The effects of tumble flow generated by various intake ports on the lean burn characteristics in a four-valve pentroof engine were investigated. Tumble intensities were tested for three intake ports of different entry angle; 25\mD, 20\mD and 15\mD. The flow characteristics in cylinder were measured under motored conditions with laser doppler velocimetry. The lean burn performances were examined in an operating 4-cylinder engine. The combustion duration under lean mixture conditions were calculated by heat release analysis of the pressure trace. The flame developments in a single-cylinder optical engine were visualized with an image-intensified CCD camera to see the effect of different flow patterns with three inlet ports of strong and weak tumble.
It was found that there is a correlation between the stronger tumble during induction and the higher convective velocity and turbulence levels at the time of ignition, which result in faster and stable combustion under lean mixture conditions. Images of the developing flame also revealed faster flame propagation in the stronger tumble port under lean mixture conditions, than that in weak tumble port. By comparing tumble intensities and combustion durations together with flame images, the design of intake port with the optimal entry angle was discussed.