Have you ever been pouring motor oil and wondered what would happen if there was a sudden spark or someone walked past with a lit cigarette? One would assume motor oil is flammable, but is it? You don’t have to wonder any more because we can tell you exactly what will happen and it may surprise you.
Motor oil can catch fire, but because it has a flashpoint above 199.4 degrees Fahrenheit (93 degrees Celsius), it is not classified by OSHA as a flammable liquid. The flashpoint of motor/engine oil is approximately 419 degrees Fahrenheit (215 degrees Celsius).
So, here’s what you need to know about burning motor oil, synthetic oil, and how to stay safe around them.
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Also read: What Makes Something Flammable?
Table of Contents
Flammability of Motor Oil
No. Motor oil is not technically flammable, that’s because OSHA defines “flammable liquids” as those that ignite when they are in the presence of “an ignition source” below 199.4 degrees Fahrenheit (93 degrees Celsius).
Motor oil, on the other hand, requires a much higher temperature to combust. In fact, it is not classified as a flammable liquid because it will burn, in the presence of an ignition source (flashpoint), at around 300-400 degrees Fahrenheit (150-205 degrees Celsius).
The temperature at which a substance will give off enough fumes to burn when exposed to a spark or other ignition source is called the flashpoint. The flashpoint of motor oil (can vary by type) is around 400 degrees Fahrenheit.
Note: This is not to be confused with auto-ignition temperature. Auto-ignition temperature is the temperature at which a substance will spontaneously (without an ignition source) catch fire. This is usually a much higher number.
Why Isn’t It Flammable?
Most flammable liquids consist of what we consider to be “short” chains of hydrocarbons. These short molecules find it easy to release vapor, and it is the vapor that allows them to be ignited at low temperatures.
The release of vapor comes from weak “intermolecular forces” which is a fancy way of saying, it takes less energy to break two molecules apart and for one to drift away in the form of vapor. Given that there are, quite literally, trillions of molecules present in a gallon of liquid – this means that this happens many times a second and can produce plenty of vapor.
Motor oil is a heavier liquid and that means it has much longer molecules (oil is, in fact, a mix of hydrocarbons, not one specific hydrocarbon, but the mix has molecules that tend to have as few as 18 or as many as 34 carbon atoms in each molecule) than the flammable liquids do.
That means it takes much more energy to break two of these molecules apart and thus motor oil has stronger “intermolecular forces” and has no tendency to form vapors at or around room temperature.
This means that it needs a higher temperature to “combust” because that temperature is what effectively weakens the intermolecular forces enough to produce a vapor.
This doesn’t mean that it won’t catch on fire, just that it requires higher temperatures to do so and is therefore classified as a combustible liquid instead of a flammable liquid.
Also read: Is Chlorine Bleach Flammable or Explosive?
What Temperature Does It Catch Fire? Flashpoint
It varies a little bit by brand but the flashpoint will be around 400 degrees Fahrenheit when a specific source of heat is introduced to the oil itself (flashpoint).
This test found the flashpoint of motor oil was 419 Fahrenheit (215 Celsius):
Don’t forget, though, that there is no vapor produced from motor oil at room temperatures and that means it can’t catch fire unless it gets heated above the flashpoint temperature. The flashpoint is the temperature at which enough fumes/vapors are released to catch fire.
However, we wouldn’t recommend that you encourage people to smoke in these places – there are usually other flammable liquids around and while you can’t help occasionally creating sparks, you ought to keep them to a minimum too.
Also read: Is Power Steering Fluid Flammable?
Oil Storage Matters
While engine oil might not be “flammable” in the strictest definition of the word “flammable”, it can still catch on fire relatively easily. Most sources of flame are much, much hotter than 400 degrees Fahrenheit and if the oil meets one of those – it can heat up the oil to the point that it will burn.
That means it matters how you store your oil and that means you need:
- Adequate ventilation. Obviously if you’re just keeping a single small bottle of oil around for your car, there’s not a huge amount that needs doing in this respect but if you do keep a lot of oil on hand, you should be looking at extraction systems to prevent build up of vapors.
- Spill containment. Stand the oil in a bucket of sand if you only have a single bottle, that way if it leaks – it’s not going anywhere except over some sand. If you have a lot, you need to talk to a professional supplier of spill containment facilities though.
- Keep them away from potential ignition sources. If there’s a likely source of flame, make sure you keep the oil at least 3 meters (10 yards) away from it. This reduces the risks of fire breaking out.
Also read: Is Transmission Fluid Flammable? Yes and No…
What About Synthetic Motor Oil?
No. In fact, synthetic motor oil is, generally speaking, even less flammable than standard motor oil – this is because they are designed to provide “superior temperature resistance”. But, like regular motor oil, they will still burn if they get hot enough.
Have a look at this test:
Synthetic motor oils are meant to operate at much higher temperatures without burning or breaking down into their constituent elements.
You might be surprised to learn that the synthetic motor oils used today have a flashpoint of around 450 degrees Fahrenheit and that some of the more specialist synthetic oils have a flashpoint of around 700 degrees Fahrenheit!
This is good news for the vehicle’s engine, in theory.
This is because it gets very hot in an engine, hot enough, at points that it can begin to break down ordinary motor oil. This doesn’t mean the oil catches fire (the sealed environment prevents that because there is no access to a fuel source, oxygen, to burnt he oil) but rather those nice long molecular chains start to split up into shorter ones.
Those short chains have a lower level of “viscosity”, that means they become more like water, for example. This, in turn, means the oil moves much faster through the engine and offers less protection to the individual components.
It also means that traditional motor oil has more “boil off”. That is, when these shorter chains of hydrocarbon molecules are exposed to the air – they have much weaker intermolecular forces and some of them break way to form vapor.
Now, fortunately, for the driver, at least, this does not mean that they become flammable. The ventilation through the engine ensures that this vapor cannot build up into meaningful quantities and catch fire, but unfortunately, once a molecule becomes vapor and leaves the engine, it will never return, either.
Thus, if you run an engine for about 6 hours at a temperature of around 400 degrees Fahrenheit, if it is running with traditional motor oil – it can be expected to see a loss of around 30% of that oil by volume! That’s quite a lot of oil.
In contrast, the synthetic oil operates very differently under the same conditions, it’s not to say that there’s no boil off but there’s much less and you’d expect to see a loss of 4% or less in these circumstances.
That means fewer top ups of oil and, in theory, depending on the price of motor oil and synthetic oil, it ought to mean that the driver who uses synthetic oil should save some money too.
However, in practice, synthetic oils are often more expensive (and significantly so) which means that the driver is then left looking to save money based on the longevity of their engine (the additional viscosity provided does promote longer lifetime use) which is much harder to measure.
The other advantage to the lower level of flammability in synthetic oils is that they tend to be cleaner than ordinary motor oil. Thus, you shouldn’t need to fully service the engine as often and that might be a big selling point if you’re sitting on the fence between the two at the moment.
Also read: Is Diesel Flammable? Yes and No…
Is motor oil flammable? Flammable is defined as a liquid that catches fire at 199.4 degrees Fahrenheit or less, so no motor oil is not “flammable”. However, that doesn’t mean that it is entirely risk-free and while a stray spark or cigarette nearby won’t make it catch fire – it’s not that hard to get it to burn, either.
That means you need to store motor oil and even synthetic oil carefully with the objective of reducing the risks to yourself and anyone else around. If you do that, though, you should find that working with these oils is fire-free.