Free piston engine compressor

Jan. 21, 1936. h. jAnicke 2,028,331

FREE PISTON ENGINE COMPRESSOR Filed May 15, 1934 2 Sheets-Sheet 1

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Jan. 21, 1936.

H. JANICKE

2,028,331

FREE PISTON ENGINE COMPRESSOR

Filed May 15, 1934

Sheets-Sheet 2

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inventor:

Patented Jan. 21, 1936

2,028,331

UNITED STA ES PATENT OFFICE

2,028,331

FREE PISTON ENGINE COMPRESSOR

Hermann Janicke, Dessau, Germany, assignor to Hugo Junkers, Bayrisch-Zell, Oberbayern, Germany

Application May 15, 1934, Serial No. 725,748 In Germany May 20, 1933

Claims. (Cl. 230—56)

My invention relates to free piston engines in which a free piston or a pair of such pistons is or are arranged in a cylinder for reciprocation, and more particularly to that kind of free piston engines which comprise a multistage compressor. In the hitherto known engines of this latter kind the compression chambers of the single stages are arranged in a row in alignment with the motor cylinder of the engine. This construction requires a considerable overall length because each stage increases the length of the engine by an amount corresponding to the maximum length of stroke of the free pistons. This drawback is particularly felt in free piston engines composed of two symmetrically designed halves, in which a pair of free pistons is provided which reciprocate simultaneously in opposite directions. For instance in a symmetrically constructed free piston engine comprising on each side a threestage compressor and a coupling gear connected to cross-members for coupling the free pistons of the engine with each other, the overall length amounts to the tenfold stroke of the pistons, because each of the six compressor pistons, the two motor pistons and the two crossmembers requires a portion of the total length corresponding to the maximum stroke.

It is an object of my invention to improve engines of the kind referred to in such manner that the overall length of the engine is considerably reduced. This is accomplished by arranging at least some of the cylinders of the multistage compressor in such manner that one cylinder is arranged within another one so that the overall length of the multistage compressor does not substantially exceed the length of a single-stage' compressor operating with a like stroke. Preferably the dimensions are chosen in such manner that in the inner dead centre position the piston of the outer stage closely approaches the outer end of the motor cylinder. Since the diameters of the cylinders and of the pistons operating in the inner cylinders are relatively small, the inner cylinders should be the high pressure cylinders, the outer ones the low pressure cylinders of the compressor. In a preferred embodiment of my invention the diameters of the inner cylinder piston are so small that the inner cylinder can be arranged in the hollow piston rod connecting the motor piston with the piston of the outer compressor cylinder. In this case the piston of the high pressure cylinder may be stationary, the hollow piston rod forming a reciprocating cylinder of the inner stage. If desired, however, the diameters of the high pressure cylinder may be chosen so small that a stationary cylinder may be provided for the high pressure stage and may be arranged within the hollow piston rod. . 5

In engines according to my invention comprising free pistons reciprocating in opposite directions of pistons may be coupled with each other by means of a well known coupling gear, which may be arranged within the outer low pressure cylinder, so that it may be directly connected to the pistons of the low pressure cylinder or to a member connected thereto. By arranging the coupling gear in this manner the dimensions of the engine are still further reduced. 15

