A method and apparatus for determining a characteristic of an injector are set forth. In one example, a compressed gas is applied to the inlet chamber of the injector while the spoolvalve is closed until the pressure of the compressed gas in the inlet chamber reaches a predetermined pressure. A change in pressure of the compressed gas in the inlet chamber is measured over time while the spool valve is closed. The pressure change measurement corresponds to the characteristic of the injector that is to be determined.

A method for correcting injection behavior of a fuel injector includes calculating a nominal value of a fuel injector family characteristic for an average fuel injector from a family of fuel injectors as a multi-variable function of engine operating conditions, calculating a corrected value of the fuel injector family characteristic as a function of the nominal value, and employing the corrected value when actuating the fuel injector to inject fuel.

A hydraulically driven fluid injection system comprising a reservoir having a first and second chamber and a piston separating the first and second chamber. A reservoir control valve to control the input of a first fuel source to the first chamber. The reservoir control may operate by having an open and a closed state. Alternatively, the reservoir control may have a first mode, a second mode, and a third mode. A second fuel may be provided to the second chamber and a metering valve may be utilized to control the flow of the second fuel to an injector. When the secondary fuel is desired, the reservoir control valve may open to allow the primary fuel to pressurize the first chamber. The result of the pressure on the first chamber causes the piston to apply a pressure to the second chamber. The metering valve may then allow the secondary fuel to flow from the second chamber to an injector.