Automotive air-conditioner having sub-compressor driven by electric motor

This application is based upon and claims benefit of priority of Japanese Patent Applications No. 2001-131604 filed on Apr. 27, 2001, No. 2001-152771 filed on May 22, 2001 and No. 2001-277184 filed on Sep. 12, 2001, the contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to an air-conditioner for use in an automotive vehicle in which an internal combustion engine mounted thereon is automatically stopped when the automotive vehicle comes to a temporary halt at an intersection or the like.

2. Description of Related Art

Recently, an automotive vehicle having a so-called idle-stop device, which automatically stops an engine when the vehicle comes to a temporary halt at an intersection or the like for saving fuel consumption, is being introduced in the market. Since a compressor of an air-conditioner mounted on such a vehicle is driven by the engine, operation of the compressor is also stopped when the engine is temporarily stopped. If the compressor comes to a halt, the air-conditioner does not perform a desired function.

JP-A-2000-127753 proposes an air-conditioner system which solves the above problem. In this system, a sub-compressor which is driven by an electric motor during a period in which the main compressor is not driven by the engine is connected in parallel to the main compressor. In this manner, the air-conditioner is able to continue its operation during such a period. Since sub-compressor is connected in parallel to the main refrigeration circuit in this system, refrigerant mixed with lubricant circulating in the main refrigeration circuit tends to enter into a sub-circuit including the sub-compressor when the main compressor is operated and the sub-compressor is not operated. The refrigerant entered into the sub-circuit still remains therein when the operation of the main compressor resumes. Therefore, an amount of refrigerant including lubricant becomes short in the main refrigeration circuit, and the main compressor cannot be properly lubricated. This results in not only insufficient cooling ability of the air-conditioner but also reduction in an operable life of the main compressor.

Since the main compressor and the sub-compressor are alternately operated, the same problem also occurs in the sub-circuit including the sub-compressor. The refrigerant mixed with lubricant enters into the sub-circuit or the main refrigeration circuit and remains therein is called “sleeping refrigerant”. In order to reduce the amount of the sleeping refrigerant, the above-mentioned JP-A-2000-127753 also proposes to dispose one-way check valves in both the main circuit and the sub-circuit, or to install valves switching flow directions of the refrigerant at junctions connecting the sub-circuit to the main refrigeration circuit. If one-way check valves are used, four of them are necessary to control the flow directions of the refrigerant. This makes the air-conditioner expensive and complex, and further makes it difficult to add the sub-circuit including the sub-compressor to the existing main refrigeration circuit.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved automotive air-conditioner having a sub-compressor driven by an electric motor, and more particularly to reduce an amount of refrigerant remaining both in a main refrigeration circuit and in a sub-circuit of such an air-conditioner without making its structure complex. Another object of the present invention is to improve durability of the compressor by providing the compressors with sufficient lubrication. Another object of the present invention is to provide an air-conditioner in which the sub-circuit including the sub-compressor is easily added to an existing refrigeration circuit. Yet another object of the present invention is to provide an air-conditioner having a sub-compressor which is driven quietly during a vehicle is not driven.

The air-conditioner of the present invention is mounted on an automotive vehicle which has a so-called idle-stop mechanism. An internal combustion engine for driving the vehicle is stopped every time the vehicle temporarily stops at an intersection or the like for saving fuel consumption.

The air-conditioner includes a main refrigeration circuit and a sub-circuit. The main refrigeration circuit is composed of a main compressor, condenser, an expansion valve and an evaporator, all connected in series. The main compressor and the evaporator is connected through a low pressure tube, and the main compressor and the condenser is connected through a high pressure tube. A first junction and a second junction are formed in the low pressure tube and in the high pressure tube, respectively. The sub-circuit is composed of a sub-compressor and inlet and outlet pipes connected to the sub-compressor. The sub-compressor is connected in parallel to the main compressor by connecting the sub-circuit between the first and the second junctions.

The main compressor is driven by the engine while the engine is in operation, and the sub-compressor is driven by an electric motor while the engine is not in operation. Thus, the air-conditioning ability of the air-conditioner is maintained at a required level even when the engine is not in operation. The capacity of the sub-compressor is smaller than that of the main compressor, and the pipe size in the sub-circuit is made smaller than that of the main refrigeration circuit.

