How an Air Conditioner Works

Would you like to know how your vehicle’s air conditioner works? The system looks complicated, but it's really quite simple. Once you know how it delivers cold air you will be better informed when problems occur and how to fix them, or understand more on what a garage is telling you when having it repaired.

Your heater and air conditioner (HVAC) have three jobs, cool the interior of the car down, heat the interior up and defrost the windshield. All three modes work together as a climate control system. This system is controlled by a main computer which sits in the dash and also houses the controls for the mode and temperature settings.

The computer gives commands to the compressor to start pressurizing the refrigerant along with the blend door actuators to direct air flow from the floor, mid and defrost vents. It also controls blower fan speed. Actuators also control air temperature by mixing the hot air from the heater and the cold air from the A/C.

When you turn the system on a voltage supply is given to the climate controller, blower motor fan and the compressor clutch or internal valve. These components then begin to operate connected through a climate control wiring harness fuse and relay.

How Does It Work?

Let's start with the refrigerant (r134a) inside the system. This particular refrigerant was specifically created for the automotive industry sometime in the late 1980’s which is slightly different than home or industrial refrigerant applications. Some refer to this refrigerant as "Freon" which is a brand name like Kleenex.

This part fluid, part gas is held under pressure inside the system. When the system is not running the refrigerant remains a gas under low pressure, about 70 psi. The optimum property of this gas is its ability to go from a hot gas when it's compressed (250 psi at about 180° F or 82° C), and when cooled in the condenser located in front of the radiator it transforms into a warm high pressure liquid which is then is released through an expansion valve or orifice tube (small hole). The high pressure liquid instantly transforms to a low pressure gas vapor (35 psi at 32° F or 0° C) which creates the coldness inside the evaporator in which case the blower motor passes air through it and into the ventilation system and into the interior of the car.

This entire action is basically a high pressure liquid, then released into a low pressure gas. If you take a can of hair spray and then release the product along with the propellant the can will get cold. This is the exact same chemical reaction used by the air conditioner the only exception is the gas is recovered inside the evaporator and then recycled into the system.

The evaporator core looks like a small squat radiator which is located inside the vehicle. Once the refrigerant has spent its cold charge it is gathered into the low side hose and onto the compressor where its once again processed.

The diagram below shows the entire system and refrigerant flow (high pressure in red, and low pressure in blue). These systems are closed (sealed) which continuously re-circulates the refrigerant. A receiver dryer or an accumulator is used to filter and remove moister from the refrigerant to help prevent the system from being damaged by rust or corrosion.

The refrigerant is transferred from one component to another by using hoses or tubes. This would be the high side and low side pressure hoses. The high side hose connects the compressor to the condenser, from the condenser to the expansion valve or orifice tube which are both located in front of the evaporator. The low side low connects the evaporator back to the compressor. The expansion valve or orifice tube do basically the same job, the system will have one or the other. An expansion valve can vary its size due to temperature whereas an orifice tube is a fixed size.

Air Conditioner Components

The AC system has four basic parts: A compressor, which is powered by the engine using the serpentine belt and has a low side port that is connected to the evaporator along with a high pressure side port which is connected to the condenser using rubber hoses. The compressor is the main mechanical part of the system with internal workings that must be lubricated by peg oil (synthetic).

The compressor is fitted with a clutch that is activated when the system is switched on. The internal parts of the compressor start to turn via the engine power and pressurizing the refrigerant.

On hybrid engines the compressor is electrically powered. A hybrid and electric vehicle’s compressor is powered by the vehicle's battery pack not the gasoline engine. A small electric motor is fitted inside of the compressor which pressurizes the refrigerant. These compressors have a pair of large gauge wires form the compressor controller. Check out the picture below:

A condenser is located in front of the engine radiator and cools the refrigerant from the compressor before it heads to the evaporator located inside the vehicle. Some condensers are designed with a receiver dryer attached which acts as a refrigerant cleaner and moisture remover which helps the system last longer. Some systems will have an accumulator that does the same job, each car maker is a little bit different, but do the same operation.

Some air conditioner systems have a separate fan for the condenser instead of the radiator cooling fan, or they will have an additional fan to help the radiator fan cool the refrigerant. These fans are controlled by the climate control computer which gives a command to the control relay to power the fan on. When these fans fail the system will not cool as well.

The condenser below is placed right in front of the engine's radiator so it can get air drawn through it much like the radiator by the engine cooling fan.

The evaporator is where the coldness is created using an expansion valve or orifice tube which is like a small hole and is where the high pressure liquid is released into the evaporator.

Air conditioner hoses are used to transfer refrigerant to from various components and is a popular place to find leaks.

High and low side service ports are used to add Freon and vacuum down and recharge and the system. These ports are usually located on each hose but can be on the compressor or accumulator as well.

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