Petrol engines -
one of the types of ICE
(engines of internal
combustion) in which ignited
mixtures of air and fuel,
carried out in
cylinders, through
sparks from spark plugs.
The role of the power regulator
performs throttle
valve that regulates
flow of incoming
air.
two-stroke and four-stroke.
Two-stroke engines have more power per unit
volume, but lose in efficiency. So they found their way
where compactness is important, not efficiency (motorcycles, motor
boats, chainsaws and other motorized tools).
Four-stroke engines dominate the rest
movement. Fuel-air system
The main task of the fuel-air system is uninterrupted
delivery of a mixture of fuel and air to the engine. Fuel supply system
also called fuel system or fuel system.
Such a system is designed to power the engine, store and clean
fuel.
Structural structure
fuel tank
fuel pump
fuel filter
injection system
fuel lines
The principle of operation of the fuel-air system
The whole scheme of the fuel supply system is as followsway:
The driver turns on the ignition;
The fuel pump pumps fuel into the system and creates a working
pressure;
Fuel enters the injection system;
Atomization and the formation of fuel-air
mixtures;
mixture formation
Under the mixture formation in engines with spark ignition is meanta complex of interrelated processes accompanying dosing
fuel and air, atomization and evaporation of fuel and its mixing
with air. Quality mixing is necessary condition
obtaining high power, economic and environmental
engine performance.
Mixture formation of injection internal combustion engine
Provides storagefuel needed
to power the engine
cars. Specified
tank in cars
often located in
back and fixed
on the bottom of the body.
Responsible for cleaning
fuel.
Responsible for supplying fuel to the injection system and
supports the necessary operating pressure in
fuel system. The principle of operation of the injector is that the ECU
(electronic control unit) supplies it with
electrical impulse. Under the impulse
the injector opens and injects gasoline into
intake manifold. Received air fuel
the mixture is sucked in through the intake valves by the piston
on the intake stroke. Point in time and duration
injection for the injector is determined by the ECU.
The mixture formation of a carburetor internal combustion engine
The formation of a mixture of gasoline withair takes place in
carburetor where gasoline
mixed with suction
air into the engine
the right amount,
sprayed and partially
evaporates. Further
evaporation and mixing
take place in the intake
pipeline and in themselves
engine cylinders.
10.
Way of education combustible mixture in the simplestcarburetor (Fig. 71)
Fuel from the tank under pressure enters through the channel,
closed by needle valve 4, into the float chamber
2. Float 3 measures the fuel level in the float
chamber, and consequently, the pressure of the fuel is maintained
almost constant so that this level is somewhat
below the nozzle hole 7; thus, when
When the engine is not running, there is no fuel leakage. At
suction stroke of the piston 10, i.e. when moving it down
air passes through the pipe 8 diffuser 6, in which it
the speed increases significantly, and consequently, the pressure
goes down. Due to rarefaction, the fuel from the float
chamber through a calibrated through hole 1,
called a jet, and nozzle 7 gushing into
diffuser, breaking up into small droplets,
evaporating in the air stream. The amount of mixture
sucked in through the inlet valve 9, is regulated by the throttle valve 5.
Under the mixture formation in engines with spark ignition is meant a complex of interrelated processes that accompany the dosing of fuel and air, atomization and evaporation of fuel and mixing it with air. High-quality mixture formation is a necessary condition for obtaining high power, economic and environmental performance of the engine.
The course of mixture formation processes largely depends on the physicochemical properties of the fuel and the method of its supply. In engines with external carburetion, the carburetion process begins in the carburetor (injector, mixer), continues in the intake manifold and ends in the cylinder.
After the fuel jet exits from the carburetor atomizer or nozzle, the jet begins to disintegrate under the influence of aerodynamic drag forces (due to the difference in air and fuel speeds). The fineness and uniformity of atomization depend on the air velocity in the diffuser, the viscosity and the surface tension of the fuel. When starting a carburetor engine at its relatively low temperature, there is practically no fuel atomization, and up to 90 percent or more of the fuel in the liquid state enters the cylinders. As a result, to ensure a reliable start, it is necessary to significantly increase the cyclic fuel supply (bring b to values of ? 0.1-0.2).
