Components Climate Unit
Evaporator
The function of the evaporator is to cool and dry the air. Refrigerant (volume is measured by the expansion valve) is injected into the evaporator. When the refrigerant reaches the cooling coils, the refrigerant is evaporated due to the pressure drop and it then absorbs heat from the cooling coils. The cooling coils and the entire evaporator are then cooled down immediately. The refrigerant gas is then sucked out using the compressor.
When hot air routed into the passenger compartment meets the cold evaporator, the humidity in the air is condensed on the evaporator. The water drains out under the vehicle via a drain hose. In the event of high humidity, this may involve considerable quantities of water, which is often mistaken for a water leak from the engine, for example. The heat energy that is released during the condensing is transferred to the refrigerant, which is evaporated. The temperature difference between the air and the refrigerant decreases. High air humidity therefore involves increased cooling needs.
The surface of the evaporator is polyurethane coated to reduce bad odors. The polyurethane coating reduces the surface tension of the water and makes it easier for the water to run off.
The evaporator consists of a tubular coil in which the refrigerant flows. The coils are equipped with flanges to increase the heat absorbing area. The evaporator is tilted to make it easier for the condensation water to drain off.
There is a low pressure in the evaporator on account of the orifice on the expansion valve and the compressor's suction effect.
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 approx. 3 °C (at which stage, the temperature of the refrigerant in the evaporator is approx. -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 compressor 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 means that the refrigerant is only partially evaporated. 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.
Compressor
The compressor is mechanical and is driven by the vehicle's engine. When the compressor is operating it requires an output of 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.
The task of the compressor is to:
- draw gaseous refrigerant from the evaporator
- compress the gas thereby increasing its pressure and temperature
- expel the gas with high pressure and high temperature to the condenser.
In ideal conditions, the compressor compresses the refrigerant from approx. 200 kPa (2 bar) to 1.2-2.1 MPa (12-21 bar). During the process, the refrigerant is heated up from 0 °C to between 70 °C and 110 °C. These pressure and temperature values apply to normal working pressure in the system's high-pressure side, when the system works under optimal conditions.
The pressure relief valve located at the rear of the compressor, acts as an extra safety device. The valve opens and releases refrigerant when the pressure in the system becomes too high (at approx. 3.8 MPa (38±3 bar)). The valve closes again when the pressure has returned to its normal value.
In the refrigerant circuit, the compressor is located between the evaporator and the condenser. The temperature of the refrigerant gas can increase to as much as 125 °C.
The compressor can only compress gases, since liquid would destroy the compressor. The compressor has variable swept volume. The compressor's brackets and pipe connections have a different appearance depending on the vehicle's engine. Compressors with variable swept volume do not engage during normal operation. The flow of refrigerant is adapted continuously according to need. The compressor works between min. and max. swept volume by means of the following:
- the pistons are driven by a cam disc with a variable angle
- the angle is determined by springs if the compressor is disengaged
- if the compressor is engaged, the angle is determined by the pressure exerted on the top of the pistons (= intake pressure) and the bottom of the pistons (= pressure in the crankcase) during the intake phase
- the pressure on the underside of the pistons (= in the crankcase) is regulated by a valve, which attempts to keep the intake pressure constant
High intake pressure = large displacement.
- The valve opens and lowers the pressure in the crankcase. The counterpressure on the back of the pistons then reduces and the cam disc angle increases.
- The increased angle generates increased stroke, which causes a greater amount of refrigerant to be "sucked in" and a drop in intake pressure.
Low intake pressure = small displacement.
- The valve closes and pressure in the crankcase increases. The pressure is built up by refrigerant that is led from the outlet side to the crankcase via a calibrated duct. The counterpressure on the back of the pistons then increases and the cam disc angle decreases.
- The decreased angle generated reduced stroke, which causes a smaller amount of refrigerant to be "sucked in" and an increase in intake pressure.
The compressor is lubricated using specially developed refrigerant oil. This oil (synthetic PAG oil) is mixed with the refrigerant when the air conditioning system is in operation. The purpose of the oil is to lubricate and seal the compressor (piston and cylinder).
Magnetic clutch assembly
- nut
- Carrier disc
- Shim discs, shims
- Spring ring
- Pulley.
- Magnet coil.
The compressor is driven by the engine's crankshaft via the drive belt. As soon as the engine starts, the belt pulley (5) on the compressor's drive shaft runs unimpeded. When the climate system is switched on, the current runs through the magnetic coil (6) which becomes magnetic. This presses the carrier disc (2) on the compressor's drive shaft forward against the pulley.
The clutch engages and the compressor accelerates with the speed of the engine. When the current to the magnetic coil is interrupted, the carrier disc (2) is released from the pulley (5) with the aid of return springs. When the compressor is disengaged the pulley rotates freely without any influence from the compressor. When the current flows through the magnetic coil, the flexible plate is drawn into engagement with the pulley. Power is then 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 (2) and the pulley (5) must be exactly correct. Shim discs (3) are therefore placed between them.
Expansion valve, TXV (Thermal Expansion Valve)
- Diaphragm
- Valve housing
- Refrigerant in liquid state, high pressure, from the receiver drier
- Refrigerant in liquid state, low pressure, to the evaporator
- Refrigerant in gas state, low pressure, to the compressor
The expansion valve checks the refrigerant flow in the evaporator in relation to the temperature and pressure. The expansion valve gives an even temperature control because the amount of refrigerant is controlled as necessary.
The expansion valve is installed between the evaporator's input and output pipes and the side member pipe. The expansion valve's connections are of the double O-ring type. In the refrigerant circuit it is located on the high pressure side between the receiver drier and the evaporator.
The expansion valve's default setting and regulation characteristics are different depending on the refrigerant used. On the outside, this is not visible and can only be distinguished by different part numbers.
The expansion valve is a pressure and temperature dependent flow regulator of the constriction type. At low cooling demand the amount of refrigerant is reduced by the valve closing. If the cooling demand increases the valve opens slightly to allow more refrigerant to the evaporator.
Systems with an expansion valve have a greater register than systems with a fixed orifice because the amount of refrigerant can be better regulated to the cooling demand.
The expansion valve consists of a diaphragm (1) and a valve housing (2).
Cross section of the expansion valve
| 1. Valve inlet from receiver drier | 6. Diaphragm |
| 2. Valve slide | 7. Valve inlet from evaporator |
| 3. Valve outlet to compressor | 8. Valve outlet to evaporator |
| 4. Temperature sensor | 9. Ball valve |
| 5. Diaphragm (filled with refrigerant) | 10. Spring |
Liquid refrigerant comes from the receiver drier and flows through the valve inlet (1). The ball valve (9) mists the refrigerant before the refrigerant flows on through the valve outlet (8) and into the evaporator.
The ball valve opening is controlled by the temperature and pressure in the refrigerant gas coming out of the evaporator.
If for example the temperature of the refrigerant gas, coming from the evaporator in the expansion valve intake (7), increases the temperature sensor (4) and increases the temperature of the refrigerant in the diaphragm head.
The refrigerant in the diaphragm head expands and compresses the ball valve in the valve seat downwards using the diaphragm (6) and a valve slide (2) so that the valve seat opens. The flow of the refrigerant to the evaporator increases. The evaporator is cooled and the temperature of the refrigerant gas is reduced.
The ball valve closes as soon as the temperature in the evaporator falls and therefore the temperature of the refrigerant in the diaphragm head also falls.