Design

Air conditioning

General




There are five main components in the climate control system:
- evaporator
- receiver
- compressor
- condenser
- throttle valve.
The system is filled with a suitable quantity of refrigerant and oil. The role of the oil is to lubricate and seal the compressor (piston and cylinder). A fluorescent leak-tracing agent is added to the refrigerant. This makes it possible to trace leaks using UV light.
In vehicles with the B418S11 engine the compressor is located on the rear edge of the engine. In vehicles with other engine variants the compressor is located on the front edge of the engine.

Principle sketch of the climate control system




Included in the climate control system:
1. Compressor
2. Condenser
3. Engine cooling fan (FC)
4. Fixed throttle valve
5. Evaporator
6. Blower fan
7. Receiver.
Explanation:
A - High pressure, warm fluid
B - Low pressure, cold fluid
C - Low pressure, cold gas
D - High pressure, warm gas
Dark arrow - Warm air
Light arrow - Cold air
The system is divided into:
- a low pressure side (B, C)
- a high pressure side (A, D).
The two sides are separated by the compressor (1) and the throttle valve (4). The evaporator (5) is located on the low pressure side and the condenser (2) is on the high pressure side.
The design of the climate control system in a vehicle is based on the laws of nature, which mean that liquids evaporate when they are subjected to a temperature increase or a reduction in pressure and that heat is absorbed during this process.
If the hot vapor is then cooled again, the heat that has been absorbed is released and the gas reverts to liquid form. This process is repeated as many times as necessary, so that "cooling is produced" continually. This is the same type of process as used in refrigerators, for example.

Refrigerant
In order to remove heat from the passenger compartment a medium that has a lower evaporating temperature than air is used, because heat will always move from a hotter body to a cooler body. The medium that is used is refrigerant R134a.
R134a is a gaseous fluorocarbon. It is chlorine free and does not damage the ozone layer. However, R134a is still environmentally hazardous as it contributes to the greenhouse effect. It is therefore important that all service work is performed by trained personnel.
R134a retains its gaseous form at normal atmospheric pressure, and only condenses if it is cooled down to below -26 °C.
R134a has the following properties:
- can only be mixed with synthetic PAG oils (polyalkyl glycol) and not with mineral oils
- does not affect metals
- affects some types of plastic, so only special seals that are intended for R134a should be used
- is not explosive
- is odorless
- is not toxic in low doses
- effectively absorbs moisture
- is not flammable
- is heavier than air in gaseous form.

Compressor




Vehicles with 5-cylinder engines have a compressor with fixed displacement (fixed cylinder displacement) (2).
Vehicles with 4-cylinder engines have a compressor with variable displacement (1 and 3).

Note! The mountings on the compressors may look different.

In vehicles with the B418S11 engine the compressor is located on the rear edge of the engine. In vehicles with other engine variants the compressor is located on the front edge of the engine.
The compressor is in the refrigerant circuit, located between the receiver and the condenser.
Compressors with fixed displacement have two limits:
- off
- on.




Air conditioning (A/C) pressure switch
The compressor is controlled by the air conditioning (A/C) pressure switch, which is a mechanical low pressure switch. The air conditioning (A/C) pressure switch is located between the evaporator and the receiver. In vehicles with electronic climate control (ECC), the compressor is also controlled by the temperature after the evaporator.
Compressors with variable displacement (variable cylinder displacement) are not switched off during normal driving. The flow of refrigerant is continually adapted to requirements. The compressor operates between min. and max. cylinder displacement by:
- the pistons are driven by a cam pulley, the angle of which can vary
- when the compressor is disengaged, the angle is determined by the springs
- when the compressor is engaged, the angle is determined by the pressure affecting the top side of the pistons (= intake pressure) and lower side (= pressure in the crank case) during the intake phase
- the pressure on the lower side of the pistons (= in the crank case) is governed by a valve which keeps the intake pressure constant.
High intake pressure = large cylinder displacement
- The valve opens and lowers the pressure in the crank case. The back pressure on the reverse of the pistons is then reduced and the angle of the cam pulley increases
- An increased angle gives an increased cylinder stroke which results in a greater volume of refrigerant being "sucked in" and the inlet pressure dropping.
Low intake pressure = small cylinder displacement
- The valve closes and the pressure in the crank case increases. The pressure is built up by the refrigerant which is led via a calibrated channel from the outlet side to the crankcase. The back pressure on the reverse of the pistons is then increased and the angle of the cam pulley reduces
- A reduced angle gives a reduced cylinder stroke which results in a smaller volume of refrigerant being "sucked in" and the inlet pressure increasing.
The two compressor variants are lubricated with specially developed refrigerant oil. This oil (synthetic PAG oil) is mixed with the refrigerant when the air conditioning system is in operation.
The compressor is also controlled by the air conditioning (A/C) pressure sensor (gasoline engines) or the air conditioning (A/C) pressure switch (high pressure switch) (diesel engines), which switches off the system if the pressure goes up to 3.1 MPa (31 bar).
The sensor or switch also controls the engine cooling fan (FC).
The air conditioning (A/C) pressure sensor or air conditioning (A/C) pressure switch is located by the firewall on the high pressure pipe.
The appearance of sensors and switches differs slightly.