The process of atomizing the liquid phase of the fuel also proceeds in the passage section of the intake valve, and when not fully open throttle- in the gap formed by it.
Part of the fuel droplets, carried away by the flow of air and fuel vapors, continues to evaporate, and the other part settles in the form of a film on the walls of the mixing chamber, intake manifold and channel in the head of the block. Under the action of a tangential force from interaction with the air flow, the film moves towards the cylinder. Since the velocities of the air-fuel mixture and fuel droplets differ insignificantly (by 2–6 m/s), the intensity of droplet evaporation is low. Evaporation from the film surface proceeds more intensively. To speed up the evaporation process of the film, the intake manifold in carburetor and central injection engines is heated.
The different resistance of the intake manifold branches and the uneven distribution of the film in these branches lead to uneven composition of the mixture in the cylinders. The degree of non-uniformity of the composition of the mixture can reach 15-17%.
When the fuel evaporates, the process of its fractionation proceeds. First of all, light fractions evaporate, and heavier ones enter the cylinder in the liquid phase. As a result of the uneven distribution of the liquid phase in the cylinders, there may be not only a mixture with a different fuel-air ratio, but also fuel of a different fractional composition. Consequently, the octane numbers of the fuel in different cylinders, will not be the same.
The quality of mixture formation improves with increasing speed n. The negative effect of the film on the performance of the engine in transient conditions is especially noticeable.
The uneven composition of the mixture in engines with distributed injection is determined mainly by the identity of the operation of the injectors. The degree of non-uniformity of the composition of the mixture is ±1.5% when working according to the external speed characteristic and ±4% on Idling with a minimum rotational speed n x.x.min.
When fuel is injected directly into the cylinder, two methods of mixture formation are possible:
With obtaining a homogeneous mixture;
With charge separation.
The implementation of the latter method of mixture formation is associated with considerable difficulties.
In gas engines with external mixture formation, fuel is introduced into the air stream in a gaseous state. A low boiling point, a high diffusion coefficient and a significantly lower value of the amount of air theoretically necessary for combustion (for example, for gasoline? 58.6, methane - 9.52 (m 3 air) / (m 3 fuel) provide an almost homogeneous combustible mixture The distribution of the mixture over the cylinders is more uniform.
Combustion of fuel can proceed only in the presence of an oxidizing agent, which is used as oxygen in the air. Therefore, for the complete combustion of a certain amount of fuel, it is necessary to have a certain amount of air, the ratio of which in the mixture is estimated by the excess air coefficient.
Since air is a gas, and petroleum fuels are liquid, for complete oxidation, liquid fuel must be turned into a gas, i.e., evaporated. Therefore, in addition to the four processes considered, corresponding to the names of the cycles of the engine, there is always one more - the process of mixture formation.
mixture formation- this is the process of preparing a mixture of fuel with air for burning it in the engine cylinders.
According to the method of mixture formation, internal combustion engines are divided into:
- engines with external mixture formation
- engines with internal mixture formation
In engines with external mixing, the preparation of a mixture of air and fuel begins outside the cylinder in a special device - a carburetor. Such internal combustion engines are called carburetor. In engines with internal mixture formation, the mixture is prepared directly in the cylinder. These ICEs include diesel engines.
Classification of combustion chambers 2. Mixing begins at the moment of the start of fuel injection and ends simultaneously with the end of combustion. The development of mixture formation and obtaining optimal results in a diesel engine depends on the following factors: the method of mixture formation; combustion chamber shapes; combustion chamber dimensions; temperature of the surfaces of the combustion chamber; mutual directions of motion of fuel jets and air charge. The degree of their influence depends on the type of combustion chamber.
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Lecture 9
CAUTION FORMATION IN DIESEL
2. Mixing methods
3. Fuel spray
In diesel engines, mixture formation occurs inside the cylinders.The mixing system provides:
Spraying fuel;
Development of a fuel torch;
Heating, evaporation and overheating of fuel vapors;
Mixing vapors with air.
