What is an Automotive Air-Conditioning System?

An automotive air-conditioning system is a vital component in modern vehicles, providing comfort to passengers by regulating the interior temperature and humidity. The primary function of the A/C system is to remove heat and moisture from the cabin air, creating a cooler and more comfortable driving environment. In this article, I will discuss the automotive air-conditioning system.


Automotive Air-Conditioning System

Due to varying conditions of heating, ventilating, cooling, and dehumidification in the atmosphere at various places, the air-conditioning of automobiles is essential. To maintain human comfort and improve the internal atmosphere in an enclosed space, proper control of freshness, temperature, humidity, and cleanliness of the air is required.                                                                                          ‘

In an automobile, the air-conditioning system involves three main processes heating, cooling, and dehumidification. In the heating system, the heat needed to warm the automobile is derived from the engine coolant or circulating warm water after passing through a heating coil. The system either consists of recirculatory heaters or fresh air intake systems with provisions of recirculation.

To produce a cooling effect, an evaporator coil operated by a refrigerating system is placed inside the chamber of the automobile. The air to be circulated inside the passenger compartment is cooled by the evaporator coil. Depending upon the speed of the vehicle, frequency of stops, and atmospheric and traffic conditions, the air conditioning loads vary from instant to instant and place to place. Due to the moving nature of the vehicle, the equipment used should be light, compact, quickly serviceable, and unaffected by vibrations.

The dust particles are entrapped by the wet surfaces of the evaporator core and are drained off with the condensed moisture. This provides clean and pure air for breathing.

In comparison to domestic air-conditioning, a very large amount of air-conditioning capacity is required for a car. This is due to the metal construction of the car, the flow of air around a moving car, and relatively large glass areas in the passenger compartment. Typically, a car air-conditioning system may be between 1 to 4 tons.

A refrigerating machine driven by the car engine is used in the automobile air conditioner to maintain the desired comfortable temperature level. For heating purposes, the warm water from the engine cooling system is used. The heat required to warm the automobile is generally provided by circulating warm water through a heating coil.

The air-conditioning system of most cars consists of a 1.5 kW compressor located under the engine hood and a belt driven from the crankshaft. The evaporator and the blower unit were located in the trunk compartment while the air-cooled condenser was placed in front of the radiator.

A typical air-conditioning system used in automobiles primarily consists of a compressor, condenser, receiver, expansion, evaporator, dehydrator, filter, sight glass, blower, and air distribution system as well as certain types of capacity control.

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Operating conditioning of Automotive Air-Conditioning System

In an automobile, the air-conditioner should provide comfort and control conditions in the car during mild or damp weather as well as hot weather. The operation should include heating, defogging, as well as removing dust, smoke, and odor.

The compressor is belt-driven from the engine. Since the engine speed changes with the change in car speeds, therefore, the compressor speed will also vary. Only on the hottest day in the sun as well as underside wind conditions the compressor as well as the cooling is at idling speed. Considerable excess capacity, particularly under cool weather conditions, is provided by the above capacity condition.

The problems of controlling the temperature as well as the refrigerant flow (both liquid and vapor) within the system are provided by this condition. The high side pressure is liable to build up while the low side pressure will drop very low when the compressor is operating very rapidly; and little or no refrigeration is needed.

The evaporator temperature will be lowered by dropping the low side pressure. Since the evaporator surface is liable to circulation through it, it is, therefore, necessary that the evaporator temperature should not be allowed to drop below 0°C. The compressor valves are liable to damage as well as result in burning out if the system is operated under a low side pressure.

To overcome these problems, different types of cycle and mechanical systems are used. To obtain maximum cooling during high heat loads in the cooling season as well as in washing or during heavy rainstorms, the fresh air ducts must be closed. About 10 minutes are taken by a standard automobile air-conditioner to cool an automobile from 43.3°C to 29.44°C standing in the sun.

Generally, the inside temperature of a car parked in the sun with windows closed is 65.5°C. The sun load and the heat conducted through the car windows is the highest heat load or the heat gains. Usually 100% fresh air or no fresh air (all circulated) to 25% fresh air is used by different units.

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Performance Rating

The compressor capacity will decrease when the automobile slows down, while it will increase when it speeds up. Except during the critical interval of parking or moving in slow-moving traffic, the variation in compressor capacity is directly proportional to the changing heat loads. This changed capacity is below the required capacity. To obtain the required capacity, the engine may be idled at higher speed and for higher engine speeds, travel in traffic in some intermediate gear.

At high speeds, the refrigerating system consumes as much as 6 kV with a unit capacity of 3 tons or 9072 kcal/h approximately. For each ton of refrigeration effect, about 0.4 kW to 2 kW are required which is comparably more than a motor-driven constant speed compressor comparably built and with the evaporator coil and condenser more ideally located requiring 0.75 kW for each ton of refrigeration.

