Media coverage of the recent Tesla Model S fires has been comprehensive, and according to Elon Musk (Chairman, Product Architect and CEO of Tesla Motors), disproportionate by several orders of magnitude when compared with conventional car fires. It is true that there are a lot of flammable materials in conventional cars and fires can break out from time to time. Although arson or other forms of criminal damage are sometimes involved, fires can also be caused by defects or routine damage to the car (including normal wear and tear as well as collisions). Most fires start in the engine, but the firewall between the engine compartment and the passenger compartment provides a protective barrier and increases the time for occupants to escape.
Fuel tank fires and explosions are very rare, largely due to the legislative testing that tanks (and whole vehicles) undergo. The minimum ignition energy of petrol vapour is relatively low; a spark can be sufficient to cause a conflagration. However, the fuel tank is cut-off from the rest of the rest of the fuel system in the event of a crash and is located away from the likely damage zones, in the majority of collisions at least.
Electric vehicles comprise a very small proportion of the fleet and it is probably too early to say (with any statistical confidence) whether their risk of fire is different to that of conventional vehicles. Some high-profile fires have been reported in the media, with the battery usually being the source of the fire. However, electric vehicles are also equipped with firewalls and these seem to have directed the flames away from the passenger compartment in these incidents, at least until the occupants were able to leave their cars safely.
Battery fires can be caused by a number of ‘abuse’ mechanisms, such as overcharging, over-discharging, short circuit, or physical damage to the cells. These mechanisms are quite different to those that cause the majority of conventional vehicle fires. However, electric vehicles are equipped with a range of systems to prevent such abuse. There are also cell- and pack-level monitoring and safety features that are designed to prevent the build-up of heat and pressure within cells that can lead to fire (and explosion). New vehicle legislation is mandating a range of battery safety and abuse tests for electric vehicles, which will establish minimum safety standards for manufacturers to meet.
This is a guest blog by Dinos Visvikis, Head of Low Carbon Vehicle Safety at the Transport Research Laboratory. His research interests include the safety of new vehicle propulsion technologies and their implications for vehicle legislation.