Abstract:
A control device for a variable capacity compressor in an automotive air conditioning system. The control device includes a first sensor disposed forward of the evaporator and a second sensor disposed behind the evaporator. The control device compares the air temperature detected by the sensors with predetermined temperatures, and controls the capacity of the compressor in accordance with the compared results.

Description:
TECHNICAL FIELD 
     The present invention relates to a device for controlling the capacity of a variable capacity type compressor in an automotive air conditioning system, and more particularly, to a device which controls the capacity of the compressor in accordance with the air conditioning load. 
     BACKGROUND OF THE INVENTION 
     Generally, the air conditioning system of an automobile is driven by the vehicle engine through an electromagnetic clutch. The air conditioning system is designed to achieve a predetermined air conditioning performance at a predetermined air conditioning load when the automobile is driven at an average speed. Thus, when the vehicle engine is idling or is being driven at low speeds, the rotational speed of the compressor is correspondingly low. Therefore, the performance of the air conditioning system is adversely effected. On the other hand, when the vehicle is driven at high speeds, the rotational speed of the compressor is to high for efficient performance. Thus, electromagnetic clutches are used to control the rotational speed of the compressor under varying drive speeds by intermittently stopping and starting the compressor. 
     However, there are many problems associated with continuously cycling the clutch on and off. For example, when the engine is driven at high speeds and the capacity of the air conditioning system is large, it is necessary for the electromagnetic clutch to be turned on or off frequently. On the other hand, at low speed or when the vehicle engine is idling, the compressor is not sufficiently driven to maintain the desired temperature in the vehicle. 
     In order to solve the abovementioned problems, a system which controls the capacity of a compressor by detecting the temperature at the outlet side of the air conditioning system evaporator is proposed in published Japanese Patent Application No. 58-30. In such a system, the performance of the air conditioning system is not directly detected. For example, even though the temperature in the inside of the vehicle may be high, the capacity of the air conditioning system is reduced when the temperature at the outlet side of the evaporator becomes lower than a predetermined temperature. Thus, the capacity of the system is insufficient to cool the vehicle. In addition, when the vehicle is running, the capacity of the air conditioning system is changed frequently, thereby placing great stress and strain on the air conditioning system. 
     SUMMARY OF THE INVENTION 
     It is therefore the overall object of the present invention to provide a device for controlling the capacity of a variable type compressor in an automotive air conditioning system in order to provide a more reliable and durable system than those known in the prior art. 
     It is another object of the present invention to provide a more reliable and durable automotive air conditioning system than those known in the prior art without increasing the complexity or cost of the system. 
     The above objects of the present invention are achieved by providing a control device which includes a first temperature detecting sensor disposed forward of the evaporator for detecting a first air temperature at the inlet side of the evaporator, a second temperature detecting sensor disposed behind the evaporator for detecting a second air temperature at the outlet side of the evaporator and a control unit. The control unit compares the detected air temperature with predetermined temperatures and controls the capacity of the compressor in accordance with the compared results. 
     Further objects, features and advantages of the present invention will be understood from the following detailed description of the preferred embodiments of the invention with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an automotive air conditioning system in accordance with the present invention. 
     FIG. 2 is a flow chart illustrating the operation of the control system of the present invention. 
     FIG. 3 is a graph illustrating the relationship between a high air conditioning load and normal vehicle speed. 
     FIG. 4 is a graph illustrating the relationship between a high air conditioning load and high vehicle speed. 
     FIG. 5 is a graph illustrating the relationship between a low air conditioning load and normal vehicle speed. 
     FIG. 6 is a graph illustrating the relationship between a low air conditioning load and high vehicle speed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, there is shown an automotive air conditioning system which is driven by engine 1. The air conditioning system comprises compressor 1, condenser 3, receiver-dryer 4, expansion valve 5 and evaporator 6 located between outlet port 21 and inlet port 22. Compressor 2 is driven by engine 1 and is a variable capacity compressor of the scroll type or swash plate type design. The capacity of compressor 2 can be varied by operating capacity changing mechanism 11 upon a signal from control unit 10. Where compressor 2 is of the scroll type, capacity changing mechanism 11 comprises an electromagnetic bypass valve which connects the inlet of the compressor to the intermediate fluid pockets through an intermediate chamber as shown in published Japanese Patent Application No. 57-148089. 