The mixture formation begins at the moment of the start of fuel injection and ends simultaneously with the end of combustion. In this case, the time for mixture formation is 5-10 times less than in carbureted engine. And a heterogeneous mixture is formed throughout the volume (there are areas of a very depleted composition, and there are areas of a highly enriched composition). Therefore, combustion proceeds at large total values of the excess air coefficient (1.4-2.2).
The development of mixture formation and obtaining optimal results in a diesel engine depends on the following factors:
mixing method;
Combustion chamber shapes;
The dimensions of the combustion chamber;
Temperatures of the surfaces of the combustion chamber;
Mutual directions of motion of fuel jets and air charge.
The degree of their influence depends on the type of combustion chamber.
1. Classification of combustion chambers
Along with ensuring optimal mixture formation, combustion chambers should contribute to obtaining high economic indicators and good starting qualities of engines.
Depending on the design and the method of mixture formation used, the combustion chambers of diesel engines are divided into two groups:
Undivided and divided.
Undivided combustion chambersare a single volume and usually have a simple shape, which is generally consistent with the direction, size and number of fuel jets at injection. These chambers are compact, have a relatively small cooling surface, which reduces heat loss. Engines with such combustion chambers have decent economic performance and good starting qualities.
Undivided combustion chambers are distinguished by a wide variety of shapes. Most often they are performed in the bottom of the pistons, sometimes partly in the bottom of the piston and partly in the cylinder head, less often in the head.
On fig. 1 shows some designs of undivided combustion chambers.
In the combustion chambers shown in fig. one, a—e the quality of mixture formation is achieved solely by atomizing the fuel and matching the shape of the chambers with the shape of the fuel injection jets. These chambers most often use multi-hole nozzles and use high injection pressures. Such chambers have minimal cooling surfaces. They have a low compression ratio.
Rice. 1. Combustion chambers of undivided diesel engines:
but - toroidal in the piston; b - hemispherical in the piston and cylinder head; in - hemispherical in the piston; G - cylindrical in the piston;
d - cylindrical in the piston with lateral placement;
e — oval in the piston; well - ball in the piston;
h - toroidal in a piston with a neck;
And - cylindrical, formed by the bottoms of the pistons and the walls of the cylinder;
to - vortex in the piston; l - trapezoidal in the piston;
m - cylindrical in the head under the exhaust valve
f—h , have a more developed heat transfer surface, which somewhat worsens the starting properties of the engine. However, by displacing air from the over-piston space into the volume of the chamber during compression, it is possible to create intense vortex charge flows that contribute to good mixing of fuel with air. This ensures high quality mixing.
The combustion chambers shown in fig. one, to-m , are used in multi-fuel engines. They are characterized by the presence of strictly directed charge flows, which ensure the evaporation of fuel and its introduction into the combustion zone in a certain sequence. To improve the working process in the cylindrical combustion chamber in the head under the exhaust valve (Fig. 1, m ) is used heat exhaust valve, which is one of the walls of the chamber.
Separated combustion chambers (rice. 2) consist of two separate volumes interconnected by one or more channels. The cooling surface of such chambers is much larger than that of undivided chambers. Therefore, due to large heat losses, engines with divided combustion chambers usually have worse economic and starting qualities and, as a rule, higher compression ratios.
Rice. 2. Combustion chambers of diesel engines of a divided type:
a - prechamber; b - vortex chamber in the head; in - vortex chamber in the block
However, with separate combustion chambers, due to the use of the kinetic energy of gases flowing from one cavity to another, it is possible to ensure high-quality preparation of the fuel-air mixture, due to which a fairly complete combustion of the fuel is achieved and exhaust smoke is eliminated.
In addition, the throttling effect of the connecting channels of the divided chambers can significantly reduce the "rigidity" of the engine and reduce the maximum load on the parts of the crank mechanism. Some reduction in the "rigidity" of engines with separated combustion chambers can also be achieved by increasing the temperature of individual parts of the combustion chambers.
2. Mixing methods
Depending on the nature of evaporation, mixing with an air charge and the method of introducing the bulk of the injected fuel into the combustion zone in diesel engines, volumetric, film and volumetric-film mixing methods are distinguished.
