Compression and rarefaction of gases. Natural gas - motor fuel
Natural gas consists mainly of methane (at least 90%) with small impurities of ethane (up to 6%), propane (up to 1.7%), and butane (up to 1%).
Methane gas is colorless and odorless, slightly soluble in water, lighter than air. It refers to saturated hydrocarbons, the molecules of which consist only of carbon and hydrogen. The high hydrogen content ensures more complete combustion of the fuel in the engine cylinders compared to gasoline and liquefied petroleum gas, so methane is a complete fuel for cars with good anti-knock characteristics.
Characteristics of methane.
Molecular formula - CH 4
Molar mass, kg / mol - 16.03
Density at a temperature of 15°C and a pressure of 0.1 MPa:
- in a gaseous state, kg / m 3 - 0.717
– in liquid state, kg/l – 0.42
Carbon number - 2.96
Boiling point, ° С - -161.7
Self-ignition (flash) temperature, ° С - 590
Net calorific value:
- in a gaseous state, kJ / m 3 - 33800
– in liquid state, kJ/l – 20900
Relative density (by air) - 0.554
Corrosive activity - none
Toxicity - non-toxic
Combustion temperature, ° С - 2030
For reference . Heat of combustion.
Heat of combustion- the amount of heat released during the complete combustion of 1 m 3 of gas, with atmospheric pressure and a temperature of 20°C.
There is a higher and lower calorific value of gas combustion. When determining the gross calorific value, all the heat released during combustion and removed from the combustion products by cooling them to the initial temperature is taken into account. In practice, the resulting water vapors do not condense and carry away part of the heat spent on heating 1 kg of water from 0 to 100 ° C, which is equal to 418.6 kJ.
During combustion, heat is consumed to evaporate the moisture contained in the fuel and obtained from the combustion of hydrogen. Therefore, to characterize gas fuels, in practice, the lower calorific value of gas combustion is used, which is a standard value.
Before being used as a motor fuel, natural gas undergoes preliminary preparation for compliance with its parameters for engine performance (removal of impurities) and storage conditions on a vehicle.
Since natural gas liquefies at -161.7°C, and in normal conditions it is impossible to do this, on cars it is stored in cylinders in a compressed state of up to 20 MPa (200 kg / cm2).
Compressed gases are characterized by the fact that at a temperature of 20°C and high pressure (20 MPa) they remain in a gaseous state.
Gas natural fuel compressed (compressed natural gas).
In terms of physical and chemical parameters and the content of impurities, natural fuel gas must comply with GOST 27577-2000 “Compressed natural fuel gas for internal combustion engines”.
In terms of physical and chemical parameters, the gas according to this GOST must comply with the requirements and standards given in Table 1.
Table 1.
| №№p/n | Indicators | Meaning |
| 1 | 2 | 3 |
| 1. | The lowest volumetric calorific value, kJ/m 3 , not less than | 31800 |
| 2. | Relative density to air | 0,55-0,70 |
| 3. | Estimated octane number (according to the motor method), not less than | 105 |
| 4. | Hydrogen sulfide concentration, g/m 3 , no more | 0,02 |
| 5. | Mercaptan sulfur concentration, g/m 3 , not more than | 0,036 |
| 6. | Mass of mechanical impurities in 1m 3, mg, no more | 1,0 |
| 7. | Total volume fraction of non-combustible components, %, max | 7,0 |
| 8. | Volume fraction of oxygen, %, no more | 1,0 |
| 9. | Concentration of water vapors, mg/m 3 , no more | 9,0 |
Disadvantages and advantages of using compressed natural gas in comparison with gasoline.
1. Disadvantages.
1.1. The content of gas under high pressure requires the use of high-strength cylinders that have a significant mass and are made of high-quality steels. The weight of one cylinder with a capacity of 50 liters with 10 m 3 of gas is about 70 kg. The installation of gas cylinders on a car entails a decrease in the carrying capacity of the car by 10-12%, and the range of the car is also reduced.
Cylinders for LNG are high-pressure vessels, for alloy steel cylinders, the test period is set once every 5 years, and for carbon steel - once every 3 years.
1.2. Since the calorific value of the gas-air mixture of methane is less than the calorific value of the gasoline-air mixture (3.22 MJ / m 3 for methane with air and 3.55 MJ / m 3 for gasoline with air), and due to the lower filling ratio of the cylinders, the engine power when converted to compressed gas is reduced by 18-20%.
1.3. When using gas fuel, it is difficult to start the engine in winter time at temperatures below 15°C. Reason is more heat ignition of the gas-air mixture and a lower flame propagation speed.
1.4. For the maintenance and repair of gas-balloon vehicles, a higher qualification of the service personnel is required. Compared to the maintenance of gasoline and diesel engines, the labor intensity of maintenance and repair of gas equipment increases by 13-15%, and costs - by 4-6%.
