Driving with the windows down in hot weather can be adding hundreds of dollars to your annual fuel bill.
The extra drag caused by open windows is a killer for fuel economy, as modern cars operate more efficiently with the air-conditioning working for cabin comfort. Open windows can be as bad as running with a roof rack, another overlooked cause of poor economy during summer holiday runs.
"A car uses less fuel to run an air-conditioning system than it will use due to the aerodynamic drag on a vehicle when its windows are open," says James Bertrand, who heads the thermal systems division at Delphi Corporation, one of the world's largest automotive suppliers. “It is often more fuel efficient to drive with the air conditioning on than with the aircon off and the windows rolled down, particularly when driving on the highway."
Delphi admits an air-conditioning system increases the load on an engine, but the latest designs of pumps are far more efficient and effective than aircon in the early days. American testing shows aircon can account for approximately five per cent of a car's annual fuel bill, but Delphi says modern energy-efficient systems can halve the running costs.
It has three big tips to maximise the effectiveness of aircon: opening windows to vent hot air before driving, using the recirculation system to improve cold-air flow, and setting the temperature to fully cold and using the fan to regulate the airflow. “These simple steps allow the car air and seats to be cooler which helps the air conditioning provide cooling comfort faster and save fuel at the same time,” says Bertrand.
He says it is important to purge hot air if a vehicle has been parked in the sun, before closing the windows to let the aircon go to work. Then, hitting the 'recirc' button — or Max on some cars — improves the effectiveness of a system because it is recirculating air which is already cooler than ambient.
Finally, a manual aircon system will run best on the fully cold setting to get maximum cooling. The blower fan is the best way to regulate the airflow. Delphi says aircon systems need to be monitored and should be checked for leaks if there is any loss of efficiency, while refrigerant needs to be topped-up because of losses through hoses over the years.
Also, newer vehicles have air filters which sometimes need cleaning to prevent reduced airflow from debris blockages.
History of airconditiong
1. The first modern airconditioning system is credited to Willis Haviland Carrier, who developed a system in Buffalo, New York in 1902.
2. The term 'air-conditioning' was first coined by Stuart Cramer in 1906.
3. Freon, the gas most often used in airconditioning, was created by Thomas Midgley in 1928.
4. Packard was the first carmaker to instal automotive airconditioners, in 1939. They cost $US274 and filled the whole boot space, pumping cool air through the rear parcel shelf.
5. General Motors fitted the first front-mounted aircon system as an option on its V8-powered Chevrolet and Pontiac models in 1954.