US2558444A

US2558444A - Pressurized breather for fref-piston units

Info

Publication number: US2558444A
Application number: US753943A
Authority: US
Inventors: Kunz Warren
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1947-06-11
Filing date: 1947-06-11
Publication date: 1951-06-26
Anticipated expiration: 1968-06-26

Description

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June 26, 1951 w. KUNZ 2,558,444

PRESSURIZED BREA'I'HER FOR FREE'PISTON UNITS Filed June 11, 1947 4 Sheets-Sheet 4 INVENTQR BYtXtZZZEIZKLQZ.

ATTORNEY Patented June 26, 1951 PRESSURIZED BREATHER FOR FREE-PISTON UNITS Warren Kunz, Glastonbury, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application June 11, 1947, Serial No. 753,943 12 Claims. (01. 60-13) This invention relates to free-piston units and particularly to an arrangement for controlling the length of the piston stroke to compensate for changes in the compressor pressures.

In adapting a free-piston unit to use in aircraft, it has been found that the unit, if designed for sea level operation, has an excessively long piston stroke at high altitude by reason of the decreasing ambient pressure which would necessarily reduce the mass flow of air through the unit, if the piston stroke were to remain constant. A feature of this invention is an arrangement for compensating for the changes in compressor inlet pressure by maintaining a pressure on the inner sides of the compressor pistons and adjusting this pressure in response to changes in compressor inlet pressures.

Another feature of this invention is an arrangement for adjusting the length of the piston stroke for the purpose of maintaining a long enough stroke to assure the uncovering of the scavenge and exhaust ports on each stroke of the piston assemblies. In accordance with this feature the length of the stroke is changed by varying the pressurization of the breather, that is the amount of pressure on the inner surface of the compressor pistons.

Where the breather is pressurized, the problems of starting the unit are increased, since with pressure on the inner surfaces of the compressor pistons the starting unit which functions for moving the pistons rapidly together on the compression stroke for the purpose of causing compression ignition of the fuel in the engine cylinder must do a greater amount of work dependent upon the pressure on the inner surfaces of the compressor pistons and the length of the piston stroke required for effectively starting the unit.

A feature of this invention is to unload the inner. surfaces of the compressor pistons by connecting the breather space to the compressor inlet during starting of the unit. A further feature of the invention is the automatic control of the unloading device so that the breather is opened automatically to the compressor inlet in the operation of the starting system.

Other objects and advantages will be apparent from the specification and claims, and from the accompanying drawings which illustrate an embodiment of the invention.

Fig. 1 is a sectional view through the freepiston unit showing the pressurized breather and the starting valves.

Fig. 2 is a sectional view at a different angle through the unit to show the restraining linkage.

Fig. 3 is a diagrammatic view showing the operation of the starting system.

Fig. 4 is an enlarged view, in section, of the pressure valve.

With reference to Fig. l, the unit shown includes an engine cylinder 10 having reciprocating pistons 12 and It to which compressor pistons l6 and I8 in

cylinders

20 and 22 are integrally connected. Sleeves 24 and 26 attached to the compressor piston complete the reciprocating piston assemblies. The sleeves in combination with

stationary pistons

28 and 30 form air spring cylinders which serve to move the piston assemblies toward each other on the engine compression stroke.

The piston assemblies are moved apart by burning fuel injected into the engine cylinder by a fuel nozzle 32 operated by a fuel pump 34. Air compressed in the air spring cylinders on the power stroke returns the piston assemblies. The assemblies are always maintained at equal distances from the center of the engine cylinder by a linkage which includes racks 36 extending from the compressor pistons and engaging a centrally located pinion 38.

Intake manifold 40 which may surround the

compressor cylinders

20 and 22 conducts air to intake valves 42 at the outer ends of the compressor cylinders. The compressed air leaves the cylinders through discharge valves 44 also at the outer ends of the compressor cylinders and pass through scavenge manifold .46 to ports ll in the engine cylinder which are uncovered by the piston l2 adjacent the end of the power stroke. When these ports are uncovered, air is blown through the engine cylinder and through exhaust ports 50 which are uncovered by the piston H into the exhaust duct 5|.

