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Study of diesel soot formation in fuel-rich combustion using a rapid compression machine


In recent years regulatory limits on emissions from diesel engines have become more and more severe. Particular attention has been devoted to soot formation, due to its dangerous impact on the environment and human health. The mechanisms leading to soot formation are not completely understood yet, despite much attention focused on this topic. Past research has shown that during diesel combustion, soot is formed in the fuel-rich zone downstream the liquid core spray; even before the diffusion flame subsists. It seems also that this premixed fuel-rich zone, where soot precursors exist due basically to pyrolysis and condensation phenomena, affects soot formation during the whole combustion process.

A specially designed rapid compression machine (RCM) has been built with the aim of studying this zone in detail and providing a set of soot formation kinetics data in an diesel engine-like pressure and thermal environment. Optical diagnostics (laser line-of-absorption technique) is used to keep track of soot particles and determine their mean diameter. Initial tests have shown soot volume loading history for various fuels equivalence ratio, pressure and temperatures. In parallel soot formation kinetics modelling is proceeding in collaboration with the Chemical Dept at M IT.

The aim of the present study is mainly to develop a better understanding of combustion conditions which may lead to smaller soot formation, based on the experimental arrangement described. Objectives consist also in studying the behaviour of different fuels (longer, branched HC's, polyaromatics, oxigenate and CN improvers) and understanding soot formation threshold due to equivalence or C/O ratio. The data set produced is also aimed at the verification of a complex (multi-zone, taking into account temperature non-uniformities) model currently being developed at MIT.

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