Air conditioning compressor


  • General
  • Vane / Vane Pump
  • Reciprocating compressor (reciprocating, crankshaft type)
  • Tilting plate compressor (general)
  • Fixed Stroke Tilt Plate Compressor
  • Tilting Plate Compressor with Variable Stroke
  • Lubrication of the compressor
  • Magnetic clutch
  • Sounds

The compressor pumps the air conditioning refrigerant through the entire system. The pressure and temperature are increased when leaving the compressor. There are a number of types of compressors that can be used for air conditioning. Reciprocating compressors are used in the air conditioning systems in today's cars. Reciprocal means that the components in the compressor reciprocate. The operation of these compressors can be compared to the piston engine technology. The reciprocal compressors also consist of 2 types, namely the crankshaft type and the tilt plate compressor.
In today's cars, tilt plate compressors are used, which are again divided into 2 types; namely the tilt plate compressor with fixed stroke and with variable stroke. Just like the alternator and the power steering pump, the air conditioning pump is driven by the multi-ribbed belt (see image below).

For more information about the V-ribbed belt, see the chapter v-ribbed belt / V-belt.

Vane / Vane Pump:
This pump is rarely used in the air conditioning system of a car. However, this pump can be used in certain product cooling installations.

Health Benefits: The (grey) disc turns right, clockwise. The yellow plungers are pressed against the wall by the centrifugal force (center of flying force). The different rooms are thus separated from each other. At the bottom right, the refrigerant enters and starts its way into the small blue space. By turning this space becomes larger, which creates negative pressure. The pump continues to run, causing the refrigerant to enter the red area. Here the space of the room becomes smaller and smaller and the refrigerant is pressurized (compressed). At the end of the red chamber is the exhaust valve, through which the refrigerant is forced out.

Reciprocating compressor (reciprocating, crankshaft type):
Just like the vane / vane pump, this pump is rarely used in the air conditioning system of a car. However, this pump can also be used in certain product cooling installations.
The image below shows a reciprocating compressor, with 1 as the inlet valve and 2 as the exhaust valve. The piston / crankshaft movement can be compared to a normal Otto or diesel engine.

Health Benefits: The piston moves from TDC to ODP (from top to bottom) and thereby sucks open inlet valve 1. The refrigerant is also drawn into the cylinder by means of underpressure. The piston then moves from ODP to TDC and presses the inlet valve back into its seat. The upward movement also lifts the outlet valve 2 from its seat. The refrigerant can now leave the cylinder. The exhaust valve closes again. Then the cycle starts again.

Tilting plate compressor (general):
Tilting plate compressors are almost always used in the air conditioning systems of cars. This is also a pump that is classified under the "reciprocal" chapter, because it also contains moving parts to and fro.

Health Benefits:
The piston in the picture below makes a horizontal movement (see the red surfaces). The horizontal movement of the piston depends on how slanted the round plate is. In this image, the plate is slanted to the maximum, which means that the plunger can make maximum horizontal movement (see the red space in the cylinder). This image shows a piston making a compression stroke. In this situation, the pump delivers maximum output. When a lower flow is required, a “fixed stroke” compressor will disengage the magnetic clutch so that the compressor is no longer driven, and a “variable stroke” compressor (as shown in the figure below) will reduce the plate "tilted". The plate is now more upright, which reduces the stroke of the piston. The fixed and variable stroke compressors are described further down the page. 
In the space above each piston there are 2 valves which are mounted in a disc plate spring. These are the suction valve and the discharge valve. When the piston goes to TDC (up), it forces the refrigerant out through the discharge valve (to the high pressure line.
The compressor only moves gas (vapour). All the refrigerant will have to be completely condensed, because too much liquid can cause a liquid hammer. This will cause the pump to malfunction. Lubricating oil does enter the cylinder (atomized) in liquid form, but that is a minimal amount. That can't hurt.
Tilt plate compressors can contain between 4 and 8 pistons/plungers.


The tilting plate compressor also consists of 2 types; namely the fixed stroke compressor, and the variable stroke compressor.
These are described below.

Tilting plate compressor
Tilt plate compressor cutaway

Fixed stroke tilt plate compressor:
The compressor is driven by the multi-ribbed belt of the engine and therefore rotates at the same speed as the engine speed (600-6000 rpm). The magnetic coupling ensures that the compressor is switched on and off. This will be discussed later.
When the compressor is turned on, the tilt plate, as it rotates, will move the pistons up and down. The suction valve and the discharge valve on each cylinder allow gas to be drawn in and moved under pressure to the high pressure part of the system. The disadvantage of a compressor with a fixed stroke is that at a higher speed the output of the pump will become much too high. The yield cannot be regulated. A certain working area can only be reached by continuously switching the compressor on and off; when the pressure drops, the compressor turns on and when the pressure gets too high, the compressor turns off. Switching on and off can be a minor shock as the motor load changes.