1.5. The operation of engines on compressed gas is accompanied by a deterioration in the traction, dynamic and operational characteristics of vehicles: acceleration time increases by 25-30%; maximum speed decreases by 5-7%.
2. Benefits.
2.1. Gas fuel burns more completely in engine cylinders due to the wider ignition limits of gas compared to gasoline. If the ignition limits of gasoline mixed with air are 6.0 and 1.5%, respectively, then the ignition limits of compressed gas mixed with air are 15% for the upper limit and 5% for the lower limit. This makes it possible to deplete the combustible mixture to α=1.2-1.3 in the operating modes of the engines.
As a result, the toxicity of exhaust gases is significantly reduced (in terms of the content of carbon oxides - by 2-3 times, by the content of nitrogen oxides - by 1.2-2.0 times, by the content of hydrocarbons - by 1.1-1.4 times).
2.2. Compressed gas does not dilute the oil in the crankcase, does not wash the oil off the cylinder walls and does not worsen the lubrication conditions. Therefore, the wear of parts of engines running on gas is lower than that of gasoline engines. As a result, the motor resource of engines increases by 1.3-1.5 times. The service life of the oil is also increased by 1.5-2 times, and the cost of it is reduced by 25-35 percent.
2.3. CNG prices are lower than petrol: Fuel cost savings are available despite the loss of engine power and reduced vehicle payload.
Autotrans-consultant.ru.
Compressed compressed gas is obtained different ways: directly from gas wells, as a product of oil refining and by fractionation of gas condensate or petroleum associated gas. Compressed natural gas not only can successfully replace liquid motor fuels, but also surpasses them in a number of parameters. Its main advantage is that compressed natural gas can be used in road transport without expensive technological processing.
The composition of natural gases produced at domestic fields is quite similar. Basically (82-98%) it is methane CH 4 with small impurities (up to 6%) ethane C 2 H 6, up to 1.5% propane C 3 H 8 and up to 1% butane C 4 H 10. In associated gases produced at oil fields, depending on the production area, the content of methane can vary from 40 to 82%, and butane and propane - from 4 to 20%.
The main component - methane CH 4 is characterized by the highest critical temperature (-82°C). Therefore, when normal temperatures even at high pressure, methane cannot be liquefied: this requires a low temperature.
The properties of methane are determined by its molecular structure. Gas refers to simple hydrocarbons. Its molecule contains a maximum of hydrogen per carbon atom. This is due to the high thermal conductivity of methane, a wide range of flammability and a low content of toxic components. Due to the high content of hydrogen in the compressed gas, it burns more completely in the engine cylinders than HPS and gasoline. Compared to other hydrocarbon gases, methane is much lighter than air, so in the event of a leak it accumulates in the upper part of the room. The high detonation resistance of methane allows forcing the engine in terms of compression ratio (9.5-10.5).
In terms of energy parameters, 1 m3 of natural gas is equated to 1 liter of gasoline. At the same time, natural gas has a very low volumetric energy concentration. If the heat of combustion of 1 liter of liquid fuel is 31426 kJ, then for natural gas it is 33.52-35.62 kJ, i.e. almost 1000 times smaller. Therefore, natural gas must be compressed to high pressure.
At automobile gas-filling compressor stations in Russia, the working pressure is 20 MPa.
For compressed gas, gas-cylinder installations (cylinders, valves, reducers, gas pipelines, etc.) are used, designed to operate at high pressure - 19.6 MPa (200 kgf / cm 2). As the gas from the cylinder is consumed, the working pressure in it continuously decreases.
CNG cylinders have a capacity of 34-400 liters, designed for a pressure of 19.6 MPa.
Since compressed gas storage cylinders are made of thick walls, a battery of eight such cylinders is quite heavy. Consequently, the payload capacity of vehicles is also reduced. At the same time, the mileage of cars running on CNG becomes 2 times less than on gasoline. Therefore, the cryogenic technology of storing CNG in a car is considered more promising. In addition, this direction is considered a milestone on the way to the creation of hydrogen engines.
Compressed (compressed) natural gas (CNG), previously called compressed natural gas (CNG), is regulated according to GOST 27577-2000 "Compressed fuel gas for internal combustion engines" determines the physicochemical and operational parameters of CNG (Table 5.7).
Table 5.7 Physical, chemical and performance characteristics of CNG
Note. The value of the indicators is set at a temperature of 293K (20 ° C) and a pressure of 0.1013 MPa .
In accordance with GOST for CNG, the temperature of the gas filled into car cylinders should not exceed 40°C. At a temperature environment above 35°C, the temperature of the charged gas must be no more than 5°C higher than the air temperature. The CNG temperature during filling is determined at the request of consumers.
