US3777722A

US3777722A - Free piston engine

Info

Publication number: US3777722A
Application number: US00068698A
Authority: US
Inventors: K Lenger
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-09-11
Filing date: 1970-09-01
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11
Also published as: DE1945924A1

Abstract

A piston engine comprises a cylinder defining a combustion chamber, a ringless piston reciprocally mounted within the cylinder, the cylinder having means spaced from the combustion chamber and defining a zone of reduced clearance around the piston. This zone-defining means includes means for lubricating the zone to reduce friction. The lubricating means may comprise a gas-flow arrangement, or a bushing formed of self-lubricating material, or both of these features. The cylinder and piston are fabricated from materials having a maximum coefficient of thermal expansion of 30 X 10 7/* C. The engine may include a compressor piston reiprocal in a compressor cylinder which are designed for minimal heat expansion.

Description

Ul lite Stats Lenger rear 91 Dec. 11, 1973 FREE PISTON ENGINE [76] Inventor: Karl Werner Lenger, Uhlandstrasse 66, Hamburg, Germany [22] Filed: Sept. 1, 19711 [21] Appl. No.1 68,698

[30] Foreign Application Priority Data Sept. 11, 1969 Germany P 19 45 924.6

[52] US. Cl...... 123/46 R, 123/193 C, 123/193 CP, 92/86.5, 92/127, 92/153, 184/68, 308/5 [51] Int. Cl...... F02b 711/00, F02d 39/10, F02f1/20 [58] Field of Search 123/46 R, 46 SC, 123/46 A, 46 B, 193 CP, 193 R, 193 C;

[56] References Cited UNITED STATES PATENTS 2,426,613 9/1947 Jackson 92/153 2,869,524 1/1959 Spier 123/46 2,878,990 3/1959 Zurcher 92/153 3,314,403 4/1967 Bouvier et al. 123/46 3/1970 Braun 123/46 R 3,538,815 11/1970 Clarke et al. 123/193 R X FOREIGN PATENTS OR APPLICATIONS 1,022,493 12/1952 France 92/153 Primary Examiner-A1 Lawrence Smith Att0rneyAlan E. Kopechi [57] ABSTRACT 1 A piston engine comprises a cylinder defining a combustion chamber, a ringless piston reciprocally mounted within the cylinder, the cylinder having means spaced from the combustion chamber and defining a zone of reduced clearance around the piston. This zone-defining means includes means for lubricating the zone to reduce friction. The lubricating means may comprise a gas-flow arrangement, or a bushing formed of self-lubricating material, or both of these features. The cylinder and piston are fabricated from materials having a maximum coefficient of thermal expansion of 30 X 10 C. The engine may include a compressor piston reiprocal in a compressor cylinder which are designed for minimal heat expansion.

23 Claims, 7 Drawing Figures PAIENIEnncc I I max 3777; 722

sum 2 or 3 FREE PISTON ENGINE BRIEF SUMMARY OF THE INVENTION At the present time the maximum pressures developed in a free piston engine are in the range of 1,700 to 2,100 pounds per square inch. Obviously such high combustion pressures result in high temperatures. For this reason some cooling is applied to cylinders and pistons, which usually are provided with appropriate piston rings, the cylinders being cooled from outside and the pistons .internally. This in effect makes the construction of such a free piston engine compartively heavy and complicated with a consequent lowering of the cyclic piston frequency and a decrease in developed engine power, as these are interrelated. Any effort to increase the engine power by increase of cylinder pressure leads to higher stresses, particularly in the piston rings.

The piston rings, being relatively frail engine parts, are usually the first to be affected by high thermal and mechanical stress. An increased wear of pistons and cylinders is another immediate result. There were efforts made to use pistons with no piston rings in internal combustion engines. The pistons and cylinders were made of the usual standard materials and the pistons were guided by a special guide on the piston rods. As the piston and cylinder materials had relatively high and dissimilar thermal expansion coefficients it is obvious that during design this must have been taken into account, creating at least in the cold condiiton, some considerable clearance between pistons and cylinders. An undesirable high loss from the slipping-by of gases was a result. Another problem is a possible high burnout rate of pistons and cylinders caused by a large amount of hot gas slipping with high speed past the gap between pistons and cylinders.

