Friday, April 27, 2007

Q&A: Flammability of Vegetable Oil - Updated

On Monday, we visited with the 5th, 6th, 7th, and 8th graders at the St. Mary's School in Goldsboro, NC.

A common questions we get from students - almost always from boys - is: does adding the vegetable oil tank in the trunk of the car increase the risk of explosion during car accidents?" I then have to convince everyone that adding a vegetable oil tank does not make Minnie The Car a rolling stick of dynamite, and that vehicle explosions pretty much never happen outside of Hollywood, California.

While trying unsuccessfully to explain this fact (much lamented by 8th grade boys everywhere), Miss Caroline from the 5th grade raised her hand and asked a question that completely stumped me: "what is the flammability of vegetable oil?"

I sputtered for a moment and managed to say that "well, it's much less than gasoline, and less than diesel, but I don't know how much less." I congratulated her on stumping me, and promised her a much more thorough explanation in a Q&A post.

So, let's first look at what flammability means, a good way of measuring it, and finally, we'll compare the flammability of vegetable oil to other common liquids and fuels.

By asking how flammable something is, we are asking how easily can it catch fire. The best way to look at the flammability of a liquid fuel is to look at its flashpoint. The flashpoint is the temperature above which a liquid can form an ignitable vapor above the surface of the fuel.

Without going into the science of vapor pressure, if a fuel is warmer than its flashpoint, if you hold a match to the surface, it will catch fire and burn. The lower the flashpoint, the easier it is to catch a fuel on fire. Gasoline, with a flashpoint of -43F is extremely flammable, while Canola oil with a flashpoint of 600F is flame retardant by comparison.

Flashpoint is measured by slowly heating a sample of the fuel in a chamber with a spark plug. As the temperature rises, the spark plug makes sparks - the temperature where the first flames form is the flash point.

Flashpoints of some common fuels:
Gasoline: -45F/-43C
Diesel: 143F/62C
Biodiesel: 300F/149C
Canola Oil: 600F/320C
Soybean Oil: 491F/255C

So, Miss Caroline, if you look at the numbers above, you'll see that vegetable oil is much less flammable than gasoline, diesel, or biodiesel.

Source research:
Wikipedia: Vapor_pressure

Wikipedia: Flammability

Wikipedia: Flash_point

MSDS definition of flashpoint's biodiesel facts's safety FAQ

Looks like I messed up the flashpoint for biodiesel - it is actually 260 to 300F, not 212F as I previously wrote. Thanks anonymous commenter for keeping me on my toes!

However, I believe that 143F is the correct flash point for petroleum diesel - can you please tell where you found 77F?


  1. the flashpoint of biodiesel is incorrect. 212F / 100C is the boiling point of water. Biodiesel (B100) has a flashpoint of about 300F, while petroleum diesel is 77F.

  2. I have a question. waht is the most flammable cooking oil