Because the sub-compressor is not in operation while the main compressor is in operation, refrigerant mixed with lubricant circulating in the main refrigeration circuit enters into the sub-circuit and retained therein. This causes shortage of refrigerant in the main refrigeration circuit. On the other hand, the refrigerant may be retained in the main refrigerant circuit while the main compressor is not in operation, causing refrigerant shortage in the sub-circuit.

To prevent the refrigerant from entering into the sub-circuit and being retained therein, the inlet pipe and the outlet pipe of the sub-circuit are branched out upwardly from the first and the second junctions. This structure suppresses refrigerant communication between the main refrigerant circuit and the sub-circuit. The inlet pipe of the sub-compressor may be branched out from the first junction first downwardly and then upwardly. In this manner, the refrigerant held in the downwardly branched out portion is sucked into the sub-circuit when the sub-compressor resumes its operation thereby to sufficiently lubricate the sub-compressor.

The sub-compressor and the electric motor for driving the sub-compressor may be contained in an unitary package. The unitary package is mounted on the vehicle via a shock absorbing member to alleviate vibration transferred from the vehicle to the sub-compressor. Looped or cranked portions may be formed in the inlet and the outlet pipes of the sub-compressor, respectively, to alleviate vibration transferred from the sub-compressor to the main refrigeration circuit. The sub-circuit composed of the sub-compressor and the inlet and outlet pipes is easily added to the main refrigeration circuit, e.g., to an existing refrigeration circuit.

Alternatively, the electric motor for driving the sub-compressor may be controlled by an electronic controller to return the refrigerant remaining in the sub-circuit to the main refrigeration circuit, or vice versa. The sub-compressor may be driven for a short predetermined period when the main compressor is in operation. In this manner, the refrigerant retained in the sub-circuit while the sub-compressor is not in operation is returned to the main refrigeration circuit. The sub-compressor may be intermittently and repeatedly operated for that short predetermined period with a certain interval in order to sufficiently return the refrigerant to the main refrigeration circuit. The sub-compressor may be driven for a short predetermined period immediately after the main compressor resumes its operation to lubricate the main compressor. The predetermined short period may be changed according to operating conditions of the engine or a required heat load of the air-conditioner. In case the refrigerant is retained in the main refrigerant circuit, the sub-compressor may be driven for a short time immediately before the sub-compressor resumes its operation. In this manner, the refrigerant retained in the main refrigerant circuit is returned to the sub-circuit to lubricate the sub-compressor.

Alternatively, a compressor having a variable capacity may be used as the main compressor. The capacity is increased to a level higher than a level for attaining a required cooling ability for a predetermined period of time immediately after the main compressor resumes its operation. In this manner, the refrigerant retained in the sub-circuit is instantly sucked into the main refrigerant circuit. The capacity may be increased up to a maximum capacity of the variable-capacity compressor. The capacity increase may be intermittently repeated during a period in which the variable-capacity compressor is in operation.

According to the present invention, the refrigerant mixed with lubricant is prevented from entering into the circuit which is not in operation by a simple and inexpensive structure in the air-conditioner. Both the main compressor and the sub-compressor are sufficiently lubricated, and thereby durability of the compressors is improved. The sub-circuit including the sub-compressor is easily mounted on the vehicle while suppressing noises generated by the sub-compressor. Further, the refrigerant retained in the sub-circuit or the main refrigeration circuit is efficiently returned to right positions requiring the refrigerant.

Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.

FIG. 1 is a schematic block diagram showing an automotive air-conditioner having a sub-compressor;

FIG. 2 is a cross-sectional view showing a sub-compressor unit used in the air-conditioner shown in FIG. 1;

FIG. 3 is a schematic view showing a structure of junctions at which a sub-circuit including the sub-compressor is connected to a main refrigeration circuit, as a first embodiment of the present invention;

FIG. 4 is a schematic view showing a modified form of inlet and outlet pipes of the sub-circuit;

FIG. 5 is a block diagram showing a relay circuit for operating the sub-compressor in an ON-and-OFF fashion;

FIG. 6 is a schematic view showing a structure of junctions at which the sub-circuit is connected to the main refrigeration circuit, as a second embodiment of the present invention;

FIG. 7 is a schematic view showing a structure of junctions at which the sub-circuit is connected to the main refrigeration circuit, as a third embodiment of the present invention;