Even with the useof like, or similar, materials for the piston and the cylinder the big clearence can not be avoided because in operation the pistons are much hotter than the cylinders. I

The aim of this invention is to create a free piston engine, of the type mentioned above, but without the above problems.

DETAILED DESCRIPTION The subject of this invention is a free piston engine of the type described above, in which at least the power piston and the power cylinder are made from material with a low thermal expansion coefficient, and the clearence beyond the combustion chamber and the gas ports is smaller than in the combustion chamber itself. The pistons have no piston ring.

According to another preferred design, the compressor cylinder and the cylinder guided compressor piston are also made from material with low thermal expansion' characteristics.

Preferably the material used for the construction of the pistons and cylinders should have, at maximum, a small thermal expansion coefficient of 30 X l C. Suchmaterial can be quarzite glass-or some ceramic material or a nickel or chromium nickel steel such as lnvar Steel or Elinvar Steel.

The coefficient of thermal expansion of those steels is somewhat higher than of the ceramics. Generally ceramic materials are preferable, as the use of ceramics results in a lighter construction and this allows a higher piston stroke frequency with a related increase in engine power output. The ceramic materials have a lower heat transfer coefficient with the advantage of lower heat losses to the outside of the engine.

I A useful ceramic material is represented by Pyroceram with a'thermal expansion coefficient of 10 X l0"'/ C., which can be operated up to l,200 C.

The application of materials with very low thermal heat expansion, as proposed above, makes it possible to keep the clearance'between the piston and cylinder very small and thus largely avoiding the gas blow-by and virtually eliminating the afore-mentioned burn-out damage.

When the power is made from a ceramic material it will be of advantage to build around it a shrunk-un metal shell, which will take the high pressure force and prevent damage or destruction of the ceramic.

A guiding device for the power piston, made prefera bly from a self-lubricating material with maximum thermal expansion coefficient of 40 X l0 C., is provided outside of the combustion chamber area. Graphite or artificial carbon preferably may be used, with embed ded ceramic grains to enhance the wear properties, if necessary. The guiding device can be designed as a cylindrical bearing and located on the cold" end of the power cylinders. It can also be, according to the invention, built-up on the inner wall of the cylinder by the flame spraying process.

According to one preferred design of the invention, some gas under pressure is introduced into thepower piston guiding device, which acts as a gaslubricating bearing. If such a gas bearing is used it is desirable that the bearing area of the power piston guiding device he subdivided into smaller sections by an arrangement of longitudinal and circumferential ribs. Each of the sections is provided by one or more pressure gas inlet openings andone or more pressure gas vent openings. To maintain the uniform gas pressure around the guide, which tends to hold the power piston in a radially central position, it is important that the gas vent openings are large enough to vent the gas quickly. In the case of some momentary displacement of the piston towards the cylinder wall, the gas with inadequate venting may cause a local buildup of pressure, which would result in an unbalance of the pressure in the guide area and prevent the reestablishment of the power piston into the central position. In other words, if owing to sideward displacement of the piston, the gas is allowed to penetrate behind the sections and ribs on one side of the guide area, due to the lack of proper venting it will act on a bigger area than on the other side where it acts within sections only, thus creating an unbalance of the force, pressing the piston against the cylinder wall and preventing a frictionless operation. To prevent this from happening, the gas which crept behind the ribs must be removed as fast as possible.

According to anotherpreferred design of this invention, the compressor piston can also be made without piston'rings, guided centrally by a bearing bush, fabricated from a self-lubricating or a lubricant impregnated material, running on a central pin. The power piston and the compressor piston should preferably be made as separate parts and the power piston mounted on a boss of the compressor piston with proper clearance.

' The design of a free piston engine, according to this invention, allows us to use comparatively high pressures without a danger of damage to the principal parts. Also a higher stroke frequency of the piston can be used with consequent higher power output. Application of ceramic materials to the construction of pistons and cylinders results in weight reduction with a further possibility of higher piston frequency and subsequent higher power output.

The invention will be further explained by the following drawings:

FIG. 1 is a partial section, showing the part of the free piston Engine, which is important for the explanation of the invention.

