A Numerical Investigation Into the Effects of Equivalence Ratio and Exhaust Gas Recirculation on Performance and Operating Range in High Compression HCCI Engines

Abstract This study investigates the performance of a high compression ratio engine converted to homogeneous charge compression ignition (HCCI) operation. A total of 24 case studies were conducted with φ = 0.22–0.29 and exhaust gas recirculation (EGR) = 0–30%. Numerical analyses were conducted using three-dimensional computational fluid dynamics simulations in avlfire software. The results indicated that increasing the equivalence ratio improved combustion efficiency, with the indicated mean effective pressure (IMEP) increasing from 3.74 bar at φ = 0.23 to 4.92 bar at φ = 0.28, while thermal efficiency improved from 46.13% to 49.88%. However, the sharp increase in the maximum pressure rise rate (MPRR) at φ = 0.28 suggested that engine operation approached the knock limit, showing the tradeoff with knock. Furthermore, increasing the EGR rate led to a reduction in combustion temperatures, with the maximum temperature dropping from 1779 K at 0% EGR to 1588 K at 20% EGR. While IMEP decreased from 4.92 bar at 0% EGR to 3.98 bar at 20% EGR, CO and HC increased due to low temperatures. On the other hand, NOx emissions, which are inherently low in HCCI combustion due to low temperatures, were further reduced with increasing EGR rates, because of both the temperature decrease and the reduced oxygen availability. Higher EGR levels also raised the risk of misfire, especially at lower equivalence ratios, narrowing the engine's stable range. These findings highlight the potential of HCCI engines to improve both environmental sustainability and energy efficiency, requiring precise control to avoid knock and misfire.