Gasoline - it is difficult to remember something more familiar to a motorist. Every day, cars burn hundreds of thousands of liters of this fuel, but few car owners seriously thought about how it is produced, about the features of the fuel composition and other aspects.
Some terminology
- Aromatic;
- Olefinic;
- Paraffin and others.
These hydrocarbons are combustible. The boiling point of the mixture varies from 33 to 250 °C, depending on the additives used.
What is gasoline made from?
Scheme of gasoline production
Fuel is produced at the facilities of oil refineries. The production process itself is very complex and is divided into several cycles.
First, crude oil enters the enterprise through pipelines, is pumped into huge tanks, and then settles. Next, oil washing begins - water is added to it, and then an electric current is passed. As a result, salts settle to the bottom and walls of the tanks.
During the subsequent atmospheric-vacuum distillation, the oil is heated and divided into several types. There are 2 stages of processing:
- vacuum;
- Thermal.
Upon completion of the primary processing process, catalytic reforming begins, during which the next purification of gasoline and the extraction of fractions of the 92nd, 95th and 98th gasoline take place.
Photo: aif.ru
This process, which is also called recycling, includes 2 main stages:
- Cracking - purification of oil from sulfur impurities;
- Reforming - endowing the substance with an octane number.
Video: How gasoline is made from oil. Just about complex
At the end of these stages, the quality of the fuel passes, which takes several hours.
It is noteworthy that domestic plants (in the majority) from 1 ton of oil receive 240 liters of gasoline. The rest is gas, fuel oil and aviation fuel.
What is octane number
This phrase is known to very many, but not everyone knows what exactly this term means and why it is so important.
The octane number is the ability of a fuel (including gasoline) to resist spontaneous combustion under pressure. In other words, its detonation resistance.
During engine operation, the piston compresses the air-fuel mixture (compression stroke). At this point, when the finished mixture is under pressure, it can spontaneously ignite even before the spark plug has sparked. In the people, this phenomenon is called in one word -. A characteristic sign of detonation is the noise in the engine - metallic ringing.
Therefore, the higher the octane number, the higher the ability of the fuel to resist detonation.
Gasoline labeling
At gas stations you can find a variety of names, not excluding the most familiar to most motorists. Usually gasoline is marked with the letters "A" and "AI". Their decryption:
- "A" - this designation indicates that;
- "AI" - the letter "I" means the method by which the octane number was determined.
There are 2 ways to determine the octane number - research (AI) and motor (AM).
Research method - It is determined by testing the fuel on a single cylinder power plant, under variable compression ratio, crankshaft speed of 600 rpm, ignition timing of 13 ° and air (intake) temperature of 52 ° C. These conditions are similar to light and medium loads.
Motor method - its determination is carried out on a similar installation, but other conditions are different. The air (intake) temperature is 149°C, the crankshaft speed is 900 rpm, and the ignition timing is variable. This mode is similar to high loads - driving uphill, running the engine under load, etc.
Consequently, the number of AM is always lower than AI, and the difference in readings indicates the sensitivity of the fuel to the operation of the power unit in different modes. It is noteworthy that in some states in the West, the octane number is defined as the average between the values \u200b\u200bof "AM" and "AI". In the Russian Federation, only a higher value of "AI" is indicated, which can be seen at all gas stations.
Brands of gasoline
Most often, the following designations are found at domestic gas stations:
- Gasoline AI-98. Differs Unlike AI-95, which is produced in accordance with GOST, the 98th is produced in accordance with TU 38.401-58-122-95, as well as TU 38.401-58-127-95. In the production of this brand of gasoline, the use of alkyl lead antiknock agents is prohibited. The production of this high-octane gasoline is carried out using a number of components - toluene, isopentane, isooctane and alkyl gasoline.
- Extra AI-95 is a high-quality gasoline, which is achieved through the use of anti-knock type additives. Produced from distillate raw materials, catalytic cracking gasoline, with the addition of isoparaffin elements (aromatic) and gas gasoline. The composition does not contain lead, which ensures the high quality of gasoline.
