Can Gasoline Ignite Without a Spark?


Gas is one of the most flammable materials, so understanding what causes it to burn is wise for your safety and that of your family. You’ve heard that gasoline can ignite without a spark, but you’re not sure if that’s a myth. Does gasoline need a spark to burn?

gasoline on a fire

Gasoline does not always require a spark for ignition. Heating the gas to a high enough temperature (500 degrees Fahrenheit or higher) can start a spontaneous fire. Gas fires are considered very dangerous, as they contain the toxins hexane, toluene, heptane, xylene, and benzene.

This article will be your introduction to all things gas fires, including what causes gasoline to burn and what you should do if you’re ever exposed to a gas fire. There’s lots of helpful information ahead, so keep reading! 

How Does Gasoline Ignite?

In most applications, gasoline ignition is quite benign. For example, motorcycles, light trucks, sport utility vehicles, and cars use gas ignition to operate.

Emergency generators, some aircraft, boats and recreational vehicles, and equipment for landscaping, forestry, farming, and construction all require gasoline ignition as well. 

So how does gas ignite? It’s all about the flash point of gasoline. 

The flash point is a chemistry term. It refers to the lowest possible temperature in which ignition of vapor will occur with a source of ignition. The flash point for gasoline is between -35 and -40 degrees if the gas is 500 to 100 octane. Unleaded gas has a flash point of -45 degrees.

Compared to other fuel sources, gasoline’s flash point is among the lowest. Here are some fuel source flash points so you can see what we mean.

  • Biodiesel – 266 degrees
  • Vegetable oil/canola oil – 621 degrees
  • Kerosene – 100 to 162 degrees
  • A or A-1 jet fuel – 100 degrees
  • 2-D diesel – 126 degrees
  • White gas or Coleman fuel – 25 degrees
  • 70-percent ethanol – 61.9 degrees 

Going together with fuel flash points is the autoignition temperature. This is a concept that’s similar enough to flash points that some people get them confused. 

The autoignition temperature refers to the lowest possible temperature that will cause spontaneous ignition without a spark. In other words, autoignition is the crux of what we’re talking about here today!

The autoignition temperature of a gas is usually higher than the flash point. Let’s use a spark-ignition engine powered by gas as an example.

The gas combines with air in the engine chamber. There, compression warms up the gas. 

Then there’s another science theory at play, Boyle’s law. Under Boyle’s law, as a container’s volume goes up, a gas’s pressure goes down. This allows the gas in the spark-ignition engine chamber to have a lower flash point. 

The spark plug ignites the gasoline. Its autoignition temperature raises to prevent excessive combustion chamber heat that would lead to preignition or engine knocking. 

Does Gasoline Need a Spark to Ignite?

Although the last section was a bit of a science lesson, it’s now much clearer to you that gasoline can indeed ignite without a spark. This is autoignition in a nutshell. 

What temperature is required for autoignition to occur? For gasoline to ignite the temperature must be between 477 and 536 degrees fahrenheit.  

It’s not only gasoline that undergoes autoignition. All the other fuel sources listed above can as well. Here’s the autoignition temperature of the gases we discussed in the last section:

  • Vegetable oil – 795 degrees
  • Kerosene – 428 degrees
  • Jet fuel – 410 degrees
  • Diesel – 410 degrees
  • Coleman fuel – 419 degrees
  • Ethanol – 685 degrees

If you compare these temperatures to the flashpoint temperatures of the fuel sources, you’ll see that the autoignition temperature is higher, as we said. 

So what exactly is happening when gasoline autoignites? There’s an equation that breaks the premise down. It’s a rather complicated equation, so we won’t delve too much into it here, only cover the basics.

Essentially, time and a heat flux cause the gasoline to heat up to a specific heat capacity. Specific heat capacity is a thermodynamics concept that dictates a substance’s top heat capacity by its mass. 

In other words, specific heat capacity measures the amount of mass that must be applied to a substance for that substance to become warmer in temperature by a single unit. The temperature of a specific heat capacity varies, which is why the autoignition point of gasoline is a range rather than one specific number.

The Risks of Gasoline Fires

Although any fire is a risk to human life, gasoline fires are especially harmful. Here are the potential side effects of an autoignited gas fire. 

