Computational Analysis of the Effects of Spray Parameters and Piston Shape on Syngas-Diesel Dual-Fuel Engine Combustion Process

Abstract

In this study, a 3D CFD analysis method for the combustion process was established for a low calorific value syngas-diesel dual-fuel engine operating under very lean fuel-air mixture condition. Also, the accuracy of computational analysis was evaluated by comparing the experimental results with the computed ones. To simulate the combustion for the dual-fuel engine, a new dual-fuel chemical kinetics set was used that was constituted by merging two verified chemical kinetic sets: n-heptane (173 species) for diesel and Gri-mech 3.0 (53 species) for syngas. For dual-fuel mode operations, the early stage of combustion was dominated by the fuel burning inside or near the spray plume. After which, the flame propagated into the syngas in the piston bowl and then proceeded toward the syngas in the squish zone. With the baseline injection system and piston shape, a significant amount of unburned syngas was discharged. To solve this problem, effects of the injection parameters and piston shape on combustion characteristics were analyzed by calculation. The change in injection variables toward increasing the spray plume volume or the penetration length were effective to cause fast burning in the vicinity of TDC by widening the spatial distribution of diesel acting as a seed of auto-ignition. As a result, the unburned syngas fraction was reduced. Changing the piston shape with the shallow depth of the piston bowl and 20% squish area ratio had a significant effect on the combustion pattern and lessened the unburned syngas fraction by half.

Fig. 1 Piston bowl geometry

Fig. 2 Computational mesh

Fig. 3 Experimental and numerical comparison of the incylinder pressure and HRR of syngas-diesel dual-fuel mode using new mechanism and n-heptane chemical kinetics set.

Fig. 4 Comparison of Experimental and numerical in-cylinder pressure and HRR of diesel only and dualfuel mode

Fig. 5 Comparison between the molar fraction of n-Heptane, OH, and CH with changes in crank angle

Fig. 7 Velocity vector plot at Syngas35, IMEP5, and 6G (center plane of the 72° sector mesh)

Fig. 9 Comparison of the in-cylinder pressure and HRR of dual-fuel mode at a different SMD

Fig. 10 Diesel spray penetration for syngas45, IMEP5, and 8G at a different SMD

Fig. 11 Velocity vector plot at Syngas45, IMEP5, and 8 G (center plane of the 72° sector mesh)

Fig. 12 CO emissions and unburned syngas fraction at a different SMD

Fig. 13 Comparison of the in-cylinder pressure and HRR of dual-fuel mode at a different spray cone angle

Fig. 14 Diesel spray penetration for syngas45, IMEP5, and 8G at a different spray cone angle

Fig. 15 CO emissions and unburned syngas fraction at a different spray cone angle

Fig. 18 CO emissions and unburned syngas fraction at a different fuel injection rate

Fig. 20 Diesel spray penetration at a different crank angle for syngas45, IMEP5, and 8G at small nozzle diameter

Fig. 19 Comparison of the in-cylinder pressure and HRRof dual-fuel mode at a different nozzle diameter

Fig. 22 CO emissions and unburned syngas fraction at a different nozzle diameter

Fig. 26 Velocity vector plot for Syngas45, IMEP5, and 8 G at a different piston bowl (center plane of the 72° sector mesh)

Fig. 6 In-cylinder temperature distribution at Syngas35, IMEP5, and 6G

Fig. 8 CO emission and unburned syngas fraction at IMEP 5 bar

Fig. 16 Comparison of injection rate

Fig. 17 Comparison of the in-cylinder pressure and HRR of dual-fuel mode at a different fuel injection rate

Fig. 21 Comparison of the in-cylinder temperature distribution for different nozzle diameter (center plane of the 72° sector mesh)

Fig. 23 Piston bowl geometry A and B

Fig. 24 Comparison of the in-cylinder pressure and HRR of dual-fuel mode at a different piston bowl geometry

Fig. 25 CO emissions and unburned syngas fraction at a different piston bowl

Fig. 27 Comparison of the in-cylinder temperature distribution at a piston bowl (center plane of the 72° sector mesh)

Table 1 Engine specifications

Table 2 Syngas composition

Table 3 Engine operating conditions