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. However, the application of high tumble flow incurs the reduction of intake mass flow rate. Toyota considers that increasing engine output (performance) is also important to achieve a “Fun to Drive” characteristic for customer satisfaction. So the development of technologies that generate tumble flow effectively while maintaining high intake flow coefficient is essential.
This paper describes the key requirements for engine design to generate high tumble flow with high flow coefficient by using both simulation and experiment. It is important that the air flow of tumble is straight and runs along the wall of the combustion chamber near the exhaust valves for high tumble flow efficiency. As a result, better performance can be achieved by changing the layout of the valve angle and the intake port. The generated tumble flow improved by 10%.