OR... How to P&G your A/C
I split this into 3 sections since I'm verbose.
1. Reasons to use it
2. Theory behind a/c and how it works
3. Steps you need to take
1. This is an easy method of reducing your FE losses due to A/C while retaining some of its benefits. I have to use my A/C when I'm dressed up in a shirt and tie as well as when my wife and/or baby are in the car, and I use this to keep everyone, including my wallet, happy.
2. Basic air conditioner theory: Your air conditining is operated by an engine driven compressor, then two "radiators"; a high pressure, hot one that is warmer than the outside air, and a low pressure, cold one that is cooler than outside air. Thermodynamically speaking, when the compressor does work on vaporized refrigerant to compress it into a liquid, its temperature increases. This is an adiabatic process. The high pressure refrigerant travels from the compressor to the condenser, which is in front of your car's engine-cooling radiator. Since it's now very hot, it loses energy to the outside air flowing through it. After losing energy, it travels to the inside of the car, passes through an expansion valve that allows a small amount of refrigerant to pass through. The refrigerant then enters the evaporator core, an area of low pressure. The liquid refrigerant evaporates, absorbing energy from the air inside your car, and then returns to the compressor.
Your compressor does not run all the time your car is on; it also doesn't run all the time your a/c is turned on. Most cars' expansion valves are thermostatic; they work to maintain a constant temperature within the evaporator core. If the expansion valve only allows a small amount of refrigerant through, the pressure in the high side of the system doesn't drop. A sensor detects this and disengages the clutch on your a/c compressor.
Therefore, if you do what you can to keep the evaporator core cool, you can decrease the percent of time your compressor is running.
3. What to do: Two things will allow you to keep your evaporator core cool and your compressor clutched off.
First, always use the recirculate function on your climate control. Already cool, dry air flowing through the evaporator won't warm it up as much as warming fresh air from outside.
If your car doesn't have a recirculate switch, chances are that it has a "normal a/c" and a "max a/c". Use the MAX! This seems counter-intuitive, but since the recirculating a/c works better, it's labeled as such, and it's also more efficient.
Second, once you get your car to a cool enough temperature, adjust the blower fan to the lowest speed setting. You still get cold air blowing on you, but the smaller volume of air that flows through the evaporator, the cooler it will stay.
Using these two steps, you can have your a/c and afford to eat for dinner too!
