The effects of spray targeting on mixing, combustion, and pollutant formation under a low-load, late-injection, low-temperature combustion (LTC) diesel operating condition are investigated by optical engine measurements and multi-dimensional modeling. Three common spray-targeting strategies are examined: conventional piston-bowl-wall targeting (152° included angle); narrow-angle floor targeting (124° included angle); and wide-angle piston-bowl-lip targeting (160° included angle). Planar laser-induced fluorescence diagnostics in a heavy-duty direct-injection optical diesel engine provide two-dimensional images of fuel-vapor, low-temperature ignition (H2CO), high-temperature ignition (OH) and soot-formation species (PAH) to characterize the LTC combustion process. Multidimensional simulations, which agree well with the optical engine measurements, provide a three-dimensional picture of the fuel-air mixing processes and quantitative analysis of the soot, UHC and CO formation and oxidation.
The combined optical measurements and model simulations show that jet interactions with the piston bowl (jet-bowl) and with neighboring jets (jet-jet) can significantly influence pollutant formation and oxidation processes. With a conventional piston-bowl-wall targeting, the fuel jet impinges at the piston-bowl wall prior to ignition and merges with neighboring jets, where substantial fuel-rich soot-formation regions develop. The jet-bowl interaction also leads to a rebound of the jet-head away from the piston bowl during late-cycle oxidation processes, enhancing the oxidation of soot, UHC and CO. By using a narrow-angle floor targeting, the fuel jet impinges at the piston-bowl floor and is redirected up along the bowl wall rather than towards neighboring jets, reducing jet-jet interactions. The suppression of these jet-jet interactions leads to less soot formation within the piston bowl. However, this altered jet trajectory eventually causes impingement at the cylinder head, creating a fuel-rich recirculation region where high levels of soot and CO form. By using a wide-angle piston-bowl-lip targeting, the jet is split between the piston bowl and squish region, which also reduces jet-jet interactions and soot formation within the bowl, but additional soot formation occurs in the squish region and total soot formation is not reduced. In addition, there is less rebound of the jet away from the piston-bowl wall and late-cycle oxidation of soot, UHC and CO is compromised.
