A second SAE-administered MRB CRP has been working since early 2011 to evaluate the potential adoption for MAC application of two low-GWP, zeotropic refrigerants originally developed by Mexichem Fluor as medium low pressure fluids having similar operating pressures to R-134a.

These fluids, AC5 (now designated R-444A by ASHRAE) and AC6 (provisionally designated R-445A by ASHRAE), are both based on the HFO R-1234ze(E). Both fluids studied in this project are ternary refrigerant mixtures.

• AC5 is a formulation of R-32/R-152a/R-1234ze(E) in the nominal proportions (by weight) 12%/5%/83%. AC5 was formulated to give a close performance match to R-134a whilst having similar flammability characteristics to R-1234yf.

• AC6 is a formulation of R-744, R-134a and R-1234ze(E) in the nominal proportions 6%/9%/85%. AC6 was formulated to give a fluid having somewhat higher refrigeration capacity than R- 134a (originally for a stationary refrigeration application) but with reduced flammability compared to R-1234yf. AC6 is not flammable at normal ambient temperature, whereas both R-1234yf and AC5 are flammable at ambient temperature.

They are both zeotropic mixtures, meaning that they exhibit a difference in compositions between liquid and vapor at equilibrium, and undergo a temperature change—often referred to as temperature glide—during evaporation or condensation.

The MRB CRP team is now sharing a technical White Paper for these blends, based upon the information previously reported at SAE Thermal Management System Symposium (TMSS) in October 2012, and more recently updated at UNEP Round Table on New Technology in Refrigeration and Air Conditioning held in Milan on 7 June 2013.

The AC5/AC6 project has been operated in three phases.

1. February to October 2011. A basic feasibility assessment of both fluids was carried out during this time. The testing methodology was developed from the framework of the previous CRP-1234 and the same outside laboratories were used.

   In addition to the scope of investigation covered by the CRP-1234 activities it was also agreed to extend the safety and risk assessment to car systems having a dual (rear) evaporator and to electric or hybrid vehicles.

   In phase one, the team covered most of the equivalent scope carried out by CRP-1234, but for two fluids and in a period of 9 months rather than two years. At the end of phase one both fluids seemed to be acceptable on the basis of the initial flammability, materials compatibility and performance tests.

   In particular, they found that the hot metal surface temperature required to ignite a spray of either fluid mixed with PAG lubricant was about 100K higher than that required to ignite a R-1234yf/lubricant spray, with other flammability characteristics such as ignition energy found comparable or more favorable than those of R-1234yf.

2. October 2011 to October 2012. In this phase, which saw more team members joining, the focus switched to more in-depth assessment of remaining key technical issues likely to be found in using and handling zeotropic refrigerants: a more detailed study of ignition source and underhood flammability; study of potential for selective leakage to alter refrigerant composition, investigation of different heat exchanger designs for use with AC6, and longer term testing of compressor durability. The fault tree analysis was also continued and extended as more data was generated by the program activities.

   The conclusion of Phase 2 activities was that both fluids could be candidate alternatives to R-134a or R-1234yf for MAC application, with acceptable safety and environmental performance.

3. The third phase of the study is focusing on the AC6 fluid as it had demonstrably lower risks associated with its flammability but also required some further effort in addressing questions around composition maintenance in car and during service.

   The goals of this phase are: to enable the construction and demonstration of prototype equipment for handling the refrigerant; to review and consolidate understanding of the flammability characteristics of AC6 in real world scenarios (such as potential for ignition in engine compartments); to develop recommendations for working standards on the use of the refrigerant in MAC applications; to produce supporting information to enable SNAP submission of the fluid, and to demonstrate engineering approaches to further improve performance by modifying system design to allow for the zeotropic fluid characteristics.

   The project aims to report final findings at the SAE TMSS symposium in Troy, Michigan in October 2013.

Sponsors of the MRB CRP include: Behr; Bosch; Chrysler; Cinetic Filling; Daimler; Denso; Doowon; General Motors; Halla Visteon Climate Control; Hyundai; Jaguar Land Rover; Mexichem; Nissan; PSA; Renault; SAIC Motors; Sanden; Schrader International; TEXA; and Volvo Cars.