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Recently, vehicles with high capacity traction batteries for driving such as Ni-MH, lithium-ion have come to be produced in the world. However, the damage has not been clarified in the case of these vehicles fire by a traffic accident or arson. In particular, lithium-ion cells in high temperature environment may cause a thermal runaway which emits smoke or flare. Therefore, it is necessary to examine what kinds of phenomena occur when a vehicle with a lithium-ion battery catches fire. In this paper, the authors studied the situation and temperature characteristics of a hybrid electric vehicle fire to investigate the temperature input to a lithium-ion battery on a vehicle. We assumed an accident that gasoline leaking from other vehicle would be spread under a vehicle and ignited. In a vehicle fire test, gasoline was spread under a hybrid electric vehicle without the traction battery instead of a vehicle with lithium-ion battery, and ignited.

At the time of a vehicle fire, high pressure hydrogen gas in a tank (a high pressure hydrogen gas cylinder) of a fuel cell vehicle (FCV), which is a passenger vehicle, must exhaust through a pressure relief device (PRD) as quickly as possible in order to prevent any accidental bursts by a temperature rise of hydrogen gas in the cylinder. The high temperature region surrounding a vehicle develops when the hydrogen gas is released through a small nozzle to the air directly. Therefore, to suppress the high temperature region, the effectiveness of a diffusion box is considered further. A pressure relief device (PRD) detects differences in temperature of the environment surrounding an FCV on fire and releases hydrogen gas in a tank to the air by which the valve opens when the temperature in the environment becomes high. The PRD also releases hydrogen gas through a nozzle, e.g. installed upward or downward, to the outside of the vehicle. The PRD is required to be installed in an FCV.

For fuel cell vehicles, which have attracted attention in recent years, the prevention of hydrogen leakage is an essential safety issue. Large fuel cell buses will require a large space to store the hydrogen system. The behavior of hydrogen that has leaked into such a large space is unknown. In this report, we studied hydrogen concentration distribution by leaking hydrogen into simulated spaces in two cases: (1) when hydrogen gas tanks are installed on the roof of the bus, and (2) when an electricity-generating system, such as fuel cell stacks, etc., is installed at the rear of the bus. The results of the experiments show that hydrogen concentration distribution is kept at a constant level throughout each location in the simulated space, depending on the opening area and hydrogen leakage rate. It was also found that the diffusivity of hydrogen in air is extremely high.

Fuel cell vehicles that use high pressure hydrogen gas as a fuel should be able to immediately release hydrogen gas from the cylinder through pressure relief devices (PRDs) in the event of a vehicle fire. The release through PRDs prevents the cylinder from exploding due to the increased pressure of hydrogen gas, but the method of releasing the gas needs to be specified in order to avoid secondary disaster due to the spread of fire. Since hydrogen cylinders for fuel cell buses are different in terms of installation location and size from those for ordinary vehicles, the location of PRDs and the release direction of hydrogen gas should be separately examined. For example, the improper locations of PRDs would raise the possibility of explosion because of a delay in temperature rise, and the direct release of hydrogen gas from a cylinder installed on the rooftop of the bus may disperse the flame over a wide area.