EP1455090A1

EP1455090A1 - Variable displacement compressor

Info

Publication number: EP1455090A1
Application number: EP03100541A
Authority: EP
Inventors: Giorgio Paolillo
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2003-03-05
Filing date: 2003-03-05
Publication date: 2004-09-08

Abstract

Variable displacement compressor (10) comprising a crankcase, housing a tilted cam plate (26) connected to a rotary shaft (20); at least one piston (30) associated to the tilted cam plate (26), the tilted cam plate (26) transferring rotary movement of the rotary shaft (20) to reciprocating movement of the piston (30), the piston (30) being arranged in a cylinder bore (28), wherein, in operation, the piston (30) draws refrigerant at suction pressure into the cylinder bore (28) and compresses the refrigerant to a discharge pressure. The compressor (10) further comprises a control valve (52) for changing the crankcase pressure with respect to the suction pressure so as to change the angle of tilt of the cam plate (26); and a control unit (60) for operating the control valve (52). According to an important aspect of the invention, the control unit (60) comprises means (66, 68) for determining a pressure differential between a crankcase pressure and a suction pressure; and means for operating a control valve (52) based on the determined pressure differential.

Description

Field of the invention

The present invention generally relates to a variable displacement compressor, in particular for use in vehicle air-conditioners.

Background of the invention

Variable displacement compressors generally comprise a crankcase, housing a tilted cam plate connected to a rotary shaft. Upon rotation of the shaft driven by the engine, the tilted cam plate transforms rotary motion of the shaft to reciprocating movement of at least one piston arranged in a cyl inder bore and connected to the cam plate. Due to the reciprocating motion of the piston, refrigerant in the cylinder bore can be compressed. By changing the angle of tilt of the cam plate, the stroke or displacement of the piston, and thereby the discharge capacity of the compressor, can be changed.
Such a compressor further comprises a control apparatus for controlling the discharge capacity of the variable displacement compressor. The control apparatus changes the angle of tilt of the cam plate by changing the pressure of the refrigerant pushing onto the back of the cam plate -also referred to as crankcase pressure-, with respect to a suction pressure, i.e. the pressure of the low pressure refrigerant delivered to the compressor by the refrigerant circuit.
The control apparatus can comprise a bellowed control valve, which senses the suction pressure and, based on the sensed suction pressure, adjusts the crankcase pressure, thereby changing the pressure differential between crankcase pressure and suction pressure and changing the angle of tilt of the cam plate. Such bellowed control valves generally have good control stability; they do however have the disadvantage of not being suitable for heat pump systems or high-pressure systems.
When the cooling load acting on the compressor is heavy, i.e. when the difference between the temperature in the passenger compartment and the temperature set by a temperature setting unit is large, the preload on the bellow is changed in such a way that a lower setting for the pressure differential is achieved. The control valve is operated so as to reduce the pressure differential between crankcase pressure and suction pressure, whereby the angle of tilt of the cam plate increases. As a result, the discharge capacity of the compressor increases.
When the cooling load acting on the compressor is low, i.e. when the difference between the temperature in the passenger compartment and the temperature set by a temperature setting unit is small, the preload on the bellow is changed in such a way that a higher setting for the pressure differential is achieved. The control valve is then operated so as to increase the pressure differential between crankcase pressure and suction pressure, whereby the angle of tilt of the cam plate decreases. As a result, the discharge capacity of the compressor is reduced.
In high-pressure systems, e.g. in CO₂ systems, the control apparatus can comprise a bellowless control valve. Such a control valve can, as e.g. disclosed in EP-A-1 087 137, use temperature information of a passenger compartment of a vehicle and information on the rotational speed of the engine for calculating an optimal value for the pressure differential based on the temperature information and the rotational speed information. This information can be obtained from the bus, a database or additional detectors.
The change in pressure differential is achieved by means of an electric driving mechanism of a control valve, the latter being in fluid communication with the crankcase, a suction area of the compressor and a discharge area of the compressor. When more cooling capacity is needed, the control valve is operated so as to further open a communication passage between the crankcase and the suction area, so that some of the refrigerant can flow from the crankcase to the low-pressure area of the suction area, thereby lowering the crankcase pressure. When less cooling capacity is needed the control valve is operated so as to further open a communication passage between the crankcase and the discharge area, so that some of the refrigerant can flow from the high-pressure area of the discharge area to the crankcase, thereby increasing the crankcase pressure.
In order to maintain the pressure differential at the optimal value, the control valve internally comprises a control passage and a valve obturator to control the opening of a communication between the discharge chamber and the crankcase chamber. The valve obturator can e.g. be a valve ball associated with a spring for urging the valve ball down onto its valve seat.
When the suction pressure rises to make the difference between the pressure in the crankcase and the suction pressure fall below the optimal value, the control valve actuates the valve body to further open the control passage. This increases the amount of the high-pressure refrigerant supplied to the crankcase from the discharge area, thus raising the pressure in the crankcase. On the other hand, when the suction pressure falls to make the difference between the pressure in the crankcase and the suction pressure rise above the optimal value, the control valve actuates the valve body to further close the control passage. This decreases the amount of the high-pressure refrigerant supplied to the crankcase from the discharge area, thus dropping the pressure in the crankcase. As a result, the pressure differential between crankcase pressure and suction pressure is maintained at the optimal value.
Although such a bellowless control valve might work sufficiently well in some operating conditions, this cannot be guaranteed for all operating conditions. Under the latter operating conditions, fluctuations of pressure at suction and discharge of the compressor can still occur, leading to an unstable system. Such instability has a negative effect on the torque of the compressor and on noise levels. There is hence room for further improvement.

