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AVL DiTest is an approved R-744 service machine for the Mercedes-Benz system.


R-744 A/C refrigerant tries to make the OE grade

Is automotive air conditioning with CO2 as the refrigerant—designated as R-744—an environmental achievement in its infancy, or is it destined to be another unsuccessful attempt to defy Mother Nature?  R-744 auto A/C projects have been put on hiatus several times in the quarter century since they have been proposed, while being reactivated in 2013 by a consortium within the VDA (German auto industry association).

R-744 is non-flammable and has an ultra-low global warming (LGW) number of 1.0. The worldwide industry’s switch to mildly-flammable and expensive R-1234yf, with its LGW of  4.0 would seem to question R-744's future. But the first system with CO2 as the refrigerant finally is available. Initially it’s only in Europe as a Daimler option, reportedly at a $350 premium vs. under $100 OE cost for R-1234yf, on the Mercedes-Benz S-Class coupe.

Audi is next in line to offer an R-744 option on its Audi A8 this summer, with Mercedes following with a system for the full S-Class line and the volume E-class.

Pricing for an "environmentally-friendlier natural" refrigerant in an all-new system obviously can't be directly compared with 0.45 kg (1.0 lb) of a chemical fluid, even if the R-1234yf price is more than 10 times that of the R-134a it replaces along with the moderate cost of small changes for the system itself. 

SAE J2771 completion sought 

BMW, VW/Audi and Porsche engineering members of the VDA consortium attended the SAE Interior Climate Control Standards Committee (ICCC) meeting in Detroit during WCX17. The Daimler member of the group was unable to attend due to flight cancellations.  Although they had no estimated date for R-744 introduction in the U.S., they urged the committee to “populate” the mothballed SAE J2771 (refrigerant recovery equipment for R-744) with detail that includes the English version of a related DIN standard, because an applicable ISO standard (No. 13043) references J2771.

With the turmoil in the U.S. EPA regarding CO2 status as a pollutant and the controversy over fuel economy standards, the primary question might be the U.S. changeover to R-1234yf itself.  Members of the ICCC told Automotive Engineering they felt the "train had left the station," noting that several OEMs had already changed over the systems in  a majority of cars and others had it programmed into production schedules.

Further, they noted that the European Commission and several Asian countries had legislation prescribing an LGW refrigerant. Additionally, California (along with nine northeastern states that follow its lead on emissions) were committed to LGW refrigerants for cars and light-duty trucks, and California is developing a program for heavy-duty trucks. Any attempt to overturn the longstanding California exemption to EPA emissions initiatives would engender lengthy legal action, they said.

Recycling unecessary 

Technically, EPA requires recycling of all automotive refrigerants. But the agency has told the ICCC that CO2 would be an exception because, as a product of human respiration, it’s a natural refrigerant.  As a result, the recycling equipment would only be required to recover and vent safely, then accurately recharge a system with virgin refrigerant.

Daimler has approved two machines, the Texa Konfort 744 and  the AVL DiTEST ADS 310.  This equipment meets VDA consortium specifications, including measuring weight of recovered (as well as recharged) refrigerant to within ±10 g, so the extent of any leak can be assessed.  The machines, which use double-piston vacuum pumps and are designed to vent at a controlled rate, also perform a vacuum hold test per VDA specifications.

Additionally, the machines perform oil separation, which must be measured accurately to within 2.0 g. To ensure operator safety within the work area, the machines have a CO2 sensor, and if there is a 700 ppm  buildup (over a normal 406 ppm), the machine stops for an hour.

The typical system charge, AE was told, will be in the range of up to 500 g.  Although the compressor is designed to retain the oil, there will be some oil in circulating refrigerant (amount depending on driving cycle), which amount of oil will determine the performance of trace dye that is installed for leak detection.

Leak detection issues 

Automotive R-744 systems operate at about 10 times the pressures of chemical refrigerant systems in use today (approximately 2500 psi/17,237 kPA on the high pressure side, 500 psi/3448 kPA on the low-pressure side).  The current state-of-the-art on R-744 system sealing indicates the designs will need a recharge at 100,000 km/60,000 mi, perhaps earlier depending on the individual system.  So effective leak detection is a priority.

The VDA consortium has tried a number of approaches in addition to trace dye, but has not made specific determinations. It told the ICCC that at this time not to attempt to write an applicable version of SAE J2774, which was the proposed performance standard for R-744 electronic leak detection. CO2 poses the obvious difficulty of underhood detection because, with technicians present and working there, the natural concentration from respiration could create a detection problem. Many electronic detectors have false-triggered on carbon dioxide, although one CO2-specific detector (an infra-red unit from Inficon) features an ambient concentration sensor to calibrate it vs. the sample detected at the tip of the probe.

Porsche Engineering’s Moritz Knoetzele, who leads the service procedure development for the VDA consortium, said he has been using a Waeco Dometic multi-gas detector specifically calibrated for CO2, with some apparent success.

Initially, a tracer gas detection system using forming gas (95% nitrogen, 5% hydrogen), which alarmed on the hydrogen, was identified as a possible choice but was set aside after testing.

Also available are color-change foaming sprays, which the VDA consortium found would identify large leaks, but Knotezle said, left a messy residue that was difficult to clean.

The first R-744 systems will be limited in efficiency, as they are designed to keep the system cost down. R-744 does not condense until 31ºC/88º F, so liquefying at least some of it for improved system Coefficient of Performance is enabled by an internal heat exchanger, and the Mercedes system does have that.

Further, there is research that indicates the more complex parallel compression systems (long used in commercial systems) also help, as noted in a presentation by Ford at the SAE WCX17. If used with a diesel engine or electric vehicle, where cabin heat is an issue, the R-744 A/C could be modified for heat pump operation, as research has shown the refrigerant’s superiority in that application.

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