Compressor
Compressor
Compressors used in car air conditioners circulate refrigerant a cycle inside a closed system. They suck refrigerant from the outlet of the evaporator and deliver it under high pressure to the condenser. The relationship between the high and low pressure sides within the system varies depending on the rpms at which the compressor is turning in relation to the vehicle speed, the ambient temperature and the vehicle interior temperature.
When the pressure drops excessively on the low pressure side, the evaporator temperature decreases. This causes moisture in the air. passing through the evaporator to freeze, so that air can no longer flow through the evaporator. In order to prevent such freezing, conventional compressors utilize a thermo-switch which turns the unit on and off repeatedly.
However, a variable displacement compressor continually senses the pressure on the low pressure side and the amount of refrigerant so as to prevent the pressure from dropping too low. The variable compressor is basically a swash plate type that changes piston stroke in response to the required cooling capacity. The tilt of the swash plate allows the piston's stroke to change so that refrigerant discharge can be continuously changed between 3 cm3 (0.18 cu in) and 146 cm3 (8.91 cu in).
1) Operation Control Valve
The operation control valve is located in the suction port (low-pressure side) and opens or closes in response to changes in refrigerant suction pressure. Operation of the valve controls the internal pressure of the crankcase. The angle of the swash plate is controlled by the difference between the crankcase's internal pressure and the piston cylinder pressure.
2) Maximum Cooling
Refrigerant pressure on the low-pressure side increases with an increase in heat loads. When this occurs, the control valve's bellow compress to open the low-pressure side valve and close the high-pressure side valve. This causes the crankcase's internal pressure to equal the pressure on the low-pressure side and the cylinder's internal pressure to be greater than the crankcase's internal pressure. Under this condition, the swash plate is set to the maximum stroke position.
3) Capacity Control
Refrigerant pressure on the suction side is low during high speed driving or when ambient or interior temperature is low. The bellows expands when refrigerant pressure on the suction pressure drops below approximately 177 kPa (1.8 kg/cm2, 26 psi).
Since suction pressure is low, it makes the suction port close and the discharge port open. Thus, crankcase pressure becomes high as high pressure enters the crankcase.
The moment acts around the journal pin near the swash plate. It is generated by the pressure difference before and behind the piston.
The drive lug and journal pin are located where the piston generates the highest pressure. As the drive lug and journal pin receive this high pressure no moment occurs. Piston pressure is between suction pressure (Ps) and discharge pressure (Pd), and is near suction pressure (Ps). If crankcase pressure (Pc) rises due to capacity control, the moment around the journal pin makes the swash plate angle decrease and also the piston stroke decrease.