- AI-95 - the main difference from Extra AI-95 in the concentration of lead, which is 30% higher;
- AI-93 - is divided into 2 categories: leaded and unleaded. Leaded fuel is produced on the basis of catalytic reforming gasoline (soft mode) with the addition of toluene and alkyl gasoline, as well as butane-butylene fraction. Unleaded is produced from the same catalytic reforming gasoline (hard mode), with the addition of butane-butylene fraction, alkyl gasoline and isopentane;
- AI-92 is the most common medium-quality gasoline on the market, containing anti-knock type additives. The maximum density is 0.77g/cmA-923. Can be either leaded or unleaded;
- AI-91 - differs in the content of anti-knock type additives. This is unleaded gasoline with an irregular density and a certain percentage of lead in the composition;
- A-80 - the composition of this gasoline is similar to that of AI-92. Maximum density - 0.755g / cmA-803;
- A-76 - usually used in agriculture. Leaded and unleaded A-76 with non-standardized density is produced. It contains additives of various types (antioxidant and antiknock), straight-run gasoline, as well as final, pyrolysis and cracking (thermal and catalytic).
Video: AI-92 or AI-95? Acceleration to 100 km and fuel consumption on the Mazda Demio (Ford Festiva Mini Wagon)
What gasoline to fill?
Many are looking for the answer to this question, so as not to inadvertently harm the engine. In this case, everything is simple - the fuel requirements are indicated in the operating instructions for a particular vehicle, and are also duplicated on the back of the gas tank flap. If the manufacturer indicated AI-95 as the recommended fuel, then you can refuel with the 92nd only at your own peril and risk. However, it is worth remembering that both the octane number and the brand of fuel can be indicated in the manual and on the label.
Also, different types of gasoline can be recorded in the manual. For example:
- AI-92 - valid;
- AI-95 - recommended;
- AI-98 - to improve performance.
As you can see, only the fuel recommended by the auto manufacturer should be filled into the tank. However, the use of gasoline with a higher octane number will not cause any harm to the engine. After all, the higher the octane number, the slower the burning rate and the greater the fuel efficiency, which has a beneficial effect on engine output, economy and other points. As a rule, the increase in power and efficiency reaches 7%. In addition, modern cars are equipped with ECUs that take into account the quality of the fuel and its octane number, adjusting the settings.
This means that it is necessary to pour AI-95 into the tank of a modern car with an atmospheric engine at a high-quality gas station. As a last resort, AI-92 is allowed. You can also focus on the degree of compression - if it is below 10 units, you can fill in AI-92. If higher - only the 95th.
As for turbocharged engines, the recommended fuel for them is AI-98 or Extra AI-95, but not AI-92.
Can gasoline be mixed?
Many people ask this question. In general, nothing catastrophic will happen from mixing fuel with different octane numbers, but only if you mix the recommended gasoline with a higher one (by octane number). For example, the 92nd recommended for the car should be mixed with the 95th. However, you don't need to downgrade. It is also worth remembering that the density of gasoline with different octane ratings is different, so that it may not mix at all - fuel with a higher octane rating will simply end up at the top of the tank, and with a lower one at the bottom.
Vladimir Khomutko
Reading time: 11 minutes
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What is the combustion temperature of gasoline?
Gasoline is used as fuel for many cars. It is a mixture of hydrocarbons that has a boiling point of 30 to 205 degrees. In addition to hydrocarbons, gasoline contains impurities of nitrogen, sulfur and oxygen. Depending on the number of certain components, gasoline for cars is divided into different brands that have different performance qualities:
- AI 92.
- AI 95.
- AI 98.
With the tightening of environmental requirements, gasolines with a lower octane number (A 76 or AI 80), which means a dirtier chemical composition, are not manufactured today.
Main qualities
The main qualities of the fuel are its chemical composition, the ability to evaporate, burn, self-ignite, deposit, as well as corrosion resistance and resistance to fire.
Physico-chemical characteristics depend on which hydrocarbons and in what proportions are present in the fuel. The freezing point of the fuel is -60 degrees, in the case of using special additives, this figure can be reduced to -71 degrees.
Fuel actively evaporates at a temperature of +30 degrees, and with increasing temperature, the evaporation process is more active. When the degree of its vapor in the air is 74-123 grams per m3, an explosive mixture is formed.
The composition of the fuel fraction affects the performance. During manufacture, it is essential to obtain an optimal ratio of light and heavy compounds in order to obtain a sufficiently high evaporation at low temperatures and prevent engine malfunctions due to the creation of vapor locks in the fuel line, which can occur due to the active evaporation of a large number of light compounds.
