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How to cool the interior of a hot car efficiently

What is the most efficient way to cool down the interior of a car which has been parked in the hot sun? Also, is it more economical to drive with the windows down and air-conditioner switched off?

When a car is parked in the sun, the air trapped inside absorbs solar heat and will begin to rise in temperature.

Interior parts like the door pads, dashboard, steering wheel and gear lever will also absorb a considerable amount of heat.

Most drivers will get in and immediately turn on the air-conditioning, set to minimum temperature and maximum blower output.

What the air-conditioner has to do is first, absorb all the heat from the cabin air, and gradually begin to reduce the temperature. Just how long this process takes varies from car to car and, of course, depends on the efficiency of the air-con.

Initially, air thrown out of the vents will be uncomfortably warm.

In most cases, the whole process of bringing down the interior temperature takes a much shorter time if the car is driven with the windows down for a few kilometres.

This helps to replace the heated interior volume with the ambient air, which is of a lower temperature. The fast-moving air also carries away some of the heat from the interior.

Meanwhile, the draft will also ease the discomfort of the heat and humidity.

Switching on the air-con thereafter gives it a better chance at hastening the cooling process.

In Singapore, driving with the windows down is usually uncomfortable. Also, this increases aerodynamic drag on the car.

Hence there will be little or no benefit in fuel economy from not switching on the air-con. In fact, for a highly streamlined car, it could even be detrimental to fuel efficiency.