An investigation into fires in hybrid vehicles and the assessment of effective fire suppression systems

The technological transformation in the automotive sector has highlighted Hybrid Electric Vehicles (HEVs – Hybrid Electric Vehicles) and Battery Electric Vehicles (BEVs – Battery Electric Vehicles) not only as engineering achievements but also as critical components of global energy policies and environmental sustainability. The Armstrong Phaeton, produced in the late nineteenth century, is recognized as the first hybrid vehicle, integrating an Internal Combustion Engine (ICE – Internal Combustion Engine) with an electric motor. Modern hybrid vehicles emerged with the market introduction of the Toyota Prius in 1997, which represented a significant turning point and accelerated the widespread adoption of hybrid technologies. However, alongside this technological progress, the issue of fire safety became increasingly visible. In the early 2000s, incidents of small-scale battery fires were reported in models such as the Toyota Prius and Honda Insight due to insufficient cooling systems. In 2012, the Chevrolet Volt was recalled over battery safety concerns, clearly indicating that hybrid vehicle fires had become a serious industrial and regulatory issue. With the expansion of hybrid and electric vehicles, the number of battery-related recalls has increased substantially. Most of these recalls were directly related to safety issues such as insulation failures, malfunctions in the Battery Management System (BMS – Battery Management System), coolant leakage, and wiring faults. Between 2010 and 2020, millions of hybrid and electric vehicles were recalled worldwide because of fire-related risks. This reveals that, while battery technologies provide considerable advantages in terms of energy density and performance, they still present significant safety challenges that must be resolved. In conventional ICE vehicles, fires typically originate from fuel leaks, electrical short circuits, or overheating of mechanical components and are often brought under control within minutes using relatively small quantities of suppression agents. In contrast, HEVs that rely on high-voltage lithium-ion batteries exhibit much more complex and long-lasting fire behavior. The key mechanism responsible for this is thermal runaway, which unfolds through distinct stages and leads to uncontrolled temperature escalation and chain reactions. The process begins with the decomposition of the Solid Electrolyte Interphase (SEI) layer and gas release, followed by separator degradation, electrolyte decomposition, and finally the liberation of oxygen from cathode materials such as Nickel-Manganese-Cobalt oxide (NMC – Nickel-Manganese-Cobalt Oxide) or Nickel-Cobalt-Aluminum oxide (NCA – Nickel-Cobalt-Aluminum Oxide). These reactions can sustain themselves and escalate to very high temperatures, generating jet flames and propagating to adjacent cells. As a result, hybrid vehicle fires may last for hours and require extraordinary suppression efforts. Statistical analyses confirm that HEVs present the highest fire risk among vehicle categories. Data from the National Highway Traffic Safety Administration (NHTSA – National Highway Traffic Safety Administration) in the United States indicate that HEVs experience more fire incidents per 100,000 sales compared to BEVs and ICE vehicles. In China, the prevalence of Lithium Iron Phosphate (LFP – Lithium Iron Phosphate) batteries has contributed to a relatively lower frequency of fires due to their greater thermal stability. Conversely, manufacturers such as Tesla employ NCA batteries to achieve higher energy density, though these chemistries offer narrower safety margins compared to LFP. European studies highlight the particular risk posed by HEV fires in confined infrastructures such as underground car parks and tunnels, where toxic smoke and re-ignition events create severe challenges for emergency response. Hybrid vehicle fires are not limited to batteries and fuel; other flammable components also play a significant role. Seat cushions made of polyurethane foam, plastic and polymeric materials such as polypropylene and ABS, wiring with PVC insulation, tires, rubber elements, engine oils, hydraulic fluids, and transmission fluids all contribute to fire intensity. Lightweight materials like magnesium alloys, often used for weight reduction, can ignite under extreme heat and amplify fire spread. Thus, hybrid vehicle fires represent a multidimensional hazard rather than a battery-only phenomenon. In this study, NMC-based cylindrical (18650 format) and prismatic lithium-ion cells were examined. NMC cells were selected because they combine high energy density with relatively lower thermal stability compared to LFP. The 18650 cylindrical cells are