     As shown in FIG. 1, evaporator 6 is disposed in duct 7. Sensor 8 is disposed at the inlet side of evaporator 6 and sensor 9 is disposed at the outlet side of evaporator 6. Sensors 8 and 9 are connected to control unit 10. Control unit 10 compares the detected temperature valves with predetermined values and then sends appropriate capacity control signals to capacity changing mechanism 11 to effect a change in the capacity of the compressor or to start or stop the operation of the compressor. 
     Heater 12 disposed in duct 7 is connected to engine 1 and receives coolant from engine 1 for heating the vehicle when the outside temperature is cold. 
     A damper 13 disposed forward of heater 12 controls the temperature of the discharged air by the angle of its opening being controlled. Blower 14 is also disposed forward of evaporator 6. With reference to FIG. 2, there is shown a flowchart which illustrates the operation of control unit 10. 
     When the air conditioning system is turned on in step 1, compressor 2 is operated at a predetermined small capacity (step 2). After the air conditioning system is operated for a predetermined time T (step 3), control passes to step 4. In the present invention time T may be, e.g., three seconds. In step 4, temperature TODB is detected by sensor 9 at the outlet side of evaporator 6 and is compared to predetermined temperature T4 in step 5. If the temperature TODB is higher than temperature T4, control pases to step 7. If temperature TODB is equal to or lower than temperature T4 control passes to step 6. 
     In step 6, a determination is made whether compressor 2 is operating. If compressor 2 is operating, control passes back to step 4. If, however, compressor 2 is not operating, control passes to step 7. 
     In step 7, temperature T air  is detected by sensor 8 at the inlet side of evaporator 6. A predetermined temperature T1 is substracted from temperature T air  and the resulting temperature is compared with a predetermined change in temperature ΔT. If the resulting temperature is greater than temperature ΔT, control passes to step 11 where the capacity of the compressor is changed to a high capacity. Control is then passed to step 16. If the resulting temperature is not greater than temperature ΔT, control is passed to step 8. 
     In step 8, temperature T air  is compared to predetermined temperature T1 and if T air  is greater than T1, control passes to step 10. Otherwise, control passes to step 9. In step 10, a predetermined temperature T2 equal to T2 MIN  is established and control is passed to step 12. In step 9, a predetermined temperature T2 equal to T2 MAX  is established and control is also passed to step 12. 
     In step 12, temperature TODB is compared to temperature T2 and if T2 is greater than TODB, control is passed to step 14 otherwise control is passed to step 13. In step 13, the capacity of compressor 2 is changed to a large capacity and control returns to step 4. In step 14, temperature TODB is compared to predetermined temperature T3 and if TODB is greater than T3, control is passed to step 15. In step 15, the capacity of compressor 2 is changed to a small capacity and control passes to step 16. 
     In step 16, temperature TODB is compared to predetermined temperature T5 and if TODB is greater than T5 control is returned to step 4. Otherwise, control is passed to step 17. In step 17, the operation of the compressor is stopped, e.g., by deactivating the electromagnetic clutch. Control is then returned to step 4. 
     FIGS. 3, 4, 5 and 6, shown the relationship between air conditioning load, temperature and compressor torque over time. The solid lines represent a compressor controlled in the manner of the present invention and the dotted line represents a compressor controlled in the manner known in the prior art by cycling the electromagnetic clutch. As the figures clearly show, the present invention provides an air conditioning system which is more efficient in its operation and more responsive to variations than such systems known in the art. 
     The invention has been described in detail in connection with preferred embodiments. These embodiments are examples only and the invention is not restricted thereto. It will be easily understood by those skilled in the art that variations and modifications can be made to the invention within the scope of the appended claims.