Mixing system
In unseparated combustion chambers, the entire compression space is a single volume bounded by the piston crown, cover 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 flares emerging from the openings of the nozzle atomizer. The vortex motion of air created during the period of gas exchange is small towards the end of compression and is of secondary importance in chambers of this type. Unseparated chambers are characterized by simplicity of design and high efficiency. The simplicity of the chamber configuration makes it possible to provide relatively low thermal stresses in its walls.
Volumetric mixture formation ensures uniform distribution of the entire cycle 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 a two-stroke engine, vortex formation is provided by the inclined or tangential arrangement of the scavenging ports.
Advantages of volumetric mixture formation: simplicity of the combustion chamber with high quality of its cleaning; small heat loss through the walls of the combustion chamber due to the relatively small surface; good starting qualities of a diesel engine, which do not require additional ignition devices; high efficiency of a diesel engine with a fuel consumption of 155 - 210 g / (kW h). Disadvantages: high excess air ratio (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 small values of the cycle fuel supply.
Volumetric mixture formation 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 is that the outlet and purge ports are located on different sides of the cylinder sleeve. They are connected respectively to the exhaust manifold and to the purge air receiver. The blowdown windows are tilted upward, and therefore the air moves first to the cylinder cover, then, displacing the exhaust gases, changes direction to the opposite.
So that by the time the purge ports are opened, the pressure in the cylinder has time to decrease and become lower than the purge air pressure, the outlet ports are provided above the purge ports. However, in this case, the piston, moving upward, will first close the purge ports, the exhaust ports will still be partially open. The purging process after the closure of the purge ports ends, therefore, a fresh charge of air will escape (partial leakage) through the not completely closed exhaust ports. To avoid this phenomenon, in large engines, the exhaust and purge windows are of the same height, but non-return valves are installed in the purge air receiver, which prevent the exhaust gases from being thrown from the cylinder into the receiver when the windows are opened; The purge starts only when the pressure in the cylinder drops after the outlet ports are opened. When the piston moves up, the purge air will flow until both windows are closed. For the same purpose, in some large engines, a drive spool is installed at the outlet pipe, the drive of which is adjusted so that at the moment the piston closes the purge ports, the spool closes the outlet ports.
The cross-slot blowing method is widespread due to its simplicity.
Camshaft steel. It has two pairs of symmetrical cam washers for each cylinder (front and reverse) to drive the fuel pumps and air distributors. Cam washers of 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 at camshaft placed reversible cylinders. The camshaft consists of a series of sections. Each individual section consists of a cam shaft section of the exhaust valves and fuel pumps and fittings.
The camshaft drive is chain; it is located at the first cylinder. The sprocket, fixed to the crankshaft, drives the sprocket, which sits on the camshaft coupling, through a single roller chain. The chain runs through two guide rails and two idler sprockets, which are attached to the swing arm. The chain is tensioned by turning the bracket using an adjusting bolt with a ball nut.
Mixing in diesel engines occurs inside the cylinder and coincides in time with the injection of fuel into the cylinder and partially with the combustion process.
The time allotted for the processes of mixture formation and fuel combustion is very limited and amounts to 0.05-0.005 sec. In this regard, the requirements for the mixing process are primarily reduced to ensuring the complete combustion of the fuel (smokeless).
The mixing process in marine diesels it is especially difficult, since the mode of operation of the diesel engine on the propeller with the highest speed, i.e., the mode with the shortest time interval during the mixture formation process, corresponds to the smallest excess air ratio in the working mixture (full engine load).
The quality of the mixture formation process in a diesel engine is determined by the fineness of the atomization of the fuel supplied to the cylinder and the distribution of fuel droplets there over the combustion space.
Therefore, let us first consider the process of fuel atomization. The jet of fuel flowing out of the nozzle into the compression space in the cylinder is influenced by: external forces of aerodynamic resistance of compressed air, surface tension and adhesion forces of the fuel, as well as disturbances arising from the outflow of fuel.
