![Pescara avec turbine](https://upload.wikimedia.org/wikipedia/commons/b/b5/Pescara_avec_turbine.gif)
A free-piston engine is a linear, 'crankless'
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
, in which the piston motion is not controlled by a
crankshaft
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
but determined by the interaction of forces from the
combustion chamber
A combustion chamber is part of an internal combustion engine in which the fuel/air mix is burned. For steam engines, the term has also been used for an extension of the firebox which is used to allow a more complete combustion process.
Interna ...
gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a
gas compressor
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...
or a
linear alternator
A linear alternator is essentially a linear motor used as an electrical generator.
An alternator is a type of alternating current (AC) electrical generator. The devices are often physically equivalent. The principal difference is in how they are ...
).
The purpose of all such piston engines is to generate power. In the free-piston engine, this power is not delivered to a crankshaft but is instead extracted through either
exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
pressure driving a turbine, through driving a linear load such as an air compressor for
pneumatic
Pneumatics (from Greek ‘wind, breath’) is a branch of engineering that makes use of gas or pressurized air.
Pneumatic systems used in Industrial sector, industry are commonly powered by compressed air or compressed inert gases. A central ...
power, or by incorporating a linear alternator directly into the pistons to produce electrical power.
The basic configuration of free-piston engines is commonly known as single piston, dual piston or
opposed piston
An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head. Petrol and diesel opposed-piston engines have been used mostly in large-scale applications such as ships, military tanks, and ...
s, referring to the number of combustion cylinders. The free-piston engine is usually restricted to the
two-stroke
A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a Thermodynamic power cycle, power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being comple ...
operating principle, since a power stroke is required every fore-and-aft cycle. However, a
split cycle four-stroke
A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...
version has been patented, GB2480461 (A) published 2011-11-23.
First generation
![US1657641-figure-1](https://upload.wikimedia.org/wikipedia/commons/c/cb/US1657641-figure-1.png)
The modern free-piston engine was proposed by
R.P. Pescara and the original application was a single piston
air compressor
An air compressor is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces ...
. Pescara set up the ''Bureau Technique Pescara'' to develop free-piston engines and
Robert Huber
Robert Huber (; born 20 February 1937) is a German biochemist and Nobel laureate. known for his work crystallizing an intramembrane protein important in photosynthesis and subsequently applying X-ray crystallography to elucidate the protein's st ...
was technical director of the Bureau from 1924 to 1962.
The engine concept was a topic of much interest in the period 1930–1960, and a number of commercially available units were developed. These first generation free-piston engines were without exception opposed piston engines, in which the two pistons were mechanically linked to ensure symmetric motion. The free-piston engines provided some advantages over conventional technology, including compactness and a vibration-free design.
Air compressors
The first successful application of the free-piston engine concept was as air compressors. In these engines, air compressor cylinders were coupled to the moving pistons, often in a multi-stage configuration. 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.
Gas generators
After the success of the free-piston air compressor, a number of industrial research groups started the development of free-piston gas generators. In these engines there is no load device coupled to the engine itself, but the power is extracted from an exhaust turbine. The turbine's rotary motion can thus drive a pump, propeller, generator, or other device.
In this arrangement, the only load for the engine is supercharging the inlet air, albeit in theory some of this air could be diverted for use as a compressed-air source if desired. Such a modification would enable the free-piston engine, when used in conjunction with the aforementioned exhaust-driven turbine, to provide both motive power (from the output shaft of the turbine) in addition to compressed air on demand.
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. Attempts were made to use free-piston gas generators for vehicle propulsion (e.g. in
gas turbine locomotive
A gas turbine locomotive is a type of railway locomotive in which the prime mover is a gas turbine. Several types of gas turbine locomotive have been developed, differing mainly in the means by which mechanical power is conveyed to the driving w ...
s) but without success.
