Diesel Fuel: Is It Flammable?

It’s the burning question that many have: Is diesel flammable? 

Diesel, a fuel that makes up around 3% of all vehicles throughout the United States is even more popular in other regions of the world. Wherever you are, diesel is a common sight in gas stations.

Diesel Fuel Is It Flammable

As diesel is a fuel, many people automatically think that it is flammable but is this true? While petrol burns and explodes, many are unsure if diesel will react in the same manner.

Well, if you’re in a hurry, the answer is, yes. Diesel fuel is classified as a flammable liquid and can easily catch fire. This is according to the U.S. Department of Labor.

It states that a flammable liquid is a liquid that has a flashpoint below 140 degrees Fahrenheit with a vapor pressure that doesn’t exceed 40 pounds per square inch at 100 degrees Fahrenheit.

The reason diesel is flammable is that its most common grades have flashpoints of around 140 degrees Fahrenheit or 60 degrees Celsius.

However, in most climates with gentle temperatures, diesel will not ignite. While diesel fuel is technically a flammable liquid, it remains less flammable than other fuels such as gasoline, propane, and ethanol.

In the following article, we will discuss how diesel catches fire and what the differences are between flammable and combustible liquids. 

Diesel: What is its flashpoint and what does this mean?

So, what is the flashpoint of something? It is simply the lowest temperature at which a substance can give off enough vapor to catch on fire or ignite.

Diesel fuel tends to have a flashpoint between 100 and 180 degrees Fahrenheit (37 to 82 degrees Celsius). This range is because not all diesel is the same. There are different types of diesel fuel. Some of these include:

  • Petroleum diesel
  • Synthetic diesel
  • Biodiesel

Types of diesel fuel are usually broken up into 3 classes: 

  • 1D (#1)
  • 2D (#2)
  • 4D (#4)

The difference in these classes is the viscosity of each one as well as the pour point (the temperature at which fuels will flow). 

When we consider the most commonly used flashpoint figure with diesel, we find it is 140 degrees Fahrenheit or 60 degrees Celsius. Therefore, it is classed as a flammable liquid. 

This is in stark difference to petroleum which has a flashpoint of -45 degrees Fahrenheit (-43 degrees Celsius). This lower flashpoint is critical for gasoline to work efficiently in engines. However, gasoline is also classified as a flammable liquid.

Gasoline engines work because gasoline combines with oxygen. This produces a spark found in the spark plugs which ignites this mixture.

These little sparks then cause small explosions that help push the pistons and begin powering the vehicle or machine’s motor. Therefore, gasoline must be able to ignite in ambient temperatures. If the temperatures are too extreme, the engine will not run properly. 

On the other hand, diesel is very different.

Diesel motors require no spark plugs. Alternatively, diesel engines use glow plugs. These heat the fuel mixture to produce the combustion that the engine requires. Because a spark is not needed for combustion to take place, diesel fuel needs a higher flashpoint.

Flammable vs Combustible

Many people get the terms “flammable” and “combustible” mixed up. However, these do not have the same meaning.

You may be forgiven for thinking that if something is flammable, it will catch fire, or if something is combustible, it will simply explode when it gets ignited. Guess what, this isn’t technically true.

The OSHA (Occupational Safety and Health Administration) has criteria set out for flammable and combustible liquids in their standard 29 CFR 1910 regulations.

While flammable liquid was once defined as any liquid that has a flashpoint below 100 degrees Fahrenheit (37.8 degrees Celsius) and combustible liquid was any liquid that has a flashpoint at or over 100 degrees Fahrenheit (37.8 degrees Celsius), such beliefs have now changed.

OSHA now states that any liquid that has a flashpoint below 199.4 degrees Fahrenheit or 93 degrees Celsius) is deemed a flammable liquid.

It’s important to note that solids and liquids do not burn in their original form. They send off flammable vapors that can ignite under certain temperatures and conditions. This is why flammable liquids are deemed more dangerous than their combustible counterparts.

A flammable liquid can catch fire more easily at lower temperatures. This isn’t to say that non-flammable liquids are non-hazardous, however. The main difference is how flammable these liquids can be and at what temperature they can ignite. 

Is it possible to set diesel on fire with a match or a lighter?

This entirely depends on the conditions the diesel finds itself in. If the environment’s temperatures or other possible heat sources cause the diesel to heat up above its flash point of 140 degrees Fahrenheit (this can vary with different types of diesel fuel), then it will begin to give off diesel fumes which are flammable. The result? The diesel can catch fire with a spark or a flame.

Nonetheless, diesel is usually under the 126 to 205 degrees Fahrenheit flashpoint range. In these conditions, it will not ignite with just lighter or other ignition sources. 

So, if diesel reaches temperatures that are equal to or more than its flashpoint, it will catch fire. At normal, ambient temperatures, it will not.

Is diesel safer than other fuels?

Diesel has a higher flashpoint than most other fuels. Therefore, it is considered to be a safer fuel, in terms of flammability. 

Diesel liquid is not only less flammable than gasoline, propane, and ethanol. It is also less toxic. Therefore, diesel is one of the safest options to choose from when considering different fuel types. 

In Summary

Diesel is a unique type of fuel. Compared to gasoline, propane, and ethanol, it is a safer, less flammable option. While diesel can be a fire hazard, only gasoline, propane, and ethanol are technically flammable liquids.

However, diesel will and can burn easily. In certain conditions, it can be highly dangerous and an effective way to fuel fires.