2.1. Volumetric mixing method
With the volumetric mixing method, the fuel is introduced in a finely atomized droplet-liquid state directly into the air charge of the combustion chamber, where it then evaporates and mixes with air, forming a fuel-air mixture.
In volumetric mixing, as a rule,undivided combustion chambers (so-called direct injection). The quality of mixture formation in this case is achieved mainly by matching the shape of the combustion chamber with the shape and number of fuel torches. In this case, fuel atomization during injection is important. The excess air coefficient for such engines is limited to 1.5-1.6 and higher.
The operating cycle with this mixture formation is characterized by a high maximum combustion pressure p, and high pressure rise rates w p = dp / dφ ("rigidity" of work).
Direct injection engines have the following advantages:
high economy ( g e from 220 to 255 g/(kWh));
Good starting qualities;
Relatively low compression ratio (ε from 13 to 16);
The relative simplicity of the design of the combustion chamber and the possibility of forcing boost.
The main disadvantages of these engines are:
Increased values of the excess air coefficient (1.6-2) at nominal modes and, as a result, a moderate value of the average effective pressure;
High "rigidity" of work ( wp up to 1 MPa/°);
Sophisticated fuel equipment and difficult working conditions due to high pressures.
At pre-chamber volumetric mixing methodThe combustion chambers are divided into two parts: the pre-chamber and the main chamber.
The prechamber is usually located in the cylinder head (Fig. 2, but ), their shape is a body of revolution. The volume of the prechamber is 20-40% of the volume of the combustion chamber. The prechamber is connected to the main chamber by a channel of small cross section.
Mixing is carried out due to the kinetic energy of gases flowing at high speeds from the main chamber to the pre-chamber during compression and from the pre-chamber to the main one during combustion. Therefore, in this case, there are no high requirements for the quality of atomization and uniformity of fuel distribution during injection. This allows the use of an injection pressure of 8-15 MPa and nozzles with a single-hole atomizer.
To the advantages of pre-chamber volumetric mixing can be attributed to:
Low maximum combustion pressure in the cylinder cavity
( pz = 4.5–6.0 MPa) and a slight “rigidity” of work ( w p \u003d 0.25-0.3 MPa / °);
Low sensitivity to changes in speed modes and the possibility of forcing the frequency of rotation of the crankshaft;
Low requirements for the quality of fuel atomization, the possibility of using low injection pressures and injectors with atomizers with a single hole at large flow sections of the channels;
Fuel combustion occurs at a relatively small excess air ratio (α min = 1.2).
The disadvantages of pre-chamber volumetric mixing are:
Low economic performance due to increased heat removal with a significant heat transfer surface and additional gas-dynamic losses during gas flow from one chamber to another;
Difficulties in starting a cold engine due to large heat losses with a large surface of the combustion chamber. To improve starting qualities in pre-chamber diesel engines, higher compression ratios are used.
(ε = 20-21), and glow plugs are sometimes installed in the pre-chambers;
Complex designs of the combustion chamber and engine head.
Vortex chamber volumetric mixingdiffers in that the combustion chamber consists of the main and vortex chambers.
Swirl chambers are most often performed in the cylinder head (Fig. 2, b ) and less often in the cylinder block (Fig. 2, in ). They are spherical or cylindrical in shape. The vortex combustion chambers are connected to the main chambers by one or more tangential channels of a round or oval shape with relatively large flow sections. The volume of the vortex chambers is 50-80% of the total volume of the combustion chamber.
A feature of vortex chamber engines is a relatively small pressure drop between the vortex and main combustion chambers and, accordingly, low gas flow rates from one part of the chamber to another. Therefore, the quality of mixture formation is ensured mainly by the intense vortex motion of the charge, which is organized in periods of compression and combustion.
Intensive vortex motion of the charge is provided good use air oxygen and smokeless operation of the engine at low values of the excess air coefficient (α = 1.15). At the same time, the requirements for the quality of fuel atomization are reduced, it becomes possible to use injection pressure of relatively low values
( p vpr = 12–15 MPa) in nozzles with one nozzle hole of large diameter (1–2 mm).
