Investigation of inversed-delta injection rate shaping diesel spray flame structure and combustion characteristics towards thermal efficiency improvement

Reduction of the late combustion is of interest for further diesel engine thermal efficiency improvement. In this work, “inversed-delta” (i.e., progressive reduction of injection rate during injection period) and conventional “rectangle”-injected diesel spray flames are investigated from simultaneous high-speed imaging and in-cylinder pressure trace in a constant volume combustion chamber CVCC at modern diesel engine conditions utilizing a novel injection rate shaping TAIZAC – TAndem Injectors Zapping ACtivation injector. Optically thinner spray tip rich mixture, shorter spray tip penetration and more relatively even UV emissions distributions are noticed in 175 MPa inversed-delta injections compared to than that of 175 MPa rectangle injection profiles, probably due to much even fuel-gas mixture distribution owing to following spray velocity reduction. However, in near-zero end-of-injection EOI pressure reduction condition as in 135 MPa inversed-delta injection profile, optically thicker and broader spray mixture accompanied with intense UV emissions and long soot residence are observed, suggesting poor atomization, mixing and soot oxidation processes. Based on pressure derived apparent heat release rate AHRR, inversed-delta injection exhibits slightly faster combustion than that of rectangle injection in fixed injection duration cases. However, for constant initial injection pressure comparison cases, combustion duration is faster in rectangle injection profiles. In similar initial injection pressure cases, inversed-delta injection combustion exhibits a distinct “shoulder”-like shape, where its AHRR departs from that of rectangle injection, results in lower peak pressure but longer combustion duration. Initial similarity and departing timing of AHRR shoulder are highly identical to that of the spray tip penetration, suggesting that the inversed-delta AHRR progress is restrained from the shoulder formation timing, possibly due to smaller volumetric reactive zone at the spray periphery.