The forces of aerodynamic resistance impede the movement of the jet, and under their influence the jet breaks up into separate drops. With an increase in the velocity of the outflow and the density of the medium into which the outflow occurs, the aerodynamic forces increase. The greater these forces, the earlier the jet loses its shape, breaking up into separate drops. The forces of surface tension and the forces of adhesion of the fuel, on the contrary, by their action tend to maintain the shape of the jet, that is, to lengthen the solid part of the jet.
Initial perturbations of the jet arise due to: turbulent motion of the fuel inside the nozzle nozzle, the influence of the edges of the nozzle hole, the roughness of its walls, the compressibility of the fuel, etc. Initial perturbations accelerate the disintegration of the jet.
Experiments show that the jet at some distance from the nozzle breaks up into separate drops, and the length of the solid part of the jet (Fig. 32) can be different. In this case, the following forms of jet decay are observed: jet decay without the action of aerodynamic air resistance forces (Fig. 32, a) occurs at low outflow velocities under the action of surface tension forces and initial disturbances; disintegration of the jet in the presence of some influence of the forces of aerodynamic air resistance (Fig. 32, b); decay of the jet, which occurs with a further increase in the outflow velocity and the appearance of initial transverse perturbations (Fig. 32, c)] decay of the jet into separate drops immediately after the jet exits the nozzle opening of the nozzle.
The last form of jet disintegration must be in order to obtain a high-quality mixture formation process. The decay of the jet is mainly influenced by the rate of fuel outflow and the density of the medium into which the outflow takes place; to a lesser extent, turbulence of the fuel jet.
The jet decay scheme is shown in Fig. 33. The jet at the exit from the nozzle breaks up into separate threads, which in turn break up into separate drops. The jet section is conventionally divided into four annular sections; the outflow velocities in these annular sections are expressed by ordinates 1, 2, 3 and 4. The outer annular section, due to the greatest air resistance, will have the lowest velocity, and the inner (core) has the highest outflow velocity.
Due to the difference in velocities in the cross section of the jet, movement occurs from the core to the outer surface of the jet. As a result of the disintegration of the fuel jet, droplets of various diameters are formed, the size of which ranges from a few microns to 60-65 microns. According to experimental data, the average droplet diameter for low-speed diesel engines is 20-25 microns, and for high-speed ones about 6 microns. The fineness of the spray is mainly influenced by the rate of flow of fuel from the nozzle of the nozzle, which is approximately determined as follows:
To obtain an atomization of fuel that meets the requirements of mixture formation, the flow rate should be in the range of 250-400 m / s. The outflow coefficient φ depends on the condition of the nozzle surface; for cylindrical smooth nozzle holes with rounded entrance edges (r? 0.1.-0.2 mm) is 0.7-0.8.
To evaluate the perfection of atomization of fuel, atomization characteristics are used, which take into account the fineness and uniformity of atomization.
In fig. 34 shows spray characteristics. The ordinate shows the percentage of drops of a given diameter from the total number of drops located on a certain area, and the abscissa shows the diameters of the drops in microns. The closer the vertex of the characteristic curve to the ordinate axis, the greater the fineness of the atomization, and the uniformity of the atomization will be the greater, the steeper the rise and fall of the curve. In fig. 34 characteristic a has the finest and most uniform atomization, and characteristic b - the coarsest, but homogeneous and characteristic 6 - medium fineness, but non-uniform atomization.
The droplet sizes are determined empirically, as the most reliable, since the theoretical path presents significant difficulties. The method for determining the number and size of droplets can be different. The most widely used technique is based on capturing onto a plate covered with any liquid (glycerin, liquid glass, a mixture of water with tanning extract), drops of a sprayed jet of fuel. The micrograph taken from the plate allows one to measure the diameter of the droplets and to count their number.
The required value of the injection pressure, with an increase in which the fuel flow rate increases, is finally set during the period of engine adjustment tests. Usually, for low-speed diesel engines it is about 500 kg / cm 2, for high-speed diesel engines it is 600-1000 kg / cm 2. When using a pump-injector, the injection pressure reaches 2000 kg / cm 2.
