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A new nitrogen oxide (NOx) reduction concept is suggested. A strong vertical vortex generated within the combustion bowl can mix hot burned gas into the cold excess air at the center of the combustion-bowl. This makes the burned gas cool rapidly. Therefore, it is possible to reduce NOx, which would be produced if the burned gas remained hot. In this paper the effect was verified with a 3D-CFD analysis of spray, air, combustion gas, and thermal efficiency as well as experiments on a 4-cylinder 2.0-liter direct injection diesel engine. The results confirmed that the vertical vortex was able to be strengthened with the change of spray characteristics and the combustion bowl shapes. This strengthened vertical vortex was able to reduce NOx by approximately 20% without making smoke and thermal-efficiency worse. Above results proved the effectiveness of this method.

In-cylinder flow and fuel behaviors in the piston cavity of a direct injection SI engine were measured by using PIV and LIF. The effect of the cavity wall on the mixing process was the focus in this study. The optical prism was installed inside piston to observe air flow and fuel behavior on a horizontal plane of the cavity combustion chamber in the piston. The fuel spray mainly impinged on the cavity bottom surface and rolled up along the cavity wall near the spark plug by it's own momentum. Then it was evaporated and diffused by swirl flow. The effect of fuel injection timing on the mixing process was also investigated. Earlier injection timing made fuel momentum small up to the time of impingement. Therefore, the fuel vapor was considerably diffused by swirl flow in the piston cavity and fuel vapor concentration near the spark plug was low.

In order to control the mixture formation, a mixture injected 4-valve SI engine was developed with a small mixture chamber and mechanically driven mixture injection valve installed into the cylinder head. The mixture injection valve was located at the center of the combustion chamber. The mixture was injected from the final stage of the intake stroke to the beginning of the compression stroke. The mixture distribution and in-cylinder flow field inside the combustion chamber were measured by a pair of laser two-dimensional visualization techniques. A planar-laser-induced exciplex fluorescence technique was used to visualize the in-cylinder mixture formation by obtaining spectrally separated fluorescence images of liquid and vapor phase fuel distribution. Particle image velocimetry (PIV) was used to obtain flow field images. In the case of the mixture injected SI engine, the mixture injected into the swirl center was retained during the compression stroke.