Abstract:
The present invention relates to a method for controlling the operation of a compressor for preventing noise that occurs in case of the shortage of refrigerant flow caused by less discharge capacity of the compressor of an air conditioner for a vehicle. In a method for controlling a compressor of an air conditioner for a vehicle according to the present invention, wherein the compressor includes a cylinder bore  13  for compressing a refrigerant, a piston  15  inserted into the cylinder bore  13,  a swash plate  48  having a controllable slope with respect to a drive shaft  40,  and a control valve  35  for controlling the slope of the swash plate  48,  the discharge capacity of the compressor is increased if it is determined that vehicle speed and accelerating rate satisfy predetermined conditions. According to the present invention, a low flow rate state of refrigerant of the compressor is predicted, and then ECV duty is increased to prevent the compressor from entering the low flow rate state of refrigerant, thereby improving the workability of the compressor and solving noise problem.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to, and claims priority to Korean patent application No. 10-2008-0094351 filed on Sep. 25, 2008, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an air conditioner for a vehicle, and more particularly, to a method for controlling the operation of a compressor for preventing noise that occurs in case of the shortage of refrigerant flow caused by less discharge capacity of the compressor of an air conditioner for a vehicle. 
         [0004]    2. Description of the Related Art 
         [0005]    An air conditioner is installed to heat or cool an interior of a vehicle. In a cooling system of such an air conditioner, a swash plate type compressor is generally employed as a compressor for compressing a vapor refrigerant of low temperature and low pressure, which is introduced from an evaporator, into a high temperature and high pressure refrigerant and then transferring it to a condenser. 
         [0006]    The swash plate type compressor is operated according to on/off of an air conditioner switch. If the compressor is operated, temperature of the evaporator is lowered, and if the compressor is stopped, the temperature of the evaporator is raised. 
         [0007]    Meanwhile, swash plate type compressors are classified into a fixed capacity compressor and a variable capacity compressor. These compressors are operated by power transmitted from rotating force of a vehicle engine. Here, the fixed capacity compressor is provided with an electronic clutch to control the operation of the compressor. 
         [0008]    However, in a case where the electronic clutch is provided, engine RPM may be varied when the compressor is operated or stopped, which may disturb stable running of the vehicle. 
         [0009]    Thus, recently, a clutch is not provided, but the variable capacity compressor that is always operated while the engine is driven and capable of varying the refrigerant discharge capacity of by changing a slope of a swash plate of the compressor is used. 
         [0010]    In such a variable capacity swash plate type compressor, a pressure control valve is generally used to control a slope of the swash plate to regulate the refrigerant discharge amount. In recent, a swash plate slope control valve (hereinafter, referred to as “ECV”), of which the operation is controlled in an electric manner, is used. 
         [0011]    Thus, in a case where a variable capacity swash plate type compressor employing an ECV, the slope of the swash plate is changed by the duty of the ECV or an applied current value, and the refrigerant discharge amount of the compressor is determined depending on the slope of the swash. 
         [0012]    As a result, it means that the duty of ECV or the applied current value is an important factor to determine the evaporator temperature (hereinafter, the case where the compressor operates means that the ECV duty is more than zero (0) and a refrigerant is discharged). 
         [0013]    The aforementioned ECV duty is a percentage value exhibiting a time period, during which the ECV is on, per the entire time. Thus, in a case where the duty is high, the refrigerant discharge of the compressor increases, while in a case where the duty is low, the refrigerant discharge decreases. 
         [0014]      FIG. 1  is a sectional view showing an inside configuration of a conventional variable capacity swash plate type compressor. 
         [0015]    As shown in the figure, in a conventional variable capacity swash plate type compressor, a center bore  11  is formed through a center of a cylinder block  10 , and a plurality of cylinder bores  13  are formed through the cylinder block  10  to radially surround the center bore  11 . Also, a piston  15  is movably installed in each cylinder bore  13 , thereby compressing a refrigerant in the cylinder bore  13 . 
         [0016]    Meanwhile, a front housing  20  is installed at one end of the cylinder block  10 . The front housing  20  and the cylinder block  10  define a crank chamber  21  therein. 
         [0017]    In addition, a rear housing  30  is installed at the other end of the cylinder block  10 , i.e., at a side opposite to the front housing  20 . A suction chamber  31  is formed in the rear housing to selectively communicate with the cylinder bore  13 . At this time, the suction chamber  31  serves to transfer a refrigerant to be compressed into the cylinder bore  13 . 
