Octane number of gasoline

Gasoline is a flammable liquid that does not have a color or with a slight yellowish tint (if no special additives are present). In order to improve the working properties of motor fuel used in internal combustion engines, manufacturers are trying to increase the octane number of gasoline. To do this, add the appropriate components. If, during the combustion of fuel, the characteristic metallic sound produced by the detonation wave is observed when it is repeatedly reflected from the walls of the cylinder, the efficiency of the engine decreases and, in addition, its wear is accelerated.

In this case, it is said that the quality of the fuel is unsatisfactory, since the octane number is low. The detonation resistance of motor fuel, that is, the ability to withstand self-ignition due to compression in the cylinders of the operating engine, characterizes this index. It is numerically equal to the volume fraction of isooctane (its other name is 2,2,4-trimethylpentane) in a two-component mixture containing also n-heptane. Such a mixture, when determining its detonation resistance under standard test conditions, creates an effect equivalent to the fuel under study.

Isooctane is the limiting hydrocarbon of the C8H18 isostructure. Due to its properties, it is difficult to oxidize even at a high compression ratio, so its detonation resistance was conventionally assumed equal to 100 units. H-heptane in the engine behaves differently: even at low compression ratios, its combustion process is accompanied by a detonation effect, so the value of this index is assumed for it in 0 units. In cases where the octane number of gasoline is above 100 units, a conditional scale of mixtures is used to assess its quality, where various doses of tetraethyl lead are added to the main component, isooctane.

Therefore, it can be argued that this figure is the most significant characteristic of motor fuel. It shows how much gasoline is resistant to detonation, that is, to an arbitrary ignition in the cylinders of the motor. The probability of a fuel explosion in the engine will be minimized if this figure is high. If the octane number of gasoline corresponds to 95 units, it detonates as a mixture consisting of 95% of 2,2,4-trimethylpentane and 5% of normal heptane. After primary oil treatment for a product called straight-run gasoline, this characteristic rarely exceeds 70 units. Therefore, in order to improve the quality of low-grade gasoline, not only compounding (mixing with high-octane products) is used, but antiknock additives of up to 0.3% are introduced.

The quality of fuel depends largely on the composition of the natural fossil from which it is obtained by fractionation or distillation - oil. It is important for such indicators as viscosity, fractional composition, the presence of sulfur compounds and paraffins, as well as the water content and dissolved salt in it. However, the above factors more likely affect the processing features and the choice of technology. The quality of gasoline, that is, the value of its octane number, determines the hydrocarbon composition of the oil, and then the way it is processed, which also determines how much isoctane, n-heptane, aromatic compounds will be in the gasoline, and so on.

The anti-knock properties of gasoline increase the aromatic hydrocarbons contained therein . The high content of benzene, first of all, affects the state of the environment, since it is a source of benzapyrene (carcinogen). The high content of high-boiling aromatic compounds contributes to the increase of carbon formation in the combustion chamber and on the engine valves. All this worsens performance indicators such as engine power, efficiency, environmental and economic aspects of its operation. The carbon that forms in the combustion chamber causes the car owner to choose motor fuel in such a way that the octane number of gasoline is high. Otherwise, the power is significantly reduced or the motor works with detonation.