Abstract: A method of operating a turbo charged diesel engine is provided where a viscosity increasing component in a diesel fuel composition is used, to improve the acceleration performance, at low engine speeds (for example up to 2200 rpm). This method may be used for reducing the engine speed at which the turbo charger reaches its maximum speed when accelerating at low engine speeds, or reducing the time taken for the turbo charger to reach its maximum speed. It may mitigate a deterioration in the acceleration performance of the engine due to another cause. The VK 40 of the resultant fuel composition is suitably 2.8 mm2/s (centistokes) or greater. The viscosity increasing component may in particular be a Fischer-Tropsch derived fuel component, an oil or a fatty acid alkyl ester. A density increasing component may be used in the fuel composition together with the viscosity increasing component.