As summer winds down, a professional association of transportation engineers is offering a few pointers for the efficient operation of your vehicle's air conditioning system.

Representatives of the Climate Control Standards Committee of SAE International recently authored a report on the air conditioning system's fuel consumption and how drivers can operate it at its most efficient levels. The organization says air conditioning is one of the most energy-intensive accessories in a vehicle.

A number of variables affect how much fuel an air conditioning system needs to use to bring a vehicle's cabin temperature to a comfortable level. These factors include the ambient outside temperature, vehicle speed, humidity, fan setting, and traffic.

SAE International says that while drivers seek to cool a vehicle as quickly as possible, doing so puts a greater strain on an air conditioning system since vehicles that have been at rest are likely to have scorching temperatures. A house with no air conditioning will only warm up by about eight degrees on a 90-degree day, but temperatures inside a vehicle on a 94-degree day can heat up by 60 to 70 degrees. Certain parts of the vehicle will also be extremely uncomfortable for the driver, including seats that can reach 130 degrees and instrument panels that can get as hot as 212 degrees under these conditions.

Drivers might also have difficulty getting to a comfortable temperature on days with high humidity. While some vehicles have humidity control to reduce energy consumption, others lack this feature.

Opening the windows provides a passive way to cool down a vehicle's interior, but it also increases wind resistance. This increased drag leads to more fuel consumption, meaning the use of open windows burns more fuel at higher speeds than air conditioning.

Open windows are a more feasible alternative for slow speeds. SAE International says systems with a high blower speed and maximum cooling load resulted in a fuel consumption of 4.1 to 8.9 miles per gallon at 30 miles per hour. By contrast, the energy use fell to three to six miles per gallon at 50 miles per hour.

Minimal air conditioning settings resulted in similar fuel consumption rates at either speed. When recirculated air was used and the fan was at its lowest level, fuel consumption was 0.5 to 3.9 miles per gallon at 30 miles per hour and 0.5 to 3.1 miles per gallon at 50 miles per hour.

The report says that using outside air and high fan speeds in an air conditioning system results in the highest energy use, while recirculated air and lower fan speeds use the least fuel. Using outside air for the first few minutes after starting a vehicle can help get rid of hot air in the interior, but switching to recirculated air afterward will cut down on energy consumption.

When traveling in city conditions, SAE International recommends opening the windows for a few minutes to vent as much heated air as possible. While using the air conditioning can further purge hot air from the vehicle, it is more efficient to travel with open windows in city conditions to keep the interior cool.

On long trips, use the outside air setting for five to 10 minutes every hour. The report says this setting will help improve air exchange in the cabin, improving the air quality in the vehicle.