A GAS SAMPLING STUDY ON THE FORMATION PROCESSES OF SOOT AND NO (NITRIC OXIDE) IN A DI (DIRECT INJECTION) DIESEL ENGINE

Abstract

Concentrations of soot, nitric oxide (NO), and other combustion products were measured by in-cylinder gas sampling in a direct-injection diesel engine. Effects of injection timing, swirl ratio, and combustion chamber geometry on the formation and emission processes of soot and NO were studied. Findings revealed that soot is promptly formed in the flame during the early combustion period where the equivalence ratio in the flame is high (over 1.0). Thereafter, almost all formed soot is quickly burned up by oxidation during the middle combustion period. Nitric oxide is formed in the flame during the early and middle combustion periods where the flame temperature was high (over 2000 K). The highest NO concentration was observed at the flame tip swept by the air swirl. Although the concentration of formed NO decreases by dilution it is nearly constant during the later combustion period; thus, the concentration of exhaust NO is almost completely governed by the formation process. Retarded injection timing not only decreases pressure and flame temperature but also increases the flame's equivalence ratio. Because of this, the maximum NO concentration decreases while the maximum soot concentration increases in the combustion chamber. The optimum swirl condition produces a good fuel distribution in the combustion chamber and reduces the maximum equivalence ratio.