Emission Formation Mechanisms in a Two-Stroke Direct-Injection Engine 982697
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 NOx due to both burning rate and phasing considerations which affected the thermal NOx 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.