Abstract This study analyzes the combustion performance of a syngas-fueled homogenous charge compression ignition (HCCI) engine using a toroidal piston (baseline piston), square bowl, and flat piston shape, at low, medium, and high loads, with a constant compression ratio of 17.1. In this study, the square bowl shape is optimized by reducing the piston bowl depth and squish area ratio (squish area/cylinder cross-sectional area) from 34 to 20, 10, and 2.5% and compared with the flat piston shape and toroidal piston shape. This HCCI engine operates under an overly lean air–fuel mixture condition for power plant usage. ansys forte cfd package with GRI Mech3.0 chemical kinetics is used for combustion analysis, and the calculated results are validated by the experimental results. All simulations are accomplished at maximum brake torque (MBT) by altering the air–fuel mixture temperature at intake valve closing (IVC) (TIVC) with a constant equivalence ratio of 0.27. This study reveals that the main factors that affect the start of combustion (SOC), maximum pressure rise rate (MPRR), combustion efficiency, and thermal efficiency by changing the piston shape are the squish flow and reverse squish flow effects. Therefore, the square bowl piston D is the optimized piston shape that offers low MPRR and high combustion performance for the syngas-fueled HCCI engine, due to the weak squish flow and low heat loss rate through the combustion chamber wall, respectively, when compared with the other piston shapes of square bowl piston A, B, and C, flat piston, and toroidal (baseline) piston shape.