Abstract: A free-piston engine formed by one or more units, each comprising a pair of opposed pistons connected to a common piston rod and movable within two opposed cylinders, with a load, such as the inductor of a linear alternator, connected to an intermediate point of the piston rod, and in which there is provided at least a cam controlled by an operatively independent motor and engaging at least a tappet connected to the piston rod of a unit of pistons. This cam is profiled and controlled in such a way as to substantially follow the movement of the tappets which is produced by the free-piston engine during normal operation, but to limit the travel of the piston units and provide the energy for the compression stroke, in case of anomalous operation. The cam serves also to start the engine, as well as to mutually synchronize the various units of pairs of pistons in engines having a plurality of units. The engine may also be in the form of an adiabatic engine fed by coal dust or other solid fuel.