Thanks to the automotive air-conditioning unit, the “weather” inside a car can change rapidly once we’re on the move – a great boon given our tropical climate.
Making a debut on cars in the 1930s, it was the Americans who first introduced the idea of automotive air-conditioning. Once found only on pricey, full-size luxury cars (because automotive air-con systems used to be bulky and expensive), the air-con has since made its way into just about every vehicle, regardless of size or price. But it wasn’t always this way.
Back in the 1960s and 1970s, if you wanted to beat the heat (and didn’t happen to own a luxury car), you might have to install an aftermarket air-con unit. However, most vehicles then were not designed to handle the extra load imposed by such a unit on its engine, cooling and electrical systems.
Common characteristics of such air-conditioners were severe loss of power (especially on smaller cars) and rough idling, and it made vehicles they were installed in prone to overheating.
Thankfully, auto air-cons have evolved very quickly, downsizing and down-pricing on the way, resulting in the modern split-unit air-conditioning system widely used in cars today.
In operating principle, they bear many similarities with the systems used in most homes, getting their name from the way the two main heat-exchanger sections are located apart from each other. This is opposed to the single-packaged air-con, which houses all the components in one box. Such units are more compact and easier to install, but its drawbacks are increased noise and inefficiency against split-unit types.
Whether it’s a split unit or single air-con, common to all are three main components – the evaporator, heat exchanger and condenser.
All air-cons are fully closed systems with refrigerant flowing through them, taking heat away from the interior of the car (through the evaporator) and releasing it into the atmosphere via the condenser. Refrigerant is never consumed or depleted, unless there is a leak somewhere in the system.
This is how it works… Driven by the belt off the engine’s crankshaft, the compressor, uh, compresses the refrigerant, which is in a gaseous state. This significantly increases the temperature of the gas, allowing it to be cooled in the condenser by the relatively cooler ambient air. What happens then is that most of the refrigerant, still under pressure, changes into a liquid state, and on its way to the evaporator, experiences a sudden drop in pressure, cooling further as it encounters the expansion valve.
The evaporator then absorbs heat from the air that’s channelled past by the blower, but more useful than said heat absorption is the dehumidification that takes place. That drop in the air’s humidity is what makes an air-conditioned car such sweet relief on a hot day.
Bringing things back to automotive air-cons, they come in two major “flavours”: manual or automatic.
In a manual air-conditioner (the more basic of the two), the ambient air temperature is controlled by the compressor, and when the thermostat signals that the desired temperature (set by the user) has been reached, the system goes into idle mode by disengaging the compressor’s electromagnetic clutch.
More sophisticated versions, known as climate control (or more simply as automatic air-con), use a combination of systems to maintain a cool, pleasant environment inside the vehicle.
Firstly, the temperature, set by the user, is constantly monitored by the system’s electronic “brain”. Temperature sensors scattered throughout the cabin then check left and right zones, and in some cases, the rear bench as well, comparing readings against the set temperature.
Modulation of that temperature isn’t just left to the compressor’s thermostat, unlike with manual air-con systems. With a climate control air-con, the fan’s blower speed varies automatically, too. Its “nervous system” can sense changes in the ambient temperature and is clever enough to respond accordingly, by keeping the compressor on, adjusting fan speed or modulating the fresh-air intake.
A further development of that is multi-zone air-conditioning. This used to be exclusive to premium cars, but is becoming an increasingly standard feature, even among more budget offerings.
They operate in a similar way to the automatic air-con mentioned earlier, in that such systems have just one compressor and condenser, but the piping for the refrigerant is arranged such that it feeds more than one evaporator. This allows for temperature zones that can be independently set and controlled (for example, 24 deg C for the driver’s side and 20 deg C for the front passenger’s).
In tropical Singapore, where the weather is more often hot than not, air-cons are only required to perform a cooling function, because the ambient temperature is almost always higher than that set for the air-conditioning.
However, in places where the reverse happens (that is, ambient temperature is lower than that set by the air-conditioner), the air-con system will automatically switch to heating mode.
But that’s something we needn’t worry about too much, unless global climate change takes its toll and it starts snowing in Singapore!
