Numerical Study on a High Efficiency Gasoline Reformed Molecule HCCI Combustion Using Exergy Analysis 2017-01-0735
In this study, the characteristics and the advantages on engine performance of the reformed molecule HCCI (RM-HCCI) combustion fueled with gasoline were investigated by exergy analysis. The processes of fuel reforming and the closed portion of the engine cycle were simulated integrated with chemical kinetics mechanism at varied compression ratio (CR) and constant speed conditions. Results showed the fuel reforming under high temperature and oxygen-free condition by the exhaust heat recovery and electric heating assistance could drive gasoline to transform to the small-molecule gas fuels, meanwhile enhanced the chemical exergy of the fuel. The reformed fuel contributed to extending ignition delay, so less dilution required in RM-HCCI engine when expanding high load compared with gasoline HCCI engine. Thus, RM-HCCI engine could achieve higher load than gasoline HCCI engine, with the improvements by 12%, 26%, and 31% at CR17, CR19, and CR21, respectively. Under the conditions of high compression ratio, boosting, lean burn, gasoline HCCI engine could achieve the highest exergy efficiencies of 50.2%~51.4% at CR17~CR21, which were further improved to 50.9%~52.6% at the same CR conditions when RM-HCCI combustion employed. These improvements came from chemical exergy gain of the fuel, reduction of exergy destruction, and the increase of work-extraction efficiency by fuel property changed and less exhaust dilution used improving specific heat ratio, while the extra electric heating consumption lowered some improvements. Furthermore, RM-HCCI combustion improved the flexibility to employ high CR than gasoline HCCI combustion for expanding high load more easily. The engine at CR21 employed RM-HCCI combustion to expand high load and transferred to gasoline HCCI combustion to extend low load, which achieved a wider operation range than gasoline HCCI engine at CR17, meanwhile, exergy efficiencies improved with the increases of 1.8%~2.4%-units. Overall, RM-HCCI combustion employed in the engine promoted to achieve higher load, higher efficiency, and more flexibility to high CR.