Safety belt pretensioner

A belt pretensioner in a vehicle, comprising a piston-cylinder device (1) with a cylinder chamber (9) containing an explosive charge (10). The piston (3) has a piston rod (4), which projects out of the cylinder on the side of the piston opposite to the cylinder chamber with the explosive charge, and is joined to a cable (24, 25), which drives, via a drum and a planetary gear set, a belt reel mechanism in the winding-up direction when the explosive charge is detonated. The cylinder chamber has an evacuation opening (11), which communicates with a spill valve (12), which is controlled by an electronic control unit in response to a sensor, which senses the weight of the occupant.