Abstract: The fill metered hydraulically actuated fuel injection system of the present invention utilizes an actuation fluid, preferably lubricating oil, as its actuation medium that is separate and different from the fuel fluid which is actually injected into the engine. Injectors according to the present invention are hydraulically actuated and include a conventional VOP type needle check, and fuel is pressurized by a plunger driven by an intensifier piston. The end of each injection event is achieved when the plunger and piston reach the end of their strokes, which provides for an abrupt end to each injection event. An abrupt end to injection is further accomplished by providing a pressure relief passage in the plunger that relieves pressure acting on the needle check at the end of injection in order to both quickly dissipate residual fuel pressure and allow the needle check to close more rapidly.

Abstract: In a hydraulically-actuated electronically-controlled unit injector, the viscosity of the actuating fluid used to actuate the unit injector varies with ambient temperature and affects the magnitude of pressure drops in the actuating fluid circuit. Such variations in viscosity affect the magnitude of the fuel delivery command pulsewidth and/or actuating fluid pressure required for engine startup versus that required for normal engine operation. The present invention provides an improved hydraulically-actuated electronically-controlled unit injector and methods of operation which compensate for variations in the viscosity of the actuating fluid used to hydraulically actuate a piston. The stroke of the piston is controlled electronically and/or mechanically in order to achieve an appropriate amount of fuel injection.