         [0018]    Also, a discharge chamber  33  is formed in the rear housing  30 . The discharge chamber  33  is formed in a region corresponding to a center of a side of the rear housing  30  that faces the cylinder block  10 . The discharge chamber  33  gives a place where the refrigerant compressed in the cylinder bore  13  stays temporarily. A control valve  35  is provided at one side of the rear housing  30 , wherein the control valve  35  controls an angle of a swash plate  48 which will be described later, by adjusting a degree of opening of a channel between the discharge chamber  33  and the crank chamber  21 . 
         [0019]    Meanwhile, a drive shaft  40  is rotatably installed through the center bore  11  of the cylinder block  10  and a shaft hole  23  of the front housing  20 . The drive shaft  40  is rotated by driving force transferred from an engine. The drive shaft  40  is rotatably installed to the cylinder block  10  and the front housing  20  by a bearing  42 . 
         [0020]    Also, a rotor  44  is installed to the crank chamber  21  such that the drive shaft  40  passes through a center of the rotor and the rotor rotates together with the drive shaft  40 . At this time, the rotor  44  is substantially disk-shaped and is fixed to the drive shaft  40 . Also, a hinge arm  46  is formed to protrude on one side of the rotor  44 . 
         [0021]    A swash plate  48  is hinged to the rotor  44  and installed to the drive shaft  40  to rotate together with the drive shaft  40 . The swash plate  48  is installed to the drive shaft  40  to have an angle changed according to discharge capacity of the compressor. In other words, the swash plate  40  is positioned between a state that it is perpendicular to a lengthwise direction of the drive shaft  40  and a state that it is inclined at a predetermined angle with respect to the drive shaft  40 . The swash plate  48  has an edge connected to the pistons  15  through shoes  50 . In other words, the edge of the swash plate  48  is connected to a connector  17  of the piston  15  through the shoe  50  such that the piston  15  is linearly reciprocated in the cylinder bore  13  by the rotation of the swash plate  48 . 
         [0022]    A connection arm  52  connected to the hinge arm  46  of the rotor  44  is formed to protrude on the swash plate  48 . A hinge pin  54  is installed at a front end of the connection arm  52  in a direction perpendicular to the lengthwise direction of the connection arm  52 , and the hinge pin  54  is movably hooked to a support  47  formed at a front end of the hinge arm  46  of the rotor  44 . 
         [0023]    A back spring  56  is installed to give elastic force between the rotor  44  and the swash plate  48 . The back spring  56  is installed around an outer surface of the drive shaft  40  and gives elastic force in a direction in which a slope of the swash plate  48  decreases. 
         [0024]    A swash plate stopper  58  is formed to protrude on one surface of the swash plate  48 . The swash plate stopper  58  serves to regulate a degree of inclination of the swash plate  48  with respect to the drive shaft  40 . 
         [0025]    A shaft stopper  60  is provided at one end of the drive shaft  40 . The shaft stopper  60  is installed around the outer surface of the drive shaft  40 , and the shaft stopper  60  serves to regulate an installation location of the swash plate  48  when it erected in a direction perpendicular to the lengthwise direction of the drive shaft  40 . 
         [0026]    Meanwhile, Japanese Laid-open Patent Publication No. 2005-104305 discloses a technique of maximizing the discharge capacity of a compressor regardless of a target cooling temperature when a vehicle is decelerated such that deceleration energy may be maximally used as compressor driving energy when a vehicle is decelerated. However, in this technique, there is a problem in that the compressor discharge capacity is unnecessarily increased to the maximum to thereby hinder refrigerant from flowing smoothly. 
         [0027]    In addition, when the refrigerant discharge amount of the compressor is small, the refrigerant may not circulate smoothly in the compressor if the above state is continued, which may cause noise. 
         [0028]    Thus, the ECV duty is kept in a low state, so that noise is generated when flow rate of refrigerant of the compressor is small and the workability of the compressor is deteriorated, thereby causing a problem in use. 
       SUMMARY OF THE INVENTION 
       [0029]    The present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a method for controlling a compressor of an air conditioner for a vehicle, wherein the workability of a compressor is improved by predicting a low flow rate state of refrigerant of the compressor and then increasing ECV duty. 
         [0030]    According to an aspect of the present invention for achieving the objects, there is provided a method for controlling a compressor of an air conditioner for a vehicle, comprising the steps of: monitoring whether vehicle speed and accelerating rate satisfy predetermined conditions and determining whether a variable capacity swash plate type compressor enters a low flow rate state of refrigerant; and controlling discharge capacity of the compressor to a predetermined value if it is determined that the compressor enters the low flow rate state of refrigerant. (In a method for controlling a compressor of an air conditioner for a vehicle according to the present invention, wherein the compressor includes a cylinder bore for compressing a refrigerant, a piston inserted into the cylinder bore, a swash plate having a controllable slope with respect to a drive shaft, and a control valve for controlling the slope of the swash plate, the method comprises the step of controlling discharge capacity of the compressor to a predetermined value if it is determined that vehicle speed and accelerating rate satisfy predetermined conditions and thus the compressor enters a low flow rate state of refrigerant. 
