VDOT A/C System

Refrigeration System
The Variable Displacement Orifice Tube (VDOT) refrigeration system has a V7 compressor that can maintain the A/C demand under all conditions without cycling. The basic compressor mechanism is a variable angle wobble-plate with axially oriented cylinders. The control of the compressor displacement is a bellows actuated control valve located on the rear head of the compressor. The control valve senses suction pressure. The crankcase to suction pressure differential controls both the wobble-plate angle and the compressor displacement. When the A/C capacity demand is high, the suction pressure will be above the control point. The control valve will maintain a bleed from crankcase to suction. A low crankcase-suction pressure differential gives the compressor maximum displacement. When the A/C demand is lower and the suction pressure reaches the control point, the valve will bleed discharge gas into the crankcase and close off a passage from the crankcase to the suction plenum. A force balance on the pistons controls the angle of the wobble-plate. A slight increase in the crankcase-suction pressure differential creates a force on the pistons, resulting in a movement about the wobble-plate pivot pin that reduces the plate angle.

The compressor has a unique lubrication system. The crankcase-suction bleed is routed through the rotating wobble-plate for lubrication of the wobble-plate bearing. The rotation acts as an oil separator, removing some of the oil from the crankcase-suction bleed, and then reroutes the oil to the crankcase where the oil lubricates the compressor mechanism.

Depending on engine usage, the compressor is cut off. Following are some examples of conditions resulting in compressor cutoff:

- Wide open throttle

- Low idle speed

- Low air temperature

- High P/S loads

When the engine is turned to OFF with the A/C system running, the refrigerant in the system will flow from the high pressure side of the evaporator tube orifice to the low pressure side until the pressure is equalized. This may be detected as a hissing sound for 30-60 seconds and is a normal condition.