Soot Formation in Low-Temperature Compression Ignition.

Abstract

A lean n-heptane/air mixture is compressed up to hot ignition using a rapid-compression machine. Attenuation and scattering of an incident laser light are measured to evaluate soot formation in compression ignition processes. Even though the mixture is lean, soot is produced in the post-flame period of compression ignition. The red coloration of combustion products is mostly due to the thermal radiation of the soot. It is also observed that the soot precursor is formed correspoonding to the cool and blue flame onsets during the preflame peried, only in the cases where the compression temperatures for these low-temperature flames belong to those of the negative-temperature coefficient regime. The soot precursor first appears just after the cool fiame has degenerated. Soot-precursor formation is closely related to ion generation, and the behavior of soot formation is deduced to depend on fuel pyrolysis in preflame processes of low-temperature ignition.