         [0031]    At this time, in the step of controlling discharge capacity of the compressor to a predetermined value, the discharge capacity of the compressor may be controlled to the predetermined value if the vehicle speed, the accelerating rate and the discharge capacity of the compressor satisfy predetermined conditions, and the predetermined value may be an approximately middle value in a discharge capacity range of the compressor. 
         [0032]    In addition, after the step of controlling discharge capacity of the compressor to a predetermined value, the method may further comprise the steps of determining whether a time of maintenance during which the discharge capacity is maintained to the predetermined value elapses; controlling the discharge capacity in a common manner if the time of maintenance elapses; and determining whether a preset time of cycle elapses, wherein after the time of cycle elapses, the above steps are repeated from the step of determining whether a compressor enters a low flow rate state of refrigerant. 
         [0033]    Here, the approximately middle value in the discharge capacity range may be a predetermined value between 45% and 65% in the discharge capacity range. 
         [0034]    Further, in the step of controlling discharge capacity of the compressor to a predetermined value, the discharge capacity of the variable capacity swash plate type compressor may be increased by a predetermined value. 
         [0035]    In the meantime, after the discharge capacity of the compressor is increased, the discharge capacity of the compressor may be linearly reduced according to a predetermined reduction rate. 
         [0036]    Here, the discharge capacity of the compressor may be increased by increasing ECV duty by about 10% thereof. 
         [0037]    In addition, the reduction rate of the discharge capacity of the compressor may be about 20% of ECV duty per minute. 
         [0038]    In the meantime, when it is determined whether the compressor enters a low flow rate state of refrigerant, it may be considered whether refrigerant pressure of the compressor satisfies a predetermined condition. 
         [0039]    Preferred embodiment according to the present invention will be described in more detail with reference to the accompanying drawings. Accordingly, the features and advantages of the present invention will become apparent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a sectional view showing an inside configuration of a conventional variable capacity swash plate type compressor; 
           [0041]      FIG. 2  is a block diagram showing an air conditioner control unit and an engine control system provided in the present invention; 
           [0042]      FIG. 3  is a graph showing a change in ECV duty according to the operation in accordance to a first embodiment of the present invention; 
           [0043]      FIG. 4  is a flowchart illustrating a method for controlling a compressor of an air conditioner for a vehicle according to the first embodiment of the present invention; 
           [0044]      FIG. 5  is a graph showing a change in ECV duty according to the operation in accordance to a second embodiment of the present invention; and 
           [0045]      FIG. 6  is a flowchart illustrating a method for controlling a compressor of an air conditioner for a vehicle according to the second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0046]    Hereinafter, preferred embodiments of a method for controlling a compressor of an air conditioner for a vehicle according to the present invention will be described in detail with reference to the accompanying drawings. 
         [0047]    First, an air conditioner control unit for controlling operation of the air conditioner and an engine control system according to embodiments of the present invention will be described in detail with reference to  FIG. 2 . 
         [0048]      FIG. 2  is a block diagram showing an air conditioner control unit and an engine control system provided in the present invention. 
         [0049]    As shown in the figure, the air conditioner for a vehicle according to the present invention includes an air conditioner control unit  100  for controlling operation of a compressor  300 . The air conditioner control unit  100  detects the state information on a vehicle and controls the discharge capacity of the compressor  300  such that the temperature inside the vehicle is adjusted as desired by a user. 
         [0050]    To this end, the air conditioner control unit  100  includes a main controller  110 . The main controller  110  calculates the discharge capacity of the compressor  300  according to the state information including vehicle interior temperature, outside temperature, evaporator temperature, solar flux, engine RPM, vehicle speed, refrigerant pressure (hereinafter, referred to as “APT”)) and refrigerant temperature, and controls the operation of the compressor  300  using an ECV driver  120 , which will be described later. 
         [0051]    In addition, the main controller  110  increases the discharge capacity of the compressor  300  when it is determined that the compressor  300  is in a low flow rate state using low flow rate determination factors (vehicle speed, accelerating rate, APT, compressor operating rate, and so on) in order to prevent the compressor  300  from entering the low flow rate state of refrigerant. 
         [0052]    To this end, the main controller  110  includes therein a buffer memory  115  for storing the low flow rate determination factors. In other words, the buffer memory  115  stores the vehicle speed, the accelerating rate and the APT, which are transmitted from an engine control system  200 , which will be described later, and the ECV duty (corresponding to the compressor operating rate) collected from the ECV driver  120  in time sequence. 
