Free piston machine

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The free piston machine combines a heat engine with internal or external heat supply and a working machine ( pump , compressor , generator ). Characteristic is the direct transmission of the cyclical movement of the piston of the heat engine to the working part without a drive mechanism; this eliminates any mechanical drive, such as a crank drive. The power is not delivered mechanically. Free piston engines usually work as two-stroke engines (internal combustion), but there are also free-piston Stirling engines (external heat supply, not necessarily combustion).

Free piston machines can be built simply (few moving parts) and compact and have a high power-to-weight ratio.

Such machines were manufactured in series by Junkers (around 1926), SIGMA (Pescara) (1934) and Sulzer (1942) , among others .

Application forms

If a linear generator, a compressor or a hydraulic pump is integrated into the free piston engine, a compact unit is created.

Motor / generator

A free piston generator, i.e. a free piston engine with an integrated linear generator for generating electrical energy, is one of the most common free piston machines. The term free-piston linear generator was coined for certain embodiments of free- piston generators .

Systems with combined heat and power are mostly driven by Stirling free-piston engines. The most important manufacturers of free-piston machines were Sigma in France and Alan Muntz & Co. from Great Britain until the 1970s.

The Pescara engine was used by the US Navy in 1952 as a gas generator for a gas turbine and achieved an output of around 900 kW. In 1956, a team from General Motors under the direction of Arthur F. Underwood investigated a free-piston engine for the GM XP-500 test vehicle for generating gas for the actual drive turbine with an output of at least 184 kW (250 hp). As early as 1940, the British aircraft designer LE Baynes planned the use of Pescara gas generators (Pescara-Muntz P.42) to drive a 130-ton flying boat, with the drive turbines each supposed to deliver 2000 to 3000 hp. However, the project did not get beyond the planning stage.


The most common application is the direct drive compressor. For small compressors, pistons that work against a spring and are electrically driven are built. Junkers free piston compressors were used to generate compressed air on German submarines during World War II . Such compressors were then mass-produced. They are still being built today (E + JE compressors).

hydraulic pump

Schematic structure of a thermohydraulic free piston machine (TU-Dresden 2003)

In the thermohydraulic F rei k olben m achine (FKM), the released on the combustion engine side is discharged to the hydraulic power side in the form of hydraulic working. The name comes from the combination of thermodynamics (combustion) and hydraulics. In the end, it is a hydraulic pump that is powered by an internal combustion engine.

A variant of the free piston machine as a hydraulic pump is the so-called water column machine . Here backwater acts directly on the pumping medium, e.g. salty brine, via one (or more) pistons.

Soil compactor and pile driver

One application of free piston machines that can still be found today are:

  • Rams , with which piles for fixing foundations or sheet pile walls are driven essentially vertically into the ground. The drive can take place via pressurized steam, compressed air or ignition of a fuel-air mixture. During the working stroke, the piston or cylinder accelerates the lying bear, a linear hammer mass of up to 20 tons , so strongly upwards that the bear is thrown up to a few meters and falls back under the constant effect of the gravitational acceleration and finally on the piling material being held bounces. The aim is to achieve a high level of impulse transmission to the pile in order to gradually drive it into the ground.
  • Explosion rammers, cobblestone rams and pile drivers that can be operated and moved by one person like the legendary DELMAG H2S. or the DELMAG frog. The piston is supported on the floor and with the cylinder accelerates the device upwards, which then reaches a height of about 25 cm, taking the piston with it via spring force. Under the constant influence of gravity, the device falls to the ground, which is compacted when it is pushed off and when it lands. The working frequency is about 1 Hz. A dosed and manually influenced inclination causes horizontal progress with every tamping step. These soil rammers or paving rammers are rarely found on construction sites, because their safe handling requires some practice and they have been replaced by hydraulic, pneumatic, electromechanical or four-stroke machine-driven systems that vibrate much faster.

Advantages and disadvantages

The advantages of a free piston machine are:

  • Integration of two machines in a common housing
  • Compact unit with few individual parts (in principle, a free-piston heat engine can get by with a moving part, the piston, which means that it can be manufactured inexpensively if the problem of coupling out the energy is solved)
  • no complex and expensive mechanical drives (e.g. crank drive)
  • no lateral piston force due to inclined position of the connecting rod, no bearing friction
  • low mass
  • easy to assemble and maintain
  • unconventional operating modes possible (variable stroke; pulse pause modulation, ...)


  • starting can be problematic as a rotating starter is not easy to use
  • in the event of a misfire, a free-piston machine inevitably stops (the crankshaft motor can continue to rotate due to the kinetic energy of the flywheel)
  • the movement of the free piston must be stabilized with complex control (the dead centers of the piston movement are not specified by the crankshaft)
  • The extremely high accelerations of the piston in the combustion engine's top dead center influence the combustion process in a previously unknown way
  • limited power range (approx. 15 to 50 kW)
  • problematic drive of the auxiliary units (fuel pump, cooling water pump, generator, ...)
  • Noise and vibration behavior completely different from established rotating combustion engines
  • single-acting machine causes considerable inertia forces

Research and Development

Research on free piston machines is currently being carried out at the following locations, among others:

  • Innas BV (Breda, Netherlands)
  • German Aerospace Center ( DLR , Stuttgart)
  • Technical University of Dresden in cooperation with Bosch Rexroth
  • Tampere University (Finland)
  • Caterpillar (Peoria / Illinois, USA)
  • Volvo (Gothenburg, Sweden; Swedish name: Frikolvmaskin)
  • Sandia National Laboratories, USA
  • Newcastle University (Great Britain)
  • Toyota Central R&D Labs, Inc. (Japan)
  • Beetron GmbH (Switzerland)

See also

Web links

Wiktionary: Free piston machine  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  2. Markus Gräf: The free piston linear generator FKLG power generation with high efficiency. DLR, APU workshop, Erlangen, October 26, 2005, accessed July 16, 2013 (PDF; 4.2 MB).
  3. Florian Kock, Alex Heron, Frank Rinderknecht, Horst E. Friedrich: The free piston linear generator - potentials and challenges. In: Motortechnische Zeitschrift mtz , 10/2013
  4. ^ William Morse: Baynes: The unknown innovator - Part 6. In: Airplane Monthly , September 1992, pp. 46ff
  5. ^ Website E + JE
  6. Note. Bear and cylinder can also form a constructive unit.
  7. See z. B. DELMAG diesel bears
  8. See e.g. B. YouTube video DELMAG ramming frame H2S
  9. See e.g. B. YouTube video Delmag Frog F5
  11. ^ Newcastle University free-piston engine project ( Memento from October 21, 2008 in the Internet Archive )
  12. Toyota Free Piston Engine Linear Generator "FPEG"
  13. The transition to sustainable power generation