Advantages of vortex chamber volumetric mixing:
Possibility of operation at low values of the excess air coefficient, which provides better use of the working volume compared to other engines and obtaining higher values of the average effective pressure;
Lower than the engines with direct injection, the maximum combustion pressure and a decrease in the "rigidity" of work;
The possibility of forcing the engine according to the frequency of rotation of the crankshaft;
Low requirements for the type of fuel;
Low injection pressure and the possibility of using simpler fuel equipment;
Stability of engine operation under variable conditions.
The disadvantages of vortex-chamber volumetric mixing are the same as for pre-chamber mixing.
2.2. Film and volume-film mixing methods
The method of mixture formation, in which the fuel does not enter the center of the air charge, but on the wall of the combustion chamber and spreads over its surface in the form of a thin film 12-14 microns thick, is called film. Then the film evaporates intensively and, mixing with air, is introduced into the combustion zone.
With volume-film mixing fuel-air mixture It is prepared simultaneously by both bulk and film methods. This method of mixture preparation takes place in almost all diesel engines and can be considered as a general case of mixture formation.
Film mixing eliminates two of the main disadvantages of diesel engines: the "rigidity" of operation and smoke when exhaust gases are released.
In film mixing, a spherical combustion chamber is used (Fig. 3), in which an intensive charge movement is carried out: rotational around the axis of the cylinder and radial in the transverse direction.
Rice. 3. The combustion chamber of the engine with film mixing:
1 - nozzle; 2 - combustion chamber; 3
- fuel film
Fuel injection is carried out by a single-nozzle nozzle with a pressure of 20 MPa at the beginning of the needle lift. The injected fuel meets the wall surface under acute angle and, almost not reflected from it, it spreads and is “stretched” by passing air currents into a thin film. Having a large surface of contact with the heated walls of the combustion chamber, the film quickly warms up and begins to evaporate intensively, and thereby is sequentially introduced into the center of the combustion chamber, where a combustion center has formed by this time.
The advantages of film mixing include the following:
"soft" work wp = 0.25–0.4 MPa/° at maximum pressure cycle pz = 7.5 MPa);
High economic performance at the level of engines with volumetric mixing and direct injection;
Relatively simple design of fuel equipment.
The main disadvantage of film mixture formation is the low starting qualities of the engine in a cold state due to the small amount of fuel involved in the initial combustion.
An example of volume-film mixture formation is the combustion chamber shown in Fig. 4.
Rice. 4. Engine combustion chamber with volumetric film
mixture formation: 1 - nozzle; 2 - combustion chamber
Fuel from the nozzle holes at an acute angle is directed to the walls of the combustion chamber. However, the air flow flowing from the over-piston space into the combustion chamber is directed towards the movement of the fuel, prevents the formation of a film and contributes only to the rapid evaporation of the fuel.
The "rigidity" of engine operation with this method of mixture formation reaches 0.45-0.5 MPa / °, and the specific fuel consumption - 106-170 g / (kW h).
2.3. Comparative evaluation of various mixing methods
Each mixing method has its own advantages and disadvantages.
Thus, engines with direct injection have good starting qualities, the highest economic performance and allow significant boosting of boost.
At the same time, these diesel engines are characterized by high “rigidity” of operation, noise level, loads on parts and excess air coefficient values, increased requirements for fuel grade and limited opportunities forcing on the frequency of rotation of the crankshaft without special changes in the design.
Engines with film and volumetric-film mixture formation, with sufficiently high efficiency, "soft" operation and undemanding fuel, have poor starting qualities.
“Soft” operation, relatively low loads on parts, lower values of the excess air coefficient and wide possibilities for boosting the crankshaft speed are inherent in engines with separated combustion chambers, however, there are significant deterioration in economic indicators and poor starting qualities.
In table. 1 shows some parameters of diesel engines with different ways mixture formation.