Modern applications
Modern applications of the free-piston engine concept include hydraulic engines, aimed for off-highway vehicles, and free-piston engine generators, aimed for use with hybrid electric vehicles.
Hydraulic
These engines are commonly of the single piston type, with the hydraulic cylinder acting as both load and rebound device using a hydraulic control system. This gives the unit high operational flexibility. Excellent part load performance has been reported.
Generators
Free-piston linear generator The free-piston linear generator (FPLG) uses chemical energy from fuel to drive magnets through a stator and converts this linear motion into electric energy. Because of its versatility, low weight and high efficiency, it can be used in a wide rang ...
s that eliminate a heavy crankshaft with electrical coils in the piston and cylinder walls are being investigated by multiple research groups for use in hybrid electric vehicles as
range extenders. The first free piston generator was patented in 1934. Examples include the
Stelzer engine
The Stelzer engine is a two-stroke opposing-piston free-piston engine design proposed by Frank Stelzer. It uses conjoined pistons in a push-pull arrangement which allows for fewer moving parts and simplified manufacturing. An engine of the same des ...
and the Free Piston Power Pack manufactured b
Pempek Systems
based on a German patent. A single piston Free-piston linear generator The free-piston linear generator (FPLG) uses chemical energy from fuel to drive magnets through a stator and converts this linear motion into electric energy. Because of its versatility, low weight and high efficiency, it can be used in a wide rang ...
was demonstrated in 2013 at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR).
These engines are mainly of the dual piston type, giving a compact unit with high power-to-weight ratio. A challenge with this design is to find an electric motor with sufficiently low weight. Control challenges in the form of high cycle-to-cycle variations were reported for dual piston engines.
In June 2014 Toyota announced a prototype Free Piston Engine Linear Generator (FPEG). 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 piston generates electricity on both strokes, reducing piston dead losses. The generator operates on a two-stroke cycle, using hydraulically activated exhaust poppet valves
A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...
, gasoline direct injection
Gasoline direct injection (GDI), also known as petrol direct injection (PDI), is a mixture formation system for internal combustion engines that run on gasoline (petrol), where fuel is injected into the combustion chamber. This is distinct fro ...
and electronically operated valves. The engine is easily modified to operate under various fuels including hydrogen, natural gas, ethanol, gasoline and diesel. A two-cylinder FPEG is inherently balanced.
Toyota claims a thermal-efficiency rating of 42% in continuous use, greatly exceeding today's average of 25-30%. Toyota demonstrated a 24 inch long by 2.5 inch in diameter unit producing 15 hp (greater than 11 kW).
Features
The operational characteristics of free-piston engines differ from those of conventional, crankshaft engines. 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. This does, however, also present a control challenge, since the position of the dead centres must be accurately controlled in order to ensure fuel ignition and efficient combustion, and to avoid excessive in-cylinder pressures or, worse, the piston hitting the cylinder head. The free-piston engine has a number of unique features, some give it potential advantages and some represent challenges that must be overcome for the free-piston engine to be a realistic alternative to conventional technology.
As the piston motion between the endpoints is not mechanically restricted by a crank mechanism, the free-piston engine has the valuable feature of variable compression ratio, which may provide extensive operation optimization, higher part load efficiency and possible multi-fuel operation. These are enhanced by variable fuel injection timing and valve timing through proper control methods.
Variable stroke length is achieved by a proper frequency control scheme such as PPM (Pulse Pause Modulation) control
in which piston motion is paused at BDC using a controllable hydraulic cylinder as rebound device. 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.
Since there are fewer moving parts, the frictional losses and manufacturing cost are reduced. The simple and compact design thus requires less maintenance and this increases lifetime.
The purely linear motion leads to very low side loads on the piston, hence lesser lubrication requirements for the piston.
The combustion process of free piston engine is well suited for
Homogeneous Charge Compression Ignition Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releas ...
(HCCI) mode, in which the premixed charge is compressed and self-ignited, resulting in very rapid combustion, along with lower requirements for accurate ignition timing control. 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 .