FIG. 2 is a partial section showing the piston guide device, according to the invention.

FIG. 3 is an alternative design of the piston guide device, as described in the invention.

FIG. 4 is a partial section of another alternative design of the piston guide device, as described in the invention.

FIG. 5 is a partial section of another variation in design of the piston guide device as claimed in the invention.

FIG. 6 8L 7 are two partial sections in which the use of the gas bearing, as described in the invention, is explained.

FIG. 1 shows, in a schematic way, all the parts of a free piston engine, working on the two cycle uniflow principle, which are required to explain the invention. Free piston engines of this type are well known.

According to the drawing, power 1 is shown in a power cylinder 2 in which it can move in longitudinal direction. The power cylinder 2 is provided with the usual scavenging ports 3, through which the exhaust gases and the scavenging air leave the cylinder. These exhaust gases, by way of an exhaust-duct 4, enter the gas storage tank (not shown). The back or cold end of the power piston 1, is mechanically connected to the compressor piston 5, which can move back and forth in the cylinder 6. The movement of the compressor piston 5, towards the left, as in FIG. 1, expels air from the cylinder. This air had entered the cylinder in the previous piston 5 stroke, i.e., toward the right, through the generally circular inlet valves 7. By the way of an air outlet valve, 8, this air is exhausted into an air storage tank, 9, from which, with appropriate timing it enters the cylinder. 2. The compressor piston, 5, operates the compression stroke in a known way under the force of an air cushion. As may be viewed in FIG. 1, this air cushion is formed by the air, trapped on the right side of the piston, 5, in cylinder, 6.

The power piston, l, and the power cylinder, 2, in the proposed design are made from a material with a very low thermal expansion, the thermal expansion coeficient being of the maximum order of 30 X l0 C., preferably from a ceramic material with thermal expansion coeff. of the order of IO lO' C. and which can be operated in temperatures up to 1,200 C. The power piston, 1, and the power cylinder, 2, can also be made of a nickel steel or chrom-nickel steel such as Invar or Elinvar. Those steels have a somewhat higher thermal expansion than quarzite glass or ceramic material such as Pyroceram.

To prevent damage or destruction of the ceramic cylinder material by the high cylinder pressures, a cylindrical reinforcement shell made of metal or steel is shrunken over the ceramic. This shell, 20, takes the cylinder pressure stresses and prevents damage to the ceramic material. The power piston, 1, is guided in the area beyond the scavenging ports, 3, in a special piston guiding device, 10. This guiding device, according to FIG. 1. is in the form of a bearing bush made from a self-lubricating material with a maximum thermal expansion coefficient of X 10/ C., such as carbon, graphite or artificial carbon, which may have some ceramic particles included to enhance the wear resistance. To create better sliding conditions, and to diminis h friction, a device, 11, is added by which a gas under pressure is delivered to the piston guiding device in the direction of the power piston. This gas under pressure may come as air from a seperate source or may be taken from the gas delivered by the engine.

As is visible in FIG. 1 there is, on the wall of the power cylinder, 2, at the point where the guide device, 10, starts, a small step. This step is located in such a way that the clearence between the power piston, 1, and the cylinder wall in the area, 12, just in front of guiding device, 10, is slightly larger than in the area, 13, inside the guiding device, 10. The clearance in the area, 13, of the guiding device, 10, is large enough so that any direct contact between the power piston, 1, and the guiding device, 10, is largely avoided. The clearance between the power piston, 1, and the cylinder wall in the area 12, is somewhat larger than in the area 13. This clearance in the area 12 is large enough so that if the power piston, l, is deflected within limits the of its clearance in the guiding device, it would not come in contact with the cylinder wall of the combustion chamber, and yet small enough such that the loss of pressure through this gap is still within permissible limits. This step is also intended to assure the presence of uniform gas pressure all around the circumference of the piston, so that in case the piston is however slightly deflected into an askew position, the pressure would not act on one side of the piston only, which would result in a contact between the power piston and the cylinder wall. In an absence of such a step it would be impossible to avoid this contact.

The FIGS. 2 8L 3 show two alternative designs of the piston guide device namely 10a or 10b.