Tilting plate compressor with variable stroke:
With this compressor, the angle at which the tilting plate is positioned is adjustable by means of an adjusting device. By setting up the tilting plate as straight as possible, the stroke of the pistons becomes small and there is as little output as possible. By tilting the tilting plate as much as possible, the pistons make a much larger stroke and the yield is a lot higher.

The image above shows how the position of the tilting plate can influence the stroke of the piston. When the engine has a higher speed, there is more output from the compressor. As a result, the pressure in the entire system becomes higher and the adjusting device will increase the pressure in the tilt plate chamber. This pressure makes the tilting plate more upright, which reduces the capacity. When the flow has decreased, the adjusting device closes and the pressure in the tilt plate chamber also decreases. As a result, the plate will again be inclined, so that the pistons can make a larger stroke again.

In this way the yield is constantly controlled. No need to turn the compressor on and off. In fact, not all compressors of this type have a magnetic coupling and will always run, even when the air conditioning is turned off.
The tilting plate chamber then remains as straight as possible as long as the air conditioning is switched off. There is always a low circulation of the refrigerant. With other types of compressors, the systems are combined; these have both a variable stroke and a magnetic coupling.

Lubrication of the compressor:
Heat is always generated between moving parts and must therefore be provided with a lubricant. The compressor lubricant is contained in the refrigerant and thus circulates continuously throughout the system. The synthetic oil PAG (Polyalkylene glycol) has been specially developed for the refrigerant R134a and must never be exchanged for another type of oil.
When mounting a new compressor, the oil (approx. 300ml) is already in the compressor. When the system is turned on for the first time, the oil is absorbed throughout the system. If the system is subsequently emptied and then refilled, for example when replacing another part or during maintenance, the oil can be added to the refrigerant by the filling station. Too much oil must never get into the compressor. In capillary air conditioning systems, an accumulator is mounted just in front of the compressor, which constantly adjusts the oil quantity according to the quantity of refrigerant (see page on the accumulator).

Magnetic coupling:
The air conditioning pump pulley is constantly driven by the multi-ribbed belt. With fixed stroke and some with variable stroke tilt plate compressors, the switching on and off of the air conditioning compressor is controlled by the magnetic coupling. When the compressor is turned on, an electromagnet in the clutch is energized. As a result, this magnet attracts the spring-mounted clutch disc, so that a firm connection is created between the pulley and the pump. When the air conditioning is turned off, the solenoid is no longer energized and its magnetic action stops. The spring of the clutch disc pushes free from the pump again. The pulley now continues to rotate with the multi-ribbed belt, while the pump (internally) is stationary.

Magnetic clutch open
Magnetic clutch closed

When the air conditioning is on, it is best when the engine is running low (ie with the clutch depressed or the engine idling). This limits the wear of the magnetic coupling. If the air conditioning is operated at e.g. 4500 rpm. is suddenly switched on, the solenoid energizes the clutch and the speed difference is very large between the stationary pump and the rotating pulley. Slip occurs, resulting in increased wear.

A few characteristic sounds may occur:

  • Clapping noise when the air conditioning is switched on:
    If a (quite loud) chattering sound is heard when the compressor is switched on, there is a chance that the magnetic coupling needs to be adjusted (if this is possible; this depends on the type of compressor). By adjusting the air gap according to the factory data, this noise will be as little as possible.
  • Humming noise from the air conditioning pump:
    There is a chance that there is a fault with the pump. There may also be too little refrigerant, so too little oil in the system. It is then advisable to visit an air conditioning specialist and empty the system and fill it with the correct amount of refrigerant and compressor oil.
  • Flapping noise from the air conditioning pump:
    Even now there is a chance that there is a defect in the pump. In this case, check whether the magnetic coupling is properly attached to the pump, because it is possible that the central bolt vibrates loose.
  • Buzzing noise, audible in the passenger compartment depending on speed:
    If there is a buzzing sound when the air conditioning is switched on, which goes along with the engine speed, it means that a resonance (vibration) is present. This can be due to a low refrigerant quantity, or it can be a resonance due to the air conditioning pipes. If the amount of refrigerant is correct, you can look for a pipe that causes a resonance. If one person accelerates and another person holds a pipe or hose, the sound may change. By gripping the pipe or hose, you remove it from the natural frequency. The vibration and thus also the sound will change or disappear. There are special vibration dampers on the market (eg for MINI where this is a known problem). By mounting this damper to the air conditioning line / hose (this is nothing more than a weight), this vibration has been eliminated.

Click here to go to the air conditioning (main) page, where the operation of the whole system with the parts is described.

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