Name one car convenience feature you absolutely couldn't live without, and it'd probably be air conditioning.

Whether you live in a hot or cold climate its cooling and demisting benefits can't be understated--it has safety benefits as much as comfort and convenience ones.

It also has an effect on your gas mileage--by as much as ten percent.

More load = more gas

When your air conditioning pump is running, it has to take its power from your engine. Running an extra belt to an A/C pump might not sound like a lot to do, but it means your engine is working a little harder to drive you along, all other things being equal.

This is easiest to understand if you've ever driven a low-powered car with air conditioning.

Sometimes, you'll experience a noticeable drop in power when you turn on the A/C, as you give your engine another task to do. Turn it off again, and you'll immediately feel those lost horsepower return...

New EPA tests

So significant are these 'parasitic' losses that the EPA is working on a new AC17 test procedure for vehicles taking A/C's effect on gas mileage into account.

According to Automotive Engineering Online, the EPA will then use these measurements to provide carmakers with credits for installing efficient systems that use as little extra fuel as possible.

The tests, one with A/C on automatic and the other with it off, consist of a 23-minute preconditioned period, a 30-minute solar hot soak, and a 23-minute EPA drive cycle. The tests are performed at 77 degrees Fahrenheit and 50 percent relative humidity.

These credits will be in addition to credits earned for using refrigerants with a global warming number below 150--i.e. those that contribute least to the greenhouse effect.

Initial results will be run without penalty or credit, to evaluate the procedure. From 2017-2019 the results will set a baseline, used from 2020 onwards to determine whether a car is doing better or worse. The testing is just one way carmakers will improve economy to meet 2025's 54.5 mpg standards.

First tests

GM has unveiled details from an initial run of the 2.4 liter Buick LaCrosse.

With A/C off, the LaCross achieved 24.6 mpg during the drive cycle. The best result with A/C on was just 22.9 mpg--the worst only 21.5.

The effects of a parasitic draw like air conditioning vary from vehicle to vehicle, but the net result is the same--a rise in fuel consumption.

While few of us can live without the cooling blast of A/C or its ability to clear a frosted window, there's still no reason it should be overlooked in the pursuit of efficiency--just what the new tests intend to achieve.


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