According to FIG. 2 the piston guide device 10a is in the form of a bearing bush made from a self-lubricating material, such as graphite, artificial carbon or similar material, and is located on the cold end of the power cylinder, 2. The bearing face of the bush, 10a, is as shown more distinctly in FIG. 6 & 7, is subdivided into a number seperate sections by means of longitudinal ribs, 14, and circumferential ribs, 15. Each section is provided with a gas inlet opening, 16, and each section is also provided with gas outlet openings, 17. The gas outlet openings are located in the space between adjacent sections, for example, in circular grooves. The flow of gas is schematically indicated by arrows in FIG. 6. It is important that the gas supplied through the inlet openings, 16, can be vented fast enough through the gas outlet openings, 17. For this reason, and also to lower slightly the gas pressure and increase the volume of outgoing gas, the gas vent openings should be made larger than the gas inlet opeings, 16. It is also important that no more gas is supplied than can be easily removed.

An inadequate size of vent openings, 17, may result in some serious trouble, in particular when the power piston, 1, under the influence of some outside forces is deflected slightly from the central position to an askew one. In such a case the clearence of the longitudinal and circ'umferetial ribs on one part of the piston will be greater, than on the other side. This in effect will cause. an inbalance of forces and if the gas cannot be vented fast enough, the force deflecting the piston from the center will be permanently greater than the force trying to reinstate the piston in the central position.

In general the piston, 1, slides largely frictionless on a layer of bearing gas in power cylinder, 2. In case of failure of the bearing gas pressure or as a resut of short acting factors only, piston 1 starts gliding on the selflubricating surface of the guide device, 10. In this way the wear of the piston, 1, of cylinder2, can be almost entirely eliminated.

The pressure of the gas supplied through the inlet openings, 16, should be kept slightly higher then the pressure of the scavenging air. The pressure difference depends on the design of the piston guide device. In general it was found that the higher the scavenging air pressure the higher the difference of the gas bearing pressure, is required.

The. design as shown in FIG. 3 is operationally the same as in FIG. 2 and in FIG. 6 & 7. The arrangement in FIG. 3 is different in that there the self-lubricating material, preferably with a maximum thermal expansion coefficient of 40 X l0' C. is deposited on the cylinder wall by the flame spraying process. The arrangement in FIG. 4 differs from FIG. 2 in that the step in front of the piston guide device, 10, as described before, is replaced partially by a somewhat longer guide bearing bush, 10c, extending to the inside, and in part by a shoulder formed on the outside face of the piston, l. The clearance between the power piston, l, and the wall of the cylinder, 2, is also slightly larger in area 12, in front of the piston guide device, 10, than in area 13, within the piston guide.

FIG. 5 shows a simplified design of the piston guide device which now consists of a simple cylindrical bearing bush made from a self-lubricating material, as specified before. No bearing gas is used with this design.

In some casesit ispossibleto build the piston guide device from a material with no special self-lubricating characteristics. Such a design would be similar to the one shown in FIG. 3, no self-lubricating material is deposited on the cylinder wall, however the gas bearing device should be incorporated. The compressor piston, 5, moving in the cylinder, 6, is guided on a centrally located guide pin, 18, with an interposed cylindrical bush 1') made from some self-lubricating or lubricant impregnated material. If a self-lubricating material is used it can be the same material as the power piston guide bearing bushing, 10. If an oil absorbant material is used, it may be a sintered metal with graphite lubrication, or an oil impregnated metal, or an oil impregnated ceramic. To keep the thermal expansion coefficient low, nickel-graphite or chrome-nickel graphite with interspersed Invar or Elinvar particles may be used. Special bearing bushes from similar materials can be made.

The compressor piston, 5 on the side towards the power piston, l, is provided with a boss, 21, that matches a properly designed end face, 22, of the power piston. As shown, the end, 22, of the power piston fits over the boss, 21, on the compressor piston, 5, with a radial clearance, so that the ringless compressor piston can find a radially independent position. A mechanical connection of piston land piston 5 is'effected by an interlock of the piston end, 22, with the face of compressor piston 5.

