A Numerical Investigation into the Effects of Equivalence Ratio and EGR on Performance and Operating Range in High Compression HCCI Engines

This study aims to determine the operating range and optimize the performance of a high compression ratio engine following its conversion to Homogeneous Charge Compression Ignition (HCCI) technology. A total of 24 case studies were conducted with different equivalence ratios (0.22–0.29) and EGR rates (0%, 10%, 20%, 30%). Numerical analyses were conducted using three-dimensional CFD simulations in AVL FIRE software. The results indicated that increasing the equivalence ratio improved combustion efficiency, with IMEP increasing from 3.74 bar at f = 0.23 to 4.92 bar at f = 0.28, while thermal efficiency improved from 46.13% to 49.88%. However, the sharp increase in the maximum pressure rise rate at f = 0.28 suggested that engine operation approached the knock limit, highlighting the balance between higher efficiency and the risk of 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 emissions increased due to the lower combustion temperatures. Higher EGR levels also raised the risk of misfire, especially at lower equivalence ratios, thereby narrowing the engine's stable operating range. These findings highlight the potential of HCCI engines to improve both environmental sustainability and energy efficiency, while stressing the need for precise control of combustion parameters to mitigate the risks of knock and misfire.