Air Conditioning: Compressor

Vehicles with 5-cylinder engines have a compressor with fixed displacement (fixed cylinder displacement) (2).

Vehicles with 4-cylinder engines have a compressor with variable displacement (1 and 3).

In vehicles with the B418S11 engine the compressor is located on the rear edge of the engine. In vehicles with other engine variants the compressor is located on the front edge of the engine.

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

Compressors with fixed displacement have two limits:

• off
• on.

Air conditioning (A/C) pressure switch 

The compressor is controlled by the air conditioning (A/C) pressure switch, which is a mechanical low pressure switch. The air conditioning (A/C) pressure switch is located between the evaporator and the receiver. In vehicles with electronic climate control (ECC), the compressor is also controlled by the temperature after the evaporator.

Compressors with variable displacement (variable cylinder displacement) are not switched off during normal driving. The flow of refrigerant is continually adapted to requirements. The compressor operates between min. and max. cylinder displacement by:

• the pistons are driven by a cam pulley, the angle of which can vary
• when the compressor is disengaged, the angle is determined by the springs
• when the compressor is engaged, the angle is determined by the pressure affecting the top side of the pistons (= intake pressure) and lower side (= pressure in the crank case) during the intake phase
• the pressure on the lower side of the pistons (= in the crank case) is governed by a valve which keeps the intake pressure constant.

High intake pressure = large cylinder displacement

• The valve opens and lowers the pressure in the crank case. The back pressure on the reverse of the pistons is then reduced and the angle of the cam pulley increases
• An increased angle gives an increased cylinder stroke which results in a greater volume of refrigerant being "sucked in" and the inlet pressure dropping.

Low intake pressure = small cylinder displacement

• The valve closes and the pressure in the crank case increases. The pressure is built up by the refrigerant which is led via a calibrated channel from the outlet side to the crankcase. The back pressure on the reverse of the pistons is then increased and the angle of the cam pulley reduces
• A reduced angle gives a reduced cylinder stroke which results in a smaller volume of refrigerant being "sucked in" and the inlet pressure increasing.

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

The compressor is also controlled by the air conditioning (A/C) pressure sensor (gasoline engines) or the air conditioning (A/C) pressure switch (high pressure switch) (diesel engines), which switches off the system if the pressure goes up to 3.1 MPa (31 bar).

The sensor or switch also controls the engine cooling fan (FC).

The air conditioning (A/C) pressure sensor or air conditioning (A/C) pressure switch is located by the firewall on the high pressure pipe.

The appearance of sensors and switches differs slightly.

A/C pressure sensor (gasoline engines) 

A/C pressure switch (diesel engines) 

The compressor also has a relief valve, located on the rear section of the compressor, which functions as an extra safety device. The valve opens and releases refrigerant when the pressure in the system is too high (at approx. 3.5 MPa (35 bar)). The valve then shuts again when the pressure has returned to normal.

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