Electric Vehicles

Electric Vehicles

Summary:

  • The emergence of electric vehicles has led to a number of unique safety concerns, including issues of crashworthiness (i.e. how the structural and weight differences of EVs compared with conventional internal combustion vehicles affect vehicle collision behaviour), post-impact vehicle safety (i.e. the challenges associated with high-voltage circuits, batteries or hydrogen fuel-cells following a vehicle collision) and low noise emission (i.e. the impact on vulnerable road users who rely on auditory cues to respond to approaching vehicles).

Crashworthiness

  • Crashworthiness, also known as passive safety, of electric vehicles (EV) differs from conventional vehicles because of the lack of internal combustion engine (ICE) in (typically) the front compartment, the addition of battery packs and the structural modifications to the vehicle designed to cope with the associated increase in vehicle weight (Lassfolk et al., 2010)

  • Electric vehicles have been submitted for consumer testing (Euro NCAP) and have demonstrated that they are capable of achieving a good score for occupant protection (Paine).

  • As a result, there are number of aspects of safety which are specific to EV crashworthiness. These include the structural behaviour of the vehicle (e.g. weight distribution, mass, and layout), mechanisms for protecting against high-voltage electrical circuits (to maintain integrity of the battery and electrical parts during a collision), mechanisms for protecting against chemical materials (e.g. to avoid electrolyte leakage and fumes into the passenger compartment), and mechanisms for protecting against fire (e.g. due to high-voltage cables, short circuits, or chemical reaction) (Widmann and Seibert, 2010).

  • There are too few electric vehicles on the road to allow meaningful study of the risk of occupant injury compared with conventional ICE vehicles (Visvikis, 2012).

  • If purely-electric vehicles penetrate the fleet in significant numbers, the market share of small cars may increase further relative to other vehicles. This may have an effect on casualty statistics, unless improvements in the “compatibility” of vehicles can be achieved (Visvikis, 2012).

Post-impact safety issues

  • There are additional risks associated with electric and hybrid vehicle technologies which are relevant to post-impact safety concerns. These include, for example, battery safety and concerns over high-voltage electrical circuits in EVs and the implications of hydrogen leaks in hydrogen fuel-cell vehicles.

  • The UN Regulation 100 specifies requirements for the safety of electric vehicles ‘in-use’ and the protection of users against electric shock. UN Regulations 94 (front impacts) and 95 (side impacts) have recently been amended to include requirements for post-impact electrical safety. But, many hazards associated with RESS are not covered by type-approval regulations; for example, there is currently no legislation relating to risks of extreme heat or fire on EVs (Visvikis et al., 2010).

  • Electric propulsion in EVs is achieved via rechargeable energy storage systems (RESS) such as batteries, capacitors and electromechanical flywheels. The hazards associated with batteries have been given particular focus, which can include electrolyte spillage due to damage to cell casing, chemical reactions as a result of extreme temperature or fire, and electrical risks such as short circuit, over voltage, or electroshock (Visvikis et al., 2010).

  • Lithium, used in lithium ion batteries for example, is highly reactive and the electrolytes are highly flammable. The safety of these batteries may be assessed via appropriate abuse tests (such as overvoltage, overdischarge, heating, arcing, crush, nail penetration and internal/external short). Results of such tests have identified a number of issues with lithium ion batteries including thermal runaway, electrolyte leaks, smoke, venting, fire and explosion (Pesaran et al., 2009; Tjus, 2011).

  • It is important to be aware of the properties of hydrogen when dealing with hydrogen fuel-cell vehicles. Crucially, hydrogen gas is odourless and colourless meaning it cannot be detected by humans without the use of hydrogen sensors. The addition of odorants, like those introduced to natural gas, is not appropriate for use in vehicles because they can contaminate fuel cells. Hydrogen is also more buoyant, smaller in molecular structure and more flammable in air than gasoline vapour, meaning a greater risk of leakage through materials and a greater risk of fire when mixed with air (Visvikis et al., 2010).

  • Because of the unique hazards associated with technologies present in alternative fuel vehicles, emergency responders must be able to easily distinguish EVs, HEVs, hydrogen fuel-cell vehicles and conventional ICE vehicles when attending incidents (Grant, 2010).

Risks associated with low vehicle noise emission

  • Recent advances in ‘green’ alternative fuel technologies are increasing the prevalence of hybrid and fully-electric vehicles in today’s marketplace. These technologies bring about reduced fuel consumption, reduced carbon dioxide emission and reduced vehicle noise emission. The latter has important implications for the safety of vulnerable road users, such as cyclists and pedestrians, particularly those who are visually-impaired, since the usual noise produced by an internal combustion engine is now absent. In low-speed environments particularly, aerodynamic and road-tyre sounds are negligible meaning road users have few available auditory cues to an approaching vehicle. Collision with pedestrians even at speeds of 30 km/h can be serious, so the lack of noise emission may present a substantial risk (Voigt et al.).

  • The detection of electric vehicles with low noise emission may be improved via the addition of synthetic sounds to act as a warning in the absence of real engine sounds. There is an important trade-off for the optimisation of such synthetic sounds; they must be optimal for health and social well-being of the neighbourhood whilst also conveying useful information about the driver’s intentions to vulnerable road users. This must include speed, acceleration and deceleration patterns, and braking (Chamard et al., 2012).

  • Evidence suggests that the greatest risks to vulnerable road users occur when the EVs are travelling at low speeds, since the noise emitted from the friction between the tyres and the road surface, and the aerodynamic noise, are both minimal. Thus, at low-speeds, an EV may be near silent. It follows that these risks will be most prevalent in urban areas which enforce low speed limits (Cocron et al., 2010).

  • However, there is mixed evidence that the number of accidents involving pedestrians is any different for electric vehicles and conventional ICE vehicles. The National Highway Traffic Safety Administration (NHTSA) in the USA reported that HEVs were found to be twice as likely to be involved in a slow-moving accident with pedestrians as equivalent ICE vehicles. However, in a review of vehicle accident statistics, the Transport Research Laboratory found that the likelihood of being involved in a collision with a pedestrian was comparable between EVs/HEVs and conventional ICE vehicles. It was also not possible to determine whether those accidents that did occur between pedestrians and EVs/HEVs were associated with the low noise emission of those vehicles (NHTSA, 2009; Morgan et al., 2011).

  • Despite the lack of strong evidence that there is currently an increased risk to vulnerable road users posed by vehicles with low noise emission, the number of electric vehicles on the road may rise in the future which could increase the risk so investigation of measures designed to mitigate those risks are of value (Visvikis et al, 2010).

  • There is good evidence that the addition of synthetic sounds is beneficial for the detectability of electric vehicles. The sound profile is an important consideration; ‘engine’ and ‘hum’ sounds appear to be preferred by visually-impaired pedestrians. For example, Nissan’s approaching Vehicle Sound for Pedestrians (VSP) system has been shown to be successful at increasing the detectability of a production Nissan electric vehicle (e.g. Goodes et al., 2008; Kim et al., 2012; Parizet et al., 2013; Wogalter et al., 2013).

  • It is argued that the addition of synthetic sounds to electric vehicles may only be necessary until the majority of vehicles on the road have low noise emissions. After that turning point, a shift in public awareness may negate the need to add warning sounds to the vehicles (Voigt et al.).

  • Date Added: 03 Apr 2012, 08:22 AM
  • Last Update: 05 Feb 2014, 04:43 PM