Introduction:
The compressor pumps the refrigerant gas from the air conditioner throughout the entire system. The pressure and temperature of the refrigerant increase as it leaves the compressor. Various types of compressors can be used for air conditioning. Modern car air conditioning systems use reciprocating compressors. “Reciprocating” means that the parts in the compressor make back-and-forth movements. The operation of these compressors can be compared to that of a piston engine. Reciprocating compressors are also available in two types: the crank type and the slant plate compressor. In modern cars, slant plate compressors are used, which are categorized into two types: slant plate compressors with fixed stroke and those with variable stroke. The air conditioning pump, like the alternator and power steering pump, is driven by the serpentine belt in combustion engines (see image below). In hybrid and fully electric vehicles, electric air conditioning compressors are found. An electric motor is powered by the HV system and drives the compressor.
The air conditioning compressor draws in gaseous refrigerant from the evaporator, keeping the pressure in the evaporator low and assisting the refrigerant to evaporate even at low temperatures. The compressor compresses the gaseous refrigerant, causing a transition from low to high pressure. This pressure increase and temperature rise cause the refrigerant to change from a gaseous to a liquid state.
The pressure provided by the air conditioning compressor is influenced by various factors, including:
- Engine RPM (in combustion engines);
- The type and amount of refrigerant;
- The temperature of the refrigerant;
- The type and design of the air conditioning compressor, which determines the capacity;
- The setting of the magnetic clutch;
- The ambient temperature.
After compression, the refrigerant leaves the compressor with a temperature of about 70 degrees Celsius. This temperature is then reduced in the condenser.
The following sections discuss various versions of air conditioning compressors, some of which are used in the automotive industry.
Vane/Slide Pump:
This pump is rarely used in a car’s air conditioning system. However, it can be applied in specific cooling installations for various products.
Operation: The (gray) disk rotates to the right, clockwise. The yellow plungers are pressed against the wall by centrifugal force, which separates the different chambers. At the bottom right, the refrigerant flows in and follows its path to the small blue space. The rotation causes this space to enlarge, leading to a vacuum. The pump continues to rotate, causing the refrigerant to enter the red section. Here, the chamber space becomes progressively smaller, compressing the refrigerant. At the end of the red chamber, there is the outlet valve, which forces the refrigerant out.
Reciprocating Compressor (crank type):
Like the vane/slide pump, this pump is rarely used in a car’s air conditioning system. However, it can also be applied in specific cooling installations for various products. In the image below, a reciprocating compressor is shown, with 1 representing the intake valve and 2 the exhaust valve. The movement of the piston and crankshaft is similar to that of a regular Otto or diesel engine.
Operation: The piston moves from TDC (Top Dead Center) to BDC (Bottom Dead Center) (from top to bottom), causing the intake valve 1 to open. The refrigerant is drawn into the cylinder by vacuum. The piston then moves from BDC to TDC, pressing the intake valve back against its seat. The upward movement also lifts the exhaust valve 2 off its seat. The refrigerant can now exit the cylinder. The exhaust valve closes again, and the cycle begins anew.
Slant Plate Compressor Introduction:
Slant plate compressors, also known as wobble plate compressors, are almost always used in automotive air conditioning systems. They belong to the “reciprocating” category due to their moving parts that move up and down.
In the illustration, we see a schematic and cross-section of a slant plate compressor. The piston makes a horizontal stroke, determined by the angle of the slant plate. In this image, the plate is maximally tilted, meaning that the piston can make a maximum horizontal movement (indicated by the red compression area in the cylinder). In the three drawings (from top to bottom), we see a complete compression stroke of a piston as a result of the rotation of the slant plate.
In this situation, the pump delivers maximum output because the slant plate has made a maximum stroke. If lower output is desired due to excessive pressure, causing the evaporator to potentially freeze due to too much refrigerant, in a compressor with a “fixed stroke,” the magnetic clutch is disengaged, stopping the compressor drive. In a compressor with a “variable stroke,” the plate is adjusted to be less “tilted.” The angle at which the plate tilts is smaller, reducing the piston stroke. Compressors with fixed and variable stroke are described later on the page.
Above each piston are two valves attached to a dish plate spring: the intake valve and the exhaust valve. When the piston moves from TDC to BDC, it pushes the refrigerant past the exhaust valve into the high-pressure line toward the condenser.
Slant plate compressors can have between 4 and 8 pistons/plungers and come in two versions:namely the compressor with a fixed stroke and the one with a variable stroke.0These are described below.0
Slant Plate Compressor with Fixed Stroke:
This compressor is driven by the engine’s serpentine belt and runs in sync with the engine RPM (between 600 and 6000 RPM). The magnetic clutch controls the engagement and disengagement of the compressor, which is further explained later.
When the compressor is engaged, the rotating slant plate moves the pistons up and down. Intake and exhaust valves at each cylinder allow the pistons to draw in gas and pressurize it into the system’s high-pressure section.