A/C pressure sensor (gasoline engines)




A/C pressure switch (diesel engines)
The compressor also has a relief valve, located on the rear section of the compressor, which functions as an extra safety device. The valve opens and releases refrigerant when the pressure in the system is too high (at approx. 3.5 MPa (35 bar)). The valve then shuts again when the pressure has returned to normal.
The compressor is mechanical and is driven by the vehicle's engine. When the compressor is operating it "steals" 0.5 - 8 kW (0.7 - 11 hp) from the engine. This may be noticed as slight jerks when driving as the compressor is engaged / disengaged.

Magnetic clutch assembly




1. Carrier disc
2. Shim discs
3. Spring ring
4. Pulley
5. Magnet coil.
The compressor is driven by the engine camshaft via the drive belt. As soon as the engine has started, the pulley (4) on the compressor drive shaft operates without hindrance.
Then the climate control system is switched on, the current passes through the magnetic coil (5), which is magnetized. This causes the carrier disc (1) on the compressor drive shaft to be pressed forward towards the pulley.
The coupling is closed and the compressor accelerates with the engine speed.
When the current to the magnetic coil is interrupted, the carrier disc (1) is released from the pulley (4) with the aid of return springs.
When the compressor is disconnected the pulley rotates freely without affecting the compressor. When the solenoid coil is supplied with current the flexible plate is drawn into contact with the pulley. Power is transferred from the pulley via the plate to the compressor shaft.
In order for the magnetic clutch to function correctly, the distance between the carrier disc (1) and the pulley (4) must be exactly correct. Shim discs (2) are therefore placed between them.

Fixed throttle valve (orifice)




1. Inlet on the high pressure side
2. Filter insert
3. Internal diameter of the throttle valve
4. O-ring
5. Outlet on the low pressure side.
The fixed throttle valve is a passive flow regulator. It consists of a thin metal pipe which is positioned in a plastic housing with a filter on both the intake and outlet sides. The filter collects any impurities in the system.
The throttle valve governs the volume of refrigerant which is released from the evaporator. The volume is affected by the dimensions of the pipe (diameter, length) and the pressure and temperature on both sides of the pipe.
The throttle valve is located in the evaporator inlet pipe.
The location of the throttle valve can be seen from the outside, as there is a projection in the line. When the air conditioning system is in operation, this point constitutes the transition between the hot and the cold areas.

Note! If the compressor is damaged, the throttle valve can be blocked by metal particles.

Upstream of the choke valve is the refrigerant in liquid form at high pressure.
The liquid refrigerant flows out under high pressure from the compressor to the inlet side of the throttle valve (1). Two O-rings prevent the refrigerant from flowing past the throttle valve.
Two filter inserts in the inlet and the outlet of the throttle valve clean the refrigerant of particles. The filter insert on the outlet side also serves to further distribute the refrigerant.
The calibrated inner diameter (3) of the throttle valve only allows a volume of refrigerant to pass that corresponds to the pressure. This limits the volume of refrigerant that flows through.

