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What You Need to Know about Fuel's Cetane and Octane Ratings

Even 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.

The definition of octane is the fuel’s ability to resist combustion from pressure or heat and wait for the arcing of the spark plug. The higher the octane, the better the ability of the fuel 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. This is a flame that was the result of auto-combustion along with the proper event created by the arcing of the spark plug.

What causes a ping
In engineering terms, multiple combustion events are identified as abnormal combustion. In a proper event, it is called normal combustion. When the two flames collide, it is heard as a 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 antiknock characteristic under light load and low rpm; 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, 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 engine is to not be cetane-sensitive. The engine design should promote a short amount of ignition delay even with low-cetane fuel.

To get the most life from any engine, it is important to understand the difference between the two fuel components and the way they’re rated.

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