         [0053]    Hereinafter, the term “low flow rate state of refrigerant” used herein means a state that the workability of the compressor is deteriorated due to a shortage of lubrication caused by less refrigerant flow and noise is accordingly generated. 
         [0054]    Meanwhile, the air conditioner control unit  100  controls the discharge capacity of the compressor by adjusting the ECV duty such that the slope of the aforementioned swash plate  48  is controlled. To this end, the air conditioner control unit  100  is provided with the ECV driver  120 . 
         [0055]    The ECV driver  120  is to control the discharge capacity of the compressor  300  and adjusts a degree of opening of a channel between the discharge chamber  33  and the crank chamber  21  through the control valve  35  to control the angle of the swash plate  48 , which will be described later, and thus to control the discharge capacity of the compressor  300 . 
         [0056]    Also, the embodiment of the present invention includes an engine control system  200  for controlling output of an engine  500  to adjust an engine rotation rate (hereinafter, referred to as “RPM”)) of a vehicle. 
         [0057]    The engine control system  200  is to adjust output of the engine  500  according to a driving state of the vehicle, and adjusts the amount of fuel and air input into the engine  500  through a throttle valve  400  by the manipulation of a driver to control output of the engine  500 . 
         [0058]    In addition, the engine control system  200  provides the main controller  110  of the air conditioner control unit  100  with the vehicle speed and accelerating rate among the low flow rate determination factors. To this end, the engine control system  200  comprise a vehicle speed sensor  210  for measuring vehicle speed and transmitting the measured value to the main controller  110 , and a valve sensor  220  for measuring a degree of opening of the throttle valve  400  to calculate accelerating rate and then providing the measured accelerating rate to the main controller  110 . 
         [0059]    As mentioned above, the values measured by the vehicle speed sensor  210  and the valve sensor  220  are stored in the buffer memory  115  in time sequence. 
         [0060]    Hereinafter, it will be explained in detail with reference to  FIGS. 3 to 6  how air conditioner control units according to embodiments of the present invention detect a refrigerant shortage state of a compressor and increase the discharge capacity of the compressor. 
         [0061]    First, a first embodiment of the present invention will be described with reference to  FIGS. 3 and 4 . 
         [0062]      FIG. 3  is a graph showing a change in ECV duty according to the operation in accordance to a first embodiment of the present invention, and  FIG. 4  is a flowchart illustrating a method for controlling a compressor of an air conditioner for a vehicle according to the first embodiment of the present invention. 
         [0063]    In  FIG. 3 , the uppermost graph represents vehicle speed, the graph thereunder represents vehicle accelerating rate that is measured by the valve sensor  220 , and the lowermost graph represents ECV duty of the compressor. An x axis represents time. 
         [0064]    At this time, the vehicle speed is expressed in the unit of km/h, the accelerating rate is expressed as a percentage of opening of the throttle valve  400  (TPS %), and the ECV duty of a compressor is expressed in the unit of %. 
         [0065]    Here, when continued over a predetermined time (hereinafter, referred to as “time of datum”) are conditions that the vehicle speed is not more than a predetermined speed (hereinafter, referred to as “low speed reference value”), the accelerating rate is not more than a predetermined value (hereinafter, referred to as “non-acceleration reference value”) and the ECV duty is not more than a predetermined value (hereinafter, referred to as “operating rate reference value”), the compressor enters a low flow rate state of refrigerant to generate noise, and the main controller  110  determines that the workability is deteriorated. Accordingly, if the low flow rate state of refrigerant is detected, the main controller  110  increases the ECV duty to a predetermined value. 
         [0066]    At this time, the low speed reference value is expressed in the unit of Km/h, the non-acceleration reference value is a reference value, which defines a state where a vehicle is not accelerated or the accelerating rate is very low, and which is expressed as the percentage of opening of the throttle valve  400  (TPS %), and the operating rate reference value is an arbitrary value pertaining to a middle section in a compressor operable range and expressed as ECV duty in the unit of %. For example, assuming that the ECV duty of a vehicle compressor is varied between 35% and 85%, the operating rate reference value is an arbitrary value close to 60%. 
         [0067]    In addition, the time of datum (TD), which is calculated by experiments, is a time period in the unit of sec. And it is determined that the compressor enters a low flow rate state of refrigerant, if the state satisfying the above conditions is continued during the time of datum (TD). 
         [0068]    In the graph shown in  FIG. 3 , upon seeing a point where the vehicle speed is lowered than the low speed reference value for the first time, the accelerating rate is not greater than the non-acceleration reference value, and the compressor operating rate, i.e., the ECV duty, is not greater than the operating rate reference value, so that the three low flow rate determination factors satisfy the conditions. However, the state of satisfying the conditions is not continued during the time of datum, but the accelerating rate is increased over the non-acceleration reference value before the time of datum elapses. Thus, the main controller  110  determines that the compressor is not in the low flow rate state of refrigerant, thereby not increasing the ECV duty. 