FIG. 8 is a block diagram showing an entire structure of an air-conditioner as a fourth embodiment of the present invention;

FIG. 9 is a flowchart showing a process of controlling a sub-compressor used in the air-conditioner shown in FIG. 8;

FIG. 10 is a timing chart showing operation of a sub-compressor in relation to operation of a main compressor;

FIG. 11 is a timing chart showing operation of a sub-compressor as a fifth embodiment of the present invention;

FIG. 12 is a block diagram showing an entire structure of an air-conditioner as a sixth embodiment of the present invention;

FIG. 13 is a flowchart showing a process of controlling a main compressor used in the air-conditioner shown in FIG. 12;

FIG. 14 is a timing chart showing controlled operation of the main compressor used in the sixth embodiment;

FIG. 15 is a timing chart showing a process of controlling a sub-compressor as a seventh embodiment; and

FIG. 16 is a flowchart showing a process of controlling a sub-compressor as a eighth embodiment.

A first embodiment of the present invention will be described with reference to FIGS. 1-3. First, referring to FIG. 1 an entire structure of an air-conditioner having a refrigeration circuit and a sub-circuit including a sub-compressor connected in parallel to the refrigeration circuit will be described. A refrigeration circuit “A” used in the air-conditioner is a known one that includes a main compressor 3 , a condenser 4 , an expansion valve S and an evaporator 6 , all connected in series in this order. The main compressor 3 is driven by an internal combustion engine 2 via a belt in an ON-and-OFF fashion. Refrigerant mixed with lubricant (referred to as refrigerant) is compressed by the main compressor 3 , and the compressed refrigerant is cooled down in the condenser 4 and condensed therein. The condenser 4 is cooled by a cooling fan 4 a driven by a motor. The condensed refrigerant is depressurized by the expansion valve 5 and is sent to the evaporator 6 . The condensed refrigerant is expanded in the evaporator 6 thereby to cool down air in a passenger compartment. An air-conditioner compartment unit 7 including the evaporator 6 and the expansion valve 5 is installed in a passenger compartment.

The main compressor 3 and the evaporator 6 are connected through a low pressure tube 8 consisting of a flexible hose 9 disposed at the compressor side and a metallic tube 10 positioned at the evaporator side. The flexible hose 9 is made of a resilient material such as rubber, and the metallic tube 10 is made of a metallic material such as aluminum. One end of the flexible hose 9 is fixed to a vehicle body by a fixing member 11 . The main compressor 3 and the condenser 4 are connected through a high pressure tube 12 consisting of a flexible tube 12 a made of a flexible material such as rubber and a metallic tube 12 b . The flexible tube 12 a is connected to the main compressor 3 while the metallic tube 12 b is connected to the condenser 4 . A first junction “X” is formed in the metallic tube 10 at a position close to a point connecting the metallic tube 10 to the flexible hose 9 . A second junction “Y” is formed in the metallic tube 12 b at a position close to a point connecting the metallic tube 12 b to the flexible tube 12 a.

A sub-compressor 20 c driven by an electric motor is connected in parallel to the main compressor 3 . More particularly, an intake pipe 21 of the sub-compressor 20 c is connected to the first junction X, and an outlet pipe 22 of the sub-compressor 20 c is connected to the second junction Y. The sub-compressor 20 c , the intake pipe 21 , and the outlet pipe 22 constitute a sub-circuit which is connected between the first junction X and the second junction Y as a whole.

A sub-compressor unit 200 that includes the sub-compressor 20 c will be described with reference to FIG. 2 . The sub-compressor unit 200 is composed of the sub-compressor 20 c and an electric motor 20 b , both contained in a cylindrical housing 20 a . The sub-compressor unit 200 is installed on the vehicle so that the sub-compressor 20 c is positioned at a upper side and the electric motor 20 b at a lower side. The sub-compressor 20 c is driven by the electric motor 20 b , and the refrigerant flowing into the sub-circuit is compressed in the sub-compressor 20 c and supplied to the refrigeration circuit A.

