This paper focuses on the energy, exergy, exergoeconomic, and sustainability performance of oxygen-hydrogen (HHO) gas utilization for flame jet ignition in a propane-fueled SI engine. The required HHO gas is produced instantaneously using an alkaline electrolyzer. A significant portion of its energy was provided by electrical energy generated from exhaust waste heat using a thermoelectric generator (TEG). The HHO assisted flame jet ignition (HHO_AJI) system ignites the propane-air mixture in the main combustion chamber using HHO gas delivered in different proportions to the pre-chamber. The HHO flow rates used in flame jet ignition were 0.4 L/min (HHO_AJI-0.4), 0.7 L/min (HHO_AJI-0.7), and 1.0 L/min (HHO_AJI-1.0). The experimental studies were conducted at a constant engine speed of 2300 rpm, 50% throttle opening, and six different lambda values (λ = 0.9, 1.0, 1.1, 1.2, 1.3, and 1.4). The results indicate that the HHO_AJI system enhances exergetic performance under lean mixture conditions. At λ = 1.4 and an HHO flow rate of 1.0 L.min-1 thermal efficiency, exergy efficiency, and sustainability index increased by 66.7%, 67.2%, and 14.2%, respectively, compared to conventional ignition, while lost energy and exergy destruction decreased by 18.3% and 14.3%. Moreover, the exergoeconomic analysis showed that the specific exergy cost of shaft power decreased by 6.5%, confirming that operating the SI engine with a lean mixture under HHO_AJI mode is not only thermodynamically but also economically advantageous.