In view of this, gasolines that are used in areas with a hot climate and in the regions of the Arctic Circle have different chemical composition in order to provide the desired performance. Gasoline is obtained in several ways:
- by direct distillation of oil;
- by selecting specific fractions;
- cracking;
- reforming.
The main component of the fuel, which is obtained by direct distillation, is alkane compounds. During cracking and reforming, they are transformed into branched alkanes and aromatic components. The last two methods make it possible to obtain fuel with a high octane number of grades AI 92 and 95.
Octane number
The name of the brand of fuel consists of letters and numbers. The letters A or AI mean the method for detecting the octane number:
And the number means octane number (92, 95).
The name of the octane number shows such a quality as the resistance of the fuel to ignition. This number is conditional. Isooctane is used as a reference, the fire resistance of which is very high, and equals 100. The octane rating was created at the beginning of the last century. It was revealed by the composition of isooctane mixed with normal heptane.
Accordingly, AI 92 brand fuel is equivalent in its resistance to ignition to 92% of a mixture of isooctane with heptane, AI 95 - 95%. The octane number can be higher than 100 if the anti-knock qualities of the gasoline are higher than those of pure isooctane.
This value is very important, since ignition leads to a rapid deformation of the cylinder-piston group. This is due to the speed of the spread of flames - up to 2.5 km per second, while under optimal conditions the fire spreads at a speed of no more than 60 meters per second.
To increase the anti-knock qualities, you can either add additives that contain lead, or change the fractional composition upon receipt. The first option can be easily obtained from AI 92, AI 95 or 98 fuel, but today it has been abandoned.
Since, although such additives greatly increase the performance of gasoline and have a low cost, they are also very toxic and have a detrimental effect on the environment than pure fuel.
They also destroy the catalytic converter of the vehicle (the combustion temperature of leaded fuel is higher than that of unleaded fuel, as a result, the ceramic compounds of the converter are sintered, and the device is damaged).
Other compounds that are less toxic, such as acetone or ethyl alcohol, can also be used as additives. For example, if you pour 100 ml of alcohol into a liter of AI 92 fuel, then the octane number will increase to 95. But the use of such funds is not economically feasible.
Chemical stability
Considering the chemical qualities of gasoline, the main emphasis should be on how long the composition of hydrocarbons will remain unchanged, since with long storage, lighter components disappear and performance is greatly reduced.
In particular, the problem is acute if gasoline with a minimum octane number is used to produce fuel of a higher grade (AI 95) by adding propane or methane to its composition. Their antiknock qualities are higher than that of isooctane, but they dissipate instantly.
According to GOST, the chemical composition of fuel of any brand must be unchanged for 5 years, subject to storage rules. But in fact, often even newly purchased fuel already has an octane number below the specified one.
Unscrupulous sellers are to blame for this, who add liquefied gas to containers with fuel, the storage time of which has expired, and the content does not meet the requirements of GOST. Usually, different amounts of gas are added to the same fuel to obtain an octane rating of 92 or 95. Confirmation of such tricks is the sharp smell of gas at gas stations.
Boiling temperature of gasoline
Any person who decides to find information about the temperature of boiling, burning or ignition of a fuel will find an interesting thing: even in fairly well-known sources, there is a difference between the indicated indicators of the same parameter. Why does this happen and what are the real indicators?
Boiling point of gasoline
The boiling point of gasoline is an interesting quantity. Today, few young motorists know that at one time, at high air temperatures, fuel that boiled in a fuel line or carburetor could block a vehicle. This phenomenon contributed to the formation of failures in the system.
The light fractions were strongly heated and separated from the heavier fractions in the form of combustible gas bubbles. The car cooled down, the gases turned into liquid - and it was possible to continue moving. Today, gasoline used at gas stations will boil at about +80 degrees.
Fuel flash point
The flash point of a fuel is the thermal threshold at which freely separated, lighter fuel fractions begin to burn from an open fire source when this source is located above the test sample.
In practice, it is shown that the flash point is determined by the method of heating in an open crucible. The fuel to be tested is poured into a small open container. Then it is slowly heated without involving an open flame.
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At the same time, the temperature is monitored in real time. Each time when the temperature of the fuel rises by 1 degree, a flame source is carried out at a small height above its surface. At this moment, when a fire occurs, the flash point is determined.
