The difference between octane and cetane
January 10, 2019Even deep within farm country where compression ignition engines are dominant, little to no conversation is ever heard about a fuel’s cetane rating. In contrast, even the most disconnected motorist is familiar with the term octane.
A very common misconception is that the terms refer to almost the same aspect of the respective fuel. It is true that both identify a characteristic of combustibility, but they are as different as soybeans are from corn in terms of nitrogen scavenging.
Learning the difference
The definition of octane is the fuel’s ability to resist combustion from pressure and/or heat and wait for the arcing of the spark plug — the higher the octane the better the ability to resist auto-combustion. A low octane fuel will be prone to auto-combust via heat or pressure.
When gasoline auto-combusts there are multiple flames expanding in the bore instead of the intended one: A flame that was the result of auto-combustion along with the proper event created by the arcing of the spark plug.
In engineering, multiple events are identified as abnormal combustion and in contrast, a proper event is normal combustion. When the two flames collide it is heard as knock or ping.
Gasoline is currently rated as an average of the Research and Motor octane (R+M/2). Research octane is the fuel’s anti-knock characteristic under light load and low rpm while the Motor octane rating is under full throttle and high thermal loading.
The octane rating of gasoline has nothing to do with its energy content. Often the components used to raise the octane steal some of the potential energy when measured in Btu. If an engine makes more power on higher octane fuel it is due to the ability to enjoy a normal combustion event and not a greater energy density.
Cetane number is the exact opposite. It is the fuel’s eagerness to ignite from the heat created in the bore from the compression of the air. The higher the cetane number the shorter the ignition delay and the better the ignition quality.
As with octane, cetane provides an audible qualifier. Higher cetane fuel will create a quieter combustion event that enjoys less diesel knock than a fuel with a low number. As an aside, the engine will also run smoother, be more responsive and usually return better fuel economy.
When designing a gas engine the goal is for it to be octane tolerant, meaning it can have a normal combustion event on low octane fuel. The edict for a diesel is to not be cetane sensitive. The design should promote a short amount of ignition delay even with low cetane fuel.
For the most life from any engine it is important to understand the difference between the two fuels and how they are rated.
Dear Mr. Bohacz,
Love reading your articles in Hemmings. I have been a Caterpillar Marine Diesel Mechanic for almost 40 years out here in Tacoma, Wa. (S. of Seattle) where I work on commercial work and fishing boats. A few years ago you wrote your slow poke story…that was me! My whole career has been working on the Alaska fishing fleet. If my job isn’t preformed correctly, lives are at stake (have worked on some of the tv show’s Deadliest Catch boats…when they come to shore). The few new kids that we hire do not have this mindset and that half-ass work is good enough.
My management won’t make the time for me to show them how to measure with a micrometer or dial indicator. The work ethic you write about is how I was trained. There’s always enough time to do it again but never enough time to do it right the first time!
Thanks for letting me vent.
Pete Warren.