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Improving vehicle fuel economy is a central part of efforts toward achieving a sustainable society, and an effective way of 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 vehicles and conventional vehicles in recent years. Cooled EGR has the potential to achieve further lower fuel consumption if the EGR ratio can be increased. Fast combustion is an important and effective way for expanding the EGR ratio. The engine combustion enhancement can be categorized into measures to improve ignition characteristics and methods to promote flame propagation.

Improving vehicle fuel economy is a central part of efforts toward achieving a sustainable society. An effective way of accomplishing this is to enhance the engine thermal efficiency. Mitigating knock and reducing engine heat loss are important aspects of enhancing the 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 vehicles and conventional vehicles in recent years. In EGR equipped engines, fast combustion is regarded as one of the most important technologies, since it realizes higher EGR ratio. To create fast combustion, generation of strong in-cylinder turbulence is necessary. Strong in-cylinder turbulence is achieved through swirl, squish, and tumble flows. Specifically high tumble flow has been adopted on a number of new engines because of the intense effect of promoting in-cylinder turbulence.

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.

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.