Numerical study to improve the combustion and thermal efficiencies of off-gas/synthesis gas-fueled HCCI-engine at different loads: Piston-shape and crevice design

This numerical work aims to improve combustion efficiency and thermal efficiency by modifying the piston shape and crevice volume ratio of the HCCI engine operated under excessively lean air-fuel mixture conditions for power generation employment. The baseline piston, optimized for the diesel engine, gives strong reverse squish flow during the expansion stroke, causing a reduction in the efficiency of the engine. Therefore, the flow field in the squish area of the piston bowl should be weakened to facilitate auto-ignition at different points to boost the burning rate. Hence, the baseline piston shape was modified by bringing down the piston bowl depth and reducing the squish area ratio from 34% to 5% with a constant compression ratio. Crevice volume ratio Vcrevice/V_TDC is also optimized for the HCCI engine to achieve high performance. All calculations are executed at MBT conditions by changing the inlet temperature of the mixture. Finally, the computational model results are validated with literature data. The results indicate that combustion phasing advances by changing the piston shape, and it also reports that the highest engine combustion efficiency of about 96%, 97%, and 99% are achieved by changing the piston shape and reducing the 40% crevice volume ratio of the engine, at low load, medium load, and high loads, respectively. The results also show that thermal efficiency is improved from 40% to 46% at low load while enhancing from 36 to 38.5% at 10 bar IMEP, respectively.