Air Conditioning System
General Description
Temperature Distribution System
- The air conditioning unit case consists of three parts, the rear of which carries the evaporator, the front is then split in two to enable the rotary flaps to be inserted. Foot outlet boxes are moulded in plastic and are removable for access.
- The method used to achieve the required air temperature is known as a series parallel system. All the air into the unit passes through the evaporator, then depending on the position of the flaps either passes through the heater matrix to be heated, or bypasses the heater matrix completely, or a combination of both to achieve the air temperature required (see images).
- The system employs two flaps that are driven to the required position (determined by the control system) by servo motors and gear-box assemblies.
- The motors can rotate in either a clockwise or anti-clockwise direction depending on the direction of current flow through the motor.
- The flap positions are monitored by feedback potentiometers which supply a voltage signal to the control module indicating the flap position.
Air Conditioning Switch Control Panel
- The switch control panel contains a blower motor control rotary switch, auto/manual mode switch, air switch, humidity control switches, temperature differential selector and an in-car temperature selector. The control panel produces digital outputs which are supplied to and used by the electronic control module.
- The airflow of the system from the blower motors is controlled by the rotary switch located on the left hand side of the control panel. The switch has five positions: OFF, LOW, NORM, HIGH, and DEFROST. In the OFF position the system is not operational. Power to the electronic control module (E.C.M.) and to the control potentiometers is removed, but a signal is sent to the E.C.M. to ensure the flaps are closed preventing outside air from entering the system.
- In the LOW, NORMAL and HIGH positions, information regarding the range of fan speed selected is received by the control module from the control switch. Signals are also received by the E.C.M from the temperature selector, feedback circuits and the various sensors. The fan speed is then electronically controlled by the E.C.M. So by selecting LOW, NORM or HIGH a level of speed in the range selected is received dependent on the vehicle requirements. Should a low fan speed be selected the control module will maintain the speed of the fan motors within a range of low speeds depending on the temperature requirement of the vehicle. There are no distinct steps between the fan speeds, but a gradual increase or decrease of speed.
- When DEFROST is selected the fans are electronically controlled to operate at maximum speed, the screen vents open, maximum heating is obtained and the lower flaps fully close (it can take up to a maximum period of 30 seconds to complete this operation).
- The control situated on the right hand side of the control panel is the temperature demand control. The temperature requirements of the vehicle within the range of 19 to 29°C can be selected by the setting of this control. With the control turned to the fully anti-clockwise position maximum cooling is obtained, and to obtain maximum heating turn the control fully clockwise.
- When the mode button is set in the auto mode the control system use all the temperature sensors in relation to the temperature selected by the temperature demand control. The system then automatically adjusts the flaps for heating or cooling modes to achieve the desired results. Blower fan speeds are also selected to increase or decrease the volume of air flow, depending upon the differences between the various temperature sensors and the selected temperature.
- When the button is set in the manual mode it overrides the temperature sensors and the system will obtain the temperature selected.
- The air button (demist) when pressed will cause the defrost flaps to the screen to open.
- The maximum and minimum limits of the evaporator temperature are determined by the setting of the humidity controls. When the switch with one teardrop is selected the evaporator temperature is controlled between 3.9 to 4.15°C. When the switch with two teardrops is selected the evaporator temperature range is between 7.1 to 7.35°C. If neither of the two humidity buttons are selected, the evaporator temperature is controlled between 0 to 0.25°C.
- With the application of the econ button, the compressor clutch is switched off and the compressor is disengaged rendering the air conditioning inoperative. A second application of this button gives maximum air conditioning with the evaporator temperature controlled between 0 to 0.25°C.
- The slide control (Air Differential Control) is used to alter the temperature of air being distributed through the face level vents. To increase the temperature of air being delivered through the vents, the slide control is moved to the right and to decrease the temperature the control is moved to the left.
Temperature Sensors
- There are three temperature sensors fitted into the system, the AMBIENT TEMPERATURE SENSOR, the IN-CAR TEMPERATURE SENSOR and the EVAPORATOR TEMPERATURE SENSOR. All three sensors are electrically identical, but the EVAPORATOR TEMPERATURE SENSOR is physically different and is not interchangeable with either the AMBIENT or the IN-CAR TEMPERATURE SENSORS.
- An input voltage is supplied to the sensors from pin 43 of the control module. The temperature sensing signal from the sensors is then fed back into the control module via pin 34 for the ambient temperature, pin 4 for the in-car temperature sensor and pin 5 for the evaporator temperature sensor. The sensors are semi-conductor devices which are accurate over a wide range. The semi-conductor is similar to a zener diode in as much as it allows current to flow in reverse bias, but the current flow varies with temperature. The sensor assembly has a built in potentiometer which is preset and must not be adjusted.
Refrigeration Cycle
- The belt-driven compressor draws in the refrigerant vapor at low pressure and compresses it. The pressurized refrigerant is then forced into the condenser located in front of the engine cooling radiator so that it receives the full flow of ram air.
- The refrigerant enters the condenser as a high pressure very hot vapor. As this hot vapor passes through the condenser coils, heat will follow its natural tendency and move from the hot refrigerant vapor into the cooler ram air as it flows across the condenser coils.
- When the refrigerant vapor reaches the temperature and pressure that will induce a change of state a large quantity of heat will be transferred to the outside air. The refrigerant then changes from a high pressure vapor to a high pressure warm liquid. This high pressure warm liquid flows from the outlet of the condenser to the dryer bottle.
- The dryer bottle is a storage tank which receives the high pressure warm refrigerant liquid from the condenser. It filters and retains any foreign particles or moisture from the refrigerant which would be harmful to the system if allowed to circulate with the refrigerant. The clean refrigerant passes to the expansion valve via the sight glass.
- The refrigerant flow to the evaporator must be controlled to obtain maximum cooling while assuring complete vaporization of the liquid refrigerant within the evaporator. This is accomplished by the expansion valve which is a metering device that changes the pressure of the liquid refrigerant from a high pressure to a low pressure liquid. As the refrigerant passes through the metering orifice in the valve, the pressure and the temperature of the refrigerant is reduced so the refrigerant leaves the expansion valve as a low pressure, cold, atomized liquid. The temperature of the refrigerant is then lower than the temperature of the air in the vehicle so that a heat exchange from the warm air to the refrigerant can take place in the evaporator.
- As this cold, atomized liquid, refrigerant passes through the evaporator coils, heat will follow its natural tendency and move from the warm air into the cooler refrigerant. When the refrigerant reaches a temperature and pressure that will induce a change of state, a large quantity of heat will move from the air into the refrigerant. The refrigerant will then change from a low pressure cold liquid to a low pressure cold vapor. The refrigerant vapor then returns to the low (suction) side of the compressor.