How Does Using the Air Conditioner Affect My Fuel Consumption?

Posted on | February 4, 2011 | 1 Comment

When talking about fuel consumption and car air conditioners, it is a good idea to first know how an automotive air conditioning system works. A car’s air con system is basically composed of:

1. A compressor, which is the heart of the system. The job of the compressor is to circulate the R-134 refrigerant through the rest of the system. It is run off the engine by a belt and is turned on and off by an electric clutch which is tied to the air con’s thermostat.
2. The condenser is basically a radiator, and is mounted on the front of the car’s radiator. It usually has an electric fan in front of it. The condenser’s job is to cool the hot compressed gas that is being pumped through it by the compressor.
3. The evaporator is located inside the car and does the job of circulating the cooled refrigerant or gas. The evaporator unit has a blower to pass the air through the tubes and fins of the evaporator. This air is what comes out of the vents in the car.
4. The expansion valve controls the flow of cold refrigerant to the evaporator. This regulates the cold air passing through the evaporator.
5. The drier, also known as the receiver-drier, is a safety mechanism for the air con system. The drier catches the liquid and other contaminants that may be circulating in your system before it can reach your compressor.

Switches, hoses and relay make up the rest of the system. So, the air conditioning system then is electro-mechanical in nature and is not just a blower (the evaporator) or a compressor (which is run off the engine). To the question asking if the AC increases fuel consumption, the answer would be, it depends on the current condition of the car while the AC is on. Are you in traffic idling your engine? In the city going with the (slow) traffic flow? Or in the interstate going 70 mph?

With our understanding of the AC system, not turning on your AC logically cuts the drag on the engine and the load on the electrical system (which loads up the alternator). Depending on the efficiency of the system, turning on the AC uses from 15-30 horsepower. Logic would say that we should get better fuel economy with the AC off. That is true, but only up to a certain point. That is why the qualified answer to the question in the previous paragraph was, it depends. In slow driving or in traffic, the car will definitely consume more fuel with the AC on. However, it has been proven that if you are traveling at interstate speeds, driving with the AC off and the windows down increases drag to the point of actually increasing your fuel consumption. A study in Europe actually documented a 7 percent increase in fuel consumption when driving at 70 mph with the windows down.

So, in this case, it would be better to turn on the AC if you are on the highway and roll the windows up. Obviously, the best situation for fuel efficiency would be driving at a steady state on the interstate with the windows up and the AC off. But that would be ridiculous unless it is winter because you would be sweating profusely in no time at all.

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