In other words, the flash point determines the threshold at which the concentration of easily evaporating fuel in the air reaches a value sufficient to ignite under the influence of an open fire source.
This indicator reveals the maximum temperature created by burning gasoline. And there is also no unambiguous information that answers this question with one number. Surprisingly, but it is for the combustion temperature that the conditions of the process, and not the composition of gasoline, play a key role.
Automobile gasolines
Gasolines are intended for use in piston internal combustion engines with forced ignition (from a spark).
Depending on the purpose, they are divided into automobile and aviation.
Despite the differences in the conditions of use, automobile and aviation gasolines are characterized mainly by general quality indicators that determine their physicochemical and operational properties.
Modern automobile and aviation gasolines must meet a number of requirements that ensure economical and reliable operation of the engine, and operating requirements: have good volatility, which makes it possible to obtain a homogeneous air-fuel mixture of optimal composition at any temperature; have a group hydrocarbon composition that ensures a stable, knock-free combustion process in all modes engine operation; not change its composition and properties during long-term storage and not have a harmful effect on fuel system parts, tanks, rubber products, etc. In recent years, the environmental properties of fuel have come to the fore.
Range, quality and composition of motor gasolines
The bulk of motor gasoline in Russia is produced in accordance with GOST 2084-77 and GOST R51105-97 and TU 38.001165-97. Depending on the octane number, GOST 2084-77 provides for five grades of motor gasoline: A-72, A-76, AI-91, AI-93 and AI-95. For the first two brands, the numbers indicate octane numbers determined by the motor method, for the latter - by the research method. Due to the increase in the share of passenger vehicles in the total vehicle fleet, there is a noticeable trend towards a decrease in the demand for low-octane gasolines and an increase in the consumption of high-octane gasolines. Gasoline A-72 is practically not produced due to the lack of equipment operated on it.
The greatest need exists for gasoline A-92, which is produced according to TU 38.001165-97, although the share of A-76 gasoline in the total production volume remains very high. These specifications also provide for brands of gasoline A-80 And A-96 with research octane numbers of 80 and 96, respectively. These gasolines are intended mainly for export. Petrol AI-98 with an octane number of 98 according to the research method is produced according to TU 38.401-58-122-95 and TU 38.401-58-127-95. Gasoline A-76, A-80, AI-91, A-92 and A-96 can be produced using ethyl liquid. Low-lead gasoline AI-91 with a lead content of 0.15 g/dm3 is produced according to separate specifications (TU 38.401-58-86-94). In the production of AI-95 and AI-98 gasolines, the use of alkyl lead antiknock agents is not allowed.
The requirements of GOST 2084-77 for the quality of motor gasoline are given in the table. All gasolines produced in accordance with GOST 2084-77, depending on the volatility indicators, are divided into summer and winter. Winter gasolines are intended for use in the northern and northeastern regions during all seasons and in other regions from October 1 to April 1. Summer - for use in all areas except for the northern and northeastern regions from April 1 to October 1; in the southern regions it is allowed to use summer gasoline during all seasons.
The parameters of motor gasoline produced in accordance with GOST 2084-77 differ significantly from accepted international standards, especially in terms of environmental requirements. In order to increase the competitiveness of Russian gasolines and bring their quality to the level of European standards, GOST R 51105-97 "Fuels for internal combustion engines. Unleaded gasoline. Specifications" was developed, which comes into force on 01.01.99. This standard does not replace GOST 2084 -77, which provides for the production of both leaded and unleaded gasoline. In accordance with GOST R 51105-97, only unleaded gasoline will be produced (the maximum lead content is not more than 0.01 g/dm3).
Characteristics of motor gasolines (GOST 2084-77)
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Depending on the octane number, four grades of gasoline have been established according to the research method: "Normal-80", "Regular-91", "Premium-95", "Super-98". Gasoline "Normal-80" is intended for use on trucks along with gasoline A-76. Unleaded gasoline "Regular-91" is intended for the operation of cars instead of leaded A-93. Automobile gasolines "Premium-95" and "Super-98" fully meet European requirements, are competitive on the oil market and are intended mainly for foreign cars imported into Russia.