The electric motor 20 b is composed of a stator 20 e having a magnet fixed to the housing 20 a and a rotor 20 f having an armature winding. A driving shaft 20 d connected to the rotor 20 f and supported by bearings 20 n drives the sub-compressor 20 c . The rotor 20 f includes a commutator 20 g to which electrical power is supplied from brushes 20 h which slidably contact the commutator 20 g . The sub-compressor 20 c is a known one, e.g., a compressor having a swash-plate, a scroll rotor, vanes or rolling pistons. An inlet port 20 j of the sub-compressor 20 c is connected to the first junction X through the inlet pipe 21 , and an outlet port 20 k is connected to the second junction Y through the outlet pipe 22 . The driving shaft 20 d is hermetically sealed by a sealing member 20 m so that the refrigerant in the sub-compressor 20 c does not enter into the motor 20 b.

Now, referring to FIG. 3, how the sub-circuit having the sub-compressor 20 c is connected to the main refrigerant circuit A will be described. The sub-compressor unit 200 is installed in the engine compartment 1 (refer to FIG. 1) via a shock-absorber 23 to suppress vibration transferred to the sub-compressor unit 200 from the vehicle body. In this particular embodiment shown in FIG. 3, the sub-compressor 20 c is positioned below both the first junction X and the second junction Y. The inlet pipe 21 is connected to the first junction X via an uprising portion 21 a formed in an inverse-U-shape, one side of which rises upward from the first junction X. Similarly, the outlet pipe 22 is connected to the second junction Y via an uprising portion 22 a formed in an inverse-U-shape, one side of which rises upward from the second junction Y. The inlet pipe 21 includes a looped portion 21 b for absorbing vibration, and similarly the outlet pipe 22 includes a looped portion 22 b . The looped portions 21 b , 22 b may be changed to cranked portions 21 b ′, 22 b ′ as shown in FIG. 4 .

Since the inlet pipe 21 and the outlet pipe 22 are connected to the main refrigeration circuit via the uprising portions 21 a , 22 a , respectively, the refrigerant is prevented from entering into the sub-circuit when the main compressor 3 is operated and the sub-compressor 20 c is not operated. Thus, an amount of refrigerant retained in the sub-circuit (the sleeping refrigerant) is reduced. The height of the uprising portions 21 a , 22 a is determined according to the amount of refrigerant delivered from the main compressor 3 .

The inner diameter of the inlet pipe 21 is made approximately one-third of the inner diameter of the low pressure tube 10 . Similarly, the inner diameter of the outlet pipe 22 is made approximately one-third of an inner diameter of the high pressure tube 12 . To reduce the amount of refrigerant entering into the sub-circuit, it is necessary to make the size of the inlet pipe 21 and the outlet pipe 22 at least smaller than the size of the low pressure tube 10 and the high pressure tube 12 , respectively. On the other hand, if the size of the inlet pipe 21 and the outlet pipe 22 is smaller than one-third of the size of the low pressure tube 10 and the high pressure tube 12 , respectively, the flow resistance in the sub-circuit excessively increases. Accordingly, it is not adequate to make the size of tubes in the sub-circuit smaller than one-third of the size of the pipes in the main refrigerant circuit.

Referring to FIG. 5, ON-and-OFF control of the sub-compressor 20 c will be described. Electric power is supplied from a battery 26 to the electric motor 20 b through a relay 27 . The sub-compressor 20 c is driven by the electric motor 20 b . The rely 27 which is made separately from other components is disposed in the engine compartment 1 at a position close to the sub-compressor unit 200 . The relay 27 is opened or closed according to a signal fed from a controller 25 positioned in the air-conditioner compartment unit 7 (refer to FIG. 1 ). When the automotive vehicle temporarily stops at an intersection or the like, its engine 2 is also temporarily stopped by means of the idle-stop device. At that time, a signal to close the relay 27 is given to the relay 27 . Upon closing the relay 27 , the motor 20 b in the sub-compressor unit 200 is rotated thereby to drive the sub-compressor 20 c.

The air-conditioner 100 structured as described above operates in the following manner. The main compressor 3 is driven by the engine 2 during a period in which the engine 2 is in operation, and the refrigerant circuit “A” performs its function to cool the air in the passenger's compartment. During a period in which the engine 2 is in operation, the sub-compressor 20 c in the sub-circuit is not driven. Since the sub-circuit is connected to the main refrigeration circuit “A” via the rising portions 21 a , 22 a , the refrigerant circulating in the main refrigeration circuit is prevented from entering into the sub-circuit. During a period in which the automotive engine 2 is not operated, the sub-compressor 20 c is driven by the motor 20 b powered by the battery 26 to maintain air-conditioning ability in the air-conditioner 100 . The capacity of the sub-compressor 20 c is made lower than the capacity of the main compressor 3 because the sub-compressor 20 c operates only for a relatively short period in which the engine 2 is in a temporary halt.