Table 1. Parameter values of diesel engines with different mixing methods
|
Mixing method |
The combustion chamber |
Average effective |
Specific ef- |
Limit frequency |
Max- |
"Rigidity" of work, MPa/° |
|
Directly |
Inseparable |
0,7-0,8 |
220-255 |
3000 |
7-10 |
0,4-1,5 |
|
Volume-ple- |
Same |
0,7-0,8 |
220-255 |
3000 |
0,4-0,5 |
|
|
Film |
Same |
0,7-0,8 |
220-240 |
3000 |
0,25-0,4 |
|
|
prechamber |
Divided |
0,65-0,75 |
260-300 |
4000 |
0,2-0,35 |
|
|
Whirlpool |
Same |
0,7-0,85 |
245-300 |
4000 |
0,25-0,4 |
3. Fuel spray
The properties of mixture formation, especially with volumetric mixture formation, are greatly influenced by the quality of fuel atomization during injection.
The criteria for evaluating the quality of spraying are spray dispersion and uniformity.
Sputtering is considered fine if the average droplet diameter is 5–40 µm.
The fineness and uniformity of the spray is determined by the injection pressure, the back pressure of the medium, the speed of the pump shaft and design features atomizer.
In addition to the quality of atomization, the depth of penetration of the atomized fuel torch into the air charge (the so-called “range” of the torch) has a great influence on the mixture formation process in diesel engines. With volumetric mixture formation, it should be such that the fuel "pierces" the entire air charge, without settling on the walls of the combustion chamber.
The shape of the torch (Fig. 5) is characterized by its length l f , taper angle β f and width b f .
Rice. 5. The shape of the fuel flame and its position in the combustion chamber
The formation of the torch occurs gradually during the development of the injection process. Length l f the flame increases as new fuel particles move to its top. The speed of the top of the torch with an increase in the resistance of the medium and a decrease in the kinetic energy of the particles decreases, and the width b f torch increases. Angle β f taper with a cylindrical shape of the nozzle opening of the sprayer is 12-20 °.
The maximum length of the torch must correspond to the linear dimensions of the combustion chamber and ensure complete coverage of the combustion chamber space by the torches. With a small flame length, combustion can proceed near the nozzle, i.e., under conditions of lack of air, which does not have time to flow from the peripheral zones of the chamber to the combustion zones in a timely manner. With an excessive length of the torch, the fuel settles on the walls of the combustion chamber. The fuel deposited on the walls of the chamber under the conditions of an irrotational process does not burn out completely, and carbon deposits and soot are formed on the walls themselves.
The fuel introduced into the cylinder in the form of torches is distributed unevenly in the air charge, since the number of torches determined by the design of the atomizer is limited.
Another reason for the uneven distribution of fuel in the combustion chamber is the uneven structure of the torches themselves.
Usually, three zones are distinguished in a torch (Fig. 6): core, middle part and shell. The core consists of large particles of fuel, which have the highest speed during the formation of the torch. The kinetic energy of the particles of the front part of the torch is transferred to the air, as a result of which the air moves in the direction of the torch axis.
Rice. 6. Fuel torch:
1 - core; 2 - the middle part; 3 - shell
The middle part of the torch contains a large number of small particles formed during the crushing of the front particles of the core by the forces of aerodynamic resistance. Atomized particles that have lost their kinetic energy are pushed aside and continue to move only under the action of an air flow entrained along the axis of the torch. The shell contains the smallest particles with a minimum speed of movement.
Fuel atomization is influenced by the following factors:
Atomizer design;
injection pressure;
The state of the environment into which the fuel is injected;
fuel properties.
Despite the fact that the design of sprayers is very diverse, sprayers with cylindrical nozzle holes are most widely used (Fig. 7, but ) and pin atomizers (Fig. 7, b ). Sprayers with oncoming jets are less commonly used (Fig. 7, in ) and with screw swirlers (Fig. 7, G ).
Rice. 7. Spray nozzles:
but — with a cylindrical nozzle hole; b - pin;
in — with oncoming jets; G - with screw swirlers
Atomizers with cylindrical nozzle holes can be multi-hole and single-hole, open and closed (with a shut-off needle). Pin atomizers are made only single-hole closed type; counter-jet sprayers and screw swirlers can only be open.
Cylindrical nozzle holes provide relatively compact flames with small expansion cones and high penetrating power.
