INTRODUCTION Countries around the world are introducing more stringent fuel economy standards as part of measures to address issues related to energy security and climate change. To correspond to these requirements, automakers have been making a lot of efforts to develop new vehicles. One comprehensible example is that HVs have been spreading widely throughout the world because the fuel economy of HVs is much better than that of conventional vehicles. As for the engine development, enhancing the maximum engine thermal efficiency is especially essential for HVs because the engine operating points are mainly on the high engine load area where engine thermal efficiency is close to the maximum engine thermal efficiency . Therefore the maximum engine thermal efficiency for HVs has been enhanced prior to the engines for conventional vehicles. In these days, the technologies which have been developed for HVs are being adopted to the engines for conventional vehicles [ 2,3]. Figure 1 shows engine thermal efficiency history and the future direction. As described before, the maximum engine thermal efficiency comes close to 40%. Main technologies which have been developed to contribute to enhance engine thermal efficiency are Atkinson cycle, cooled EGR, and low friction technologies. In the short term, it is expected to enhance the maximum engine thermal efficiency more than 40% by modifying the current technologies [ 5]. However, since it is considered that much higher engine thermal efficiency is required to meet more stringent social requirements in the future, new technologies should be developed. In the following sections, this paper describes the future technologies for enhancing engine thermal efficiency. Fig.1. Engine Thermal Efficiency History and Future Direction THERMAL EFFICIENCY EMHANCEMENT Theoretical engine thermal efficiency is well known as the Otto cycle equation. This equation means a higher expansion ratio or a higher specific heat ratio leads to higher engine thermal efficiency. The former one can be achieved by raising the compression ratio [ 6] or by retarding EVO (Exhaust Valve Open timing) and the latter one can be achieved by adopting lean burn [ 7]. However raising the compression ratio and lean burn has some issues to be solved such as knocking and the enhancement of combustion. Actual engine thermal efficiency results from various losses such as mechanical loss, pumping loss, cooling heat loss, exhaust loss, and unburned loss, differing from theoretical engine thermal efficiency. Engine Technologies for Achieving 45% Thermal Efficiency of S.I. Engine Koichi Nakata, Shinichiro Nogawa, Daishi Takahashi, Yasushi Yoshihara, Atsunori Kumagai, and Tetsushi Suzuki Toyota Motor Corporation ABSTRACT To correspond to the social requirements such as energy security, and climate change, enhancing engine thermal efficiency is strongly required in these days. As for the specific engine technologies to improve engine thermal efficiency, Atkinson cycle, cooled EGR (Exhaust Gas Recirculation), and low friction technologies have been developed [ 1, 2, 3, 4]. As a result, the current maximum thermal efficiency comes close to 40%. However, since it is considered that much higher engine thermal efficiency is required in the future to meet more stringent social requirements, a new prototype L4 engine which features a long stroke design with a high tumble is investigated to clarify the future direction in this paper. In regard to combustion, the lean boosted concept with cooled EGR is examined. In consequence, it is shown that more than 45% engine thermal efficiency can be achieved. This paper describes the means to enhance engine thermal efficiency and a future possibility. CITATION: Nakata, K., Nogawa, S., Takahashi, D., Yoshihara, Y . et al., "Engine Technologies for Achieving 45% Thermal Efficiency of S.I. Engine," SAE Int. J. Engines 9(1):2016, doi:10.4271/2015-01-1896.2015-01-189