As shown in Fig. l, the innerends of the

compressor cylinders

20 and 22 are interconnected by a passage 52 to form the pressurized breather space. Pressure is maintained in this space by means of a control valve 54, the function of which is to adjust the pressure in such a manner as to maintain the desired length of stroke for the piston assemblies under normal operating conditions.

As shown in Fig. 4, valve 54 includes a plunger 56 sliding in a bore 58 and normally closing a port 60 communicating with the breather space by apassage .62 and conduit 63. Bore 58 has another port 64 communicating by a passage 8 to a conduit 68 from the scavenge manifold 46.

Plunger 66 is moved in a direction to connect

ports

60 and 64 to admit air from the scavenge manifold to the breather by a spring 10 acting on the stem 12 of the plunger, and by peak breather pressure in a chamber 14 acting on a diaphragm 16 also connected to the stem 12.

As shown, breather pressure in passage 62 acts on a check valve 18 in a connecting passage 80 between passage, 62 and chamber 14. This check valve provides for the maintaining in chamber 14 of a pressure close to the peak pressure in the breather. A small vent passage 82 bypasses the check valve for slowly bleeding oi the pressure in the chamber between successive compressor strokes.

In addition to the breather pressure acting on diaphragm 16, scavenge pressure is admitted by a passage 84 to a chamber 86 on the other side of the diaphragm. The stem 12 also has mounted thereon another smaller'diaphragm 81 forming another wall of chamber 86. On the other side of diaphragm 81 is atmospheric pressure in a chamber 88 vented as at 90, one end of plunger 56 being in this chamber. As the pressure in the breather increases the pressure in chamber 14 increases and moves plunger 56 downward to vent port 60 allowing a drop in pressure in the breather. Similarly a reduction in pressure in the breather causes a drop in pressure in chamber 14 thus moving plunger 56 upwardly to connect

ports

60 and 64 for admitting scavenge air to the breather.

The breather control may be made responsive to changes in altitude. To this end, stem 12 of the plunger is connected to a lever arm 92, one end of which engages with a sealed bellows -94 which may be located in a vented chamber 96. The bellows may be acted upon by a spring 98 shown within the bellows. As the atmospheric pressure decreases, bellows 94 expands, thus moving plunger 56 downward to decrease the pressure in the breather.

In addition to controlling the pressure in the breather during the operation of the unit, air under pressure may also be delievered to the breather space when the unit is not operating for the purpose of cranking the pistons into starting position. To this end, as shown in Fig. 3, air under pressure in a conduit I from the startin air supply is delivered through a conduit I02 to the starting devices I04, and through a conduit I06 which connects with conduit 68. A pressurereducing valve I08 is provided in the conduit I06 to reduce the pressure delivered through conduit I06 to the port 60 and thence to the breather space.

The reducing valve I08 which may be of any suitable construction comprises, in the arrangement shown, a diaphragm IIO, one side of which is vented as at II2 to the atmosphere and the other side of which is exposed to the pressure in the conduit I06. The diaphragm is connected to a valve element II4 which is adapted to close a passage II6 for the air entering the conduit I06 and a spring II8 acting on the diaphragm normally tends to hold the valve element II4 open.

' As pressure builds up in the conduit I06, the pressure on the diaphragm overcomes the action of the spring H8 and more or less closes the valve element II4, thereby maintaining a limit on the pressure in the conduit I06. A solenoid valve I I8 in a control circuit opens the conduit I06 to admit air to the breather during the starting operation, as will be pointed out, for the purpose of moving the pistons into the outer end positions shown.

