42 and 43 and flow likewise into the reserve chamber formed within cylinder 44 via the path shown by the arrows in fig5 thus lifting the ball valve 45 this chamber retains the gases at their highest pressure due to the non return function of ball valve 45 and communicates via passage 49 with the pair of circular cavities 50 and 51 controlling the retraction of both pistons 42 and 43 the maximum pressure is thus exerted against the upper annular surfaces 42a and 43a of pistons 42 and 43 respectively and also against the annular surfaces provided under the heads of these pistons to sum up the initial useful surface area is reduced to the difference between the sum of the annular surfaces 42a and 43a and the annular surface 50a under the head of piston 42 it will be noted that the gas pressure is exerted simultaneously on both pistons and that these pistons can only move down simultaneously when the head of piston 42 abuts with its surface 50a the edge of plug 59 this movement being somewhat damped due to the pressure prevailing in cavity 50 the piston 43 is positioned beyond the seal 56 as shown in fig6 consequently the gases partially expanded during the first phase of the stroke will exert a relatively reduced pressure against the bottom 60 of piston 43 and the additional useful surface area thus available will compensate the pressure drop while regularizing the thrust as already mentioned in the applicants 39 patents referred to hereinabove and.