         [0069]    Thereafter, upon seeing a point where the vehicle speed is lowered than the low speed reference value at the second time, it would be understood that the time of datum elapses while the accelerating rate is not greater than the non-acceleration reference value and the ECV duty is not greater than the operating rate reference value. In this case, the main controller  110  determines that the compressor is in the low flow rate state of refrigerant and increases the ECV duty. 
         [0070]    At this time, the main controller  110  increases the ECV duty to a preset forced value and then maintains the value for a certain time (hereinafter, referred to as “time of maintenance”). 
         [0071]    Here, the forced value is suitably calculated by experiments in advance such that the compressor may escape from the low flow rate state of refrigerant when it is determined that the vehicle speed, the accelerating rate and the ECV duty are respectively detected lower than their reference values during the time of datum and thus the compressor is in the low flow rate state of refrigerant. The forced value is at least equal to or greater than the operating rate reference value. 
         [0072]    In addition, the time of maintenance (TM), which is calculated by experiments, is a time period enough to allow the compressor to be prevented from entering the low flow rate state of refrigerant again, by permitting the refrigerant of the compressor to smoothly flow. 
         [0073]    Thus, the ECV duty is increased to prevent the compressor from entering the low flow rate state of refrigerant and generating noise. However, if the main controller  110  did not increase the ECV duty, the compressor would enter the low flow rate state of refrigerant and thus cause noise. 
         [0074]    In addition, after the time of maintenance, the ECV duty is lowered again, so that the original state is maintained. 
         [0075]    At this time, as shown in the figures, the vehicle speed, the accelerating rate and the ECV duty are maintained over the time of datum as being not greater than the low speed reference value, the non-acceleration rate reference value and the operating rate reference value, respectively, but the main controller  110  does not increase the ECV duty until a predetermined time (hereinafter, referred to as “time of cycle”) is passed. This is because the compressor does not enter the low flow rate state of refrigerant during the time of cycle as the ECV duty is increased. 
         [0076]    In other words, the time of cycle is calculated by experiments and represents a minimal time required for the compressor to enter the low flow rate state of refrigerant after the compressor is operated with the forced value during the time of maintenance. 
         [0077]    A method for controlling a compressor of an air conditioner for a vehicle according to the first embodiment of the present invention will be described. The method for controlling a compressor according to this embodiment firstly starts together with driving the air conditioner (S 100 ). 
         [0078]    Then, it is determined whether the compressor  300  satisfies the entry conditions according to the present invention (S 110 ). 
         [0079]    At this time, the entry conditions are satisfied when the state of satisfying all of the compressor operating rate condition, the vehicle non-accelerating condition and the low vehicle speed condition are maintained over the time of datum. 
         [0080]    Here, the compressor operating rate condition means that the ECV duty is not greater than the operating rate reference value, the non-accelerating condition means that the vehicle accelerating rate (the degree of opening of a throttle valve) is not greater than the non-acceleration reference value, the vehicle speed condition means that the vehicle speed is not greater than the low speed condition, and the above state is maintained over the time of datum (TD), as mentioned above. 
         [0081]    In other words, step S 110  is executed in such a manner that the time is counted from the moment when the vehicle speed, the vehicle accelerating rate and the compressor operating rate become not greater than the reference values, respectively, and then it is monitored whether the vehicle speed, the vehicle accelerating rate and the compressor operating rate are maintained as being not greater than the reference values, respectively, until the counted time becomes equal to the time of datum (TD). 
         [0082]    In step S 110 , if it is determined that the entry conditions are not satisfied, the air conditioner control unit  100  executes a common ECV control (S 120 ). Here, the common ECV control means that after the state information on a vehicle is detected, ECV duty is controlled according to the discharge capacity of the compressor  300  such that the temperature inside the vehicle is changed to a desired temperature. 
         [0083]    Meanwhile, if the entry conditions are satisfied, the time is counted again (S 130 ). The time counted here is not accumulated to the time counted in step S 110  for determining whether the entry conditions are satisfied. In other words, the time is initiated and then counted from zero (0) again. 
         [0084]    Also, since at the same time, the entry conditions are satisfied in step S 110 , the ECV duty is output as the forced value to control the compressor (S 140 ). At this time, the forced value is set as being not smaller than the operating rate reference value and not smaller than an approximately middle value in the compressor operable range, as mentioned above. Since the entry conditions are satisfied in step S 110 , it is determined that the compressor is in the low flow rate state of refrigerant, thereby preventing noise by outputting the ECV duty as a high forced value. 
