Fundamentals of The Automotive Cabin Climate Control System
Fundamentals of The Automotive Cabin Climate Control System
Fundamentals of The Automotive Cabin Climate Control System
Heating Ventilation and Air Conditioning (HVAC) is the technology for indoor and automotive
ambient comfort. HVAC facilitates in managing the pleasant climate inside the cabin by controlling
the degree of hotness/coolness.
There were times when having an air conditioner in a car was considered one of the big features, but
today air conditioners have become standard equipment even in entry-level automobiles. The desire
for even more comfort and luxury has led to the development of the climate control system inside an
automobile. The primary purpose of automatic climate control is to manage the temperature of a
given area for the comfort of onboard passengers.
HVAC was first introduced into automobiles in the early 1960s, and is available in most of the highend vehicles today. It is a complex system consisting of mechanical/electronic switches or knobs in
the frontend. The backend of the system comprises one or more blower motors, actuators (for fresh
air circulation control, air-flow control and temperature control), and refrigeration unit coupled with
many ducts through which air is transferred to the cabin.
The basic principle behind the operation of HVAC unit is conduction and convection. Heat is
transferred from a low-temperature region to a high-temperature region in the vehicle, due to the
pressure difference. This process of heat transfer is called Refrigeration. Figure 1 shows the cycle
diagram of the complete refrigeration process.
Evaporator
Compressor
Condenser
Receiver/Drier
Expansion device
The five major components are divided into two pressure regions: the high-pressure side is the
condenser and receiver/drier unit, and the low-pressure side is the air conditioning evaporator. The
dividing point between high and low pressure cuts through the compressor and the expansion valve.
The following section discusses in detail each piece of the HVAC system (see Figure 2).
removes heat of compression, motor winding heat, mechanical friction, and other heat absorbed in
the suction line. Another key task of the air conditioner compressor is to generate the flow of
refrigerant in the system.
The tasks performed by compressor can be summarized as follows:
1.
2.
3.
4.
Superheat
Remove latent heat or (condense)
Remove more sensable heat or (subcooled)
Generate the flow of refrigerant
Condenser
Condenser
The hot, high-pressure vapor makes its next stop at the condensing coil. The condenser is just like
the evaporator it is a heat exchanger. Inside the condensing coil the refrigerant flows from top to
the bottom of the coil.
Since the refrigerant is at much higher temperature than ambient temperature it cools down as it
passes through the coil. By the time the super heat refrigerant reaches the lower third of the coil, it
cools down enough to change back into a liquid. This process is known as sub-cooling.
As the refrigerant condenses to liquid form by releasing the heat, the outside temperature of the
copper tube becomes very high and with the help of the blower/radiator fan heat is blown out of the
system. This heated air in some vehicles becomes the source of warm air in cooler climate
conditions.
The placement of the condenser is also important for better efficiency, as it is very hot, so maximum
surface area needs to be exposed to ensuring cooling at a faster rate.
Drier/Receiver
Drier/Receivers are located in the high-pressure section of the system, usually in the plumbing
between the condenser outlet and the expansion valve inlet, although some may be connected
directly to the condenser.
The drier/receiver serve three very important functions:
1. They act as storage containers for extra refrigerant during periods of low cooling demand. This is
the receiver function of the receiver/drier.
2. They contain a filter that can trap contaminants inside the A/C system.
3. The Drier/Receiver contains a material called a desiccant. It is used to absorb moisture (water)
that may have gotten inside the A/C system during manufacture, assembly or service. This is the
drier function of the drier/receiver.
Expansion Device
The expansion device is required to generate the pressure difference for liquid refrigerant to boil off
into gas. The expansion device creates a pressure drop by restricting the flow of refrigerant around
the system.
Slowing down the flow of refrigerant causes the compressor to partially evacuate one side of the
system. This low pressure void is called the suction side or the low side of the system.
Figure 3: In an automated climate control system, the data from temperature and humidity
sensors in the vehicle cabin is continuously compared by an MCU to the setting defined by
the occupants, and the heating/cooling is adjusted appropriately.
Each manufacturer has its own distinct method for delivering the perfect climate for individual
passengers; however, they all rely on certain similar components, like additional controls in the
drivers HVAC control unit, an additional HVAC control unit in the rear seating area, individual
temperature sensors for each zone, lots of extra hidden ducting to carry the air where its needed
and extra vents - lots and lots of extra vents. For example, the Lexus LX570 has 28 of them.
About the authors
Nitin Gupta is a Sr. Application Engineer with experience in the field of embedded software
development for digital multimedia both consumer and automotive.
Manish Jindgar is a Sr. Application Engineer responsible for developing reference applications
related to automotive domain.
Ravinder Dasila has been working in the Automotive MCU group at Freescale as an Application
Enginner since July 2011 and is involved in designing automotive reference solutions.