Free-piston engine

Internal-combustion engines – Free piston – Single chamber; one piston

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F02B 7100

Patent

active

058293930

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a free-piston engine according to the preamble of claim 1.
Engines of this type are known and are used to convert chemical energy of liquid and gaseous fuel into mechanical energy, for example in the form of hydraulic or pneumatic pressure or electrical energy.
Energy conversion in known free-piston engines is accomplished by ignition of compressed air mixed with fuel in the combustion space, whereupon the expanding combustion mixture sets the combustion piston in motion. The movements of the freely moving combustion piston are controlled by the engine control system by means of a hydraulic control system. The supply of energy can be consumed by means of a hydraulic system, which may be combined with the hydraulic control system, by means of a pneumatic system or by means of an electric system.
Free-piston engines of this type, in which the chemical energy is converted into hydraulic energy, are for example known from EP 0 254 353 or WO 93/10342. In the engines described in these publications, the movements of the free piston are controlled by means of a hydraulic system, in which a hydraulic piston forming a piston assembly together with the combustion piston stands still on the outer dead centre before the start of the compression stroke, and starts a new stroke after receiving a control signal. This signal switches on a starting valve, for example valve 26 shown in FIG. 1 in EP 0254353, or valve 24 shown in FIG. 3 in WO 93/10342, whereupon the compression stroke starts.
A disadvantage of these known free-piston engines is the rebound of the piston assembly at the end of the expansion stroke. At the end of the expansion stroke, i.e. at the moment the combustion piston comes to a standstill, the pressures reigning in the first chamber and in the second chamber are the same, namely the pressure of the pressure accumulator. Since the first surface is larger than the second surface, the piston assembly will rebound and the pressure in the first chamber will quickly decrease because there is no oil supply. The pressure in the second chamber remains equal to the pressure in the pressure accumulator, so that the piston assembly will come to a standstill and subsequently move back again.
The present invention aims to reduce this rebound to a minimum so that the piston assembly can come to a standstill more quickly and less time is lost between successive strokes. As a result of this, the position of the piston assembly on the outer dead centre will also be better defined.
To this end, the free-piston engine according to the invention is designed in accordance with the characterizing part of claim 1.
By placing a non-return valve in the conduit between the pressure accumulator and the second chamber, one achieves that, during a rebound of the piston assembly, the pressure in the second chamber can become higher than that in the pressure accumulator. This high pressure acts on the second surface of the hydraulic piston and stops the movement in the direction of the combustion chamber. This way a considerable reduction of the rebound, a better control of the movement of the piston assembly and a higher frequency of the engine can be achieved.
A further improvement of the free-piston engine is achieved when the engine is designed in accordance with the characterizing part of claim 2 or 3.
By providing an open connection between the first chamber and the pressure accumulator or between the first chamber and the second chamber in the region where the piston assembly moves at a high speed, wherein said open connection can be closed by moving the hydraulic piston, the flow rates in the starting valve are kept donw, as a result of which the losses in the hydraulic system remain small.
The invention will hereinafter be further explained with reference to the drawing with the following figures:
FIG. 1 shows a free-piston engine with a hydraulic control system according to the prior art.
FIG. 2 shows a schematic section of a first embodiment of the hydraulic control system of the free-piston engine

REFERENCES:
patent: 4308720 (1982-01-01), Brandstadter
patent: 5287827 (1994-02-01), Almendinger et al.
patent: 5540194 (1996-07-01), Adams

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