| Summary: | 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.
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