Free-piston electric current generator

A medium-power, free-piston electric current generator is disclosed in which the piston is slidable in a vertical cylinder open at its top end, with a combustion chamber at its lower end. Induction coils are provided along the length of the cylinder. The piston is formed at least in part of magnetic material and the piston return is effected by the force of gravity. Valves for exhausting combustion gases are provided in the lower part of the cylinder and the opening and closing of the valves is synchronized with the movement of the piston. The cylinder and piston are cooled appropriately. The generator may employ gasoline, heating oil, a gas fuel or explosive substance.

FIELD OF THE INVENTION 
The present invention relates to free-piston electric generators. 
BACKGROUND OF THE INVENTION 
Free-piston electric generators are already known which, according to the 
Diesel principle, supply medium-power turbo-alternators with hot gases at 
about 450.degree. C. and an average pressure of about 3.times.10.sup.5 
pascals. Such generators have an overall efficiency of 35-36%, which is 
comparable to a conventional Diesel alternator system. 
More recently there have been disclosed horizontal free-piston engines 
having one or two pistons in opposed movement, carrying annular magnetic 
members which reciprocate between induction coils provided in the cylinder 
associated therewith, thereby generating electric current in the coils. In 
such engines the compression of the Diesel fuel mixture at a suitable 
pressure is ensured by the return of the piston or pistons which is 
produced by the energy stored in springs or hydraulic accumulators of 
appropriate design as to their function and to the maintenance of the 
energy storage capacity over a period of time. 
U.S. Pat. No. 3,105,153 and French printed patent application No. 2,204,068 
both relate to free-piston electric current generators. In the first the 
free piston is suspended by oscillating cross member and in the second the 
free piston may be horizontal. Such apparatus have closed cylinders. 
Moreover, they require either an explosion in a second combustion chamber 
(U.S. Pat. No. 3,105,153) or metal springs (French printed patent 
application No. 2,204,068) to ensure the return of the piston or pistons 
and the compression of the gaseous mixture necessary for the following 
explosion. 
SUMMARY OF THE INVENTION 
It has been found that a medium-power free-piston electric current 
generator may be constructed with a single vertical piston wherein the 
return stroke of the piston is ensured by the force of gravity and the 
resulting apparatus, while being of simple design, permits the production 
of electric current at an exceptionally high efficiency and in normal 
operation produces very little pollution as it contains little or no 
unburnt substances. 
According to the invention there is provided a medium-power free-piston 
electric generator apparatus, in which the free piston is slidable in 
sealing contact with the internal wall of a cylinder. The generator 
apparatus comprises a vertically oriented cylinder open at its upper end 
and having a combustion chamber at its lower end, with induction coils 
provided along at least part of the length of the cylinder, means for 
feeding a fuel-and-combustion supporting gas mixture, and means for 
ensuring ignition and combustion thereof in the cylinder. The free piston 
is made at least in part of magnetic material, and the return stroke of 
the free piston is effected by the force of gravity. Valves exhausting 
combustion gases are provided in the lower part of the cylinder. Means 
synchronize the opening and closing of the valves with the movement of the 
piston. Ports are located in the upper part of the cylinder above the top 
dead center position of the piston in normal operation. Cooling means are 
disposed in the body of the cylinder and/or on the outer surface thereof, 
along at least part of the length of the cylinder and on the piston. 
Such an apparatus comprises a thermal machine which uses the known 
principle of free-piston engines and more particularly that of free-piston 
electric current generators which produce direct current having variable 
sign, voltage and strength. 
According to the invention the return stroke of the piston is not ensured 
as in present-day apparatuses by a complex assembly of connecting rods, 
shafting or hydraulic accumulators but merely by the force of gravity, 
which is made possible by the vertical orientation of the cylinder. In 
addition this arrangement reduces friction to the extent possible. 
The kinetic energy which is produced by the explosion and converted into 
potential energy when the piston reaches its top dead center position 
ensures, at least in part, during the return stroke of the piston to its 
low position, the compression of the fluids in the combustion chamber for 
the following explosion. The compression and explosion of the 
air-gasoline, air-heating fuel or air-gas mixture is carried out as is 
known in Diesel engines or internal-combustion engines. 
According to an embodiment of the present invention the explosion which 
constitutes the source of energy which is converted into electric current 
is produced by combustion or detonation of a suitable metered quantity of 
an explosive substance selected among those known in the art as suitable 
for the characteristics of the apparatus and introduced into the 
combustion chamber at the bottom of the cylinder during each return stroke 
of the piston to its low position. 
According to a particularly preferred embodiment the apparatus comprises a 
two-stage piston having an upper stage provided with piston rings having a 
diameter substantially equal to that of the inner diameter of the cylinder 
and a lower stage having a smaller diameter which alone is engageable into 
the actual combustion chamber which is substantially at the same diameter. 
