Performance, Efficiency and Emissions Comparison of Diesel Fuel and a Fischer-Tropsch Synthetic Fuel in a CFR Single Cylinder Diesel Engine during High Load Operation

Fischer-Tropsch (FT) synthetic fuels have been shown to produce lower soot and oxides of nitrogen emissions than petroleum-based diesel #2 (D2) in previous studies. This performance is frequently attributed to the very low aromatic content as well as essentially zero sulfur content. The objective of this empirical study was to investigate the high engine load regime using a military FT and D2 fuel in a CFR diesel engine at fueling levels approaching stoichiometric. A testing matrix comprised of various injection advance set points, fueling amounts (e.g. load) above 6 bar gross indicated mean effective pressure (IMEPg), and three different compression ratios (CR) was pursued. The results show that oxides of nitrogen emissions are always equal to or lower running FT compared to diesel. This result is attributed to the higher cetane number of FT leading to lower peak in-cylinder pressures as compared to D2. FT fuel CO₂ emissions are always lower than D2 as a result of the higher H/C ratio of the FT fuel. At CRs of 15 and 16.5 as well as for near maximum brake torque (MBT) injection advance timings, the FT cumulative particulate matter (PM) emission levels are lower than diesel approaching 8 bar IMEPg, but are similar at the highest loads attained (∼8 bar IMEPg) with both fuels (at this load excessive sooting resulted). At intermediate-advanced injection timings and high CR, the FT fuel showed no PM advantage with possibly worse levels at some operating conditions. An accompanying heat release analysis showed that the pre-mix burn fraction of FT is always less than D2, and that this pre-mix fraction increases with decreasing CR and injection advance. PM was seen to always decrease with increasing pre-mix burn fraction. However, FT always yielded much less soot that D2 for similar pre-mix burn fractions indicating a more effective diffusion burning phase for FT.