Oak Ridge Laboratory answers the A/C-on fuel use questions

  • 17-Apr-2013 12:23 EDT
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This is a fuel-economy graph for the Toyota Corolla. Notice crossover from slightly less to slightly more fuel-efficient with A/C-on at about 75 mph (121 km/h).

Which is more fuel efficient: rolling down the windows or keeping them closed with the air conditioning turned on? And how much fuel do you really save vs. windows closed with A/C off and perhaps the HVAC in vent with low blower speed?

There have been many studies on fuel consumption during U.S. EPA driving cycles, and a forthcoming drive cycle (AC17) that specifically will target the issue from an official standpoint. However, Oak Ridge National Laboratory's (ORNL) Fuels, Engines, and Emissions Research Center couldn't find the answers to these typical "motorists questions." So it tackled them from what it considered a motorist's perspective. That is, it sought to answer the questions for a steady highway cruise in summer weather, and a report on its results was given at SAE 2013 World Congress by Shean Huff, a member of the research team.

Obviously because of the variability in what motorists do, both in driving and setting the controls on the A/C, there is no single set of answers. And although windows rolled down was more economical at lower speeds, there was a crossover point at higher road speeds when A/C produced better results, but only with one of the two vehicles tested. In any case, the testing showed that a motorist might as well opt for comfort. In A/C-on vs. A/C-off testing, only the most extreme situation, with windows closed and the A/C off, was there anything resembling a "major" difference in both vehicles tested.

However, the testing did produce many interesting results, among them that maximum possible cooling vs. motorist's true comfort level was a large factor in fuel usage.

Explorer, Corolla tested

ORNL researchers had a 2009 Ford Explorer and Toyota Corolla for the tests. The original intent was to do all the testing on public roads, but even with knowledge of local roads, there was likely to be too much variation from such factors as humidity, wind direction, and grade (and the resulting transmission shifting activity) over the period of time necessary to provide enough data.

As a result, the decision was made to use the facility's Burke Porter 300-hp (224-kW) two-wheel-drive dynamometer. Also at the dyno setup was a 40-hp (30-kW) variable-speed fan, rated capable of supplying up to 30,000 ft3 (850-m3) of air from a 25 x 25-in (635 x 635-mm) outlet duct that can be raised or lowered and has variable inclination. The fan can be adjusted to match dyno road speed (up to 80 mph/129 km/h). The laboratory is climate-controlled, and the testing was done at 95°F (35°C).

The dynamometer calibrations were set up with results derived (per SAE procedure) from an 80-0 mph rolldown road test. The fuel economy was calculated with the same carbon balance method used for the EPA numbers, although of course this was a simple procedure, not a five-cycle fuel-economy measurement.

'Worst case' to start

The test was designed to start with "worst case," that is, with the system in Max Cool, Recirculation, High Blower, airflow set from vents (to face) and 1500 W of heat added to the cabin. This was deemed certain to maintain a 100% A/C duty cycle, and to reflect the highest parasitic loss. The configuration was intended to produce a maximum fuel-economy penalty, although something more typical of short-trip operation, which would include initial cool down.

The testing did confirm that the aerodynamic drag effect of rolled-down windows increases with road speed, while the effect of A/C operation decreases.

The A/C clutch operation was determined through the OBD II connector and vehicle data bus on the Explorer. This was not possible on the Corolla, which had a dealer-installed A/C system, so ORNL monitored the clutch status via test instrumentation (a current probe) connected to the clutch coil wiring. For both vehicles, however, the OBD II connector was used to measure vehicle speed, mass airflow, and throttle position. The cabin was instrumented to measure air temperatures at the A/C outlet vent and rear seat "breath" area. In addition, ambient temperature and temperatures at the condenser inlet and outlet were monitored.

Despite the added cabin heat, the researchers found that the 100% A/C duty cycle could produce uncomfortably cold compartment air that eventually dropped to 58°F (14°C). That still translated to a crossover, where use of A/C was more fuel efficient than rolling down the windows at about 75 mph (121 km/h)—the extreme result of the drag effect that has long been assumed.

On the Explorer, there was no crossover, because of the large cabin.

But with both vehicles, the researchers determined that cabin comfort was best at an average 50% duty cycle, so this was modeled. The results were that the A/C fuel consumption at 60 mph (97 km/h) was reduced by 50-67% on the Corolla, by 50% on the Explorer. So at that typical highway cruising speed and a reasonable setting for the A/C, the motorist will be comfortable and there's likely to be no fuel economy penalty vs. driving with the windows down. Yes, there is a drop in fuel economy with A/C-on vs. A/C-off and windows rolled up, approximately a 6-7%. But no A/C operation and closed windows at 95°F (35°C) is not realistic, of course.

No surprise: relative fuel consumption from A/C operation was highest at idle.

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