By running multiple engines in parallel, vibrations due to balancing issues may be reduced, but this requires accurate control of engine speed. Another possibility is to apply counterweights, which results in more complex design, increased engine size and weight and additional friction losses.
Lacking a kinetic energy storage device, like a flywheel in conventional engines, free-piston engines are more susceptible to shutdown caused by minute variations in the timing or pressure of the engine cycle. Precise control of the speed and timing is required as, if the engine fails to build up sufficient compression or if other factors influence the injection/ignition and combustion, the engine may misfire or stop.
Advantages
Potential advantages of the free-piston concept include:
* Simple design with few moving parts, giving a compact engine with low maintenance costs and reduced frictional losses.
* The operational flexibility through the variable compression ratio allows operation optimisation for all operating conditions and multi-fuel operation. The free-piston engine is further well suited for
homogeneous charge compression ignition Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releas ...
(HCCI) operation.
* High piston speed around top dead centre (TDC) and a fast power stroke expansion enhances fuel-air mixing and reduces the time available for heat transfer losses and the formation of temperature-dependent emissions such as nitrogen oxides (NOx).
Challenges
The main challenge for the free-piston engine is engine control, which can only be said to be fully solved for single piston hydraulic free-piston engines. Issues such as the influence of cycle-to-cycle variations in the combustion process and engine performance during transient operation in dual piston engines are topics that need further investigation. Crankshaft engines can connect traditional accessories such as alternator, oil pump, fuel pump, cooling system, starter etc.
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.
Opposing piston engine
Most free piston engines are of the
opposed piston
An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head. Petrol and diesel opposed-piston engines have been used mostly in large-scale applications such as ships, military tanks, and ...
type with a single central combustion chamber. A variation is the Opposing piston engine which has two separate combustion chambers. An example is the
Stelzer engine
The Stelzer engine is a two-stroke opposing-piston free-piston engine design proposed by Frank Stelzer. It uses conjoined pistons in a push-pull arrangement which allows for fewer moving parts and simplified manufacturing. An engine of the same des ...
.
Recent developments
In the 21st century, research continues into free-piston engines and patents have been published in many countries. In the UK,
Newcastle University
Newcastle University (legally the University of Newcastle upon Tyne) is a UK public university, public research university based in Newcastle upon Tyne, North East England. It has overseas campuses in Singapore and Malaysia. The university is ...
is undertaking research into free-piston engines.
A new kind of the free-piston engine, a
Free-piston linear generator The free-piston linear generator (FPLG) uses chemical energy from fuel to drive magnets through a stator and converts this linear motion into electric energy. Because of its versatility, low weight and high efficiency, it can be used in a wide rang ...
is being developed by the German aerospace center.
In addition to these prototypes, researchers at West Virginia University in the US, are working on the development of a single cylinder free-piston engine prototype with mechanical springs at an operating frequency of 90 Hz.
[Bade, Mehar, Nigel N. Clark, Matthew C. Robinson, and Parviz Famouri. "Parametric Investigation of Combustion and Heat Transfer Characteristics of Oscillating Linear Engine Alternator." Journal of Combustion 2018 (2018).]
References
Sources
* Mikalsen R., Roskilly A.P. A review of free-piston engine history and applications. ''
Applied Thermal Engineering
''Applied Thermal Engineering'' is a peer-reviewed scientific journal covering all aspects of the thermal engineering of advanced processes, including process integration, intensification, and development, together with the application of thermal ...
, Volume 27, Issues 14-15, Pages 2339-2352, 2007.'
External links
DLR researchers unveil a new kind of range extender for electric carsExtensive homepage about Free Piston EnginesNewcastle University
"Engine of Tomorrow - Goes to Work Today."''Popular Science'', September 1957, pp. 138–141/294, detailed article/cutaway drawing on free-piston diesel engines.
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