Modeling HCCI combustion: Modification of a multi-zone model and comparison to experimental results at varying boost pressure

The present study presents a comparison of the results obtained from a modified HCCI multi-zone model to experimental measurements, at different load and boost pressure conditions. The multi-zone model includes a modified sub-model for the wall heat transfer and accounts for the heat transfer between zones. Gas mixing between cold and hot regions of the combustion chamber, which is of major importance for the emissions formation, is also accounted for throughout compression, combustion and expansion. Combustion is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver. A refined zone configuration near the combustion chamber wall was used, in order to obtain a high resolution at the emissions formation regions. The pressure traces and emissions of nine experimental cases were compared to the multi-zone model results. In these cases the equivalence ratio and the boost pressure were varied, while maintaining constant engine speed. The results show adequate agreement with the pressure traces. The emissions trends are also adequately captured, with the absolute values presenting some deviation from the experimental cases especially for the HC and CO emissions at the relatively low air-fuel equivalence ratios.HCCI Calibration Multi-zone model Supercharged Emissions Hydrocarbons CO

Investigating the importance of mass transfer on the formation of HCCI engine emissions using a multi-zone model

The focus of the present study is to investigate the importance of mass transfer on the formation of the most important HCCI engine emissions, i.e. unburned HC and CO emissions. A multi-zone model is used for this purpose. The multi-zone model includes sub-models for the heat transfer between zones and to the cylinder wall and for the mass transfer between the hotter and colder regions of the combustion chamber. The combustion mechanism is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver. The results indicate that mass transfer during combustion and expansion plays a significant role on the formation of both the unburned HC and the CO emissions and therefore must be taken into account for the closed part of the engine cycle, i.e. compression, combustion and expansion. According to the model results, the formation of these emissions is located mainly at the crevice and the near-the-wall regions and is determined by the temperature field and mass transfer. It is shown that neglecting mass transfer in these regions during combustion and expansion would result to a significant deviation from the values predicted with the inclusion of mass transfer.HCCI Multi-zone model Mass transfer Emissions formation Hydrocarbons CO

Heat transfer in HCCI multi-zone modeling: Validation of a new wall heat flux correlation under motoring conditions

The present study focuses on the development and a preliminary validation of a heat transfer model for the estimation of wall heat flux in HCCI engines via multi-zone modeling. The multi-zone model describes heat flow between zones and to the combustion chamber wall. Mass, species and enthalpy transfer, which affect the temperature field within the combustion chamber, are also considered between zones, accounting for the convective heat transfer terms. The multi-zone heat transfer model presented herein has been developed for HCCI combustion simulation and although it has been used in the past, its validation was based on cylinder pressure data under firing conditions. In the present study a more accurate validation of the model is conducted. This is achieved by comparing the multi-zone model heat loss rate predictions to the corresponding predictions of a validated CFD code. The cases examined correspond to actual motoring cases, against which the CFD code has been validated in a previous work. Moreover, a sensitivity analysis is presented, to assess the effect of the zone configuration, i.e. zone thickness and number, on the predicted heat loss rate and temperature profiles. In addition, a comparison is made between the results obtained from the proposed heat flux correlation and one in which the temperature gradient at the wall is approximated via finite differences.HCCI Heat transfer Wall heat flux Multi-zone model Motoring Validation