1. Field of the Invention
The present invention relates to a control unit and a method for operating a valve, e.g., a fuel injector of an internal combustion engine in a motor vehicle, which is activated with the aid of an actuator, in which the actuator is activated using a control variable which has a control period.
2. Description of Related Art
Methods and control units of the aforementioned type are used, for example, in high-pressure injectors for gasoline direct injection, in which a motion of a valve needle is controlled, for example, by energizing a magnetic circuit. The magnetic circuit is a component of an electromagnetic actuator which exerts a magnetic force on the valve needle when energized. Common types of high-pressure injectors are designed in such a way that the energization of the solenoid of the electromagnetic actuator causes the injector to open, i.e., lifts the valve needle from its closed position and subsequently holds the valve needle at a lift stop corresponding to its open position. When the energization ends, initially the magnetic force rapidly decreases in a manner known per se, so that a return spring which acts upon the valve needle in the closing direction accelerates the valve needle toward its closed position. The closing process ends when the valve needle reaches its valve seat. After this point in time the return spring force no longer has an accelerating effect on the valve needle, and acts only as a sealing force which transfers the valve needle to its valve seat.
In addition to the motion characteristic of the valve needle and the stroke throttle curve of the valve, the quantity of fuel injected during the above-described control process is determined primarily by an opening duration of the valve, i.e., the time interval between the lifting of the valve needle from its closed position and the return to its closed position.
However, the hydraulic opening duration of the valve is usually not directly known in a control unit which controls the valve; rather, only a control period of the actuator which drives the valve needle is known. There is a so-called opening delay time between a start of the control period and the actual hydraulic opening of the injector, and there is a so-called closing delay time between an end of the control period and the point in time of the actual hydraulic closing of the injector.
These delay times of the valve are not known, and instead are a function of operating and setting parameters of the valve and other criteria. In particular for very short desired opening durations of the valve, in which the valve needle does not reach a lift stop which corresponds to its completely open position, but instead undergoes a ballistic trajectory, the metering accuracy of the conventional systems is inadequate.