Advantages of the air-conditioner 100 described above are summarized as follows. Since the sub-circuit having the sub-compressor 20 c is connected to the main refrigeration circuit via the rising portions 21 a , 22 a , the amount of the refrigerant entering the sub-circuit when the sub-compressor 20 c is not operated is reduced, and therefore the amount of “sleeping refrigerant” in the sub-circuit is reduced. In a conventional air-conditioner, as explained above, it has been necessary to use check valves for preventing the refrigerant from entering the sub-circuit.

Since the inner diameter of the inlet pipe 21 and the outlet pipe 22 in the sub-circuit is made smaller than the inner diameter of the lower pressure tube 10 and the high pressure tube 12 in the main refrigerant circuit, respectively, the amount of refrigerant entering into the sub-circuit is further reduced, and the amount of refrigerant remaining in the sub-circuit is also reduced. Since the sub-circuit composed of the sub-compressor 20 c , the inlet pipe 21 and the outlet pipe 22 is simply connected to the main refrigeration circuit at junctions X, Y, and the relay 27 is a component separate from the controller 25 , the sub-circuit can be easily added to an existing main refrigeration circuit.

Since the sub-compressor 20 c is mounted in the engine compartment via a shock-absorber 23 , and the inlet pipe 21 and the outlet pipe 22 include the looped or cranked vibration absorbing portions 21 b , 22 b , respectively, vibration generated by the sub-compressor 20 c is effectively absorbed. Because the sub-compressor 20 c operates when the engine 2 is not operating, it is important to quietly operates the sub-compressor 20 c . Since the inlet pipe 21 and the outlet pipe 22 are made of a metallic material, they are manufactured at a low cost compared with pipes made of a resilient material such as rubber. Since the sub-compressor 20 c is packaged integrally with the electric motor 20 b in a single sub-compressor unit 200 , it can be easily mounted on the vehicle. Further, since the electric motor 20 b is hermetically sealed from the sub-compressor 20 c by the sealing member 20 m , it is not necessary to use a specially designed electric motor, but a motor common to other applications can be used.

A second embodiment of the present invention will be described with reference to FIG. 6 . In this embodiment, the sub-compressor 20 c is positioned above the first junction X and the second junction Y. Other structures are the same as those in the first embodiment. Though the looped or cranked vibration-absorbing portions are not shown in FIG. 6, they may be formed in the similar manner as in the first embodiment. An uprising portion 21 a of the inlet pipe 21 is connected to the first junction X, and similarly, an uprising portion 22 a of the outlet pipe 22 is connected to the second junction Y. The refrigerant circulating in the main refrigeration circuit when the sub-compressor 20 c is not operating is prevented from entering into the sub-circuit by the uprising portions 21 a , 22 a . Accordingly, an amount of refrigerant remaining in the sub-circuit is reduced, and shortage of the refrigerant in the main refrigeration circuit is avoided.

A third embodiment of the present invention will be described with reference to FIG. 7 . In this embodiment, the uprising portion 21 a connected to the first junction X (shown in FIG. 3) is modified to an uprising portion 21 a ′ as shown in FIG. 7 . That is, the uprising portion 21 a ′ is once bent downwardly from the first junction X, forming a U-shaped portion, and the U-shaped portion is connected to an inverse U-shaped portion. The refrigerant retained in the downwardly bent U-shaped portion is sucked into the sub-compressor 20 c through the inlet tube 21 when the sub-compressor 20 c resumes its operation, and thereby the sub-compressor 20 c is properly lubricated.

A fourth embodiment of the present invention will be described with reference to FIGS. 8-10. An entire structure of an air conditioner 100 is shown in FIG. 8 . The sub-circuit composed of the sub-compressor 20 c , the inlet pipe 21 and the outlet pipe 22 is connected between the first junction X and the second junction Y in the same manner as shown in FIG. 1 . The sub-compressor 20 c is driven by an electric motor 20 b . In this embodiment, the electric motor 20 b is controlled by an electronic controller 130 to which various sensor signals are fed. The sensor signals include a signal indicating whether air-conditioning is required, vehicle speed, engine speed, temperature in the passenger's compartment, and temperature measured immediately after the evaporator 6 . Control programs explained below are also stored in the electronic controller 130 .