A compressor with a fixed stroke displaces a fixed volume per revolution. Therefore, output depends on the compression speed, i.e., engine RPM. To regulate the output, the compressor is continuously cycled on and off: engaging when pressure drops and disengaging when pressure is too high. Particularly in small engines, engagement can feel like a “shock” due to the required power. Abrupt engagement causes higher mechanical load and disrupt regulation, resulting in variations in cooled air temperature for occupants.
At high engine RPM and thus increasing discharge pressure, more refrigerant flows through the evaporator. This delays cooling and can freeze the evaporator. In such cases, the magnetic clutch is disengaged by the thermostat or pressure switch.
Slant Plate Compressor with Variable Stroke:
In this type of compressor, the angle of the slant plate is adjustable thanks to an adjustment mechanism. By setting the slant plate as upright as possible, the piston stroke is limited, and the output is minimal. Conversely, by placing the slant plate as inclined as possible, the pistons make a much larger stroke, significantly increasing the output. We see the following versions of the slant plate compressor with variable stroke:
- with internal control and magnetic clutch;
- external control with and without magnetic clutch.
Internal Control and Magnetic Clutch:
The image shows how the position of the slant plate can influence the piston’s stroke. Higher engine RPM results in greater compressor output. This causes a pressure increase throughout the system, prompting the adjustment mechanism to increase the pressure in the slant plate chamber.
The increased pressure forces the slant plate to become more upright, reducing capacity. If output decreases, the adjustment mechanism closes, reducing pressure in the slant plate chamber. This causes the plate to tilt again, allowing the pistons to make a larger stroke. The greater the angle, the greater the stroke and output.
In an internal (mechanical) control system for adjusting the position of the slant plate in a variable stroke air conditioning compressor, the suction pressure is typically used to automatically adjust the position. This system uses a pressure-controlled mechanism that responds to changes in the compressor’s suction pressure.
The control mechanism typically consists of one or more diaphragm or bellows chambers connected to the suction side of the compressor and the drive shaft of the slant plate. When the suction pressure changes, this causes movement in the diaphragm or bellows. This movement is then transmitted to the mechanism that adjusts the angle of the slant plate.
- At higher suction pressures, such as when cooling demand increases, the pressure-controlled mechanism adjusts the slant plate angle. This results in a larger piston stroke length and thus higher refrigerant compression. This leads to higher discharge pressure and greater cooling capacity.
- At lower suction pressures, the mechanism reduces the slant plate angle, resulting in a shorter piston stroke length and lower refrigerant compression. This lowers discharge pressure and adjusts cooling capacity to the reduced cooling demand.
In a variable output air conditioning compressor, a valve controls the connection to the crankcase (in the slant plate chamber) and both the high and low-pressure sides of the compressor. The low-pressure side is influenced by the measured suction pressure. Below is an explanation of how the control valve works when increasing and decreasing output.
Increase Output:
As cooling capacity decreases, the temperature on the suction side rises and suction pressure increases. This suction pressure compresses the elastic bellows, making them smaller. When the bellows compress, ball valve A closes and valve B opens. This creates a connection to the crankcase. This allows the pressure in the slant plate chamber to escape to the low-pressure side (on the suction side), causing the slant plate to tilt more. This results in greater compressor output and an increase in cooling capacity.
Decrease Output:
If cooling capacity increases, suction pressure decreases. The suction pressure lowers, and the bellows expand in volume, closing opening B and opening ball valve A. High-pressure gas then flows in and goes via ball valve A and the opening to the slant plate housing. This causes the slant plate to stand upright. Consequently, compressor output decreases and cooling capacity is reduced.
The control valve adjusts the pressure in the slant plate chamber. The resulting pressure difference compared to the pressure in the compression chambers leads to the tilting of the slant plate, affecting the compressor’s output. Stroke length is controlled by the pressure in the low-pressure section of the air conditioning system. Compressors with variable stroke (output) usually don’t have a thermostat switch on the evaporator. The inlet pressure is held at 2 bar in these compressors.
External Control, without Magnetic Clutch:
In a compressor with external control, an electromagnetic valve is used to regulate the pressure in the compressor housing. The electromagnetic valve is controlled by an ECU (the engine ECU or air conditioning ECU) using a PWM signal. However, the suction pressure continues to play a role in the control process. The air conditioning ECU receives signals such as the desired air conditioning mode (dehumidifying, cooling), the desired and actual temperature, and the outside temperature.0
Based on this, the computer calculates the optimal setting for the control valve and thus the compressor output. The suction pressure may also vary if necessary. Practically speaking, the suction pressure varies between 1.0 and 3.5 bar. A low suction pressure improves cooling capacity at low compression RPM. A higher than average suction pressure at low heat load results in more efficient operation and thus lower fuel consumption. The heavy magnetic clutch can now be omitted, saving about 1 kg. Usually, the clutch is equipped with a vibration damper and a slip mechanism.
