Recent work has demonstrated the potential of gasoline-like fuels to reduce NOx and particulate emissions when used in Diesel engines. In this context, straight-run naphtha, a refinery stream directly derived from the atmospheric crude oil distillation process, has been identified as a highly valuable fuel.
The current study is one step further toward naphtha-based fuel to power compression ignition engines. The potential of a cetane number 25 fuel (CN25), resulting from a blend of hydro-treated straight-run naphtha CN35 with unleaded non-oxygenated gasoline RON91 was assessed.
For this purpose, investigations were conducted on multiple fronts, including experimental activities on an injection test bed, in an optically accessible vessel and in a single cylinder engine. CFD simulations were also developed to provide relevant explanations.
Among multiple results, evaluation of full-load performance on the single cylinder engine showed that with a conventional nozzle configuration designed for Diesel fuel, CN25 was able to meet the maximum low-end torque target. However, at maximum power rate, a 15% power loss was noted. CFD results revealed a poor fuel distribution between the piston bowl and the squish area, a difficulty that was overcome by increasing the hydraulic flow of the nozzle along with the number of injector holes.
At part load, an optimization was carried out on six operating points, and dedicated injection strategies were established to properly manage the combustion. Finally, global assessments performed on the WLTC cycle showed encouraging results: 5% CO2 emission reduction was measured compared to Diesel while achieving an engine-out NOx and particulates level compliant with Euro 6 standard (no NOx after treatment but DPF requested). Excessive UHC and CO emissions were nevertheless measured (UHC *2.5 and CO *1.1). Dedicated efforts are still in progress to further reduce these, mainly by adapting the cetane number of the fuel to within the range 30-35.