After the pistons have been cranked into the starting position of Fig. 1, they are moved rapidly together by the starting devices I04. As shown, each of these devices, which may be located within the

air spring pistons

28 and 30, includes a chamber I20 communicating with the air spring (the space I22) by a poppet valve I24. This valve is connected to a piston I26 by a piston rod I28 and is normally held closed by a spring I30 acting on the piston. The cylinder I32 in which the piston I26 is slidable communicates with the chamber I20 so that when air under pressure is delivered to the chamber I20, the valve I24 will remain closed. As shown, air under pressure reaches the chamber I20 through the conduit I02 and a branch conduit I34 interconnects the opposed starting devices at opposite ends of the unit so that both devices will be supplied with starting air simultaneously.

After a predetermined pressure has been built up in the chambers I20 of the opposed starting devices, they are both vented simultaneously by delivering air under pressure through a conduit I36 connected with the starting air supply and communicating with the outer ends of the cylinders I32 in which the pistons I26 are positioned. By the application of air under pressure to these pistons, the valves I24 are opened rapidly and air from the chambers moves the pistons rapidly together.

For unloading the breather during the operation of the starting devices, after the pistons have been moved into starting position, the unit is provided with one or more unloading valves I38, each of which, as best shown in Fig. 1, includes a movable valve element I40 adapted to close ports I42 in the wall of the breather space 52. Each valve element I40 is connected by a rod I44 to a piston I46 in a cylinder I48. The valve is normally urged into an open position by a spring I50 acting on w the piston.

Air under pressure entering the outer end of each cylinder I48 acts to move the valve I40 into 7 closed position against the action of the spring I50. The outer ends of the cylinders I48 may be connected by conduits I52 to the conduit 68 which, as above stated. communicates with the conduit I06 from the starting air supply. Thus, while pressure is being supplied through the conduit I06 to the space 62 for cranking the pistons to the outer ends of the strokes in readiness for starting, air pressure is simultaneously delivered to the cylinders I48 to hold the valves I40 closed. A check valve I54 may be provided in the conduit 68 to prevent the flow of air under pressure from the conduit I06 into andthrough the conduit 68, thereby making it possible to build up the necessary pressure for cranking in the space 52. A bleed passage I55 may bypass check valve I54.

To make the starting system entirely automatic, the operation of the cranking and starting mechanism may be electrically controlled. In the first place the fuel supply for the free-piston unit is provided by starting the motor I56 which drives the fuel supply pump I58. This is accomplished by closing a manually operated switch I60 on a control panel I62. This completes a control circuit through leads I64 to a relay I66 which, when I I20 enters the air springs and to a relay I88 which operates a switch I82. This switch controls a power circuit including a normally closed switch I84 and a solenoid I88 which controls the valve II 9 in the conduit I88. Thus. when the switch I82 is closed, the solenoid I83 is energized to cause cranking air to enter the breather space and, acting on compressor pistons I8 and I8, to move the piston assemblies into the positions of Fig. 1. As the pistons reach the outer ends of their stroke in readiness for starting, a

cam I98 on the pinion 38, Fig. 2, closes a switch I92 which completes a circuit I94 through a relay I98 that controls the switch I94. The relay I98 opens the switch I84 and breaks the circuit through the solenoid I88 thereby cutting oil any further air supply to the breather space and, by reducing the pressure in cylinders I48, venting the breather space.

The circuit I94 also includes the solenoid I98 that controls a valve 288 in the conduit I82 which supplies starting air under pressure to the starting devices. Thus, when the solenoid I98 is energized, starting air is delivered to the starting devices to build up the desired pressure in the chambers I28.

