This experimental and numerical study evaluates how ducted fuel injection (DFI) and pilot injections interact to impact soot formation and the premixed heat release pressure spike in diesel combustion. Experiments showed that pilot injections reduced the premixed heat release spike of a free spray by approximately 70%, while DFI configurations only experienced a decrease of approximately 25%. Similarly, pilot injections reduced the initial lift-off length (LOL) of the main injection of a free-spray by approximately 30%, while DFI’s initial LOL had little to no change when pilot injections were utilized. Regardless of whether a standalone-main or pilot-main strategy was used, DFI was able to reduce the spatially integrated natural luminosity (SINL) of the flame relative to a free spray, indicating a likely reduction in soot formation. Both duct configurations studied produced steady SINL signals which were approximately 30% and 70% of the free-spray’s, respectively. For DFI, pilot injections further reduced the peak SINL compared to a standalone main by approximately 16%. The decrease in peak SINL correlated with increased spray head penetration rates. The numerical study revealed that pilot injections led to leaner mixtures near the tip of the penetrating spray for all configurations. Thus, unaffected LOLs, leaner penetrating spray tips, and a reduced time for soot formation possibly led to less soot in the head of the transient penetrating spray when DFI is used in conjunction with pilot injections.
GehmlichRKDumitrescuCEWangY, et al. Leaner lifted-flame combustion enabled by the use of an oxygenated fuel in an optical ci engine. SAE Int J Engines2016; 9(3): 1526–1543.
2.
MusculusMPLachauxTPickettLM, et al. End-of-injection over-mixing and unburned hydrocarbon emissions in low-temperature-combustion diesel engines. Technical Report, SAE Technical Paper, 2007.
3.
PolonowskiCJMuellerCJGehrkeCR, et al. An experimental investigation of low-soot and soot-free combustion strategies in a heavy-duty, single-cylinder, direct-injection, optical diesel engine. SAE Int J Fuel Lubricants2011; 5(1): 51–77.
4.
SiebersDHigginsB.Flame lift-off on direct-injection diesel sprays under quiescent conditions. SAE Transactions2001; 400–421.
PickettLMSiebersDL.An investigation of diesel soot formation processes using micro-orifices. Proc Combust Inst2002; 29(1): 655–662.
7.
PickettLMSiebersDL.Fuel effects on soot processes of fuel jets at di diesel conditions. SAE Transactions2003; 2044–2066.
8.
MusculusMPBMilesPCPickettLM. Conceptual models for partially premixed low-temperature diesel combustion. Prog Energy Combust Sci2013; 39(2–3): 246–283.
9.
BuurmanNJNyrenstedtGMuellerCJ.Ducted fuel injection provides consistently lower soot emissions in sweep to full-load conditions. Technical report, SAE Technical Paper, 2023.
10.
PickettLMSiebersDL.Orifice diameter effects on diesel fuel jet flame structure. J Eng Gas Turbine Power2005; 127(1): 187–196.
11.
SaxenaSBedoyaID.Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog Energy Combust Sci2013; 39(5): 457–488.
12.
MuellerCJNilsenCWBilesDE, et al. Effects of fuel oxygenation and ducted fuel injection on the performance of a mixing-controlled compression-ignition optical engine with a two-orifice fuel injector. Appl Energy Combust Sci2021; 6: 100024.
13.
KamimotoTBaeM. High Combustion Temperature for the Reduction of Particulate in Diesel Engines. SAE Trans1988; 97: 1988, pp. 692–701.
14.
MuellerCJNilsenCWRuthDJ, et al. Ducted fuel injection: a new approach for lowering soot emissions from direct-injection engines. Appl Energy2017; 204: 206–220.
15.
FitzgeraldRPSvenssonKMartinG, et al. Early investigation of ducted fuel injection for reducing soot in mixing-controlled diesel flames. SAE Int J Engines2018; 11(6): 817–833.
16.
GehmlichRKMuellerCJRuthDJ, et al. Using ducted fuel injection to attenuate or prevent soot formation in mixing-controlled combustion strategies for engine applications. Appl Energy2018; 226: 1169–1186.
17.
SvenssonKIMartinGC.Ducted fuel injection: effects of stand-off distance and duct length on soot reduction. SAE Int J Adv Curr Pract Mob2019; 1(3): 1074–1083.
18.
SvenssonKFitzgeraldRMartinG.Ducted fuel injection: an experimental study on optimal duct size. Technical report, SAE Technical Paper, 2022.