In order to accelerate the transition to the production of unleaded gasoline, instead of ethyl liquid, it is allowed to use a manganese antiknock agent at a concentration of not more than 5 mg Mn/dm3 for the Normal-80 brand and not more than 18 mg Mn/dm3 for the Regular-91 brand. In accordance with European requirements for limiting the content of benzene, the indicator "volume fraction of benzene" was introduced - no more than 5%. A standard has been established for the indicator "density at 15 ° C". The norm for the mass fraction of sulfur has been tightened - up to 0.05%. To ensure the normal operation of vehicles and the rational use of gasoline, five volatility classes have been introduced for use in various climatic regions in accordance with GOST 16350 - 80. Along with determining the temperature of gasoline distillation at a given volume, it is also possible to determine the volume of evaporated gasoline at a given temperature of 70, 100 and 180 ° C. The indicator "evaporability index" has been introduced. GOST R 51105-97, along with domestic ones, includes international standards for test methods (ISO, EN, ASTM).
The norms and requirements for the quality of motor gasoline and volatility characteristics in accordance with GOST R 51105-97 are given in the table.
Norms and requirements for the quality of motor gasoline in accordance with GOST R 51105-97
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By composition, motor gasolines are a mixture of components obtained as a result of various technological processes: direct distillation of oil, catalytic reforming, catalytic cracking and hydrocracking of vacuum gas oil, isomerization of straight-run fractions, alkylation, aromatization of thermal cracking, visbreaking, delayed coking. The component composition of gasoline depends mainly on its brand and is determined by a set of process units at the refinery.
The basic component for the production of motor gasolines are usually catalytic reforming or catalytic cracking gasolines. Catalytic reforming gasolines are characterized by a low sulfur content, they contain practically no olefins, so they are highly stable during storage. However, the high content of aromatic hydrocarbons in them is a limiting factor from an ecological point of view. Their disadvantages also include the uneven distribution of detonation resistance by fractions. The share of the catalytic reforming component in the gasoline fund of Russia exceeds 50%.
Catalytic cracked gasolines are characterized by a low mass fraction of sulfur, octane numbers according to the research method of 90-93 units. The content of aromatic hydrocarbons in them is 30-40%, olefins - 25-35%. There are practically no diene hydrocarbons in their composition, so they have a relatively high chemical stability (induction period 800-900 min.). Compared to catalytic reforming gasolines, catalytic cracking gasolines are characterized by a more uniform distribution of knock resistance over fractions. Therefore, it is advisable to use a mixture of catalytic reforming and catalytic cracking components as a base for the production of motor gasoline.
Gasolines from thermal processes such as cracking, delayed coking have low knock resistance and chemical stability, high sulfur content and are used only to produce low-octane gasolines in limited quantities.
In the production of high-octane gasolines, alkyl gasoline, isooctane, isopentane and toluene are used. AI-95 and AI-98 gasolines are usually obtained with the addition of oxygen-containing components: methyl tert-butyl ether (MTBE) or its mixture with tert-butanol, called feterol. The introduction of MTBE into gasoline makes it possible to increase the completeness of its combustion and the uniformity of the distribution of detonation resistance by fractions. The maximum allowable concentration of MTBE in gasolines is 15% due to its relatively low calorific value and high aggressiveness towards rubbers.
To achieve the required level of detonation properties of leaded gasolines, ethyl liquid is added to them (up to 0.15 g of lead/dm3 of gasoline). It is allowed to add Agidol-1 or Agidol-12 antioxidants to secondary process gasolines containing unsaturated hydrocarbons to stabilize them and meet the requirements for the induction period. To ensure safe handling and labeling, leaded gasolines must be colored. Gasoline A-76 is dyed yellow with fat-soluble yellow dye K, gasoline AI-91 is dyed orange-red with fat-soluble dark red dye Zh. Leaded gasolines intended for export are not dyed.
Approximate component compositions of motor gasolines of various grades are given in the table.
Medium component compositions of motor gasolines
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Recently, the assortment of motor gasolines has been significantly replenished due to new grades produced according to technical specifications. This was due to a sharp increase in the production of unleaded gasoline and a reduction in the production of leaded gasoline.
At the same time, tetraethyl lead is replaced by various non-traditional additives and additives, previously produced by the chemical and microbiological industries for other purposes.
Such substances include various ethers, alcohols, organometallic compounds, etc. The need for the production of such gasoline according to specifications is dictated by the fact that all additives and additives can be introduced in strictly defined concentrations. To control the content of these components in the technical specifications, special indicators are provided and additional control methods are introduced.
