Difference Between Octane Number And Cetane Number

What Is Octane Number?

An octane rating also referred to as octane number, is a standard measure of an engine or aviation gasoline capability against compression. The higher the octane number, the more compression the fuel can withstand before detonating. Octane rating does not relate directly to the power output or the energy content of the fuel per unit mass or volume, but simply indicates gasoline’s capability against compression. In broad terms, fuels with a higher octane rating are used in higher-compression gasoline engines that potentially yield higher power. Note that such higher power comes from its higher compression of the engine, and not necessarily from the gasoline.

In contrast, fuels with lower octane numbers (but higher cetane numbers) are ideal for diesel engines, because diesel engines (also referred to as compression-ignition engines) do not compress the fuel, but rather compress only air and then inject fuel into the air which was heated by compression.

Gasoline engines rely on ignition of air and fuel compressed together as a mixture, which is ignited near the end of the compression stroke using electrically activated spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of gasoline with lower octane numbers may lead to the problem of engine knocking.

What You Need To Know About Octane Number

• Octane number of a petrol sample is defined as the percentage of iso-octane Octane in the mixture of iso-octane and n-heptane, which has similar knocking to the petrol sample.
• Octane Number measures the ability of a fuel to resist pre-ignition of the fuel in the gasoline engine. Fuel having high Octane number is considered as Ideal fuel for the Gasoline engine.
• Octane rating is used for petrol.
• Octane number of good petro should be at least, 85 for motorcycle and cars 100 for aeroplanes and helicopters.
• Octane number of petrol can be increased by adding benzene or toluene.
• The fuel with high octane rating has low cetane number.
• Higher octane number gives greater resistance of gasoline for premature combustion which is known as knocking.
• Octane rating is decided according to the octane number of Iso-octane which is 100. The octane number of Iso-butane is hundred and zero for n heptane hence every fuel is rated between zero to the hundred.  Iso-octane has maximum octane number the hundred and N-heptane has minimum octane number zero. Commonly octane number of fuel ranges from 87 to 95 in gasoline fuel.

What Is Cetane Number?

 Cetane number also referred to as Cetane Rating is a measure of the ignition value of a diesel fuel that represents the percentage by volume of cetane in a mixture of liquid methylnaphthalene that gives the same ignition lag as the oil being tested. Cetane number of particular of a fuel can be defined as the percentage volume of n-hexadecane in the mixture of n-hexadecane and 1-methylnaphthalene which is responsible to provide ignition delay period. It is exactly opposite of the Octane number and all the diesel fuels are indexed to cetane based on their performance of ignition under compression. Cetane number measures how quickly the fuel starts to burn inside the compression engine.

Cetane has a cetane number defined to be 100, while heptamethylnonane’s measured cetane number is 15, replacing the former reference fuel alpha-methylnaphthalene, which was assigned a cetane number of 0. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane’s 100 and heptamethylnonane’s 15.

Cetane number = % n-cetane + 0.15(% heptamethylnonane)

Cetane number is an inverse function of a fuel’s ignition delay, the time period between the start of ignition and the first identifiable pressure increase during combustion of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower Cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils.

Generally, diesel engines operate well with a CN from 48 to 50. Fuels with lower cetane number have longer ignition delays, requiring more time for the fuel combustion process to be completed. Hence, higher speed diesel engines operate more effectively with higher cetane number fuels.

What You Need To Know About Cetane Number

• Cetane number of diesel sample is defined as percentage of nhexadecane in the mixture of n-hexadecane and 2-methyl naphthalene which has same ignition character like the ignition character of the diesel under test.
• Cetane Number measures the ignition delay of the fuel in the diesel engine. When Cetane number is higher, the ignition delay is short and the fuel having high Cetane number is considered as the Ideal fuel in the diesel engine.
• Cetane rating is used for diesel.
• Cetane number for good diesel should be 25 for low speed egine,35 for medium speed engine and 45 for high speed engine.
• Cetane number diesel can be increased by adding by adding ethyl nitrate or acetone peroxide.
• The fuel with high cetane rating has low octane number.
• Higher cetane number is responsible for the rapid ignition of the diesel which gives excellent performance of the diesel engine.
• Cetane rating is measured according to the ignition of the cetane. N-hexadecane consists maximum cetane number which is the hundred and 1-methylnaphthalene has minimum cetane number zero.  It ranges from 30 to 35, fuel exceeding this number is known as poor fuel.

Also Read: Difference Between Diesel And Petrol Engine

Difference Between Octane Number And Cetane Number In Tabular Form