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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.

The authors inserted an optical fiber into the combustion chamber of a DI engine and investigated the diesel flame spectra. The engine, attached with a water spray device in the intake pipe, was operated under various ignition timings, and the relations were investigated among the intensity of CN band spectra, NOx concentration in the diesel exhaust and the flame temperature. As the result, a certain correlation was found between CN band spectra and NOx concentration. From this correlation, they confirmed that the NOx in the diesel exhaust contains not only the generation by “the Zeldvich mechainism”, but also “the Prompt NOx”. They also confirmed that to reduce “the Prompt NOx”, it is not effective to retard the ignition, but effective to spray water into the combustion chamber in high load operations.