Of the structural elements of the fuel supply system, the size of the nozzle orifice diameter has the greatest effect on the spray fineness.
With a decrease in the diameter of the nozzle hole, the fineness and uniformity of the atomization increase. In high-speed engines with single-chamber mixture formation, the diameter of the nozzle openings is usually 0.15-0.3 mm, 2 in low-speed engines it reaches 0.8 mm, depending on the cylinder power of the engine.
The ratio of the length of the nozzle hole to the diameter, within the limits used in engines, has almost no effect on the quality of fuel atomization. The smooth cylindrical nozzle opening of the injector provides the least resistance to the outflow of fuel, and therefore the outflow from such a nozzle occurs at a higher speed than from nozzles of other shapes. Therefore, the smooth cylindrical nozzle provides finer atomization. For example, a nozzle with helical grooves has an outflow coefficient of the order of 0.37, while a smooth cylindrical nozzle has an outflow coefficient of 0.7-0.8.
An increase in the number of revolutions of the engine shaft, and, accordingly, the number of revolutions of the shaft fuel pump, increases the speed of the plunger of the fuel pump and therefore increases the discharge pressure and the flow rate of the fuel.
Consideration of the process of disintegration of the flowing jet of fuel allows us to conclude that the viscosity of the fuel also affects the fineness of the spray. The higher the viscosity of the fuel, the less perfect the atomization process will be. Experimental data show that the higher the viscosity of the fuel, the more sizes droplets of atomized fuel.
The jet of fuel at the exit from the nozzle of the nozzle, as described earlier, breaks up into separate threads, which in turn break up into separate droplets. The entire mass of droplets forms a so-called fuel torch. The fuel torch expands with distance from the nozzle, and, consequently, its density decreases. The density of the torch within one section is also not the same.
The shape of the fuel flame is shown in Fig. 35, which shows the torch core 1 (denser) and shell 2 (less dense). Curve 3 shows the quantitative distribution of droplets, and curve 4 - the distribution of their velocities. The torch core has the highest density and speed. This droplet distribution can be explained as follows. The first drops entering the space of compressed air quickly lose their kinetic energy, but create more favorable conditions for the movement of subsequent drops. As a result, the rear drops catch up with the front ones and push them to the sides, continuing to move forward themselves until they are removed from behind by the moving drops, and. etc. Such a process of pushing away some drops by others goes on continuously until an equilibrium occurs between the energy of the jet in the outlet section of the nozzle and the energy spent on overcoming the friction between the fuel particles, on pushing the leading droplets of the fuel jet, on overcoming the friction of the jet about air, to entrainment of air and to create vortex movements of air in the cylinder.
The depth of penetration of the fuel torch, or its range, plays a very significant role in the process of mixture formation. The depth of penetration of the fuel flame is understood as the depth of penetration of the tip of the flame for a certain period of time. The flame penetration depth must correspond to the shape and dimensions of the combustion space in the engine cylinder. With a short range of the torch, the air near the cylinder walls will not be involved in the combustion process, and thus the conditions for fuel combustion will worsen. With a long range, fuel particles, falling on the walls of the cylinder or piston, form carbon deposits due to incomplete combustion. Thus, correct definition the range of the torch is of decisive importance in the formation of the mixture formation process.
Unfortunately, the solution of this issue theoretically encounters enormous difficulties, consisting in taking into account the influence on the range of the effect of facilitating the movements of some drops by others and the movement of air in the direction of the jet.
All the formulas obtained for determining the range of the torch L f do not take into account these factors and are essentially valid for individual droplets. Below is the formula for determining bf, which is obtained from an empirical law:
Here? - the speed of the fuel jet;
0 - speed of movement in the channel of the nozzle of the nozzle;
k is a coefficient that depends on the injection pressure, on the back pressure, on the nozzle diameter, on the type of fuel, etc .;
T is the range time.