         [0085]    In addition, while the ECV duty is controlled as the forced value, it is monitored whether the counted time which is being counted from step S 130  is equal to or greater than a predetermined time of maintenance (S 150 ). 
         [0086]    The time of maintenance means a time period for maintaining the ECV duty as the forced value, and is a time period calculated by experiments for allowing the refrigerant of the compressor to smoothly flow and thus preventing the compressor from entering the low flow rate state of refrigerant. 
         [0087]    If it is determined that the counted time of step S 150  is equal to or greater than the time of maintenance, the compressor is controlled according to a common ECV control method (S 160 ). Here, the common ECV control means that after the state information on a vehicle is detected, the ECV duty is controlled according to the discharge capacity of the compressor such that the temperature in the vehicle is changed to a desired temperature, as mentioned above. 
         [0088]    On the other hand, if it is determined that the counted time of step S 150  does not pass the time of maintenance, the ECV duty for controlling the compressor is maintained as the forced value. 
         [0089]    Meanwhile, while the counted time exceeds the time of maintenance and thus the compressor is controlled in a common ECV control manner, the time counting initiated in step S 130  is continued to determine whether the counted time is equal to or greater than the time of cycle (S 170 ). 
         [0090]    Then, if the counted time is equal to or greater than the time of cycle as a result of step S 170 , the series of steps starting from step S 110  for determining whether a vehicle state satisfies the entry conditions is repeated. 
         [0091]    In addition, the second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 . 
         [0092]      FIG. 5  is a graph showing a change in ECV duty according to the operation in accordance to a second embodiment of the present invention, and  FIG. 6  is a flowchart illustrating a method for controlling a compressor of an air conditioner for a vehicle according to the second embodiment of the present invention. 
         [0093]    First,  FIG. 5  shows vehicle driving and compressor operation states when an air conditioner operates while a vehicle is driven. 
         [0094]    The uppermost graph represents compressor refrigerant pressure, the second graph represents vehicle speed, the graph thereunder represents vehicle accelerating rate that is measured by the valve sensor  220 , and the lowermost graph represents compressor operating rate. Also, an x axis represents time. 
         [0095]    At this time, the refrigerant pressure is expressed in the unit of Kgf/cm 2 , the vehicle speed is expressed in the unit of Km/h, the vehicle accelerating rate is expressed as a percentage of opening of the throttle valve  400  (TPS %), and the ECV duty of a compressor is expressed in the unit of %. 
         [0096]    Here, when continued over a predetermined time (hereinafter, referred to as “time of datum”) are conditions that the refrigerant pressure is less than a predetermined value (hereinafter, refereed to as “refrigerant pressure reference value”), the vehicle speed is less than a predetermined speed (hereinafter, referred to as “low speed reference value”), the accelerating rate is less than a predetermined value (hereinafter, referred to as “non-acceleration reference value”) and the ECV duty is less than a predetermined value (hereinafter, referred to as “operating rate reference value”), the compressor enters a low flow rate state of refrigerant to generate noise, and the main controller  110  determines that the workability is deteriorated. 
         [0097]    However, the above process is just an example of determining the low flow rate state of refrigerant of a compressor, and all the above parameters of the refrigerant pressure, the vehicle speed, the accelerating rate and the compressor operating rate may be not used simultaneously in determining the low flow rate state of refrigerant of a compressor. In other words, the refrigerant pressure may be not selected as a basis for determining the low flow rate state of refrigerant of a compressor. 
         [0098]    At this time, the refrigerant pressure reference value is expressed in the unit of Kgf/cm 2 , the low speed reference value is expressed in the unit of Km/h, the non-acceleration reference value is a reference value, which defines a state where a vehicle is not accelerated or the accelerating rate is very low, and which is expressed as the percentage of opening of the throttle valve  400  (TPS %), and the operating rate reference value is an arbitrary value pertaining to a middle section in a compressor operable range and expressed as ECV duty in the unit of %. For example, assuming that the ECV duty of a vehicle compressor is varied between 35% and 85%, the operating rate reference value is an arbitrary value close to 60%. 
         [0099]    Here, the refrigerant reference value, the low speed reference value, the non-acceleration reference value and the operating rate reference value are calculated by experiments as reference values for determining the low flow rate state of a compressor. As an example, the refrigerant reference value may be 9 Kgf/cm 2 , the low speed reference value may be 30 Km/h, the non-acceleration reference value may be 6%, and the operating rate reference value may be 60% ECV duty. 
