Gasoline is a flammable liquid that is colorless or has a slight yellowish tint (unless special additives are available). 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 to it. If during the combustion of fuel a characteristic metallic sound is heard, created by a detonation wave when it is repeatedly reflected from the cylinder walls, then the engine's efficiency decreases, and its wear is accelerated.
In this case, they say that the fuel quality is unsatisfactory, since the octane number is low. The knock resistance of motor fuel, that is, the ability to withstand self-ignition due to compression in the cylinders of a running engine, characterizes this indicator. Numerically, it is equal to the volume fraction of isooctane (its other name: 2,2,4-trimethylpentane) in a two-component mixture containing n-heptane as well. Such a mixture, when determining its knock resistance under standard test conditions, creates an effect equivalent to the test fuel.
Isooctane is the ultimate hydrocarbon of C8H18 isostructure. Due to its properties, it is difficult to oxidize even with a high degree of compression; therefore, its detonation resistance was conventionally assumed equal to 100 units. N-heptane in the engine behaves differently: even at low compression ratios, the combustion process is accompanied by a detonation effect, therefore the value of this indicator for it is taken as 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 indicator is the most significant characteristic of motor fuel. It shows how gasoline is resistant to detonation, that is, to arbitrary ignition in the cylinders of the engine. The probability of a fuel explosion in the engine will be minimized if this indicator is high. If the octane number of gasoline corresponds to 95 units, then it detonates as a mixture consisting of 95% 2,2,4-trimethylpentane and 5% normal heptane. After the initial processing of oil 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 agents are also added in an amount of up to 0.3%.
The quality of the fuel depends largely on the composition of the natural mineral from which it is obtained by fractionation or distillation - oil. For her, such indicators as viscosity, fractional composition, the presence of sulfur compounds and paraffins, as well as the water content and the salts dissolved in it are important. However, these factors are more likely to affect the characteristics of processing and the choice of technology. The quality of gasoline, that is, the value of its octane number, determines the hydrocarbon composition of oil, and then the method of its processing, which also determines how much isooctane, n-heptane, aromatic compounds and so on will be in gasoline.
The antiknock properties of gasoline increase the aromatic hydrocarbons contained in it . A high benzene content primarily affects the environment, as it is a source of benzapirene (a carcinogen). The increased content of high boiling aromatic compounds contributes to an increase in carbon formation in the combustion chamber and on the engine valves. All this worsens such performance indicators as engine power, efficiency, environmental and economic aspects of its operation. The carbon deposits formed in the combustion chamber force the car owner to choose motor fuel so that the octane number of gasoline is high. Otherwise, the power decreases significantly or the motor works with detonation.