Basics

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.

1. INHERENTLY BALANCED ENGINES WITH TWO OPPOSED MOVING MASSES
Outward compressing engine
Outward compressing engine
Outward compressing, opposed piston engine
E.g. used at Junkers, Cooper-Bessemer, Sulzer, Baldwin-Lima-Hamilton, and the early Pescara compressors.

Inward compressing engine
Inward compressing engine
Inward compressing, opposed piston engine
This amazingly simple and thermodynamically optimal design was almost exclusively in use for Pescara engines and Pescara licensors.

Schematic view of In / Outward compressing
Schematic view of In / Outward compressing
A schematic view of outward, respectively inward compressing engines. All free-piston engines ever serial produced were based on these two principles. The most successful free-piston engine was an inward compressing type, that is by far the most economic free piston design.

2. ENGINES WITH ONLY ONE MOVING MASS
Single-acting Free-Piston Engine
Single-acting Free-Piston Engine
Single-acting Free-Piston Engine, recoil-operated 
metallic springs, air bouncers and oil accumulators were used
metallic springs, air bouncers and oil accumulators were used
Double-acting Free-Piston Engine
Double-acting Engine
Double-acting free-piston engine
This engine has a centralized uniflow-scavenging and toroidal combustion chambers.

Stelzer Engine
Stelzer Engine
Double-acting free-piston engine Stelzer type
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.

Vibration-compensated engine
Vibration-compensated engine
Vibration-compensated Free-Piston Engine 
Patent dated 1994 (Sampower Finland)

Download Patent Application Publication

Vibration-compensated engine
Vibration-compensated engine
Vibration-compensated Free-Piston Engine 
Patent dated 2011 (Vienna Engineering)
    
Download Patent Application Publication
Vibration-compensated engine
Vibration-compensated engine
Vibration-compensated Free-Piston Engine 

A russian proposal

Vibration-compensated engine
Vibration-compensated engine
Vibration-compensated Free-Piston Engine (Pempek)
The vibrations are eliminated by connecting 4 units that run in 2 pairs at 180 degrees phase shifted.