US2869524A

US2869524A - Free piston engine

Info

Publication number: US2869524A
Application number: US702024A
Authority: US
Inventors: Hans G Spier
Original assignee: Baldwin Lima Hamilton Corp
Current assignee: Baldwin Lima Hamilton Corp
Priority date: 1957-12-11
Filing date: 1957-12-11
Publication date: 1959-01-20
Anticipated expiration: 1976-01-20

Description

Jan. 20, H G SPIER FREE PISTON ENGINE 2 Sheets-Sheet 1 Filed Deo. ll, 1957 ATTORNEYS Jam 20, 1959 H. G. SPIER 2,869,524

EREE PIsToN ENGINE 2 Sheets-Sheet 2 Filed Dec. ll, 1957 POWER GAS D\SCHARGE INVENTOR.

BY @MEQW ArroRA/Efs United tates Patent i FREE PHSTUN ENGINE Hans G. Spier, Hamilton, hio, assigner to Baldwin- Linla-Hamiiton Corpnration, Hamilton, Ohio, a corporation of Pennsylvania Application December 11, 1957, Serial No. 702,024

2 Claims. (Cl. 12S-46) This invention relates to free piston engines and has for its primary object to provide a free piston engine having greater efficiency than known engines.

It is known that the heat rejection to the coolants and by radiation in normally cooled free piston power gas generators amounts to about `17 percent in the power cylinder and about 7.5 percent in the power pistons. It has been found that a specific fuel consumption of about 0.350 poundsvper gas horsepower per hour or a thermal eflieiency of 38 percent can be practically obtained with these levels of heat rejection, the remaining losses thus amounting to about 37.5 percent. Assuming that these internal power and heat losses of 37.5 percent are unavoidable in practice, the present invention proposes to increase the v`eiiiciency of a free piston power gas generator by eliminating or greatly reducing the power cylinder and piston heat losses. It will be apparent that if these losses, amounting to 24.5 percent of the heat in the cycle,

could be Vcompletely eliminated, the thermal efciency of the cycle would rise to 62.5 percent, and the specific fuel consumption would drop to 0.22 lbs./ G. H. P./hr.

In accordance with the present invention, the power pistons and the power cylinder are each substantially uncooled. These portions then operate at a mean ternperature approximately equal to or somewhat higher than that of the gas discharged by the machine. Since piston ring contact or any other contact of the hot pistons with the equally hot cylinder is impossible, the present invention provides means to guide the pistons internally and maintains a small, free running clearance between the power pistons and the power cylinder in which they reciprocate.

The invention provides a free piston power gas generator having numerous advantages over known units, which advantages may be briefly enumerated as follows: (l) higher gas pressures are permitted if desired, (2) higher thermal efficiencies are attainable, (3) the service life of Van engine is extended by the elimination of piston rings, (4) oil consumption is reduced, (5) firing pressures are increased for increased output and efficiency, (6) accessory requirements are reduced, (7) the engine is less expensive to construct initially, and (8) the friction losses are reduced.

Other objects and advantages will become apparent from the following description of a preferred embodiment of the invention, reference being had to the accompany ing drawings, in which- 4Figure 1 is a diagrammatic ,side elevation, with parts broken away, of a free piston power gas generator incorporating the present invention;

Figs. 2, 3, and 4 are enlarged sectional views of a portion of a piston crown seal; and

Fig. 5 is a somewhat diagrammatic central vertical sectional view of one end of the machine shown in Fig. l.

The drawings show a free piston power gas generator embodying the present invention. The machine is essentially symmetrical about a vertical center line passing through the injector nozzle as is common in the art so Fei-tentati Jan. 20, 1959 that the more detailed figures show only one end. As is customary, the engine comprises a central power cylinder portion 1li and, on each end, large compressor cylinders 12 and 14. The spaces at the outer end of the compressor cylinders, in the configuration shown, are used as

bounce spaces

16 and 18 so that initial compression ofl the scavenging and charging air takes place on the inward stroke of the pistons.

The power section of the pistons working in the power cylinders is designated 20 in each instance, and each of the powe'r sections is connected by a trunk section 22 to a larger compressor piston 24 working in its respective oneV of the compressor cylinders. As is customary, each larger section of the piston compresses air in the bounce space behind it during the outward stroke and thus stores up the energy needed for the next succeeding inward stroke. Also on the outward stroke, a charge of fresh air is drawn into the compressor cylinders through intake valves 30; on the inward stroke of each compressor piston this charge of air is discharged through valves 32 to a scavenging air receiver or space 345. in the form of the gas generator shown in the drawings the scavenging air space 34 communicates with the bounce spaces 16 and 1S, but this is not necessarily true in more conventional machines.

As is also customary, the intake and discharge ports for the power cylinder are opened and closed by the power pistons. The intake or scavenging ports are in the form of Va peripheral series designated 36, at one end of the power cylinder iii, and discharge ports 38 are in a similar series at the opposite end. The spacing of thesev ports with respect to the center line of the machine is such that the discharge ports open somewhat ahead of the intake ports, and remain open for the passage of scavenging air through the power cylinder. The discharge passage for power gas is designated i0 in the drawings and may be connected to an appropriate turbine or other gas consumer in a manner well known in the art.

Fuel is injected into the power cylinder by a conventional injector 42. The fuel pump and other conventional accessories, as well as the synchronizing means fcr the pistons have been omitted from the drawings in the interests of clarity.

in the usual free piston power gas generator the power piston carries a number of conventional piston rings, and the interior of the power cylinder is lubricated. Since the function of the machine is to suppiy gas at elevated temperatures and pressures, the operating temperature of the cycle is kept as high as possible commensurate with satisfactory operation. The high temperatures burn the lubricating oil from the cylinder walls with the result that piston ring failure is the most common form of breakdown of a free piston machine.

