Air Conditioning

Going back to the start of my driving career and air conditioning was available on comparitively few cars. It has been widely available on British specification machines since the late 1990s. Personally, it’s been an essential for me since I started commuting by car, mainly because of the summer heat; I’ve spent too long sitting in traffic on a hot day, with the fan blowing moist, warm air at me, the windows down, slowly overheating, to consider anything but an air conditioned car. Actually I said that when I replaced the Cinquecento with the Mondeo but that’s another story.

This is only one benefit from air conditioning.

The Ka is one of the least powerful cars available for sale with standard air conditioning (certainly not the least powerful though). In many respects this better illustrates the impact on performance of air conditioning.

What is it and how does it do it?

There are two benefits to air conditioning. One is that the system de-humidifies the air. The second is that it also cools the air. Many people assume that it merely chills the air, but it is the de-humidifying aspect that makes it much more comfortable. The human body cools itself by evaporation and the less water vapour there is in the air, the more efficient our own cooling system is. There are other useful benefits such as if the air inside the car is dry, the windows do not mist up.

I won’t go into too much detail about how the air conditioning system works because one, it’s boring and two, I’ll quickly get out of my depth. The cornerstone to how the system works is that when a liquid is allowed to expand to form a gas, it absorbs a large amount of heat energy. It is this cooling effect that is used to chill the air and a separate dehumidifier is used to remove moisture.

Since the system is sealed the gas must be compressed back into a liquid so that is may be reused. This process uses the compressor, which in most cars is driven by an auxiliary belt attached to the engine. Just as expanding the liquid cools the gas down, compressing it heats it up. The gas is compressed using the compressor, which in most cars is driven by an auxiliary drive belt attached to the engine. The gas is converted into a liquid in the condenser, which typically sits right at the front of the engine bay in a front-mounted engine. The condenser is cooled by one or more fans.

Because the expansion of a fluid into a gas causes a powerful cooling, the compressor is usually not constantly running, instead the system cycles the compressor in and out to prevent the fluid from freezing. Frozen air conditioning fluid will damage the system. This is why air conditioning systems will not activate at low temperatures, typically from anywhere below 4°C. Since changing the status of the compressor also changes the power being drawn from the engine, most ECUs /link/ will adjust the engine so as to smooth over the transition between the compressor running and not running.

The air conditioning system must be used to keep it at good operating efficiency. Every month, the air conditioner must first be used for at least half an hour, and then switched off and the heater used (in order to dry the ventilation system out). If the system isn’t used, the seals will dry up (and so the system will leak fluid). If the system isn’t dried out, mould and bacteria will grow in the system, spreading disease, infection, and smelling really bad. In the summer, this might not be the most comfortable thing to do, but to prove just how humid the ventilation system gets, after using the system for half an hour, switch it off but keep the ventilation fan running. You’ll soon notice that the vents are dripping wet.

If you want to read up more about air conditioning, this link to this article on the How Stuff Works website.

What are the disadvantages?

The first disadvantage is that the compressor requires power and in most cars, this is provided via an auxiliary belt attached to the engine. The amount of power required to drive the compressor depends according to the design and the cooling effect required but in most vehicles you do feel the impact in acceleration. You may also notice a small difference in fuel consumption.

The second disadvantage is that when the system is in use the cooling system efficiency is reduced because the air flowing past the radiator has already been heated up by flowing though the condenser. If the engine starts to overheat, the air conditioner is shut down. Many systems also shut down if the driver requires full power.

Some cars’ ECUs do not smooth over the transition from loading the compressor to not, which results in a rather jerky experience under certain conditions. The Ka has a relatively jerky transition, the Honda was seamless unless on the overrun and the Saab is somewhere between the two, but closer to the Accord.

The air conditioning system requires maintenance and therefore adds to the running costs of the car. Depending on who you ask, the system should be re-gassed every other year. It should also be regularily run, again depending on he manufacturer, some believe it should be run for a minimum of thirty minutes a month. This is to keep the seals lubricated and so prevent the gas or fluid from escaping.

The final disadvantage is that the system adds a little weight and complexity to the engine bay.

In Use

I use the air conditioning system as frequently as I want to throughout the year but with one caveat – I try not to use it for windscreen demisting, since if you consistently use air conditioned air for demisting purposes the car will mist up very quickly if the air conditioning system stops working.

If you get to a car after it has been sat in the sun for a while I would recommend you open the windows to let the trapped hot air escape when first getting into it, rather than simply switching the air conditioner on and letting it do all of the cooling work.