Operation of the sub-compressor 20 c is controlled by controlling the electric motor 20 b in an ON-and-OFF fashion. A process of controlling the sub-compressor 20 c will be described with reference to FIG. 9 . At step S 10 , the sub-compressor 20 c is stopped. At step S 20 , whether cooling is required or not is checked. If it is not required, the process returns to step S 10 , and the sub-compressor 20 c is kept at a non-operating state. If cooling is required, the process proceeds to step S 30 where whether the main compressor 3 is under operation is determined. If the main compressor 3 is operating, the process proceeds to step S 70 , where the sub-compressor 20 c is intermittently operated, details of which will be explained later.

If it is determined that the main compressor is not under operation at step S 30 , the process proceeds to step S 40 , where whether the vehicle is at a halt (not moving) is determined based on the vehicle speed. If the vehicle is at a halt, the process proceeds to step S 50 , where whether the engine 2 is at a non-operating state is determined based on the engine speed. If the engine is not operating, the process proceeds to step S 60 , where the sub-compressor 20 c is driven by turning on the motor 20 b . Thereafter, the sub-compressor 20 c is kept operated until the engine 2 resumes its operation. That is, the cooling ability of the air-conditioner 100 is maintained by operating the sub-compressor 20 c during a period in which the main compressor 3 is not driven by the engine 2 .

If it is determined that the vehicle is not at a halt at step S 40 , the process returns to step S 10 , and the sub-compressor 20 c is stopped. Similarly, if it is determined that the engine 2 is not at a halt at step S 50 , the process returns to step S 10 , and the sub-compressor 20 c is stopped.

FIG. 10 shows a timing chart of the sub-compressor control explained above. The vehicle speed is shown in (a), the engine speed in (b), operation of the main compressor 3 in (c), and the ON-and-OFF control of the sub-compressor 20 c in (d). As shown in (c) and (d), the main compressor 3 and the sub-compressor 20 c are alternately operated. In other words, the sub-compressor 20 c is driven by the motor 20 b during a period in which the main compressor 3 is not driven by the engine 2 . However, as shown in (d), the sub-compressor 20 c is intermittently driven in the period in which the main compressor 3 is being driven. This intermittent operation is performed at step S 70 shown in FIG. 9 . As shown in (d), the sub-compressor 20 c is driven for a predetermined short period T 1 in order to return the refrigerant entered into the sub-circuit to the main refrigeration circuit. The intermittent operation is periodically repeated with a predetermined interval T 2 .

Since the refrigerant (mixed with lubricant) entered into the sub-circuit composed of the sub-compressor 20 c , the inlet pipe 21 and the outlet pipe 22 when the sub-compressor 20 c is not operating is returned to the main refrigeration circuit including the main compressor 3 , a shortage of lubricant in the main refrigeration circuit is avoided. Accordingly, not only is the cooling ability of the refrigeration circuit kept stable, but the main compressor 3 is properly lubricated and its durability is improved. Further, since the refrigerant is periodically returned to the main refrigeration circuit, an amount of the refrigerant remaining in the sub-circuit is effectively reduced. The periods of time T 1 and T 2 are predetermined according to conditions of the remaining refrigerant in respective air-conditioners.

The timing for returning the refrigerant in the sub-circuit to the main refrigeration circuit may be modified according to individual needs. It is necessary, however, that the refrigerant be returned to the main refrigerant circuit at least every time when the main compressor 3 resumes it operation.

A fifth embodiment of the present invention will be described with reference to FIG. 11 . In this embodiment, the sub-compressor 20 c is operated only as much as necessary to maintain the cooling ability of the air-conditioner 100 during a period in which the main compressor 3 is not in operation. For example, if the cooling ability of the main refrigeration circuit is higher than necessary, the sub-compressor 20 c may not be operated even in a period in which the main compressor 3 is not operating. In this case, there is a possibility that the sub-compressor 20 c remains at a non-operating state for a long time, and a large amount of the refrigerant may remain in the sub-circuit. If this occurs, the refrigerant becomes short in the main refrigeration circuit, causing insufficient lubrication of the main compressor 3 .

