Optical studies of combusting diesel sprays were done on three different alternative liquid fuels and compared to Swedish environmental class 1 diesel fuel (MK1). The alternative fuels were Rapeseed Oil Methyl Ester (RME), Palm Oil Methyl Ester (PME) and Fischer-Tropsch (FT) fuel.
The studies were carried out in the Chalmers High Pressure High Temperature spray rig under conditions similar to those prevailing in a direct-injected diesel engine prior to injection. High speed shadowgraphs were acquired to measure the penetration of the continuous liquid phase, droplets and ligaments, and vapor penetration. Flame temperatures and relative soot concentrations were measured by emission based, line-of-sight, optical methods. A comparison between previous engine tests and spray rig experiments was conducted in order to provide a deeper explanation of the combustion phenomena in the engine tests.
Results pertaining to spray behavior show that high viscosity fuels have wider spray cone angles, smaller discharge coefficients (Cd) and shorter vapor penetration than low viscosity fuels. Continuous liquid phase penetration is related to differences in surface tension, viscosity and density; while the penetration of droplets and ligaments is related to volatility, their penetration is short for highly volatile fuels and long for low-volatility fuels.
Engine tests show that particle matter (PM) emissions are generally lower when these alternative fuels are used, but the use of RME leads to increased NOx emissions correlating with elevated flame temperatures.