Fire protection of electric and hybrid vehicles

More and more vehicles on our roads are electric or hybrid vehicles (EV and HEV) as there is a global transition to renewable fuels. Knowledge is limited about fire risks with vehicles having these new energy sources. Consequences connected to risks of Li-ion batteries in case of a malfunction may imply serious outcomes, for example, if the driver is affected by gas emissions or by fire/explosion and therefore more research in this area is needed.

Storing large amounts of energy, whether it is in large batteries used in energy-storage applications or batteries used in electric vehicles can be inherently dangerous as energy stored in a confined space tries to escape, sometimes violently. As a result, Li-ion batteries are susceptible to spontaneous ignition and subsequent explosion due to overheating. Overheating may be caused by electrical shorting, rapid discharge, overcharging, manufacturing defects, poor design, or mechanical damage, among many other causes. This might cause thermal runaway and the release of a flammable electrolyte. A thermal runaway produces enough heat to cause adjacent battery cells to ignite thus producing a fire that repeatedly flares up at each cell. Li-ion fires have different characteristics and spread quickly in comparison to normal fires which makes extinguishing very difficult.

Risk perspectives and risk management

The difference between vehicles with combustion engines and EV/HEV in a risk perspective is the location and the character of the potential fire sources. In vehicles with combustion engines the location of most fire sources is concentrated in the area of the engine compartment. In EV/HEV vehicles the potential fire sources can be located in different sections of the vehicle which means that more protection zones have to be considered and often together with a combination of different extinguishing agents due to the sensitivity of electrical components and the type of fire that might occur.

Prior to any installation of fire-suppression systems, a fire-risk analysis must be conducted in order to identify potential fire hazards. The severity and probability are then assessed in order to design the optimal fire-protection solution for the vehicle. SP method 5289 – Fire Risk Management procedure for vehicles – issued by RISE Research Institutes of Sweden, is used as a basis for conducting fire-risk assessments. A fire-risk management process includes the following steps:

1. Hazard identification
2. Risk estimation
3. Risk evaluation
4. Risk reduction

A fire hazard exists where there is a possible interaction between fuel, ignition sources and oxygen. The hazard identification process aims to answer where this could happen, when it could happen, how it could happen and why it could happen.

Following the hazard identification, risks shall be quantified by a risk-estimation method. The objective is to quantify the risks such that they can be sorted with respect to priority and actions needed. A method often used in the vehicle industry is Failure Mode and Effects Analysis (FMEA). This method can also be applied for quantification of the vehicle fire risks. The identified hazards (failure modes) are given risk priority numbers based on quantifications of probability of occurrence, severity and probability of detection failure. Each of the quantifications is made on a relative scale where a higher rating contributes to a higher estimated risk priority number. Fire severity can be quantified by estimating the potential consequences for the vehicle, the driver and any passengers, as well as to the surrounding environment.

When the fire risks have been quantified, they shall be sorted to provide an overview of the risk image. The risk evaluation, which is done based on the fire-risk assessment matrix, aims to provide this overview and to separate risks that need to be addressed from risks that are acceptable.

Following the risk-evaluation process an action plan for each identified fire risk must be laid out. Risk-reduction measures can be arranged in a hierarchic structure where measures at each level should be considered.

1. Risk elimination or minimisation by design
2. Passive and active fire-protection systems
3. Improved maintenance and cleaning procedures
4. Improved training and quality procedures

Interim multi-zone fire-protection solution

Currently there is no comprehensive solution for fire mitigation available on the market. However, Dafo Vehicle Fire Protection (Dafo VFP) has developed a full-coverage interim multi-zone fire-protection solution in response to the urgent need. The Dafo FORREX liquid-based extinguishing agent, which is non-corrosive, is used as one part of the complex system solution, and it provides effective cooling capacity to slow the fire development, allowing safe evacuation of passengers.

