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Unlike the old fixed displacement compressor (FDC), the variable displacement compressor (VDC) automatically varies its pumping capacity to meet air conditioning demands. When the temperature of the car cabin is high, it increases its cooling capacity until it reaches the desired temperature. Once the desired temperature is reached, it automatically reduces its ability to maintain the desired temperature.
With a VDC, there is no motor jerk caused by the compressor clutch turning on and off (as in FDC). In fact, some VDCs don’t have a clutch at all. This results in very smooth operation and improved fuel consumption.
Two types of variable displacement compressor (Fig. 1a and Fig. 1b)
There are two types of VDC in common use: internally controlled variable displacement compressor (ICVDC) and externally controlled VDC (ECVDC). Figure 1a shows an ICVDC and Figure 1b shows an ECVDC. They have basically the same internal structure. They differ only in the way the travel control valve is actuated. In an ICVDC, the control valve is actuated by the pressure of the refrigerant in the suction chamber of the compressor by means of a bellows or diaphragm. In an ECVDC, activation of the control valve is done by the engine ECU or by an external electronic module using a solenoid actuator. Observe the solenoid actuator harness (Fig. 1b). The solenoid is inside the valve.
Externally controlled variable displacement (ECVDC) compressors have much better control of piston displacement, and therefore temperature, compared to ICVDC. This makes the clutch completely unnecessary in ECVDC, as shown in Fig. 1b.
The internal structure of a variable displacement compressor (VDC) (Fig. 2)
Figure 2 shows the internal components of a VDC.
Internally Controlled Variable Displacement Compressor (ICVDC) – Unloaded and Stopped (Fig. 3)
When the compressor is not charged with refrigerant, the swash plate is held in the minimum angle position by the spring on the shaft (Fig. 3). The displacement control valve (DCV) bellows is in an expanded condition, closing the low-side port while opening the high-side port.
ICVDC loaded but not working (Fig. 4)
When the system is loaded and the compressor is not running, the pressure in all the compressor chambers is the same. This pressure causes the DCV bellows to contract, opening the low-side port, while closing the high-pressure port. See figure 4.
ICVDC loaded and running: reaches its maximum displacement (Fig. 5)
The slightly inclined position of the swash plate creates a small offset in the compressor’s compression chamber (Fig. 4). When the compressor runs, the swash plate that rotates with the shaft wobbles a bit. This wobbling action causes the pistons to move back and forth with a short stroke.
On each suction stroke of the pistons, a small volume of refrigerant is drawn into the compression chamber through the suction reed valve, which is then pumped into the discharge chamber through the discharge reed valve at each compression stroke of the pistons. This increases the pressure in the compressor discharge chamber, while reducing the pressure in the suction chamber. Since the low side port of the displacement control valve (DCV) is open at this stage, the same reduced pressure is present in the control chamber.
The pressure of the refrigerant in the control chamber and the spring around the shaft exert a combined force (F2) on the back of each piston. As pumping continues, there comes a time (and it won’t take long) when the force at the rear of each piston (F2) is less than the force exerted on the head of each piston (F1) by the coolant in the compression. camera.
When F1 is greater than F2, the pistons opposite the pivot are pushed to the left by the resultant force (F1 minus F2). This increases the swash plate angle and, of course, the piston displacement. In effect, more refrigerant is drawn from the suction chamber and pumped into the discharge chamber. This further increases the pressure in the discharge chamber and decreases the pressure in the suction chamber. And since the low side port of the control valve is still …