Introduction:
The compressor pumps the refrigerant – gaseous refrigerant from the air conditioning system – through the entire system. The pressure and temperature of the refrigerant increase when it leaves the compressor. There are various types of compressors that can be used for air conditioning. In modern automotive air conditioning systems, reciprocating compressors are used. “Reciprocating” means the components inside the compressor make back-and-forth movements. The operation of these compressors can be compared to that of a piston engine. Reciprocating compressors also come in two types: the crank type and the swash plate compressor. In modern cars, swash plate compressors are used, which in turn are divided into two types: the swash plate compressor with fixed stroke and the variable stroke version. The A/C pump, like the alternator and power steering pump, is driven by the multi-rib belt in combustion engines (see image below). In hybrid and fully electric vehicles we find electric A/C compressors. An electric motor is powered by the HV system and drives the compressor.
The A/C compressor draws in the gaseous refrigerant from the evaporator, keeping the pressure in the evaporator low and thereby promoting the evaporation of the refrigerant, even at low temperatures. The compressor compresses the gaseous refrigerant, causing the transition from low to high pressure. This pressure increase and temperature rise cause the refrigerant to change from gaseous to liquid.
The pressure delivered by the A/C compressor is influenced by several factors, including:
- Engine speed (for combustion engines);
- The type and quantity of refrigerant;
- The temperature of the refrigerant;
- The type and design of the A/C compressor, which determine the capacity;
- The adjustment of the magnetic coupling;
- The ambient temperature.
After compression, the refrigerant leaves the compressor at a temperature of approximately 70 degrees Celsius. This temperature is then reduced in the condenser.
In the following paragraphs, various types of A/C compressors are discussed, which may or may not be used in the automotive industry.
Vane / slide pump:
This pump is rarely used in a car’s A/C system. However, it can be used in specific cooling installations for various products.
Operation: The (grey) disc rotates to the right, clockwise. The yellow plungers are pressed against the wall by centrifugal force, separating the different chambers from each other. At the bottom right, the refrigerant flows in and follows its path to the small blue chamber. Due to the rotation, this chamber becomes larger, creating a vacuum. The pump continues to rotate, and the refrigerant enters the red section. Here, the chamber volume becomes smaller and smaller, so the refrigerant is pressurized (compressed). At the end of the red chamber there is the outlet valve, through which the refrigerant is forced out.
Piston compressor (reciprocating, crank type):
This pump, like the vane / slide pump, is rarely used in a car’s A/C system. However, it can also be used in specific cooling installations for various products. In the image below, a piston compressor is shown, where 1 indicates the inlet valve and 2 the outlet valve. The movement of the piston and crankshaft is similar to that of a conventional Otto or diesel engine.
Operation: The piston moves from TDC (Top Dead Centre) to BDC (Bottom Dead Centre) (from top to bottom), causing inlet valve 1 to open. The refrigerant is drawn into the cylinder by vacuum. The piston then moves from BDC to TDC and pushes the inlet valve back onto its seat. Due to the upward movement, outlet valve 2 is also lifted from its seat. The refrigerant can now leave the cylinder. The outlet valve closes again. The cycle then starts over.
Swash plate compressor introduction:
Swash plate compressors, also known as wobble plate compressors, are almost always used in automotive air conditioning systems. They fall under the category of “reciprocating” because of their moving components that move up and down.
In the illustration we see a line drawing and cross-section of a swash plate compressor. The piston makes a horizontal stroke, which is determined by the angle of the swash plate. In this image the plate is tilted to the maximum, which means the piston can make a maximum horizontal movement (indicated by the red compression space 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 swash plate.
In this situation, the pump delivers maximum output because the swash plate has made a maximum stroke. If a lower output is desired because the pressure becomes too high and, due to too much refrigerant, freezing of the evaporator may occur, then in a compressor with a “fixed stroke” the magnetic coupling is disengaged so that the compressor is no longer driven. In a compressor with a “variable stroke” the plate is tilted less. The angle at which the plate tilts is smaller, so the stroke of the piston is also smaller. Compressors with fixed and variable stroke are described later on this page.
Above each piston there are 2 valves attached to a valve plate spring: the suction valve and the discharge valve. When the piston moves from TDC to BDC, it pushes the refrigerant out past the discharge valve into the high-pressure line towards the condenser.
Swash plate compressors can have between 4 and 8 pistons/plungers and come in two versions: namely the compressor with fixed stroke, and the one with variable stroke. These are described below.
Swash plate compressor with fixed stroke:
This compressor is driven by the engine’s multi-rib belt and rotates in sync with engine speed (between 600 and 6000 revolutions per minute). The magnetic coupling controls the switching on and off of the compressor, which is explained in more detail later.
When the compressor is engaged, the rotating swash plate moves the pistons up and down. Suction and discharge valves at each cylinder allow the pistons to draw in gas and transfer it under pressure to the high-pressure side of the system.
