A systematic review on sustainability assessment of internal combustion engines

Internal combustion engines (ICEs) have served as the primary powertrain for mobile sources since the 1890s and also recognized as significant contributors to CO2 emissions in the transportation sector. In order to achieve "carbon neutrality" for transportation sectors, ICE vehicles (ICEVs) are facing substantial challenges in meeting CO2 regulations and intense competition from battery electric vehicles and fuel cell vehicles. Consequently, new technologies of ICEs are continually emerging to enhance competitiveness in reducing environmental impacts. However, the limited studies on the life cycle assessment (LCA) of ICEs make it difficult to evaluate the actual contributions of the emerging technologies from a life cycle perspective. Conducting a systematic review of ICEs LCA studies could identify weaknesses and gaps in these studies for new scenarios. Therefore, this article presents the first systematic review of the LCA of ICEs to provide an overview of the current state of knowledge. A total of 87 life cycle assessment studies between 2017 and 2023 using the Scopus database were identified after searching for the keywords "Sustainability assessment" OR "Life cycle assessment" AND "Internal combustion engine*" OR "ICE*" and carefully screening, and then classified and analyzed by six aspects including sustainability indicators, life cycle phases, life cycle inventories, ICE technologies (including alternative fuels), types of mobile sources and powertrain systems. It is concluded that there are quite limited studies solely focusing on LCA of ICEs, and the LCA assessment lacks consideration of: 1. environmental pollution, human health and socio-economic aspects, 2. fuel production process and maintenance & repair phase, 3. small and developing countries, 4. the emerging ICE technologies and zero carbon/carbon-neutral fuels, 5. large and high-power mobile sources and heavy-duty hybrid technologies.