Graduate Thesis Or Dissertation
 

Partial recirculation of diesel engine exhaust for the reduction of oxides of nitrogen

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  • Oxides of nitrogen are a concern as effluents to atmospheres because they participate in photochemical reactions which produce smog, because they attenuate sunlight which gives the atmosphere a reddish brown cast, and because of their toxic properties and the health hazards associated with the photochemical products they produce. Oxides of nitrogen are formed during the heterogeneous combustion of the diesel engine. When the locally overlean regions are subjected to high temperatures, nitric oxide (NO) is formed from the nitrogen and oxygen in the air. Nitrogen dioxide (NO₂) is formed slowly from the nitric oxide. The concentration of oxides of nitrogen formed is a function of the fuel-air ratio; the concentration magnitude is limited at low fuel-air ratios by the peak-cycle temperature of combustion and at high fuel-air ratios by the amount of excess air available. Oxides of nitrogen were measured by the spectrophotometric method, which was originally conceived by California Research Corporation. An instrument based on this method was built at this institution by Douglas E. Johnson and was modified and developed as a part of this thesis. The tests were conducted by setting the engine to initial conditions of speed and load close to peak power but below the exhaust smoke point. This initial run was made without any recirculation. The subsequent runs allowed increasing amounts of recirculated gas to flow to the engine intake; the engine speed was maintained constant by varying the load on the engine. The quantity of recirculation was independently varied and the temperature, which was dependent on the quantity of recirculation, was allowed to vary simultaneously. The results of these recirculation tests, which allowed both recirculation variables to increase simultaneously, show that the concentration of oxides of nitrogen in the exhaust gas remained essentially constant, at 300 parts per million (ppm), up to eight percent of exhaust gas recirculation from which the concentration increased rapidly to 700 ppm at 11 percent of exhaust gas recirculation. Increasing the percent of exhaust recirculation from zero to 11 percent caused the fuel-air ratio to increase from 0.050 to 0.065. The engine performance parameters were also affected by the exhaust recirculation. The horsepower (hp) output decreased from 32 hp at zero percent recirculation to 27.7 hp at 11 percent recirculation, a decrease of 12.5 percent. The exhaust smoke intensity decreased slightly between zero and about four or five percent recirculation but then increased rapidly becoming objectionable before reaching 11 percent recirculation. The temperature of the exhaust gas in the exhaust stack remained essentially constant at 1,000° F from zero to seven percent recirculation and then increased to 1,100° F at 11 percent recirculation. Concentrations of carbon monoxide, 0.1 percent, and unburned hydrocarbons, 20 ppm, were not substantially affected by the recirculated exhaust; carbon dioxide concentrations increased from 9.6 percent at zero percent recirculation to 11.6 percent at 11 percent exhaust recirculation. These tests show that there are no reductions in concentration of oxides of nitrogen to be gained by recirculation of exhaust gas, as conducted in the manner of these tests. In addition, the engine parameters of horsepower and smoke intensity were not benefited; the concentration of carbon monoxide and hydrocarbons was virtually unaffected, and the concentration of carbon dioxide increased by two percent. It is recommended that further studies of diesel exhaust recirculation be conducted under conditions where the recirculation temperature is held to a low constant value, because theory and earlier work, under unknown operating conditions, indicate that oxides of nitrogen can be reduced by exhaust gas recirculation.
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