Premixed Diesel Combustion Analysis in a Heavy-Duty Diesel Engine

Optimizations were performed on a Heavy-Duty diesel engine equipped with a conventional electronic unit injector in order to minimize fuel consumption, and emissions of NOx and particulate matter. A low speed light load case and a high speed light load case were optimized with these considerations in mind. Exhaustive parametric studies were performed in order to find sets of operating conditions that resulted in low emissions and high fuel economy. It was found for the low speed light load case (Mode 2, 25% load and 821 rev/min) that low emissions operating conditions existed at either very early or very late start-of-injection timings and high EGR (PM = 0.018 g/kW-hr, NOx + HC = 1.493 g/kW-hr with SOI = -21 degrees ATDC, 48% EGR; or 0.085 g/kW-hr PM, 1.02 g/kW-hr NOx with SOI = 4 degrees ATDC, 39% EGR). For the high speed light load case, it was found that low emissions were available when operating with an early start-of-injection timing and high EGR (0.059 g/kW-hr PM, 2.52 g/kW-hr NOx with SOI = -28 degrees ATDC, 49% EGR). Particulate is known to be formed in fuel rich regions, but if the local equivalence ratio is kept below approximately 2, very little particulate is formed. Analysis of the present data shows that if there is an optimal delay between the end of injection and the start of combustion (2 to 4 crank angle degrees), then enough mixing takes place so that the maximum local equivalence ratios are reduced. NOx is shown to decrease steadily as the EGR rate is increased, or as the start-of-injection timing is retarded.