Evaporator




The evaporator is an air heated heat exchanger located in the distribution housing for the climate control system. The evaporator consists of pipe loops in which the refrigerant flows. The loops have flanges to increase the heat absorbing surface. The evaporator is inclined to make it easier for the condensation to run off. Every hour, 10 - 11 liters of condensation are formed. The condensation is led via a drainage house out of the distribution housing and down under the vehicle.
In the refrigerant circuit, the evaporator is located on the low pressure side between the throttle valve and the intake side on the receiver.
In the evaporator there is a low pressure due to the choke valve and the suction effect of the compressor.
When the refrigerant enters the evaporator via the choke valve, it expands and both its pressure and temperature lower.
When the warm air encounters the cold evaporator, the humidity condenses on the evaporator. The heat (energy) that is released during condensation is transferred to the refrigerant which evaporates. The temperature difference between the air and the refrigerant reduces. High humidity results in an increased need for cooling.
To obtain sufficient cooling capacity, the refrigerant's evaporation temperature must be considerably lower than the temperature desired in the passenger compartment. But to prevent the moisture that has condensed on the evaporator from freezing to ice, the air is not cooled below approximately +3 °C (at which stage the temperature of the refrigerant in the evaporator is approximately -3 °C). There is a temperature sensor after the evaporator. The temperature sensor detects the temperature of the air that has passed through the evaporator. When the temperature is too low, the system is switched off. This is to prevent ice forming on the evaporator.
It is important that there is the correct amount of refrigerant in the climate control system.
- Too much refrigerant in the evaporator causes the refrigerant to heat up, but it only evaporates partially. This means that a smaller amount of heat is absorbed from the air, which results in reduced cooling capacity
- Too little refrigerant in the evaporator causes the refrigerant to evaporate and the vapor to overheat. This means that a smaller amount of heat is absorbed from the air, which results in reduced cooling capacity.

Receiver.




1. Outlet line to compressor
2. Inlet line from evaporator
3. Cover
4. U-pipe
5. Filter insert
6. Refrigerant oil
7. Drying element
8. Leak detection dye (LDD).
The receiver is basically a reservoir containing a drying agent.
The receiver is located to the right (in front of the right-hand front wheel) in the engine compartment.
To avoid damage to the compressor, the receiver is located on the low pressure side to separate the fluid from the gas. The receiver outlet is located so that only refrigerant in gas form can leave the receiver. Refrigerant in liquid form is collected in the bottom of the container.
The refrigerant gas passes from the evaporator through the inlet line (2) to the receiver on the intake side. The gas flows along the cover (3) and creates an eddy current. The drying element (7) binds the moisture that is absorbed by the refrigerant.
The refrigerant gas collects under the cover, where it is routed out through the opening to the outlet line (1).
Behind the filter insert (5) there is a small hole in the U-pipe (4). The refrigerant oil (6) that collects in the bottom of the receiver is sucked out through this opening and mixed with the refrigerant gas (oil/gas mixture ratio: 3%). The hole plays a very important role in the return of oil to the compressor. This means that the moving parts in the compressor are supplied with sufficient lubrication.
The receiver also contains a small amount of leak detection dye (8).

Condenser (air cooled heat exchanger)




The condenser is located in front of the engine radiator. The airflow and engine cooling fan (FC) blow air through the condenser. In the cooling system, it is located on the high pressure side opposite the compressor and the throttle valve.
The condenser consists of pipe loops in which the refrigerant flows. The loops have flanges to increase the heat radiating surface.
The pipe connection is made of aluminum. To reduce the rotation when connecting pipe/hoses to the junction block, there is a reinforcement (made of plastic or aluminum) on the block. When disconnecting pipes/hoses, a counterhold must be used. There is a socket for this in the junction block.
When the refrigerant enters the condenser it is in gas form and has both high pressure and high temperature. In the condenser the hot refrigerant loses some of its heat to the cooler air. The refrigerant then condenses and changes into liquid form. The amount of heat given off = the amount of heat taken up in the evaporator + the amount of heat supplied by the compressor. In order to supply sufficient cooling capacity, the condensation temperature of the refrigerant must be greater than the temperature of the outside air.

Service valves




1. Service valve for emptying and vacuum pumping.
2. Service valve for emptying, vacuum pumping and filling of refrigerant.
There are two service valves, one on the low pressure side and one on the high pressure side. To avoid incorrect connection the valves have different dimensions.
Filling of refrigerant normally takes place on the low pressure side. The liquid refrigerant then stays in the accumulator and there is no risk that liquid refrigerant will flow to the compressor.
In an orifice system such as this, filling can also take place on the high pressure side.

Hoses, pipes
All connections, apart from the evaporator, have double O-rings as a seal. The connection to the evaporator has a single O-ring.
The O-rings are manufactured in a specially developed material (HNBR = hydrated nitrile rubber).