Intra-cycle recirculation of partial oxidation products (RePOx): A concept for internal combustion engine combustion control

This work studies the potential for using intra-cycle recirculation of partial oxidation products (RePOx) to control the combustion phasing of advanced internal combustion engine combustion systems such as HCCI. The partial oxidation products are created by extracting a portion of the lean charge products during the expansion stroke and mixing these with the fuel in an auxiliary chamber (RePOx prechamber). The equivalence ratio of the recirculated reactants is controlled by varying the amount of mass extracted. The recirculated partially oxidized products are then reintroduced into the main chamber and mixed with compressed air to facilitate the main chamber reaction. This process was modeled using the CHEMKIN-PRO software package combined with an external program to balance mass and energy for the RePOx system. n-Heptane, modeled with a detailed reaction mechanism, was used as the fuel. The effects of extraction mass, extraction timing, injection timing, prechamber volume and overall equivalence ratio were examined. Varying prechamber volume did not show any effect on the performance or combustion phasing under the conditions and assumptions of this study, whereas injection timing, extraction mass and extraction timing were found to have significant effects on combustion phasing indicating the RePOx concept could possibly be used to control HCCI and derivative internal combustion engine combustion systems. The most significant effect was achieved by varying the extraction mass, where increasing the extraction mass from 5% to 7% advanced the combustion phasing by 8° crank angle. RePOx system was also found to increase the lean limit by 0.05 equivalence ratio compared to conventional HCCI without any specific optimization for lean operation.