Free-piston engine
The free-piston engine is a linear, 'crankless' combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion cylinder gases, a rebound device and a load device.
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 practise restricted to the two-stroke operating principle, since a power stroke is required every revolution.
First generation free-piston engines
The free-piston engine was first proposed by Pescara (1928) 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.
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. (Toutant (1952).)
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 the supercharging of 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 (London et al. (1952)). Attempts were made to use free-piston gas generator for vehicle propulsion, but without success (Underwood (1957), Frey et al. (1957)).
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 free-piston engines
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 (Achten et al. (2000), Brunner et al. (2005)).
Free-piston engine 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. 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. (Clark et al. (1998), Tikkanen et al. (2000)).
Free-piston 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 HCCI operation.
Challenges
The main challenge for the free-piston engine is engine control, which can only can 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.
References
- Mikalsen R., Roskilly A.P. A review of free-piston engine history and applications. Applied Thermal Engineering 2007:27:2339-2352.
- Underwood A.F., The GMR 4-4 ‘‘HYPREX’’ engine – A concept of the free-piston engine for automotive use, SAE Transactions 1957:65:377–391.
- Frey D.N. et al., The automotive free-piston-turbine engine, SAE Transactions 1957:65:628–634.
- London A.L., Oppenheim A.K., The free-piston engine development -- Present status and design aspects, Transactions of the ASME 1952:74:1349–1361.
- Achten P.A.J. et al., Horsepower with brains: The design of the Chiron free piston engine, SAE Paper 2000–01–2545, 2000.
- Brunner H. et al., Renaissance einer Kolbenmachine, Antriebstechnik 2005:4:66–70.
- Clark N. et al., Modelling and development of a linear engine, Proc. ASME Spring Conference, Internal Combustion Engine Division, 1998:30:49–57.
- Tikkanen S. et al, First cycles of the dual hydraulic free piston engine, SAE Paper 2000–01–2546, 2000.
- Toutant W.T., The Worthington–Junkers free-piston air compressor, Journal of the American Society of Naval Engineers 1952:64:583–594.
- Pescara R.P., Motor compressor apparatus, US Patent 1,657,641, 1928.