particularly suitable for modeling cascading thermal runaway in compact, series-connected formats, while prismatic cells better reflect the large surface-area exposure typical of automotive modules. The combination of these two cell formats provided a more realistic representation of hybrid battery packs. Full HEV battery packs often contain hundreds of cells with operating voltages between 200–400 volts (V – volt), whereas Plug-in Hybrid Electric Vehicles (PHEVs) typically employ larger capacities in the range of 8–18 kilowatt-hours (kWh – kilowatt-hour). The experimental setup was carefully designed to replicate the conditions of hybrid vehicle fires. In open-area trials, gasoline flames were introduced to simulate combined fuel-battery fire scenarios, while in closed-chamber experiments, a reinforced steel enclosure was used to monitor battery responses under controlled conditions. Key measurements included thermal response, flame development, and gas release. Comparative suppression experiments were carried out with water, NOVEC 1230, BIOVERSAL, and a High-Viscosity Suppression Solution (YVSS – High-Viscosity Suppression Solution). The experimental findings revealed that open-air tests rapidly triggered battery involvement, with cylindrical cells displaying sequential thermal runaway while prismatic cells exhibited delayed but more intense thermal responses. In closed-chamber scenarios, YVSS demonstrated superior performance, delaying ignition and preventing re-ignition through its viscous adherence, which limited oxygen transport and vapor release. Water provided fast cooling but was insufficient in preventing re-ignition; BIOVERSAL had ecological advantages but limited performance under sustained heating; and NOVEC 1230 showed low effectiveness in hybrid fire scenarios. In addition to the direct suppression challenges, the study also emphasizes the toxic and environmental hazards associated with HEV fires. The breakdown of electrolytes generates hazardous products such as hydrogen fluoride (HF – Hydrogen Fluoride), carbon monoxide (CO), hydrogen (H₂), volatile organic compounds (VOC – Volatile Organic Compounds), and fine metal oxides. When combined with the products of gasoline combustion, the toxicity of the smoke and gases increases substantially. Moreover, thousands of liters of suppression agents—if not properly managed—may lead to contaminated runoff containing heavy metals, solvents, and fluorinated compounds, with long-term risks for soil, groundwater, and ecosystems. What distinguishes this dissertation from earlier research is its dual experimental framework. By combining open-air scenarios, where the interaction of gasoline flames with battery modules was observed, and closed-chamber trials, which allowed controlled analysis of thermal runaway and suppression behavior, the study bridges laboratory conditions with real-world fire dynamics. The simultaneous use of both cylindrical and prismatic NMC cells provides a methodological novelty, as it more accurately represents the diversity of cells integrated into HEV battery packs. This dual-cell approach not only enhanced the validity of the findings but also created a foundation for future scaling studies on full battery packs. Beyond its experimental contributions, the research offers forward-looking recommendations. Engineering strategies such as improved module spacing, advanced cooling systems, and fire-resistant enclosure materials are discussed as potential ways to mitigate fire risks. Furthermore, the study emphasizes the importance of training for emergency response teams, who must adapt suppression tactics to the unique challenges of HEV and BEV fires. Finally, the results provide evidence that may inform the refinement of international standards and regulatory frameworks, including NFPA guidelines and European TS EN norms, ensuring that hybrid vehicles can be safely integrated into the transport systems of the future. In conclusion, this dissertation provides an original and comprehensive analysis of hybrid vehicle fires, combining historical background, literature review, experimental research, and comparative evaluation of suppression methods. The findings demonstrate that HEV fires are multidimensional events fueled not only by high-voltage lithium-ion batteries but also by fuel and various flammable vehicle components. Among the suppression agents tested, YVSS exhibited the highest potential for enhancing fire safety. These findings contribute significantly to the academic literature and also provide practical insights for engineering design, emergency response planning, environmental management, and policy-making. The work ultimately highlights the need for integrated strategies to ensure the safe and sustainable integration of hybrid vehicles into modern transportation systems.