Observation and analysis of the formation of diesel soot with a laser light extinction method

The soot emission from diesel engines generally increases with the shortening of the ignition lag. However, the mechanism of this phenomenon in the combustion chamber is obscure and difficult to follow. The objective of this investigation is to observe and analyze the in-chamber soot formation process under varied ignition lag with simultaneous photography of the direct flame image and laser shadowgraph. In the experiment, the qualitative measurement of the soot concentration was carried out by substracting the laser light extinction through a non-firing chamber from that through a firing chamber. It was found that the soot concentration in the divided chamber showed a maximum immediately after the start of combustion, and reduced rapidly. With a shortening of the ignition lag, the maxmum soot concentration in the divided chamber increased while the apparent soot formation rate decreased.