A fixed-stroke compressor displaces a fixed volume per revolution. Output therefore depends on compressor speed, i.e. engine speed. To control the output, the compressor is continually switched on and off: switched on when pressure drops and switched off when pressure is too high. Especially in small engines, engagement can be felt as a “jolt” due to the power demanded. The abrupt engagement causes higher mechanical load and disrupts regulation, resulting in variations in the cooled air temperature for occupants.
At excessively high engine speed and the resulting increase in discharge pressure, more refrigerant flows through the evaporator. This slows cooling and can cause the evaporator to freeze. In such cases, the magnetic coupling is disengaged by the thermostat or pressure switch.
Swash plate compressor with variable stroke:
In this type of compressor, the angle of the swash plate is adjustable thanks to an adjustment mechanism. By positioning the swash plate as upright as possible, the stroke of the pistons is limited and the output is minimal. Conversely, by positioning the swash plate as steeply as possible, the pistons make a much larger stroke and the output increases significantly. We see the following versions of the variable-stroke swash plate compressor:
- with internal control and magnetic coupling;
- external control with and without magnetic coupling.
Internal control and magnetic coupling:
The illustration shows how the position of the swash plate can influence the piston stroke. A higher engine speed results in a greater output of the compressor. This causes an increase in pressure throughout the system, which prompts the adjustment mechanism to increase the pressure in the swash plate chamber.
The increased pressure forces the swash plate to move more upright, reducing capacity. When the output drops, the adjustment mechanism closes and the pressure in the swash plate chamber decreases. This causes the plate to tilt more, allowing the pistons to make a larger stroke. The greater the angle, the larger the stroke and the higher the output.
With an internal (mechanical) control system for adjusting the position of the swash plate in a variable-stroke A/C compressor, the suction pressure is usually used to automatically regulate this adjustment. This system uses a pressure-controlled mechanism that responds to changes in the compressor’s suction pressure.
The control mechanism usually consists of one or more diaphragm or bellows chambers that are connected to the suction side of the compressor and to the drive shaft of the swash plate. When the suction pressure changes, this causes movement in the diaphragm or bellows. This movement is then transferred to the mechanism that adjusts the angle of the swash plate.
- At higher suction pressures, such as when the cooling demand increases, the pressure-controlled mechanism will adjust the swash plate angle. This results in a greater stroke length of the pistons and thus in higher compression of the refrigerant. This leads to higher discharge pressure and greater cooling capacity.
- At lower suction pressures, the mechanism will reduce the swash plate angle, resulting in a shorter stroke length of the pistons and lower compression of the refrigerant. This lowers the discharge pressure and adjusts the cooling capacity to the reduced cooling demand.
In a variable-output A/C compressor, a valve controls the connection with the crankcase (in the swash plate chamber) and both the high- and low-pressure sides of the compressor. The pressure on 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.
Increasing output:
When cooling capacity decreases, the temperature on the suction side rises and suction pressure increases. This suction pressure compresses the elastic bellows, making it smaller. When the bellows is compressed, ball valve A closes and valve B opens. This creates a connection to the crankcase. As a result, the pressure in the swash plate chamber can escape to the low-pressure side (on the suction side), causing the swash plate to tilt further. This results in a higher output of the compressor and an increase in cooling capacity.
Decreasing output:
As cooling capacity increases, suction pressure decreases. The suction pressure becomes lower and the bellows expands in volume, closing opening B and opening ball valve A. High-pressure gas now flows in and passes through ball valve A and the opening into the swash plate housing. This causes the swash plate to move to an upright position. As a result, pump output decreases and cooling capacity is reduced.
The control valve adjusts the pressure in the swash plate chamber. The resulting pressure difference relative to the pressure in the compression chambers causes the swash plate to tilt, which affects the pump’s output. Stroke length is controlled by the pressure in the low-pressure section of the A/C system. Variable-stroke (variable-output) compressors usually do not have a thermostat switch on the evaporator. The inlet pressure in these compressors is maintained at 2 bar.
External control, without magnetic clutch:
With a compressor using external control, a solenoid valve is used to regulate the pressure in the compressor housing. The solenoid valve is controlled by an ECU (the engine ECU or A/C ECU) by means of a PWM signal. However, the suction pressure still plays a role in the control process. The A/C ECU receives signals such as the desired A/C mode (dehumidifying, cooling), the desired and actual temperature, and the outside temperature.
Based on this, the computer calculates the optimal setting for the control valve and therefore the compressor output. If necessary, the suction pressure can also vary. In practice, the suction pressure varies between 1.0 and 3.5 bar. A low suction pressure improves cooling capacity at low compressor speed. A higher than average suction pressure with low heat load results in more efficient operation and therefore lower fuel consumption. The heavy magnetic clutch can now be omitted, saving around 1 kg. The clutch is usually equipped with a vibration damper and a slip mechanism.