In order to cover all risk areas in an EV or HEV bus, the main risk scenarios are divided into four protection zones:

• Zone 1 Auxiliary heater and AC – protection for potential fires due to leakage of flammable fuels sprayed on hot surfaces etc.
• Zone 2 Battery compartment – protection of batteries from outside fire, fire containment at battery fire/thermal runaway.
• Zone 3 Electrical cabinet – protection for electrical-related fires due to short circuits, arcs etc.
• Zone 4 Engine compartment – protection installed for combustion engine (HEVs) and compartment with electrical components (EVs) for potential fires due to leakage of flammable fuels sprayed on hot surfaces etc.

The different zones are then protected in various ways by robust detection and suppression systems – both liquid- and gas-based solutions.

Mandatory installation of fire-suppression systems in buses and coaches

In 2016 the documents Addendum 106 – UNECE Regulation No. 107 – Revision 6 – Amendment 3 and 5 were published. The documents included a fire-testing procedure for fire-suppression systems for engine compartments of buses and coaches with four tests extracted from SP method 4912.

Since 11 July 2018, it has been mandatory to install fire-suppression systems in new vehicle types of single-deck, double-deck, rigid or articulated vehicles of category M2 or M3 and specifically vehicles having a capacity exceeding 22 passengers in addition to the driver – ‘Class III’. Class III vehicles are constructed exclusively for the carriage of seated passengers, more commonly referred to as ‘coaches’. As of 11 July 2019, this will apply to all new Class III vehicles.

As of 1 September 2020, it will be mandatory to install fire-suppression systems in new vehicle types of single-deck, double-deck, rigid or articulated vehicles of category M2 or M3 and specifically vehicles having a capacity exceeding 22 passengers in addition to the driver – ‘Class I and Class II’. Class I vehicles are constructed with areas for standing passengers, to allow frequent passenger movement, and Class II vehicles are constructed principally for the carriage of seated passengers and are designed to allow the carriage of standing passengers in the gangway and/or in an area which does not exceed the space provided for two double seats, more commonly referred as ‘city buses and inter-city buses’. As of 1 September 2021, this will apply to all new Class I and Class II vehicles.

The requirement for installation of fire-suppression systems applies to the vehicle. The fire-suppression system manufacturer’s UNECE approval (fire-suppression system as component with regard to UNECE Regulation No. 107) will be used as a part of the bus manufacturer’s vehicle approval for UNECE Regulation No. 107.

The requirement applies in the case of vehicles having an internal combustion engine or a combustion heater located to the rear of the driver’s compartment. Consequently, EV and HEV buses and coaches are currently excluded from this requirement.

Li-ion battery fire-safety research

Li-ion batteries are still a relatively new technology and Li-ion battery safety is a recent research area. Regulations and standards are to some extent lagging behind. Dafo VFP is at the forefront of the research and is participating in several projects dealing with fire hazards of Li-ion batteries in vehicles in order to provide solutions for reducing the risks and consequences of a thermal incident in or in connection with Li-ion batteries in heavy commercial HEVs and EVs such as buses and trucks. Dafo VFP is leading one of the most advanced research projects called Li-IonFire^® funded by the EU Framework Program for Research and Innovation – H2020 – under the SME Funding Scheme.

The ongoing research will lead to future safety solutions, including system design, battery placement, monitoring and analysing existing data through the Battery Management System (BMS) to achieve early detection of battery malfunction in order to shut the battery down before a full thermal runaway occurs. Further on, the objective is to investigate the extent to which fire-suppression systems can be applied to vehicles powered by Li-ion batteries in an optimised way, including refining the formulation of extinguishing agents and possible integration within the Li-ion batteries. In addition, investigations will be conducted on the possible use of sensitive smoke sensors with control systems integrated into the vehicle’s CAN-bus control system.

The main objective is to implicate the development of future standards and regulations such as UNECE Regulation 100 and UNECE GTR 20.

Fires in Li-ion batteries are rare but severe and very difficult to suppress once they are fully developed. Therefore, early detection will be key in any upcoming solution.

For more information, go to www.dafo-vehicle.com