The coupling between the power piston, 1,-and compressor piston 5, is achieved by spring23, (preferably of a helical type), connected on one end 'to the power piston, l, and on the other end to the compressor cylinder, 6. The spring, 23, extends through the hollow guide pin, 18, through compressor piston 5. The guide pin, 18, is made in one solid piece with the compressor cylinder, 6, or it is connected with it permanently, in some other way. For limiting the travel of the compressor piston, 5, to the right, as per FIG. 1, a limiting ring, 24, is provided in the compressor cylinder, 6, as shown in FIG. 1.

If some additional cooling of the engine is required water can be injected in the combustion chamber in the standard way, seperately or combined with fuel. The equipment for such a water injection system is well known so that further explanation is not necessary. The free piston engine, built according to these plans can be operated with twicethe piston stroke frequency as other existing free piston engines. Thus the use of this engine permits a much higher power output to be obtained from an engine of the same size. Through the use of this invention it is possible to build free piston engines with a high specific power output, which makes them applicable to motor traction purposes, as well as stationary and ship propulsion engines.

What is claimed is:

1. A free piston engine comprising:

power cylinder means including internal wall means,

a combustion chamber and scavenging port means in said internal wall means;

a ringless power piston disposed for reciprocal movement in said power cylinder means;

compressor cylinder means;

a compressor piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said ringless power piston;

said power cylinder means having guide means spaced axially from the combustion chamber and extending radially inwardly of said wall means to define a zone of reduced clearance around said ringless power piston;

,said guide means beingrigidly mounted relative to said wall means and arranged co-axially with said power piston;

said guide means having means including opening means communicating with said zone, for supplying said zone with a flow of lubricating gas around said power piston to inhibit the occurrence of friction between said ringless power piston and said power cylinder means.

2. An engine according to claim 1 wherein the power piston and the power cylinder means are comprised of a material having a maximum coefficient of thermal expansion of 30 X l0 C..

3. An engine according to claim 2 wherein the compressor piston and the compressor cylinder means are comprised of a material having a maximum coefficient of thermal expansion of 30 X lO C..

4. An engine according to claim 1 wherein-the power piston, the power cylinder means, the compressor piston, and the compressor cylinder means are comprised of a ceramic material having a coefficient of thermal expansion of approximately 10 X l0' C..

5. An engine according to claim 4 and further including a metal shell disposed around an outer periphery of said power cylinder means for reinforcing said power cylinder means. I

6. An engine according to claim 1 wherein the power piston, the power cylinder means, the compressor pis-' ton, and the compressor cylinder means are comprised of lnvar steel.

7. An engine according to claim 1 and further including a limiting means at one end of said compressor cylinder means to limit longitudinal travel of said power piston and said compressor piston.

8. An engine according to claim 1 wherein a section of said power piston disposed between said zone of reduced clearance and said combustion chamber is of larger diameter than the section disposed in such zone.

9. A free piston engine comprising:

power cylinder means including a combustion chamber and scavenging port means;

compressor cylinder means;

a compressor piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said power piston;

said power cylinder means having means spaced from the combustion chamber and defining a zone of reduced clearance around said power piston; said zone-defining means including;

a plurality of longitudinally spaced, circumferential ribs for defining a plurality of sections;

inlet and outlet passage means communicating with said sections to conduct a flow of gas along said sections; and

means for supplying a flow of gas to said zone at a pressure higher than the pressure of scavenging air in the combustion zone to inhibit the occurrence of friction between said power piston and said power cylinder means.

10. An engine according to claim 9 wherein said inlet passage means are smaller in size than said outlet passage means.

ll. A piston engine comprising:

a cylinder defining a combustion chamber and including internal wall means; a ringless piston disposed forreciprocal movement in said cylinder; said cylinder having guide means spaced axially from the combustion chamber and extending radially inwardly of said wall means to define a zone of reduced clearance around said ringless piston; said guide means being rigidly mounted relative to said wall means and arranged co-axially with said piston;

said guide means having means, including opening means communicating with said zone, for supplying said zone with a flow of lubricating gas around said piston to inhibit the occurrence of friction between said ringless piston and said cylinder.

