In a "free-piston engine", a piston moves freely, without being linked to a crankshaft. The force of the burned gases is converted into a movement of the piston. The relatively heavy piston takes over the kinetic storage function of the absent flywheel. The energy can be used e.g. to generate compressed air. It can also be used to operate a gas turbine, or - more recently - to generate electricity directly via permanent magnets on the moving piston and copper windings on the surrounding cylinder.
This engine has a centralized uniflow-scavenging and toroidal combustion chambers.
The main problem of all these types of free-piston machines is visible in the following movie: The moving mass of these engines is not counterbalanced. Therefore, the engines vibrate due to their basic construction. Such types engines are therefore not really suitable for commercial applications.
Unless one combines two engines and run them by 180° phase shifted. However, some of the greatest advantages of these engines get lost: space, cost, and weight savings. Whereas, a complex electronic synchronization is required.
A russian proposal