Homogeneous charge compression ignition is a promising alternative when facing future requirements for internal combustion engines. Even though it has been investigated for many years it is not yet fully understood. To evaluate the impact of low temperature chemistry, different simulation tools have been developed and applied. Additionally, a reduced primary reference fuel mechanism has been reviewed and modified with respect to new cool-flame delay measurements. Based an simulations of ignition delay times in homogeneous reactors and laminar burning velocities of freely propagating flames, it was found that the intermediate species produced by low temperature reactions primarily have a significant influence an auto-ignition. Zero-dimensional engine simulations have been used to represent engine-like pressure and temperature conditions without accounting for influences of the flow field and inhomogeneities. They turned out to be an effective tool to investig ate kinetics in internal combustion en ines, whereas their limitations always must be considered. Finally, a new multi-zone model for CFD simulations has been introduced and validated against engine experiments. In contrast to common multizone models the model presented is coupled to the flow solver at all times and adapts the number of zones automatically to the complexity of the in-cylinder conditions. The grouping of zones is repeated at every CFD timestep based an the distribution of mixture fraction and temperature. This leads to an appropriate resolution whenever it is required. Therefore, the model was found to reliably predict gasoline HCCI combustion with and without the use of a pre-injection during the negative valve overlap. Besides pressure and temperature, intermediate species as well as emissions predicted by the model are in very good agreement to the values measured in engine experiments. Generally, the pre-injection strategy allows to operate the engine at very lean conditions. At both high and low temperature conditions, the intermediate species produced during the negative valve overlap were found to be decisive for auto-ignition to occur in the main combustion stroke. Larger amounts of intermediates lead to earlier ignition but, nevertheless, a certain temperature level is the basic requirement for auto-ignition. In all cases investigated, hydrogen peroxide was found to be a key species. In the low temperature case, the influence of the intermediate species was found to be significantly stronger than in the high temperature case. But in all cases it was observed that without the presence of intermediates the combustion was incomplete. This leads to the conclusion that also in the high temperature case the intermediate species do have a stabilizing effect on the combustion. As a consequence, it is possible to operate the engine under very lean conditions with very low cycle-to-cycle variations. The ability to postpone auto-ignition by applying an additional late injection has been found to be of importance in homogeneous systems only. CFD simulations revealed that local enrichment by the fuel spray is responsible for even earlier ignition. It may therefore be applied to enhance ignition instead. The results show that simulation of homogeneous charge compression ignition engines must be accomplished in conjunction with detailed chemistry, including low temperature reactions.


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    Titel :

    Low temperature chemistry in gasoline compression ignition engines


    Weitere Titelangaben:

    Niedrigtemperaturchemie in Benzinmotoren mit Kompressionszündung


    Beteiligte:
    Röhl, Olaf (Autor:in)

    Erschienen in:

    Erscheinungsdatum :

    2010


    Format / Umfang :

    144 Seiten, Bilder, Tabellen, 106 Quellen




    Medientyp :

    Hochschulschrift


    Format :

    Print


    Sprache :

    Englisch





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