Abstract In recent years, enhancing engine thermal efficiency is strongly required. Since the maximum engine thermal efficiency is especially important for HVs, the technologies for improving engine thermal efficiency have been developed. The current gasoline engines for hybrid vehicles have Atkinson cycle with high expansion ratio and cooled exhaust gas recirculation (EGR) system. These technologies contribute to raise the brake engine thermal efficiency to more than 38%.In the near future the consumers demand will push the limit to 40% thermal efficiency. To enhance engine thermal efficiency, it is essential to improve the engine anti-knock quality and to decrease the engine cooling heat loss. To comply with improving the anti-knock quality and decreasing the cooling heat loss, it is known that the cooled EGR is an effective way. Therefore the cooled EGR technology is widely spread throughout the world and ESTEC (Economy with Superior Thermal Efficient Combustion) has been introduced in some papers. This paper describes some concepts like high tumble that are able to expand the EGR limit and to realize the engine thermal efficiency of 40%. Introduction Countries around the world are introducing more stringent fuel economy standards as part of the measures to address energy security and climate change issues. For this reason, automakers are making a lot of efforts to improve the fuel economy of their vehicles by reducing vehicle weight, CVT, and developing hybrid vehicles. Especially hybrid vehicles are an important technology to improve the fuel economy of vehicles. Figure 1 shows the operation area of a HV engine and of a conventional engine. It can be shown that the HV engine uses higher brake mean effective pressure (BMEP) area and higher engine thermal efficiency area compared toconventional engines. Therefore improving the maximum engine thermal efficiency leads to better fuel economy of the hybrid vehicles. Figure 2 shows the historical and future trends of maximum engine thermal efficiency in gasoline engines. As it can be seen, the maximum engine thermal efficiency level for conventional vehicles was recently around 35%. A new engine for conventional vehicle reached 38% using the high expansion ratio and the cooled EGR which were originally introduced in HVs [ 1]. In the future, it is expected that technologies for HVs will be applied to most conventional vehicles and the engine thermal efficiency will be further increased. It means that the role of combustion becomes even more important for engine development, in addition to the improvement of traditional concepts, such as low friction and valve train system. Figure 1. Engine operation range for different drivetrainsCombustion Development to Achieve Engine Thermal Efficiency of 40% for Hybrid Vehicles2015-01-1254 Published 04/14/2015 Daishi Takahashi, Koichi Nakata, Yasushi Yoshihara, Yukinori Ohta, and Hiroyuki Nishiura Toyota Motor Corp. CITATION: Takahashi, D., Nakata, K., Yoshihara, Y ., Ohta, Y . et al., "Combustion Development to Achieve Engine Thermal Efficiency of 40% for Hybrid Vehicles," SAE Technical Paper 2015-01-1254, 2015, doi:10.4271/2015-01-1254. Copyright © 2015 SAE InternationalFigure 2. History and future direction of engine thermal efficiency Engine Thermal Efficiency Improvement Measures and Future Direction This section describes adopted methods for improving the engine thermal efficiency and then discusses the future direction by using engines for HVs as an example. Gasoline engine thermal efficiency can be expressed theoretically using Equation (1) , which represents the efficiency of the Otto Cycle. (1) (2) Here, ε is the compression ratio or the expansion ratio, cv is the specific heat at constant volume, and cp is the specific heat at constant pressure. These equations show that a higher expansion ratio or higher specific heat ratio leads to an improvement in the engine thermal efficiency.