The circuit through the solenoid I88 is controlled by a switch 282 actuated by a relay 284. This relay and another relay 288 are controlled by the contactbar 288 of a pressure switch 289 which is moved in response to pressure in a bellows 2 connected to the conduit I82. The bar 288 normally completes a circuit through the solenoid I98. As the pressure is built up in conduit I82, this circuit through solenoid I98 is broken and a circuit 2III through the relay 284 is completed to open the switch 282. The contact bar also completes another circuit through the relay 288 which operates a switch 2 I4 to complete a circuit through a solenoid 2I8 which operates a valve 2I8 in the conduit I38 to admit air under pressure to the cylinders I82 for opening valves I24. Thus, when the pressure in the chambers I28 has reached the predetermined value, the air in these chambers is released by the operation of the valve 2I8, thereby causing the pistons to be moved rapidly toward each other on the start ing stroke. When the switch 2 is moved to close the circuit through the solenoid 2I8, it breaks a circuit through a solenoid 228 which controls a normally open valve 222 on a vent conduit 224 from the conduit I98. The starting system having functioned, the push button "8 is released and the parts are restored to the positions shown in readiness for restarting the unit.

In operation the free-piston unit while running has the breather space 52sealed by the valves I48 so that this space acts as an inverted air cushion. The maximum pressure in this breather space is adjusted as a function of the scavenge pressure by the action of the diaphragm 18 which is exposed to scavenge air pressure in the chamber 88. Thus, as scavenge pressure increases, the plunger 55 is moved upward, Fig. 3, to admit scavenge air through the conduit. 88 to the breather space.

When the pistons approach the inner ends of their strokes, the pressure in the breather is sufficient to cause the check valve I8 to open and admit pressure to the chamber I4 so that the pressure of the scavenge air in chamber 88 is balanced by the breather air pressure in chamber I4 and the spring I8. In addition to adjusting the breather pressure as a function of scavenge pressure, it may also be adjusted as a function of atmospheric pressure through the function of the sealed bellows 94 which expands as the atmospheric pressure decreases to move the plunger 58 downward and vent the breather space through the passage 83.

Assuming now that the unit has been stopped and is to be started, the switch I88 is closed to start the fuel supply pump I58 thereby assuring the necessary fuel for the injection pump 34. The Push button "8 is then held down to cause the starting operation. The first step in the operation is the opening of the solenoid valve I I29 which admits air to the cylinders I48 for holding the vent valves I48 closed and also admits starting air through the conduit 88 and past plunger 58 to the breather space 52. It willbe apparent that as the high starting air pressure reaches conduit 88, it also enters chamber 88 thereby moving the plunger 58 upward, Fig. 4, to interconnect

ports

88 and 84. Pressure in the breather will cause outward movement of the pistons into the starting position of Figs. 1 and 2. When the pistons reach starting position, the cam I98 closes switch I92 which operates the solenoid valve 288 for charging the starting devices I84. This same switch operates the relay switch I84 to cause the solenoid valve I I9 to be closed and permit the air pressure in the breather space to vent through the bypass I55 into the scavenge system.

When the pressure in the starting devices reaches a predetermined value, the contact bar 288 of the pressure switch completes a circuit which opens the solenoid valve I98 thereby admitting air under pressure to the outer ends of the cylinders I32 thereby opening the valves I24 to admit air under high pressure to enter the air springs and move the pistons rapidly together. Obviously when the cylinders I48 have been vented by closing of the valve I I9, the vent valves I48 are opened by the springs I58 so that when the starting devices operate for moving the pistons rapidly together, the breather space is connected with the inlet manifold so that there is no 'pressure acting on the inner side of the compressor pistons. When the unit is operating, the push button "8 is released thereby restoring the starting system to its original position in readiness for a subsequent starting operation.

The electrical control mechanism by which the starting operation is obtained is an invention of Meitzler and is claimed in a copending application, Serial No. 753,945 filed June 11, 1947.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described, but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. A free-piston unit including an engine cylinder and piston, a compressor cylinder and piston, said pistons being connected together to form a piston assembly, and an air spring including cylinder and piston parts, one part being connected to the engine-and-compressor piston assembly, said air spring causing movement of the piston assembly on the engine compression stroke, said compressor cylinder having inlet and discharge valves controlling the admission and discharge of gas to one end of the compressor cylinder, the other end of the compressor cylinder being closed to form a chamber, means including a pressure source and a valve connecting said source to said chamber for maintaining a predetermined pressure in said chamber at the other end of the compressor cylinder, and means for shifting said valve for adjusting the pressure in said chamber in response to changes in the discharge pressure from the compressor.