With an increase in the diameter of the nozzle opening, the penetration depth of the torch increases. An open type atomizer provides a lower atomization quality than a closed one. The lowest atomization quality is observed when using open-type nozzles at the beginning and end of fuel injection, when fuel flows into the cylinder at low pressure drops.
Pin atomizers have a needle with a cylindrical or conical pin at the end. Between the pin and the inner surface of the nozzle hole there is an annular gap, which is why the torch of sprayed fuel takes the form of a hollow cone. Such torches are well distributed in the air charge medium, but have a low penetrating power. Such atomizers are used in divided combustion chambers with small dimensions.
The higher the injection pressure, the greater the penetration and length of the fuel jet, the finer and more uniform the fuel spray.
The medium into which the fuel is injected affects the quality of atomization through pressure, temperature and swirl. With an increase in the pressure of the medium, the resistance to the advance of the torch increases, which leads to a decrease in its length. In this case, the quality of spraying changes slightly.
An increase in air temperature leads to a decrease in the flame length due to more intensive evaporation of fuel particles.
The more intense the movement of the medium in the cylinder, the more evenly the fuel is distributed in the volume of the combustion chamber.
An increase in the temperature of the fuel leads to a decrease in the length of the torch and a finer atomization, since when the fuel is heated, its viscosity decreases. Fuels with a higher viscosity are less atomized.
4. Formation of a combustible mixture and ignition of the fuel
Atomized fuel, falling into layers of hot air, heats up and evaporates. In this case, first of all, fuel particles with a diameter of 10–20 μm evaporate, and larger particles evaporate already during the combustion process, gradually being involved in it. Fuel vapors, mixing with air, form a combustible mixture of heterogeneous composition. The closer to the surface of not yet evaporated fuel particles, the richer the mixture and vice versa. In this case, the values of the coefficient of excess air throughout the entire volume of the combustion chamber vary over a very wide range. Promotion of fuel particles in air layers contributes to some leveling of the composition of the mixture over the volume of the combustion chamber, since in this case vapors are dispersed along the trajectory of fuel movement.
Since the size of the fuel particles in the flame shell is minimal, and the temperature is the highest in comparison with the entire structure of the flame, the process of mixture formation in the shell occurs most intensively. As a result, the entire shell of the torch evaporates before the start of combustion. Nevertheless, some amount of air manages to get into the middle part of the torch, as well as into the core. However, due to the significant concentration of fuel in this zone, the evaporation process is slowed down.
After ignition, the mixture formation process is accelerated, as the temperature and the rate of mixing of the fuel with air increase sharply. The mixture formation that took place before the start of combustion has a greater influence on the operation of the engine.
Before combustion, the evaporated fuel goes through a stage of chemical preparation. In this case, critical concentrations of intermediate oxidation products arise in separate zones of the mixture, which leads to a thermal explosion and the appearance of primary flames in several places. Zones with an excess air coefficient of 0.8-0.9 are the most favorable for the appearance of such foci. These zones are most likely on the periphery of the torch, since the chemical and physical processes of fuel preparation for combustion end here earlier.
Thus, ignition in a diesel engine is possible at any total excess air ratio. Consequently, in a diesel engine, the excess air coefficient does not characterize the ignition conditions of the mixture, as is the case in a carburetor engine (ignition limits).
test questions
1. At what values does the combustion of the mixture occur in diesel engines?
2. What determines the perfection of the combustion process in diesel engines?
3. What is the difference between split combustion chambers and undivided ones?
4. Name the forms of undivided combustion chambers known to you.
5. Advantages and disadvantages of separated combustion chambers.
6. What mixing methods do you know?
7. Advantages and disadvantages of direct injection.
8. Tell us about the film and volumetric-film mixing methods.
9. Advantages and disadvantages of film mixing.
10. What are the criteria for assessing the quality of spraying the mixture?
11. What factors influence fuel atomization?
12. What types of fuel atomizers are most widely used?
13. Why does the excess air coefficient in a diesel engine not characterize the conditions for ignition of the mixture (by limits)?