When deriving formula (26), it was assumed that k = const, and therefore it does not reflect reality and, moreover, does not take into account the influence of the previously indicated factors. This formula is more likely to be valid for determining the flight of an individual drop, and not for the jet as a whole.
More reliable are the results of experiments to determine the range. In fig. 36 shows the results of experiments to determine the range L f, the largest width of the torch B f and the speed of movement of the top of the torch? depending on the angle of rotation of the fuel pump roller? at different backpressures in the bomb r b.
The diameter of the nozzle nozzle is 0.6 mm. Injection pressure p f = 150 kg / cm 2 ; the amount of injected fuel? V = 75 mm 3 per turn. The pump shaft rotation speed is 1000 rpm. Torch range at p b = 26 kg / cm 2 reaches L f = 120 cm, and the speed is about 125 m / s and quickly drops to 25 m / s.
Curves? = f (?) and L f = f (?) show that with an increase in back pressure, the range and speed of the outflow of the flame decrease. The torch width B f varies from 12 cm at 5 ° to 25 cm at 25 ° rotation of the pump shaft.
Reduction of the fuel supply period, increase in the flow rate contribute to an increase in the initial speed of the front of the flame and the depth of its penetration. However, due to the finer spray pattern, the torch speed decreases more quickly. With an increase in the nozzle diameter, while maintaining a constant flow rate, the range of the torch increases. This is due to an increase in the density of the torch core.
With a decrease in the nozzle diameter, while the total area of the nozzles remains unchanged, the angle of the cone of the torch increases, and therefore the drag also increases, while the range of the torch decreases. With an increase in the total area of the nozzle openings of the nozzle, the atomization pressure decreases, the flow rate decreases, and the range of the fuel flame decreases.
The experiments of V.F. Ermakov show that preheating the fuel before injecting it into the cylinder significantly affects the size of the flame and the fineness of the spray.
In fig. 37 shows the dependence of the length of the flame L f on the temperature of the injected fuel.
The dependence of the flame length on the fuel temperature in 0.008 s from the start of injection is shown in Fig. 38. It was found that with an increase in temperature, the width of the flame increases and the length decreases.
This change in the shape of the flame with increasing fuel temperature indicates a finer and more homogeneous fuel atomization. With an increase in the fuel temperature from 50 to 200 ° C, the flame length decreased by 22%. The average droplet diameter decreased from 44.5 microns at a fuel temperature of 35 ° C to 22.6 microns at a fuel temperature of 200 ° C. These experimental results allow us to conclude that heating the fuel before injecting it into the cylinder significantly improves the process of mixture formation in a diesel engine.
Numerous studies show that the process of fuel spontaneous combustion is preceded by its evaporation. In this case, the amount of evaporating fuel until the moment of spontaneous ignition depends on the size of the droplets, on the pressure and temperature of the air in the cylinder, and on the physicochemical properties of the fuel itself. An increase in the volatility of the fuel contributes to an increase in the quality of the mixture formation process. The method for calculating the process of evaporation of the fuel flame, developed by prof. DN Vyrubov, makes it possible to assess the influence of various factors on the course of this process, and it is especially important to quantitatively estimate the concentration fields of fuel vapors in a mixture with air.
Assuming that the medium surrounding the drop at a sufficient distance from it has the same temperature and pressure everywhere, with concentration.
When deriving formula (27), it was assumed that the drop has a spherical shape and is motionless with respect to the environment. vapors equal to zero (at the same time, the medium directly at the droplet surface is saturated with vapors, the partial pressure of which corresponds to the droplet temperature), a formula can be obtained that determines the time of complete evaporation of the droplet:
The air temperature in the cylinder has the greatest influence on the rate of fuel evaporation. With an increase in the compression ratio, the evaporation rate of the droplet increases due to an increase in the air temperature. An increase in pressure in this case slows down the rate of evaporation somewhat.