Therefore, in this embodiment, the sub-compressor 20 c is operated every time when the main compressor 3 resumes its operation, while keeping operation of the sub-compressor at a low level which only satisfies the necessary cooling ability. FIG. 11 shows the operation of the sub-compressor 20 c during the period in which the main compressor 3 is in operation. As shown in FIG. 11, the sub-compressor 20 c is driven every time after the main compressor 3 resumes its operation. The sub-compressor 20 c is driven only for a predetermined short period of time T 1 . Since the refrigerant is returned to the main refrigeration circuit every time the main compressor 3 resumes its operation, the main compressor 3 can be properly lubricated. Further, since the sub-compressor 20 c is periodically operated, components of the sub-compressor 20 c are protected from sticking to one another.

A sixth embodiment of the present invention will be described with reference to FIGS. 12-14. In this embodiment, the main compressor 3 which is operated in an ON-and-OFF fashion is replaced with a compressor 3 a having a variably controllable capacity (referred to as a variable-capacity compressor). As shown in FIG. 12, the variable-capacity compressor 3 a is also controlled by the electronic controller 130 . Other structures of the air-conditioner 100 are the same as those of the foregoing embodiments. The variable-capacity compressor 3 a used as the main compressor and the sub-compressor 20 c are alternately operated in this embodiment, too. However, the capacity of the variable-capacity compressor 3 a is changed during a period in which it is in operation.

Referring to FIG. 13, a process of controlling the variable-capacity compressor 3 a and the sub-compressor 20 c will be explained. At step S 100 , the sub-compressor 20 c is stopped. Then, at step S 200 , whether there is a request for cooling is determined. If not, the process returns to step S 100 , where the sub-compressor 20 c is kept at a non-operating state. If there is a request for cooling, the process proceeds to step S 300 , where whether the engine 2 is at a non-operating state is determined. If the engine 2 is in operation, the process proceeds to step S 400 and then to step S 500 , where the variable-capacity compressor 3 a is controlled to change its capacity according to the temperature in the passenger's compartment and the temperature measured downstream of the evaporator 6 . Details of the capacity control will be explained later, referring to a timing chart shown in FIG. 14 . Then, the process returns to step S 200 to repeat the above-described process. On the other hand, if it is determined that the engine 2 is at a non-operating state at step S 300 , the process proceeds to step S 600 , where the variable-capacity compressor 3 a is stopped. Then, the process returns to step S 200 .

The controls of the variable-capacity compressor 3 a and the sub-compressor 20 c explained above are shown in a timing chart in FIG. 14 . The vehicle speed is shown in (a), the engine speed in (b), the controlled capacity of the variable-capacity compressor 3 a in (c), and the ON-and-OFF control of the sub-compressor 20 c in (d). The variable-capacity compressor 3 a and the sub-compressor 20 c are alternately operated as shown in (c) and (d). The capacity of the variable-capacity compressor 3 a is changed during a period in which it is in operation as shown in (c). That is, every time when its operation starts, the capacity is increased to a level higher than a level that attains a required cooling ability for a predetermined period of time T 3 . In this particular embodiment, the capacity is increased to a maximum level of the variable-capacity compressor 3 a . The capacity increase for the period of time T 3 is repeated thereafter with a predetermined interval T 4 .

Since, in this embodiment, the capacity of the variable-capacity compressor 3 a is increased to a high level every time when its operation resumes for a predetermined period T 3 , the refrigerant retained in the sub-circuit including the sub-compressor 20 c is sucked into the main refrigeration circuit by the increased sucking ability of the variable-capacity compressor 3 a . Accordingly, shortage of the refrigerant in the main refrigeration circuit is avoided, and the cooling ability is properly maintained, and the variable-capacity compressor 3 a is properly lubricated. In other words, the refrigerant remaining in the sub-circuit is quickly returned to the main refrigeration circuit by the increased sucking ability of the variable-capacity compressor 3 a every time when its operation resumes. Thereafter, the capacity increase is repeated with a predetermined interval T 4 as shown in FIG. 14 . In this manner, the variable-capacity compressor 3 a used as the main compressor is sufficiently lubricated without fail. The sixth embodiment may be combined with the fourth embodiment. That is, the intermittent operation of the sub-compressor 20 c may be performed also in the sixth embodiment.