All gasolines produced according to specifications must comply with the requirements of GOST R 51313-99 "Automotive gasolines. General technical requirements", which is introduced from July 01, 2000.
Compliance of gasolines produced according to technical specifications with the requirements of GOST R 51313-99 is checked during their certification, which is mandatory.
Automobile gasolines. General specifications
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Evaporation occurs from the surface of liquids (and even solids). Among the many molecules, there are those whose speed is randomly sufficient to fly out of the liquid and mix with air. These particles form steam. The higher the temperature of the liquid, the greater the average speed of the molecules and the more often they fly out into the atmosphere (and less often condense back). Thus, the system is in thermodynamic equilibrium, and the vapor above the liquid is saturated.
Flash and fire point
For a mixture of air and steam (fuel) to ignite in the presence of fire, it must have a sufficient concentration of combustible molecules. Oil consists of many different fractions - more or less volatile. Thus, the composition of an oil product determines at what temperature its saturated steam will ignite. This is one of the main characteristics of the fuel.
The minimum temperature at which vapors above the surface of a flammable liquid can ignite from a fire is the flash point. The mixture burns quickly, new molecules do not have time to fly out, and the flame dies out. With further heating, the ignition temperature can be reached. Instead of a flash, a steady burning will be observed on the surface. Finally, there is the self-ignition temperature (it is even higher), at which a source of fire is not needed to cause a flame or explosion.
Flash Point Determination
There are several methods for different substances. The details of the test may vary (type of apparatus used, heating and stirring rates, etc.), but the idea is the same.
The sample (combustible liquid) is placed in a special container - a crucible. It is a brass (or similar material) vessel of a certain shape and size (like a mug with a flange). The crucible has a lid with holes for a thermometer, ignition source, etc. The container is placed inside the apparatus, which provides the necessary conditions for testing and the accuracy of the results.
The liquid is stirred and heated at a constant rate. At certain temperature (or time) intervals, an ignition source is lowered into the crucible through a hole in the lid. When a flash occurs, the temperature is recorded. Bring its value to standard atmospheric pressure.
The flash point of diesel fuel in a closed crucible is measured according to GOST 6356. This is a normalized value, it is indicated in the quality certificate. It can also be determined according to the international standard ISO 2719, which is adopted in Russia. The document establishes 2 methods for various substances; using the Pensky-Martens test apparatus. The open cup can also measure the flash point; it will be slightly higher. Heat and fuel molecules are dissipated in the external environment.
Classification of flammable liquids by flash point
The division into categories in different countries may vary. In Russia, flammable liquids are distinguished from the group of flammable liquids: ≤ 61ºС in a closed crucible (in an open - no more than 66ºС). In turn, LVZH are divided into 3 categories. The flash point in a closed crucible is given, in brackets - in an open one.
- Particularly dangerous: ≤ -18 (-13)ºС - gasoline, acetone, diethyl ether.
- Permanently dangerous: -18 ... + 23ºС (-13 ... + 27ºС) - naphtha, toluene, ethylbenzene, ethyl alcohol.
- Dangerous at elevated air temperatures: +23…+61ºС (+27…+66ºС) – kerosene, turpentine, propylbenzene.
The lighter the oil fractions (the earlier they boil in the distillation column), the lower their flash point. For general purpose diesel fuel, it should be from 40ºС (L, E) or from 30ºС (З, А) and higher. Summer and off-season diesel for ships, diesel locomotives and gas turbines is at least 62ºС, i.е. it no longer applies to flammable liquids.
The flash point of gasoline is below -40ºС. This indicator is not standardized by GOST. For kerosene it is +28…+60ºС, for motor oil – from +130 to +325ºС. The flash point of oil is usually in the range of -35 ... 0ºС and depends on the composition.
Gasoline is used as fuel for most cars. It is a mixture of hydrocarbons having a boiling point of 30 to 205 degrees Celsius. In addition to hydrocarbons, gasoline contains impurities containing nitrogen, sulfur and oxygen.
Depending on the number of certain compounds, motor gasoline is divided into different grades that have slightly different performance properties:
- AI-92;
- AI-95;
- AI-98.
With the tightening of environmental requirements, gasolines with a lower octane number, such as A-76 or AI-80, and, consequently, a more “dirty” chemical composition, are not currently being produced.
