Is auto A/C a parasite? GM engineer says that's the fuel-economy-centric view

  • 25-Apr-2013 07:23 EDT
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Buick LaCrosse testing with A/C off vs. A/C on. Graph shows results on SCO3 (EPA city procedure) but at moderate 77°F, with temperatures set as shown.

“We’re parasites,” a room full of air-conditioning engineers at the SAE 2013 World Congress in Detroit on April 17 was told by Jeffrey Bozeman, a General Motors advanced A/C engineer. The audience didn't get offended, because it knew what he meant.

A/C may be a necessity for the motoring public—including the safety factor of defrost. But from a fuel-economy standpoint, it consumes fuel that otherwise could propel a car further along the road, so it's a parasitic draw.

Work is under way that is leading to AC17, a U.S. EPA test procedure that actually will measure A/C fuel use and provide credits to the extent efficiency is demonstrated (test illustrated at here).

This is part of the mandated route to the 54.5-mpg-by-2025 CAFE (corporate average fuel economy) mandate staring in carmakers' faces. They recognize that they must make many changes in other areas, but that A/C also is a target for improvements in fuel economy. A/C accounts for perhaps 5% of total automotive fuel consumption but may use up to twice that percentage or more when turned on. And A/C impact on electric vehicle range is enormous, estimated at up to half in very hot weather.

'Indirect' credits

There are EPA incentives for improving fuel efficiency of A/C and reducing the heat loading of the cabin. They are in a group of "indirect" credits, meaning they apply to technologies that reduce fuel consumption. Some appear hard to quantify adequately in a vehicle-based test, and if a carmaker introduces something not listed in the "indirect" group it can get extra credits by submitting evidence to EPA.

Credits are in g/mi of carbon dioxide, for which there is a conversion formula to mpg. The A/C credits for fuel economy are capped at 5.7 g/mi through 2016 and thereafter are 5.0 for cars, 7.2 for trucks. "Direct credits" apply only to the refrigerant and are 6.3 g/mi for low-leakage systems (per a menu in SAE J2727). There also is a credit of 13.8 g/mi for cars, 17.2 g/mi for trucks, if they contain an alternative refrigerant with a global warming number below 150 (R-1234yf is the only choice to date).

All listed credits presently are applied on an "if a car has the technology, it gets the credit" basis, except for the minimal evaluation of an "A/C at idle" test. That will change with the adoption of AC17, a test developed by USCAR (U.S. Council for Automotive Research), a Chrysler-Ford-GM group, plus EPA and CARB (California Air Resources Board).

AC17 consists of two 2-h tests, one with A/C on, one with A/C off. Each starts with a 23-min preconditioning period, followed by a 30-min solar hot soak (at 850 W/m²) and then 23 min of EPA drive cycle—10 min city/13 min highway. Fuel economy between the city/highway periods is compared (A/C on vs. A/C off). AC17 is performed at 25°C (77° F) and 50% relative humidity, which were deemed more representative of A/C use than the ambient conditions in EPA fuel economy cycles designed to evaluate emission controls.

Carmakers' AC17 results go to EPA on a no-penalty (or credit) basis, to evaluate the procedure and consistency, up through 2016. In 2017-2019, the results set the baseline, and starting in 2020 a car must do as well or better, using the efficiency technologies in its baseline, such as: externally controlled compressor with reduced HVAC reheat, default system to increase recirculation, compressor oil separator, improved evaporator and/or condenser, pulse-width-modulated blower motor, and internal heat exchanger (IHX).

The test is time-consuming, so only one volume-representative model per platform must be tested. AC17 only includes those features likely to be accurately quantified in a "road testing" cycle. EPA also will continue with off-cycle credits for such features as active seat ventilation, passive and active cabin ventilation, and solar reflective paint. All credits add up to 8.3 g/mi for cars, 10.3 g/mi for trucks, but are capped at 3.0 g/mi for cars and 4.3 g/mi for trucks.

If a carmaker introduces new technology in 2020 and seeks extra credit, it must demonstrate value in AC17, or if that's impractical, by engineering analyses.

GM's Bozeman shared some test results (approximately comparable to AC17) with a 2.4-L Buick LaCrosse, following EPA city and highway schedules at 25°C. GM started with one test at 76°F (24°C) setting on the Automatic Temperature Control and switched to 72°F Auto (22°C) for the second, third, and fourth test, and finally a 68°F Auto (20°C). Theoretically, all 72°F Auto tests should have produced the same results, and Bozeman attributed increases to powertrain computer adaptive learning, as the HVAC was unchanged.

IHX an exception

To date, AC17 evaluations seemingly show reasonable correlation with the fuel economy improvements the carmakers anticipated, except for the IHX, which receives a 1.1 g/mi CAFE credit.

The IHX has demonstrated considerable improvement in A/C coefficient of performance in OE testing, said Bozeman. AC17 failure to reflect improvement is being attributed by researchers to moderate temperature/humidity test conditions. However, a particular vehicle also may be a factor. Recent GM testing, with a two-evaporator system, is the latest to show a clear advantage for the IHX. Perhaps supplemental evaluation methods are needed.

The two-evaporator results, for an R-134a system, were described at the SAE Congress by Lothar Seybold of Opel. The company tested four plumbing setups, and one (labeled C1) he said was a clear winner—lightest, easiest to manufacture and service, lowest cost, and least complex to package.

The IHX, also called a suction line (low-pressure line) heat exchanger, is incorporated in the refrigerant suction line, plumbed with the high-pressure A/C liquid line, to subcool the liquid. Heat is transferred from the hot liquid to the cold vapor of the suction (low-side) line, and this subcooling of the liquid increases the amount of heat it can absorb when it vaporizes in the under-dash evaporator. Seybold said the design chosen (C1 in table) had highest heat transfer in most test points, 25% higher heat transfer compared to the second design. At 77°F ambient, it showed 6.4% higher efficiency compared with no IHX and up to 2.2% better than the C3 IHX installation.

The C1, Seybold said, had the best front evaporator cooling capacity.

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