In hot weather, cars tend to get lower gas mileage than at other times. That’s not a flaw in the engine control system. It’s the extra work that the air conditioning makes the engine do.
The cold season is very different. Every engine produces heat, and we simply route that waste heat into the car’s interior. Heating the car during driving comes with no extra cost.
But air conditioning requires removing heat from the interior. So an extra radiator is required. The big energy consumer, though, is the compressor. It squeezes a refrigerant gas into a very small volume. The compressed gas is then cooled in its radiator. When a valve system lets it expand again, it gets very cold. A fan then cycles the indoor air across another type of radiator — the “evaporator” — which gradually cools the air inside the car. All this costs extra gasoline.
A real waste of money occurs when the air conditioning puts out lukewarm instead of cold air. Most of the time, it needs a recharge of refrigerant, but the compressor pump also wears over the years. In that case, gas is wasted running it, but no real cooling results. Servicing the system is best left to a professional shop, because some parts are under very high pressure, and injuries can occur easily.
In stop-and-go traffic, where conditions are often the most sweltering, the system operates at its lowest efficiency because there is little wind to cool the radiator. Steady highway speed usually lets the engine achieve its highest gas mileage. Turning on the air conditioning at 60 mph can be more efficient than having the windows open because the car doesn’t have to fight air turbulence. Motorists who want to save gas know to leave the a/c off in dense city traffic, then roll the windows up and turn the cooling on when things are moving swiftly.
The new electric cars are a different story. Both heating and cooling cost money, because the heater as well as the air conditioner draw power from the battery pack. It’s still rare to get 100 miles out of a single charge. If mileage is critical, an electric car needs to be driven as much as possible without the comfort features of heating or air conditioning. Improvements in battery technology hopefully will provide higher mileage per charge in the years to come.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His email is email@example.com