The uniform distribution of fuel particles over the combustion space is mainly determined by the shape of the combustion chamber. In marine diesel engines, non-divided chambers (mixing in this case is called single-chamber) and separated chambers (with pre-chamber, vortex chamber and air-chamber mixing) have been used. One-chamber mixture formation has the greatest application.
Single-chamber mixture formation is characterized by the fact that the volume of the compression space is limited by the bottom of the cylinder head, the walls of the cylinder and the bottom of the piston. Fuel is injected directly into this space, and therefore the spray pattern should, as far as possible, ensure that the fuel particles are evenly distributed over the combustion space. This is achieved by coordinating the shapes of the combustion chamber and the fuel spray plume, while observing the requirements for the range and fineness of the spray of the fuel plume.
In fig. 39 shows diagrams of various undivided combustion chambers. All these combustion chambers have a simple configuration, do not require complicating the design of the cylinder head and have a small value of the relative cooling surface F cool / V c. However, they have serious drawbacks, which include: uneven distribution of fuel over the space of the combustion chamber, as a result of which, for complete fuel combustion, it is necessary to have a significant excess air ratio (? = 1.8? 2.1); The required atomization fineness is achieved by a high fuel injection pressure, which increases the requirements for the fuel equipment and the mixture formation process will be sensitive to the type of fuel and to changes in the engine operating mode.
Combustion chambers can be divided into the following groups: chambers in the piston (schemes 1-5); chambers in the cylinder cover (diagrams 6-8); between the piston and the cover (schemes 11-15); between two pistons in engines with RPM (Schemes 9-10).
Of the chambers in the piston in medium-speed and multi-turn diesel engines, the most widely used is the chamber of form 2, in which the recesses in the piston reproduce the shape of the spray pattern and thereby increase the uniformity of the distribution of fuel particles. In order to improve the mixture formation in the unseparated chambers, the air charge of the cylinder is given a vortex motion.
In four-stroke diesel engines, the vortex movement is achieved by setting screens on the intake valves or by the appropriate direction of the intake channels in the cylinder head (Fig. 40). The presence of screens on the inlet valve reduces the flow area of the valve, as a result of which hydraulic resistances increase, and therefore it is more expedient to use the curvature of the inlet channels to form a vortex air movement. In two-stroke diesel engines, air swirl is achieved by the tangential arrangement of the purge windows. A very uniform mixture formation is achieved in chambers, most of which are located in the piston (see Fig. 39, schemes 4 and 5). In them, when air flows from the sub-piston space (during the compression stroke) into the chamber, radially directed vortices are created in the piston, which contribute to better mixture formation. Cameras of this type also called "semi-separated".
Chambers located in the cylinder cover (see Fig. 39, Scheme 6-8) are used in two-stroke engines. The chambers between the piston and the cylinder cover (Fig. 39, schemes 11-15) are of the most advantageous shape without large indentations in the piston or in the cylinder cover. Such cameras are used mainly in two-stroke diesel engines.
In combustion chambers between two pistons (see Fig. 39, diagrams 9 and 10), the axis of the nozzles is directed perpendicular to the axis of the cylinder, with the location of the nozzle holes in the same plane. In this case, the injectors have a diametrically opposite arrangement, thereby achieving a uniform distribution of fuel particles over the space of the combustion chamber.
Mixing is the preparation of a combustible mixture to prepare fuel for combustion in an internal combustion engine cylinder. The combustion process lasts a very short time, for example, in the MOD it is 0.05-0.1 seconds, in the FOS - 0.003-0.015 seconds. In order to ensure complete combustion of the fuel in this short period of time, it is necessary to prepare a working mixture consisting of finely atomized liquid fuel ( diesel internal combustion engines) or fuel vapors (carburetor internal combustion engines) mixed with air. To provide High Quality mixture, which is estimated by the coefficient of air surplus (α), the fuel must be finely atomized and evenly distributed throughout the entire volume of the combustion chamber. The chamber should be configured to match the shape and range of the torch from the nozzle.