In reducing these constructions to practice it becomes necessary to suitably choose the volume and the pressure ratios in the stages. As a rule in a two-stage compressor the total pressure ratio, i. e. the ratio between the discharge pres- 20 sure of the second stage and the suction pressure of the first stage is distributed uniformly over the stages, and similarly the volume ratios, 1. e. the ratio between the volume of the dead space and the stroke volume are made equal in both 25 stages, so that the volumetric efficiency is the same in both stages also. Now, obviously, in an engine according to my invention the pressure ratios and the volume ratios cannot be chosen in this manner, because, in order to attain this, 30 the diameter of the second stage must be made so large that the second stage cannot be arranged within the hollow piston rod, the outer diameter of which must not exceed the diameter of the motor piston. However, the diameter of 35 the motor piston cannot be chosen at will, because it is determined by the energy to be fed to the compressor cylinders during each stroke, by the stroke itself and by the average specific pressure in the motor cylinder. In consequence 40 thereof in an engine according to my invention the pressure ratios or the volume ratios or both are made to differ from each other in the single stages. For instance, in order to sufficiently reduce the diameter of the second stage, the volume 45 ratio of this stage is reduced as far as possible by reducing the dead space. In consequence thereof the volumetric efficiency is increased and the required stroke volume is decreased so that the diameter is small at a given stroke. The 50 stroke volume of the second stage can be reduced further by providing for a relatively large pressure ratio in the first stage, the gas in the first stage being compressed to a relatively high pressure and a correspondingly small volume during 65

2,028,331 every stroke at which it is delivered to the second stage.

The energy required for returning the pistons from their outer to their inner dead centre posi5 tions and for compressing the charge in the motor cylinder (hereinafter called feed-back energy), is delivered in free piston engines substantially by the expansion of the gas bodies remaining over in the dead spaces of the com10 pressor cylinders. Now, if the sect nd stage of the compressor is constructed with a low volume ratio as above described, this stage can feed only a small amount of feed-back energy, and there-¹fore the amount to be fed by the first stage must 15 be correspondingly greater. In consequence thereof the dead space and the volume ratio of the first , stage must be relatively large. This requires a very large stroke volume in the first stage In order to suck in the required amount of 20 gas during every stroke. In order to obtain a large stroke volume at a predetermined length of stroke the diameter of the piston of the first stage must be sufficiently large. This is advantageous, since the arrangement of the coupling 25 gear within the cylinder of the low pressure stage is facilitated thereby. In some cases, however, in order to arrange the coupling gear within the cylinder of the low pressure stage, it is necessary to increase the diameter of this cylinder 30 in excess of the value required for obtaining a large volume ratio, so that the efficient stroke volume and the energy demand of the compressor would be increased to an undesirable extent. In order to overcome this difficulty, I 35 reduce the efficient length of the piston stroke by discharging from this cylinder, during the initial portion of the working stroke, a portion of the sucked gas. If the compressor operates with air, the discharged air may be used as 40 scavenging air in the motor cylinder.

It should be understood that my invention may be used as well in connection with symmetrically designed free piston engines as with non-symmetrical engines comprising a multi45 stage compressor.

In the drawings affixed to this specification and forming part thereof some diagrams designed to illustrate the operation of an engine according to my invention and an embodiment 50 of such an engine are shown diagrammatically by way of example.

In the drawings:

Figs. 1 to 3 are pressure volume diagrams, in which the abscissae designate the volume v and 55 the ordinates the pressure p of the gas.

Fig. 4 is an axial section of a symmetrically designed free piston engine comprising two motor pistons.

Referring first to Figs. 1 to 3, the influence of 60 the pressure ratios and the volume ratios on the sucked-in gas volume and the amount of feedback energy shall be explained. Fig. 1 shows the diagram of an ordinary compressor stage. During the working stroke, beginning at the point 65 B, the gas is compressed along the curve B—C from the pressure pi to the pressure p₂. During the last portion of the working stroke the compressed gas is discharged, this portion being indicated by the horizontal line C—E. The energy 70 fed to the compressor stage during the working stroke is rendered in the diagram by the area of the surface A—B—c—E—F—A. At the end of the working stroke a body of gas is enclosed in the dead space having the volume vt at the 75 pressure pi, in which is stored a portion of the energy fed to the compressor during the working stroke. During the initial portion of the back stroke this gas body is expanded to the initial pressure pi along the curve E—G. Thereafter a new gas body is sucked in, this portion of the 5 operating cycle being indicated by the horizontal line G—B. The energy fed to the free pistons during the back stroke (the so-called feed-back energy) is rendered in the diagram by the area of the surface A—B—G—E—F—A. ₁₀

The influence of a variation of the volume ratio to the feed-back energy may be guessed from Fig. 2. The volume ratio is expressed by the formula _t 15 vt+vh in which vt is the volume of the dead space and tin the stroke volume. If the dead space is increased to the value ut+«'t and the volume ratio is correspondingly changed to ²⁰^L yr+v'r

Vt+v't+vh the diagram is changed to the operating cycle BC'EG'B. In consequence thereof the energy fed ²⁵to the compressor during the working stroke is diminished by a value indicated in Fig. 2 by the area of the surface B—C—C'—B, while the feedback energy is increased by an amount corresponding to the area of the surface E—G—G'_E. ³⁰

The length of the suction stroke is decreased to G'—B, which may be compensated for by increasing the diameter of the piston.

Referring now to Fig. 3, the influence of a variation of the pressure ratio shall be discussed. If ³⁶the pressure ratio is increased from

Pi Pi the volume ratio ₄₀ vt ντ+ν_αbeing left unaltered, the operating cycle is changed to B—C”—E—G. Hereby the discharging stroke is diminished from CE to C”E, ⁴⁵so that the stroke volume of the next following compressor stage may be correspondingly smaller. Since, however, the length of the stroke has a predetermined value, the diameter of the cylinder of the next following stage can be reduced. ⁵⁰The above shows that by suitably choosing the pressure ratios and the volume ratios, the riimensions, viz. the stroke volume and the diameter of a cylinder, may be varied to a considerable ex- -_rtent, so that one or a plurality of high pressure cylinders may be arranged within the hollow piston rod connecting the motor piston to the piston of a low pressure cylinder. Besides this the diameter of the low pressure cylinder may be _eomade so large that the coupling gear can be arranged within the low pressure cylinder and directly connected to the free pistons, for instance of the low pressure cylinders. It can further be seen from the diagrams that by suitably choos- ₆₅ing the pressure ratios and the volume ratios the feed-back energy delivered by the single stages may be varied and chosen in correspondence with the required amount.

In Fig. 4 I is a motor cylinder formed with 70 scavenging ports 4 and exhaust ports 5. 2 and 3 are free pistons arranged for reciprocation in opposite directions within the cylinder I, the pistons being shown in Fig. 4 in their outer dead centre position. The ports 4 and 5 are controlled 75

2,028,331 by the pistons 2 and l, respectively. Each piston is connected with a two-stage compressor by means of a hollow piston rod β having, in the embodiment shdwn in Fig. 4, the same diameter 5 as the pistons. Each piston rod 6 is connected to the piston 1 of the associated low pressure compressor cylinder, each piston 1 being arranged for reciprocation In a cylinder 8. The cylinders 8 are made integral with a cylindrical 10 casing enclosing the motor cylinder I. The lengths of the cylinder I and of the piston rods 6 are so chosen that in their inner dead centre positions the pistons Ί closely approach the ends of the motor cylinder I. Each cylinder 8 18 is closed at its outer end by means of an end wall 10 provided with suction valves 14 through which fresh air is sucked into the cylinders 8 during the return strokes of the pistons. The high pressure stage of each compressor comprises a 20 cylinder 11 rigidly secured to the associated end wall 10 and a piston 12 connected to the associated motor piston by means of a piston rod 13. If desired, the piston rods 13 may be connected to the pistons 2, 3 and 12 by means of links or the 25 like, in order to avoid jamming of the pistons.