12. An engine according to claim 11. wherein said zone-defining means is comprised of a self-lubricating carbon material having a maximum coefficient of thermal expansion on the order of 40 X lO'/ C..

13. An engine according to claim 11 wherein said cylinder and said piston are comprised of material having a maximum coefficient thermal expansion of 30 X l"/ C.

14. A free piston engine comprising:

power cylinder including internal wall means, a combustion chamber and scavenging port means, in said internal wall means;

compressor cylinder means;

said guide means being rigidly mounted relative to said wall means and arranged co-axially with said power piston;

said guide means including means for lubricating said zone to inhibit the occurrence of friction between said power piston and said power cylinder; and the power piston and the power cylinder means being comprised of material having a maximum coefficient of thermal expansion of 30 X 1O C.

15. An engine according to claim 14 wherein said guide means is comprised of a self-lubricating material having a maximum coefficient of thermal expansion of 40 X l0 C.; said guide means being disposed on the cold end of the power cylinder means beyond the scavenging ports.

16. An engine according to claim 15 wherein said self-lubricating material comprises carbon having ceramic particles added thereto for resisting wear.

17. An engine according to claim 15 wherein said self-lubricating material comprises a flame-sprayed coating on an innerwall of the power cylinder means.

18. A free piston engine comprising:

power cylinder means including a combustion chamber and scavenging port means;

compressor" cylinder means;

a compressor cylinder piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said power piston;

said power cylinder means having means spaced from the combustion chamber and defining a zone of reduced clearance around said power piston; said zone-defining means including means for lubricating said zone to inhibit the occurrence of friction between said power piston and said power cylinder means;

said compressor cylinder means includes a pin extending substantially co-axially with said power piston; said compressor piston being provided with a cylindrical bushing mounting said compressor piston for reciprocal movement on said pin; said lastnamed bushing being comprised of self-lubricating material;

said power piston including a hollow portion; and

' a spring being fastened at one end to the power piston and at the other end being fastened to the compressor cylinder means to maintain the power piston and the compressor piston in contact during starting of the engine.

is hollow; and said spring comprises a helical spring passing through the pin.

23. An engine according to claim 22 wherein said power piston is provided with a bore; said compressor piston being provided with a boss disposable with clearance within said bore such that said compressor piston is radially movable relative to said power piston to radially independent positions.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 7'77, 722 I Dated December 11, 1973 Inventor(s) Thaddeus G. Csaky and Karl Werner Lenger 7 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN THE TITLE PAGE In the listing of Inventors, before the name of Karl Werner Lenger, add the name and address of co-inventor Thaddeus G. Csaky as follows: THADDEUS G. CSAKY, 15 H1111 Ave. nnapolisQMd. 21408 e-.

Signed and sealed this 16th day of April l97L (SEAL) Attest:

EDWARD PLFLETGHERJ'HQ G. MARSHALL DANN Attesting Officer Commissioner of Patents Few PO-105O (10-69)

Claims

1. A free piston engine comprising: power cylinder means including internal wall means, a combustion chamber and scavenging port means in said internal wall means; a ringless power piston disposed for reciprocal movement in said power cylinder means; compressor cylinder means; a compressor piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said ringless power piston; said power cylinder means having guide means spaced axially from the combustion chamber and extending radially inwardly of said wall means to define a zone of reduced clearance around said ringless power piston; said guide means being rigidly mounted relative to said wall means and arranged co-axially with said power piston; said guide means having means including opening means communicating with said zone, for supplying said zone with a flow of lubricating gas around said power piston to inhibit the occurrence of friction between said ringless power piston and said power cylinder means.

2. An engine according to claim 1 wherein the power piston and the power cylinder means are comprised of a material having a maximum coefficient of thermal expansion of 30 X 10 7/* C..

3. An engine according to claim 2 wherein the compressor piston and the compressor cylinder means are comprised of a material having a maximum coefficient of thermal expansion of 30 X 10 7/* C..

4. An engine according to claim 1 wherein the power piston, the power cylinder means, the compressor piston, and the compressor cylinder means are comprised of a ceramic material having a coefficient of thermal expansion of approximately 10 X 10 7/* C..