Increasing the control current to the control valve closes the passage from the high-pressure chamber to the crankcase. The variable opening provides space to discharge the pressure-increasing leakage gas via the suction pressure chamber. This equalizes the pressure in the crankcase (Pc) and the suction pressure Ps, causing the swash plate to move into the position for maximum output.
Reducing the output is done by increasing the pressure in the crankcase. The control valve opens, creating a connection between the crankcase and the high-pressure chamber. The control valve has a bellows that is influenced by the suction pressure, which changes the setpoint. The control current to the control valve works together with the bellows setting. A small variable opening allows a limited flow of refrigerant to the suction pressure chamber.
Lubrication of the compressor:
Moving parts always generate heat and therefore must be lubricated. In addition to its lubricating properties, the oil also provides sealing and noise damping. Initially, the compressor is filled with oil, and lubrication is achieved via mist lubrication. This oil mist also reaches the plungers and is then carried along with the refrigerant throughout the entire system. During condensation, a mixture of refrigerant and a liquid oil mist is formed. This oil mist is again drawn in by the compressor.
The synthetic oil PAG (Polyalkylene Glycol) is specially designed for the refrigerant R134a and must never be replaced by another type of oil. However, the different viscosities specified by manufacturers must be taken into account. 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 YF for use with the refrigerant R1234YF, because of the sensitivity to moisture in the system.
In addition to PAG oils, there are also mineral, PAO and POE oils.
- Mineral oil was used in the old R12 systems.
- PAO oil (PolyAlphaOlefin) is fully synthetic and not hygroscopic. This is in contrast to PAG oil, which is highly hygroscopic.
- POE oil (Polyester) is used in electric A/C compressors of HV vehicles. When the wrong oil (PAG) is used, the insulated lacquer layer of the copper wire of the electric motor is damaged.
When a new compressor is installed, it already contains oil (approximately 200 to 300 ml). The manufacturer specifies this oil quantity in the documentation.
Without evacuating the system, it is not really possible to determine how much refrigerant and oil are present in the system. In the event of a repair, for example when replacing a condenser, a small amount of oil will be lost. The manufacturer usually indicates how the distribution in the system is. In general, we can use the following distribution:
• compressor approx. 50%
• condenser approx. 10%
• flexible suction line approx. 10%
• evaporator approx. 20%
• filter/drier approx. 10%
When the system is switched on for the first time, the oil is distributed throughout the entire system. If the system is later emptied and then refilled, for example when replacing another component or during maintenance, the oil can be added to the refrigerant via the charging station. It is essential to ensure that not too much oil ends up in the compressor. Too much oil in the system can cause the compressor to suffer liquid slugging. In air conditioning systems with a capillary tube, an accumulator is fitted directly in front of the compressor, which continuously adjusts the quantity of oil to the amount of refrigerant (see the page about the accumulator).
Magnetic clutch:
The pulley of the A/C pump is continuously driven by the multi-rib belt. In swash plate compressors with fixed displacement and some with variable displacement, the magnetic clutch controls switching the A/C compressor on and off. When the compressor is switched on, an electromagnet (1) in the clutch is activated. This causes the magnet to pull in the spring-mounted clutch plate (4), creating a fixed connection between the pulley and the pump. When the air conditioning is switched off, the electromagnet is no longer activated and its magnetic effect stops. The spring of the clutch plate pushes it away from the pump. The pulley now continues to rotate with the multi-rib belt, while the pump (internally) stands still.
Switching on the air conditioning is most favourable when the engine speed is low, such as with the clutch pedal depressed or when the engine is idling. This minimises wear on the magnetic clutch. If the air conditioning is switched on at, for example, 4500 rpm, the electromagnet will activate the clutch and there will be a large difference in speed between the stationary pump and the rotating pulley. This can cause slip, leading to increased wear.
Noises:
A few characteristic noises may occur:
Clacking sound when switching on: A loud clacking sound when the compressor is switched on may indicate a possible adjustment of the magnetic clutch. Depending on the type of compressor, this adjustment can reduce the air gap and minimise the noise.
Humming noise from the A/C pump: A humming noise indicates a defect in the pump or possibly a shortage of refrigerant and oil in the system. Consult an A/C specialist to check the system, evacuate it and refill it with the correct amount of refrigerant and oil.
Rattling sound from the A/C pump: A rattling sound can also indicate a pump defect. Check whether the magnetic clutch is firmly attached to the pump to prevent the centre bolt from coming loose.
Buzzing noise linked to engine speed: A buzzing noise audible in the interior and following the engine speed indicates resonance or vibration. This can be caused by too low a refrigerant quantity or by A/C lines that resonate. If the refrigerant level is in order, a vibration-causing line can be identified by holding it while accelerating. Special vibration dampers, such as those available for specific issues like with MINI, can correct this type of vibrations.
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