The present invention supports and guides the pistons on inwardly extending tubular members Sii which are a part of the frame structure of the machine and are bolted or otherwise ixed to the end walls or frame members. These tubular guide members are s-o proportioned and designed that a negligible distortion occurs at operating temperatures. 52 and 54 of carbon or similar material capable of withstanding high temperatures and reciprocating with a mini mum of lubrication ride on the guide members Si) and'- Spaced shoe or bearing rings cient volume to cool that portion of the piston between the shoe rings 52 and 54 if desired, being drained from the interior of the tubular guide member 50 in a manner well known in the art. For this purpose the rings 52 and 54 are drilled axially.

The piston being supported and guided on the tubular member 50, there need be no contact whatever between the piston and the cylinder in which it operates. Thus piston rings are dispensed with and no lubrication of the cylinder walls is used. In the crown or forward portion of the power piston 20, the interior walls are insulated as at 62 where in other machines this portion is cooled either by air or by circulating oil. Any oil circulated into the piston through the ports 60 in the present machine thus performs no cooling function so far as the piston crown is concerned. The exterior portion of the crown is spaced from the cylinder wall by a circumferential clearance, and can operate at the mean temperature of the combustion cycle.

g The circumferential clearance around the power piston crown allows for a certain amount of leakage of gas from the high pressure part of the power cylinder cycle directly to the turbine inlet pressure level without supplying its expansion energy to the compressor piston. This energy loss is partly regained in the resulting temperature rise in the gas llow to the turbine, and it will be noted that none of the gas leaves the ow pattern of the machine. Whatever leakage occurs from the power cylinder in the direction of the scavenging ports 36 merely increases the scavenging pressure available when these ports are next opened.

It has been found that excellent sealing of the power cylinder, or low leakage, can be obtained if the exterior of the power piston is made in the form of an elongated labyrinth seal. That is, if the exterior of the piston is formed with a series of circumferential grooves 65 of the forms shown in Figs. 2, 3 and 4. Of these groove forms, the preferred one appears at present to be that shown in Fig. 3. In this form the leakage velocity past the crest of the groove is high and the gas expands into the base 66 or larger volume of the groove with some turbulence. In cross section the groove slopes gently away from its crest in the direction of gas ow, but the rear wall is either undercut as in Fig. 3 or radial as in Fig. 4 so that gases expanding into the groove having passed the crest are baffled and must make a substantial change in direction before flowing out of one groove into the next. The rear wall of each groove thus makes an angle of at least 90 with respect to the axis of the piston.

This passage and expansion of the gas is repeated from groove to groove in the series and the specific leakage of gas during the high pressure part of the cycle is low. It has been found that the loss of energy in the cycle due to leakage is much less than the gain in energy due to reduction in cooling losses so that there is a net gain in eiciency over a conventional cooled unit. Tests indicate that in a typical small machine a running clearance of .020 all around the power piston results in leakage losses of approximately the same value as the eliminated cooling losses, so that any lesser clearances give rise to an increase in thermal eticiency over a cooled machine with piston rings. The seal area formed by the grooves must at least equal the length of stroke of the pistons. Thus if the machine operates with a nominal l inch stroke, the front 10 inches of the power piston at least should be formed with the labyrinth seal grooves.

It is, of course, understood that in a free piston machine the stroke length varies from part load operation to full load operation.

The power cylinder is insulated as at 67 and the exhaust port header or manifold is also insulated as at 68.

In some instances it may be desirable to eliminate piston rings from the compressor piston also. This can be done since the compressor piston (being integral with the power piston) is supported by the central tubular guide member 50. A seal ring 69 of carbon or the like can be supported on the periphery of the compressor piston and serve to isolate the compressor cylinder from the bounce cylinder. A labyrinth seal on the periphery of the compressor piston may alsobe used.

The shank or trunk portion 22 of the piston which connects the power or crown portion 20 with the compressor piston 24 can be cooled if desired to reduce the quantity of conducted heat into the compressor piston.

If cooled, for example by oil owing through the dlstributor ring 56, this portion should be of substantially smaller diameter than the hot crown or power portion to reduce heat transferby radiation from the hot cylinder wall during the inner portion of the stroke.

It will thus be seen that the present invention provides a free piston power gas generator in which lubrication is confined to only a small exterior surface of a single piston supporting and guide member, and the power piston and its cylinder operate dry and at the mean temperature of the combustion cycle. If satisfactory dry carbon or other bearings are developed even the lubrication of the guide 50 may be dispensed with. Thus the quantity of lubricating oil used is greatly reduced, and the adverse effects of carbonized or deteriorated oil entirely eliminated.

While the invention has been disclosed in connection with a particular form and disposition of the parts, it should be expressly understood that it is capable of numerous modifications and changes without departing from the scope of the appended claims.

What I claim is:

l. In a free piston power gas generator, the combination of a frame, aligned power, compressor and bounce cylinders carried by said frame, connected power and compressor pistons working in said cylinders, said power piston having a series of peripheral sealing grooves extending from its tiring end, said peripheral sealing grooves increasing in cross-sectional area in the direction of gas leakage and having rear walls disposed at least from the axis of the piston, a body of insulation disposed within said power piston radially opposite to said peripheral sealing grooves, and means carried by said frame to support at least the grooved portion of said power piston entirely out of contact with its respective cylinder.

2. The combination of elements defined in claim l in which the rear wall of each of said peripheral sealing grooves is undercut with respect to the crest of the next succeeding groove.

References Cited in the tile of this patent UNITED STATES PATENTS 2,645,213 Huber July 14, l953 2,671,606 Ricardo Mar. 9, 1954 FOREIGN PATENTS 605,191 Great Britain July 19, 1948