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Driving tips

Tech Talk:
The cost of keeping your cool

Published July 24, 2013

As we’ve discussed previously in this column, using the air-conditioner in your vehicle increases fuel consumption.

That’s because the air-conditioning compressor is typically driven by the vehicle’s engine, usually via some form of belt drive. Consequently, when the A/C is on, it adds a load to the engine that increases its fuel usage.

However, just how much additional fuel it takes to run the A/C is a matter of debate. Not surprisingly, the answer varies dramatically with such factors as the vehicle itself, driving speed, wind conditions and the temperature, both outside and in.

From a driver’s perspective, it’s a matter of comfort versus dollars and cents. Knowing just how great that trade-off is could make a difference in the decision on when and how much to use the A/C.

In broad strokes, using A/C for highway driving accounts for about 5 per cent of your overall fuel usage in a gasoline-powered car or truck. But it can be twice that percentage, or more, during the initial cool-down stage or on short trips.

The results of separate studies conducted by General Motors and the Oak Ridge National Laboratory in the U.S., both presented at a recent conference for the Society of Automotive Engineers, help to quantify the issue.

Temperature setting effect

In the GM study, a Buick LaCrosse with a 2.4-litre four-cylinder engine was tested in city and highway driving at an outside temperature of 25C, with and without the A/C on.

The driving schedule approximated that of a new test, called AC17, that will be adopted by the U.S. Environmental Protection Agency to determine the official fuel-economy ratings for vehicles in 2017. It was jointly developed by the U.S. Council for Automotive Research, the California Air Resources Board and the EPA.

With the A/C turned off, the Buick burned 9.56 L/100 km.

With the A/C on Auto, and set to a temperature of 24C, fuel use increased by 7.4 per cent. Setting the temperature at 22C increased fuel use by 10.7 per cent, and lowering it to 20C boosted fuel use by 14.4 per cent.

Those are significant increases, greater than the accepted norms for highway use. But the test incorporated both city and highway driving, with multiple accelerations and decelerations, so it is probably a realistic reflection of many drivers’ real-world experience.

Regardless of the exact increases, the study demonstrated that the lower the temperature control is set, the more fuel is used.

A/C versus open windows

The Oak Ridge study took a different approach, addressing the oft-repeated argument that it is more fuel-efficient to drive with the windows open than to use air-conditioning.

The counter-argument is that the added aerodynamic drag resulting from open windows offsets the increased power requirement of the A/C, making the latter the preferred choice.

To test those theories, the lab ran fuel-consumption tests on a Toyota Corolla at constant speeds ranging from 65 to 130 km/h, at a very high outside temperature of 35C.

As might be expected, the tests revealed that fuel consumption was always lowest with the A/C off and the windows closed. But that combination would be impractical in the real world.

Lowering the windows increased fuel use by 4.8 per cent at 65 km/h, 5.1 per cent at 80 km/h, 5.4 per cent at 100 km/h, 7.0 per cent at 120 km/h and 7.9 per cent at 130.

Using the A/C greatly increased fuel use at lower speeds — about 20 per cent more than with the A/C off at 65 km/h. But that negative effect reduced as speed increased, to just 5.3 per cent at 130 km/h.

The effects of windows down or A/C on crossed over at about 120 km/h, with A/C becoming the more fuel-efficient option above that speed.

Presumably, A/C would be more comfortable than windows down well before that speed, but researchers did not consider occupant comfort.

The bottom line is, keeping comfortable in your vehicle when it gets hot, however you do it, will incur some cost in terms of energy use, just as it does in a building. These studies provide some idea of what those costs really are.

wheels@thestar.ca

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