Is Engine/Motor Oil Flammable? Will It Catch Fire?

Have you ever considered what is and what isn’t flammable? Like, have you ever been filling up the oil in your car and wondered just how flammable that is? What would happen if there was a sudden spark, or if someone lit up a cigarette next to you while you were filling up the tank? 

Is EngineMotor Oil Flammable

You might assume that motor oil is flammable, and that is just the cautious soul in you. That is not a bad thing, being cautious about what may or may not be flammable is very wise.

However, you need not wonder if it is flammable or not anymore, because we will tell you exactly what you need to know about this, no more guessing. The true facts might take you by surprise. 

It is possible for motor oil to catch fire, it has a flashpoint above 199.4 degrees Fahrenheit, or 93 degrees Celsius. It is, however, not considered as being a flammable liquid by OSHA. The flash point of motor and engine oil is approximately 419 degrees Fahrenheit, and 215 degrees Celsius. 

Now, we are going to give you all the information that you need to know about burning motor oil, and synthetic oil, as well as how to stay safe around them. 

How flammable is motor oil?

First off, motor oil is not technically flammable, this is because OSHA defines flammable liquids as ones that will ignite when they are in the presence of an ignition source that is below 199.4 °F/ 93 °C. 

Motor oil requires a much higher temperature in order to combust. It is not classified as being a flammable liquid because although it will burn, it will only burn in the presence of an ignition source, or a flash point, that is around 300- 400 °F, or 150-205 °C.

The flash point is the temperature at which a substance will give off enough fumes to burn when it is exposed to a spark or source of ignition. The flash point of motor oil can vary by type, but typically it is around 400 °F.

Do keep in mind that this is not to be confused with auto-ignition temperature. Auto ignition temperature is the temperature that a substance will spontaneously catch fire, meaning catching fire without an ignition source. This is usually a much, much higher number. 

It’s not? How come?

Okay, so now you are wondering, ‘how come it is not flammable?’ We do not blame you for asking this, you would expect it to be, and the fact that it isn’t will leave you with more questions.

The truth is that most flammable liquids will consist of what we would consider to be ‘short’ chains of hydrocarbons. These are short molecules that find it easy to release vapors, and it is this vapor that will allow them to ignite at lower temperatures. 

The release of this vapor comes from ‘weak intermolecular forces’, which is just a high-tech scientific way of saying that it takes less energy to break two molecules apart and for them to drift apart in the form of vapors. 

Given that there are literally trillions of molecules found in a gallon of liquid, this means that this can happen as many times per second, or even millisecond, and they can produce a very significant amount of vapor. 

Motor oil is also a heavier liquid, and this means that it has much longer molecules. In fact, oil is a mix of hydrocarbons, and not just one hydrocarbon. But the mix has molecules that tend to have as few as 18 or as many as 34 carbon atoms in any one molecule.

So these molecules are significantly longer than flammable liquids have. 

This means that it takes much more energy to break two of these molecules apart and therefore motor oil has much stronger intermolecular forces, and has a smaller tendency to form any vapor around room temperature. 

Basically, it is more resilient to heat than other, more flammable, liquids are.

This simply means that it needs a much higher temperature to combust, because the temperature is what will effectively weaken these intermolecular forces enough for them to produce a vapor. 

By this we aren’t saying that it won’t catch on fire, it simply requires much higher temperatures to catch on fire, and therefore it is classified as being combustible, not flammable.  

At what temperature is it flammable?

The temperature at which motor oil becomes flammable varies brand by brand, however the flash point is typically around 400 °F when any specific heat source is introduced to the oil itself. 

Remember, there is no vapor that is produced from motor oil when it is at room temperatures, and this means that it simply cannot catch fire unless it is heated above the flash point temperature. 

The flash point temperature is the temperature at which there are enough fumes and vapors released for it to catch fire. 

However, it is still not encouraged to have caution around these places, especially around the oils and other liquids used in motors, because while motor oil is not flammable there are usually other much more flammable liquids around, which could more easily catch fire.

And when you are smoking a cigarette, you can not always help to create the occasional spark, so you ought to keep these to the minimum. 