A larger control current to the control valve closes off the passage from the high-pressure chamber to the crankcase. The variable opening allows the pressure-increasing leakage gas to be exhausted via the suction pressure chamber. This equalizes the pressure in the crankcase (Pc) and the suction pressure Ps, causing the wobble plate to position for maximum output.
Output reduction occurs by increasing the pressure in the crankcase. The control valve opens, creating a connection between the crankcase and high-pressure chamber. The control valve has a bellows influenced by the suction pressure, changing the setpoint. The control current to the valve works together with the bellows setting. A small variable opening permits a limited flow of refrigerant to the suction pressure chamber.
Compressor Lubrication:
Moving parts always generate heat, so they need lubrication. Besides providing lubrication, the oil also ensures sealing and noise damping. Initially, the compressor is filled with oil, and lubrication is achieved through mist lubrication. This oil mist also reaches the plungers and is then carried with the refrigerant throughout the system. During condensation, a mixture of refrigerant and a liquid oil mist forms. This oil mist is then reabsorbed by the compressor.
The synthetic oil PAG (Polyalkylene Glycol) is specially designed for refrigerant R134a and should never be replaced with another type of oil. However, one must consider the different viscosities recommended by manufacturers. Refer to the specifications for this.
Common PAG oils are:
- PAG 46 (lowest viscosity)
- PAG 100
- PAG 150 (highest viscosity)
- PAG oil with the addition of YF for use with refrigerant R1234YF, due to its sensitivity to moisture in the system.
In addition to PAG oils, there are also mineral, PAO, and POE oils.
- Mineral oil was used in old R12 systems.
- PAO oil (PolyAlphaOlefin) is fully synthetic and non-hygroscopic, unlike PAG oil, which is highly hygroscopic.
- POE oil (Polyester) is used in electric air conditioning compressors of HV vehicles. Using the wrong oil (PAG) can damage the insulated varnish layer of the copper wire in the electric motor.
When installing a new compressor, it already contains oil (about 200 to 300 ml). The manufacturer specifies this oil amount in the documentation.
Without evacuating the system, it is not easy to determine the amount of refrigerant and oil present in the system. In case of a repair, such as when replacing a condenser, some oil may be lost. The manufacturer usually indicates the distribution in the system. Generally, this distribution can be adhered to:
2 compressor approx. 50%
2 condenser approx. 10%
fluid suction line approx. 10%
evaporator approx. 20%
filter/dryer approx. 10%
When the system is first activated, the oil is distributed throughout the system. If the system is later evacuated and then refilled, such as when replacing another component or during maintenance, the oil can be added to the refrigerant via the filling station. It’s essential to ensure that too much oil does not end up in the compressor. Too much oil in the system can cause the compressor to suffer from liquid slugging. In air conditioning systems with a capillary tube, an accumulator is mounted just before the compressor, which constantly adjusts the oil amount to the refrigerant amount (see the page on the accumulator).
Magnetic Clutch:
The pulley of the air conditioning pump is continuously driven by the serpentine belt. In slant plate compressors with a fixed stroke and some with a variable stroke, the magnetic clutch controls the engagement and disengagement of the air conditioning compressor. When the compressor is engaged, an electromagnet (1) in the clutch is activated. This causes the magnet to pull the spring-mounted clutch plate (4) towards it, creating a solid connection between the pulley and the pump. When the air conditioning is turned off, the electromagnet is no longer activated, and the magnetism ceases. The spring of the clutch plate pushes it away from the pump. The pulley continues to rotate with the serpentine belt, while the pump (internally) stands still.
Engaging the air conditioning is most favorable when the engine RPM is low, such as when the clutch is depressed or when the engine is idling. This minimizes wear on the magnetic clutch. If the air conditioning is engaged at, for example, 4500 RPM, the electromagnet will activate the clutch, resulting in a significant speed difference between the stationary pump and the rotating pulley. This can cause slippage, leading to increased wear.
Sounds:
Several characteristic sounds can occur:
Clattering sound when engaging: A loud clattering sound when engaging the compressor may indicate a possible adjustment of the magnetic clutch. Depending on the compressor type, this adjustment can minimize the air gap and reduce noise.
Humming sound from the air conditioning pump: A humming sound suggests a defect in the pump or possibly a shortage of refrigerant and oil in the system. Consult an air conditioning specialist to check and evacuate the system and refill it with the correct refrigerant and oil amounts.
Clattering sound from the air conditioning pump: A clattering sound can also indicate a pump defect. Check whether the magnetic clutch is tightly attached to the pump to prevent the central bolt from becoming loose.
Buzzing sound linked to engine speed: A buzzing sound audible inside and synchronized with engine speed indicates resonance or vibration. This can be caused by too low refrigerant quantity or resonating air conditioning lines. If the refrigerant level is adequate, a vibrating pipe can be identified by holding it during acceleration. Special vibration dampers, like those available for specific issues such as with MINI, can correct these types of vibrations.
Related page: