Vehicle Climate Control: Then and Now

The gasoline heater was made popular by Ford in the 1930s. a blowtorch-like flame and a fuel line inside the passenger compartment would seem dangerous today, but this system was very effective in dealing with frost.

A car’s interior is very successful in collecting solar energy. There were attempts to chill the passenger compartment long before refrigeration—or even the automobile—was invented. In 1884, William Whiteley placed blocks of ice in trays under horse carriages. A fan attached to a wheel forced air to the interior of the carriage. A bucket of ice in front of a floor vent was the automotive equivalent to this early comfort control system.

The first car with real air conditioning was the 1939 Packard. A huge evaporator was mounted in the trunk, leaving very little space for luggage. Cadillac was the next car with air conditioning. In 1941, 300 Cadillac’s were built with the air-conditioning option.

A recent change in modern air-conditioning systems has been the switch from R-12 Freon (which damages the earth’s ozone layer) to a more environmentally friendly refrigerant, R134A. Freon production has been outlawed, and older cars are being converted to the new refrigerant.

The automotive heating and air conditioning system is called a heating, ventilation, and air conditioning system (HCAC).
Heating, ventilation, and air-conditioning systems share the same blower motor. The motor usually has multiple speed settings. On many cars, instead of an “off” setting, the blower runs at low speed when the ignition key is on. This maintains a fresh flow of air into the interior of the car.


Hot air is provided by routing engine heat from the engine coolant is passenger compartment.

Air Conditioning

Is the process in which air inside of the passenger compartment is cooled, dried, and circulated. Comfort depends on temperature, humidity, and air movement. Heat is removed from inside the vehicle and transferred to the outside air.

In the 1920s, household refrigerators were first introduced. The first refrigeration unit on an automobile was available as a luxury extra on the Packard automobile in 1939, but air conditioning did not become popular with the motoring public until the 1960s. Today, over 80% of cars sold have air conditioning. Refrigeration systems are also used on off-road and farm machinery.

Air-conditioning systems originally were manually controlled by the passengers. Many of today’s systems are automatic. At one time it was thought that air conditioning was not energy efficient because it used power. But today’s aerodynamic cars are designed to be run on freeways with the windows up for the least wind resistance. A car gets better freeway fuel economy with the air conditioning on and the windows up than it does with the windows down and the air conditioning off. In fact, at 40 mph more gas is used with the windows down.

Air Conditioning Principles

Automotive air conditioning works on the same principles on which household refrigerators and air conditioners work. A liquid refrigerant is changed to a gas and then back to a liquid again. If there is to be a change of state of the refrigerant to take place, there must be a transfer of heat. Two principles apply here:

  • For a liquid to change to a gas, it must absorb heat
  • For a vapor to change to a liquid, it must release heat

Heat Transfer

Whenever there is a difference in the temperatures of two objects, heat can be transferred. Heat will flow to anything that has less heat. Heat transfer occurs by convection, radiation, or evaporation.


When a body gives off heat, the surrounding air becomes warmer and moves upward. Air that contains less heat takes its place. This is convection, one of the processes of removing heat. Two principles of convection are:

  • Heat rises (notice how steam or smoke rises)
  • Heat always flows from hot to colder

In a car air-conditioning system, air is not actually cooled. Heat is taken away. Have you ever walked past the outside of a house air conditioner installed in a window? Did you feel the hot air coming off the back of it? Heat is being removed from the house.


Another way heat is transferred is by radiation. The effects of the sun’s radiation are felt by the human body when moving from the shade into bright sunlight. Dark car colors absorb and radiate heat more than light colors.


Heat is also transferred by evaporation. As water vaporized it absorbs heat, cooling the surface.


High humidity makes evaporation it the human body is most comfortable at to 80°F. (22.2°C to 26.6°C) with humidity 50% in street clothes. People feel just as cool at 79°F with: Just as they do at 72°F and 90% humidity.

States of Matter

All metals can exist in one of three different states. The three states of matter are solid, liquid, and gas. When a solid is heated to a certain point, it begins to melt become a liquid, as more heat is applied, the temperature remains the same until the entire solid is liquefied.

When the temperature of the liquid, say water ix heated some more, the temperature will start to rise again. The temperature of the water rises until it changes into steam. Steam will remain at 212°F at sea level. This is called the heat of vaporization.


Moisture from steam condenses on a cool glass or mirror, a vapor changes to a liquid. As a vapor condenses, it releases its
Heat. When steam condenses back to water, it Les 970 BTUs of heat per pound. This heat released during condensation is called latent heat of condensation.

Cooling means “taking away heat.” During evaporation, heat is absorbed. The lower the boiling temperature of a liquid, the easier it evaporates. Alcohol feels cooler on your hands than water does. This is because alcohol has a lower boiling point. As it evaporates, it pulls heat from your hands.

An A/C system consists of three major devices in a closed system: the compressor, the condenser, and the evaporator. Refrigerant circulates among these devices. A flow control device regulates the flow of refrigerant among them. For an air-conditioning system to operate there must be large differences in pressure within the system. Changing pressure and the gas or liquid state of the refrigerant regulates the operation of the cooling cycle. The air-conditioning has four stages: compression, condensation, expansion and vaporization.

Absorbing Heat

Located inside of the car’s interior is a device called the evaporator, who’d refrigerant is circulated thru the evaporator. Refrigerant enters the evaporator as a liquid and changes to a gas as it removes heat from the air passing through it.

Reducing Humidity

Humidity can enter the car’s interior from the outside air. When moisture condenses on surfaces, visibility problems can result. Vaporized refrigerant is pumped through the suction side of the evaporator to the compressor in the engine compartment. The compressor is driven by a belt from the engine crankshaft. It pressurizes the refrigerant, further increasing its temperature. The temperature of the refrigerant before it enters the condenser makes the condenser more efficient at removing heat.

Compressor Clutch

The compressor has a clutch that connects and disconnects from the crankshaft by a electromagnet mounted on the compressor pulley.

Transferring Refrigerant Heat to Outside Air

The refrigerant is pumped to the condenser, located in front of the engine’s radiator. The condenser has metal tubes/fins. The condenser is a radiator for refrigerant. Its job is to transfer the heat that was absorbed in the passenger compartment to the cooler air blowing through it. The refrigerant’s boiling temperature becomes higher when it is pressurized. This is the same idea as pressurizing radiator coolant with a radiator cap to raise its boiling point. Cooling the pressurized refrigerant in the condenser causes it to change from a gas to a slightly cooled but still warm liquid.

Pressure in the condenser must be high enough to operate the flow control device and raise the refrigerant temperature well above that of the Ambient (surrounding). The refrigerant becomes concentrated and very hot when it is compressed. It must be hotter than the air coming across the condenser or heat transfer cannot take place. This allows rejection of the heat that was absorbed as the refrigerant was flowing through the evaporator.

Air Conditioning System Parts

All air-conditioning systems use an evaporator, condenser, and compressor. The system will also use one or the other of these flow control devices. The system will use a dryer also. The expansion tube system uses a receiver dryer and the orifice tube system uses an accumulator dryer.

Flow Control Devices

To raise pressure, there must be a restriction in the air-conditioning system. The restriction divides the system into the hi and the low. The terms high side and low side refer to high pressure and low pressure within the system. The flow control device, located in the inlet pipe of the evaporator, lets the high pressure off of the refrigerant as it changes to vapor. When the evaporator becomes too cold, moisture that has accumulated on the fins freezes, preventing airflow and hampering heat dissipation. The evaporator can also freeze up if its drain is plugged, allowing water to build up in the evaporator case.

Orifice Tube

An orifice tube system cycles the compressor clutch, instead of using an expansion valve. It has a fixed orifice just before the evaporator.

Air Conditioning Compressors

Compressors were large and heavy. Today’s compressors are small and light. There are several types of compressor designs. Three of the designs use pistons. Earlier crankshaft type compressors have cylinders and resemble a gas engine. They have cast iron rings. Later compressors use Teflon® rings, they provide low friction so they can be used in an aluminum bore while causing very little wear.

The larger cast iron A-6 compressor used by GM up until the early 1980s weighed nearly 35 pounds. Its aluminum replacement weighs only 12 pounds.

A radial compressor is like a radial aircraft engine. It has multiple cylinders with double-ended pistons and one eccentric crankshaft throw called a Scotch yoke.

Another compressor type is the scroll compressor that has a fixed and a movable scroll. As the scroll orbits, a pumping chamber forms that is open at the outer end. The chamber becomes smaller as the scroll rotates. The advantage to this compressor is its smooth operation. One other compressor style, the rotary vane, has blades like a power steering pump or smog pump.

Compressor Lubrication

Another way that compressors are similar to two-stroke gasoline engines is the way they are lubricated. Most compressors are lubricated by oil that is carried in the refrigerant. The old, heavy A-6 has a sump and pump.


Some air-conditioning systems have a muffler installed on the outlet of the compressor. This is because some systems make pumping noises due to high or low side pressure vibrations. Mufflers are occasionally wrapped with insulation to further reduce noise.

Accumulator or Receiver Dryer

Air conditioners use either a receiver/dryer or an accumulator. Both devices have a desiccant that removes moisture from the system. Moisture is a contaminant in an air conditioning systems because it can freeze refrigerant and corrode the inside of the system. A receiver/dryer is installed in the system, its functions is to be sure that pure liquid ant is supplied to the expansion valve. An accumulator is installed in the low side. One of its functions is to be sure that pure refrigerant vapor is supplied to the compressor.

When a receiver/dryer is used, it is installed after the condenser where it provides a storage place for excess liquid refrigerant until it is needed again by the evaporator. Remember, refrigerant is a high pressure liquid after the condenser.
Some systems have a sight glass on the receiver dryer or in the high pressure liquid line so that unwanted vapor bubbles can be spotted as the system operates. This can give a quick check of whether the refrigerant level is too low. A sight glass is only found on the high pressure side of the system.

If the system has an accumulator instead of a receiver/dryer it will be found in the low side at the outlet to the evaporator. All of the refrigerant does not vaporize in the evaporator core so the accumulator must capture any liquid to keep it from going to the compressor. Liquid refrigerant would “slug” the compressor, ruining it. The compressor is only supposed to pump vapor. A sight glass will not work on the low side because it is full of vapor, not liquid.
An accumulator is used with an orifice tube. The hole in the orifice tube is only 0.072″. This sounds small, but 200 psi will blow a substantial amount of refrigerant through that size hole.

Evaporator Icing Control

There are several methods of shutting off the clutch to keep the evaporator from freezing on an orifice tube system. A thermostatic switch on the evaporator is one method. It has a mercury tube that senses when the temperature of the evaporator falls to about 32°F. The switch turns off the compressor clutch.

Pressure Cycling Switch

Remember, the evaporator accumulates moisture as it dehumidifies the air in the passenger compartment. This water would freeze if the air-conditioning system were to continue to pump a fresh supply of refrigerant to the evaporator core. Air from the blower would not be able to carry heat away from the passenger compartment and the system would cease operation.
Sometimes a pressure cycling switch is used instead of a thermostatic switch. The temperature in the evaporator can be predicted by the pressure of the refrigerant inside. The pressure cycling switch is mounted on the accumulator at the outlet of the evaporator.

Some systems, mostly in the 1960s and 1970s, use a suction throttling valve (STV) installed between the evaporator and compressor. These systems did not cycle the clutch. Lower pressure means that the system is getting colder. An STV monitors pressure in the evaporator and shuts off refrigerant flow when the pressure drops below 30 psi with R12 refrigerant. An STV can also be called a pilot-operated absolute (POA) valve, or an evaporator pressure regulator (EPR) valve.

System Switches

There are many different switches and controls used to protect the system. Some of them are listed here.

Some air-conditioning systems run in the defrost position to dry the air going to the windshield. An ambient temperature switch keeps the compressor from working when outside temperatures are cold (about 35°F). This protects the compressor from damage.

When pressure in the system drops too low, usually because the system is low on refrigerant, the low pressure cutout switch will open the circuit to the magnetic clutch. This switch is mounted in the compressor inlet. When there is little or no refrigerant, the system must not be allowed to operate because there will be no lubrication for the compressor.
A high pressure cutout switch mounted on the compressor outlet shuts off the compressor if discharge pressure becomes too high. This causes compressor damage.

A pressure relief valve bleeds off excessive pressure and can be located on the compressor high side, or anywhere else on the high side of the system. A cutoff switch can be used to shut off the compressor during wide open throttle on cars with small engines.

Refrigerants and The Environment

Refrigerant that was widely used until recently is R-12 or Freon®. It has been around for over 60 years is nonpoisonous, stable, and inexpensive to manufacture. Because it has been outlawed, its cost is very high. The major problem with this refrigerant was that it was not discovered until recently, depletion of the earth’s stratosphere (10-30 miles above the surface of the earth) is its protective ozone layer filters out most of the sun’s harmful ultraviolet rays. The amount of ozone, which makes the sky appear blue, has remained constant in the stratosphere for centuries. Freon® CFCs are depleting the protective ozone layer through a chemical reaction When CFCs are released into the atmosphere; they slowly travel into the stratosphere where they may remain for 100 years or more.

In 1987, the United States and twenty-two other countries signed an agreement known as the Montreal Protocol. The agreement set limits on the production of ozone-depleting chemicals, which were totally phased out by the year 2000. In Europe and the United States, they have been banned since 1996.

Beginning in 1994, almost all cars were equipped with R-134A refrigerant. This refrigerant is a hydro fluorocarbon, rather than a CFC. It is simply a greenhouse gas and does not damage the ozone layer. There have been changes made to the systems to accommodate the different characteristics of the refrigerants. The most notable is that the condenser is larger. The system will cool effectively with the new changes.

This article is the intellectual property of Simmons BOSS CREATIONS. Any reuse of the contents must include the following attribution:

Marcus Simmons, ASE Certified
Phone: (248) 461-6977