19.
YraguenBFSteinbergAMNilsenCW, et al. Parametric evaluation of ducted fuel injection in an optically accessible mixing-controlled compression-ignition engine with two- and four-duct assemblies. Int J Engine Res2024; 25: 245–261.
20.
OgrenRMBaumgardKJRadhakrishnaV, et al. First investigation of ducted fuel injection on a retrofitted heavy-duty multi-cylinder production engine. SAE Int J Fuel Lubricants2025; 19(1): 1–26.
21.
FengYShangTCaiJ, et al. Leaner lifted-flame combustion with ducted fuel injection: the key role of forced two-stage mixing. Fuel2023; 347: 128431.
22.
GuoJBrouzetDChungWT, et al. Analysis of ducted fuel injection at high-pressure transcritical conditions using large-eddy simulations. Int J Engine Res2024; 25(2): 305–319.
23.
LucchiniTZhouQD’ErricoG, et al. Modeling fuel-air mixing, combustion and soot formation with ducted fuel injection using tabulated kinetics. Technical report, SAE Technical Paper, 2022.
24.
LiuXMohanBImHG.Numerical investigation of the free and ducted fuel injections under compression ignition conditions. Energy Fuels2020; 34(11): 14832–14842.
25.
MilloFPianoAPeiretti ParadisiB, et al. Ducted fuel injection: experimental and numerical investigation on fuel spray characteristics, air/fuel mixing and soot mitigation potential. Fuel2021; 289: 119835.
26.
MilloFSegatoriCPianoA, et al. An engine parameters sensitivity analysis on ducted fuel injection in constant-volume vessel using numerical modeling. Technical report, SAE Technical Paper, 2021.
27.
MilloFPianoAPeiretti ParadisiB, et al. Ducted fuel injection: a numerical soot-targeted duct geometry optimization. SAE Int J Engines2021; 15(2): 297–317.
28.
NilsenCWBilesDEYraguenBF, et al. Ducted fuel injection versus conventional diesel combustion: an operating-parameter sensitivity study conducted in an optical engine with a four-orifice fuel injector. SAE Int J Engines2020; 13(3): 345–362.
29.
OngJCZhangMJensenMS, et al. Large eddy simulation of soot formation in a ducted fuel injection configuration. Fuel2022; 313: 122735.
30.
GodboldCGuptaIKurtzE, et al. Experimental investigation of mixing phenomena for ducted fuel injection. Proc Combust Inst2024; 40(1–4): 105385.
31.
SegatoriCPianoAPeiretti ParadisiB, et al. Exploiting the potential of large eddy simulations (les) for ducted fuel injection investigation in non-reacting conditions. Int J Multiphase Flow2024; 171: 104686.
32.
ChenTPeiYAnY, et al. Rethinking spray combustion: the role of ducted fuel injection in enhancing mixing and reducing emissions. Renew Sustain Energ Rev2025; 218: 115816.
33.
RajasegarRSrnaA.A review of current understanding of the underlying physics governing the interaction, ignition and combustion dynamics of multiple-injections in diesel engines. SAE Int J Adv Curr Pract Mob2022; 5(1): 117–134.
34.
LiuXImHGMuellerCJ, et al. A computational parametric study of ducted fuel injection implementation in a heavy-duty diesel engine. Fuel2024; 358: 130228.
35.
KnoxBWGenzaleCL.Scaling combustion recession after end of injection in diesel sprays. Combust Flame2017; 177: 24–36.
36.
NoriVSeitzmanJ. Evaluation of chemiluminescence as a combustion diagnostic under varying operating conditions. In 46th AIAA Aerospace Sciences Meeting and Exhibit, 2008, p. 953.
37.
SegatoriCOrlandoMPianoA, et al. Optical diagnostics-based numerical modeling of a compression-ignition optical research engine implementing ducted fuel injection. Appl Energy Combust Sci2025; 24: 100408.
38.
LaunderBESharmaBI.Application of the energy-dissipation model of turbulence to the calculation of flow near a spinning disc. Lett Heat Mass Transf1974; 1(2): 131–137.
39.
PopeSB.An explanation of the turbulent round-jet/plane-jet anomaly. AIAA J1978; 16(3): 279–281.
40.
ReitzRD.Mechanism of breakup of round liquid jets. Encycl Fluid Mech1986; 10: 223–249.
