Means for reducing the particulate matter (PM) from swirl chamber type diesel engines were searched out, and the reducing mechanisms were investigated using an optically accessible engine. Both of the swirl chamber and the main chamber were simultaneously observed by the shadowgraph method. The following points were clarified in this study.

(1) At a light load, the suppression of the initial injection rate reduces PM, because SOF is reduced by the change in ignition point and smoke is reduced by the retarded flowout of the dense soot from the swirl chamber.

(2) Over a medium load, the main cause of the exhaust smoke is fierce spray-wall impingement which leads to fuel adhesion on the wall and the stagnation of a rich fuel-air mixture.

(3) Enlarging swirl chamber volume ratio suppresses the formation of dense soot in the swirl chamber. In the main chamber, however, the soot oxidization becomes insufficient due to the mixing effect reduced by the essentially decreased chamber depth.

(4) The most effective method for obtaining the sufficient mixing of effect in the main chamber is to maintain the necessary depth of the trench area, which prevents the widening of the flame jet from the connecting passage and consequently ensures the twin-vortex formation.

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