The lower stage of the two-stage piston may advantageously be equipped 
with piston rings though this is not absolutely necessary.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The free-piston electric generator illustrated in the drawing comprises a 
cylinder 1 positioned vertically with its upper end open to the 
surroundings. The combustion or explosion chamber 2 is located in the 
lower part and bottom of the cylinder which is closed. The combustion 
chamber 2 has a volume calculated as a function of the power to be 
produced for each explosion and as a function of the type and properties 
of the combustible mixture employed. Depending on availabilities of the 
types of fuels, the combustion chamber 2 is supplied with an air-gasoline 
mixture, an air-heating oil mixture or an air-gas mixture, under the best 
conditions known in the art, which have long been proven in 
internal-combustion and Diesel engines, namely: mechanical injection, 
feeding turbocharged with exhaust gases, etc. 
Cylinder 1 is equipped, in the conditions which preliminary tests have 
shown to be the highest efficiency conditions, with a series of induction 
coils 3, the density of the turns of the coils being calculated as a 
function of the desired voltage at the output terminals of the circuit, 
and taking into account the fact that the speed of the piston 4 carrying 
the permanent magnetic material 5 diminishes when the piston rises in the 
cylinder to its top dead center position which corresponds to the 
equilibrium pressure between the inside of the expansion chamber for gases 
under the piston and the atmosphere. 
For reasons of safety the cylinder 1, whose overall length may, for 
example, be about 490 cm for an internal diameter of about 60 cm must be 
provided with one or more ports 6 in its upper part at a level slightly 
above that the piston reaches at the end of its stroke if the operation is 
normal. The ports 6 ensure that the piston does not rise substantially 
higher than normal in case of accidental overpressure. It must be 
emphasized that these ports 6 are not used for exhausting combustion 
products in normal operation. The exhausting of combustion products is, as 
indicated above, effected through valves 7 disposed in the lower part of 
the cylinder. The opening of the valves is controlled by synchronizing 
means which is designed to take as its starting point for the opening 
substantially the moment the piston is going to descend from the top dead 
center position. The closing of the valves 7 is controlled by the same 
synchronizing means and tripped by the passage of the piston at a point in 
the cylinder previously calculated to be where the volume contained in the 
piston has been suitably scavenged, the compression of residual gases, 
ensured by the kinetic energy of the piston adding to the pressure of the 
combustion-supporting gas and fuel introduced, provides in the actual 
combustion chamber 2 ignition or self-ignition conditions. 
The combustion-supporting gases, the fuel or the explosive material are 
introduced by conventional means in metered amounts at 8 into the lower 
part of the combustion chamber. 
The apparatus is cooled by any suitable conventional means. The cylinder 1 
may, for example, be cooled by a flow of water in the lower part thereof 
which is at the highest temperature and in the upper part thereof only by 
external fins 11 which increases the heat transfer surface with the 
surroundings. As for the cooling of the piston it may be improved, if 
desired, by a fin member on the upper surface of the piston for taking 
advantage of the vertical reciprocation to give off as much heat as 
possible from the body of the piston 4. The fin member 9 is preferably 
conical or helical in shape, is made of good heat transfer material, and 
causes slight rotation of the piston which contributes to uniform wear of 
the cylinder wall. 
Nevertheless it must be noted that the expansion of the combustion gases 
produced by the explosion in the combustion chamber may continue to 
completion in the system, contrary to what occurs in conventional systems, 
and cools the resulting combustion gases naturally so that the temperature 
thereof does not exceed 250.degree. C. 
The piston 4 is equipped with permanent magnets 5 of annular configuration 
embedded in the nonmagnetic body which defines the piston 4. The strength 
of the permanent magnets 5 is calculated so as to ensure with the 
induction coils 3 in the cylinder the fastest braking or deceleration of 
the piston and the generation of direct current having the desired 
strength and voltage. With Diesel type engine operation the mass of the 
piston must be such that the kinetic energy at the end of its return 
stroke ensures in the actual combustion chamber a pressure of 
1.2-2.times.10.sup.6 pascals necessary for self-ignition of the Diesel 
fuel mixture. Such a piston advantageously has two stages as indicated 
above, as is known at the present time for certain types of engines. This 
two-stage configuration ensures good scavenging conditions during the 
descent of the piston. The lower part of the piston of smaller diameter 
penetrates into the actual combustion chamber. After the explosion in the 
combustion (i.e. explosion) chamber the piston moves upwardly, permitting 
the expansion of gases in the larger diameter space which enables use of 
high power permanent magnets and coils. Thus, the height of the cylinder 
is limited to the optimum length from the standpoint of expansion and the 
quality of combustion. The combustion is substantially complete since 
there is nothing to interfere with it in practice, the pollution due to 
any possible unburnt substances is reduced to a minimum. 
The permanent magnets and the nonmagnetic material of the body of the 
piston must be able to resist repeated shocks and impacts and the elevated 
temperature at the beginning of the combustion. The piston may 
advantageously be provided with a high temperature resistant insulating 
lining on its lower end surface in contact with the hot combustion 
chamber. 
