Summer’s longest day has passed and the roads are now packed with families heading off for their summer holidays. You might be looking forward to the relaxation of going away on holiday, but the journey? Traffic jams, fractious families and delays getting to airports can all be guaranteed to send a driver’s blood pressure skywards. Driving in stop-start traffic is probably the worst possible route to good fuel consumption. In fact, you can look forward to watching your fuel economy reading trickle downwards if you just sit there, air conditioning on, engine ticking over. And that is exactly what will happen if you are stopped, with air conditioning continuing to pump cool air around the car.

In cooler weather, if you have automatic stop-start, it would probably switch your engine off most of the time that you are not moving, helping to cut fuel consumption. If your car doesn’t have the automated system installed, you can simply do it yourself and turn the engine off when you aren’t going anywhere. It would not only cut fuel consumption and air pollution, but also remove the background noise of idling engines and cooling fans cutting in to keep engines from overheating. But in warmer weather, the stop-system will operate less, because of the parameters set-up by the car manufacturer. To ensure that the battery isn’t drained excessively by climate control and air conditioning systems, quite simply, the engine just keeps ticking over on a hot day, and in the process, guzzling fuel. Idling in this way is one of the guaranteed ways of keeping fuel consumption up.

But what alternatives do you have on a hot summer’s day? You could just open the windows. Assuming you have been rolling along at a decent temperature up to that point, the chances are that air conditioning will have cooled the cabin to your chosen temperature anyway. If you’re not moving, why not just switch off and open the windows to get some fresh air? A nice breeze will help to keep the car cool, then when you start to move again, use the air conditioning then to bring the temperature back down again. If you are driving alone, you can always open the windows on the passenger side of the car to keep the air flowing while you are stopped to keep the noise down. There are few things more upsetting when you’re stuck in traffic than the sound of free flowing traffic heading in the opposite direction. But there may be a downside to all of this, in that other drivers may well be sitting in their cars, engine on, air conditioning working away, and filling the area with exhaust fumes.

So why does air conditioning use up so much fuel? Think of it as the reverse of heating the car. Just as much energy is needed to remove unwanted heat as it is to introduce wanted warmth. The difference is that while cars can tap into waste heat from the engine to warm the car, we can’t reverse that process. The air conditioning system works just like your household fridge, with a refrigerant that can absorb the heat, pumped around a pair of heat exchangers to transfer the unwanted warmth from inside the car to somewhere else.