in the hot space 40 and causes some gas convection flow the disturbance caused by the flow of convection currents within the hot space provides the needed small movement or perturbation of the displacer piston to regeneratively initiate further movement and consequent starting of the engine the substantial movement of the piston during steady state operation will cause the sealing ring 54 to slide to and fro between the opposite side walls of the groove 52 sealingly seating against the side wall during most of the piston movement as illustrated in fig3 and 4 therefore complete sealing between the ring and the opposite sliding surfaces may be accomplished by the seating of the ring against the wall of the groove alternatively the sealing may be accomplished by using a fluid impervious resilient support 60 as described above fig5 through 11 illustrate alternative seal mechanisms embodying the present invention for example the seal 70 of fig5 may be resiliently attached to sliding member 72 by means of a plurality of circularly spaced pairs of axially aligned springs 74 and 76 or other resilient means spaced about the cylindrical surface and extending between the ring and the walls of the groove for complete sealing the resilient means may be a pleated annularly ridged impervious sheet material forming a bellows fig6 illustrates a sealing ring 78 mounted to a groove 80 by means of an elastic ring 82 fig7 illustrates a sealing ring 84 having a plurality of inwardly directed.