Powertrain electrification has been proven as an effective solution to public concerns about fossil fuel usage and carbon emissions. As a midterm technology from conventional internal combustion engine vehicles to electric vehicles, hybrid electric vehicles have received wide research attention from industry and academics alike and are sharing an increasing percentage of vehicles in the market. As a crucial component in hybrid powertrains, the internal combustion engine has important impacts on vehicle performance. Recent years have witnessed tremendous effort towards hybrid electric vehicle-specific engine technologies. Low temperature combustion concept, alternative fuels, over-expansion Atkinson cycle, and waste heat recovery technique provide the most promising solutions to clean and efficient hybrid-specific engines. In this paper, recent progress in these four technologies on hybrid powertrain platforms is reviewed comprehensively. Working principles, influencing factors, benefit potentials, advantages, and disadvantages of each technology are discussed in detail. Although these technologies present improvement in fuel economy and emissions, there are several issues as well. Challenges and concerns associated with system complexity, restricted operating conditions, controls, cost, safety, etc., are specified, and corresponding recommendations are provided from technology level to powertrain level. Future research opportunities are suggested at the end to encourage more research activities in this area.

中文翻译：

---

混合动力电动汽车专用发动机：最先进的审查

动力总成电气化已被证明是解决公众对化石燃料使用和碳排放担忧的有效解决方案。混合动力汽车作为传统内燃机汽车向电动汽车过渡的中期技术，受到了工业界和学术界的广泛研究关注，在汽车市场中所占的比例也越来越大。内燃机作为混合动力总成的关键部件，对车辆性能具有重要影响。近年来，人们在混合动力电动汽车专用发动机技术方面做出了巨大的努力。低温燃烧概念、替代燃料、过度膨胀阿特金森循环和废热回收技术为清洁高效的混合动力专用发动机提供了最有前途的解决方案。本文全面综述了这四项技术在混合动力总成平台上的最新进展。详细讨论了每种技术的工作原理、影响因素、效益潜力、优点和缺点。尽管这些技术改善了燃油经济性和排放，但也存在一些问题。详细说明了与系统复杂性、受限操作条件、控制、成本、安全性等相关的挑战和问题，并从技术层面到动力总成层面提供了相应的建议。最后建议了未来的研究机会，以鼓励在该领域开展更多的研究活动。