Abstract

Contents

  1. Obtaining base gasolines
    1. Variety of gasoline
    2. Marking of gasolines
  2. Prohibited ways to increase the octane number of gasoline
  3. Methods for measuring the octane number
    1. Motor method for determining the octane number
    2. The research method for determining the octane number
    3. Express method
  4. Conclusion
  5. List of sources

1. Obtaining base gasolines

Gasolines – flammable colorless or slightly yellow (in the absence of special additives) liquids having a density (700-780) kg/m. Gasolines are highly volatile, and the flash point is within (20-40) degrees Celsius. The boiling point of gasolines ranges from 30 to 200°C. Pour point – below -60 degrees. When burning gasolines, water and carbon dioxide are formed. At concentrations of vapors in air (70-120) g/m3 explosive mixtures are formed.

For a long time, gasoline was produced by rectification (distillation) and selection of fractions of oil boiling in certain temperature ranges (up to 100 C grade I petrol, up to 110 C petrol, special, up to 130 C grade II petrol). However, the common property of these gasolines is a low octane number. Generally, the production of straight-run gasoline with an octane number above 65 by the motor method is rare and is possible only from the oil of Azerbaijan, Central Asia, the Krasnodar Territory and Sakhalin. However, even for distillates from these oils, a sharp decrease in the octane number is characteristic with an increase in the temperature of the end of the selection. Therefore, the entire gasoline fraction (ca. 180°C) is rarely used. For oils. The Ural-Volzhsky Basin, Kazakhstan, as well as the fields of Western Siberia, are characterized by the predominance of normal paraffin hydrocarbons, therefore, straight-run gasolines from them are characterized by low octane numbers. This prompted refiners to select the fraction up to (90-95)°C in the 1930s, so that n-heptane does not enter it, or include heavier fractions with their subsequent clear rectification to remove normal paraffins. A similar denormalization of straight-run gasolines makes it possible to bring the octane number to (74-76) points with a significant, however, lower yield of the target product. At present, the fraction of NK-180°C is distilled from the oil, which is then divided into fractions of NK-62°C or NK-85°C. These latter distillates are used as components of commercial gasolines, or are directed to refining (isomerization).

Automobile gasolines due to their physical and chemical characteristics should have the following properties:

1.1 Variety of gasoline

Automobile gasolines are divided into summer and winter (in winter gasolines there are more low-boiling hydrocarbons).

The main brands of motor gasolines:

The norms and requirements for the quality of motor gasolines are presented in Table 1.

Indicators Normal-80 Regular-92 Premium-95 Super-98
Octane number, not less than: motor method 76,0 82,5 85,0 88,0
Octane number, not less than: research method 80,0 91,0 95,0 98,0
Lead content, g/dm3, not more than 0,010 0,010 0,010 0,010
The content of manganese, mg/dm3, not more than 50 18 – –
Content of actual resins, mg/100 cm3, not more than 5,0 5,0 5,0 5,0
Mass fraction of sulfur, %, not more than 0,05 0,05 0,05 0,05
Volumetric fraction of benzene, %, not more than 5 5 5 5
Appearance Clean, transparent Clean, transparent Clean, transparent Clean, transparent
Density at 15°С, kg/m3 700-750 725-780 725-780 725-780

1.2 Marking of gasolines

In accordance with ГОСТ Р54283-2010, motor gasolines are marked with three groups of signs separated by a hyphen (for example, АИ-92-4):

There are also other types of gasoline:

2. Prohibited ways to increase the octane number of gasoline

Octane number – this is actually the level of detonation, in which gasoline ignites and explodes in the combustion chamber of the car.

If the gasoline ignites earlier than necessary, while the intake valves are not yet fully closed and the cylinder is not at the top point, then naturally the engine, not that it will not work at full power, but will work incorrectly, what is even worse, in fact we will get detonation but about this further. With such a low octane number, we will get a bunch of problems with parts of the engine for a long period of time – wear of valves, saddles for them and additional carbon deposits, etc. In addition, the mismatch of the octane number for the engine entails the very additional detonation, which is often confused with the knocking of the valves.

