MIXING- (in engines internal combustion) formation of a combustible mixture. External mixture formation (outside the cylinder) is carried out by the carburetor (in carburetor engines) or mixer (in gas engines), internal mixing nozzle ... ... Big Encyclopedic Dictionary
mixture formation- I am; Wed The process of forming mixtures. Accelerated with. C. in internal combustion engines (mixing fuel with air or another oxidizer for the most complete and rapid combustion of fuel). * * * mixture formation (in engines of internal ... ... encyclopedic Dictionary
Mixture formation- (in internal combustion engines), the formation of a combustible mixture. External mixture formation (outside the cylinder) is carried out by a carburetor (in carburetor engines) or a mixer (in gas engines), internal mixture formation by a nozzle ... ... Automotive Dictionary
MIXING- the process of obtaining a working (combustible) mixture in internal engines. combustion. There are 2 mains. type C: external and internal. With external S. the process of obtaining a working mixture is carried out by hl. arr. outside the engine slave cylinder. With internal S., ... ... Big Encyclopedic Polytechnic Dictionary
In carburetor engines, the combustible mixture is prepared in a special device called carburetor.
A schematic of an elementary downdraft carburetor is shown in Fig. 16.9.
In a float chamber 2 using a float 4 and needle valve 3 constant fuel level is maintained.
When the engine is running, due to the suction action of the piston in the diffuser 6 vacuum is created. Fuel from the float chamber 2 through a calibrated hole 1, called e jet, is sucked to the spray nozzle 5, which sprays it.
To prevent fuel leakage from atomizer 5 when the engine is not running, its upper edge is located 2-3 mm above the fuel level in the float chamber 2. The latter happens balanced and unbalanced. In the first case, the float chamber communicates with the at-
Rice. 16.9.
1 - jet; 2 - float chamber; 3 - needle valve; 4 - float; 5 - sprayer; 6 - diffuser; 7 - throttle valve; 8 - pipeline with mospheric air through an air cleaner, in the second - directly with atmospheric air, as shown in Fig. 16.9.
The advantages of balanced float chambers include the fact that, regardless of resistance air filter, the air and gasoline consumption is better balanced and the chamber becomes less dirty.
Formed in the diffuser 6 combustible mixture in the intake manifold 8 through the intake valves it is directed into the engine cylinders. Fuel evaporation and mixture formation begin in the diffuser 6 of the carburetor and continue when the combustible mixture moves along the suction pipe 8 and end when it is compressed in a cylinder. In four-stroke engines, this process occurs during two piston strokes, which corresponds to 330-340 ° of rotation. crankshaft... During suction and compression, turbulence is formed, as a result of which the evaporated fuel mixes well with the air.
For better evaporation of the fuel during mixture formation, the combustible mixture is sometimes heated in the intake manifold, which ensures economical combustion of fuel with low excess air ratios and high crankshaft rotation speed.
The amount of combustible mixture entering the engine, and therefore its power, is regulated throttle 7. With a larger opening, the air speed in the diffuser increases. 6, the rarefaction and the intensity of the outflow of fuel from the atomizer 5, as well as the amount of the combustible mixture entering the cylinder, increase.
Depending on the design of the engine and its load, the air velocity in the diffuser ranges from 50 to 150 m / s. The composition of the combustible mixture prepared in the carburetor is characterized by the excess air coefficient a. A combustible mixture with a = 1 is called normal, for a = 1 - = - 1.15 - depleted, for a> 1.15 - poor. Engine operation from medium to full load on a lean mixture provides the lowest specific fuel consumption. At a> 1.3, the combustible mixture does not ignite due to lack of fuel. A combustible mixture with an excess amount of fuel at a = 1.00-ID 5 is called enriched, and for a rich. For a
When working on a rich mixture, the greatest engine power is provided due to an increase in the combustion heat of the charge and a higher flame propagation speed. However, when operating on this mixture, the fuel does not burn completely, which leads to its increased specific consumption.
On a rich mixture, the engine must operate during the start-up period, idle speed and at maximum power.
When the engine is running with an elementary carburetor during the start-up period, due to the low vacuum in the diffuser and the location of the fuel level in the atomizer 2-3 mm below its mouth, no fuel flows out of the atomizer, and clean air enters the engine (a -? °°) ... Thus, starting the engine with an elementary carburetor is impossible.
An elementary carburetor cannot provide the engine start and its stable operation on Idling, as well as the required composition of the mixture when switching from one mode of operation to another. Therefore, it is equipped with devices that ensure the receipt of the most advantageous composition of the mixture under various operating conditions of the engine. Such devices include compensation jets, economizers, booster pumps, etc.
