Free-piston engine
A free-piston engine is a linear, 'crankless' internal combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion chamber gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a gas compressor or a linear alternator).
The free-piston engine is usually restricted to the two-stroke operating principle, since a power stroke is required every fore-and-aft cycle.
Some of these engines utilised the air remaining in the compressor cylinders to return the piston, thereby eliminating the need for a rebound device.
Free-piston air compressors were in use among others by the German Navy, and had the advantages of high efficiency, compactness and low noise and vibration.
A number of free-piston gas generators were developed, and such units were in widespread use in large-scale applications such as stationary and marine powerplants.
[10] Examples include the Stelzer engine and the Free Piston Power Pack manufactured by Pempek Systems [1] based on a German patent.
[11] A single piston Free-piston linear generator was demonstrated in 2013 at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).
As the piston is forced downward during its power stroke it passes through windings in the cylinder to generate a burst of three-phase AC electricity.
The engine is easily modified to operate under various fuels including hydrogen, natural gas, ethanol, gasoline and diesel.
The main difference is due to the piston motion not being restricted by a crankshaft in the free-piston engine, leading to the potentially valuable feature of variable compression ratio.
The frequency can therefore be controlled by applying a pause between the time the piston reaches BDC and the release of compression energy for the next stroke.
Also, high efficiencies are obtained due to nearly constant volume combustion and the possibility to burn lean mixtures to reduce gas temperatures and thereby some types of emissions.
Another possibility is to apply counterweights, which results in more complex design, increased engine size and weight and additional friction losses.
Rotational movement to spin conventional automobile engine accessories such as alternators, air conditioner compressors, power steering pumps, and anti-pollution devices could be captured from a turbine situated in the exhaust stream.
