Detection method for detecting a gap size of a gap between an injector valve assembly of an internal combustion engine and a piezo stack for activating the injector valve assembly, having the steps of: - providing a piezo stack having an actuator unit and a sensor unit that are coupled to one another with a force fit, wherein the sensor unit is configured to detect force gradients acting on the actuator unit; - providing an injector valve assembly that is activated during operation by means of the actuator unit, wherein the injector valve assembly and the piezo stack are arranged at a distance from one another across a gap having an unknown gap size; - detecting a voltage signal of the sensor unit; - applying a defined voltage pulse to the actuator unit, so that the actuator unit deflects along an actuator unit longitudinal axis while decreasing the gap; - detecting a period (Δt) for which the voltage pulse is applied to the actuator unit starting from a first time (t 1), at which the voltage pulse begins to be applied, up to a second time (t 2), at which a voltage gradient (dU) occurs in the detected voltage signal of the sensor unit; - ascertaining the gap size of the gap from the detected period (Δt) and the defined voltage pulse; characterized in that - a second voltage gradient (dU) is detected in the voltage signal of the sensor unit at a third time (t 3), at which an injector needle of the injector valve assembly lifts off from a needle seat, wherein a negative voltage gradient (dU) is detected in the voltage signal of the sensor unit, and - a third voltage gradient (dU) is detected in the voltage signal of the sensor unit at a fourth time (t 4), at which the injector needle comes into force-fitting contact with the needle seat, the fourth time (t 4) and the second time (t 2) enclosing the third time (t 3), wherein a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit at the fourth time (t 4). - providing a piezo stack having an actuator unit and a sensor unit that are coupled to one another with a force fit, wherein the sensor unit is configured to detect force gradients acting on the actuator unit; - providing an injector valve assembly that is activated during operation by means of the actuator unit, wherein the injector valve assembly and the piezo stack are arranged at a distance from one another across a gap having an unknown gap size; - detecting a voltage signal of the sensor unit; - applying a defined voltage pulse to the actuator unit, so that the actuator unit deflects along an actuator unit longitudinal axis while decreasing the gap; - detecting a period (Δt) for which the voltage pulse is applied to the actuator unit starting from a first time (t 1), at which the voltage pulse begins to be applied, up to a second time (t 2), at which a voltage gradient (dU) occurs in the detected voltage signal of the sensor unit; - ascertaining the gap size of the gap from the detected period (Δt) and the defined voltage pulse; characterized in that - a second voltage gradient (dU) is detected in the voltage signal of the sensor unit at a third time (t 3), at which an injector needle of the injector valve assembly lifts off from a needle seat, wherein a negative voltage gradient (dU) is detected in the voltage signal of the sensor unit, and - a third voltage gradient (dU) is detected in the voltage signal of the sensor unit at a fourth time (t 4), at which the injector needle comes into force-fitting contact with the needle seat, the fourth time (t 4) and the second time (t 2) enclosing the third time (t 3), wherein a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit at the fourth time (t 4).