Sprinkler Age A Publication of the American Fire Sprinkler Association
On the news nearly every day, one can hear a report about a vehicle fire. These fires occur in residential garages, commercial garages, and open parking lots, among other places. Fires in automobiles are not new. However, the attention and severity of the fires have increased the attention these fires receive. What has caused this?
Since the earliest days of the automobile, vehicle designs have evolved in response to demands for performance, efficiency, safety, and cost reduction. Central to this evolution has been a steady shift in materials and a reduction in vehicle weight. These changes have significantly altered the nature of automotive fire hazards. Understanding how modern vehicles burn requires an appreciation of how and why these materials replaced earlier, heavier construction methods.
Early automobiles were built primarily from steel, iron, and wood. Thick metal frames, heavy body panels, and minimal interior components resulted in vehicles with substantial weight and relatively low combustible content. Fires in these early vehicles were typically caused by fuel leaks or engine failures, but fire growth tended to be slower and more localized. The predominance of metal allowed heat to dissipate, and the absence of synthetic materials limited flame spread and toxic smoke production.
As mass production expanded in the mid-20th century, manufacturers began reducing vehicle weight to improve fuel efficiency and lower costs.
Thinner steel panels replaced heavier components, and plastics, rubber, and synthetic foams were introduced into interiors, wiring insulation, and trim. These materials dramatically reduced weight but increased the combustible fire load. Plastics and foams ignite more readily than metal and burn with higher heat release rates, producing dense, toxic smoke that accelerates fire spread and reduces survivability.
One of the most significant weight-saving changes was the transition from steel fuel tanks to plastic fuel tanks. This shift began in Europe in the late 1960s and 1970s, and became widespread by the 1980s and 1990s. Plastic fuel tanks, typically made from high-density polyethylene, offered substantial weight savings, corrosion resistance, and design flexibility. Their ability to be molded into complex shapes allowed for improved vehicle packaging and compliance with emissions standards.
From a fire safety perspective, plastic fuel tanks present both benefits and risks. Their flexibility can help absorb impact energy during minor to moderate crashes, potentially reducing immediate rupture. However, plastic tanks are combustible and vulnerable to heat exposure. In fire conditions, they can soften, melt, and fail more rapidly than steel tanks, releasing fuel that intensifies fire growth. Molten plastic can further spread burning fuel, increasing fire severity and reducing the time available for occupant escape and firefighting intervention.
Modern lightweighting strategies have continued into the 21st century with the use of aluminum, magnesium alloys, advanced plastics, and composite materials. While many of these materials are non-combustible, they often lose structural integrity at lower temperatures than traditional steel. Reduced vehicle mass also means less thermal mass to absorb heat, allowing fires to escalate more rapidly. At the same time, increased use of plastics and synthetics continues to raise interior fire loads.
Electric, hybrid, and other alternative-fuel vehicles introduce additional complexity. While eliminating liquid fuel fires in some scenarios, lithium-ion battery systems create prolonged, high-energy fire events. Propane, hydrogen, and other gaseous fuels create their own concerns. Lightweight and combustible surrounding materials can accelerate fire spread once a failure occurs, and if a battery system is involved, the battery itself sustains the fire.
The evolution of automotive materials and the drive to reduce vehicle weight have transformed vehicle fire behavior. Modern vehicles burn faster, hotter, and with greater complexity than earlier designs. Recognizing how material choices, particularly plastics and fuel system components, affect fire hazards is essential for fire protection engineers, sprinkler system designers, AHJs, and fire departments adapting to the realities of today’s vehicle fires.
Now that we know the historical factors, how do fire protection engineers and sprinkler designers address these problems? Up to now, most of the designs were based on historical designs with some educated guesses. Research into these designs has been conducted, and while not at all comprehensive or complete, the preliminary data is clear. (Note: AFSA is a proud sponsor of the parking garage research being conducted by the NFPA Fire Protection Research Foundation.) Sprinkler systems do work to control the fire until the responding fire department takes control of the situation. Again, “control” is the magic word. A sprinkler system, in most cases, will not extinguish a fire in a vehicle. If one looks at the definition of “fire control” in NFPA 13, Standard for the Installation of Sprinkler Systems, 2025 edition, it states:
3.3.82 Fire Control – Limiting the size of a fire by distribution of water so as to decrease the heat release rate and pre-wet adjacent combustibles, while controlling ceiling gas temperatures to avoid structural damage.
One thing was obvious from personally witnessing many of the vehicle tests: the vehicle’s hood, roof, and trunk lid shielded the fire from the sprinkler’s water spray. A vehicle fire is a shielded fire—period. No amount of water on the exterior of the vehicle will extinguish the fire, but it will control the fire, which meets the purpose of the sprinkler system design goals.
The remaining question is how the fire department handles the fire. Well, just as almost everything in the world, the answer is “it depends.” How is the fire department trained? What equipment do they have at their disposal? Do they have sufficient water? What type of vehicle is burning? While these questions and answers are important to the AHJ and fire department, they have almost no impact on the sprinkler design. The only item I think could be affected is the water supply duration. Given that most parking garages are in urban areas with public water supplies, water duration is usually not a concern. In a rural setting, the water duration should be evaluated by the AHJ and the fire department.
Look for changes in the upcoming 2027 edition of NFPA 88A, Standard for Parking Structures, which should be released in the fall of this year, and the 2028 edition of NFPA 13, which should be released in the fall of 2028.
Just like everything in life, change is going to happen. My grandson is turning two years old, and my granddaughter is now seven months old. I’m getting older myself, but I still love what I do. Drop a line if you have comments on this topic or ideas for future columns. I look forward to seeing everyone as I travel around the country making presentations and attending events. Feel free to say hello to me as I enjoy meeting members on the road!
ABOUT THE AUTHOR: John August Denhardt, P.E., ET, CWBSP, FSFPE, is the vice president of engineering and technical services for the American Fire Sprinkler Association (AFSA). He is responsible for strengthening AFSA’s engineering and technical approaches to meeting member, industry, and operational priorities, with an emphasis on service, quality, and integrity. Denhardt is a registered professional engineer (P.E.) in the District of Columbia and the states of Delaware, Maryland, Pennsylvania, and Virginia. He is NICET Level III certified in water-based systems layout, NICET Level III certified in inspection and testing of water-based systems, and a certified water-based system professional through NFPA. Denhardt is a member of the NFPA 13 technical committee on sprinkler system discharge criteria, a fellow in the Society of Fire Protection Engineers (SFPE), a member of the SFPE Board of Directors, a member of the Board of Trustees for NFPA’s Fire Protection Research Foundation and sits on the University of Maryland Department of Fire Protection Engineering’s Board of Visitors. A native of Maryland, Denhardt holds a Bachelor of Science degree from the University of Maryland College Park in fire protection engineering. Prior to this role, Denhardt was employed by Strickland Fire Protection in Forestville, Maryland, since 1994, overseeing large-scale projects and assisting with design and installation technical issues.