Mixture formation is called the preparation of a working mixture of fuel and air for combustion in the engine cylinders. The mixture formation process occurs almost instantly: from 0.03 to 0.06 s in low-speed internal combustion engines and from 0.003 to 0.006 s in high-speed ones. To achieve complete combustion of fuel in the cylinders, it is necessary to ensure that the working mixture of the required composition and quality is obtained. In case of unsatisfactory mixture formation (due to poor mixing of fuel with air) with a lack of oxygen in the working mixture, incomplete combustion occurs, which leads to a decrease in efficiency ICE operation... Economical operation of the engine is achieved primarily by ensuring the most complete and fastest combustion of fuel in the cylinders near to. m. t. In this case, it is very important to atomize the fuel into the smallest possible homogeneous particles and their uniform distribution throughout the entire volume of the combustion chamber.
Currently in ship internal combustion engines mainly single-chamber, pre-chamber and vortex-chamber methods of mixture formation are used.
At single chamber mixture formation fuel in a finely dispersed state under high pressure is injected directly into the combustion chamber formed by the piston crown, cover and cylinder walls. Direct injection fuel pump a pressure of 20-50 MPa is created, and in some types of engines 100-150 MPa. The quality of mixture formation depends mainly on matching the configuration of the combustion chamber with the shape and distribution of the fuel combustion flares. For this, the nozzles of the nozzles have; 5-10 holes with a diameter of 0.15-1 mm. During injection, the fuel, passing through small holes in the nozzle, acquires a speed of more than 200 m / s, which ensures its deep penetration into the air compressed in the combustion chamber.
Combustion chamber of the Gesselmann type:
The quality of mixing of fuel particles with air depends primarily on the shape of the combustion chamber. Very good mixing is achieved in the chamber shown in the figure above and first proposed by Gesselman. It is widely used in four- and two-stroke internal combustion engines. Bumpers 1
at the piston edges prevent fuel particles from entering the bushing walls 2
cylinder having a relatively low temperature.
High power ICEs often have concave pistons. The combustion chamber formed by the cylinder head and the piston of this design allows good mixture formation.
In the case of mixture formation with direct injection of fuel into an undivided chamber, the latter can have a simple shape with a relatively small cooling surface. Therefore, internal combustion engines with a single-chamber method of mixture formation are simple in design and the most economical.
The disadvantages of a single-chamber method of mixture formation are as follows: the need for increased excess air ratios to ensure high-quality fuel combustion; sensitivity to change speed mode(due to the deterioration in the quality of spraying with a decrease in the engine speed); very high pressure of injected fuel, which complicates and increases the cost of fuel equipment. In addition, due to the small openings of the injector nozzles, it is necessary to use carefully cleaned fuel. For the same reason, it is very difficult to carry out single-chamber mixture formation in low-power high-speed internal combustion engines, since with low fuel consumption, the diameters of the nozzle openings of the injectors must be significantly reduced. It is very difficult to manufacture multi-hole nozzles with a very small diameter of nozzle holes; moreover, such holes quickly become clogged during operation and the nozzle breaks down. Therefore, in low-power high-speed internal combustion engines, mixture formation with separate combustion chambers (pre-chamber and vortex chamber), carried out with a single-hole nozzle, is more effective.
The figure shows an internal combustion engine cylinder with pre-chamber mixing... The combustion chamber consists of a prechamber 2 located in the lid and the main camera 1 in the space above the piston, interconnected. The volume of the pre-chamber is 25-40% of the total volume of the combustion chamber. When compressed, the air in the cylinder enters at high speed through the connecting channels 4 into the antechamber, creating intense vortex formation in it. Fuel under a pressure of 8-12 MPa is injected into the pre-chamber by a single-hole nozzle 3 , mixes well with air, ignites, but burns only partially due to lack of air. The remaining (unburned) part of the fuel, together with the combustion products under a pressure of 5-6 MPa, is thrown into the main combustion chamber. In this case, the fuel is intensively atomized, mixed with air and burned. The advantages of ICEs with pre-chamber mixture formation include the fact that they do not require fuel equipment operating under very high pressure and do not require highly purified fuel.
The main disadvantages of these ICEs are: a more complex design of the cylinder covers, which creates the risk of cracking due to thermal stresses; difficulty starting a cold engine; increased consumption fuel due to imperfect mixture formation. The relatively large surface of the prechamber walls causes a strong cooling of the air when it is compressed during engine start, which makes it difficult to obtain the temperature required for spontaneous combustion of the fuel. Therefore, in engines with a pre-chamber method of mixture formation, higher compression is allowed (the compression ratio reaches 17-18), and also electric glow plugs are used and the intake air is heated during the start-up period.
