Air con helps you keep a cool head

The hotter your car's cabin gets, the worse your reactions and decision making become. Photo / John Borren

Once upon a time car air conditioning was a luxury. Now it's all but standard as buyers rapidly realise it's useful not only to cool the cabin in summer, but to demist those windows in winter.

So how does it work? Much the same as your fridge. Typically, they use a compressor, a condenser that's often fitted behind the grille to benefit from air flow, an expansion valve and evaporator, plus pipes to connect the lot.

The compressor pressurises the refrigerant gas, which heats it, then pumps it through the condenser's coils. As it travels heat radiates into the passing air and the now cool gas condenses to a liquid.

As it goes through the expansion valve the sudden pressure loss turns it again to a gas, now a very cold one as it's drawn heat from its surroundings - much as you'll get cold if liquid evaporated to vapour on your hot skin.

Still following us? That cold gas then passes through the evaporator's coils, then goes back to the compressor.

Effectively, air going over the evaporator and heading to your car's vents has given up its heat en route. Meanwhile, the moisture it carries condenses on those coils, just as dew forms on cold glass. The dry air makes its coolness more effective - helping to evaporate your sweat.

But the same system also works in winter to beat condensation inside the car -allied to the heater, of course.

Considering saving money by leaving the air con off? It'll work better if you run it a few times a week to keep the system's seals lubricated. Don't use it and they'll dry out, shrink, and then leak. Yes, you'll use about 10 per cent more fuel with it on than off. But at around 85kph, depending on your car, the extra drag created by opening the windows costs more fuel than the air con would.

Mind you, air con isn't perfect. Early refrigerant gases released chlorine when they broke down, thus contributing to ozone depletion, and they're now illegal. The hydrofluorocarbons that replaced them are a greenhouse gas and the EU plans to ban them by 2018, in the meantime cracking down on regs controlling gas leakage.

You could go without, of course, but there's a safety factor. If you're not at a comfortable temperature and atmospheric humidity, your driving - and the ease with which you shrug off fatigue - will suffer.

People work at their best between 20C and 22C. But when it's 30C outside and sunny, an hour of driving can see cabin temperatures lift to 42C around your head.

An increase from 25C to 35C can reduce your powers of reasoning by 20 per cent - equivalent to a blood alcohol level of 0.5ml. So use your air con, and stay both cool and safe this summer.