Fischer-Tropsch (FT) conversion of gasification products to liquid hydrocarbon fuel commonly includes FT synthesis followed by mild refining of the FT synthetic oil into diesel, kerosene, and naphtha, each defined by a specific boiling range. These FT products are derived typically by mixing condensable gaseous reactor effluent (∼C5-C20) with hydrocracked liquid reactor effluent (∼C20+). This mixture of FT liquids (FTL), is then hydrotreated and distilled to yield the desired products.This paper evaluates compression-ignition engine performance during preliminary tests of the condensable gaseous reactor effluent (∼C5-C20) portion of the (FTL) from a Syntroleum Corporation plant. Engine operability, maximum torque, fuel economy and emissions were evaluated for both FTL and mixtures of FTL with gasoline, hexanes, diethyl ether, and ethanol. Emphasis was placed on particulate and NOx emissions. However, hydrocarbon, carbon dioxide, and oxygen concentrations were also followed.Engine tests were performed with neat FTL, as well as with FTL formulations containing (1) 25% gasoline; (2) 25% hexanes; (3) 20%, 25%, and 33% of a equal volume mixture of ethanol and diethyl ether; and (4) 20% and 25% ethanol. Performances were compared to US 1D and US 2D diesels. A three-mode test sequence was performed consistent with heavy duty diesel engine applications and each of the fuels met or exceeded goals on reduced emissions and operability of the unmodified engine. Significant emission performance advantages were observed with several of these formulations.