Octane Number and the Cetane Number are used as reference points for determining the quality of gasoline and diesel fuel and how likely a fuel is to catch fire on its own. The efficiency of gasoline fuel is determined by its octane number, whereas diesel is evaluated using its cetane number. Gasoline with a high cetane rating has a low octane rating, the opposite is also true. Because of this, neither gasoline nor diesel can be used in a vehicle’s engine that runs on either of those fuel types.
What is the Octane Number of Gasoline Fuel?
The octane number of a fuel specifies the amount of iso-octane (2,2,4-trimethylpentane) in the blend of iso-Octane (Octane number 100) and normal-heptane (Octane number 0).
Octane rating is the measure of the gasoline fuel’s resistance to knock during combustion in an engine. The higher the resistance higher will be the Octane number and the lower will be the knocking in an engine. Higher octane number or high resistance to knock will cause efficient combustion of fuel and as a result low knocking by the engine.
What is the Cetane Number of Diesel Fuel?
The cetane number or cetane rating is a measure of the ignition quality or knocking tendency of diesel fuel in a diesel engine. It is used to predict how easily the diesel fuel will ignite and burn when injected into the combustion chamber of a diesel engine. A high cetane number indicates a fuel that burns smoothly in the engine cylinder with minimum delay between fuel injection and autoignition.
The cetane number scale is based upon two hydrocarbons, Cetane (n-hexadecane, C16H34) and isocetane (2,3,4,5,6,7,8-heptamethylnonane, C11H10). The cetane number of cetane is 100 which has a short delay period during ignition. Whereas the cetane number of heptamethylnonane is 15 and has a long delay period. The cetane number of a fuel is a measure of how much n-hexadecane is present in a mixture of n-hexadecane and 1-methylnaphthalene, which slows the ignition process. The cetane number calculation formula is as follows;
Cetane number = % n-cetane + 0.15(% heptamethylnonane)
Cetane = 40+0.15 (60) = 53.5
Differentiate between Octane Number and Cetane Number
The ratio of octane to cetane is the inverse of the ratio of cetane to octane. A few factors allow for easy differentiation between them. Following are some common differences between octane and cetane numbers.
| Description | Octane Number | Cetane Number |
| Fuel Type | It measures the Gasoline fuel quality. | It measures the diesel fuel quality. |
| Measuring Rating |
The octane number of a fuel specifies the amount of iso-octane in the blend of iso-Octane and normal-heptane. The Octane number of iso-octane is 100 whereas n-heptane has 0. |
The cetane number is based on two hydrocarbons, Cetane and isocetane. The cetane number of cetane is 100, whereas the cetane number of heptamethylnonane is 15. |
| Function | Octane Number measures gasoline engine fuel pre-ignition resistance and ensures that fuel ignites only from a spark plug. | Cetane Number monitors diesel fuel delay time in the ignition and ensures fuel ignition on compression. |
| Engine Type | Octane rating is used for engines called spark ignition engines or gasoline engines. | Cetane rating is applied for engines called Compression Ignition Engines or Diesel engines. |
| Number Scale | Gasoline fuel with a higher Octane number means fuel resists premature ignition and only ignites upon spark and low knocking in the gasoline engine. | Diesel fuel with a higher cetane number means a short delay in the ignition and rapid ignition upon the compression stroke hence low knocking in the compression ignition Engine. |
| Octane and Cetane | The fuel with high Octane number has a low Cetane rating. | The fuel with a high Cetane number has a low low Octane rating. |
| Normal Value | The normal value of the Octane number is 92~98. | The normal value of Cetane is 45~55. |
| Engine Efficiency | A high Octane rating ensures complete combustion of gasoline fuel and hence high efficiency of the engine. |
A high Cetane number also ensures complete combustion of the diesel fuel, hence the high efficiency of diesel engines. |
| Knocking | A higher octane number gives greater resistance to gasoline fuel and avoid premature combustion. |
A higher cetane number gives rapid combustion upon the compression stroke with a short delay. Longer delays would cause abnormal combustion and knocking in the engine. |
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