How does an auto AC compressor work?
The compressor is the heart of the air-conditioning system because it circulates or pumps refrigerant and oil through the system.
The compressor is responsible for two main functions required by air-conditioning system.
- One function of the compressor is to raise the pressure of the refrigerant.
- It is critical that the refrigerant be hotter than the temperature of the ambient (surrounding) air; otherwise, no heat transfer would take place.
- The second function of the compressor is to create low pressure in the evaporator. The lower pressure condition in the evaporator allows the refrigerant to vaporize (boil), enabling the refrigerant to absorb large quantities of heat energy from the cab of the vehicle.
- The compressor also circulates the refrigerant and compressor oil, which are mixed together throughout the air-conditioning system.
Failure of either of the compressor’s two main functions would result in a loss or reduction of refrigerant circulation within the air-conditioning system.
Without proper refrigerant circulation, the cooling process of the air-conditioning system will be reduced or will stop working altogether.
Air conditioning compressors come in may different styles, but all perform the same function. In most cases, the compressors are mounted near the front of the engine. The compressor is belt-driven from the engine crankshaft. The compressor’s drive pulley is incorporated into an electromagnetic clutch. This clutch provides a means of turning the compressor on or off, depending upon the temperature requirements of the system.
Two refrigerant lines are attached to the compressor, one being the discharge line and the other the suction line. The refrigerant lines can always be identified by their physical size. The suction line is always larger in diameter than the discharge line.
The suction (inlet) side draws in low-pressure, low-temperature refrigerant gas from the evaporator. The compressor then pumps out high-pressure, high-temperature refrigerant gas to the condenser.
Two-piston-type compressor operation.
Intake Stroke: Whenever the piston is moving in a downward direction, it can be considered to be on an intake stroke. As the piston moves down in the cylinder, the volume becomes greater and the pressure is reduced. The refrigerant gas is then drawn into the cylinder from the suction (low pressure) side. To accomplish this, the refrigerant must first enter the intake port and push the intake reed valve down (open) so that it may be drawn into the cylinder.
Compression Stroke. When the piston then starts its upward travel, the volume within the cylinder is reduced, causing a rise in pressure. The instant the pressure within the cylinder exceeds that of the suction line, the intake reed valve closes. The closing of the inlet valve allows the piston to continue to raise the pressure within the cylinder. The discharge reed valves are held closed by springs so that they act like a check valve. Once the pressure within the cylinder exceeds the force of the closing springs and the refrigerant pressure in the discharge line, the valve opens, allowing the high pressure refrigerant gas to be pushed out into the discharge port and eventually into the discharge line.
Swash Plate Compressor Operation.
Intake Stroke. As the swash plate pulls the piston down in the cylinder, it creates a drop in pressure. This pressure is lower than that of the refrigerant in the suction line. This imbalance in pressure forces the higher pressure of the refrigerant within the suction line to open the suction reed valve, allowing the cylinder to be filled with refrigerant gas.
Compression Stroke.Further rotation of the swash plate changes the direction of the piston within the cylinder. As the refrigerant is compressed slightly, it creates a pressure higher than that contained within the suction line. This pressure imbalance causes the suction reed valve to close, preventing refrigerant from exiting the cylinder through the suction port. The pressure within the cylinder rises to a point where the discharge reed valve opens, allowing the superheated refrigerant to be pushed out of the cylinder into the discharge port and on out the other components of the air-conditioning system.
Rotary Vane Compressor. The compressor drive pulley is belt-driven from the engine’s crankshaft pulley. Turning the rotor assembly causes the vanes to extend by centrifugal force, and causes them to seal against the cylinder wall. As the vane passes the suction inlet, the volume of the chamber increases, causing a lower pressure that draws refrigerant into the cylinder. Once the vane passes the largest portion of the eccentric, the vane will be pushed back into its bore as if follows the eccentric shape. This causes the volume of the chamber to decrease, squeezing the refrigerant between the vane and the cylinder wall, and increasing the pressure and temperature of the refrigerant. The high-pressure, high-temperature refrigerant is then pushed out the discharge port and through the discharge valve out into the discharge line.
Variable displacement compressor operation.
The pistons of the compressor are driven by a variable angle swash plate. The angle of the swash plate is changed by a bellows-activated control valve located in the rear head of the compressor. The control valve senses the suction pressure and controls the swash plate angle based on crankcase suction pressure differential. Operation of the control valve is dependent on differential pressure.
Scotch Yoke Compressor operation.
The operation of this compressor is similar to that of all the other piston compressors. During the intake stroke, the piston moves down in its bore and refrigerant vapor is drawn into the cylinder through the suction reed valve. The piston changes its direction of travel on the exhaust stroke and compresses the refrigerant vapor to a high-pressure gas, pushing it through the discharge valve plate on the exhaust stroke.
Scroll Compressor Operation.
