A Study of Nox Generation Mechanism in Diesel Exhaust Gas 901615

The authors inserted an optical fiber into the main combustion chamber of a DI engine and led the diesel flame to the outside of the chamber. They operated the engine under various conditions and investigated into the relations between the intensity of CN band spectra in the flame, the flame temperature and the Nox concentration in the exhaust gas. As the result, a certain correlation was observed between the Nox cencentration and CN spectra, but no remarkable correlation was found between Nox and flame temperature. In view of the above facts, the Nox generation is presumably related to CN, which is an intermediate product in diesel combustion flame.

IN ORDER TO MAKE CLEAR the state of combustion in diesel engines, the analysis of the phenomena must be done both in the physical and in the chemical aspects. However, diesel combustion phenomena take place instantaneously in the totally closed chamber at high temperature and high pressure, and it has never been a subject to be readily elucidated by the conventional method of chemical analysis. Accordingly, spectroscopic process was selected as an effective method for analyzing the chemical reaction in diesel combustion, and measurements have been made by means of this method ^{1) 2)}. Unfavorably, however, in case of the chemical reaction of hydrocarbon compounds which has been regarded as the main factor in diesel combustion, majority of the various band spectra are in the ultraviolet and visible ranges. Spectroscopy in those ranges has long been regarded impossible because of the adverse effects of soot, bright flame and the like.

The authors recently employed “streak camera”, a special apparatus for high speed spectroscopic analysis, and carried out a series of experiments on the combustion flame in a DI engine. As the result, state of the existing band spectra in the diesel flame and its behavior at the moment of fuel ignition have been partly made clear³⁾. However, quantitative evaluation of the light energy of those spectra has been left for the future experiments.

For the purpose of reducing the amount of Nox in diesel engine exhaust, analysis of Nox generation mechanism is essential. With regard to the mechanism of Nox generation in diesel flame, so called “thermal Nox theory” has been generally accepted, in which it is presumed that the nitrogen in the suction air is exposed to high temperature and oxidized in the combustion chamber, resulting in the formation of Nox. However, verification of this theory through chemical method has not been done yet.

In the present experiment, the authors tried to quantitatively evaluate the intensity of band spectra existing in diesel flames by means of the streak camera. Special attention was given to the CN band spectra in the flame while running the engine under various conditions, and the relation between the intensity of CN band spectra and the concentration of Nox in the exhaust was investigated. At the same time, correlation between the diesel flame temperature and Nox concentration was examined. As the result, a certain of correlation was observed between the Nox concentration and CN band spectra. But no remarkable correlation was found between Nox and flame temperature.