2. A free-piston unit including an engine cylinder and piston, a compressor cylinder and piston, said pistons being connected together to form a piston assembly, and an air spring including cylinder and piston parts, one part being connected to the engine-and-compressor piston assembly, said air spring causing movement of the piston assembly on the engine compression stroke, said compressor cylinder having inlet and discharge valves controlling the admission and discharge of gas to one end of the compressor cylinder, the other end of the compressor cylinder being closed to form a.chamber, means in cluding a pressure source and a valve connecting said source to said chamber for maintaining a predetermined pressure in said chamber at the other end of the compressor cylinder, and means for shifting said valve for adjusting the pressure in said chamber in response to changes in atmospheric pressure.

3. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders located at opposite ends of the engine cylinder and in which the compressor pistons reciprocate, each engine piston and its associated compressor piston forming a piston assembly, opposed air spring acting on the piston assemblies for moving them toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, a fluid interconnection between the other ends of the compressor cylinders, a normally open valve adjacent to and communicating directly with said other ends of the compressor cylinder for venting said other ends of the compressor cylinders and means for closing said valve when the unit is operating.

4. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders located at opposite ends of the engine cylinder and in which the compressor pistons reciprocate, each engine piston and its associate compressor piston forming a piston as.- sembly, opposed air springs acting on the piston assemblies for moving them toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the fiow of gas into and out of one end of each compressor cylinder, means for starting the unit by admission of air under pressure into the air springs, and means for venting the other ends of the compressor cylinders during the operation of the starting means.

5. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, said cylinder having inlet and exhaust ports, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders at opposite ends of the engine cylinder in which the compressor pistons reciprocate, each engine piston with the attached compressor piston forming a piston assembly, opposed air springs each including cylinder and piston parts, one part of which is connected to each piston assembly and the other part of which is connected to the opposed compressor cylinders, said air springs operating to move the piston assemblies toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, means for interconnecting the other ends of the compressor cylinders to form a sealed chamber constituting a reverse air spring, starting means for the unit, and meansfor venting said chamber during starting or the unit including at least one large size valve closing a port located directly in a wall of said chamber and so constructed as to prevent a pressure rise in said other ends when said valve is open and the unit is operating.

6. A free-piston unitincluding opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, said cylinder having inlet and exhaust ports, opposed compressor pistons connected to the e ne pistons, opposed compressor cylinders at opposite ends of the engine cylinder in which the compressor pistons reciprocate, each engine piston with the attached compressor piston forming a piston assembly, opposed air springs each including cylinder and piston parts, one part of which is connected to each piston assembly and the other part of which is connected to the opposed compressor cylinders, said air springs operating to move the piston assemblies toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, means for interconnecting the other ends of the compressor cylinders to form a sealed chamber constituting a reverse air spring, means for venting said chamber including at least one normally open valve for closing a port in said chamber, and pressure means responsive to the discharge pressure of the compressor cylinder for moving said valve into closed position.

7. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, said cylinder having inlet and exhaust ports, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders at opposite ends of the engine cylinder in which the compressor pistons reciprocate, each engine piston with the attached compressor piston forming a piston assembly, opposed air springs each including cylinder and piston parts, one part of which is connected to each piston assembly and the other part of which is connected to the opposed compressor cylinders, said air springs operating to move the piston assemblies toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, means for interconnecting the other ends of the compressor cylinders to form a sealed chamber constituting a reverse air spring, means for venting said chamber including at least one normally open valve for closing a port in said chamber, pressure responsive means for holding said valve closed, a duct in which pressure is built up as the unit operates, and a connection from said duct to said pressure responsive means for holding said valve closed while the unit is operating.

8. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, said cylinder having inlet and exhaust ports, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders at opposite ends of the engine cylinder in which the compressor pistons reciprocate, each engine piston with the attached compressor piston forming a piston assembly, opposed air springs each including cylinder and piston parts, one part of which is connected to each piston assembly and the other part of which is connected to the opposed compressor cylinders, said air springs operating to move the piston assemblies toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, means for interconnecting the other ends of the compressor cylinders to form a sealed chamber constituting a reverse air spring, means for venting said chamber including at least one valve closing a port in said chamber, and pressure responsive means for holding said valve closed, a compressed air duct into which the gas from the compressors discharges, and a connection from said duct to said pressure responsive means for closing said valve in response to pressure in said duct.

9. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, said cylinder having inlet and exhaust ports, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders at opposite ends of the engine cylinder in which the compressor pistons reciprocate, each engine piston with the attached compressor piston forming a piston assembly, opposed air springs each including cylinder and piston parts, one part of which is connected to each piston assembly and the other part of which is connected to the opposed compressor cylinders, said air springs operating to move the piston assemblies toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, means for interconnecting the other ends of the compressor cylinders to form a sealed chamber constituting a reverse air spring, means for venting said chamber including at least one valve closing a port in said chamber, pressure responsive means for holding said valve closed, a scavenge duct into which the gas from the compressors discharges, said scavenge duct being connected to the inlet ports of the engine, and a connection from said duct to said pressure responsive means for closing said valve in response to pressure in said duct.

10. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders located at opposite ends of the engine cylinder and in which the compressor pistons reciprocate, each engine piston and its associate compressor piston forming a piston assembly, opposed air springs acting on the piston assemblies for moving them toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, a scavenge duct connecting the discharge of the compressors to the engine inlet ports, a source of pressure and means for maintaining a predetermined pressure in the other ends of the compressor cy1- inders, said means including a valve for alternately connecting said other ends of the compressor cylinders to the source of pressure or to a vent, and means responsive to changes in the pressure in said scavenge duct for moving said valve.

11. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders located at opposite ends of the engine cylinder and in which the compressor pistons reciprocate, each engine piston and its associate compressor piston forming a piston assembly, opposed air springs acting on the piston assemblies for moving them toward each other on the engine compression stroke, eachcompressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, a scavenge duct connecting the discharge of the compressors: to the engine inlet ports, a source of pressure, means for maintaining a predetermined pressure in the other ends of the compressor cylinders, said means including a valve for alternately connecting said other ends of the compressor cylinders to the source of pressure or to a vent, means responsive to changes in the pressure in said scavenge duct for moving said valve, and other means responsive to changes in atmospheric pressure for moving said valve.

12. A free-piston unit including opposed engine pistons, an engine cylinder in which the engine pistons reciprocate, opposed compressor pistons connected to the engine pistons, opposed compressor cylinders located at opposite ends of the engine cylinder and in which the compressor pistons reciprocate, each engine piston and its associate compressor piston forming a piston assembly, opposed air springs acting on the piston assemblies for moving them toward each other on the engine compression stroke, each compressor cylinder having inlet and discharge valves controlling the flow of gas into and out of one end of each compressor cylinder, a scavenge duct connecting the discharge of the compressors to the engine inlet ports, a source of pressure, and means for maintaining a predetermined pressure in the other ends or the compressor cylinders, said means including a valve for alternately connecting said other ends of the compressor cylinders to the source of pressure or to a vent, and means responsive to changes in pressure in the scavenge duct and to changes in pressure in said other ends of the compressor cylinder for moving said valve.

WARREN KUNZ.

REFERENCES CITED The following references are of record in the tile of this patent:

' UNITED STATES PATENTS Number Name Datev ,434,280 Morain Jan. 13, 1948