The diameters of the cylinders 11 and of the pistons 12 are smaller than the inner diameters of the hollow piston rods 6, so that each high pressure stage may be arranged within its associated 30 low pressure stage and the cylinders 11 protrude into the hollow piston rods 6, the overall length of the engine being not substantially greater than in an .engine comprising single-stage compressors. It should be understood that the pres35 sure ratios and the volume ratios in the compressor stages are to be chosen in the manner discussed above.

are exhaust valves arranged in the walls of the cylinders 8 near the end walls 10 through 40 which the air compressed in the low pressure stages is discharged into an intermediate chamber 18, from which the air is sucked into the cylinders 11 of the high pressure stages through suction valves 15 provided in the mid portion of 45 the end walls 10. The intermediate chamber 10 may be designed as a cooler for the air compressed in the low pressure cylinder. 17 are exhaust valves arranged in the mid portion of the end walls 10 through which the air compressed in 50 the high pressure cylinders is discharged into pipes 19 leading the compressed air to a consumer (not shown). 20 are spring-loaded non-return valves for adjusting the pressures in the engine independently of the pressures outside of the en55 gine, so that the required operating pressures are always maintained in the engine.

In each half of the engine the motor piston and the compressor pistons connected with each other form a freely movable mass operating as CO a unit. These units are coupled with each other by means of a coupling gear comprising toothed racks 21,22 and a pinion 23 in engagement therewith. The racks are connected to the pistons 7, the pinion 23 being rotatably mounted in the C5 casing of the engine. The diameters of the cylinders 8 and of the casing, with which they are integral, are made so large that the coupling gear and its connections to the pistons 7 can easily be arranged within the cylinders and the casing. 70 To this end the diameters of the cylinders 8 are greater than would be necessary in order to obtain the required feeding output of the lower pressure stages. In order to hold the output of the low pressure stages at the required value, 75 exhaust valves 24 are suitably arranged in the walls of the cylinders 8, so that during the Initial portion of the working stroke of the pistons air is discharged from the cylinders 8, until the pistons 7 pass the outlet valves 24. In the embodiment shown in Fig. 4, which is operated with 5 air, the air discharged from the left hand cylinder 8 during the initial portion of the operation is used as scavenging air by connecting the exhaust valves 24 by means of the connection 25 to the scavenging air chamber 26 surrounding the io motor cylinder I and connected to its interior by means of the scavenging port 4. It should be understood that the right end cylinder 8 may, if desired, be connected to the chamber 26 in a similar manner. 15

The scavenging pump of the engine is formed by the space enclosed between the left piston 7 and the left end wall 27 of chamber 26. 28 are suction valves arranged in the circumferential wall of the pump space and 29 are pressure valves 20 arranged in the wall 27, so that air is sucked through valves 28 during the working stroke of the pistons and is pumped into the chamber 26 through valves 29 during the return stroke of the pistons. 25

In order to provide a load balance between the racks connected to the same piston, for instance between the racks 21, a rigid connection between the racks and the associated piston is provided merely at the right-hand piston shown in Fig. 4. 30 In the left-hand compressor, however, the racks 21 are connected to the associated piston 7 by means of an annular cross-member 34 shaped as a spherical segment and provided with spherical surfaces 30 and 31, which abut against corre- 35 spondingly shaped counter-surfaces provided on the piston 7 and a counter member 32 bolted to the piston 7 by means of screw bolts 33. The cross-member 34 is provided with suitably dimensioned bores 35, through which extend the 40 screw bolts 33, so that the cross-member 34 is movable with respect to the piston 7 to the extent of securing a load balance between the racks 21 pivoted to the cross-member at 36. The racks 22 are rigidly secured to the associated piston 7, 45 because otherwise the coupling gear could not operate. 37 is a fuel inlet formed in the wall of the cylinder I.

The operation of the engine shown in Fig. 4 is substantially the same as that of known free 50 piston engines. Fuel mixture is periodically fed into cylinder i and burnt therein so that the motor pistons 2 and 3 execute working strokes, during which the air pre ent in the compression chambers is compressed and partly delivered to 55 the consumers. The pistons are returned to their inner dead centre positions by the energy stored in the gas or air bodies remaining over in the dead spaces of the compression chambers, scavenging being effected in the manner described eo above. It should however be understood that the engine according to the invention does not require as large a space as other free piston engines comprising multistage compressors and that more particularly the overall length of the engine is C5 considerably reduced.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described for obvious modifications will occur to a person skilled in the art. 70