Actuator unit, in particular for injecting a fuel into a combustion chamber of an internal combustion engine

1. An actuator unit for injecting a fuel into a combustion chamber of an internal combustion engine, comprising:

an electrically conductive excitation winding disposed in a housing,

a first ferromagnetic circuit loop including a return through a projection of the housing,

a second ferromagnetic circuit loop through the housing without passing through the return

an armature configured to move along an axis between:

an idle position in which the armature completes the second ferromagnetic circuit loop by contact with a surface segment that extends in a plane perpendicular to the axis of movement of the armature,

a working position in which the armature completes the first ferromagnetic circuit loop by contact with the return,

wherein the armature is biased along the axis toward the idle position by a spring holding force of a spring element,

wherein a current flowing through the excitation winding produces a magnetic field that provides (a) a magnetic holding force acting on the armature in the same direction as the spring holding force provided by the spring element, and (b) a magnetic motive force acting on the armature in the opposite direction as the magnetic holding force and spring holding force,

wherein when the current flowing through the excitation winding is less than or equal to a predetermined current value, the current produces a magnetic holding force which, in combination with the spring holding force, is greater than the magnetic motive force, thereby holding the armature in the idle position as magnetic flux passes through the first ferromagnetic circuit loop, and

wherein when the current flowing through the excitation winding exceeds the predetermined current value, the magnetic motive force produced by the current exceeds the magnetic holding force combined with the spring holding force, which produces an air gap in the region of the surface segment, breaking the first ferromagnetic circuit loop and, resulting in the magnetic holding force decreasing with a steep gradient and the armature moving toward the working position.

2. The actuator unit of claim 1 , wherein the surface segment is defined by at least one of a projection of the armature and a projection of the ferromagnetic circuit.

3. The actuator unit of claim 1 , wherein the axis of movement of the armature extends through the surface segment.

4. The actuator unit of claim 1 , wherein the surface segment is defined by at least one of a cup-shaped projection of the armature and a cup-shaped projection of the ferromagnetic circuit.

5. The actuator unit of claim 1 , wherein the surface segment is spaced apart from the axis of movement of the armature in a radial direction.

6. The actuator unit of claim 5 , wherein the surface segment is defined by at least one of an annular projection of the armature and an annular projection of the ferromagnetic circuit.

7. The actuator unit of claim 6 , wherein the projection runs around a body of the armature, at least sectionwise.

8. The actuator unit of claim 1 , wherein the area of the surface segment is selected to provide a predetermined magnetic holding force.

9. The actuator unit of claim 2 , wherein the projection is produced from a material that becomes magnetically saturated at a particular field strength.

10. The actuator unit of claim 1 , wherein the surface segment is defined by at least one of a projection integral with the armature and a projection integral with the ferromagnetic circuit.

11. The actuator unit of claim 1 , wherein the surface segment is defined by at least one of a projection connected to the armature as a separate component and a projection connected to the ferromagnetic circuit as a separate component.

12. A method of operating an actuator unit comprising an electrically conductive excitation winding disposed in a housing, a first ferromagnetic circuit loop including a return through a projection of the housing, a second ferromagnetic circuit loop through the housing without passing through the return, and an armature configured to move along an axis between (a) an idle position in which the armature completes the second ferromagnetic circuit loop by contact with a surface segment that extends in a plane perpendicular to the axis of movement of the armature, and (b) a working position in which the armature completes the first ferromagnetic circuit loop by contact with the return, wherein the armature is biased along the axis toward the idle position by a spring holding force of a spring element, the method comprising:

applying a current through the excitation winding that produce a magnetic field that provides (a) a magnetic holding force acting on the armature in the same direction as the spring holding force provided by the spring element, and (b) a magnetic motive force acting on the armature in the opposite direction as the magnetic holding force and spring holding force,

setting the current flowing through the excitation winding to a value less than or equal to a predetermined current value, which produces a magnetic holding force which, in combination with the spring holding force, is greater than the magnetic motive force, thereby holding the armature in the idle position as magnetic flux flows through the first ferromagnetic circuit loop, and

increasing the current flowing through the excitation winding above the predetermined current value, which causes the magnetic motive force produced by the current to exceed the magnetic holding force combined with the spring holding force, which produces an air gap in the region of the surface segment, breaking the first ferromagnetic circuit loop and resulting in the magnetic holding force decreasing with a steep gradient and the armature moving toward the working position.