The octane number is obtained by shifting the components of gasoline. Isooctane – a substance that is almost not explosive with increasing pressure, and its detonation resistance was taken as 100 units. At the same time, n-heptane is not at all resistant to detonation when the pressure rises (it can be said to be self-detonating), so its detonation resistance is taken as 0. It is the mixture of these substances that allows you to adjust the octane number in gasoline. In addition, trimethylpentane is added to the gasoline, from which the octane number depends little. There are gasolines, and with an octane rating of more than 100 units, isooctane is used for them with the addition of different amounts of additives. Everything that burns here is in use: raw materials for refining, stable gasoline, gas condensate with the addition of coke-chemical products to increase the octane number.

In the basic raw materials (in order to give the fuel a normal appearance) most often add the following ingredients:

Increase the quality of motor gasoline can be due to the following activities:

The most common domestic gasoline А-76, АИ-93 (ГОСТ 2084-77) and АИ-92 (ТУ 38.001165-97) do not meet the specified requirements for lead content (for leaded gasolines), sulfur content, absence of benzene and detergent regulations additives.

3. Methods for measuring the octane number

To date, only two official methods for determining the octane number of gasoline are calculated:

3.1 Motor method for determining the octane number

The motor method characterizes the detonation resistance of gasoline under engine operating conditions at maximum capacities and with an increased thermal regime (traffic outside the city). The gasoline test is carried out at 900 rpm, the temperature of the intake mixture is 149°C, the variable ignition timing. When checking fuel by this method, the gasoline provided is compared with mixtures of reference fuels, alternately switching the engine power from one fuel to another. During the study, a mixture of reference gasoline is determined, the detonation of which completely coincides with the detonation of the sample.

3.2 The research method for determining the octane number

The research method characterizes the detonation resistance of gasoline under conditions of engine operation at a partial load (traffic in the city). The determination of the octane number of gasoline by the research method is carried out in exactly the same way as for the motor method, that is, the gasoline provided is compared with mixtures of reference fuels. However, there are some differences. In particular, the test runs at 600 rpm. In addition, the ignition timing is constant (13° to the TDC of the highest point in the distance the piston travels when the crankshaft rotates), and the intake air temperature is 52°C.

The methods presented are rather laborious, carried out in special rooms by well-trained personnel in bulky installations, and their duration is (6-8) hours.

3.3 Express method

There is also an express method for determining the octane number of gasoline. For the express analysis of fuel composition and its octane number, some manufacturers use special measuring devices digital octanometers (see Figure 1.2).

photo

Figure 1.2 – Digital octanometer

The principle of the octanometer is based on measuring the dielectric constant of automobile gasolines. Initially, the memory of this device retains the parameters of the main fuel brands. When examining a sample of gasoline, it is compared with these brands, and the results are displayed on the screen. These devices are simple enough to use, however, since this method does not apply to official ones, it is not recommended to fully take into account the results of this study.

In the express method for determining the octane number of gasoline, the gasoline temperature and its permittivity at this temperature are measured, it is established from the literature that the dependence of the permittivity on the octane number is described by expression [1]:

formula1
where A – octane number of gasoline;
Х – coefficient of proportionality, determined experimentally;
ε –dielectric permeability of gasoline;
a – a temperature coefficient of 2.5∙10-2 1/˚С;
T – temperature of gasoline.

Using the formula (1), we define the limits of the change in the dielectric constant of gasoline with a change in the octane number in the range from 80 to 100 and the temperature from -10˚С to +35 ˚С. The results are shown in Figure 1

ris1

Figure 1 – Dependence of dielectric constant on octane number and temperature

It follows from Fig. 1 that the permittivity of gasoline in the operating conditions varies from 0.5 to 3.43.

To measure the dielectric constant of gasoline, it is proposed to use a capacitive sensor of cylindrical shape, the overall dimensions of which are shown in Fig. 2.

ris2

Figure 1 – Appearance and overall dimensions of the capacitive sensor

The height of the inner cylinder is made less than the outer one to prevent gasoline transfusion across the edge of the sensor and increase its sensitivity. In this case, the working height of the capacitive sensor will be h = 100 mm.

