Method to Determine Bulk Modulus of a Fuel

A method for determining the bulk modulus of a fuel used in a fuel injection system of an internal combustion engine, said fuel injection system including a fuel accumulator volume or common rail for the supply of fuel to one or more fuel injectors, pumping means to pressurise fuel in said common rail/accumulator volume, comprising the steps of implementing a pumping event, measuring the consequential increase in pressure in said accumulator volume; and, determining the bulk modulus from said increase/change in pressure.