional diesel application in terms of pressure, temperature, heat release rate and emissions. Finally an injection process optimization has been performed. From the results it emerges the validity of the concept even though successive mixture formation process optimizations are recommended.