A hot water coil using engine heat is installed in the air duct for heating purposes. During both the cooling and heating purposes, the same fans may be used.

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Typical Installation of Automotive Air-Conditioning System


A factory-installed air-conditioner to be fitted as a compact unit or cooling unit that can be fitted in any standard make of automobile is manufactured by most automobile manufacturers. A small mixing chamber is by a V-belt from a special pulley on the engine crankshaft. In certain systems, the compressor-driven pulley contains an electrically operated clutch to declutch it from the pulley.

The condenser is mounted at the front of the car while the receiver is mounted at the side of the engine. The expansion valve, which is the controlling valve in the system is placed near the evaporator. The evaporator is fitted in the tank compartment.

The cooled air enters the passenger compartment at its rear before distribution. Below the rear seat, the warm air is returned through a grill to the cooling unit after passing through a filter. Units of generally 1 to 1.5 tons capacity, with compressors ranging from 1.5 to 3 kW (maximum) are used in most of the cars. To control the car temperature fitted under the dashboard receives air from both the evaporator coil as well as the heater coil. The passenger compartment receives the conditioned air traveling from the plenum or mixing chamber.

For heating the motor vehicle, the controls are set. The hot water from the engine block water jacket circulating through the heater coils in the passenger compartment is used for heating purposes.  To provide air-conditioning cooling, the controls are again set. The air blown through the cold evaporator coils is circulated through the passenger compartment of the motor vehicle for this purpose. Certain units are also operated by automatic systems.

The compressor is generally mounted on the side of the engine. It is motor-driven and compensates for the variable compressor speeds a thermostat is also used. A recent development in automobile air-conditioning is a complete factory-assembled unit that can be bolted near the engine.

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Air Conditioning System Components

It includes a compressor, magnetic clutch, condenser, receiver drier, strainer, expansion valve, evaporator, blower, and air-distributor system. The evaporator and the expansion valves are located inside the passenger compartment, and the other components are usually put under the car The components and the behavior of the refrigerant in them are briefly described here:


The compressor used in the automobile air-conditioning system is belt-driven from the engine crankshaft pulley. A magnetic clutch engages the compressor shaft. When the voltage is applied to the compressor clutch coil, the magnetic force generated locks the clutch plate and the pulley together as one unit; and then the compressor shaft is turned with the pulley.

Springs in a clutch plate and hub assembly automatically move the plate away from the pulley when the voltage to the clutch coil is interrupted. This causes the compressor to stop. Most compressors used are piston-type compressors with one or two cylinders. The compressor draws in low-pressure, high-temperature vaporized refrigerant and compresses it to high pressures. The refrigerant never reaches its liquid state in the compressor. The compressor compresses the refrigerant to a maximum of about 200 N/cm2 and I 00°C, and pushes it along the condenser.

The refrigerant used is a chlorine-fluorine mixture known by the trade name of ‘Freon-12 ‘. It has a boiling point of -22° at atmospheric pressure and exerts a vapor pressure of 125 N/cm at 133°F.

Magnetic Clutch

The electrically controlled magnetic clutch is housed in a pulley assembly and the system selector switch controlling the clutch operation is mounted on the control panel inside the passenger compartment. In the Off or Vent position, the compressor and its clutch are off. Other four positions of the selector switch, the clutch is engaged or disengaged depending upon the temperature of the air in the ambient temperature. Below 40°F, the ambient switch located in front of the condenser remains open and the compressor cannot be operated. Above 55°F, this switch closes and completes the circuit to the compressor clutch.

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A condenser is a fin-and-tube radiator, usually placed in front of a car’s radiator. The condenser receives heated and compressed refrigerant vapor from the compressor and is cooled by the air passing across the condenser. The refrigerant is now in the form of a high-pressure liquid which is fed to the receiver-drier unit.

Receiver-drier(or Dehydrator)

The refrigerant is stored under pressure in the receiver-drier or the dehydrator. The pressure in the receiver lines is between 50 N/cm2 to 200 N/cm2 depending upon the compressor speed and the surrounding air temperature. The drier removes any traces of moisture present in the system to avoid freezing of moisture at low temperatures and, thus, clogging the lines.

Even a small amount of moisture can cause running troubles. Drier is usually a silica gel filter that absorbs any water that may be in the refrigerant. A liquid-sight glass is mounted on the high-pressure line between the receiver and the expansion valve to ascertain the quantity of the refrigerant in the system.

Expansion Valve

The refrigerant goes from the receiver-drier to the expansion valve where a sudden expansion to a much lower pressure occurs. The refrigerant changes back to vapor state and this causes a big chilling effect that drops the surface temperature of the evaporator coils to near freezing. It is here that the cooling effect is obtained.

