TED-AJ03-289 SIGNIFICANCE OF THE SPRAY TIP REGION IN NO_x EMISSIONS OF DIESEL COMBUSTION

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In diesel combustion, NOx is one of the most difficult elements to control, because the equivalence ratio is outside of the available range of NOx catalysts and also because the lean combustion, as homogeneous charge compression ignition (HCCI), is not an option for the whole operation range due to power and efficiency concerns. It is known that NOx emissions typically increase when the fuel injection pressure is increased. This is generally explained as the fuel mixes well with air and high temperature region is established widely. When considering residence time in the jet flame, however, the available time for NOx formation[figure]would be decreased in high-pressure fuel injection due to faster diffusion of the combustion gas with the air. To answer the arguments, one of the authors made simple analysis. Figure A-1 shows changes in instantaneous NO formation speed for spray penetration with three different initial injection velocities relative to the effective nozzle diameter, i. e. with three different residence time scales. The NO increase significantly with increases in the penetration length and make a peak value, after the peak it decreases and converges to the value for fully developed sprays with the increase of the penetration length. The NO emission for the fully developed sprays is smaller with higher injection velocities. In diesel combustion, the dimensionless penetration increases with higher-pressure fuel injection and the point shifts from "A" to "B", resulting in increased NO emission. The phenomenon was explained as much larger amount of NOx than steady jet flame is formed at the tip of the jet flame shortly after start of injection and as the increased NOx by higher fuel injection speeds is due to the development of this region. The objective of the present paper is to evaluate the above predictions, and identify the significance of NO formation at the tip of the spray shortly after injection. Experiments were made to measure NOx from a jet flame injected in a closed vessel with different injection speeds and periods. The experimental results indicated agreement with the predictions, showing that the source of major NO emissions is the tip of the flame shortly after the start of injection. Two-dimensional numerical simulation was also performed and showed the appropriateness of the above conclusion.[figure]

TED-AJ03-289 SIGNIFICANCE OF THE SPRAY TIP REGION IN NO_x EMISSIONS OF DIESEL COMBUSTION

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