Effect of natural gas energy fractions on combustion performance and emission characteristics in an optical CI engine fueled with natural gas/diesel dual-fuel

Natural gas/diesel dual-fuel combustion is one of the most promising low-temperature combustion strategies. However, unstable combustion processes under low engine loads restrain the development of this strategy. To overcome this problem, a visualization technique and numerical simulations were used at low engine load conditions to investigate herein how the natural gas energy fraction (NGEF) coupled with split injection affects flame development, performance, and the emission characteristics of natural gas/diesel dual-fuel engines. The results reveal that increasing the NGEF retards the start of combustion and decreases the peak pressure. The NGEF significantly affects the distribution of the ignition kernel, with a higher NGEF scattering the ignition kernel. Combustion first occurs in regions of richer mixture near the cylinder wall and then propagates towards the interior. The amount of methane produced by diesel fuel through NC7H16 -> C7H15-2 -> PC4H9 -> C2H4 -> CH4 is relatively small compared with natural gas. Unburned methane is detected near the central axis of the cylinder, which is mainly due to the limited coverage of the flame. The lower soot volume fraction and unburned hydrocarbon emissions can be obtained by increasing the NGEF from 0% to 70%, whereas further increasing the NGEF to 85% deteriorates combustion.