Abstract: Hydraulic spring drive apparatus comprising: a rotatable cam shaft having fixed thereon a number of cams; a number of rocker arms, one following each cam, each pivotally attached at one end to a pressure bar and at another to an expansible, compressible connecting means in turn connected to a crank portion of a crank shaft. The cams, crank shaft, and expansible compressible connecting means are arranged so that a number of connecting means expand against the compression of one connecting means. The pressure bar is movable to position the rocker arms for a selected degree of connecting means compression.