When a belt-driven piston-type A/C compressor is engaged, it produces a noticeable ‘thump’ and a reduction in engine power. This can be eliminated by keeping the compressor engaged all the time and controlling refrigerant flow by varying the displacement of the compressor.
The variable displacement compressor is an axial piston design, with the pistons driven by a wobble plate or a swashplate. Since the angle of that plate determines the length of the piston stroke, changing that angle changes the length of the stroke, therefore changing the amount of refrigerant pumped (displaced) on each stroke. The plate angle is controlled with linkage and springs, and it’s adjusted by changing refrigerant pressure in the compressor housing. So the key to controlling displacement is controlling housing pressure.
When housing pressure is increased, pressure on the backside of the pistons keeps them “higher” in their bores, closer to the cylinder head. This reduces the angle of the swashplate and shortens the stroke, reducing displacement. When housing pressure decreases, a spring pushes the adjusting linkage away from the cylinder head, increasing plate angle and lengthening the piston stroke to increase displacement. Housing pressure is controlled by a valve with ports and passages that connect to the suction (low side) and discharge (high-side) chambers of the compressor head.
Two different types of control valves are used; mechanical and electronic. The mechanical valve has a precision diaphragm that senses low-side pressure. When the cabin is warm, evaporator temperature increases, which increases low-side pressure and collapses the diaphragm. A port opens to vent housing pressure to the suction side of the compressor head. This decreases housing pressure and increases piston stroke, increasing refrigerant flow through the system.
As evaporator temperature decreases, so do low-side pressure. The diaphragm expands to close the low-side vent port and at the same time opens a port that admits high-side pressure into the housing. Higher pressure reduces piston stroke and refrigerant flow volume. Remember, changing flow volume doesn’t change pressure, so a diaphragm-type control valve remains stable.
Delphi’s mechanical displacement control valve is actually two valves in the same body. When low-side pressure is high, the bellows collapse (towards the left). The cone valve opens and pressure vents from the wobble plate case (compressor housing) through the by-pass hole and to the suction side of the compressor head, reducing housing pressure. When low-side pressure is low, the bellows expands to close the cone valve and open the ball valve. Pressure vents from the discharge cavity (high-side) to the wobble plate case, increasing housing pressure.
Around 2001, DENSO introduced a pulse-width-modulated solenoid valve to control housing pressure, based on information from temperature and pressure sensors in the refrigerant system. With a computer controlling the valve’s duty cycle, compressor displacement can be used to control evaporator temperature, rather than the other way around.
The mechanical control valve is still used in some systems because it’s inexpensive and reliable, but its range of control is limited. Electronic control valves are becoming more common, and on many applications, there is no clutch, so the compressor runs continuously. Displacement can be reduced to about 1 percent when cooling is not needed, keeping seals lubricated, minimizing oil pooling and preventing other kinds of damage that result from long periods of inactivity. Ultimately there is less load on the engine when the A/C is in use, reducing the system’s contribution to tailpipe emissions. As mileage and emissions regulations continue to tighten, we can expect to see electronically-controlled variable displacement compressors on more new models.