Abstract: A fuel or liquid injector includes a needle valve and a piezoelectric actuator that bears a hydraulic acceleration piston on its distal end. The actuator expands moving the piston toward the needle valve. The needle valve has a ball end, restrained against the valve seat by a spring, sealing the valve until the spring force is overcome by mechanical and hydraulic forces on the piston. When closed, the piston rests against the needle valve. Expansion of the piezoelectric actuator mechanically actuates the piston, and then increases the hydraulic pressure, which closes the fuel check valve, and accelerates the forward movement of the piston attached to the needle valve. When the restraining force on the needle valve is overcome, the ball end of the needle valve is unseated, and compressed fluid in the lower hydraulic chamber is released in a high velocity spray.