Storing oil 

While engine oil is not considered to be flammable, in the most strict definition, it can still catch on fire fairly easily.

Most sources of flames are significantly hotter than the 400 °F that it takes for engine oil to ignite, and if oil meets a flame, it can heat up the oil to the point where it will burn. So, how you store your engine oil does matter. 

This means that in an area where you store engine oil, you need the following precautions; 

  • Ensure that there is adequate ventilation.  Obviously, if you are just keeping a single small bottle of oil around for your car, you do not need to do a huge amount. However, if you do have a lot of oil on hand, you should be seeking an extraction  system to prevent any build up of vapors. 
  • Think about spill containment.  You should stand your oil in a bucket of sand if you only have a single bottle. That way if there is a leak it will only go on some sand, and sand is the opposite of flammable. If you have a lot of oil, you should talk to a professional supplier of a spill containment facility. 
  • Keep your oil away from any potential ignition sources. If there is any likely source of a flame, ensure that you keep the oil at least 3 meters away from it. This will reduce the risk of any fires breaking out.

Okay, but, what about synthetic motor oil? 

Motor Oil

So, if standard motor oil is not very flammable, what about synthetic motor oil? Well, actually, synthetic motor oil, is in general terms, even less flammable than your standard motor oil, this is because they are designed to provide superior temperature resistance.

However, like regular motor oil, synthetic motor oil will still burn if it gets hot enough. 

Synthetic motor oils are meant to operate at much higher temperatures without burning or breaking down into their constituent elements.

Synthetic motor oils have a flash point of around 450 °F today and some of the more specialized synthetic motor oils that we use today have a flash point of around 700 F, which is quite a significant difference to standard motor oil, don’t you think? 

This is also good news for the engine of the vehicle as well, in theory. 

Simply good because it can get very, very hot inside an engine, hot enough that at a point it can actually begin to break down an ordinary motor oil.

This does not necessarily mean that the oil will catch fire, as the sealed environment will prevent this as there is no access to a fuel source, oxygen and so on to burn the oil, but simply that the long molecular chains will start to split up into much smaller ones. 

These shorter chains will have a lower level of viscosity, which means that they become more similar to water. This will then mean that the oil will move much faster around the engine and will offer less protection to the individual components. 

This also means that a traditional motor oil will have more of a ‘boil off’. This means that when these shorter chains of hydrocarbon molecules are exposed to air, they will have much weaker intermolecular forces and some of them may break away to form a vapor. 

Now, fortunately for the driver this does not mean that they become flammable, it would certainly be detrimental to the motoring industry if it did.

The ventilation through the engine ensures that this vapor can not build up to a meaningful enough quantity that it could catch fire, however, unfortunately once a molecule becomes a vapor, and it leaves the engine, it will never return either. 

What this means is, if you run an engine for about 6 hours at a temperature of around 400 °F, and it is running with a traditional motor oil, then you can expect to see a loss of about 30% of your oil by that volume! Which is quite a significant amount of oil lost! 

On the other hand, synthetic oil will operate differently under similar conditions, it is not to say that there is no boil off, but there is much less, and you would expect to see a loss of only around 4% in contrast, which is a much nicer number, don’t you think? 

Losing less oil simply means that you will need to do fewer top-ups of oil, and you will not need to spend as much money refilling your oil.

Depending on the price of motor oil and synthetic oil, it might mean that the driver who is using synthetic oil is saving a significant amount of money too! 

Although, in practice, synthetic oils are typically more expensive, and usually by quite a bit.

This  means that the driver is then left looking to save money based on the longevity of their engine, with the additional viscosity of the synthetic oil generally providing a longer lifetime use, which is really quite hard to measure.

It is also worth noting that synthetic oil is much cleaner, so you will not need to service your engine as much.  

Closing the case

Motor oil is not flammable, its point of which it will burn is much higher than 199.4 °F, however, this does not mean it won’t catch fire, lighting a cigarette nearby won’t send it up in flames, but it is still susceptible to burning if it comes into contact with a direct flame or if it goes over a temperature of 400 °F.

Caution is the best way to prevent it from combusting. 

Motor oil is not flammable, but it is combustible.