Object of the invention

The object of the present invention is hence to provide an improved variable displacement compressor. This object is achieved by the compressor as claimed in claim 1.
It is a further object of the present invention to provide an improved method for controlling a variable displacement compressor. This object is achieved by the method as claimed in claim 13.

Summary of the invention

The present invention proposes a variable displacement compressor comprising a crankcase, housing a tilted cam plate associated to a rotary shaft and at least one piston associated to the tilted cam plate, the tilted cam plate transferring rotary movement of the rotary shaft to reciprocating movement of the piston, the piston being arranged in a cylinder bore. In operation, the piston draws refrigerant at suction pressure into the cylinder bore and compresses the refrigerant to a discharge pressure. The compressor further comprises a control valve for changing the crankcase pressure with respect to the suction pressure so as to change the angle of tilt of said cam plate; and a control unit for operating the control valve. According to an important aspect of the invention, the control unit comprises means for determining a pressure differential between a crankcase pressure and a suction pressure; and means for operating a control valve based on the determined pressure differential.
The displacement of the piston is set by changing the pressure differential between crankcase pressure and suction pressure to an optimal value calculated based on the load on the compressor. The optimal value is dynamic and depends e.g. on the required cooling capacity. In order to keep the pressure differential at the optimal value, the control unit determines the pressure differential between crankcase pressure and suction pressure and operates the control valve based on the determined pressure differential, so as to maintain the pressure differential at the optimal value. If the suction pressure decreases slightly, the pressure differential increases slightly. The control unit then operates the control valve to reduce the pressure in the crankcase. If the suction pressure increases slightly, the pressure differential decreases slightly. The control unit then operates the control valve increase the pressure in the crankcase.
The present invention allows very good control of the compressor, so that improved stability of the system is obtained. As soon as a change in the pressure differential is determined, the control valve can be operated so as to bring the pressure differential back to the optimal value. This leads to improved durability, reduced noise, reduced emissions and fuel consumption and, more generally, improved performance of the compressor and the air conditioning system.
It has further to be noted that the present invention allows the use of a bellowless valve, i.e. a valve suitable for heat pump systems or for high-pressure systems, while having control stability comparable to that of a bellowed valve.
The control valve is preferably fluidly arranged between the refrigerant at crankcase pressure and the refrigerant at discharge pressure. The control valve can e.g. be a 2-port valve having a first port connected to a crankcase chamber containing the refrigerant at crankcase pressure and having a second port connected to a discharge chamber containing high-pressure refrigerant at discharge pressure. A bleed channel can be arranged between the crankcase chamber and a suction chamber containing low-pressure refrigerant at suction pressure. By controlling the degree of opening of the communication between the crankcase and discharge chambers, the amount of high-pressure refrigerant to the crankcase chamber can be regulated, thereby adjusting the pressure differential between crankcase pressure and suction pressure. In operation, if the suction pressure decreases slightly, the pressure differential increases slightly. The control unit then operates the control valve to decrease the opening of the communication passage between the crankcase and discharge chambers, thereby reducing the amount of high-pressure refrigerant flowing to the crankcase chamber and reducing the pressure in the crankcase chamber. If the suction pressure increases slightly, the pressure differential decreases slightly. The control unit then operates the control valve to increase the opening of the communication passage between the crankcase and discharge chambers thereby increasing the amount of high-pressure refrigerant flowing to the crankcase chamber and increasing the pressure in the crankcase chamber.