Vortex chamber method of mixture formation also used in low-power high-speed internal combustion engines. In these engines, the combustion chamber is also divided into two parts. The vortex chamber, which has a spherical or cylindrical shape, is placed in the cylinder cover or cylinder block and communicates with the main combustion chamber by a connecting channel tangential to the wall of the vortex chamber. Thanks to this, the compressed air flows into the vortex chamber through the connecting channel 1 , receives a rotary motion in it, which contributes to good mixing of fuel with air. The volume of the vortex chamber is 50-80% of the total volume of the combustion chamber. Fuel is supplied to the vortex chamber by a single-hole nozzle 2 under a pressure of 10-12 MPa. The diameter of the nozzle orifice is 1-4 mm.
The use of a vortex chamber method of fuel atomization ensures a fairly complete combustion of fuel in high-speed internal combustion engines. The disadvantages of such engines are increased fuel consumption and difficulty in starting. An electric glow plug is used to facilitate starting the internal combustion engine. 3 located next to the nozzle.
The specific fuel consumption of engines with pre-chamber and vortex-chamber mixture formation is 10-15% higher than that of engines with single-chamber mixture formation.
The mixture formation process is carried out as a result of fuel atomization using a high-pressure nozzle, directed vortex movement of the charge in the chamber, and sometimes also by regulating the temperature of the parts on which the fuel is evaporated.
Types of mixture formation.
Depending on the nature of fuel injection, there are volumetric, film and volume-film (mixed) types of mixture formation, which are carried out in undivided combustion chambers.
Volumetric mixing- fuel is injected into the air. This method does not allow fuel to enter the walls of the combustion chamber. This mixture formation takes place in 2-stroke engines.
Film blending- the bulk of the fuel falls on the walls of the chamber and spreads out in the form of a thin liquid film. In this case, for good ignition, about 5% of the fuel is injected into the compressed air, and the rest of it is injected onto the walls.
- part of the fuel is injected into the air, and part on the walls.
One of the methods of bulk-film mixture formation was proposed by Meurer and developed by MAN (Germany). It is characterized by the following features:
For better ignition and combustion, 5% of the fuel is injected into the compressed air, and the bulk of the fuel (95%) is applied to the walls in the form of a 10-15μm thick film;
The fuel injected into the heated air ignites spontaneously and then ignites the combustible mixture formed during the evaporation of the film from the cylinder walls and mixing the fuel vapor with air;
At the beginning of combustion, the fuel from the surface of the walls evaporates relatively slowly and combustion begins slowly. Then the processes are accelerated, while the piston goes to the BDC and therefore the engine runs smoothly and silently;
This combustion process allows the use of various fuels in the engine: gasoline, kerosene, naphtha, diesel oil, etc.
The combustion chamber has developed propellers that create an intense vortex movement of the air charge, which contributes to good evaporation and mixture formation.
Engines with a similar process are called multi-fuel engines.
Mixing in split combustion chambers
Separated combustion chambers are used to improve mixture formation. There are two types of mixture formation: pre-chamber and vortex chamber.
Prechamber mixing characterized in the following ways:
1. The combustion chamber is divided into two parts: the pre-chamber with a volume of (0.25-0.4) V s and the main chamber, which are interconnected by narrow channels that prevent the rapid flow of gases from the pre-chamber into the cylinder. As a result, the maximum combustion pressures are low and the engine runs very smoothly.
2. In the process of compression in the antechamber, a random turbulent movement of air is created due to its overflow at a high speed (200-300 m / s) through narrow channels from the cylinder. In this case, mixture formation is determined by the intensity of the air flow in the pre-chamber, and not by the quality of fuel atomization, due to which the engine is not very sensitive to the type of fuel and has a reduced injection pressure (10-13MPa).
3. The presence of narrow channels and a developed surface of the combustion chamber leads to large heat losses through the walls of the prechamber and energy losses when gases flow into the prechamber and back, which makes it difficult to start a cold engine and impairs its efficiency.
To facilitate starting, the compression ratio is increased to 20-21, and glow plugs are installed in the pre-chamber, which are turned on at start-up.
Vortex chamber mixing in contrast to the pre-chamber is characterized by:
1. A large volume of the vortex chamber (0.5-0.8) V s, in which an organized rotational movement of air is created during the compression process.
