A new low global warming refrigerant, CF3CF=CH2 (HFO‐1234yf), has been developed to replace HFC‐134a in automotive air conditioning systems. HFC‐134a is being phased out in the European Union due to high global warming potential. HFO‐1234yf is highly energy efficient, exhibits low toxicity, and can potentially be used in systems currently designed for refrigerant HFC‐134a with minimal modifications. Significant work has been completed to assess the flammability characteristics of HFO‐1234yf. Though it is flammable per ASTM E‐681, it is significantly less so than HFC‐152a which has also been considered as an HFC‐134a alternative. HFO‐1234yf has a relatively high lower flammability limit at 6.2 vol% in air. Computational Fluid Dynamics (CFD) modeling has shown it is very unlikely the lower flammability limit will be reached when the refrigerant is leaked into the passenger compartment of a vehicle. HFO‐1234yf also has a high minimum ignition energy (MIE) from 5,000 to 10,000 mJ which indicates there may be very few potential ignition sources in a vehicle. It also has very low burning velocity at 1.5 cm/sec indicating low potential for damage should an ignition occur. Several automotive industry groups have completed extensive risk assessments which have concluded HFO‐1234yf is safe to use as a refrigerant in vehicles. This article will review flammability test results and challenges that were overcome to measure flammability of a mildly flammable refrigerant as well as CFD modeling work for vehicle leakage scenarios. © 2009 American Institute of Chemical Engineers Process Saf Prog, 2010

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