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 effective 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 effective 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 effective 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.