An engine for an-auto injector for administering a dose of a liquid medicament includes a case arranged to be attached to a package syringe containing a syringe. The case contains a drive capable of, upon activation, pushing the needle from a covered position into an exposed position, and operating the syringe to supply the dose of medicament. The case contains a trigger arranged to lock the drive in a loaded state prior to manual operation and capable of, upon manual operation, releasing the drive for injections. The drive is arranged as a gas spring including a cylinder and a piston with a plunger for transmitting the spring force. A latch is arranged for preventing translating between the cylinder and the case. The latch is disengageable by the released gas spring being at least nearly fully extended thus allowing the extended gas spring to be translated in a distal direction.