Air Conditioning: Compressor

Fig 1: Identifying Compressor

Courtesy of VOLVO CARS CORPORATION

The compressor is mechanical and is driven by the vehicle engine. When the compressor is operating it "steals" 0.5-8 kW (0.7-11 bhp) from the engine. This may be noticed as slight jerks when driving as the compressor is engaged / disengaged.

The task of the compressor is to:

draw gaseous refrigerant from the evaporator
compress the gas thereby increasing its pressure and temperature
expel the gas with high pressure and high temperature to the condenser.

In ideal conditions the compressor compresses the refrigerant from about 200 kPa (2 bar) to between 1.2 MPa and 2.1 MPa (12 and 21 bar). During the process the refrigerant is heated up from 0 °C to between 70 °C and 110 °C. These pressure and temperature values apply to normal working pressure in the system's high-pressure side, when the system works under optimal conditions.

The pressure relief valve located at the rear of the compressor, acts as an extra safety device. The valve opens and releases refrigerant when the pressure in the system becomes too high (at about 3.5 MPa (35 bar)). The valve closes once again when the pressure returns to its normal value.

The temperature of the refrigerant gas can increase to as much as 125 °C.

The compressor can only compress gases, as liquid would damage the compressor.

The compressor is in the refrigerant circuit, located between the evaporator and the condenser.

The compressor has a variable displacement. The compressor mounting and pipe connections vary depending on the engine in the vehicle.

Compressors with variable displacement usually do not switch off during normal operation. The flow of refrigerant is continually adapted based on need. The compressor works between min. and max. displacement due to the following:

the pistons are driven by a cam disc with a variable angle
the angle is determined by springs if the compressor is disengaged
if the compressor is engaged, the angle is determined by the pressure exerted on the top of the pistons (= intake pressure) and the bottom of the pistons (= pressure in the crankcase) during the intake phase
the pressure on the underside of the pistons (= in the crankcase) is regulated by a valve, which attempts to keep the intake pressure constant

High intake pressure = large displacement.

The valve opens and lowers the pressure in the crankcase. The counterpressure on the back of the pistons then reduces and the cam disc angle increases.
The increased angle generates increased stroke, which causes a greater amount of refrigerant to be "sucked in" and a drop in intake pressure.

Low intake pressure = small displacement.

The valve closes and pressure in the crankcase increases. The pressure is built up by refrigerant that is led from the outlet side to the crankcase via a calibrated duct. The counterpressure on the back of the pistons then increases and the cam disc angle decreases.
The decreased angle generates a reduced stroke, which causes a smaller amount of refrigerant to be "drawn in" and an increase in intake pressure.

The compressor is lubricated with specially developed refrigerant oil. This oil (synthetic PAG oil) is mixed with the refrigerant when the air conditioning system is in operation.

A/C switch

Fig 2: Identifying A/C Switch

Courtesy of VOLVO CARS CORPORATION

The compressor is controlled by a linear sensor. It acts as a switch than switches off the compressor if the pressure in the cooling system becomes too high or too low. The A/C switch also controls the engine cooling fan. The A/C switch is located on the cooling system's high-pressure side directly before the evaporator. In the engine compartment the A/C switch is located in the cold zone under the plenum cover.

Magnetic clutch assembly

Fig 3: Identifying Magnetic Clutch Components

Courtesy of VOLVO CARS CORPORATION

Number	Designation	Number	Designation
1	Carrier disc	4	Pulley.
2	Shim discs	5	Magnet coil.
3	Spring ring	-	-

The compressor is driven by the engines crankshaft via the drive belt. As soon as the engine starts, the belt pulley (4) on the compressor's drive shaft runs unimpeded. When the climate system is switched on, the current runs through the magnetic coil (5) which becomes magnetic. This presses the carrier disc (1) on the compressor's drive shaft forward against the pulley.

The clutch engages and the compressor accelerates at engine speed.

When the current to the magnetic coil is interrupted, the carrier disc (1) is released from the pulley (4) with the aid of return springs.

When the compressor is disengaged the pulley runs freely without any influence from the compressor. When the current flows through the magnetic coil, the flexible plate is drawn in and engages with the pulley. Power is then transferred from the pulley via the plate to the compressor shaft.

In order for the magnetic clutch to function correctly, the distance between the carrier disc (1) and the pulley (4) must be exactly correct. Shim discs (2) are therefore placed between them.