This invention relates to variable capacity wobble plate compressors mainly adapted for use in air conditioning systems for automotive vehicles, and more particularly to an improved wobble plate compressor in which the crankcase pressure is controllable for varying the displacement or capacity of the compressor.
A variable capacity wobble plate compressor in general is adapted to vary its displacement or capacity through a change in the angularity of the wobble plate. It is known e.g. from U.S. Pat. No. 3,861,829 to vary the refrigerant pressure in the crankcase for changing the angularity or angle of inclination of the wobble plate relative to the drive shaft. A conventional wobble plate compressor of this type comprises a fluidtight housing, a drive shaft rotatably disposed in the housing, a cylinder block arranged in the housing and formed therein with a plurality of cylinders circumferentially arranged around the drive shaft and extending substantially parallel to the axis of the drive shaft, pistons received in the respective cylinders for reciprocating motions therein, and a wobble plate supported at its diametrically central portion by trunnion pins extending at right angles to the drive shaft and axially movable therealong and also supported at its peripheral edge by a pivot pin rotatable about the drive shaft together therewith. The wobble plate is adapted to be pivotally displaced in unison with axial movement of the trunnion pins along the drive shaft to have its angularity varied relative to the drive shaft. As the wobble plate rotates in a position inclined relative to the drive shaft, the pistons are reciprocatingly moved in their respective cylinders for pumping actions. In the compressor, the resultant reaction force exerted by all the pistons, some on their compression strokes and some on their suction strokes, acts upon the wobble plate at a point inside a half portion of the circumference described by the axes of the cylinders, which is located at the same side of the drive shaft as the pistons on their compression strokes, so that the wobble plate is acted upon by the above resultant reaction force to become inclined relative to the drive shaft about the trunnion pins as a movable fulcrum during the pumping actions of the pistons. The resultant reaction force of the pistons counteracts the pressure in the crankcase which acts upon the pistons as back pressure. Therefore, when there occurs a drop in the pressure in the crankcase, the wobble plate is displaced in the angularity-increasing direction to increase the capacity, whilst when there occurs an increase in the crankcase pressure, the wobble plate is displaced in the angularity-decreasing direction to decrease the capacity.
In this arrangement, it is known that the difference between values of the crankcase pressure corresponding, respectively, to the maximum angularity of the wobble plate and the minimum angularity thereof falls within a range from 5 to 10 percent of the difference between the suction pressure and the discharge pressure of the compressor. For example, if the compressor is operating in a condition wherein the discharge pressure is 14 kg/cm.sup.2, and the suction pressure is 2.1 kg/cm.sup.2, the crankcase pressure has to be controlled within a very small range from approximately 2.7 kg/cm.sup.2 to approximately 3.3 kg/cm.sup.2, with a small pressure difference of approximately 0.6 kg/cm.sup.2. This means that the angularity of the wobble plate varies in high response to a change in the crankcase pressure. This requires precise control of the crankcase pressure, making it difficult to control the capacity in a stable manner.