The control valve is preferably furthermore fluidly arranged between the refrigerant at crankcase pressure and the refrigerant at suction pressure. The control valve can then e.g. be a 4-port valve, wherein the first and second ports are connected to the crankcase and discharge chambers respectively and wherein the third and fourth ports are connected to the crankcase and suction chambers respectively. By controlling the degree of opening of the communication between the crankcase and discharge chambers, and the degree of opening of the communication between the crankcase and suction chambers, the amount of high-pressure refrigerant to the crankcase chamber and the amount of refrigerant from the crankcase chamber to the suction chamber can be regulated. The pressure differential between crankcase pressure and suction pressure can thereby be adjusted. In operation, if the suction pressure decreases slightly, the pressure differential increases slightly. The control unit then operates the control valve to increase the opening of the communication passage between the crankcase and suction chambers so as to reduce the pressure in the crankcase chamber. If the suction pressure increases slightly, the pressure differential decreases slightly. The control unit then operates the control valve to increase the opening of the communication passage between the crankcase and discharge chambers so as to increase the pressure in the crankcase chamber.
According to a first embodiment of the invention, the control unit comprises means for measuring the suction pressure of the refrigerant; means for measuring the crankcase pressure of the refrigerant; and means for calculating the pressure differential, by comparison of the measured suction pressure and the measured crankcase pressure. Preferably, the means for measuring the pressure are absolute pressure sensors. Simple and cheap sensors can be used. The measured pressures can simply be subtracted to obtain a precise value for the pressure differential between the crankcase pressure and the suction pressure.
According to a second embodiment of the invention, the control unit comprises means for measuring the pressure differential between crankcase pressure and suction pressure. Preferably, the means for measuring the pressure differential is a differential pressure sensor. The means for measuring the pressure differential can be arranged either in a suction chamber or in the crankcase of the compressor. Such a differential pressure sensor can be arranged in only one of the chambers, with a probe extending into the other of the chambers. The pressure differential between crankcase pressure and suction pressure can be directly measured.
According to a third embodiment of the invention, the control unit comprises means for measuring the suction pressure of the refrigerant; means for estimating the crankcase pressure of the refrigerant; and means for calculating the pressure differential, by comparison of the measured suction pressure and the estimated crankcase pressure.
The crankcase pressure estimation can be based on one or more of the following parameters: discharge pressure, suction pressure, current to control valve, suction temperature, discharge temperature. The means for estimating the crankcase pressure can e.g. be in the fashion of a Kalman filter.
Advantageously, the means for determining the pressure differential between the crankcase pressure and the suction pressure is configured so as to repeat the determination, preferably at short time intervals. It is thereby possible to rapidly react to a change in pressure differential. As soon as a change in the pressure differential occurs, the optimal value for the pressure differential can be restored.
The present invention also concerns a method for controlling a variable displacement compressor comprising the steps of calculating an optimal value for the pressure differential between crankcase pressure and suction pressure and operating a control valve to bring the pressure differential between crankcase pressure and suction pressure to the optimal value. According to an important aspect of the invention, the method further comprises the steps of determining a pressure differential between a pressure in the crankcase chamber and a suction pressure; and operating the control valve based on the determined pressure differential.
The operation of the control valve can furthermore be based on other parameters, such as e.g. suction pressure.
The step of determining the pressure differential is preferably repeated at short time intervals. It is thereby possible to rapidly react to a change in pressure differential. As soon as a change in the pressure differential occurs, the latter can be brought back to the optimal value.

