INTRODUCTION In recent years, automotive fuel efficiency improvements and reductions in emissions have been pursued in the face of ever-stricter regulations to protect the environment by reducing the CO2 emissions considered to cause global warming. Electric technologies such as EV (Electric Vehicles) have been increasing in response, but for the time being, conventional vehicles running solely on internal combustion engines and hybrid vehicles which combine conventional and electric motors is likely to continue to be the mainstream, which means improvements in the fuel consumption of internal combustion engines will remain essential to the future. There are technologies which improve vehicles fuel consumption by improving partial thermal efficiency. Typical engine technologies include the downsized turbocharged engine configuration, Variable Cylinder Management ( 1), etc. On the other hand, there are technologies which improve overall engine thermal efficiency. For this purpose, it is necessary to improve the maximum thermal efficiency with a higher compression ratio and a higher EGR rate. This study focused improvement of the maximum thermal efficiency by these methods. The target value of the maximum thermal efficiency was set to the 45%, a 5 % improvement over the current 40%. In particular, the S/B ratio had a large effect. A higher S/B ratio makes Surface-V olume ratio (S/V ratio) of combustion chamber at TDC lower, therefore cooling loss is reduced( 2). In this study, S/B ratio was varied from 1.2 to 2.0.IMPROVING THERMAL EFFICIENCY In pursuit of improvement in maximum thermal efficiency, stoichiometric air-fuel ratio, 91 RON gasoline fuel, and an engine speed of 2000 rpm were set as operating conditions. To estimate thermal efficiency, the engine specifications and efficiency were determined using a one-dimensional simulation. Using performance data on existing engines, thermal efficiency was estimated under MBT (Minimum advance for the Best Torque) condition in relation to compression ratio and EGR rate. Figure 1 shows boost pressure (vertical axis) and EGR rate (horizontal axis) in relation to brake thermal efficiency when the compression ratio is 15 (a) and 17 (b). When MBT operation is possible at a compression ratio of 17, EGR rate of about 30%, and boost pressures above 250kPa, 45% brake thermal efficiency can be achieved. Figure 1. Estimated result of brake thermal efficiency at engine speed = 2000rpm, MBTThermal Efficiency Enhancement of a Gasoline Engine Kenichiro Ikeya, Masanobu Takazawa, Taketo Yamada, Shinrak Park, and Ryutaro Tagishi Honda R&D Co Ltd ABSTRACT The goal of this research was to improve thermal efficiency under conditions of stoichiometric air -fuel ratio and 91 RON (Research Octane Number) gasoline fuel. Increasing compression ratio and dilution are ef fective means to increase the thermal efficiency of gasoline engines. Increased compression ratio is associated with issues such as slow combustion, increased cooling loss, and engine knocking. Against these challenges, a higher stroke-bore ratio (S/B ratio) and a lower ef fective compression ratio were tried as countermeasures. With respect to increased dilution, combustion of a high-EGR (Exhaust Gas Recirculation) was tried. High-energy ignition and optimized combustion chamber shape with high tumble port were tried as countermeasures against slow combustion and reduced ignitability due to a higher EGR rate. As a result of this study on single cylinder engine, a brake thermal efficiency of 45% was achieved at an engine speed of 2000 rpm with a S/B ratio of 1.5, a compression ratio of 17, an ef fective compression ratio of 12.5, and an EGR rate above 30%. Application to a four-cylinder engine was investigated, taking into account factors such as an engine height, effects between cylinders and the impact of a supercharger on the intake and exhaust systems. CITATION: Ikeya, K., Takazawa, M., Yamada,