The formation of a fuel flame is characterized by range, spray cone angle and fuel droplet size. For better use, the torch forms a droplet mist in the form of a diverging cone. This mist must penetrate into all parts of the combustion chamber, but not touch the surfaces of the CPG parts. Drops of fuel falling on the walls of the cylinder bushing dissolve the oil film, mix poorly with air and burn incompletely, forming soot and carbon deposits. According to the method of mixture formation, engines are distinguished into:
1). Single chamber- jet mixture formation with direct fuel injection, used in large and medium-power internal combustion engines with various piston head shapes. They have a small heat transfer surface and therefore little heat loss. This gives great economy and good starting properties.
Disadvantages: high fuel injection pressure (up to 1200 kg / cm 2), complicating the fuel equipment, rigidity and increased engine noise.
2). Pre-chamber- such mixture formation is applied to the FOS with a cylinder diameter D = 180-200 mm. The pre-chamber is located in the cylinder cover, the volume of which is 20-40% of the total volume of the combustion chamber. The antechamber is connected to the main chamber by channels, the number of which can be from 1 to 12. Part of the fuel burns in the antechamber, so there is no need to supply it with high pressure. Such ICEs are less sensitive to fuel quality.
Disadvantages: increased specific fuel consumption, difficulty in starting in the cold season, significant heat losses due to the large cooling surface, low efficiency of the engine.
3). Vortex chamber- It is also used for FOD in the form of a spherical or cylindrical combustion chamber located in the cylinder head. Its volume is 50-80%. It communicates with the main combustion chamber with a large-section channel. Air entering the vortex chamber during the compression stroke receives a rotational motion. Due to this, the fuel injected under a pressure of 100-140 kg / cm 2 mixes well with air and burns out. Together with hot combustion products, part of it flows into the main chamber, creating vortex flows, where it burns completely.
Advantages: reduction of α, smokeless exhaust, low injection pressure, use of single-hole nozzles of injectors, which reduces the cost of manufacturing fuel equipment.
Disadvantages: the complexity of the design of the cylinder cover, the difficulty of starting a cold engine and the need to use an incandescent coil to heat the air in the chamber.
4). Film- the combustion chamber is located in the piston head and is directly connected to the above-piston space. The chamber diameter is ≈ 0.3-0.5D of the cylinder liner. The piston head is oil-cooled, so the temperature of its outer surface is no more than 200-400 ° C. Fuel is injected at a pressure of ≈ 150 kg / cm 2 through a multi-hole nozzle. Approximately 95% of the fuel falls on the inner surface of the piston chamber in the form of a thinnest layer, the rest is sprayed into the volume of the combustion chamber. At first, the atomized fuel self-ignites, then its vapors ignite from the burning torch. Intensive mixing of fuel vapors with air occurs due to vortex formation. ICEs with such mixture formation are multi-fuel, i.e. can use light and heavy fuels.
Internal combustion engines can be classified according to various criteria.
1.By appointment:
a) stationary, which are used at power plants of low 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 kind 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 cylinder of the piston engine, 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 mixing 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 changing the load, 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 the 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 takes place; 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 motor.
9. According to the cooling method, engines are distinguished:
a) liquid-cooled;
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.
1. The process of mixture formation in spark ignition engines.
The complex of interconnected processes of dosing fuel and air, atomizing and evaporating fuel, as well as mixing fuel with air is called mixture formation. The efficiency of the combustion process depends on the composition and quality of the air-fuel mixture obtained during mixture formation.
In four-stroke engines, usually external mixture formation which begins by metering fuel and air in the injector, carburetor or mixer (gas engine), continues in the intake tract and ends in the engine cylinder.
There are two types fuel injection: central - fuel injection into the intake manifold and distributed - injection into the intake ports of the cylinder head.
Fuel atomization with central injection and in carburetors, it begins at a period when the fuel stream, after its exit from the nozzle or atomizer hole, under the influence of aerodynamic drag forces and due to the high kinetic energy of the air, breaks up into films and droplets of various diameters. As the droplets move, they break up into smaller ones. With an increase in the fineness of atomization, the total surface of the droplets increases, which leads to a more rapid conversion of fuel into steam.
