Gasoline is obtained through refining and cracking petroleum, and is composed of a mixture of hydrocarbons with five to ten carbon atoms in the chain. Thus, the quality of gasoline varies according to the hydrocarbons that compose it.
As explained in the Combustion Engine Function text, this gasoline-powered engine works on a four-stroke schedule that involves compressing the gasoline-air mixture. The more gasoline resists this compression without exploding, the better its quality. But if the gasoline is very sensitive to compression, suffering premature explosions before the piston reaches the dead center and before the spark is released from the spark plug, the result will be engine deregulation, the piston will be pushed with less force in the fourth stroke and the power of the car will decrease. When this occurs, there is a noise in the engine known as «knock pin» (knocking).
For this reason, a scale to measure the quality of gasolinewhich is called octane number.
Among the hydrocarbons that make up gasoline, the one with the best octane number or the best resistance to compression is isoctane (2,2,4-trimethylpentane). This compound was assigned the value 100 octane or 100 octane. On the other hand, the heptane is the gasoline component least resistant to compression and it was assigned the value of zero octane or octane number equal to 0.
This means that, if gasoline has an octane number equal to 87%, it behaves as if it were a mixture of 87% isoctane and 13% heptane.
To improve the compressive strength and, consequently, the octane number of gasoline, some additives, which are called antiknocks, can be added. The antiknock most used by Petrobras is methyl t-butyl ether (MTBE)which is added in up to 7% by volume.
In Brazil, octane rating is expressed in Antiknock Index (IAD), and common gasoline is also specified by MON (Engine Octane Number) or Engine method – ASTM D270, which evaluates the resistance of gasoline to detonation when the engine is at full load and at high speed.
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In addition, according to the National Agency of Petroleum, Natural Gas and Biofuels (ANP), Brazilian legislation established, through the publication of Ordinance No. 143 of the Ministry of Agriculture, Livestock and Supply, that all types of gasoline must be added with anhydrous ethanol with a percentage of 25%. The margin of error is plus or minus 1%. This is done because ethanol evaporates more easily and works as an antiknock, increasing the octane number of gasoline. Furthermore, in this percentage, the addition of ethanol to gasoline helps to reduce pollutants, such as carbon monoxide (one of the greenhouse gases), and also improves the internal cleanliness of the engine.
The addition of alcohol to gasoline generates a type of gasoline classification in A and C:
* Gasoline A: it is produced in refineries or petrochemicals and does not contain ethanol. Its density, in general, varies from 700 to 770 g/L;
* Gasoline C: it is sold at gas stations and received ethanol addition by distributors. Its density, in general, varies from 718 to 775 g/L.
Depending on the components and quality, the gasoline classification Brazilian, according to the ANP, is done as follows:
1- Common Gasoline: It has a yellowish color, an octane number of 87, no additives and a maximum sulfur content of 50 ppm (S-50 gasoline).
The color of the original gasoline varies from colorless to yellowish depending on the chemical composition and the different refining processes. Only podium gasoline is colorless.
2- Additive Gasoline: It is the same as regular gasoline (same octane number and sulfur content), however, as the name implies, gasoline with additives has detergent and dispersant additives that clean the fuel system and injector nozzles. To differentiate it from common gasoline, it receives the addition of a dye of the desired color. Petrobras adds the green dye.
Additivated gasoline from Petrobras receives the addition of green dye.
3- Premium Gasoline: it has the same detergent and dispersant additives as gasoline with additives, but its octane number is higher, being equal to 91. In addition, its sulfur content is lower, being less polluting.
4- Gasoline Podium: gasoline commercialized exclusively by Petrobras, it presents better performance because it has an octange index equal to 95, in addition to detergent/dispersant additives and low sulfur content. It is originally colorless, but is marketed with a slightly orange color because of the dye added to the anhydrous ethanol that is added to it. This is the most stable gasoline that exists today in the Brazilian market, and can be stored for a longer time than the others.
The only dye colors that cannot be used in automotive gasolines are pink and blue. Pink is not used because, in the 1980s, when ethanol production was small in Brazil, methanol was imported from the United States so that it could be used as a gasoline additive, forming the mixture MEG (methanol-ethanol-gasoline ), which received a pink dye to be differentiated. The drawback of its use is that methanol corrodes steel and is very toxic. In addition, its flame is invisible to the naked eye, which makes it difficult to control fires and accidents. Therefore, today it is not used in any type of gasoline in Brazil.
The blue color is used in aviation gasoline.
By Jennifer Fogaça
Graduated in Chemistry