One of the parameters for the classification of diesel fuel is the cetane number, which indicates the degree of its flammability. The qualitative indicator for gasoline is the octane number – determines the resistance of gasoline to detonation. Therefore, we can say that the cetane and octane number in diesel fuel and gasoline are inverse to each other.
Cetane number (CN)
In diesel engines, the fuel ignites not from a spark, as in petrol, but self-ignited as a result of high pressure. The cetane number characterizes the flammability and combustion quality of diesel fuel in the engine. A fuel mixture with a higher CN has a shorter delay period, burns calmer and fuller than with a low one. In addition, such diesel fuel is lighter and produces less smoke.
The optimal cetane number of diesel fuel for standard engines is from 40 to 55 GOST. If the CN is above 55, the fuel does not have time to completely burn in the cylinders. The cetane number of winter fuel is higher than the summer one, due to the need to work at low temperatures. Also these two types of fuel differ in chemical composition.
The cetane number is determined by calculating the volume fraction of cetane in a special fuel mixture that has the same flammability period as the test sample. To do this, cetane (in a different word hexadecane) is mixed with alfamethylnaphthalene and the cetane volume is calculated, taking its index by 100.
Cetane number increase
The value of the cetane number depends on the presence of paraffins, olefins, naphthenes, and aromatic hydrocarbons in the diesel engine. The production of fuel with a high CN is not always possible and requires more expenditure. Therefore, to increase the cetane number, special additives are used.
The additive to increase the cetane number improves the flammability of the fuel, increases the completeness of combustion, facilitates the start of the engine and reduces the amount of emissions to the atmosphere. The additives are based on alkyl nitrites, which in the future can be displaced by alkylperoxides.
Octane number (ON)
The octane number of gasoline shows its resistance to detonation – ignition upon compression in cylinders. The requirements for detonation resistance of fuel depend on engine parameters, therefore the octane number is an indicator not so much of the quality of gasoline as its compliance with the characteristics of the engine.
Methods for determining ON
The determination of the octane number is similar to the determination of cetane. But here, as a standard, a mixture of isooctane (in which ON is 100) with n-heptane (ON is 0) is used with the same characteristics as in the prototype gasoline. And then the volume of isooctane is determined, which corresponds to the desired octane number.
This procedure is called the research method of measuring the octane number, hence the letter “I” in the marking AI. A motor method can also be used, in which tests are carried out in conditions of real driving.
For each value of the octane number, the compression ratio for both measurement methods is set, these standard parameters are given in the octane number table.
How to increase the octane number
Fuel with a low octane number ignites earlier than with a high one, and if the engine does not meet the requirements, it can lead to detonation, that is, an explosion. To increase the ON, anti-knocking additives are often used, the safest of which is methyl-t-butyl ether. This non-toxic substance has an octane number above 100, ensures complete combustion of fuel and reduces carbon dioxide emissions.
Today, there are special devices for checking the octane number, which allow for a short time to check gasoline directly at the gas station.