With an increase in air speed, the fineness and uniformity of atomization improve, and with high viscosity and surface tension of the fuel, they deteriorate. So, when starting a carburetor engine, there is practically no fuel atomization.
When injecting gasoline, the atomization quality depends on the injection pressure, the shape of the atomizing nozzles and the speed of the fuel flow into them.
In injection systems, electromagnetic nozzles are most widely used, to which fuel is supplied at a pressure of 0.15 ... 0.4 MPa to obtain drops of the required size.
Spraying of the film and fuel droplets continues when the air-fuel mixture moves through the sections between the inlet valve and its seat, and at partial loads, in the gap formed by the covered throttle valve.
The formation and movement of a film of fuel occurs in the channels and pipelines of the intake system. When the fuel moves, due to interaction with the air flow and gravity, it partially settles on the walls of the intake manifold and forms a fuel film. Due to the action of surface tension forces, adhesion to the wall, gravity and other forces, the speed of the fuel film is several tens of times less speed mixture flow. Fuel droplets can be blown off the film by a stream of air (secondary atomization).
When gasoline is injected, usually 60 ... 80% of the fuel gets into the film. Its amount depends on the location of the nozzle, the range of the jet, the fineness of atomization, and in the case of distributed injection into each cylinder - and from the moment of its start.
In carburetor engines at full loads and low speed, up to 25% of the total fuel consumption falls into a film at the outlet of the intake manifold. This is due to the low air flow rate and insufficient fuel atomization fineness. When the throttle valve is closed, the amount of film in the intake manifold is less due to the secondary atomization of fuel near the valve.
Fuel vaporization it is necessary to obtain a homogeneous mixture of fuel with air and to organize an efficient combustion process. In the intake port, before entering the cylinder, the mixture is two-phase. The fuel in the mixture is in the gas and liquid phases.
With central injection and carburation for film evaporation, the inlet line is specially heated with liquid from the cooling system or with exhaust gases. Depending on the design of the intake tract and the operating mode at the outlet of the intake pipeline, the fuel is in the form of vapors by 60 ... 95% in the combustible mixture.
The fuel vaporization process continues in the cylinder during the intake and compression strokes, and by the beginning of combustion, the fuel is almost completely vaporized.
With distributed fuel injection onto the intake valve plate and the engine running at full load, 30 ... 50% of the cyclic dose of fuel evaporates before entering the cylinder. When fuel is injected onto the walls of the inlet channel, the fraction of evaporated fuel increases to 50 ... 70% due to an increase in the time for its evaporation. In this case, heating of the intake manifold is unnecessary.
The conditions for the vaporization of gasoline in cold start modes deteriorate, and the fraction of the evaporated fuel before entering the cylinder is only 5 ... 10%.
Uneven mixture composition, entering different engine cylinders, during central injection and carburation is determined by different geometry and length of channels (unequal resistance of the branches of the intake tract), the difference in the velocities of air and vapor, drops and, mainly, the fuel film.
With an unsuccessful design of the intake tract, the degree of uniformity of the mixture composition can reach ± 20%, which significantly reduces the efficiency and power of the engine.
The uneven composition of the mixture also depends on the operating mode of the engine. With central injection and in a carburetor engine, as the speed rises, the atomization and evaporation of the fuel improves, therefore the unevenness of the mixture composition is reduced. Mixing improves as the engine load is reduced.
With distributed injection, the uneven composition of the mixture over the cylinders depends on the identity of the injectors. The greatest unevenness is possible at idle at low cyclic doses.
The organization of external mixture formation of gas automobile engines is similar to carburetor engines. Fuel is introduced into the air stream in a gaseous state. The quality of the air-fuel mixture during external mixing depends on the boiling point and the diffusion coefficient of the gas. This ensures the formation of an almost homogeneous mixture, and its distribution over the cylinders is more even than in carburetor engines.