5. An engine according to claim 4 and further including a metal shell disposed around an outer periphery of said power cylinder means for reinforcing said power cylinder means.

6. An engine according to claim 1 wherein the power piston, the power cylinder means, the compressor piston, and the compressor cylinder means are comprised of Invar steel.

9. A free piston engine comprising: power cylinder means including a combustion chamber and scavenging port means; a ringless power piston disposed for reciprocal movement in said power cylinder means; compressor cylinder means; a compressor piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said power piston; said power cylinder means having means spaced from the combustion chamber and defining a zone of reduced clearance around said power piston; said zone-defining means including; a plurality of longitudinally spaced, circumferential ribs for defining a plurality of sections; inlet and outlet passage means communicating with said sections to conduct a flow of gas along said sections; and means for supplying a flow of gas to said zone at a pressure higher than the pressure of scavenging air in the combustion zone to inhibit the occurrence of friction between said power piston and said power cylinder means.

10. An engine according to claim 9 wherein said inlet passage means are smaller in size than said outlet passage means. 11. A piston engine comprising: a cylinder defining a combustion chamber and including internal wall means; a ringless piston disposed for reciprocal movement in said cylinder; said cylinder having guide means spaced axially from the combustion chamber and extending radially inwardly of said wall means to define a zone of reduced clearance around said ringless piston; said guide means being rigidly mounted relative to said wall means and arranged co-axially with said piston; said guide means having means, including opening means communicating with said zone, for supplying said zone with a flow of lubricating gas around said piston to inhibit the occurrence of friction between said ringless piston and said cylinder.

12. An engine according to claim 11 wherein said zone-defining means is comprised of a self-lubricating carbon material having a maximum coefficient of thermal expansion on the order of 40 X 10 7/* C..

13. An engine according to claim 11 wherein said cylinder and said piston are comprised of material having a maximum coefficient thermal expansion of 30 X 10 7/* C.

14. A free piston engine comprising: power cylinder including internal wall means, a combustion chamber and scavenging port means, in said internal wall means; a ringless power piston disposed for reciprocal movement in said power cylinder means; compressor cylinder means; a compressor piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said ringless power piston; said power cylinder means having guide means spaced axially from the combustion chamber and extending radially inwardly of said wall means to define a zone of reduced clearance around said ringless power piston; said guide means being rigidly mounted relative to said wall means and arranged co-axially with said power piston; said guide means including means for lubricating said zone to inhibit the occurrence of friction between said power piston and said power cylinder; and the power piston and the power cylinder means being comprised of material having a maximum coefficient of thermal expansion of 30 X 10 7/* C.

15. An engine according to claim 14 wherein said guide means is comprised of a self-lubricating material having a maximum coefficient of thermal expansion of 40 X 10 7/* C.; said guide means being disposed on the cold end of the power cylinder means beyond the scavenging ports.

18. A free piston engine comprising: power cylinder means including a combustion chamber and scavenging port means; a ringless power piston disposed for reciprocal movement in said power cylinder means; compressor cylinder means; a compressor cylinder piston disposed for reciprocal movement in said compressor cylinder means and being operably connected to said power piston; said power cylinder means having means spaced from the combustion chamber and defining a zone of reduced clearance around said power piston; said zone-defining means including means for lubricating said zone to inhibit the occurrence of friction between said power piston and said power cylinder means; said compressor cylinder means includes a pin extending substantially co-axially with said power piston; said compressor piston being provided with a cylindrical bushing mounting said compressor piston for reciprocal movement on said pin; said last-named bushing being comprised of self-lubricating material; said power piston including a hollow portion; and a spring being fastened at one end to the power piston and at the other end being fastened to the compressor cylinder means to maintain the power piston and the compressor piston in contact during starting of the engine.

19. An engine according to claim 18 wherein said bushing on said compressor piston is comprised of an oil-impregnated sintered metal.

20. An engine according to claim 18 wherein said bushing on said compressor piston is comprised of an oil-impregnated ceramic material.

21. An engine according to claim 18 wherein said bushing on said compressor piston is comprised of an artificial graphite material.

22. An engine according to claim 18 wherein said pin is hollow; and said spring comprises a helical spring passing through the pin.