For maximum cooling effect you should use the air recirculation mode, which means the system can cool air that it has already cooled. Cars with climate control do this for you.

Manual, Semi-Automatic and Automatic Systems

The majority of cars with air conditioning either have manual or automatic systems save for a few with a semi-automatic system. This generation Polo is an example of a car with semi-automatic air conditioning.

With a manual system, you do just that – you control the temperature of the air entering the cabin, its direction and speed and if you should use air recirculation or the air conditioning system. Manual systems can be relatively easy to operate or can have certain irksome traits.

There are very few semi-automatic systems in existence. Volkswagen’s marketing department dreamt up a “Semi Automatic Air Conditioning” system for the Polo, which controls the temperature of the air leaving the system according to the interior temperature but does not the speed nor direction of the air flow.

With an automatic system, also know as “climate control,” the car controls everything for you other than setting the temperature. It will adjust the temperature, direction and speed of the air entering the car to move the cabin temperature to your determined level. It will control the use of air recirculation and the air conditioning system. Most automatic systems have manual or semi-automatic modes of operation too.

Climate Control Systems / Automatic Air Conditioning

All climate control systems share a few common traits. In essence, the system accepts inputs from a variety of sensors and uses the data to determine the most suitable direction, speed and temperature to output air.

Some sophisticated systems are able to maintain different zones in the car, automatically switch air recirculation on to avoid bad smells (can they determine if the occupants are producing the smell?), maintain a desirable humidity level and avoid blowing icy cold air at the occupant(s).

The Saab’s automatic air conditioning system attempts to control the environment inside the car so as to make the passengers feel as though the temperature is as selected, which is not quite the same as putting the temperature of the vehicle at the selected temperature. In strong sunlight the interior of the vehicle may need to be cooler than the selected temperature to counter the effects of warm sunshine, for example.

The 9-3’s Automatic Air Conditioning System

The 9-3’s automatic air conditioning system takes input from the usual collection of sensors. Most of these sensors work in the generic way.

The most obvious is the solar sensor, which lives in the middle of the dashboard. If the car is in strong sunlight the AAC will reduce the temperature of the air and is more likely to increase the fan speed. I do not believe the solar sensor is directional.

The cabin air temperature sensor is located immediately below the AAC unit, in front of the gear lever. This unit consists of a small fan that draws air from the cabin over the sensor.

The mixed air sensor tells the AAC unit the temperature of the air that is leaving the heater unit.

The external temperature sensor is only partially relevant to the operation of the automatic air conditioning system, but it determines if the AAC should use the air conditioning compressor and heated mirrors and rear screen elements.

The AAC also takes an input from the vehicle speed sensor, being more likely to increase the fan speed at a low road speed.

In addition to these sensor inputs, the AAC has a number of changeable defaults and can be used in semi-automatic mode. The defaults include at what temperature to use the air conditioning compressor, either at 1°C or higher, or 11°C or higher (the second temperature can be adjusted using the Tech2 /link/ diagnostics equipment). You can have a default mode to start the car in, too. You can select if the AAC should automatically handle using the heated mirrors and rear screen.

In semi-automatic mode, you can manually determine the fan setting and / or the direction, but other than putting the car into maximum heat or maximum cooling, you cannot manually determine the heater output. Instead, you can determine what temperature the AAC should try to make the passengers feel.

I should add that the automatic air conditioning has three special modes, in common with many similar systems.

Saab’s Special Modes

Maximum Heating: push the AAC temperature beyond 27°C and you engage maximum heating mode. This directs the air to the windscreen / footwell, at high speed and full heat.

Maximum Cooling: push the AAC temperature under 15°C and you engage maximum cooling mode. This directs the air to the face level vents, at high speed, the air conditioning compressor engaged and air recirculation activated.

Demist: runs the compressor, puts the fan to high speed and directs the air to the windscreen. Useful for rapid demisting.

Chef’s Automatic Air Conditioning In Use

Most of the time, the automatic air conditioning is left set at 21°C in full Auto mode. Like this, the system will use the air conditioning compressor if the temperature is at or above 11°C. It will defrost the car if necessary.

I do occasionally change the temperature, moving it up or down one or two degrees, but most of the time I can leave the system in automatic mode.

Where I do intervene is usually when it is raining. Here, I may force the system to use theair conditioning compressor for demisting purposes even though the temperature is at or below 10°C. I may also direct the airflow to the windscreen by pushing that button twice. One press puts the car into demist mode, which runs the air conditioning compressor and the fan at high speed, directing the air at the windscreen.