Spontaneity 

Fuel fires that occur due to autoignition are often spontaneous. You could have spare gas in your garage or in your warehouse at work and the gas can ignite at any point if the temperatures rise to a certain level.

A fire can change your life in an instant. All at once, the home you’ve lived in for years or the workplace you’ve been a part of for decades can go up in literal smoke. You’ll lose most if not all your possessions, and sometimes everyone in the building doesn’t make it out alive. 

High Flammability 

Since gas is flammable, it’s highly dangerous upon burning. The fire has a very significant fuel source in the gasoline, allowing the fire to spread. A fast-spreading fire can quickly burn down more square footage of a home or building, sometimes rendering it unsavable. 

Dark Smoke

The area where the gas fire occurred will soon become engulfed in black smoke due to the low oxygen content and the elevated thermal load of the gasoline fire. Heavy, thick smoke can reduce visibility, making it hard for victims to escape even if there are clear exit points. 

The fire department, when they arrive, can also struggle with navigating the building depending on how strong the smoke is.

Breathing in smoke can lead to asphyxiation as the oxygen in the air depletes, which can be deadly. 

Toxicity 

More so than the lack of visibility and breathability, the smoke that burns from a gasoline fire is a risk because it’s highly toxic. It contains the compounds hexane, toluene, heptane, xylene, and benzene. Let’s talk about each of these toxins now.

  • Hexane: The fluid hexane has no color and only a subtle odor, making it difficult to detect. Inhaling hexane can lead to headaches, nausea, and dizziness. Your throat and eyes can be irritated as well. Chronic exposure could cause blurry vision, weakness, and numbness. 
  • Toluene: Toluene is also colorless. It starts as a liquid and can transform into a vapor when room temperature air enters the equation. Its odor is sweet or sharp but not both. The symptoms of breathing it in include headache, pupil dilation, confusion, fatigue, and even kidney and liver damage as well as nerve damage. 
  • Heptane: A type of straight-chain alkane, heptane is a primary ingredient in gas. It can cause skin irritation and potentially nervous symptom side effects as well.  
  • Xylene: The third colorless liquid in gasoline is xylene. It has a sweet smell. Since xylene is highly flammable, it’s especially harmful. Besides irritating the throat, skin, and eyes, xylene exposure can also cause muscle incoordination, confusion, and headaches. Breathing in too much xylene can be deadly. 
  • Benzene: Benzene can be colorless to light yellow depending on the temperature. It too is flammable and smells a little sweet. Exposure to benzene can lead to the onset of symptoms such as amenia or excessive bleeding. 

What to Do in a Gas Fire 

Now that you’re privy to the potentially life-threatening effects of a gasoline fire, what should you do if a gas fire ever occurs in your home or office? 

You personally should do nothing to tamp down the fire. Instead, tell your loved ones or coworkers to evacuate, exit the building immediately, get a fair distance away, and call 911 or your emergency number equivalent.

Once the fire department arrives, they’ll contend with the gas fire. They’ll usually begin by opening windows and doors, sometimes forcibly, to provide ventilation. This will mitigate the effects of the black smoke. 

Using a C2 fire extinguisher should control the flames before the gasoline fire can spread further. Whether your possessions are savable and the building someday habitable again will depend on how severe the fire is and how quickly the fire department works. 

How to Prevent Gasoline Fires

After reading the last section, you might feel rightfully terrified of a gasoline fire burning down your home. How can you prevent such an occurrence from happening? 

If you can help it, don’t keep gas sources on your property. If you must store gasoline, then you should check the gas periodically to ensure the temperature of the room is not too high. 

Gasoline can burn without a spark in a process known as autoignition. It only requires temperatures of around 450 to 500 degrees for gas autoignition to occur, which will start a spontaneous fire. 

If a gas fire ever occurs on your premises, your priority must be to get out. Do not try to combat the fire, as gasoline fires can be extremely dangerous!

Mike Pertz

I’m Mike, I’m a full time firefighter/paramedic/diver for a department just west of Cleveland, Ohio and the founder of FirefighterNOW. I’m also a columnist for FireRescue1. If you’re reading this blog my guess is you are interested in the fire service. There's information on fitness, gear, interviews, tests and more. I hope you find what you're looking for.

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