2. Large flow area and, consequently, high combustion pressure in the cylinder due to the rapid flow of burnt gases from the vortex chamber to the main one.
3. Due to the large flow cross sections, the charge energy losses during overflow are relatively small. For reliable starting, vortex chamber motors have = 17-20.
Internal combustion engines can be classified according to various criteria.
1.By appointment:
a) stationary, which are used at power plants of small and medium power, to drive pumping units, in agriculture, etc.
b) transport vehicles installed on cars, tractors, airplanes, ships, locomotives and other transport vehicles.
2.By the type of fuel used, engines are distinguished that run on:
a) light liquid fuel (gasoline, benzene, kerosene, naphtha and alcohol);
The proposed classification applies to internal combustion engines, which are widely used in the national economy. Special engines (jet, rocket, etc.) are not considered in this case.
b) heavy liquid fuel (fuel oil, diesel oil, diesel fuel and gas oil);
c) gas fuel (generator, natural and other gases);
d) mixed fuel; the main fuel is gas, and liquid fuel is used to start the engine;
e) various fuels (gasoline, kerosene, diesel fuel, etc.) - multi-fuel engines.
3.Motors are distinguished by the method of converting thermal energy into mechanical energy:
a) piston, in which the process of combustion and conversion of thermal energy into mechanical energy takes place in the cylinder;
b) gas turbine, in which the process of fuel combustion takes place in a special combustion chamber, and the conversion of thermal energy into mechanical energy occurs on the blades of a gas turbine wheel;
c) combined, in which the process of fuel combustion occurs in a piston engine, which is a gas generator, and the conversion of thermal energy into mechanical energy occurs partly in the piston engine cylinder, and partly on the blades of a gas turbine wheel (free-piston gas generators, turbo-piston engines, etc.). ).
4. By the method of mixture formation, piston engines are distinguished:
a) with external mixture formation, when the combustible mixture is formed outside the cylinder; all carburetor and gas engines work in this way, as well as engines with fuel injection into the intake pipe;
b) with internal mixture formation, when during the intake process only air enters the cylinder, and the working mixture is formed inside the cylinder; Diesel engines, spark ignition engines with fuel injection into the cylinder and gas engines with gas supply to the cylinder at the beginning of the compression process operate in this way.
5.The method of ignition of the working mixture is distinguished:
a) engines with ignition of the working mixture from an electric spark (with spark ignition);
b) engines with compression ignition (diesels);
c) engines with pre-chamber-torch ignition, in which the mixture is ignited by a spark in a special combustion chamber of a small volume, and the further development of the combustion process takes place in the main chamber.
d) engines with gas fuel ignition from a small portion of diesel fuel, igniting from compression, -
gas-liquid process.
6.According to the method of carrying out the working cycle, piston
Engines are divided into:
a) four-stroke naturally aspirated (intake of air from the atmosphere) and supercharged (intake of a fresh charge under pressure);
b) two-stroke - naturally aspirated and supercharged. A distinction is made between supercharging with a compressor driven by an exhaust gas turbine (gas turbine supercharging); pressurization from a compressor mechanically connected to the engine and pressurization from compressors, one of which is driven gas turbine and the other is the engine.
7.According to the method of regulation when the load changes, they are distinguished:
a) engines with quality control, when, due to a change in load, the composition of the mixture changes by increasing or decreasing the amount of fuel introduced into the engine;
b) engines with quantitative control, when, when the load changes, the composition of the mixture remains constant and only its amount changes;
c) engines with mixed regulation, when, depending on the load, the amount and composition of the mixture change.
8.The designs differ:
a) piston engines, which, in turn, are divided:
according to the arrangement of the cylinders into vertical in-line, horizontal in-line, V-shaped, star-shaped and with opposing cylinders;
according to the arrangement of pistons to single-piston (each cylinder has one piston and one working cavity), with oppositely moving pistons (the working cavity is located between two pistons moving in opposite directions in one cylinder), double-acting (there are working cavities on both sides of the piston) ;
b) rotary piston engines, which can be of three types:
the rotor (piston) makes a planetary motion in the housing; when the rotor moves between it and the walls of the housing, chambers of variable volume are formed in which the cycle is performed; this scheme has received predominant use;
the body makes a planetary motion, and the piston is stationary;
the rotor and the body make a rotational movement - a biro-toric engine.
9. Engines are distinguished by the method of cooling:
a) with liquid cooling;
b) air-cooled.
On cars, piston engines are installed with spark ignition (carburetor, gas, fuel injection) and with compression ignition (diesel engines). On some experimental vehicles, gas turbine and rotary piston engines are used.
