A Review on the Development of Hybrid Powertrain Systems and Alternative Clean Energy Engines

Abstract

This review examines hybrid powertrain systems and renewable-fuel internal combustion engines as transitional and long-term solutions for low-carbon transportation. As the global energy structure shifts from fossil fuels to cleaner and more sustainable energy sources, hybrid powertrain systems currently serve as an important transitional technology that combines benefits of internal combustion engines and electric motors. Therefore, this review first compares series, parallel, and series-parallel hybrid architectures in terms of energy flow, efficiency characteristics, application scenarios, and technical limitations. It then discusses energy management strategies, with particular attention to reinforcement learning methods for real-time power allocation. Finally, hydrogen and ammonia engines are reviewed as potential carbon-free combustion pathways, with a focus on combustion characteristics, storage challenges, emissions, and future development trends. Overall, this study aims to summarize the current status, key technological bottlenecks, and future development trends of hybrid and alternative clean energy engine technologies for automotive powertrains.