Brief description of the drawings

The present invention will now be described, by way of example, with reference to the accompanying drawing, wherein Fig.1 shows a section view through a variable displacement compressor according to the invention.

Detailed description of a preferred embodiment

Fig.1 shows a variable displacement compressor 10 comprising a housing having a front housing member 12, a central housing member 14 and a rear housing member 16. Between the front housing member 12 and the central housing member 14, a crankcase chamber 18 is formed.
A rotary shaft 20 passes through the crankcase chamber 18 and is coupled to an engine 22 via a clutch mechanism 24 such as an electromagnetic clutch. When the engine 22 runs and the clutch mechanism 24 is engaged, the rotary shaft 20 is rotated. A cam plate 26 is supported by the rotary shaft 20 and is inclined with respect to the latter.
A plurality of cylinder bores 28 (only one shown in Figure 1) are formed in the central housing member 14. A piston 30 is retained in each cylinder bore 28. Each piston 30 is attached to the periphery of the cam plate 26 via a shoe 32 and reciprocates forward and backward in the cylinder bore 28 as the cam plate 26 rotates with the rotary shaft 20.
A suction chamber 34, which forms a suction pressure area, and a discharge chamber 36, which forms a discharge pressure area, are arranged in the rear housing member 16. A suction port 38 with a suction valve (not shown) and a discharge port 40 with a discharge valve (not shown) are formed between the suction and discharge chambers 34, 36 and the cylinder bore 28. As the piston 30 moves from the top dead center to the bottom dead center, refrigerant from the suction chamber 34 is drawn into the cylinder bore 28 via the suction port 38 and the suction valve. As the piston 30 moves from the bottom dead center to the top dead center, the refrigerant in the cylinder bore 28 is compressed to a predetermined pressure and is discharged to the discharge chamber 36 via the discharge port 40 and the discharge valve.
The suction chamber 34 and the discharge chamber 36 are connected to an external refrigeration circuit 42, at least comprising a condenser 44, an expansion device 46 and an evaporator 48.
In order to control the discharge capacity of the compressor 10, communication passages 50, 50' connect the discharge chamber 36 to the crank chamber 18. A control valve 52 is located between the communication passages 50, 50'. A bleed passage 54 connects the crank chamber 18 to the suction chamber 34. A pressure transmitting passage 56 extends between the suction chamber 34 and the control valve 52.
The discharge capacity 10 depends on the required air conditioning system load. For instance, if a lot of cooling is required, the flow volume discharged from the compressor 10 has to be increased. The stroke or displacement of the piston 30 must be increased to increase the flow volume. In order to increase the displacement of the piston 30, the pressure in the crankcase chamber 18 is reduced with respect to the pressure in the suction chamber 34. Similarly, if only a little of cooling is required, the flow volume discharged from the compressor 10 has to be reduced. The stroke or displacement of the piston 30 must be decreased to reduce the flow volume. In order to decrease the displacement of the piston 30, the pressure in the crankcase chamber 18 is increased with respect to the pressure in the suction chamber 34.
A control unit 60 is provided for determining an optimal value for the pressure differential between crankcase pressure and suction pressure. Based on the information received from a temperature sensor 62, located e.g. in a vehicle passenger compartment, and a temperature setting unit 64, the control unit 60 determines an optimal value for the pressure differential and instructs the control valve 52 to open or close respective passages 50', 54, so as to bring the pressure differential between crankcase pressure and suction pressure to the optimal value. Other parameters, such as e.g. rotational speed of the engine, can also be taken into account by the control unit 60 when determining the optimal value. This information is generally already available in engines.
The control unit 60 further comprises an absolute pressure sensor 66 arranged in the crankcase chamber 18 and an absolute pressure sensor 68 arranged in the suction chamber 68. By means of the pressure sensors 66, 68, the control unit 60 determines, preferably at regular time intervals, a pressure differential between a pressure in the crankcase chamber 18 and a suction pressure and operates the control valve 52 based on the determined pressure differential. The pressure differential is monitored and maintained at the optimal value, so that stability of the system is ensured.
According to another embodiment of the invention, the pressure sensors 66, 68 can be replaced by a differential pressure sensor. Such a sensor can be arranged in either of the crankcase chamber 18 and the suction chamber 34 and have a probe extending into the other chamber. A single sensor can hence be used, thereby reducing costs and weight.
According to yet another embodiment, the pressure sensor 66 in the crankcase chamber 18 can be replaced by a virtual sensor estimator, e.g. in the fashion of a Kalman filter. The virtual sensor estimator, which can be integrated in the control unit 60, can estimate the crankcase pressure based on one or more of the following parameters: suction pressure, discharge pressure, current to control valve, suction temperature or discharge temperature.

