Q&A: Cetane number of vegetable oil

I met Eric over dinner in Indianapolis, and he asked me a very good question:

What is the Cetane number of Vegetable Oil as compared to Petroleum- and Bio-Diesel?

Having no idea what a cetane number was, my response was: "hmm, that would make a great Q&A post. Watch our blog for my answer!"

So, two and a half weeks later, I finally have the time to investigate and post a response to his question.

I found the answer in several places: Wikipedia's Descriptions of Cetane Number and Cetane, Journey to Forever's comparison of Vegetable Oil, Bio Diesel, and Petroleum Diesel, ChemFinder, the NIST chemistry web book, and FuelMagic's cetane discussion.

The short answer is this: the cetane number of vegetable oil varies depending on type and quality of oil, but it doesn't vary far from the cetane number of petroleum diesel.

Here is the data (from Journey to Forever):
Diesel 45
Canola Oil 40-50
BioDiesel 45-65

This is all well and good, but who the heck is cetane, and why do I care about her number?
Cetane (AKA: hexadecane) is a fuel that experiences autoignition very easily. If you were to test a fuel for use in an autoignition engine (like a Diesel engine), and it behaved exactly like cetane, the fuel's cetane number would be 100. If it were slightly worse than cetane, it would have a cetane number of 99, and so on. If the fuel does not autoignite, it has a cetane number of 0. Diesel fuel has a cetane number of about 45. Most Diesel engines run well when using fuels with cetane numbers between 40 and 55 (please note that Canola Oil is well within that range).

So, simply put: The cetane number is a fuel's ability to combust in a diesel engine, as compared to cetane.



A cetane molecule. Image taken from NIST.

Don't confuse cetane number with octane number - these are NOT equivalent measurements.
If you put 100% octane (or a fuel with a octane number of 100) into a gasoline engine it would run very well. If you put 100% cetane (or a fuel with a cetane number of 100) into a diesel engine, it would either not run at all, do some terrible damage to itself, or both.

Fuel Magic says:

There is no benefit to using a higher cetane number fuel than is specified by the engine's manufacturer

and

Diesel fuels with cetane number lower than minimum engine requirements can cause rough engine operation.

So, any fuel that has a cetane number of 40-55 will work just fine. This means that Vegetable Oil works just great as a diesel substitute (if you can deal with the viscosity).

I hope that answers your question Eric!

Please keep your questions coming - just use the contact us page to send us questions!