We determine the limits of the change in the capacitance of the sensor when the permittivity varies from 0.5 to 3.43. To do this, we use the following relationship:

formula2
where ε – the dielectric constant;
ε_0 – an electrical constant equal to 8,854185∙10-12 Ф/м;
r1, r2 – the radius of the outer and inner electrodes;
h – working height of capacitive sensor.

Graphical representation of the dependence of capacitance on the dielectric constant of gasoline is shown in Fig. 3.

ris3

Figure 3. – Dependence of the capacitance of the sensor on the dielectric constant

It follows from Fig. 3 that when the permittivity varies from 0.5 to 3.43, the capacitance of the sensor changes in the range from 15.3 pF to 104.9 pF.

We turn on the capacitive sensor in the generator of the sinusoidal signal, assembled according to the Klapp scheme (see Figure 4) [2]. When the dielectric constant of gasoline changes, the capacitance of the sensor changes and, accordingly, the frequency of the output signal of the generator changes, which is an information parameter.

ris4

Figure 4 – Schematic diagram of an autogenerator

Unlike the classical circuit of a generator with a capacitive three-point, in this circuit, an additional capacitor C3 is connected in series with the inductance of the circuit. The total capacitance of the circuit Ck becomes smaller in this case than with two capacitors. To maintain the same oscillation frequency, it is necessary to increase the inductance of the circuit Lk.

The result is a circuit with a large characteristic impedance. This circuit, while maintaining the same losses, has a large Q-factor, and, consequently, better reference properties. This helps to increase the stability of the frequencies of the generated oscillations. In addition, the inclusion of capacitor C3 reduces the coupling factor of the transistor to the circuit. This reduces the destabilizing effect of the loop parameters on the oscillator frequency [2].

To reduce the mass-dimensional parameters of the generator and increase its sensitivity to changes in the dielectric constant of gasoline, and, respectively, and the octane number of gasoline, the nominal frequency is chosen in the region of 5 MHz.

The capacitive sensor is connected in parallel to the capacitor C3, in which case the total capacitance of the circuit is:

formula3

Зthe dependence of the frequency of the output signal of the generator on the dielectric constant is described by expression:

formula4

A graphical representation of the dependence of the frequency of the generator output signal on the dielectric constant is shown in Fig. 5.

ris5

Figure 5 – Dependence of the output frequency of the generator on the dielectric constant

It follows from Fig. 5 that when the permittivity varies from 0.5 to 3.43, the frequency of the output signal of the sine-wave signal generator varies from 4.82 MHz to 4.06 MHz by a nonlinear law.

To further process the signal in the digital part of the electronic device, it is necessary to convert the sinusoidal signal into rectangular pulses using a comparator. If the amplitude value of the output signal of the generator is less than the logical one, it is necessary to pre-amplify the sinusoidal signal before feeding it to the comparator.

Using expressions (1) and (4), we obtain a mathematical model of the electronic device for determining the octane number of gasoline, created on the basis of a capacitive sensor of cylindrical shape and included in the oscillator circuit of a sinusoidal signal generator,

formula5

4. Conclusion

Thus, the magnitude and accuracy of determining the octane number will depend on the magnitude and accuracy of measuring the frequency of the generator output signal and the temperature of the sample of gasoline.

However, the disadvantage of this method of measurement is the insufficient accuracy in determining the quality of gasoline, since it does not measure the percentage of water. Therefore, in order to increase the accuracy of the measurement, the following tasks must be performed.

  1. Analyze the effect of water content in gasoline on its quality.
  2. Study methods for determining the percentage of water in gasoline.
  3. Finalize the structural diagram of the device taking into account the analysis of water content in gasoline
  4. Develop a scheme for an electronic device, taking into account the analysis of the water content of gasoline, to investigate its main characteristics

List of sources

  1. Пат. 2231780 РФ, МПК G 01 N 27 / 22, G 01 N 33 / 22. Способ определения октанового числа бензина / А.И. Кавтарадзе; Кавтарадзе Альберт Иванович (Ru). – заявл. 22.11.2002; опубл. 27.06.2004.
  2. Модифицированный генератор Клаппа с низкими искажениями [Электронный ресурс]. – Режим доступа: http://promplace.ru/article_single.php?arc=25 – Дата доступа: май 2016. – Загл. с экрана.