Soot Formation Study in a Rapid Compression Machine

A rapid compression machine was used to study the soot formation process under diesel enginelike conditions. The apparatus creates accurately controlled conditions at the end of compression (uniform mixture, temperature, and well-defined mixture composition) and, by decoupling chemistry with mixing, provides an unambiguous data interpretation for kinetics study. The soot evolution was studied by the line-of-sight absorption method (at $632.8nm$⁠), which measured the soot volume concentration evolution in the initial stage of soot growth before the optical path became opaque. For a rich butane mixture at fuel equivalence ratio of 3, the ignition delay showed a negative temperature dependence at intermediate temperatures. The soot volume fraction showed an initial exponential growth, with a growth rate depending on the compressed charge fuel concentration. A substantial amount of soot was formed after the soot cloud became opaque. By weighing the total soot particles after the experiment, only $∼10-15%$ of the soot mass was formed when the beam transmission was reduced to 5%. The final soot mass was $∼15-18%$ of the total carbon mass for compressed charge density of $250mol∕m3$ and temperature from $740to930K$⁠.