Distinguishing features: external adjustment via control valve N280; hollow pistons; belt drive (no electromagnetic clutch).
Belt drive: When the unit is switched off, the compressor continues to run. However, its performance is less than 2%. Since there is no electromagnetic clutch, checking the air conditioning system by inspection and listening is no longer possible.
Compressor at zero flow
The working volume of the compressor varies by changing the tilt of the rocker (oblique) washers. When the compressor is not engaged, the swash plate is in the vertical position (piston stroke is less than 2%). Changing the position of the swash plate is controlled by changing the ratio of different pressures in the compressor.
Inlet pressure
The pressure in the low pressure circuit of the system, i.e. the pressure of the refrigerant before the compressor.
High pressure (pressure in the high pressure circuit)
The refrigerant pressure behind the compressor is the main pressure that changes the position of the swash plate to increase capacity.
Compressor case pressure
Back pressure in the compressor housing combined with the swashplate return spring force repositioning the swash plate to reduce capacity.
The regulating valve under their influence is responsible for the balance of these pressures. The main influence on the final balance of forces is exerted by the pressures in the compressor housing and the high pressure circuit. The high pressure in the compression chamber acts on the piston and tries to bring the swashplate into an inclined position, i.e. a high displacement position. The pressure in the housing provides a force that tends to bring the swash plate into a vertical position.
Compressor at maximum capacity
The solenoid control valve N280 has a spool and pressure element. The inlet pressure determines the position of the pressure element, and therefore how far the spool is extended. If a higher cooling rate is requested, the E87 control and display unit will actuate the control valve. In this case, the spool moves down and reduces the flow area at the point where the high pressure circuit (dark green color) connects to the cavity inside the body (yellow). As a result, high pressure begins to dominate (case pressure drops) and through the piston brings the wobble washer into an inclined position.
If a lower cooling rate is required, the orifice between the high pressure circuit and the housing cavity is increased. This results in pressure equalization in the compression chamber and the compressor housing. As a result, the return spring drives the wobble plate to the zero feed position. With a duty cycle of 40-0 Hz, the N280 control valve can be forced to hold the spool in "floating" position to achieve optimum pressure control.
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