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| 7653. | Mixing in internal combustion engines | 10.61KB | |
| Mixing is the process of mixing fuel with air and forming a combustible mixture in a very short period of time. The more evenly the fuel particles are distributed throughout the combustion chamber, the more perfect the combustion process. Homogenization of the mixture is ensured by the evaporation of the fuel, but for good evaporation, the liquid fuel must be pre-atomized. Fuel atomization also depends on the speed of the air flow, but its excessive increase increases the hydrodynamic resistance of the intake tract, which worsens ... | |||
Mixing system
In undivided combustion chambers, the entire compression space is a single volume bounded by the piston crown, cap, and cylinder walls. The required quality of mixture formation is achieved by matching the configuration of the combustion chamber with the shape and distribution of fuel jets coming out of the nozzle nozzle holes. The vortex air movement created during the period of gas exchange is small by the end of compression and plays a secondary role in chambers of this type. The undivided type chambers are characterized by simple design and high efficiency. The simplicity of the chamber configuration makes it possible to provide relatively low thermal stresses in its walls.
Volumetric mixing ensures uniform distribution of the entire cyclic fuel supply in the mass of the air charge in the combustion chamber, which is achieved by the appropriate shape of the fuel flame. The quality of mixture formation in this case largely depends on the presence of organized vortex formation of air flows. IN two-stroke engine vortex formation is provided by an inclined or tangential arrangement of purge windows.
Advantages of volumetric mixture formation: simplicity of the combustion chamber with high quality of its cleaning; small loss of heat through the walls of the combustion chamber due to the relatively small surface; good starting qualities of a diesel engine that do not require additional ignition devices; high efficiency of a diesel engine with a fuel consumption of 155 - 210 g / (kWh). Disadvantages: high coefficient of excess air (b = 1.6 h2.2); high spray pressure (up to 100 - 130 MPa); increased requirements for fuel equipment; the impossibility of high-quality mixture formation with small cylinder diameters and low values of cyclic fuel supply.
Volumetric mixing is used in almost all diesel engines with a cylinder diameter of more than 150 mm.
Gas distribution system
Cross-slot blowing. The peculiarity of this method lies in the fact that the outlet and purge windows are located on different sides of the cylinder sleeve. They are connected accordingly exhaust manifold and with purge air receiver. The purge windows are tilted upwards, in connection with which the air first moves to the cylinder cover, then, displacing the exhaust gases, reverses direction.
So that by the time the purge ports open, the pressure in the cylinder has time to decrease and become lower than the pressure of the purge air, the outlet ports are provided above the purge ports. However, in this case, the piston, moving up, will first close the purge windows, the exhaust windows will still be partially open. The purge process ends after the purge ports are closed, therefore a fresh charge of air will escape (partial leakage) through the not completely closed outlet ports. To avoid this phenomenon, for large engines, exhaust and purge windows are made the same height, but non-return valves are installed in the purge air receiver, which prevent exhaust gases from being thrown from the cylinder into the receiver when the windows are opened; purge begins only when the pressure drops in the cylinder after the opening of the exhaust ports. When the piston moves upwards, purge air will flow until both windows are closed. For the same purpose, in some large engines, a drive spool is installed on the exhaust pipe, the drive of which is adjusted so that at the moment the piston closes the purge windows, the spool closes the exhaust ports.
The cross-slot blowing method is widely used due to its simplicity.
The camshaft is steel. It has for each cylinder two pairs of cam washers of a symmetrical profile (front and reversing) to drive fuel pumps and air distributors. The cam washers of the fuel pumps, as well as their rollers - pushers, have bevels at the ends, and when reversing, it is enough to move the camshaft in the axial direction so that the corresponding cam washers become under the drive rollers. At the aft end of the engine camshaft reversible cylinders are placed. The camshaft consists of a number of sections. Each individual section consists of a shaft section with camshafts for exhaust valves and fuel pumps and connecting parts.
Camshaft drive chain; it is located at the first cylinder. Chain wheel attached to crankshaft, through a single roller chain, drives a sprocket that sits on a camshaft clutch. The chain passes through two guides and two idler sprockets fixed in a swivel bracket. The chain tension is carried out by turning the bracket using an adjusting bolt with a ball nut.