PROVIDING FUEL

A method of providing fuel to an engine (12) during a period of operation of the engine. Ambient air conditions are determined in dependence upon actual measured values of ambient air conditions. The likelihood of vapour trail formation and/or vapour trail persistence is determined based upon engine operating point and the determined ambient air conditions. A desired soot emission index is determined. A fuel blender (14) is operated in fluid communication with a source of a first fuel composition and a source of a second fuel composition via at least one inlet (32, 34). A required resultant fuel composition is calculated as a function of the desired soot emission index, the required resultant fuel composition defining a percentage of the first and second fuel composition for blending together. The fuel blender mixes said percentages of first fuel composition and second fuel composition to produce the required resultant fuel composition for output to an engine via at least one outlet (36).