Free-piston linear generator

The Free-Piston Linear Generator (FPLG), is a free-piston engine coupled with a linear alternator. It converts chemical energy from fuel into electric energy. Because of its versatility, low weight and good efficiency, it can be used in a wide range of applications, although it is of special interest of the mobility industry as range extenders for electric vehicles.

Description

The free-piston engine linear generators can be divided in 3 subsystems:[1][2]

The FPLG has many potential advantages compared to traditional electric generator powered by an internal combustion engine. One of the main advantages of the FPLG comes from the absence of crankshaft. It leads to a smaller and lighter engine with fewer parts. This also allows a variable compression ratio, which makes it possible to design an engine that works ideally with different kinds of fuel.

The linear generator also allows to control the resistance force, and therefore a better control of the pistons movement of and the combustion. The total efficiency (including engine and generator) of free-piston linear generators are around 40%,[3][4][5] which is a significant improvement compared to conventional combustion engines. The latters reach about 20% under typical US driving conditions.[6]

Development

FPLG patent from 1943 - Pontus Ostenber, USA by P. Ostenberg

The first patents of free-piston linear generators dates around 1940, however in the last decades, specially after the development of the rare-earth magnets, many different research groups are working on the topic.[7][8]

Although there is a variety of names and abbreviations for the technology, the term "Free-piston linear generator" and "FPLG" particularly refers to the project at German Aerospace Center.

Operation

The free-piston linear generator generally consists of three subsystems: combustion chamber, linear generator and a return unit (normally a gas spring), which are coupled one to another through a connecting rod.

In the combustion chamber, a mixture of fuel and air is ignited, so the pressure increases and the moving parts (connection rod, linear generator and pistons) are accelerated in the direction of the gas spring. The gas spring is compressed, and while the piston is near the bottom dead center (BDC), fresh air and fuel are injected in the combustion chamber, expelling the exhaustion gases from the chamber.

The gas spring pushes the moving parts assembly back to the top dead center (TDC), compressing the mixture of air and fuel that was injected and the cycle repeats. This works similar to the two-stroke engine, however it is not the only possible configuration.

The linear generator can generate a force opposed to the movement not only the expansion but also in the compression phase. The magnitude and the force profile affects the piston movement, as well as the overall efficiency.

Variations

The FPLG have been conceived in many different configurations and line-ups, but for most applications, particularly for the automotive industry, it is interesting to use two opposed pistons to balance out the forces in order to reduce vibration and noise. In the simplest case, a second unit is just a mirror of the first, with no functional connection to the first. Alternatively, a single combustion chamber or gas spring can be used, allowing for a more compact design and easier synchronization between the pistons.

The gas spring and combustion chamber can be placed on the ends of the connection rods, or they to share the same piston, using opposite sides in order to reduce space.

The linear generator itself has also many different configurations and forms. It can be designed as round tube, a cylinder or even flat plate in order to reduce the center of gravity, and/or improve the heat dissipation.

The free-piston linear generator's great versatility comes from the absence of crankshaft, which gives the engine another degree of freedom. The combustion can be carried as a two-stroke engine as well as a four-stroke engine. However, this last one requires a much higher intermediate storage of energy: as there is no rotational inertia of the crankshaft, the gas spring would have to power the piston through the intake, compression and exhaustion phases. That is the reason why most of the current research focus on the two-strokes variant.

Also regard to the combustion phase, several variations are possible:

The DLR research

The Institute of Vehicle Concepts of the German Aerospace Center is currently developing a FPLG (or Freikolbenlineargenerator - FKLG) since 2002, and has published several papers about this subject.[1][2][16][17]

During the first few years of research, the theoretical background along with the 3 subsystems were developed separately. In 2013, the first entire system was built and operated successfully.[18]

The German center is currently into its 2nd version of the entire system, on which two opposite cylinders will be used in order to reduce vibration and noise, making it viable for the automotive industry.

References

  1. 1 2 Pohl, Sven-Erik (2007). Der Freikolbenlineargenerator - Theoretische Betrachtungen des Gesamtsystems und experimentelle Untersuchungen zum Teilsystem der Gasfeder. Hamburg: Helmut-Schmidt-Universität.
  2. 1 2 Ferrari, Cornelius (2012). Entwicklung und Untersuchung eines Freikolbenlineargenerators unter besonderer Berücksichtigung des verbrennungsmotorischen Teilsystems mit Hilfe eines neuartigen vollvariablen Prüfstands. Stuttgart: Universität Stuttgart.
  3. ↑ Sven-Erik Pohl: Der Freikolbenlineargenerator - Theoretische Betrachtungen des Gesamtsystems und experimentelle Untersuchungen zum Teilsystem der Gasfeder, Helmut-Schmidt-Universität, Hamburg, 2007
  4. ↑ Cornelius Ferrari: Entwicklung und Untersuchung eines Freikolbenlineargenerators unter besonderer Berücksichtigung des verbrennungsmotorischen Teilsystems mit Hilfe eines neuartigen vollvariablen Prüfstands, Universität Stuttgart, Stuttgart 2012
  5. ↑ Braun, Peter (2015). "Why ‘Free Piston’ engines could power your next plug-in hybrid". Road Rave. Digital Trends.
  6. ↑ "Improving IC Engine Efficiency". Courses.washington.edu.
  7. ↑ R. Mikalsen, A.P. Roskilly. "A review of free-piston engine history and applications" (PDF).
  8. ↑ Kosaka, H., Akita, T., Moriya, K., Goto, S.; et al. (2014). "Development of Free Piston Engine Linear Generator System Part 1 - Investigation of Fundamental Characteristics". SAE International.
  9. ↑ Van Blarigan, Peter (2001). "ADVANCED INTERNAL COMBUSTION ELECTRICAL GENERATOR" (PDF).
  10. ↑ DLR researchers unveil a new kind of range extender for electric cars
  11. ↑ "The Free Piston Power Pack: Sustainable Power for Hybrid Electric Vehicles". SAE international. SAE. 2003.
  12. ↑ Hansson, Jorgen (2006). "Analysis and Control of a Hybrid Vehicle Powered by a Free-Piston Energy Converter". Königlich Technische Hochschule Portal.
  13. ↑ "Linear Combustion Engine". Linear Combustion Engine. 2004.
  14. ↑ BEETRON: The transition to sustainable power generation
  15. ↑ "Toyota develops high-efficiency ‘free piston’ no-crankshaft combustion engine… to power an EV". Extreme Tech.
  16. ↑ Kock, F., Haag, J., and Friedrich, H. (2013). The Free Piston Linear Generator - Development of an Innovative, Compact, Highly Efficient Range-Extender Module. SAE International.
  17. ↑ Kock, F. (2015). Steuerung und Regelung des Freikolbenlineargenerators - Entwicklungsmethode und Regelungskonzept für den Betrieb eines neuartigen Energiewandlers. Stuttgart: Deutsche Zentrum für Luft- und Raumfahrt.
  18. ↑ "DLR team develops demonstrator of free-piston linear generator as range extender for EVs". Green Car Congress. 2013-02-20.

External links

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