The invention refers to an engine (1.1) for an auto-injector (1) for administering a dose of a liquid medicament (M), the engine (1.1) comprising a case (9) having a distal end and a proximal end arranged to be attached to a packaged syringe (1.2) containing a syringe (3) with a hollow needle (4) and a stopper (5) for sealing the syringe (3) and displacing the medicament (M), wherein the syringe (3) is slidably arranged with respect to the case (9), the case (9) containing: -drive means (10) capable of, upon activation: -pushing the needle (4) from a covered position into an exposed position, and -operating the syringe (3) to supply the dose of medicament (M), and -activating means (11) arranged to lock the drive means (10) in a loaded state prior to manual operation and capable of, upon manual operation, releasing the drive means (10) for injection, characterized in that the drive means (10) is arranged as a gas spring comprising a cylinder (10.1) and a piston (10.2) with a plunger (10.3) for transmitting the spring force, wherein a compressed gas (G) is provided in a high pressure cavity defined between the piston (10.2) and a distal end of the cylinder (10.1), wherein the cylinder (10.1) is slidably arranged in the case (9), wherein a latch (14) is arranged for preventing translation between the cylinder (10.1) and the case (9), wherein the latch (14) is disengageable by the released gas spring (10) being at least nearly fully extended thus allowing the extended gas spring (10) to be translated in distal direction (D).Linvention concerne un moteur (1.1) pour un auto-injecteur (1) pour administrer une dose de médicament liquide (M), le moteur (1.1) comprenant un étui (9) comportant une extrémité distale et une extrémité proximale agencées pour être fixées à une seringue emballée (1.2) contenant une seringue (3) avec une aiguille creuse (4) et un bouchon (5) pour fermer hermétiquement la seringue (3) et déplacer le médicament (M), dans lequel la seringue (3) est di
