Improving vehicle fuel economy is a central part of efforts toward achieving a sustainable society. An effective way for accomplishing this aim is to enhance the engine thermal efficiency. Measures to mitigate knocking and reduce engine cooling heat loss are important aspects of enhancing the engine thermal efficiency. Cooled exhaust gas recirculation (EGR) is regarded as a key technology because it is capable of achieving both of these objectives. For this reason, it has been adopted in a wide range of both hybrid and conventional vehicles in recent years. Toyota has been introducing these technologies as ESTEC (Economy with Superior Thermal Efficient Combustion).Improving cycle-to-cycle variations in combustion, in addition to fast combustion is essential for achieving high engine thermal efficiency.In this paper, the effect of cycle-to-cycle variations in high tumble combustion, which is the key technology in current SI (Spark Ignition) engines, is investigated by using results combined with the detailed phenomenon analyses of flow formation in combustion chamber during the intake and compression strokes and cycle-to-cycle variation in initial flame propagation formation. A new direction in the means of improving cycle-to-cycle variations in next-generation engines combustion is obtained because both experiment and simulation are used. And the effectiveness of high-tumble concept on generating high turbulence in combustion chamber is also described.