The expansion valve is operated by opposing pressures on either side of the diaphragm. If the evaporator outlet temperature is higher the refrigerant expands and exerts more pressure against the diaphragm, thereby, providing a greater opening to the expansion valve and hence, a greater flow of refrigerant into the evaporator for increased cooling. When the outlet temperature of the evaporator is low, the expansion valve starts closing.


The evaporator unit, where the cooling effect is obtained, is usually located inside the passenger compartment below the dashboard. A high-capacity blower circulates the air in the car interior across the evaporator coils, and this drops the temperature of the air inside the passenger compartment. The heat picked up by the refrigerant goes back to the compressor in the vapor from where the refrigerant is again compressed to a high pressure and the cycle starts again.

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Suction Throttling Valve

The suction throttling valve ensures that the refrigerant in the evaporator stays at such a pressure that the evaporator core surface temperature does not fall below the freezing point of water (0°C), thus preventing ice formation in the evaporator. A pressure equalizing tube is usually connected between the suction side of the suction throttling valve and the expansion valve diaphragm for effective expansion valve functioning.

Types of Air-Conditioning Control System

To control the function and operation of a unit, there are four basic cycles and mechanical systems used. These systems vary in size and installation procedures and are as follows :

  1. Low-side pressure control
  2. Pressure  operated  by-pass
  3. Solenoid operated by-pass
  4. Electromagnetic clutch
  5. Combination of the above controls

1. Low-Side Pressure Control

In this system, an evaporator pressure-controlled regulator valve is installed in the suction line. A constant pressure is kept in the evaporator by this valve. The evaporator tends to go below a certain setting while holding the evaporator at constant pressure and temperature, the valve would close.

This pressure control prevents the compressor from producing a high vacuum at high speeds causing the compressor to lose its oil. A system with an automatic expansion valve having a small bleeder hole in the orifice is used to overcome this problem. To prevent a very high vacuum from forming, a small amount of refrigerant enters the suction line. In this system, the compressor is run continuously.

2. Pressure Operated Hot Gas By-Pass Valve

In this system, the pressure-operated bypass valve is connected between the compressor discharge (high side) and compressor low side or suction line. When the pressure difference reaches the valve setting, the valve is set to open and bypass hot vapor from the high side to the low side.

The valve will close or open if the suction line pressure decreases or increases respectively. To maintain a certain pressure in the evaporator, hot gas (vapor) is fed into the low-pressure side. In this system, a continuously operated compressor is used.

3. Solenoid Operated Hot Gas By-Pass

To open a solenoid valve and bypass the hot gas from the high-pressure side to the low-pressure side when the temperature of the evaporator falls to 0°C, a thermostat mounted on the evaporator is used. It is located in the return air flow with a sensing bulb. When the return air temperature is lowered, so that the thermostat is at a temperature of 0°C, the solenoid valve opens to allow hot gas from the condenser to bypass back into the suction line.

This valve does not provide the throttling effect of a pressure-operated valve since it is either closed or wide-opened. In this system, either a continuous-operating compressor or a rotary compressor is used. When the circuit is closed or when the current is on, the solenoid is in a closed position. When the circuit is open, the valve also opens. Hence, the thermostat opens on temperature drop. When the pressure in the system balances to reduce the starting load and the power is off, the solenoid valve also opens up.

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4. Electronic Clutch

In certain automobile air-conditioning systems, a mechanism permitting the engine to run without the compressor running is used. To engage the compressor belt pulley to the compressor crankshaft or disengage it, a clutch is used. This clutch operated by electromagnetism by forcing the clutch disk against the pulley is known as a magnetic clutch. Two types of magnetic clutches used are -revolving magnetic coil and stationary magnetic coil.


Regular maintenance, including checking refrigerant levels, inspecting for leaks, and cleaning or replacing air filters, is essential to ensure the efficient and effective operation of the automotive air-conditioning system. Additionally, proper use of the A/C system can contribute to fuel efficiency by reducing the need for open windows at high speeds.

Frequently Asked Questions(FAQ’S)

How often should I service my car’s A/C system?

It is recommended to have your car’s A/C system inspected and serviced annually, even if there are no apparent issues. Regular maintenance helps ensure optimal performance and can prevent major problems down the road.

Can I use my car’s A/C system to cool the car faster in hot weather?

While it’s tempting to blast the A/C at maximum settings, it’s more effective to gradually lower the temperature. Starting with the windows down and the A/C on a higher setting allows hot air to escape first, and then adjusting to a cooler temperature helps maintain comfort.

How does the A/C system impact fuel efficiency?

Proper use of the A/C system, such as avoiding excessive use at low speeds and ensuring the system is well-maintained, can contribute to fuel efficiency. However, running the A/C at high speeds is generally more fuel-efficient than driving with open windows.

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