CNG ignites at a temperature of 635-645°C in the engine combustion chamber, which is 3 times higher than the ignition temperature of gasoline.
This makes it difficult to start the engine, especially at low ambient temperatures (below -5°C). Therefore, the cars have a backup fuel supply system with gasoline. At the same time, in terms of ignition and fire hazard, CNG is much safer than gasoline.
The positive factors of using CNG include the following:
The service life of engine oil is increased by 1.5-2.0 times due to the absence of its dilution and reduction of pollution; as a result, oil consumption is reduced by 30-40% compared to gasoline engines;
The motor resource of the engine increases by an average of 35-40% due to the absence of carbon deposits on the parts of the cylinder-piston group;
Increases the service life of spark plugs by 40%;
The overhaul run of the engine is increased by 1.5 times;
Significantly reduced (up to 90%) emissions with exhaust gases of harmful substances, especially CO.
The engines of LPG vehicles running on CNG, if the gas is used up, can quickly switch to running on gasoline.
Along with the advantages, the following disadvantages can be noted:
The labor intensity of maintenance and repair increases by 7-8%, and the price of a car increases by an average of 27% due to the presence of additional gas-balloon equipment;
Engine power is reduced by 18-20%. Traction-dynamic and operational characteristics of cars are deteriorating: acceleration time increases by 24-30%; maximum speed is reduced by 5-6%; the limiting angles of climbs to be overcome are reduced by 30-40%; operation of a car with a trailer is difficult; the driving range on one gas station decreases (does not exceed 200-250 km);
The carrying capacity of the vehicle is reduced by 9-14% due to the use of high-pressure steel cylinders (their number and weight may be different);
The mileage utilization rate of LPG vehicles is reduced by 8-13% compared to gasoline vehicles;
Annual productivity when working on urban transportation is reduced by 14-16% compared to gasoline.
The considered features of CNG as a fuel for cars make it possible to determine the rational area of application of gas-balloon cars: transportation in large cities and adjacent areas (the priority is the improvement of the air basin).
The effectiveness of intracity transportation on gas-balloon vehicles is obvious when servicing trade, household, communications and other institutions.
General description of reciprocating compressors. Single stage and two stage. harmful space
According to the nature of the action, reciprocating compressors can be single (or single) acting and double acting. In single-acting units, one suction or discharge is performed per piston stroke. In double-acting compressors, two suctions or discharges are carried out in one stroke of the piston.
According to the number of compression stages, reciprocating compressors are divided into three types: single-stage, two-stage and multi-stage. The compression stage is usually called the part of the compressor in which the gas is compressed to an intermediate or final pressure.
Structurally, single-stage compressors can be vertical or horizontal. As a rule, compressors with a horizontal design are double-acting machines, and compressors with vertical design belong to the units of simple action.
In a single-stage single-acting compressor with horizontal type design, the piston moves inside the cylinder. The cylinder is equipped with a cover that has suction and delivery valves. The compressor piston is connected to the connecting rod and crank. A flywheel is located on the crank shaft. During the stroke of the piston from left to right, a vacuum occurs in the area between the piston and the cylinder. The pressure difference between the suction line and the cylinder causes the valve to open, causing gas to flow into the cylinder. When the piston reverses from right to left, the suction valve closes and the gas in the cylinder is compressed to a pressure level p 2 . Further, through the valve, the gas is displaced into the discharge line. The cycle ends and repeats again.
The single-stage, double-acting compressor is equipped with four valves (two suction and two discharge). Such machines are more complex, but their performance level is twice as high. For cooling purposes, the cylinder and covers can be equipped with water jackets. To increase the productivity index, these machines can be manufactured in multi-cylinder designs. Single-stage compressors with a vertical type of design are more productive and faster than horizontal ones. In addition, they take up less floor space and are more durable.
Two-stage compressors with a horizontal design are usually equipped with a single cylinder and a stepped or differential piston type. The gas is compressed in the cylinder by the left side of the piston, after which it passes through the cooler and enters the cylinder from the other side, where it is compressed to the level p 2 .
Multi-stage designs are equipped with cylinders that are arranged in series (tandem system) or in parallel (compound system). There are also opposed compressor designs, where the pistons move in opposite directions. Cylinders in structures of this type are located on both sides of the shaft.
It should be noted that the actual process of gas compression in the compressor differs from the theory. So, between the piston, when it is in its extreme position and the cylinder cover, there is some free volume. This gap is called the harmful space. In this gap, upon completion of injection, the compressed gas expands during the reverse stroke of the piston. For this reason, the suction valve only opens after the pressure level has dropped to the suction pressure level. Thus, the piston makes an idle movement, which reduces the performance of the compressor.
