Numerical Study of Equivalence Ratio and Swirl Effect on the Performance and Emissions of an HCCI Engine

A promising combustion strategy that combines the advantages of both spark ignition (SI) and compression ignition (CI) combustion modes is homogeneous charge compression ignition (HCCI) combustion mode. A volumetric combustion of lean mixture of charge is the beauty of HCCI combustion leading to low NO_x emissions and soot. In the present work, a CI engine with HCCI combustion is analyzed to study the effect of swirl motion of intake charge at different equivalence ratios on performance and emissions using three-zone extended coherent flame combustion model (ECFM-3Z, CI). The analysis is done considering equivalence ratios ranging from 0.26 to 0.86 and swirl ratios from 1 to 4. HCCI engine with each equivalence ratio was analyzed at all the four swirl ratios using ECFM-3Z CI model. First, 594validation of the model is done with the existing literature. The present study revealed that ECFM-3Z of STAR-CD has well predicted the performance and emissions of CI engine in HCCI mode. The simulation results show that the in-cylinder pressures, temperatures, NO_x emissions and piston work were reduced with an increase in swirl ratio irrespective of the equivalence ratio. However, wall heat transfer losses, CO₂ emissions, and CO emissions increased with increase in swirl ratio irrespective of the equivalence ratio. The combined results of the equivalence ratio and swirl ratio show that higher in-cylinder pressures, higher temperatures, higher CO₂ emissions are observed at lower swirl ratios and higher equivalence ratios. The lower CO and NO_x results are observed at higher swirl ratios and lower compressions ratios. Low wall heat transfer losses are observed with high swirl ratios and low equivalence ratios. Swirl ratio and equivalence ratio are optimized for the chosen engine geometry. It is observed that there is a trade-off between the emissions and piston work.