         [0100]    In addition, the time of datum (TD), which is calculated by experiments, is a time period in the unit of sec. And it is determined that the compressor enters a low flow rate state of refrigerant, if the state satisfying the above conditions is continued during the time of datum (TD). 
         [0101]    In the graph shown in  FIG. 5 , upon seeing a point where the vehicle speed is lowered than the low speed reference value for the first time, the refrigerant pressure is less than the refrigerant pressure reference value, the accelerating rate is less than the non-acceleration reference value, and the compressor operating rate is less than the operating rate reference value, so that the four low flow rate determination factors satisfy the conditions. However, the state of satisfying the conditions is not continued during the time of datum, but the accelerating rate is increased over the non-acceleration reference value before the time of datum elapses. Thus, the main controller  110  determines that the compressor is not in the low flow rate state of refrigerant, thereby not increasing the ECV duty. 
         [0102]    Thereafter, upon seeing a point where the vehicle speed is lowered than the low speed reference value at the second time, it would be understood that the time of datum elapses while the refrigerant pressure is less than the refrigerant pressure reference value, the accelerating rate is less than the non-acceleration reference value and the compressor operating rate is less than the operating rate reference value. In this case, the main controller  110  determines that the compressor is in the low flow rate state of refrigerant and increases the ECV duty. 
         [0103]    At this time, the main controller  110  increases the current ECV duty to an ECV duty value increased by a preset ECV duty compensation value. This is to increase the ECV duty for preventing the compressor from entering the low flow rate state and generating noise. However, if the main controller  110  did not increase the ECV duty, the compressor would enter the low flow rate state of refrigerant and thus cause noise. 
         [0104]    Then, after the increased ECV duty is reduced at a certain ratio (a compensation reduction rate) to the initial ECV duty, the ECV duty is controlled in a common manner. 
         [0105]    Here, the ECV duty compensation value, which is an increment of ECV duty for preventing the compressor from entering the low flow rate state, is calculated by experiments. Preferably, 10% or so of the ECV duty is set as an ECV duty compensation value. However, in order to prevent the ECV duty from being excessively increased, it is preferred that the ECV duty increased by the ECV duty compensation value does not exceed 85%. 
         [0106]    Meanwhile, the compensation reduction rate is a reduction rate for preventing the ECV duty from being rapidly changed, when the ECV duty is reduced. Preferably, the reduction rate is set such that 20% of the ECV duty is reduced per minute. 
         [0107]    Further, the compensation reduction rate of the ECV duty is also a value calculated by experiments, and it is preferred that the ECV duty is controlled in a common manner after the ECV duty is reduced at a rate of 20% per minute. 
         [0108]    In the meantime, a method for controlling a compressor of an air conditioner for a vehicle according to the second embodiment of the present invention will be described step by step with reference to  FIG. 6 . The method for controlling a compressor according to this embodiment firstly starts together with driving the air conditioner (S 200 ). 
         [0109]    Then, if the air conditioner operates, it is determined whether a vehicle satisfies the entry conditions according to the embodiment of the present invention (S 210 ). 
         [0110]    At this time, the entry conditions are satisfied when the state of satisfying all of the refrigerant pressure condition, the compressor operating rate condition, the vehicle non-accelerating condition and the low vehicle speed condition is maintained over a preset time of datum (TD). 
         [0111]    Here, the refrigerant pressure condition means that the refrigerant pressure of the compressor  300  is less than the refrigerant pressure reference value, the compressor operating rate condition means that the ECV duty is not greater than the operating rate reference value, the non-accelerating condition means that the vehicle accelerating rate (the degree of opening of a throttle valve) is not greater than the non-acceleration reference value, and the vehicle speed condition means that the vehicle speed is not greater than the low speed condition 
         [0112]    Then, it is also determined whether such a state is maintained over the time of datum (TD), and then it is determined whether the entry conditions are satisfied. 
         [0113]    If it is determined that the state of satisfying all of the refrigerant pressure condition, the compressor operating rate condition, the vehicle non-accelerating condition and the low vehicle speed condition is maintained for the time of datum (TD) as a determination result of step S 210 , the ECV duty of the compressor is increased to the increased ECV duty value (S 220 ). 
         [0114]    At this time, the increased ECV duty value is equal to a value obtained by adding the ECV duty compensation value to the ECV duty at a time point that the entry conditions are satisfied in step S 210 , i.e., a current time point that the time of datum (TD) is passed after the refrigerant pressure condition, the compressor operating rate condition, the vehicle non-accelerating condition and the low vehicle speed condition are all satisfied. 
         [0115]    The ECV duty compensation value is an increment of ECV duty, which prevents the compressor from entering the low flow rate state, as mentioned above. The ECV duty compensation value is calculated by experiments, and about 10% of ECV duty may be set as the ECV duty compensation value. 
