Climate control engineers face new CAFE test

  • 12-Aug-2014 04:02 EDT

The AC17 test for manual temperature control system starts with full cold and high blower settings, then switches to midpoints at first idle of the SCO3 test.

Automotive climate control engineers are working to wring increased efficiency not only from the refrigeration system, but from every peripheral too. Yes, there are CAFE (corporate average fuel economy) credits that can be claimed for specific engineering features from a U.S. EPA menu, but cars and light trucks will have to earn them by passing a new EPA validation test: AC17. And with the industry facing regulatory dictates for electric drive vehicles (EVs) from California and some northeastern states, reducing the detrimental effect that climate control operation can have on vehicle range may be equally critical.

AC17 is on a complex phase-in schedule that starts with reporting only, to give industry some time to iron out wrinkles. It's a more real-world test than the basic A/C idle test in present use.

However, efficiency contributions of upgrades are small, and individual ones may be within the tolerances of test repeatability. It is hoped that along with overall results of all upgrades, certain segments of the complete AC17 test will show significant improvements that can be attributed to particular upgrades, such as improved performance during the high demands of initial cool-down.

Test initially is optional

AC17 was developed by the EPA in conjunction with USCAR (U.S. Council for Automotive Research, the Detroit 3 research consortium) with input from CARB (California Air Resources Board). Like any brand-new test that introduces many variables, AC17 is getting considerable analytic attention.

The test is optional to the A/C idle procedure for 2014 through 2016, so the industry can learn its nuances. Automakers will begin sending AC17 data to the EPA in the 2015 calendar year, showing results for efficiency improvement from reduced reheat, increased recirculation, pulse-width modulated blowers, and internal heat exchangers. Because of AC17's complexity, only one vehicle from a platform must be tested.

The A/C idle test is basically 10 min of A/C on, 10 min off, within an ambient temperature range of 68-86°F (20-30°C). Results are compared against a calculated standard—i.e., the difference in CO2 g/mi between A/C on and A/C off. If too great, reduced (or potentially no) credits are allowed. Large engines tend to show a much smaller increase in load at idle from A/C vs. a small engine (V8 vs. four-cylinder). At present, manufacturers can use either the A/C idle test or AC17. But credits initially are "gifted" with AC17, which should promote its use.


The phase-in schedule begins with the 2014-2016 model years. EPA's CAFE credits (in the form of CO2 g/mi) are a maximum of 5.7 g/mi fleet average. Manufacturers report either A/C idle test data (every configuration of vehicle or engine displacement), or results of AC17 on one configuration per platform. If the manufacturer chooses the AC17 option, it gets full credit just for reporting results. A manufacturer can "bank" some credits from early use of a low-global-warming refrigerant.

In the 2017-2019 phase, EPA's CO2 g/mi and NHTSA's gallons/mi both apply. Basically equivalent for CAFE, they are 5.0 g/mi (0.000563 gal/mi) for cars, 7.2 g/mi (0.000810 gal/mi) for trucks. AC17 must be used, but again, reporting results for one configuration per platform earns full credit. Total A/C credits (to 2025) for efficiency upgrades and using low-global-warming refrigerant are 18.8 g/mi for cars, 24.4 g/mi for trucks.

In the 2020-2025 phase, CAFE credits are unchanged but AC17 enforcement begins. It is mandatory on one configuration per platform with the efficiency technologies and one baseline vehicle (without). The Baseline need not be identical; in fact could go back as far as 2014 and be combined with engineering analysis. AC17 results must equal or exceed CAFE menu credits, or credit is lowered to reflect improvement shown.

AC17, performed at 25°C (77°F) and 50% relative humidity, incorporates versions of EPA window sticker tests, beginning with Federal Test Procedure 74 (part of FTP 75 city test, basically the preconditioning section), followed by 30 min of solar loading (850 W/m²), then SCO3 (assortment of speeds and acceleration rates) and the highway test, in a two-hr procedure. AC17 is performed twice (first with A/C on, then A/C off), for total test time of 4 h.