List of reference numerals

10: variable displacement compressor
12: front housing member
14: central housing member
16: rear housing member
18: crankcase chamber
20: rotary shaft
22: engine
24: clutch mechanism
26: cam plate
28: cylinder bores
30: piston
32: shoe
34: suction chamber
36: discharge chamber
38: suction port
40: discharge port
42: external refrigeration circuit
44: condenser
46: expansion device
48: evaporator
50, 50': communication passages
52: control valve
54: bleed passage
56: pressure transmitting passage
60: control unit
62: temperature sensor
64: temperature setting unit
66: pressure sensor
68: pressure sensor

Claims

Variable displacement compressor comprising a crankcase housing:

a tilted cam plate associated to a rotary shaft; and

at least one piston associated to said tilted cam plate, said tilted cam plate transferring rotary movement of said rotary shaft to reciprocating movement of said piston, said piston being arranged in a cylinder bore, wherein, in operation, said piston draws refrigerant at suction pressure into said cylinder bore and compresses said refrigerant to a discharge pressure;

said compressor further comprising:

a control valve for changing a crankcase pressure with respect to said suction pressure so as to change the angle of tilt of said cam plate; and

a control unit for operating said control valve

characterised in that said control unit comprises:

means for determining a pressure differential between a crankcase pressure and a suction pressure; and

means for operating a control valve based on the determined pressure differential.
Compressor according to claim 1, wherein said control valve is fluidly arranged between said refrigerant at crankcase pressure and said refrigerant at discharge pressure.
Compressor according to claim 2, wherein said control valve is furthermore fluidly arranged between said refrigerant at crankcase pressure and said refrigerant at suction pressure.
Compressor according to any of claims 1 to 3, wherein said control unit comprises:

means for measuring the suction pressure of said refrigerant;

means for measuring the crankcase pressure of said refrigerant; and

means for calculating said pressure differential, by comparison of the measured suction pressure and the measured crankcase pressure.
Compressor according to claim 4, wherein said means for measuring the pressure are absolute pressure sensors.
Compressor according to any of claims 1 to 3, wherein said control unit comprises:

means for measuring the pressure differential between crankcase pressure and suction pressure.
Compressor according to claim 6, wherein said means for measuring the pressure differential is a differential pressure sensor.
Compressor according to claim 7, wherein said means for measuring the pressure differential is arranged in a suction chamber of said compressor.
Compressor according to claim 7, wherein said means for measuring the pressure differential is arranged in said crankcase of said compressor.
Compressor according to any of claims 1 to 3, wherein said control unit comprises:

means for measuring the suction pressure of said refrigerant;

means for estimating the crankcase pressure of said refrigerant ; and

means for calculating said pressure differential, by comparison of the measured suction pressure and the estimated crankcase pressure.
Compressor according to claim 10, wherein said crankcase pressure estimation is based on one or more of the following parameters: discharge pressure, suction pressure, current to control valve, suction temperature, discharge temperature.
Compressor according to any of the previous claims, wherein the means for determining the pressure differential between the crankcase pressure and the suction pressure is configured so as to repeat the determination, preferably at short time intervals.
Method for controlling a variable displacement compressor comprising the steps of:

calculating an optimal value for the pressure differential between crankcase pressure and suction pressure;

operating a control valve to bring the pressure differential between crankcase pressure and suction pressure to said optimal value;

characterized by the further steps of:

determining a pressure differential between a crankcase pressure and a suction pressure; and

operating said control valve based on the determined pressure differential.
Method according to claim 11, wherein operation of said control valve is furthermore based on suction pressure.
Method according to claim 11 or 12, wherein the step of determining the pressure differential is repeated, preferably at short time intervals.