Free-piston engine
Encyclopedia
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 basic configuration of free-piston engines is commonly known as single piston, dual piston or opposed pistons, referring to the number of combustion cylinders. The free-piston engine is in practice restricted to the two-stroke operating principle, since a power stroke is required every fore-and-aft cycle.
and the original application was a single piston air compressor
. 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.
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. Attempts were made to use free-piston gas generators for vehicle propulsion (e.g. in gas turbine locomotive
s) but without success.
and the Free Piston Power Pack manufactured by Pempek Systems based on a German patent.
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 machine with sufficiently low weight, and control challenges in the form of high cycle-to-cycle variations have been reported for dual piston engines.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
, in which the piston motion is not controlled by a crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...
but determined by the interaction of forces from the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a gas compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
or a linear alternator
Linear alternator
A linear alternator is essentially a linear motor used as an electrical generator.An alternator is a type of alternating current electrical generator. The devices are often physically equivalent. The principal difference is in how they are used and which direction the energy flows...
).
The basic configuration of free-piston engines is commonly known as single piston, dual piston or opposed pistons, referring to the number of combustion cylinders. The free-piston engine is in practice restricted to the two-stroke operating principle, since a power stroke is required every fore-and-aft cycle.
First generation
In 1807, de Rivaz tested an early engine that operated a free piston on a chain. The modern free-piston engine was proposed by R.P. PescaraRaúl Pateras Pescara
Raúl Pateras Pescara de Castelluccio , marquis of Pateras-Pescara, was an Argentine lawyer and inventor specializing in seaplanes and helicopters, as well as motors, compressors, and the Pescara free-piston engine.-Biography:At the beginning of the 20th century, his family returned from Buenos...
and the original application was a single piston air compressor
Air compressor
An air compressor is a device that converts power into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts...
. 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 only load for the engine is supercharging the inlet air.)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
Gas turbine locomotive
A Gas turbine locomotive is a locomotive powered by a gas turbine. The majority of gas turbine locomotives have had electric transmission but mechanical transmission has also been used, particularly in the early days. The advantage of using gas turbines is that they have very high power-to-bulk...
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, and excellent part load performance has been reported for such engines.Generators
The use of a free-piston engine with a linear generator is being investigated by a number of research groups, driven by the increasing interest in the hybrid electric vehicle concept in the automotive industry. The first free piston generator was patented in 1959, and since then, a number of variations have been proposed. Examples include the Stelzer engineStelzer engine
The Stelzer engine is a two-stroke opposing-piston design proposed by Frank Stelzer. It uses conjoined pistons in a push-pull arrangement which allows for fewer moving parts and simplified manufacturing....
and the Free Piston Power Pack manufactured by Pempek Systems based on a German patent.
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 machine with sufficiently low weight, and control challenges in the form of high cycle-to-cycle variations have been reported for dual piston engines.
Features and potential advantages
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.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 (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.Sources
- Mikalsen R., Roskilly A.P. A review of free-piston engine history and applications. Applied Thermal Engineering, Volume 27, Issues 14-15, Pages 2339-2352, 2007. http://www.mikalsen.eu/papers/FPEreview.pdf.