Significant reductions of particulate matter (PM) and nitrogen oxides (NOx) emissions from diesel engines have been realized through fueling with water-fuel emulsions. However, the physical and chemical in-cylinder mechanisms that affect these pollutant reductions are not well understood. To address this issue, laser-based and chemiluminescence imaging experiments were performed in an optically-accessible, heavy-duty diesel engine using both a standard diesel fuel (D2) and an emulsion of 20% water, by mass (W20).
A laser-based Mie-scatter diagnostic was used to measure the liquid-phase fuel penetration and showed 40-70% greater maximum liquid lengths with W20 at the operating conditions tested. At some conditions with low charge temperature or density, the liquid phase fuel may impinge directly on in-cylinder surfaces, leading to increased PM, HC, and CO emissions because of poor mixing. Measurements of natural pre-combustion chemiluminescence showed that the ignition delay of W20 is 30-60% longer than that of D2, and that ignition occurs 30-40% farther downstream in the jet. As a result, significantly more premixing of fuel and air occurs prior to ignition for W20, leading to a leaner mixture for the initial premixed burn, which should decrease soot formation during the premixed combustion phase. The diffusion flame lift-off for W20, extracted from images of natural OH chemiluminescence, was consistently 20-60% longer than that for D2 at the operating conditions tested. This shift in lift-off length allows more entrainment of air upstream of the diffusion flame after the initial premixed burn phase, which results in leaner mixtures and should decrease soot formation during the mixing-controlled combustion phase, in the absence of liquid fuel impingement on in-cylinder surfaces.
These mechanisms of pollutant formation for water emulsions are shown to be consistent with emissions measurements observed under steady-state conditions in a multi-cylinder production diesel engine.