Diesel fuel and a method of operating a diesel engine

A diesel fuel blend providing a relative reduction to the emission of oxides of nitrogen when combusted in a heavy duty diesel engine, the diesel fuel blend comprising:

• (a) a diesel fuel derived from a Fischer-Tropsch process;
• and
• (b) a mineral oil based diesel fuel having a sulfur content of less than 100 ppmw and a T90 of more than 261° C.,
• wherein the weight fraction of component (a) in the blend is between 0.28 and 0.5.

A diesel fuel blend that provides a relative reduction to the emission of oxides of nitrogen when combusted in a heavy duty diesel engine, the diesel fuel blend comprising:

• (a) a diesel fuel derived from a Fischer-Tropsch process, diesel fuel derived from the Fischer-Tropsch process comprising at least 90% w of iso and linear paraffins, a boiling range of from about 150° C. to 400° C., a T90 of from 280° C. to 340° C., a density of from 0.76 g/mL to0.79 g/mL at 15° C., a cetane number of at least 60, and a viscosity of from 2.5 centistokes to 4 centistokes at 40° C.;
• and
• (b) a mineral oil based diesel fuel having a sulfur content of less than 100 ppmw;
• wherein the weight fraction of component (a) in the blend is between 0.28 and 0.5;
• and
• wherein, when combusted, said diesel fuel blend produces an emission of oxides of nitrogen from said combustion which is P % lower than an emission of oxides of nitrogen which can be calculated on the basis of a linear blending behavior of components (i) and (ii), P is relative to the emission of oxides of nitrogen caused by the said mineral oil based diesel fuel, and P is defined by the equation
• P=A·X(1−X),
• in which equation A is a number in the range of from 10 to 25, and X is the weight fraction of component (i) in the blend, expressed as a number in the range of from 0 to 1.