Free piston engines

What is a free piston engine?

A free-piston engine is a …..type of internal combustion engine where the pistons’ motion is not-mechanically controlled. Unlike conventional engines there is no crankshaft or mechanical transmission to transmit the power generated and govern the motion of the pistons.

How does that work?

Unlike conventional engines there is no crankshaft or mechanical transmission to transmit the power generated and guide the piston movement. The pistons are free to move back and forth due to combustion and the pressure that is created in the bounce chambers.

Each piston is coupled with a linear electric machine that generates electrical power. The same electric machine is also able to dynamically control the piston to optimize the compression ratio in real-time and thus allowing an advanced type of control over the thermodynamic efficiency of the engine.

Digital piston motion control

Electronic fuel injection has revolutionized the efficiency and reliability of internal combustion engines over the past three decades, turning what were once mechanically driven processes into activities under the total control of "electronic control units" (or ECUs). Even more recent developments concerning variable valve timing (VVT) and variable inlet air compression systems have also brought under the control of the electronic control units the air that is introduced into the combustion chambers, further increasing efficiency and extracted power for the same displacement.

The advent of digital piston motion control will allow free piston engines to complete the journey of internal combustion engines in abandoning their original mechanical constraints.


Developers of free piston engines in many parts of the world are investing significant resources to develop these types of engines, with the intention of realizing numerous benefits:

High efficiency

Developers of free piston engines have reported significant increases in efficiency, which come from a variety of reasons. First of all, in a free-piston engine the piston moves toward and away from TDC position with a controllable motion law, optimizing the timing for compression and expansion of the working charge. Secondly, the precise control of the piston motion allows to realize particularly advanced combustion cycles such as HCCI (homogeneous charge compression ignition).

This results in greater conversion of fuel energy into actual usable pressure to produce work. Finally, the elimination of many traditional mechanical components removes many parts and surfaces responsible for friction and heat loss typical of conventional engines. All these reasons together mean that free piston engines have the potential to produce electric power with efficiency between 40% e 50% compared to 25%-35% of the most efficient generators available today.

Low emissions

In theory, an internal combustion engine combines oxygen with the correct amount of fuel to produce carbon dioxide and water vapor. In practice, however, not all fuel is burned and some oxygen molecules combine with nitrogen in the air instead of fuel. These two undesirable and unavoidable aspects of the process produce polluting exhaust gases.

In traditional engines these processes are controlled (in order to reduce them to a minimum) by dynamically varying the amount of fuel introduced. In a free-piston engine, compression ratio management provides a means of controlling the energy possessed by each oxygen and fuel molecule. This additional control capability provides developers with the ability to adopt much more effective emission reduction strategies without compromising performance and efficiency.

Flexible fuels

The variable compression rate made possible by the digital control of piston motion, in a free piston engine, can be adapted to different types of fuel, even dynamically

Low noise and vibration

The elimination of rotating components reduces the cause of vibrations both inside the engine and towards the structures on which it is mounted. A free-piston engine with two opposite cylinders is inherently perfectly balanced, and in fact air compressors using this architecture are famous for the absence of vibrations.

No lubrication

Due to the low lateral loads on the piston shell, oil lubrication can be replaced by air bearings. This not only eliminates the need for oil but also reduces heat losses and friction, reduces emissions and the cost of engines once they are produced in high volumes.

Low cost

A free piston engine is extremely simple due to the elimination of many components: crankshaft, flywheel, connecting rods etc. The motor crankcase is also simpler and can be obtained by extrusion instead of casting, further reducing costs.


The reduction and simplification of components also translates into an extremely compact layout that allows applications in confined spaces.

Want to know more?
Contact us