Tesla Motors' Model S sedan began 2013 with the distinction of becoming Motor Trend's Car of the Year—a first for an electric car. (See related, “Tesla Motors' Success Gives Electric Car Market a Charge.”) Heading into 2014, the sleek poster child of a battery-powered future is now at the center of a U.S. government investigation into fires that began after Model S drivers struck road debris at highway speeds.
The core question in the probe is whether a safety defect in certain Model S vehicles caused the fires and must be fixed. While the investigation unfolds, here are some answers to basic questions about fires in vehicles—both electric- and petroleum-powered. (See related, “Pictures: Cars That Fired Our Love-Hate Relationship With Fuel.”)
What do we know about the Model S fires?
Three Model S cars, including two in the United States, have been involved in fires since early October. One car caught fire in Mexico after its driver crashed the Model S into a concrete wall. Two others caught fire after metallic roadway debris struck the low-slung Model S undercarriage and damaged the battery pack, which is configured as a flat, heavy pancake on the car's belly. The fires did not spread to the passenger compartment and no one was seriously injured.
The U.S. National Highway Traffic Safety Administration (NHTSA) has opened a formal investigation into the two Model S incidents in the United States. The agency said that impact damage to the battery's enclosure triggered what's called thermal runaway, in which a battery’s temperature rises uncontrollably as its cells rapidly release stored energy and heat adjacent cells.(See related, “ Quiz: What You Don’t Know About Batteries“) According to the official investigation summary posted on NHTSA's website, "In each incident, the vehicle's battery monitoring system provided escalating visible and audible warnings, allowing the driver to execute a controlled stop and exit the vehicle before the battery emitted smoke and fire."
How has Tesla responded?
Tesla has now updated software for the Model S air suspension system to keep the car's underbelly raised higher above the roadway when it's traveling at highway speeds. (Related Quiz: What You Don't Know About Cars and Fuel) "To be clear, this is about reducing the chances of underbody impact damage, not improving safety," Tesla Chief Executive Elon Musk wrote in a November 18 post on the company blog. The company has also said it will expand its warranty to cover damage from fire, even if it's due to driver error. "Unless a Model S owner actively tries to destroy the car," Musk wrote, "they are covered." (See related, “Pictures: Eleven Electric Cars Charge Ahead, Amid Obstacles.”)
While the NHTSA investigation could go on for weeks (Tesla's full response to NHTSA's request for information is not due until January 14), German authorities have already concluded an inquiry into the safety of the Model S, according to a translation of a letter dated November 27 and posted this week on Tesla's website. Germany's Federal Motor Transport Authority, Kraftfahrt-Bundesamt, wrote that it had found "no manufacturer-related defects."
How common are car fires?
Car fires are not as common as Hollywood might lead us to believe, but not as rare as we might hope. Highway vehicle fires reported to local public fire departments in the United States numbered 172,500 in 2012—down more than 50 percent from the numbers reported in the 1980s despite a steady increase in the number of miles driven. That works out to about one vehicle fire every three minutes, mostly in passenger cars. Between 2006 and 2010, automobile fires killed an average of four people each week, according to data from the National Fire Protection Association (NPFA).
Photograph courtesy Tennessee Highway Patrol, AP
The fire that destroyed this Tesla Model S in Smyrna, Tennessee in November followed two other fires, triggered a U.S. safety investigation.
For Tesla, the fires reported this fall occurred in just three of the more than 13,000 Model S sedans sold in the 2013 model year. "They had three fires in a short time, which is kind of a lot," said fire protection engineer Peter Sunderland, an associate professor at the University of Maryland who is currently fire-testing lithium battery cells for Ford. (See related, “Pictures: A Rare Look Inside Carmakers' Drive for 55 MPG.”)
Musk views the data more favorably. "Based on the Model S track record so far, you have a zero percent chance of being hurt in an accident resulting in a battery fire," he wrote in a post on the company's blog this week. "It is literally impossible for another car to have a better safety track record, as it would have to possess mystical powers of healing."
What causes car fires?
Again, reality differs significantly from what we see in movies, where fire typically follows car chases gone awry. In fact, three-quarters of car fires are due to far less cinematic mechanical and electrical failures, such as leaks, breaks, backfires, and worn out or improperly installed parts, or short circuits.
Every vehicle fire has three basic ingredients: a heat source, something that burns, and oxygen. Common sources of heat in highway vehicle fires include powered equipment, electrical arcing, smoldering objects, and sparks resulting from friction. Although a tank full of gasoline or diesel might seem like the most obvious place for fire to begin, most fires begin in the area of the engine, running gear, or wheels. Only a small fraction of highway vehicle fires begin in the area of the tank or fuel line. These fires tend to be among the most deadly, however, because gasoline can burn so quickly, said Marty Ahrens, a fire data analyst for the NFPA.