Basic properties
The main properties of gasoline are its chemical composition, the ability to evaporate, burn, ignite, form deposits, as well as corrosiveness and resistance to detonation.
The physical and chemical properties of gasoline vary depending on which hydrocarbons and in what proportions it contains. The freezing point of gasoline reaches -60 degrees Celsius, in the case of the use of special additives, this value can be lowered to -71 degrees. Gasoline actively evaporates at temperatures above 30 degrees, and with increasing temperature, evaporation occurs more intensively. When the concentration of its vapor in the air reaches 74 - 123 grams per cubic meter, an explosive mixture is formed. 
The fractional composition of gasoline directly affects the performance properties. During production, it is important to achieve the correct ratio of light and heavy fractions in order, on the one hand, to ensure a sufficiently high volatility at low temperatures, and on the other hand, to prevent interruptions in the operation of the engine due to the formation of vapor locks in the fuel line, which may occur due to intense evaporation a lot of light fractions. In this regard, gasolines used in places with a hot climate and in the Arctic Circle have a different chemical composition in order to provide the necessary performance properties.
There are several ways to get gasoline: direct distillation of oil and the selection of certain fractions (this method was used at the beginning of the era of motorization), in the middle of the last century, cracking and reforming began to be used. The main component of gasoline obtained by direct distillation is alkane chains. During cracking and reforming, they are converted into branched alkanes and aromatic compounds.
The last two methods make it possible to obtain high-octane fuel grades AI-92, 95 and higher.
Octane number
The name of the brand of gasoline consists of an alphanumeric designation. The letters A or AI indicate the method for determining the octane number:
- motor (A)
- research (AI)
and the figure determines the octane number (92, 95, etc.).
The value of the octane number indicates such a property as the resistance of gasoline to detonation. This number is relative. Isooctane is taken as a standard, the detonation resistance of which is very high and is taken equal to 100. The octane number scale was proposed at the beginning of the last century. It was determined by the content of isooctane in a mixture with normal heptane (its detonation resistance is very low and is assumed to be zero). Accordingly, AI-92 gasoline is equivalent in its resistance to detonation to a 92% mixture of isooctane with heptane, AI-95 is 95%, and so on. The octane number can be higher than 100 if the anti-knock properties of the fuel are even higher than those of pure isooctane. 
This value is very important, since detonation leads to the rapid destruction of the cylinder-piston group. This is explained by the speed of propagation of the flame front - up to 2.5 km / s, while under normal conditions the flame spreads at a speed of no more than 60 m / s.
To increase the anti-knock properties, one can either add additives containing lead compounds (tetraethyl lead) or change the fractional composition upon receipt. The first method is easily obtained from AI-92 gasoline AI-95, or 98, but it has now been abandoned. Since, although such additives significantly increase the performance properties of the fuel and have a low cost, they are also very toxic and have a much more detrimental effect on the environment than pure gasoline, and also destroy the car’s catalytic converter (the combustion temperature of leaded gasoline is higher than that of unleaded, as a result, the ceramic elements of the neutralizer simply sinter, and the device fails).
Other compounds that are less toxic, such as ethyl alcohol or acetone, can also be used as additives. For example, if you add 100 ml of alcohol to a liter of AI-92 gasoline, then the octane number will increase to 95. However, the use of such additives is not economically viable.
Chemical stability
Considering the chemical properties of gasoline, the main emphasis should be on how long the composition of hydrocarbons will remain unchanged, since during long-term storage, lighter compounds evaporate and performance deteriorates greatly. This problem is especially acute if a higher grade gasoline (AI-95) was obtained from a fuel with a lower octane number (for example, AI-92) by adding propane or methane to its composition. Their antiknock properties are higher than those of isooctane, but they also evaporate very quickly.
The state standard requires that the chemical composition of gasoline of any brand, whether AI-92, 95 or 98, remains unchanged for at least five years, subject to storage rules. However, in reality, often even freshly purchased fuel already has an octane number lower than the declared one (for example, not 95, but 92). This is due to the dishonesty of sellers who add liquefied gas to tanks with fuel whose shelf life has expired and the composition does not comply with GOST. As a rule, different amounts of gas are added to the same gasoline in order to obtain an octane rating of 92 or 95. An obvious confirmation of such tricks is the strong smell of gas at gas stations. It is likely that the performance properties of such gasoline will noticeably deteriorate right before our eyes, before the time the fuel tank is empty.