Emission Formation Mechanisms in a Two-Stroke Direct-Injection Engine

Engine tests were conducted to study the effect of fuel-air mixture preparation on the combustion and emission performance of a two-stroke direct-injection engine. The in-cylinder mixture distribution was altered by changing the injection system, injection timing, and by substituting the air in an air-assisted injector with nitrogen. Two injection systems which produce significantly different mixtures were investigated; an air-assisted injector with a highly atomized spray, and a single-fluid high pressure-swirl injector with a dense penetrating spray. The engine was operated at overall A/F ratios of 30:1, where stratification was necessary to ensure stable combustion; and at 20:1 and 15:1 where it was possible to operate in a nearly homogeneous mode. Moderate engine speeds and loads were investigated.

The effects of the burning-zone A/F ratio were isolated by using nitrogen as the working fluid in the air-assist injector. The burning-zone A/F ratio was found to be the most significant factor in the production of NO_x due to both burning rate and phasing considerations which affected the thermal NO_x production and also by chemical production mechanisms. The burning-zone A/F ratio was also found to impact the CO formation, predominately at retarded injection timings. The HC emissions were moderately affected by the burning-zone A/F ratio, and were also observed to result from overmixing of the spray, short-circuiting of the fuel, and poor combustion quality. Some evidence of anomalous injector behavior was detected in the HC and CO results.