The Model S fires are unusual in that they started after an impact rather than due to some internal malfunction. According to Ahrens' most recent analysis for the NFPA, collisions or overturns were factors in only 4 percent of all automobile fires between 2006 and 2010.
Also significant: Motorists survived more than 998 out of every 1,000 automobile fires reported from 2006 to 2010, according to NFPA.
When do car fires become deadly?
Although crashes are the least common trigger for car fires, they are the most deadly. Close to 60 percent of the 300-400 deaths in U.S. car fires each year follow a collision or overturn. In many cases, the crash itself may be survivable otherwise, but it leaves the driver unable to act before being overcome by smoke or flames. "Collisions or overturns are the scenarios that are most likely to be fatal, and because speed is such a factor,” said Ahrens. “Basically good driving can prevent an awful lot of those." Significantly, the least experienced drivers, older teens and young adults, are at the greatest risk of highway vehicle fire death.
A fire after a crash is somewhat different from a mechanical fire, but the basic ingredients are the same. "In a crash, everything gets moved around," Ahrens explained. "Maybe the fuel line breaks, or maybe something in the engine compartment breaks, so you have something that is very, very hot coming in contact with something that can burn pretty easily."
Fumes also present a hazard. Whereas older car interiors featured more cotton and natural materials, Sunderland said, today's designs rely heavily on polymers and plastics, which produce "more thick, black smoke and things like cyanide and carbon monoxide, and burn faster than the old stuff." These fast-burning materials are often the next thing to catch fire after gasoline, he said.
What's different about fires in electric cars?
The stuff that burns and the situations in which fire is likely to arise are quite different in EVs than in traditional gasoline-fueled vehicles. The rate at which technology is changing complicates matters even further. (See related, “Range Anxiety: Fact or Fiction?“)
"You need a lot of energy to propel a big, heavy car fast and far," said Sunderland. Lithium ion batteries used in today's electric cars store electrical energy as well as chemical energy, in the form of a flammable electrolyte (the medium through which ions are transported between electrodes). While automakers have designed sophisticated systems for protecting batteries from abuse and managing temperature, thermal runaway can result from excess heat, mechanical damage, charging problems, and poor design or defective manufacturing of cells. (See related, “How to Compare the Cost of Electric and Gas Cars.”)
If a fire does ignite, the fumes from batteries are different from those that come from gas. "When gasoline burns, you can breathe it—you don't want to, but you could," Sunderland said. "With lithium, the smoke is more toxic." And although gasoline and diesel fires are likely to occur when the car is being driven, "Battery fires in cars can also happen when it's charging or parked," he said. A plug-in hybrid Chevy Volt, for example, burst into flames two years ago while parked, weeks after its battery and cooling system were damaged in government testing.
It's important to remember, however, that electric car technology does not present an inherently greater fire hazard than the old internal combustion engine, Sunderland said. "Gasoline and diesel are also dangerous," he said, but people have become used to this hazard and accept the risk because of 100 years of experience. He suggests considering how we’d react today if "someone said, 'Let's use gasoline, keep it in a one-eighth-inch thick sheet metal tank.' We'd say 'Are you crazy? Gas pools everywhere. It ignites and everyone dies.' It would never get approved."
Understanding the real fire risks associated with electric cars will take time—first, because the number of battery electric vehicles on roads today is still so limited; second, because most of these cars are only a few years old; and third, because the new technology does not fit neatly into old data-collection schemes. The national system for vehicle fire reports does not have a category for the type of fire that was sparked in the Teslas. "Running over an object in the road—our coding system doesn't have a good way to capture that," Ahrens said.
"As with any new technology," she added, "It's going to be interesting to watch how it ages and how it functions when people don't do exactly what they're supposed to do, taking it in for all the regularly scheduled maintenance." (See related, “Driving the Limit: Wealthy Nations Maxed Out on Travel?")
What should I do if my car catches fire?
NFPA advises drivers to follow three steps:
Stop. Pull over, if possible. Turn off the car in order to cut the flow of gasoline and electric current. "Keep the hood closed because more oxygen can make the fire larger," NFPA warns.
Get Out. Move at least 100 feet away from the vehicle, looking out for traffic and keeping your passengers together.
Call for help at 911 or the emergency number for the local fire department. Do not try to fight the fire yourself. "Vehicle fires can be tricky," NFPA says, "even for firefighters."
This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.