The present invention relates to a variable displacement compressor used in a refrigerant circuit of a vehicle air conditioner. More particularly, the present invention pertains to a displacement control apparatus and a displacement control method for the variable displacement compressor.
A typical refrigerant circuit of a vehicle air conditioner includes a condenser, an expansion valve, an evaporator and a compressor. The compressor receives refrigerant gas from the evaporator. The compressor then compresses the gas and discharges the gas to the condenser. The evaporator transfers heat to the refrigerant in the refrigerant circuit from the air in the passenger compartment. The pressure of refrigerant gas at the outlet of the evaporator, in other words, the pressure of refrigerant gas that is drawn into the compressor (suction pressure Ps), represents the thermal load on the refrigerant circuit.
Variable displacement swash plate type compressors are widely used in vehicles. Such compressors include a displacement control valve that operates to maintain the suction pressure Ps at a predetermined target level (target suction pressure). The control valve changes the inclination angle of the swash plate in accordance with the suction pressure Ps for controlling the displacement of the compressor. The control valve includes a valve body and a pressure sensing member such as a bellows or a diaphragm. The pressure sensing member moves the valve body in accordance with the suction pressure Ps, which adjusts the pressure in a crank chamber. The inclination of the swash plate is adjusted, accordingly.
In addition to the above structure, some control valves include an electromagnetic actuator, such as a solenoid, to change the target suction pressure. An electromagnetic actuator urges a pressure sensing member or a valve body in one direction by a force that corresponds to the value of an externally supplied current. The magnitude of the force determines the target suction pressure. Varying the target suction pressure permits the air conditioning to be finely controlled.
Such compressors are usually driven by vehicle engines. Among the auxiliary devices of a vehicle, the compressor consumes the most engine power and is therefore a great load on the engine. When the load on the engine is great, for example, when the vehicle is accelerating or moving uphill, all available engine power needs to be used for moving the vehicle. Under such conditions, to reduce the engine load, the compressor displacement is minimized. This will be referred to as a displacement limiting control procedure. A compressor having a control valve that changes a target suction pressure raises the target suction pressure when executing the displacement limiting control procedure. Then, the compressor displacement is decreased such that the actual suction pressure Ps is increased to approach the target suction pressure.
The graph of FIG. 8 illustrates the relationship between suction pressure Ps and displacement Vc of a compressor. The relationship is represented by multiple lines in accordance with the thermal load in an evaporator. Thus, if the suction pressure Ps is constant, the compressor displacement Vc increases as the thermal load increases. If a level Psl is set as a target suction pressure, the actual displacement Vc varies in a certain range (xcex94Vc in FIG. 8) due to the thermal load. If a high thermal load is applied to the evaporator during the displacement limiting control procedure, an increase of the target suction pressure does not lower the compressor displacement Vc to a level that sufficiently reduces the engine load.
Thus, the compressor displacement is not always controlled as desired as long as the displacement is controlled based on the suction pressure Ps.
Accordingly, it is an objective of the present invention to provide a displacement control apparatus and a displacement control method for a variable displacement compressor that accurately controls the compressor displacement regardless of the thermal load on an evaporator.
To achieve the above objective, the present invention provides a displacement control apparatus for a variable displacement compressor used in a refrigerant circuit of a vehicle air conditioner. The compressor is driven by a drive source of a vehicle. The apparatus includes a displacement control apparatus, a first device, a second device and a controller. The displacement control mechanism controls the displacement of the compressor based on the pressure difference between the pressures at two pressure monitoring points located in the refrigerant circuit. The pressure difference represents the displacement of the compressor. The first device detects external information representing the required cooling performance of the refrigerant circuit. The second device detects external information representing the load acting on the drive source. The controller determines a target value of the pressure difference based on the external information detected by the first device. The displacement control mechanism controls the displacement of the compressor such that the pressure difference seeks the target value and judges whether to set a limit value of the pressure difference based on the external information detected by the second device. When the limit value is set and a compressor displacement that corresponds to the target value is greater than a compressor displacement that corresponds to the limit value, the controller uses the limit value as the target value of the pressure difference to limit the compressor displacement.
The present invention may also be embodied in a method for controlling the displacement of a variable displacement compressor used in a refrigerant circuit of a vehicle air conditioner. The compressor is driven by a drive source of a vehicle. The method includes determining a target value of the pressure difference between the pressures at two pressure monitoring points located in the refrigerant circuit based on external information that represents the required cooling performance of the refrigerant circuit, the pressure difference representing the displacement of the compressor, controlling the compressor displacement such that the pressure difference seeks the target value, judging whether to set a limit value of the pressure difference based on external information that represents the load acting on the drive source, and using the limit value as the target value of the pressure difference when the limit value is set and when a compressor displacement that corresponds to the target value is greater than a compressor displacement that corresponds to the limit value.