The simple construction of a free-piston engine in combination with a linear alternator represents the (theoretically) simplest possible construction of a power generator. Although - it seems that in practice it is not quite that simple, as some examples will reveal in the following.
Revolutionary proposal: The power take-off occurs on both the exhaust gases (2, 3) as well as electric (10, 4) with a linear alternator. The engine had a supercharger too (5). The proposal was never realized.
Harold Kosoff developed an opposed piston engine in which the pistons are synchronized pneumatically rather than mechanically.
As early as 1964 he had a vision of a linear generator, which powered cars and supplied houses with electricity and heat. The engine was not developed further.
Download Jarret Article (English language)
The time was not ready yet for a free-piston linear generator, because no rare-earth magnets with sufficiently high flux density were available.
Only with extremely powerful magnets, the piston mass can be reduced to an acceptable range.
In Jarret-like free-piston designs, the permanent magnets are thermally stressed.
A prototype made by Galileo, using commercially available weed wacker engine cylinders and a linear alternator from a Stirling engine. Double-acting type.
The privately funded company had to abandon its research for lack of money. Two govt institutes began their work instead:
- West Virginia University
- Sandia National Laboratories
"A High Specific Energy Output Free Piston Engine-Generator (HISEN-FPEG) for Hybrid Electric Vehicles and Electric Utility Power Generation" by Dr. Robert Law / Kenneth A. Galitello, Jr. March 1996
Second prototype (diesel), not a working engine.
"Optimization of a Brushless Permanent Magnet Linear Alternator for Use With a Linear Internal Combustion Engine", William R. Cawthorne, West Virginia University
"Free-Piston Diesel-Fueled Linear Alternator for Auxiliary Power Unit Applications, 1996-1999", Atkinson, Famouri, Clark, West Virginia University
To date, Sandia has worked one of the most comprehensive studies on free-piston engines. However, the permanent magnets of this version are thermally stressed.
Sandia chose a Jarret-like design with thermally highly stressed permanent magnets.
"ADVANCED INTERNAL COMBUSTION ELECTRICAL GENERATOR", Peter Van Blarigan, Sandia National Laboratories, Livermore, USA
"Developing a Thermodynamic Fuel Cell", Peter Van Blarigan, Sandia National Laboratories, Livermore, USA
The engine - virtually the same since 1980 - was coupled with a linear alternator developed in Chemnitz, Germany.
The engine stalled as power was taken off. The alternator did not work as a starter, a demonstration never took place.
Single-acting recoil-operated engine, later on combined to an opposed piston engine.
The engine was then combined to an opposed piston engine with a center bounce chamber and two outer expansion chambers. The proposal is similar to the '72 Jarret engine.
"Der Freikolbenlineargenerator FKLG Stromerzeugung mit hohem Wirkungsgrad", Dr.-Ing. Markus Gräf, Institut für Fahrzeugkonzepte, DLR
A single-acting design with flat type linear alternator. The alternator design was described in 2008 by the Shanghai Jiaotong University.
The engine has a single combustion chamber and an opposite bouncer chamber. Similar engines were built by INNAS and TU Dresden.
The vibrations of this engine design must be damped by a foundation, note the massive side brackets.
Based on the above-described experimental apparatus, this machine is to arise.
The machine looks very similar to EMO150 engine that was invented by Huber / Jarret in the 1960s, but the bounce chambers of this newer version seem to be surprisingly small in relation to the alternators.
This engine is designed as a two-stroke diesel. The vibrations are eliminated by connecting 4 units that run in 2 pairs at 180 degrees phase shifted.
"The Free Piston Power Pack: Sustainable Power for Hybrid Electric Vehicles", Douglas Carter and Edward Wechner, Pempek Systems, Australia
First Prototype with integrated starter / alternator.
Thermal separation of the linear alternator from the engine cylinders. Displacement 30 ccm, frequency 160 Hz, compression ratio 1:24
2004 Volvo Free Piston Energy Converter (FPEC)
EU funded project
"Analysis and Control of a Hybrid Vehicle Powered by a Free-Piston Energy Converter", Jörgen Hansson, Royal Institute of Technology (KTH), Stockholm, Sweden
"Volvo has earlier developed and built a prototype of a dual free piston engine and also made some testing on it. Conclusions made from the FPEC prototype resulted in that a free piston engine of opposed piston type would be used."
"Modelling and Control of Linear Combustion Engine (LCE)", Pavel Němeček, Ondřej Vysoký, Michal Šindelka, Czech Technical University in Prague
"SIMULATION OF A TWO-STROKE SPARK IGNITION FREE PISTON LINEAR ENGINE MOTION", N. A. N. MOHAMED, A. K. ARIFFIN & S. FONNA, Universiti Teknologi Malaysia
Single-acting Free-Piston Engine Project
Source: "The design and simulation of a two-stroke free-piston compression ignition engine for electrical power generation", R. Mikalsen, A.P. Roskilly, Newcastle University,UK (basic research)
Source: "Starting of a Free-Piston Linear Engine-Generator by Mechanical Resonance and Rectangular Current Commutation", Zulkifli, Karsiti, Rashid A-Aziz, Universiti Teknologi PETRONAS, Malaysia
Source: "The performance characteristics of a hydrogen-fuelled free piston internal combustion engine and linear generator system", Youngmin Woo, Youngjae Lee, Yonggyun Lee, Automobile Energy and Environment Research Center, Korea et al.
Source: "Flat-type permanent magnet linear alternator: A suitable device for a free piston linear alternator", Qingfeng Li, Jin Xiao, Zhen Huang, Shanghai Jiaotong University, China et al.
2012 General Motors
This picture looks very familiar, showing an opposed poston engine with an integrated linear alternator. 50 years ago, a patent for this principle was already filed in 1962 by Stirling A. Colgate.
The work of GM is based on the Pescara-SIGMA free-piston engine and the history as GM ordered the "Hyprex" engine for their XP500 Firebird at S.I.G.M.A. back in 1955. A first intention to reactivate the name "Hyprex" was dropped.
freikolben.ch was cited with text and pictures in the following extensive dissertation:
Download "TURBINE-COMPOUND FREE-PISTON LINEAR ALTERNATOR ENGINE"