The scroll compressor operates by rotating one scroll within a stationary scroll. The ends of the rotating scroll coop up the refrigerant vapor at the suction port of the compressor. As the scroll continues to rotate, the inlet passage is sealed off and the volume of the passage becomes smaller, increasing the pressure of the refrigerant vapor. The refrigerant vapor is squeezed through to the discharge passage at the center of the scrolls. The refrigerant vapor is at a higher pressure and temperature as it leaves the discharge port.
The scroll’s vapor passages are continuously at various stages of compression at the same time. This way the scroll compressor is able to supply a smooth, stable suction and discharge pressure.
Automotive A/C compressor repair & maintenance.
Two-piston-type compressor Maintenance.
This compressor has a few basic areas of maintenance:
- The oil level of this compressor should be checked, but this can only be performed when the air-conditioning system is empty (refrigerant recovered).The oil level of a new replacement compressor must also be checked before installation.
- Compressor crankshaft seals are consumable items that will eventually leak and require replacement. For this procedure, the refrigerant must be removed from the system.
- Replacing the valve plates or gaskets is another maintenance task. Again, for this procedure the refrigerant must be removed from the system.
- The compressor’s electromagnetic clutch may also be replaced. This is one of the few repair procedures that may be performed while the unit is fully charged.
- Belt tension should be checked on a regular basis. Because the compressor is belt driven, proper belt tension is important for proper air conditioning operation.
- Compressor main seals replacement.
Swash Plate Compressor Maintenance.
Just as with the two-piston-type compressors, swash plate compressors have some service/maintenance issues:
- The compressor oil must be checked whenever the air-conditioning system is discharged.
- Replacement of the compressor shaft seal is a maintenance issue.
- Replacement of the valve plate or gasket (for a non-operating or leaking compressor) is also a maintenance issue.
- Compressor main seals replacement
What type of compressor is used in car AC?
There are many different manufacturers and styles of air-conditioning compressor, but the all operate in the same basic way. Some. These compressors may again be broken down by:
- Piston and cylinder arrangement
- Compressor mounting
- Type and number of drive belts
- Compressor displacement
- Fixed or variable displacement
These compressors may be constructed of either steel or aluminum. This compressor requires approximately 14 horsepower from the engine when it is operating. You still will find these compressors on older vehicles, but because of their weight and the drag they put on the engine, they are not used much today. Today’s automotive manufacturers use much more efficient compressors to increase vehicle mileage.
This style of compressor uses a reed valve assembly located above the pistons to control the flow of refrigerant into and out of the compression chamber. The crankshaft and housing must be sealed to prevent refrigerant and oil leaks to the atmosphere.
Swash plate compressors
Swash plate compressors get their name from the means by which the pistons are driven, a swash (slanted) plate mounted on a revolving shaft. As the swash plate (sometimes called a wobble plate) turns, it pushes and pulls the pistons back and forth to draw in, compress, and discharge the refrigerant gas.
These compressors are available in different configurations and numbers of cylinders. Each piston has a rod with ball-and-socket-type bearings that fit into a slipper (foot of the piston).The rotation of the swash plate causes reciprocating (up and down) movement of each piston inside its cylinder. The valve plate contains a reed valve assembly. These reed valves allow refrigerant to flow in only one direction, not unlike a check valve. Many manufacturers use swash plate compressors because they put less drag on the engine, approximately 7 horsepower, compared to the 14 horsepower consumed by the two-piston-type compressor.
Rotary vane compressors
The rotary vane compressor does not use a piston to raise the pressure of the refrigerant. These compressors were used for many years in the light truck market. This style of compressor also does not use a suction valve.However; it does incorporate a discharge valve. The discharge valve serves as a check valve, preventing high-pressure refrigerant vapor from migrating back into the compressor discharge port during the compressor off cycle or when the air-conditioning system is not operating. This compressor performs exactly the same function in the air-condition system as the piston-type or swash-plate-type compressor, but the operation of the rotary vane compressor is entirely different. The rotary vane compressor operates much like a vane-style power steering pump.
Variable displacement compressors
Variable displacement compressors automatically adjust their displacement to match the vehicle’s air conditioning demands. These compressors are used in both the light and medium-duty truck market. A control valve senses the evaporator load and automatically changes the displacement of the compressor. Unlike cycling clutch systems these variable displacement compressors run continuously without any clutch cycling. Temperature is maintained by changing the capacity of the compressor, not by engaging or disengaging the clutch. This feature allows the system to cool more softly and uniformly. It also eliminates the noise problems associated with cycling clutch systems. Dehumidification and fuel economy are also improved.
Scotch yoke compressors
Scotch Yoke compressors have been used for many years in the air-conditioning industry. On this compressor, opposed pistons are pressed onto opposite ends of a yoke. The yoke rides on a slider block located on the shaft eccentric of the compressor. Rotating the shaft also turns the yoke with its attached pistons. This causes pistons to reciprocate, following the eccentric contour. Each of the four pistons contains a suction reed valve, and a reed valve plate is located on top of each cylinder
The beauty of the scroll compressor is that it has only one moving part. The scroll compressor has not yet been used in the heavy truck market, but because it puts less drag on the engine, it is probably only a matter of time before it gains popularity.