A seventh embodiment of the present invention will be described with reference to FIG. 15 . In this embodiment, the structures of the air-conditioner 100 are the same as those in the fourth embodiment. However, intermittent operation of the sub-compressor 20 c is performed at a different timing from that of the fourth embodiment to cope with a situation where the refrigerant is retained in the main refrigeration circuit and becomes short in the sub-circuit including the sub-compressor 20 c . As shown in FIG. 15, the sub-compressor 20 c is operated for the period of time T 1 immediately before the sub-compressor 20 c resumes its operation. In this manner, the lubricant retained in the main refrigeration circuit during a period in which the sub-compressor 20 c is not in operation is returned to the sub-circuit, and thereby the sub-compressor 20 c can be operated under sufficient lubrication when it resumes operation.

An eighth embodiment will be described with reference to FIG. 16 . In this embodiment, the period of time T 1 for returning the lubricant remaining in the sub-circuit by the intermittent operation of the sub-compressor 20 c is changed according to engine operating conditions. Though the period of time T 1 is fixed to a predetermined value in the fourth and fifth embodiments described above, it is changed, in this embodiment, in the process of controlling operation of the sub-compressor 20 c.

Steps S 31 and S 32 shown in FIG. 16 are added between steps S 30 and S 70 shown in FIG. 9 . At step S 31 , a ratio R indicating an operating condition of the engine 2 is calculated according to the following formula: R=ΣTi/ΣTr, where Ti is a period of time in which the engine 2 is not operated in each ON-and-OFF control of the engine, and Tr is a period of time in which the engine 2 is operated. Ti and Tr are summed up for a predetermined period counting from a present time to obtain ΣTi and ΣTr, respectively. Data necessary for the above calculation are obtained from the engine speed and the vehicle speed fed to the electronic controller 130 and from a timer contained in the electronic controller 130 .

The period of time T 1 is set to a longer period as the ratio R becomes smaller, and vice versa. The fact that R is small means that the engine 2 is in operation for a longer period of time compared with a period of time in which the engine is not in operation. In other words, the main compressor 3 is in operation for a longer period of time while the sub-compressor 20 c is in operation for a shorter period of time. In this situation, a larger amount of the lubricant accumulates in the sub-circuit including the sub-compressor 20 c . Therefore, the period of time T 1 for returning the lubricant retained in the sub-circuit to the main refrigeration circuit is made longer. On the contrary, the fact that the ratio R is large means that the sub-compressor 20 c runs for a longer period of time. In this situation, an amount of lubricant retained in the sub-circuit is small. Therefore, the period of time T 1 for returning the lubricant to the main refrigeration circuit is set to a shorter period. By. controlling the period of time T 1 in this manner, the main compressor 3 is sufficiently lubricated, and electric power required for intermittently driving the sub-compressor 20 c is saved.

The period of time T 1 may be changed based on a required heat load, i.e., on an actual operation ratio R 1 of the sub-compressor 20 c , which is calculated according to the following formula: R 1 =Td/Ti, where Td is a period of time in which the sub-compressor 20 c is actually operated in a given period of time Ti in which the engine 2 is not in operation. The ratio R 1 is averaged for a certain period of time counting from a present time, and the averaged R 1 is used to determine the period of time T 1 . The period of time T 1 is set to a longer period as R 1 becomes smaller, and vice versa. In a situation where R 1 is small, the sub-compressor 20 c is kept in a non-operating state for a long time, and a large amount of lubricant accumulates in the sub-circuit. Therefore, the period of time T 1 for returning the refrigerant to the main refrigerant circuit is made longer. On the contrary, in the situation where R 1 is large, the sub-compressor 20 c is in a non-operating state for a short time, and only a small amount of refrigerant remains in the sub-circuit. Accordingly, the period of time T 1 for returning the refrigerant to the main refrigerant circuit is made shorter. By changing the period of time T 1 according to R 1 , the same advantages as those obtained by changing T 1 according to R are obtained.

In the situation where the refrigerant remaining in the main refrigeration circuit is returned to the sub-circuit as done in the seventh embodiment, the period of time T 1 can be changed in a similar manner as in the eighth embodiment described above.

While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.