Laserdiagnostik in Verbrennungsprozessen

Soot Formation in Internal Combustion Engines

Flickering candle lights, campfires or internal combustion engines produce soot. Formation of soot means a loss of usable energy. Deposits of soot vitiate the thermal and mechanical properties of an engine. The distribution of soot directly affects the heat radiation and the temperature field of a flame.

setup experiment
Fig. 1: Experimental setup for the simultaneous, spatially and temporally resolved measurement of soot volume fraction (LII), NO concentration (NO-LIF) and fuel vapor concentration (Rayleigh scattering) in a high-pressure high-temperature Diesel spray chamber under engine-like conditions

The exhaust gas of Diesel engines contains fine soot particles. These are suspected causing cancer. The black soot clouds of the Diesel engines in the 1980s are gone. But the remaining, invisible fine particles are a severe toxicological problem. They are supposed to cause asthma and cardiac infarctions. [1]

Soot formation in engines driven by hydrocarbons, especially Diesel engines and aircraft turbines are in the focus of research. Soot and nitric oxide NO hereby behave like antagonists. A small soot production is at the expense of a large NO formation, and vice versa. However, while the formation of nitric oxides in internal combustion engines is quite well understood formation of soot is by far more complicated and difficult to examine. The formation of soot particles in Diesel sprays (Vaporization of Sprays) is so fast and complex that it is not sufficiently understood yet.

schematic setup
Fig. 2: Schematic setup from Fig. 1

Soot consists of agglomerates with a diameter of up to several hundred nanometers. These have a fine structure of spherical primary particles. Soot formation starts with the pyrolysis of fuel molecules and the formation of polycyclic aromatic hydrocarbons (PAH). The most important precursor of the formation of higher hydrocarbons is acetylene (C2H2). 2-dimensional condensation processes follow. Finally a rearrangement produces spherical primary particles which continue growing at their surface.

Most examinations of soot in combustion processes were based on elastic scattering, extinction or sampling. Latterly, Laser-Induced Incandescence (LII) proved to be an attractive, robust and versatile technique for measuring soot in unstable flows of complex geometry [2]. Fig. 1 and 2 show an experimental setup for the simultaneous measurement of soot volume fraction (LII), NO concentration (NO-LIF) and fuel vapor concentration (Rayleigh scattering) at a burning Diesel spray under engine-like conditions.

[1] D. H. Lamparter, Kennzeichen D - Deutschlands Autofahrer steigen reihenweise auf Diesel um - Umweltschützer warnen vor möglichen Gesundheitsgefahren, Die Zeit, No. 3, Jan 13, 2000, p. 19.
[2] J. Dec et al., Soot Distribution in a D.I. Diesel Engine Using 2-D Laser Induced Incandescence Imaging, SAE paper No. 910224 (1991).

Joachim Gronki



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