Test Setup – Second Phase

In a second level, testing the release of HFO-1234yf oil mist onto a surface with lower temperatures was investigated. The temperature was reduced to 800degC in order to simulate an operating turbo charger hit by a mixture of tetrafluoro-1-propene and PAG oil ND8. In this scenario the mixture was again ignited. The flame propagation and the flammability envelope lead to the conclusion that this pilot fire had caused a secondary fire in the engine compartments. It could be concluded that by using HFO-1234yf, today’s high safety level in terms of flammability would be significantly reduced, causing an increased vehicle damage after moderate front end collisions and putting human life at risk.

In a next step, the temperature was reduced to 600degC (simulating a hot exhaust manifold). Also in this setup the refrigerant oil mixture was ignited. The flame propagation was substantial and even so the flammability envelope was reduced, it could still be judged to be sufficient to cause a secondary fire.

Conclusion Second Phase

It can be concluded that 2,3,3,3-Tetrafluoroprop-1-ene (HFO-1234yf) used as a refrigerant in a vehicle application reduces significantly today’s safety level in terms of flammability. Possible implications are increased insurance premiums since the cost for repair will be increased. Further investigation is necessary on the implications for persons being present in confined spaces, e.g. during accidents in tunnels or garages, when such a fire happens (due to the low LC50 values of the thermal decomposition products). The risk of a pilot fire caused by a front end collision in a vehicle using HFO-1234yf is significantly increased. A risk that the pilot fire causes a secondary fire became clearly visible during testing and is highly possible (the flames consumed the polycarbonate indicator panels and the operational wire harness).

Background

Pure ND8 oil was released without refrigerant to the 600degC surface. The oil was ignited. The flame propagation and the flammability envelope were judged to be moderate but the burning oil will provide the MIE (minimum ignition energy) for the ignition of HFO-1234yf.

An additional flammability test of pure HFO-1234yf was conducted using a cigarette lighter. The refrigerant was ignited and flame propagation was observed. The person handling the test was equipped with an active carbon filter mask and fire protective gear. The reason for the mask and protective gear was that the thermal decomposition products of HFO-1234yf are Hydrogen fluoride (HF) and Carbonyl fluoride (COF2). These substances have a LC50 (lethal concentration) value at 1h of 966 ppm and Carbonyl fluoride (COF2) is having a LC50 value at 1h of only 360 ppm. LC50: 50% of an animal population dies within 1h if the atmosphere contains the indicated amount of the substance in ppm.

Technical Development Direction

When compared to R152a (A2), the HFO-1234yf flammability envelope is slightly reduced. However, the flammability envelope of HFO-1234yf requires either an oil free circuit or a secondary loop for the front end heat exchanger.

Outlook

In a next phase, side impact collision will be investigated during which refrigerant lines inside the passenger compartment are ruptured.

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