Vehicle Hot Surface Ignition and Mitigation Measures of R-1234yf Refrigerant for MAC Systems

The European Commission (EC) as well as the United States Environmental Protection Agency (EPA) published legislations to regulate or encourage the use of low Global Warming Potential (GWP) refrigerants applied to Mobile Air Conditioning (MAC) systems. Europe mandates a GWP less than 150 of MAC refrigerants for new vehicle types. The thermodynamic refrigerant properties of R-1234yf are slightly different from the properties of R-134a, currently used in MAC systems. Although the basic material data show that R-1234yf is flammable, ignition tests performed for an automotive engine under-hood environment reveal design and packaging influences of its ignition behavior. After extensive collaborative research in 2009, the Society of Automotive Engineers Cooperative Research Team (SAE CRP1234) concluded that R-1234yf is suitable for use in automotive applications. Further ignition risk assessment regarding R-1234yf usage in MAC systems was done by SAE CRP1234-4 in 2013. They concluded that “risks are still very small compared to the risks of a vehicle fire from all causes and well below risks that are commonly viewed as acceptable by the general public.”

This paper will compare under-hood hot surface ignition behavior of five different vehicles under specific test conditions. Out of these five, two vehicles were further investigated and the engine compartment designs were assessed. A broad range of flammability testing was completed to study release of the R-1234yf and Polyalkylene Glycol (PAG) oil mixture. Both vehicles were equipped with a transverse gas engine, front take down exhaust routing, turbo charger, and single evaporator MAC system. This engine architecture was chosen due to the short distance from a potential condenser leak and the hot exhaust system components in case of a frontal crash. Although the engine architecture was the same for both vehicles, the engine design and packaging were different and were described in this paper. Parameters that were considered include heat shield design, engine packaging, exhaust system surface temperatures during refrigerant release, and the impact on the time to ignition of the R-1234yf/PAG mixture. Mitigation measures were also identified and their potential integration into a vehicle were discussed. As engine coolant will be released in many real-world frontal crash scenarios, the impact of vaporized glycol water mixture on the ignition behavior was assessed. This paper further describes the MAC systems, the test facility used, and the testing procedure applied.