When operating with an explosive charge the piston must be lighter, but 
must have very good shock resistance. It may then have a single diameter, 
the explosion or combustion chamber being very small. In such an 
arrangement there is no compression of the gas during the return stroke of 
the piston, but simply the confining of the gas at the time of explosion 
(combustion or detonation). In this modified embodiment using an explosive 
charge, it is important, however, to keep the electric circuit closed so 
that the operation of the coils due to the circuit being closed during the 
downstroke of the piston slows the movement of the piston and generates 
direct current with a sign opposite to that produced during the upstroke 
of the piston. 
Conversely, when the mixture is fuel and a combustion-supporting gas, it is 
advantageous to open the electric circuit as soon as the piston is in its 
top dead center position and close it only at the instant the next 
explosion is produced. 
The apparatus is supported on a suitable base designed to absorb a large 
part of the shocks or impacts produced by the repeated explosions. The 
damping of shocks may advantageously be provided by means of a hydraulic 
shock absorber. 
The coils are connected in accordance with the intended use of the 
apparatus and the desired conversion of the resulting current. 
In operation a suitable air-gasoline, air-heating oil or air-gas mixture is 
ignited by a spark or pressure, or a suitable explosive charge is 
detonated. The piston is driven upwards at a rather high speed, thereafter 
slowed by the generation of direct current. The expanded gases are cooled 
and the piston is at zero velocity at the top of its upstroke. The piston 
then falls, scavenging the resulting combustion gases, and compressing the 
remaining combustion at the end of the downstroke, and gases are injected 
at elevated pressure into the combustion chamber. The injection of the 
fuel at high pressure or the ignition of the explosive charge produced 
rapid combustion and expansion of the gas combustion products. The piston 
then starts its upstroke for another cycle. 
The current generated in the coils 19 may be used to supply a bank of 
conventional accumulators or directly by a dynamo, or, preferably, for 
electrolysis in one of its many forms. 
The current may thus serve in an electrolyzer 20 operating under pressure 
(2-3.times.10.sup.6 pascals) and elevated temperature. The hydrogen 21 and 
oxygen 22 produced are introduced into a turboalternator 23 having a 
turbine 24 and an alternator 25. The detonation of the mixture is avoided 
and at least part of the exhaust gases are fed back into the apparatus. 
The turbo-alternator 23 thus supplied produces a.c. at the desired 
voltage. A small volume storage is sufficient to ensure the smooth 
operation of the turbine 24 and compensate for the fact that the 
generation of d.c. is cyclical. 
The oxygen produced in the electrolyzer 20 under pressure may be used 
directly in the combustion chamber of the generator. This amount of oxygen 
must be supplemented by a substantially greater amount of air at lower 
pressure (about 6.times.10.sup.5 pascals) in the combustion chamber 
slightly before the pure oxygen and the fuel. The process permits the 
volume of the combustion chamber to be reduced and substantially improves 
the overall efficiency even further, by avoiding heating to an elevated 
temperature a quantity of nitrogen which interfers with conventional 
combustion processes. In this arrangement the hydrogen exiting 21 the 
electrolyzer 20 under pressure may be consumed with air in a turbine. At 
the exit of the turbine after expansion and cooling, an amount of water 30 
is recovered necessary for the supply 31 of the electrolyzer. The hydrogen 
thus produced in particularly advantageous conditions may be used in 
hydrogeno chemistry or hydrogeno metallurgy. 
The apparatus of the invention may be used at a stationary location or 
transported (for example on aboard a ship). 
As brought out above, the improved efficiency of this type of generator 
apparatus as compared to efficiencies of about 1000-6000 KWh Diesel 
engines comes from, on the one hand, the near total elimination of 
friction which conventionally uses about 5% of the energy produced by 
engines and on the other hand, the complete use of the expansion of the 
gases which are exhausted at about 240.degree. C., directly improving the 
energy yield. Thus, nearly half of the losses due to cooling the cylinder 
are eliminated. As the customary losses found in engines of this power 
range are about 28%, the improved efficiency in this respect may be an 
increase of about 14 percentage points. 
All told, the efficiency of the actual generator may reach 57%, from which 
the losses inherent in the conversion of the electric current produced 
must be deducted. But such losses are low if the current produced is used 
directly in electrolysis apparatus. They may be only about 3% if an 
electronic synthesizer is used. In a current conversion assembly 
comprising a bank of accumulators and a generator-alternator system, the 
overall efficiency is still about 43%. 
Without these values being a limitation of the present invention, the 
length of the cylinder may be about 5-6 meters and the internal diameter 
500 mm, whereas the lower part of the two-stage piston has a diameter of 
300-400 mm. With a piston having an appropriate mass, the frequency for a 
Diesel generator is 30-40 strokes/minute. The power output is about 
200-1800 KWh. The consumption is about 150 g of heating oil or the 
equivalent thereof for about 1 KWh produced. 
The temperature of the gases at the exist is about 200.degree.-240.degree. 
C. and at least part of these gases may be recovered and conveyed to 
turbo-charger for supercharging in case of a Diesel engine. 
To overcome any defect in feeding fuel and for stopping the operation a 
damping system 12 is provided at the lower part of the cylinder outside 
the combustion chamber. 
The piston is lubricated at the top dead center position by injecting a 
suitable quantity of oil between the piston rings 13.