Noise-reduced actuation method for a piezoactuator in an injector

#This application claims priority to DE Patent Application No. 10 2010 063 667.3 filed Dec. 21, 2010. The contents of which is incorporated herein by reference in its entirety.

#The invention relates to an actuation method for a piezoactuator in an injector which is mounted on a cylinder head and has an injection valve. The method comprises the triggering of an injection by applying an injection charge current pulse to the piezoactuator in order to open the injection valve and start an injection, as well as applying an injection discharge current pulse to the piezoactuator in order to close the injection valve and end the injection. The injection charge current pulse and the injection discharge current pulse each bring about a change in length of the piezoactuator, which change in length excites an oscillation system, comprising the injector and the cylinder head, to oscillate. After an idle time interval in which no injection takes place, the steps for triggering an injection are repeated.

#In known injectors, for example diesel piezo common-rail injectors, an injection valve for injecting fuel into the combustion chamber of the cylinder head is actuated by a piezoactuator. As a result of a voltage being applied to the piezoactuator or as a result of the electrodes of a piezoactuator being charged with a charge current pulse, the piezoactuator experiences a change in length. This change in length is transmitted as an operating stroke to a valve element of the injection valve. In order to close the injection valve, the electrodes of the piezoactuator are discharged with a corresponding discharge current pulse. The change in length is thereby reversed and the movement of the piezoactuator transmitted to the valve element of the injection valve in order to close the valve. These changes in length of the piezoactuator take place in a highly dynamic fashion and generate acceleration forces and mechanical oscillations which, in the installed state on a cylinder head, are audibly transmitted to the injector and the cylinder head. This noise which is caused by the piezoactuator is frequently felt to be disruptive, in particular if it predominates over the combustion noises in the idling mode of the engine.

#The invention will be explained in more detail below with reference to FIGS. 1 and 2, in which, in a schematic form:

#FIG. 1 shows the piezoenergization (charge current of the piezoactuator) as a function of the time; and

#FIG. 2 shows the reaction forces and oscillations at the injector at the interface for the cylinder head which are caused by the piezoactuator.

#According to the actuation method according to various embodiments for a piezoactuator in an injector which is mounted on a cylinder head and has an injection valve, an injection is triggered by applying an injection charge current pulse to the piezoactuator in order to open the injection valve and start an injection. The injection valve is closed and the injection ended by applying an injection discharge current pulse to the piezoactuator. The injection charge current pulse and the injection discharge current pulse each bring about a change in length of the piezoactuator, which changes in length excite an oscillation system, comprising the injector and the cylinder head, to oscillate. After an idle time interval in which no injection takes place, the steps for triggering the injection are repeated. According to various embodiments, during the idle time interval, a damping charge current pulse and a damping discharge current pulse are applied to the piezoactuator at the time t1 and at the time t2. The damping charge current pulse and damping discharge current pulse likewise generate changes in length of the piezoactuator, but said changes in length are too small to activate the injection valve and to trigger an injection. According to various embodiments, the times t1 and t2 during the idle time interval are selected such that excitation of the oscillation system by the damping charge current pulse and the damping discharge current pulse takes place in anti-phase with respect to the oscillation which is excited by the injection charge current pulse and the injection discharge current pulse.

#Both the injection charge current pulse and the injection discharge current pulse bring about a change in length of the piezoactuator, which change in length applies a pulse to the oscillation system with the injector and cylinder head. The system is already excited to experience natural oscillation at a frequency which is typical of the system by the injection charge current pulse. After a time interval ti (injection time) after the start of the injection charge current pulse, the injection discharge current pulse is applied to the piezoactuator and as a result the injection is ended. Renewed excitation of the oscillation system is also brought about by this injection discharge current pulse. Depending on the duration of the interval ti, the amplitude of the natural oscillation varies depending on whether the excitation takes place in anti-phase or in phase with the already existing oscillation. A reduction in the oscillation amplitude could therefore be achieved in principle by adaptation of the injection period ti. However, the injection period, by means of which the quantity of the injected fuel is also controlled, should preferably be freely selectable.

#Therefore, in order to reduce the oscillation amplitude of the excited natural oscillations, according to various embodiments, an additional damping charge current pulse and a damping discharge current pulse are applied to the piezoactuator between two injections. The amplitude or duration of the damping charge current pulse and of the damping discharge current pulse are shorter here than those of the injection charge current pulses and those of the injection discharge current pulses, with the result that the change in length of the piezoactuator which is brought about as a result is not sufficient to activate the injection valve. This is intended to prevent an additional injection between the injections which are actually provided from taking place. The piezoactuator in an injector usually has a certain idle travel which has to be overcome before the actuator movement triggers a movement of the valve element. The damping charge current pulse and the damping discharge current pulse are therefore small enough for the resulting change in length of the actuator to take place within the limits of the idle travel.