         [0116]    For example, if the ECV duty at a current time point when the entry conditions are satisfied is 40% and the preset ECV duty compensation value is 10%, the increased ECV duty value becomes 50%. Also, in step S 220 , the compressor is controlled by setting the increased ECV duty value of 50% as the ECV duty of the compressor. 
         [0117]    Then, with the ECV duty increased to the increased ECV duty value, i.e., with the current ECV duty increased by the ECV duty compensation value, it is initiated to count the elapsed time (S 230 ). 
         [0118]    Here, the elapsed time is counted in order to linearly decrease the ECV duty according to the preset compensation reduction rate together with increasing the ECV duty as mentioned above. At this time, the compensation reduction rate is a reduction rate for preventing rapid change of the ECV duty and may be set as an ECV duty reduction rate with respect to time (for example, a 20% reduction rate of ECV duty per minute). Accordingly, the elapsed time is counted, so that the ECV duty is reduced at a preset constant rate according to the elapsed time. 
         [0119]    Thus, in step S 220 , with the ECV duty increased, the elapsed time is counted and the ECV duty is slowly reduced. The ECV duty is calculated as a value obtained by subtracting a value multiplying the compensation reduction rate by the elapsed time from the increased ECV duty value calculated in step S 220  (S 240 ). 
         [0120]    In other words, in a case where the ECV duty at the time point when the entry conditions are satisfied is 40%, the preset ECV duty compensation value is 10%, and thus the increased ECV duty value is 50%, if the compensation reduction rate is 20%, the ECV duty for controlling the compressor at a point that 15 seconds are elapsed is obtained by subtracting 20%/minute×15 seconds from 50%, which is 45%. 
         [0121]    In step S 240  as mentioned above, the ECV duty is continuously calculated and controlled as the elapsed time is passed. 
         [0122]    Then, while executing step S 240  as mentioned above, it is continuously monitored whether the current ECV duty calculated in step S 240  is reduced below a compensation completion value (S 250 ). 
         [0123]    At this time, the compensation completion value may be obtained by subtracting the ECV duty compensation value from the increased ECV duty value. Here, the value obtained by subtracting the ECV duty compensation value from the increased ECV duty value becomes an ECV duty value when the entry conditions are satisfied in steps S 210  and S 220 . In other words, this value may be an ECV duty before the ECV duty was forcibly increased, but may be set as another different value. For example, the compensation completion value may be equal to the operating rate reference value. 
         [0124]    In other case, step S 250  may be executed by comparing the elapsed time with a preset time. For example, if the preset ECV duty compensation value is 10% and the compensation reduction rate is 20% per minute, the increased ECV duty is recovered to the initial value after 30 seconds, so that step S 250  may be executed by monitoring whether the elapsed time is 30 seconds or more. 
         [0125]    In addition, as a result of the determination in step S 250 , when the ECV duty is not greater than the compensation completion value or the elapsed time is a preset time or more, the compressor is controlled in a common ECV control manner (S 260 ). Here, the common ECV control means that after the state information on a vehicle is detected, the ECV duty is controlled according to the discharge capacity of the compressor  300  such that the temperature in the vehicle is set to the user&#39;s desired temperature. 
         [0126]    According to a method for controlling a compressor of an air conditioner for a vehicle of the present invention as discussed above, the following advantages can be expected. 
         [0127]    That is, the compressor is prevented from entering a low flow rate state of refrigerant by predicting a low flow rate state of refrigerant of the compressor and then increasing ECV duty. Thus, there is an advantage in that the workability of the compressor is improved. 
         [0128]    Further, in the present invention, the compressor is prevented from entering a low flow rate state of refrigerant by predicting a low flow rate state of refrigerant of the compressor and then increasing ECV duty, thereby decreasing noise generated from the compressor and thus giving stability to users. 
         [0129]    Furthermore, when ECV duty is increased according to the present invention, the ECV duty is increased to a middle value in a common operable range of the ECV duty. Thus, there are advantages in that the ECV duty is prevented from being excessively increased (in comparison to a case where the ECV duty is increased to the maximum) to thereby make it possible to improve the fuel efficiency of a vehicle and prevent excessive torque and thus fluctuation of the engine RPM from occurring. 
         [0130]    Moreover, when the ECV duty is decreased after the ECV duty is increased according to the present invention, the ECV duty is decreased at a moderate rate, thereby decreasing fluctuation of the ECV duty and thus reducing an influence on a change in vehicle torque. 
         [0131]    The scope of the present invention is not limited to the embodiments described above but is defined by the appended claims. It will be apparent that those skilled in the art can make various modifications and changes thereto within the scope of the invention defined by the claims.