Specific manual A/C strategy 

However, even something seemingly as straightforward as turning on the A/C is anything but. With Automatic Temperature Control, the system is set for a particular temperature: 22°C (72°F). Although many systems have a proprietary control algorithm, particularly if the default is ECO mode, this approach is believed to yield significantly comparable results. However, manual A/C requires a different strategy: full cold for temperature and high blower speed, with temperature and blower speed turned to midpoint settings at first idle on SCO3, targeting a center register air outlet temperature of 55°F (13°C) This isn't intended to be equivalent to the ATC operation.

EPA based the CAFE credits on research performed during an SAE International Cooperative Research Group's I-MAC (improved mobile A/C) study of 2005-08. In that program, the EPA and the National Renewable Energy Laboratory (NREL) aimed to identify a 30% improvement in system efficiency and a 30% reduction in cooling loads (plus 50% reduction in leakage). AC17 testing to date has demonstrated reasonable repeatability for conventional cars and light trucks—to within 2.0 g/mi, or about 0.6%. However, comparable repeatability isn't quite there yet for hybrids, primarily during SCO3, which involves greatest use of the hybrid system.

There are CAFE credits for solar/thermal technology (glazing, seat and cabin ventilation and reflective paint). Solar/thermal credits are capped at 3.0 g/mi for cars, 4.3 g/mi for trucks. Off-cycle credits are available for thermal control/energy usage improvements, including idle stop, aero, waste heat recovery, high-efficiency lighting, and active engine and transmission warm-up. Including solar/thermal, hybrids, plug-in hybrids, and EVs, credits are capped at 10 g/mi for a manufacturer's fleet.

AC17 solar loading tests show solar film/glass has a significant effect on CO2 emissions in comparative evaluations, both in SCO3 and highway tests. On a Dodge Avenger, the film produced a 12 g/mi difference (430.4 vs. 442.4 g/mi) vs. no film in an SCO3 test, and a 5.3 g/mi difference in the highway test (251 vs. 256.3 g/mi).

NREL solar load activity

NREL has been particularly active in real-world testing of solar control film, glass and paint, and the effect of vehicle pre-ventilation—primarily with the objectives of improving EV range. It cited data on the Mitsubishi i-MiEV that indicated a drop of 46% in Max Cool, 34% in A/C Normal. It also referred Automotive Engineering to a study by FleetCarma, a fleet data-logging company that showed the Nissan Leaf range averaged 75 mi (121 km) at 65°F (18°C) and dropped to 58 mi (93 km) at 95°F (35°C).

NREL tested two Ford Focus EVs, provided under a cooperative agreement with Ford, with one vehicle a control. The other was fitted with solar control film on all glass surfaces. The cars were hot-soaked from sunrise until noon. Average interior air temperature reached 121.3°F (49.61°C) without the film, 111.7⁰F. (44.28°C) with the film, with ambient at 77°F (25°C) at noon.

The laboratory then began a 20-min simulated-drive cool-down in Max A/C and measured A/C energy savings at 48.9%. Cabin air temperature equivalency (to within 1.2°F/0.7°C) was established by lowering blower speed on the film-fitted Focus, which reduced evaporator capacity (and compressor power required, to produce the energy saving). This is apparently a maximum "because of the A/C settings, extended soak with full solar load, and a 20-minute simulated drive that weights the thermal transient fairly high," NREL's John Rugh said.

A separate pre-ventilation test during plug-in, immediately prior to drive-off, was made by NREL, and it pointed to another way to reduce A/C electric use to increase EV range. It used a Focus blower and ducts to purge cabin air following a similar hot soak, and cabin temperatures were lowered 12.6°F (7°C) within 15 min, at a cost of just 0.08 kW·h. Extending pre-ventilation to 30 min doubled energy consumption but lowered cabin temperature only an additional 1.8°F (1°C). A/C energy savings from a 20-min cool-down were estimated at about 50%.

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